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GROVES AND THORP'S
CHEMICAL TECHNOLOGY
OB
CHEMISTEY
APPLED TO ARTS AND MANUFACTURES
VOL. IV.
ELECTRIC LIGHTme AND PHOTOMETRY
Other Volumes of this Series
Ifidited by
Ghas. £. Gboybs, F.B.S., usm Wh. Thorp, B.So.
FUEL AND ITS APPLICATIONS. By E. J. Mills. D.Sc..
F.K 8., and F. J. Bowan, C E. With 600 lUuatration*.
Cloth, net» S5.00
UGHTING BT CANDLES AND OILS. Fatb ahd Oils,
by W. T. Dent. Stearinb Ihdustbt. by J. McArthnr.
Candlb MANurAOTTTRB, by L. Field and F. A. Field. The
FBTROLKUif iNDtJSTBT AND LAMPS, by Boverton B«dwood.
MlMEBS' 8AFBTT LAMPS, by BoYerton Bedwood and D. A.
Loais. With 360 lUtutrationa. Cloth, net, 94.00
GAS LIGHTING. By Charlbs Hunt, Manager of the Bir-
mingham Corporation Gas Works. With 800 Illaatrationa.
Cloth, net, 9&.B0
CHEMICAL TECHNOLOGY
OR
CHEMISTRY IN ITS APPLICATIONS TO
ARTS AND MANUFACTURES
WITH WHICH IS INOORPOSATED
RICHARDSON AND WAHS' CHEMICAL TECHNOLOGY
VOL. IV.
SDITED BT
W. J. DIBDIN, F.LC, F.C.S., &3
«/•
ELECTRIC LIGHTING
A. G. COOKE, M.A., A.MJ.E.E.
BBAD OV THB aUKIRIOAL IHQIHmntnrO AVD PHT8I0B DIPABTMBRT
or TBM BATTU8XA POLTnOHHIO
PHOTOMETRY
BT
W. J. DIBDIN, P.I.C., F.C.S., Ac.
fOSMlBLT CBXVI8T AND SrPBBIllTlirDINO GAB IXAICIIIXR TO
THB XBTROPOLITAN BOABO OV WOBKS AHD TBI
LOITDOH OOUMTT OOUNOIL (1882-1807)
PHILADELPHIA
P. BLAKISTON'S SON & CO.
1012 WALNUT STREET
1903
V.
io'^ ' '-'6 '. h
i» I
Pierce Fund.
\oL.Z\
y >
PEEFACE.
« * I
Sons apology is perhaps neceeeary for the attempt to compress mto the
gpaoe of some two hundred and seventy pages a mass of information
oovering the whole subject of Electric Lighting. The aim has been to
put together a connected, and, as far as possible, complete and scientific
account of the whole subject in such a manner as it is hoped may be useful
to specialists in one or more branches of work connected with electrical
industry who may wish for a rii^ime of all the various systems, machinery,
lamps, &c., in use for the supply of electric lighting; more especially
to Architects, Civil and Mechanical Engineers, and scientific workers, to
whom a general knowledge of the subject is of professional value. To such
readers the completeness of their general view is of the highest value, though
it may not be the detailed knowledge of the actual worker in one br«&oh,
since they wDl wish to know the reasons, not merely the bare facts, of
certain data and limitations which, originating in the achievements of a
collateral industry, may apply to their own professional work.
It is hoped that the work may be read with facility by any one who has
an elementary knowledge of Electricity and Magnetism, or even that
knowledge which every educated person will acquire by observation
without further instruction and reading. Efforts have been made to avoid
gaps in the logical continuity of the argument ; and to simplify formulee
by a reduction to the minimum of all minor corrections necessary to
professional exactness. In short, the aim is a readable treatise, as well as
a work of reference. Details which are likely to be ephemeral only appear
as examples ; and the examples of manufacture chosen for description, or
theory of design, are not necessarily those believed to be the most recent
or the best, but such as are thought serviceable for expounding the
principles and methods of manufacture and working.
The section relating to photometry has been carefully compiled with a
view to the full description of existing and proposed standard methods
of determining the visional intensity of artificial illumination, and
particularly with a view to showing the necessity for an International
Agreement as to a standard of light. The experience which has been
gained during the past two decades by the investigations carried out in
England, France, Germany, and America has succeeded in the production
VI PREFACE.
of various methods, many of Tvliich are practical and reliable; but the
danger underlying this work is that in the struggle to pioduce an
acceptable proposition, the initial value of the *' standard of light'' may be
unconsciously lowered. For this reason it is desirable that a " standard of
reference" should be agreed upon by the respective governments. The
molten platinum standard of M. Tiolle has been recommended for that
purpose, although it is objected to as an instrument for daily use. The
question of public lighting is one of so much commercial importance that it is
most desirable that a common agreement should be arrived at in regard to
the valuation of the light sold and paid for. In consequence of the introduc-
tion of the arc light, and that produced by the Welsbach Mantle system,
the quaUty of the light requires as much or even more consideration
than its quantity. That is to say, that it is by no means certain that the
photometrical equivalents as ordinarily understood are parallel with the
commercial or eyesight equivalent. For instance, the light of k candies
produced by the arc light may be the only description of light suited
for a particular purpose; no multiple of m candles produced by, say,
ordinary gas flames, being capable of taking its place. The value of a given
light must be in relation to the object for which it is employed, and
not according to some arbitrary standard having a spectroscopic character
totally unsuited for many special purposes. "Whilst this point of view
is one that must not be lost sight of, it is nevertheless undoubtedly desirable
that some basis of general agreement should be arrived at. Doubtless the
most reliable standard would be one in which the respective portions
of the spectrum were as nearly as possible comparable in their relative
intensity to sunlight, and that the photometrical ratio of each of the major
divisions of the coloured spectra should be agreed upcn acd tabulated for
general reference. This is in fact the lines upon which Abney, Kichols and
others have worked, but, unfortunately, their researches have not yet
reached the stage when the commercial world becomes aware of their
importance.
In order to make the work complete, from the point of view of the
''gas photometrist," the valuable ''notification of the Metropolitan Gas
Beferees " has been added in the form of an appendix, as it describes
the recognised methods for the estimation of the impurities in coal gas,
viz., sulphur, ammonia and sulphuretted hydrogen. Although not strictly
coming under the head of photometry, yet it is so clearly associated with it
from the point of view of the Gas "Works Manager that the subject
cannot be overlooked y>i\h. advantage.
CONTENTS.
»♦■
PAOS
Jntrodnctioii i
Pnnction of Electric Currents a
Electric Power 3
Sabdivision of Power . • • • 4
Hydraulic Analogy • • • 5
Condaotors 6
Specific Keeistanoe 6
Board of Trade Regnlations 8
Hinimiim Siae of Oonductore • 8
Current Density 10
«
Standard Sixes of Cables 11
Insulation 12
Striking Distance of Disruptive Sparks 14
Insulation Tests 15
Possible Oausee of Fire 16
Defective Insulation 17
Fusible Cot-outs 18
Size of Fuse-Wire ao
Street Mains 21
Conduits 22
Bare-strip Systems 25
Oil Insulation 26
Electrical Testing 27
Parallel DiBtribation 29
Choice of Electromotive Force 30
Fall of Potential 32
Efficiency of Distribution 33
Network and Feeder Mains 34
Multiple- Wire Systems 35
Electro-Magnetiflin 37
The Dynamo 37
Magnetic Fields ... 38
Vlil CONTENTS.
PAGE
Eleotro-Magnetisni {continued)
Magnetio Force 39
Hagnetio Induction 4^
Magnetisation 43
Magnetic Circuits 44
Reloctance 45
HystereBis 47
The Closed Dynamo (Gfreneral Theory) 52
Lens's Law 53
The Closed CoU 54
The Armature Core • . 56
Drum Winding 59
Calculation of E.M.F 60
Distortion of Magnetic Field 61
Oommntation •• 62
Armature Reaction ........... 64
Sparklessness . . . • 66
Armature Winding • • . • 68
Field Magnets • • . • 70
The Closed- Coil Dynamo (Design and Begulation) . , . 74
Leakage 76
Characteristics 77
Power Curve ••• 79
Critical Point of Series Dynamo • • 80
Shunt- wound dynamo •• 81
Speed Variation • • . 83
Compound Winding 84
Magnetisation 85
Relation of Size to Capacity •..•••.. %j
Reluctance 89
Heat, Escape of 91
Stress on Conductors 92
Hopkinson's Test for Efficiency 94
Storage Batteries 95
Elementary Forms of Batteries 95
•* Formed" and *• Pasted "Plates 97
Electrolyte 99
Resistance 100
Capacity and Efficiency loi
Chemical Actions 103
Leading Types of Accumulators 104
Secondary Batteries — E.P.S. Cells 105
Portable Storage Cell . 106
Charge and Discharge, Rate of 107
Plants Types 109
Secondary Battery Room no
CONTENTS. IX
PAGB
Storage Batteries (eonHnmed)
Use of Seoondary Batteries for Lighting • ... iii
Train Lighting 113
Central Station Practice 115
Switches 116
Begalation with "Boosters " 117
Ck)ntinuoii8 Current Transformer Systema 117
Multiple Wire-Systems • . 117
Battery Transformation 119
Oontiaaous Current Transformer I20
Efficiency of Transformation ••...... 123
Series Distribution 124
Arc Lamp Circuits . • . 126
Arc and Lighting Dynamos 127
Connection of Armature Coils 130
Sfficiency 132
The Shunt or '*TesMr'*Oironit ......... 133
Commutation witih Double Brushes 135
Bectiflcation of Alternating Currents . 136
Pulsating Onrreats 138
Incandescent Lampe In Seriee 140
Alternating Cnzxents (Theory) 141
GenenUion of Alternating Currents 142
Self-lDdnotanrBWi 145
Impedance 146
GrapUoal Methods 146
Distortion Due to Iron Cores 148
Curve Trading 149
Mntnal Inductance 150
AltsnuUing Current Transformers 153
An^Dgy to Continuous-Current Transformers . • » • 154
AlteruH^ng Currents (Machinery) 156
The Parsons' Turbo Alternator ....;... 157
Siemens Alternator 1 58
Moltipolar Fields 159
Ferranti Alternator 160
Crompton Alternator 160
Mordey Alternator 161
Kapp Alternator 164
Blwell-Parker 165
Alternating Current Transformers . . 167
Lamination 169
Eddy Currents 170
Hysteresis 171
Deterioration of Iron 172
X CONTENTS.
PAGB
Alternating CurrentB (ICaohinery) {oontinued)
Winding 173
Hedgehog and Ferranti Transformers 175
Lowrie-Hall Transformer . • • • 176
Brash Transformer 177
Eapp and Snell Transformer .178
Weston Transformer 178
Transformer Losses • • • • 179
Power Factor 181
Transformer Testing 182
Dimensions of Transformers 185
Alternating Current Distribution 187
Long Distance Transmission • • • 187
Snb-Stations 188
Transformers in Parallel 190
"Skin Effect" 191
Undergroand Mains 193
Safety Devices 194
Polyphase Carrents •• 195
Two-phase Alternator 196
Three-phase Alternator 197
Rotary Converters • . 198
The Coupling together of Generators 200
Oontinuous Current Generators 201
Dynamos in Parallel 202
Coupling of Alternators 203
A^jastment of Excitation 205
Synchronising 206
Incandescent Lamps 207
Radiation ..•••••••••. 208
Semi-incandescent Lamps 209
Carbon Filaments 211
High and Low Voltage Ijamps 213
Flashing 215
Exhaustion 217
Candle Power and Efficiency 218
Dimensions of Filaments 221
Variation of Efficiency . • . 223
The Nernst Lamp 229
Arc Lamps 231
The Electric Arc 232
Temperatare of Arc 233
Carbon Electrodes 234
Appearance of Arc 235
Distribution of Light 237
CONTENTS. XI
rACB
Arc Lamps {fiontinusd)
Candle Power .•••••••••• 239
Alternating Corrent Arcs 240
Effect of Gored Carbons 243
R.^gulating Mecbaniflm . • • • 243
B.irly Forms of Arc Lamp •• 244
Pilsen Arc Lamp 247
Brush Arc Lamp 248
Brockie-Peel Arc Lamp , 250
B.C.C. Arc Lamp 252
Crompton-Pochin Arc liamp 253
PboBoiz Arc Lamp . 254
Siemens "Band" Arc Lamp •••••••• 255
Lona Arc Lamp •••••• 257
Enclosed Arcs •••••• 258
Jandns Arc Lamp •••• 259
Searchlights ••••• 259
Central Station Economy 260
Fael 261
Boilers ••••••• 263
Irregularity of Demand ••••••••• 265
Refuse Destructors •••••••••• 266
Efficiency of Machinery ••• • 267
Value of Condensation •••• 269
Parsoni' Steam Turbine •••• 269
Willan's Engine ••••••271
Efficiency at Reduced Loads •••••••• 273
Efficiency of Distribution « 274
Methods of Charging for Electrical Energy •••••• 276
Meters 277
Photomotry ••••••••■••• 279
Law of Inverse Squares ••• 279
Bougner's Photometer •••••••••• 281
Foncault's Photometer 281
Rumford's Photometer 281
Bunsen's Photometer 281
Lethebj's Photometer 282
Toolej Street Photometer • 283
Erans* Photometer 283
Haroonrt*s Table Photometer 284
Radial Photometers 290
Dibdin*s Radial Photometers 291
Hartley's UniverMil Photometer • 292
Tests by Radial Photometer 294
Harcourt's Holophotometer 301
Preece's Phoiometer 304
Xll OONTEKTS.
PAGE
Photometry {continued)
Freece and Trotter*8 Photometer 305
Trotter's Photometer 305
Optical Photometers • 308
Weber's Photometer 311
Grosse's Photometer 313
Lummer and Brodhun's Photometer 317
Discs •••• 319
Standards of Light 319
The English Sperm Candle 319
The French " Oarcel " Lamp 324
The German Standard 325
Comparison of Various Standards 325
The Hefner Lamp • • • • • 327
Amyl-acetate • • • • • 333
Kriiss Flame Measare 335
The Patch Standard 336
Proposed Substitutes for Candles ••336
Harconrt's One Candle Pentane Flame 337
Methven's Screen 340
Sugg's Ten Candle •* Test " • " 341
Violle's Molten Platinum 341
Dibdin*s Pentane Argand 342
Harcourt's Screened Lamp (No. 2) 348
Harcourt's Ten Candle Lamp 350
VioUe's Acetylene Standard 353
Methods of Determining the Illuminating Power of Coal Gas 354
Dr. Pole's Law 354
Comparative Results 356
Appendix A.'
Sugg's London Argand Burner 357
Appendix B.
Gas Referees* Table for Correcting Volume of Gas for Barometer and
Thermometer Readings . • 358
Appendix C.
Metropolitan Gas Referees* Notification 360
As to Times and Mode of Testing for Illarainating Power . . . 360
As to Times and Mode of Testing for Purity 362
Sulphuretted Hydrogen 3^2
Ammonia 3^3
Measurement of Gas and Rate of Flow . , 3^4
CONTENTS. Xiu
PAGE
Appendix C. {eontinufid)
Snlphnr Compoands other than Sulphuretted Hydrogen • • . 364
Pressure at which Gras is Supplied 3^
Meters 3^7
Measure for Testing Meters •••••.••. 368
Beports 370
As to Illuminating Power 370
As to Maximum Amounts of Impurity 370
Sulphur Compounds other than Sulphuretted Hydrogfin . • • 370
Sulphur Test. 370
Gas Referees* Street Lamp Pressure Gauge • • . • • . 371
Provision of Pentana ••••'•••••• 273
INDEX 374
ILLUSTEATIONS.
n »»«
nc
ftaomoAL TsniHoi
OoBiiflottDn for eondnotlvllj tMfe ••••••••!
a IntulrtlflB rw^rtinw ••••••••3
Oft
llBid 3
Ifnttyrm magnedo fltiUI 4
IiMOBplito rlBg-augntl 5
XyMttMli onrrs •••••••••••6
Twiimiifint of hjitererti Ui 7
Ooiir Dtvaiio s
IMagmii of rlng!-wlBdiii|^ • 8
LiuoB of loroe through amiAtiire core 9
Dtopoaltion of Uhm of luoe xo
» w •» •••••••••II
IMognm odE dmm-^wlndliig •••.•.... la
Diitortloii of magiiodo Held 13
TfemagiMtUtag tunm of armatats •.....•. 14
]>lflerenoe of potontlal of dynamo ••••..•. 15
Xjaa*8 halAndng eoilt 16
» • 17
8ay«r*« ofBiftton 18
Sewnlttle aotion o( eommiiftator eoUa 19
Broib E.E. Co.*i two-pole rimple mAgnetlo dyiuuno ..... 21
n „ double Upohur mAgnetic dronlt as
■nltipolar djnamo 23
Choimeteristic of serles-woand dynamo 24
Power caires for seriee-wonnd dynamo 25
CMiieal point of eharaeterlstio so
Formation of eharaoterlette of drant-woond dynamo .... 07
Orttieal point of „ » » 98
Powor eor?ea of divnt-wonnd dynamo 09
Sfleot of speed on eharaoteriatics 30
Lines of foroe in air-gap 31
Determination of Ioimb 1b dynamo anoatnr* 3a
BnwAGB BATTsnns :
Storage-cell hydrometen 33
8ec<M^Uiy batteriei — eleetrle power storage cells . . • . • 34
Portable Btoimge cell 35
Beoondazy battery room (Grompton-Howell cell) • • • • . 36
Btonge battery regulating switch .....••. 37
Blonige batteiy eharge and discharge switch 38
OomoHuotrs CuBBcirr TRAVSVouinB Ststeim : •
OODtlnnoQfl current transformer •••••••••39
a8
a8
38
38
48
55
55
57
57
59
61
63
64
65
65
67
67
67
70
71
73
78
79
80
81
82
82
83
89
94
102
X05
106
ixo
117
117
121
XVI
ILLUSTBATIONS.
SSRIM l>iaTRIBnTIOK t PIC.
Bnuh aro-llghtlng: dyiutmo ••.40
Core ring of bnub Aro-IighUng dynamo 41
DlAgimm of eonnecUonfl of Bniih-Oeipel ragnlator 4a
ft M t» »»••••• 43
Thomson-Hooston aro-lSglitIng dynamo 44
Fermntl rectifier 45
Ooldfton aeriea Incandaaoent lamp ••• 40
AiTEurATijra Correhtb:
Cloek-dJa^ram of emrent ••••• •••• •47
Cnirea of aieotro-mottye force •••• 48
Cyclic eorraa of enrrent .••••••••. 49
Deformation of the cnrrentrcnrvo •••.••••5^
GrapUa xqmaentation of power mppltod to the dreott • • • • 51
▲LranvATiiro Cusbxhtb (KAOHiinBT)t
Panone* tnrbomltemator 53
Biemena* alternator ••• • • S3
CrompCun alternator .••••••••••54
Hordey alternator ••••••55
Armature of alternator ••••• 56
lield-magneia of alternator 57
Knpp alternator 58
59
Xlirell-Farker alternator 60
Portamonth electric lighting atatlon with Ferranti alternator • • • 6z
7araday*a anchor-ring 6a
Babatation of Kleetric Supply Corporation (Ferranti tnnafonnen) . • 63
Lowrie-Hall transformer •••.•64
TheBroah „ 65
Kapp and BnOil tranaformer 60
Weaton tranaformer ••••••••••• 67
Oalorlmetrio teat of tnaafoimer • ..dS
AXiTSBVATIHO CURBXHT DlSTBIBUIIOa t
Oardew earthing devioe ••••••••••69
Oerlikon three-phaae alternator •••••••••70
Ths CouvLnro togbthbb or Gknbratobi i
ElectromotiTe force in the drooit of the armataree • • • • • 71
Haximnm yalne of dlfflerence of potential • • 79
Diflerenoe of potential ••• •••••••73
Ikcaitobsobiit Lampix
High Yoltage lampe •••• ..74
Low n . 75
Beal for large lampa ...Td
Bhort aeal for small lamp •• ••77
Lamp with parallel fllamenta .••••••••78
Details of bayonet socket 79
Screw terminal holder for large lamp 80
Variation in candle power during life of incandescent lamp • • • 8z
Variation of effloiency « „ „ „ ■••8a
Cost per candle hour for lamps .••••••■•83
The Nemst lamp ••••■•••••••84
Arc Lamps:
niamination of oontinnons current arc lamp in different directions • • 85
Distribation of illumination with an alternating current arc ... 86
Brush arc lamp 87
« .. n 88
Brockie-FoU arc lamp 89
•f .• M • . • 90
Electric Construction Co/s arc lamp 91
n n WWII 92
PAGB
za8
xa9
133
133
13s
137
141
U7
147
148
148
X5«
^S7
Jg
z6z
z6a
Z63
X64
X64
z68
174
X76
X77
178
178
z8a
004
»5
•05
8x3
813
fi;6
816
ai7
817
sao
sa7
sa7
839
830
837
84Z
"49
849
8SX
8SX
858
85a
ILLUSTBATIONS.
xvu
Am LAJiPfl (oohIAmimO nc.
Ciom]ytoii-Poohlii ■tng'le-carboii are lamp .••••••93
The Phflonlx arc lamp •••.94
Btemenfl** band** are lamp • • • 95
Tbe Luna aze lamp 97
• tfWft* •••••••••• •9^
Cehtbai. STAnov Boovomt ;
WlUan'i eeatial TalTe engin* •••••••••99
t
Law of Inrene Sqcaiea xoo
„ „ „ M loi
Lethebj*B photometer • zoa
TooleySt. « '^3
Bvana « X04
HareonrtTa table photometer « • X05
Segulatlng tap of Harcoiut*a table photometer 106
Clamp for » m n x^
Fhotoped of „ « m xo8
Harooiirf 8 table photometer ••.•••••- 109
Haxtley'a nnlTersal photometer ••••••••• no
Dlbdln's radial « xzx
Haiaontal raja of light (Hat flamea) zia
Angul a r . ,,»» ^w*. ••••• X13
Hedaontal » » » » » X14
^ n n » » <I5
n n ft M n • • • • • • * aXld
9 M >t » compoond reflector •••••• Z17
M » w n wlthont reflector zx8
M it M tt from Argand burner Z19
•, « « » » • •• with reflector . . . lao
i» ••.»•• w w Mw »nd onp . ifli
Angnlar n^ from Argand bnmer 133
„ » • • • with reflector only • .133
" m n m ^ n n m and CBp . . . I34
ft n w Argand bnmer ••«••••• 135
„ „ n n n wlth oardboard ahado . • . .136
„ f> H "Chriatlana'* burner • . . 137
n n n n Wlth Opal globo .... 138
B]areoart*8 taolophotometer • • 139
» • 130
» n X3I
Freeoe'a photometer 133
Preece and Trotter'a photometer 133
Trotter^ photometer (plan) 134
« « (aection) 135
« acieen 136
X37
Weber'a photometer 138
Oroeae'a « 139
m • 140
n n 141
Lnmmer and Brodhnn's photometer 142
• n n n 143
m ** n m ........ I44
• n m m ..••••.. 145
BTAnDABDB OF LlOHT I
Careel lamp and balance 146
The Hefner lamp 147
n •• n 148
n n n 149
» n 150
• ... 151
m n n ... 152
» n right-gange ... 153
• f. n 154
rxcB
353
aS4
255
357
357
979
279
380
38a
383
283
385
386
S87
388
389
393
393
396
396
396
397
397
397
397
398
398
398
399
399
899
300
300
300
300
30X
3<M
303
304
305
305
307
307
SIX
3x3
3x4
3x5
3»7
3x7
3x7
318
335
329
330
330
33X
331
331
331
332
332
XYUl
ILLUSTEATIONa
0rAin>AKD8 ov Light (continued) fig.
The Hefner ilght-gwDge 156
157
KrfiM flame nwesare 158
Datoh ■tandard lamp 159
HarooQit'e ono-eandle pontane standard 160
Methvenli wreen ataodard z6i
Vlolle's molten platinam standard i6a
Dlbdin*B pentane^urgand 163
H I. 164
m 165
M It ft I^
ft n w >67
ft H M ..•.•••.•• 109
H n n 169
, 170
•. n » 171
«• »» n •••••••••• 173
Harooart^ pentane lamp (No. a) • • 173
n n •» w •••••••• 174
„ ten*«andle lamp
VloUe^ acetylene etandani 177
Sngg'a Standard London Argand •••••. Z78
Qas Referees* one twelfth eidiie foot measure •••••• 179
» „ snlphnrtest • • • x8o
It M pressnre gange ••■•••• zBx
TAOB
33a
335
336
337
340
349
343
344
344
344
345
945
345
346
347
347
349
349
350
35«
353
357
368
371
37a
PLATES.
PukTB I. HopkfaMon'i Yoke ; • • • epf09itepag%
„ II. Magnetic Cjole for Wroaght Iron • • • • «.
t*
»f
III. n » Annealed Iron Wire •
IV. M M Oast Iron . . • •
y. n „ Annnled Steel • • •
VI. „ « Steel Wire, Annealed .
VII. ^ ^ Steel Wire, Glaa Hard
VIII. Belatlon of HTstereels to iff^-g<«n«iw Induction .
GL Synthetic Cnrre for Dynamo liagnetio Clrenit
(Bdiaom-Hopklnion Type) ....
X Synthetic Carre for Dynamo Hagnetlo CIronIt
(Manchester Type) • • • • •
.4a
47
47
47
47
47
47
SO
77
77
CHEMICAL TECHNOLOGY.
■t» *
ELECTRIC LIGHTING.
CHAPTER L
Introduction.
Is attempting to arrange a programme which may serve as a logical hasis
for the discussion of the numerous different systems whereby electric
currents are used for the purposes of lighting, it is found that they may all
be grouped under three or four distinct headings. In each of these groups a
distinct class of machinery is required for the generation of the electric
currents, a distinct arrangement d conducting wires for their distribution,
and each group of systems has its own peculiar advantages according to the
purpose for which the lighting is required, whether pubUc or private, and
according to the distance to which transmission from the centre of genera-
tion has to be made. No system can be pronounced intrinsically better than
another, although for some special purpose the question as to the best
qrstem for that particular purpose may need most detailed consideration
before being finally adopted.
To explain the fundamental cause for the variation of these systems, it
is necessary to consider this fundamental difference that distinguishes light-
ing by means of electric currents from other systems of illumination dis-
cussed in this series. Whereas the consumption of fuel is the primary
source of all means of illumination (except in the case where wind or water-
power is employed), in other methods of illumination than that by means
of electric currents the fuel is obtained and reduced to a more or less port-
able form, and distributed in this form to the various centres of illumi-
nation where its combustion produces not only light but also heat. The
discussion of other means of illumination therefore concerns itself mainly
with the chemical preparation of the fuel in a form suitable to distribution
and combustion at various distributed centres. Now it is most important to
distinguish electricity, or the electric current, from the fuel. Neither does the
quantity or magnitude of the electric current necessarily bear any relation to
the amount of illumination obtainable by its means, nor do the valuations
of the systems of electric lighting arise from variations in the physical
composition of electricity. The combustion of the fuel, if fuel be used as
the sooroe of the electric current, takes place at a single centre^ whilst the
illumination is only produced at a number of small centres di&tributed over
a surrounding area. The fuel may be similar in nature— coal, oil, etc. — ^to
A
2 THE FUNCTION OF ELECTRIC CURRENTa
that employed and diHtributed for direct production of illuminntion ; but tho
heat-energy produced is transformed, first into mechanical energy, then into
the form of energy associated with electric currents, nnd thus transmitted
to distant points. In the latter form energy, instead of fuel in material
form, is distributed to the various centres, where once more it is transformed
into heat, and the distributed centres bf^conin sources of combined heat and
luminous radiation exactly analogous to those where similar radiation is
obtained directly by the combustion of the f u^l. The function fulfilled by
the electric current is therefore simply that of transmission or distribution
of energy. The term power, meaning the rate of production and utilisation
of energy, is more convenient in this connection, as the conception of an
electric current is generally freed from that of quantity of electricity, or
the electric fluid.
It will be seen at once that the three extra transformations of the power
which must be effected in supplying '' electric *' lighting must necessarily^
as in all physical transformations except to the lowest form of energy —
that of heat — be attended by waste; and this waste must, in order to
compete efibctively with other systems, be compensated for by certain
advantages. The advantages possessed by electric lighting may be sum-
marised as follows :
Firstly. A greater freedom in the choice of fuels, utilising cheaper or
unreduced material, and their more efiicient combustion at larger centres ;
and sometimes the utilisation of equivalent " free " sources of mechanical
power from wind or water.
Secondly. The more convenient and inexpensive distribution by the
electric conductor.
Thirdly. The more efficient production of luminous radiation with less
attendant heat.
Understanding, then, that the electric current is only the distributing
medium, it will be seen that the question of distribution must be dealt with
most carefully, and explanation of the various systems of distribution in
logical sequence should be the programme to be followed in arranging a
work on the subject of electric lighting. This course will be adopted, and
for each system or group of systems the suitable machinery for the genera-
tion of the electric current, or, as it is better expressed, the transformation
of power from the mechanical into the electric form, must be explained and
illustrated. To make the explanation intelligible, whilst giving a sketch of
the design and principles of working of this machinery, a brief sammary of
the science of electro-magnetism must be inserted. It will then be possible
to give, in addition to illustrations and dimensions of some of the principal
types now in use for various purposes, the calculations that must be made
for one special example, and some estimate of the degree of efficiency and
accurate regulation possible. Although necessarily less detailed and practical
than specialised treatises on the design of electro-magnetic machinery, a
brief treatise on the principles involved may be useful to those more con-
cerned with the selection and arrangement of the systems to which they are
applicable.
Were it necessary in this treatise to arrange the subjects discussed, as
are the Propositions of Euclid or the successive chapters in an Elementary
Science Text-book, in the order in which knowledge must be acquired by a
student, it would probably be advisable to commence with the explanation
of the various types of lamps, incandescent, arc, etc., since upon the manner
in which electric power is transformed to luminous radiation depends very
largely the form in which it must be generated and distributed. It will be
necessary to assume a general knowledge if the question of distribution is
to be dealt with immediately, or to anticipate the details which will be
ELECTRIC POWER. 3
given snbseqaentlj in the chapters oonceming electric lamps and measuring
instruments. This arrangement has been suiopted after due consideration
as the most lucid and convenient order for most readei*s.
We have emphasised the fact that the electric current is by no means
analogous to the fuel which is distributed for other means of illumination.
That which is truly analogous to fuel, in the sense of bearing a quantitative
relation to the fuel consumed in its production, we term electric energy ;
but since this physical quantity is not capable of being stored, except in
minute quantities, in the medium in which electric phenomena are pro-
duced, passing directly from and to other forms of energy, we shall have
to deal more frequently with the term electric power, indicating the rate
at which energy is transferred. It will be assumed that the reader is
acquainted with the elementary theory of electricity and magnetism, and
familiar with the expression of the power conveyed by electric current, as
in the case of all natural phenomena with which energy or power can be
associated, by the product of two factors, the measures of the electromotive
force and the current. It is possible, as will be shown, to transform
mechanical power, the primary product of the consumption of fuel, into
the power associated with and transmitted by an electric current, the
magnitude of which and the magnitude of the electromotive force producing
it are quite independent; each of these magnitudes, moreover, may be
what we will, provided the product of their measures does not exceed an
amount proportional to the number of horse-power absorbed in their pro-
duction. Utilising the commonly accepted units and notation, if E be the
number of volts, the number of amperes in an electric circuit, then
E X O is the measure of the electric power in watts, and the minimum
horse-power necessary to produce it is ~^r""' -^^^ ^*® ®^ ^^^^ ^^'
sumption, and the heat and light radiation obtainable also bearing
definite relations to the same product, save in so far as losses must inevit-
ably attend the transformation of the one form of energy into the other.
Throughout the succeeding chapters the discussion of these relations, and
the way in which the losses may be minimised, will form the principal
theme.
The multiplex systems of electrical distribution for lighting and power
purposes arise chiefly from this freedom of choice in the magnitudes of the
electromotive force and the electric current necessary for the transference
of a given amount of power. We have to consider the possibility,
or at least the convenience, of any relative magnitude for the conditions
attending —
(i) GenercUion, or, more correctly, transformation of power from the
mechanical forms or directly from fuel.
(2) Transmieeion to greater or less distances.
(3) Subdivisiony with accurate regulation, to the various lamps or centres
of illumination.
(4) UUliscUion, or transformation of the power into light.
With regard to Generation, there is little difficulty in modifying the
relations between the magnitudes of the two factors of electric power at
pleasure without serious variation in efficiency of the operation ; but this
choice gives rise to extreme variations in the design of the machinery for
the purpose.
On the other hand, the question of Transmission to a distance neces-
sitates a reduction of the current factor, the reduction being the more
imperative as distance over which the power is to be transmitted is increased.
Unless the current factor be decreased, and the electromotive force factor
be correspondingly increased, to a far greater extent than that most suitable
4 SUBDIVISION OF TOWER.
for utilisation with the present form of lamps, and to an extent wliicli
renders special preaiution necessary to render the systems free from danger
to life and property, transmission and distribution must be confined within
a very limited area in the neighbourhood of the centre of generation. It
will be seen at once that small currents require correspondingly small
conductors for their transmission. The extra insulation demanded by an
increase in the electromotive force will be shown to be a far less serious
matter than the cost of large copper conductors, so that the initial expen-
diture of a system designed to transmit a small current at high electromotive
force will be comparatively small. Moreover, the power wasted in heating
the conductors will, even with a proportionate reduction of the sectional
area of the conductors as the current is decreased, bear a reduced ratio to
the total power transmitted, so that a higher working efficiency can be
maintained. The distance of transmission is therefore the paramount con-
sideration determining the relative magnitudes of the power factors, and
excessive distances generally give rise to complicated systems to retain the
advantages, and remove the difficulties attending the use of high electro-
motive forces.
The appropriate Subdivifnon of the power associated with an electric
current among the numerous centres of illumination may be effecced by the
subdivision of either or both the power factors. The most convenient is
the subdivision of the current factor, effected by connecting the lamps '' in
parallel," that is to say, as separate circuits between two transmitting con-
ductors. Such a system may be likened to the distribution of water for the
supply of a residential district, the lamps corresponding to the taps whereby
the water escapes, and one of the conductors, with its radiating branches,
to the complete system of water mains. To complete the analo<ry, however,
it is necessary to suppose a collection of the water by a system of return
mains, similar to the distributing system, whereby all the water is returned
to the source and redistributed without loss. The circulation of the blood
in the arteries of the body and return by the veins would form, in some
respects, a more apposite analogy.
The pandlel system will be considered first in detail, and its limitations
explained. It will be seen that, although eminently suitable for interior
lighting with incandescent lamps, the electromotive force factor will be
limited by the exigencies of lamp construction and considerations of safety
to life and property, and therefore the distance of transmission will also be
confined within very reduced limits. A considerable extension of this limit
is obtained by a simultaneous subdivision of the electromotive force factor
of the power, and as the complications involved do not materially affect the
design of the generating machinery suitable to the simple parallel system,
the various '^ multiple- wii*e " systems, as they are termed, will be explained
in the same section.
Turning next to the other extreme, we have the subdivision of the
electromotive force factor, or " series " system, in which the same cur-
rent is used for all the lamps. This system will require in general a
totally distinct type of generating machinery, and is specially adapted to
public or street lighting with arc lamps. It will, moreover, be suitable
to other special conditions, which will be de.^^cribed in the chapter devoted
to this system.
For longer distances than are attainable with the multiple wire systems,
and for incandescent lighting of the interior of buildings, the various
" transformer " systems are introduced in order to combine the advantages
of the parallel system with the economy in capital and working expenses
introduced by the employment of high electromotive forces and small
currents. The numerous modifications of these systems can be grouped
HYDRAULIC ANALOGY. 5
under two headings, accoi*ding as continuous or alternating currents are
employed.
Having thus drawn up a prngiumme of the work to be done in
explaining the various systems of distribution of power for electric lighting,
each system or group of systems will be studied in detail, the general
description being followed by a study of the design of the machinery
appropriate to each. A digression from the programme will be found
necessary to explain the theory of electromagnetism with the notation
which it is preferred to adopt in applying it to the design of machinery ;
this will be introduced before dealing with the machinery suitable to the
parallel, or "low tension " systems of transmission. Sufficient has been
indicated in this chapter, in very general words, as to the function which
the electric cuirent fulfils, as the mere connecting link between the source
of power and the points at which power is converted into luminous radia-
tion.
We have stated that the mere magnitude of the current, as measured
by the galvanometer or other similar instrument, is not necessarily propor-
tional to the power transmitted, or to the luminous radiation which is
produced. Instead of fuel being distributed to various points, as in the case
of gas and oil, the electric current may be compared with a current of
water employed for hydraulic transmission of power, or to the speed of
shafting or ropes for similar purposes. To these it is exactly analogous in
requiring another factor to be known, the electromotive force corresponding
with the pressure of the water, the torque of the shafting or the tension
of the rope, before the magnitude of the power transmitted is known. As
the design of the pipe conveying the water, and of the shaft or rope, is
modified greatly by the relative value of the two factors of the mechanical
power transmitted, so is that of the electric conductor. The sectional ai-ea
of the conductor depends mainly on the magnitude of the electric current,
as the sectional area of a pipe depends on the magnitude of a water current ;
but in both cases there i» considerable modification according to the distance
to be transmitted. The necessary insulation of the conductor depends
mainly on the electromotive force, just as the thickness and tensile strength
of the metal pipe depend on the pr&^^ure of the water within it. As a
treatise on Hydraulic Trans ujission of Power would first deal with the
theory of the flow of water and then of the size and strength of pipes to
convey it, so, assuming a general knowledge of the theory of electric
currents avid electromotive force, the technical details of conductors for
electric lighting may be generally discussed in the succeeding chapter
before proceeding to their selection and arrangement under the different
systems. The actual calculations of size of conductor and insulntion will
of course be postponed but the ph> sical qualities of the various materials,
employed for all sj'stoms alike, should form our initial study. Anticipation
of some d^'tailrt and terminology will be inevitably necessary, but will be
avoided as far as possible.
6 MATERIALS FOR OONDUOTORS.
CHAPTER IL
Conductors.
Wb may commence with the following table, showing the specific resistance
of various metals when pure and tested at a temperature of 15° Centigrade.
Specific resistaoce in microhms.
Silver (annealed) 1.488
Copper (annealed) 1*580
Copper (hard drawn) 1.616
Gold 2.036
Aluminium 2.S81
Zinc •••••••• 5*8^
Platinum • 8.957
Iron 9.61 1
Tin 13.070
Lead ..•••••• 19.420
German Silver 20.710
Platinoid 32.907
Mercury 94.070
This table will show that copper is only rivalled in electrical conduc-
tivity by the valuable metals, and the only possible competitors for
electric transmission of power are aluminium and iron. The latter may be
used with economy under some exceptional circumstances, particularly for
return mains, in cases where little or no insulation is required, and leakage
current will do no harm. In mines, for example, there is often a quantity
of iron cable which has served its purpose for haulage, etc., and which may
be used. The iron or steel plates of ships, iron water pipes, etc., have been
employed to carry the return current ; but the practice is reprehenf^ible
since electrolytic action commonly ensues. Since the sectional area of an
iron conductor would require to be seven times that of a copper main, and
the weight five times, to give the same resistance, the cost of the carriage
being correspondingly greater, the cost of the copper main itself will probably
be the smaller, and the greatly increa.'^ed cost of insulation would altogether
prohibit the use of iron under ordinary circumstances.
Aluminium has recently replaced copper very extensively for electric
transmission in America, more especially for power transmission and electric
traction. Compared with copper conductors of equal sectional area the
conductivity of aluminium is about 0.6 ; compared with conductors of equal
weight, the ratio of conductivity is nearly 2. As the price of aluminium is
now but little more than twice that of copper, and likely to be further
reduced if the demand were increased by utilisation for electric conductors,
the substitution of aluminium for many purposes in connection with electric
lighting seems very probable. The two greatest objections are the difficulties
of soldering and the increased cost of insulation. For overhead bare-wires
aluminium appears specially suitable, as the tensile strength of wire may be
made very great as compared with the weight. The degree to which
aluminium is subject to corrosion under atmospheric infiuences is now
being made a subject of careful experiment, and the result will largely
determine its future value.
The conductivity of copper is greatly reduced by the presence of even the
smallest amount of impurity ; some specimens of copper wire, or that which
is commercially so-called, giving a conductivity no greater than that of
iron. The conductivity of diflerent samples of copper is usually expressed
in terms of the conductivity of "pure copper" in the hard drawn form as
determined by Dr. Matthiesson, who found that there was no appreciable
SPECIFIC RESISTANCE. 7
difference between the conductivities of silver and copper in the form of
hard drawn wire, but by annealing the conductivity of copper was raised
3.3 per cent., and that of silver as much as 9 per cent. Annealed copper
was too soft, however, for practical use, and he preferred to take the con-
ductivity of pure hard drawn copper as the standard, which he determined
as 1.634 microhms, or .000001652 B.A. units of resistance at o^ Centi-
grade. Specimens are sometimes reported as having a conductivity of loi
or 102 per cent., this meaning either that a purer specimen than that
tested by Matthiesson is obtained, or more frequently that it is tested in a
somewhat softer condition. It is commonly specified that the specific
conductivity of the copper used for electric light conductors should be of at
least 98 per cent, conductivity, and this seems to be easily attainable.
The resistance of all metals increases with the temperature, and this
increase must be taken into account when dealing with conductors for
electric power distribution. Matthiesson found that the conductivity of iion
was diminished by 39.2 per cent., and that of nearly all other metals by 29.3
per cent., when raised from o^ to 100° Centigrade; whilst German silver
loses only 3.1 of its conductivity between the same limits of temperature.
The rate of increase of resistance is not quite uniform, being more rapid at
higher temperatures. Matthiesson found that the conductivity at t° 0. could
be represented with sufficient accuracy in the form
A- B.t + 0.t«|
but it will be more convenient to adopt an expression for the resistance,
instead of the conductivity, that of copper being given by Ayrton (a
deduction from Matthiesson's results) as
r(i + .003824 ( + .00000126 fi)f
r being the resistance at o^ 0.
Dr. Siemens adopted for the resistance an expression of the form
aT* + bT + 0,
where T repref^nted the temperature in Centigrade degrees reckoned from
the absolute zero, that is from 275° C, giviug lor copper, a formula correct
througb a very wide lunge, *
.026577 T* + .0031443 T - .22751.
At 20^ C. the rate of increase is in the proportion of '003874 of the
resistance (at zero temperature) for every degree of rise, and for such
variations as will be permitted in electric light conductors we may consider
this rate uniform, and adopt the usual term '' temperature coefficient " for
this fraction.
The temperature coefficients of most other pure metals differ but
slightly from that of copper ; iron, however, gives a greater variation
(.0048), whilst alloys have a much smaller temperature coefficient (German
silver .00044; platinoid .00025). For this reason, as well as their high
specific resistance, these and other alloys are useful for the construction of
standard resistances : whilst the rapid increase of the resistance of iron with
the temperature renders it the more valuable as a material for regulating
resistances in series with constant potential arc lamps, or similar purposes
where the object is to restrain a dangerous excess of current*
The resistance of a mile of pure hard drawn copper conductor of one square
inch section at o® C. may be taken as .042 ohm, and at 15® C. as .04458 ohm.
With a current of 1000 amperes flowing in this cable, the diflerence of
8 SIZES OF CONDUCTORS,
potential between its extremities (at 15° C.) will be 44.58 volts, or a
difference of one volt for every 39.48 yards. The same difi'eience of
potential must exist with conductors of any section and length if the current
density, 1000 amperes per square inch, be retained ; and when the current
density is altered the difference of potential must be altered in the same
proportion. For rough estimates of the fall of potential along conductors^
it is convenient to make this easily remembered rasult the basis of calculation,
taking 2.5 volts as the fall (when 1000 amperes on the square inch is the
current density) for every 100 yards, adding one-hundredth of a volt for
every degree Cent, in the same dLstance when the rise in temperature is
considerable, and correcting for any alteration of the cun'ent density from
this standard amount. It will be seen later that such estimates are of great
value, and constantly recurring in dealing with the various systems of
electric lighting, the approximate result given by the rule being of sufficient
accuracy for all practical purposes.
The sectional area of electric light conductors to carry given currents will
in most cases be determined by the question of the highest permissible fall of
potential along them, the consequent wa.ste of power and difficulties placed in
the way of uniform regulation necessitating a smaller current density than that
which will be allowed by considerations of safety. These determinations are
considered in their appropriate places for the different systems of distri-
bution; but in some cases, generally those of distribution within a very
limited distance, it becomes necessary to see that the current density in the
conductors does not, even when satisfying the conditions of efficiency and
regulation, exceed the limits determined by safety. These limits are
determined by codes of rules laid down by the Board of Trade, chiefly for
street mains, &c., and by Fire Insiu-ance Companies for. house wiring.
The Board of Trade regulation with respect to the sectional area of
conductors are as follows (Rules 4 and 5, issued Feb. 1896):
" 4. Maximum Current to Conductors. — The maximum working current
in any conductor shall not be sufficient to raise the temperature of the con-
ductor or any part thereof to such an extent as to materially alter the
physical condition or specific resistance of the insulating covering, if any,
or. in any case to raise such temperature to a greater extent than 30®
Fahr. (i6.6° Cent.) : the cross sectional area and conductivity at joints
must \}e sufficient to prevent local heating, and the joints must be protected
against corrosion.
" 5. Minimum Size of Conductm's, — The sectional area of the conductor
in any electric line laid or erected in any street after the date of these
regulations shall not be less than the area of a circle of one- tenth of an
inch diameter, and where the conductor is formed of a strand of wires
each separate wire shall be at least as large as No. 20 standard wire gauge."
The fonner regulation will lead us to a discussion concerning the rise in
temperatiire likely to occur with various current densities, sizes of con-
duct rs, and types of conduit. The latter regulation is introduced to
remove or minimise the danger of fracture of strands of the conductor, or
injury to the insulation, when subjected to the necessary bending and other
rough usage of di*awiiig-in systems.
For internal wiring, the rules adopted by most of the British Fire
Offices fix the limit of the rise in temperature at 18° Fahr. (10° Cent.).
Whilst, corrasponding with the llule 5 above metitioned which determines
the minimum size of wire to be used, even with the smallest currents, on
account of mechanical strength to bear the necessary handling, and to
permit slight corrosion without dangerous reduction of the sectional area,
it is specified by the Phoenix Fire Office Rules that " All conductors of a
larger sectional area than No. 16 S.W.G. should be composed of strands.
EISE OF TEMPERATURE. 9
No conductor of less size than Ko. i8 S.W.O. should be used except in
fittings, and in fittings no conductor should be less than No. 20 S.W.G/'
Other rules raise the limit for unstraiided conductors to No. 14 S.W.G.
The limiting rise in temperature of 18^ Fahr. (10^ Cent.) is founded on
the estimate that double the current density would raise the temperature
to the limit which is considered dangerous. As it is elsewhere specified
for india-rubber insulation that it should not soften at a temperature below
160° Pahr.y this may be considered the dangerous limit. Now doubling
the current density in the conductors would raise the tf^mperature by an
increment four times that of the normal current, or 72° Fahr. (40^ Cent.)
instead of 18° Fahr. (10° Cent.), so that the dangerous temperature would
only be reached with a doubled current if the temperature of the surround-
ings were 88° Fahr., a fair estimate of the maximum likely to occur,
without artificial heating, in the climate of Great Britain.
Now the predetermination of the rise of temperature in conductors
with dififerent current densities cannot be effected with any exactness, as
much will depend on the thickness and heat conductivity of the insulating
material, of the casing or conduits in which the conductors are laid, etc ;
nor is it an easy matter to measure it when the conductors are laid. Under
similar conditions we may find a law of variation connecting the sectional area,
cun'ent density, and limiting temperature attained, by assuming that the
rate of di£>sipation of heat varies as the surface of the conductors, and the
rise in temperature above the surrounding objects at a little distance from
the wire : a law which may reasonably be expected to hold good for small
variations in temperature when the conductor is surrounded by material of
much lower heat conductivity than the conductor itself, or is radiated from
the bare surface of an uninsulated conductor. The constants of the varia-
tion may then be determined experina en tally for any given conditions.
The rate of heat generation is proportional to C^R, or - for a given
d^
length of the conductor, d being the diameter. The rate of heat dissipation
may be taken as proportional to the rise in temperature, T, multiplied by
the surface, or by the diameter, d, for a given length ; therefoie since
it will follow that
C2
aa « cl.T
Ooc d.^T.^andToc ^
The current corresponding to a given rise in temperature therefore
varies as d , and the current density/ as d ; or, in terms of the sectional
area A, as A and A respectively.
Exhaustive experimental investigations have been made independently
by Preece, Forbes, and Kenelly to determine the rise in temperature in
conductors. For bare, round, solid wires, having a tarnished surface from
which the heat escapes solely by radiation, Preece gave the following
formulae for the total current C, and the current density D, wliich gave a
lise in temperature of T degrees Centrigrade,
= 182.76 A*T*
D= 182.76 A., T*.
But the results will be considerably modified with a bright surface, from
which the radiation would be slower, and the current giving the same rise
in temperature much less. On the other hand, a sliglib current of air
lO CURRENT DENSITY.
would reduce the temperature, or allow a larger current to be used ; and a
stranded wire, having a larger surface for the same sectional area would
also allow of a larger current. In the latter case, taking as an approxi-
mation
D » 200 A"*T*,
we may estimate that, for a bare cable of one square inch sectional area, a
current density of 632 amperes per square inch would raise the temperature
18° Fahr., and for the same rise in temperature 160 amperes could be
carried on a cable of 0.16 square inch section, or with a current density of
1000 amperes per square inch.
More valuable results, at least for our purpose, were given by Kenelly
in 1889, from experiments carried out by the Eiiison Electric Light Co.
with insulated conductors enclosed in wooden mouldings, as in the common
practice of internal wiring. Under these conditions, Kenelly gave the
following formula for the current which would raise the temperature of a
conductor 10.4° Cent.
or
B 560 d% d being the diameter in inches,
a Z38 d', d being the diameter in centimetres ;
or, inverting these results, the required diameter is given in inches and
centimetres by
d = .0147 0' and .0374 0* respectively.
To compare the former result, using that in which the inch is used as
the unit of length, with those of Preece, we may reduce it to
ss .668 A for a rise of 10.4* Cent. ;
or generally
= 208 A*T*
D = 208 A"*T*.
From this it will appear that the heat is removed more quickly by con-
duction than by radiation from the tarnished metal surface, at least with
the insulating substances used (impregnated cotton). The effect of insulating
suspended wires, especially if the external surface is blackened, is to increase
the rate of radiation, the larger surface for radiation more than compen-
sating for the resistance of material to the conduction of heat. For
Kenelly's complete report, which dealt also with insulated and uninsulated
wires suspended in the interior of a room and in the open air, we may refer
to the " Electrician," Dec. 13 and 20, 1889.
As the predetermination of the rise in temperature of conductors is
subject to considerable uncertainty, it is far better to specify the current
density that may be employed with various currents. It is evident from
the experimental results given above that the formulse for the current and
current density
= 600 A and D = 600 A
will, for stranded conductor, give a rise in tempei'ature of less than 18°
Fahr. under any practical conditions ; therefore we might take as the law
for minimum current density
STANDARD SIZES OF CABLE.
II
■
which ^onld allow a current density of 500 amperes per square inch for a
conductor carrying 1000 amperes; 1000 amperes per square inch for
a conductor carrying 108 amperes; 1500 amp^rt^s per square inch for
a conductor carrying 37 amperes; 2000 amplres per square inch for a
conductor cairying 1 5 amperes.
The regulations issued by the Phoenix Fire OflSce allow a current density
of 1000 amperes per square inch for a conductor carrying 100 amperes or
less, merely specifying that the current density should be Jess with a larger
conductor. More recent regulations issued by the Liverpool and London
and Globe Insurance Co. specify more exactly as follows :
For incandescent lamps only : 1500 amperes per square inch of sectional
area for currents up to 10 amperes. 1000 amperes per square inch of
sectional area for currents from 10 to 100 amperes. 800 amperes per square
inch of sectional area for currents over 100 amperes. For arc lamps,
motors, heating appliances, etc. : 1 000 ampdres per square inch of sectional
area for cuirents up to 50 amperes. 800 amperes per square inch of
sectional area for currents over 50 amperes.
The lower current density allowed for the conductors carrying the
current for arc lamps, etc., is necessitated by the greater liability of the
curreut to exceed the normal amount. The following table of suggested
sizes of conductoi-s is also issued with these rules :
Sise.
For iDcandescent Lamps
For other
only.
Purposes.
Area
No. of 60
Watt.
100 Volt
Lamps.
Onrrent
Onrrent
8.W.G.
(solid)
in
In
iqnare io.
Amptees.
Amptees.
18
.0018
4
2.7
1.8
61/38
.0018
4
**Z
1.8
3/22
.0019
4
2.8
1.9
7/25
.0022
5
3-3
2.2
3/20
.0031
8
4.6
31
16
.0032
8
4.8
3.2
vn
.0032
8
4.8
3.2
108/38
.0032
8
4.8
3-2
7/22
.0044
II
6.6
4.4
7/214
.0050
12
7.5
S.O
14
.0050
12
7.5
5-0
7/204
.0061
15
9.1
6.1
7/20
.0072
17
10.0
7-2
7/18
.0128
21
12.8
12.8
19/20
.0198
33
19.8
19.8
7/16
.0229
38
22.9
22.9
19/18
.0349
58
34.9
34.9
7/14
.0356
59
35-6
35.6
19,17
.0479
80
47-9
47.9
19/16
.0621
104
62.9
50.0
19/15
.0789
131
78.9
631
19/14
1
.0973
162
97.3
77.8
12
INSULATING MATERIALS.
Insulation.
The insulation resistance of a cable, L centimetres in length, of circular
section, the internal and external diameters of the insulating covering being
d and D respectively, will be given by the formula,
R =
<rlog
D
2tL
<r bfting the specific resistance, or that of a cubic centimetre from one face to
the opposite. For the substances used as insulators, the value of a when
expressed in megohms will be exceedingly great, and it is convenient to
adopt the unit of specific insulation resistance suggested by Preece, that of
lo^^ c.g.s. units, or !o^^ ohms cr lo^ megohms per cubic centimetre. In
terms of this unit, the specific resistance of substances frequently employed
is given by the following table ;
Iniiulating Material.
Sppcific Bcsi stance
in Pieece Units.
Temperatnre of Test
Degrees Centigrade.
99
99
99
Dry air • •
Paraffin • •
Ebonite • •
Flint glass
India-rubber (Siemens' special)
(untreated)
(ozokeriied)
„ „ (vulcanised)
Shellac
Gutter-percha .
Vulcanised bitumen
cable)
Glass (ordinary)
Mica .
Paper (parchment)
„ (cardboard)
„ (ordinary)
(Oallender
E.L
infinity
34
28
20
16.17
10.9
6.6
1.5
9.0
0.45
0.45
0.09
0.048
.00003
.000005
,000003
46
46
20
IS
24
IS
15
28
24
15
20
20
The above list is collected from the results obtained by numerous
experimentalists. The specific resistance of insulating materials generally
decreases with the temperature, although at moderate temperatures a pre-
liminary rise is observed with some. In specifying the insulation resistance
required for conductors, it is necessary to specify also the temperature at
which the test should be taken, 75° Fahr. being customary. The test of
insulation should be taken after a minute's electrification at the maximum
potential it is proposed to use, when possible, or at least 100 volts. The
insulation will appear to improve after a short time of electrification, owing
to the cessation of the current supplying the statical charge communicated to
the dielectric, and the opposing E.M.F. of electrolysis, which seems always to
accompany the passage of the current. When intended to be impervious
to moisture, as the insulation for all conductors for electric lighting should
be except for flexible connection and the like, it should be tested after
24 hours immersion in water.
The attainment of exceedingly high insulation resistance is not, at least
with moderate electromoiive forces, of paramount importance in the con-
ductors for electric lightirig. Where the E.M.F. does not exceed that used
THICKNESS OF INSULATION. 1 3
for parallel distribution for arc or incandescent lamp, the thinnest papei
insulation, if perfectly dry and mechanically sound, would be sufficient, the
corresponding leakage current, distributed over the whole system of con-
ductorSy would not be sufficient to do any harm by its electrolytic or other
action, and the waste of power would be wholly insignificant. The require-
ments of the insulating covering, which are of more importance, are as
follows :
(i) It must be of sufficient flexibility, mechanical strength, and thick-
ness to withstand the rough usage to which it may be subjected during
laying, and subsequently.
(2) It must be impervious to moisture, and not affected, appreciably, in
fspeci£c resistance, etc., by such temperatures as that to which it may
unavoidably be subjected. For ordinary internal wiring 160** Fahr. is
specified by insurance rules, at which temperature the insulation should not
be softened so that the conductor may be displaced, and it should not be
easily inflammable.
(3) It should not tend to corrode the conductor.
(4) For high tension conductors, it should be of sufficient thickness to
prevent the passage of a disruptive spark, even if the E.M.F. be raised
considerably above the normal amount ; to ensure this, it should be tested
before use with double the maximum E.M.F.
(5) The cost of the material should be as low as possible consistent with
the previous conditions.
Now india-rubber not only comes highest in the specific insulation list
of all flexible and tenacious substances, but approximates to the above
conditions more closely than any other; therefore it is adopted for the
larger proportion of underground street conductors, and almost exclusively
for indoor wiring. Vulcanised rubber, although much inferior in specific
insulation to pure, is pieferred on account of the more important conditions
of mechanical strength, durability, etc. In order to satisfy the third con-
dition above, a thin inner coating of pure rubber should separate the
vulcanised rubber from the copper, and the latter should be carefully tinned,
otherwise the copper is liable to corrosion by the sulphur used in the
vulcanisation. In order to render the insulation impervious to moisture an
oater covering of waterproof tape is supplied, and for further mechanical
protection a braiding of tarred flax.
For low tension work, that is, for use with electromotive forces not
exceeding 250 volts, the thickness of the insulation need not be greater
than that which will ensure oontinuity throughout, and will resist the
stress produced by bending the conductor, etc. As the latter will be greater
with larger conductors, a greater thickness of insulation will be required or
better quality of rubber, and the insulation test, which makes the same
demand, and tests the continuity of the covering, remains the best means of
specifying the extent of insulation required. The Phoenix Fire Office
regulations insist that the insulation of the conductor before laying should
exceed 250 megohms per mile when intended for dry places, and 600
megohms per mile when intended for damp places ; other regulations specify
300 megohms throughout. The corresponding thickness of vulcanised
rubber is very small, and is generally greatly exceeded by that supplied by
manufacturers, as the addition to the cost of the copper conductor is
relatively unimportant. Some further standard of mechanical strength and
durability of rubber insulation, whereby it may be satisfactorily tested, is
much needed. Some engineers require that specimens of the cable should
not show any apparent injury to the insulation after the straining conse-
quent upon bending it round a drum to a curvature greatly in excess of
that to which it is likely to be bent in all succeeding manipulation.
14 SPABKING DISTANCE.
Electrical faults, however, frequently develop as a consequence of straining
in the insulation only after a long time, in some cases within the writer's
experience after months or years of regular use, undisturbed by further
manipulation. A very excessive mechanical strain has therefore to be
exerted if a satisfactory test made immediately is to be trusted.
For high tension work, that is, for use with electromotive forces exceed-
ing 500 volts if continuous, and 250 volts if alternating, the Board of Trade
regulations insist on a continuous covering of insulating material having a
thickness of not less than one- tenth part of an inch ; and in cases where
the extreme difference of potential in the circuit exceeds 2000 volts, the
thickness of insulating material must not be less in inches or parts of an
inch than the number obtained by dividing the number expressing the
number of volts by 20,000. It is also specified that no conductor should
(except with written consent) be used for the transmission of more than
300,000 watts (or 50,000 for aerial lines), so that with 2000 volts the limit-
ing current is 150 amperes. With conductors of the size suitable for
currents less than this, the minimum thickness permitted, if of high
grade rubber, the insulation resistance should exceed 5000 megohms per
mile, and this is commonly specified as the standard for high tension work.
To justify the Board of 'frade regulation as to minimum thickness, the
following experiments as to the electrio strength of air and india-rubber
made by Siemens may be referred to. Measurements of the "striking
distance ** of disruptive sparks were taken with various electromotive forces,
obtained from an alternating current transformer, the "virtual E.M.F."
being measured with an electrostatic voltmeter. In air, between fiat discsi
the striking distances were as follows :
E.M.F.
Millimetres.
Inohei.
2,000 • ,
» • 0.67
•••
.0264
4,000
. . 1.59
•••
.063
6,000 • ,
. 2-53
• a.
.100
8,000 • 4
. 3.60
...
.142
10,000 • ,
• 4.80
...
.189
12,000 . ,
1 • 6.46
...
.254
15,000
. 10.23
••.
.395
It must be observed that the maximum E.M.F., which most probably
determines the striking distance, would be about the virtual meeBure
multiplied by ^2, but as the most common use of high pressure supply
will be with alternating E.M.F., the customary method of virtual measure-
ment will be for our purpose the most valuable. The striking distance
between sharp points will he but little greater, if at all, for the same E.M.F.
Between a steel point and disc (the former a sharpened cone with an angle
of 60°) Siemens found for 2000 vertical volts the striking distance was 0.4
millimetres (or less than between the discs) ; with 8000 volts 4.08 milli-
metres (or somewhat greater); and for higher E.M.F. the distances of
sparking were about the same as above.
To test the electric strength of india-rubber the following tests were
made of the E.M.F. required to puncture the insulating covering of cables,
and india-rubber sheets of various thicknesses.
INSULATION TESm
IS
TbickDesB in Milli-
metres.
E.M.F. of Disrup.
tive Spark.
1
40 inches of 880 H. wire . •
10 jards of experimental core •
li » If M • •
I yard coDcentric cable . . •
India-rubber sheet . • • •
Experimental core • • • •
3.5
3.8
2.5
3-5
.5
1.0
10.0
21,500
20,000
16,000
28,000
7,000
10,500
38,000
Thus india-rubber seems capable of withstanding nearly three times the
elect lie pressure of a similar thickness of air.
It would appear, therefore, that with a thickness of india-rubher of one-
tenth of an inch, or 2.5 millimetres, the E.M.F. producing a disruptive
spark would be about 16,000 virtual volts alternating, or 24,000 with con-
tinuoQa E.M.F., and aci*oss a corresponding air gap 6000 or 9000. The
Board of Trade Regulations may therefore be taken to give a safety factor
of eight with 2000 virtual volts alternating E.M.F. With extra-high
tension the safety factor is less, 10,000 volts (^ inch thickness of insulation)
apparently giving a safety factor of about five, but there is then much less
danger of mechanical injury.
When an installation for the interior lighting of a building is complete,
a minimum insulation resistance for the whole system is demanded by fire
insurance offices, and generally by the supply company to whose system the
installation is to be connected, if not supplied by a separate generating
plant. The value of this insulation test is by no means a satisfactory
eriterion of the safety of the installation, only serving as a partial safe-
guard against very defective workmanship, and of little value as a proof of
durability and improbability of failure. Although the test can hardly be
omitted, it should not be substituted for thorough inspection of the work.
With conductors insulated to at least 300 megohms per mile, the leakage
current must be infinitesimally small with a moderate E.M.F., but it w^l
be impossible to prevent a much larger leakage from the fittings where the
conductors are bare, and some degree of dampness on the surface of the
porcelain or other insulators will greatly lower the insulation of the whole
installation, and cause it to be a very variable quantity. As the fittings
are therefore the main source of leakage, it would appear to be the more
justifiable requirement that the minimum insulation resistance permitted
should be in inverse proportion to the number of lamps employed, irrespective
of size, which should not afiect greatly the insulation attainable for the
corresponding fittings. This should make the' test as satisfactory a criterion
of the quality of the work as it is capable of being. Other regulations,
however, define the minimum insulation resistance as inversely propor-
tional to the total current to be used in the installation, thus making the
maximum leakage current proportional to the whole current.
The Phoenix Fire Office Regulations specify as the minimum insulation
resistance between the whole installation and earth, and between the con-
ductors, for continuous currents with an E.M F. not greater than 220 volts^
as half a megohm for installations of 25 lights, and a smaller insulation
resistance in proportion for a larger number of lights. For alternating
currents the insulation resistance must be double that given by this
definition. The Regulations adopted by most other Fire Offices specify that
the leakage from the conductors to earth over the whole installation, or
between the conductors, should not exceed the twenty-thousandth part of
ID POSSIBLE CAUSES OP FIEE.
the working current even under the most unfavourable conditions. Under
dry conditions double the insulation resistance is insisted on. With loo-vrlt
lamps of i6 c.p., taking .64 ainp6re each, the working current would be
16 amperes, and the leakage thus permitted between the conductors under
unfavourable conditions 1/1250 ampere, corresponding to an insulation
resistance of 125,000 ohms, or only a quarter of that required by the
Phoenix Regulation. The standard required by many supply companies is
much higher than those of insurance offices, that of the London £lectric
Supply Corporation alternating current being,
for less than 25 lamps • • ,2 megohms,
for from 25 to 50 „ • • •1-25 „
for from 50 to 100 „ • • • -75 m
for upwards of 100 „ • • • •S n
The Regulations recommended by the Institute of Electrical Engineers
as to the insulation resistance of complete installations is as follows :
'* The insulation rasistance in megohms of the whole of any installation
either to earth, or from any supply conductor to any return
conductor when all branches are switched on, but the lamps,
motors, <&c., removed, must in no case be less than that given by
dividing the electromotive force in volts by the number of lamps ;
thus, if the electromotive force be 100 volts and the number of
lamps be 50, then 100/50 => 2, that is, the resistance must be
2 megohms. Tests should be made of the electromotive force
intended to be used, but in no case less than 100 volts."
This regulation seems somewhat severe, in view of the fact that high
insulation is not a perfect criterion of the quality of the workmanship ; for
whilst it may easily be satisfied with very moderate care in wiring building
where plaster, brickwork, etc., are thoroughly dry, and where the fittings
need be few and may be placed in dry unexposed positions, there are many
conditions, such as in new buildings, public halls, etc., where the best of
workmanship, with the types of porcelain fittings approved by experience
for jiafety, may give much lower insulation. When the fittings are supported
on dry woodwork, a higher insulation may be secured thereby, which is
obviously no proof of diminished risk of fire. A relaxation of the regulation
in special cases is most reasonable, and due recognition of the value of an
insulation test, with its limitations is needed.
Seeking for the possible causes of fire that may arise from Electric Light
installations, we may class them as follows :
(i) Those which may arise from the lamps themselves, when plaoed near
inflammable material and improperly protected. There is little or no danger
to be anticipated from smail incandescent lamps, even in the event of
fracture, but the larger sizes and arc lamps need to be placed at a consider-
able di^'tance from woodwork, etc.
'2) Tho^e which may arise from overheating of the conductors.
^3) Those which may arise from the leakage current through imperfect
insulation.
At present we have only to deal with the two latter, since the first will
be discussed in the chapter on Incandescent Lamps. With conductors of
sufiicient sectional area, as already determined, and sufficiently protected by
fuses against undue increase of the current, there still remains a possibility
of local heating owing to increased resistance at one point. This may oocur
with loose contacts in the fittings, imperfect conductivity at the joints, or
corrosion of the conductor reducing the sectional area. Corrosion is especially
serious with conductors of small sectional area, but is not likely to occur
except after the breakdown of the insulation, when the leakage current may
{:
CASING OF CONDUCTORa 1 7
by electrolytic action produce very rapid corrosioD. Small conductors are
also liable to injiuy, or even fracture, in handling, and, although con-
tinuity is maintained in the circuit, considerable heating will ensue at that
point. The local heating may be sufficient to ignite the insulation, or the
casing or other inflammable material in the neighbourhood, and in the
pnfsence of any inflaillmable gas the danger will be extreme.
Defective insulation may be caused through insufficient mechanical
protection of the conductors, and consequent abrasion, chemical decom-
position, or decay of the insulating material. The greatest danger of this
occurring is in the hidden portions of the work, where the conductors pass
throu£rh walls and under flooring, and are subject to the attacks of vermin,
add liberated from moist plaster, etc., and where maintenance of continuity
in the wood casing, earthenware or metal tubes, or other mechanical protec-
tion, may easily be shirked by careless workmen. The insulation of joints
with pure rubber, as in common practice, is seldom up to the standard of
the rest of the insulation in durability. Moisture and dirt in the
fittings is a no less common cause of low insulation. Now a "fault" or
complete break-down of insulation at one point of a circuit will not cause
leakage of the current provided the remainder of the system is highly
insulated throughout, and is therefore less dangerous in a transformer
system which is made up of many independent circuits. Two points of
weak insulation, especially if on different branches of a parallel system,
may, however, cause considerable leakage, which may become the im-
mediate cause of a fire by the carbonisation of damp woodwork, con*
verting it into conducting material, and finally igniting. Wood casing,
most commonly adopted on account of convenience, cheapness, and
appearance, whilst perfectly satisfactory when dry, may become, when
wet and in the presence of leakage currents, a source of danger instead of
safety.
The system of wood casing has the great advantage over any other
method of mechanically protecting the conductors that it lends itself easily
to decorative effects, and therefore encourages the placing of the conductors
on the surface of walls and ceilings where they are eaisily accessible, and
always under some supervision. It should be used only in dry situations,
and the wood well seasoned and well varnished. In passing through walls
it is recommended, and generally insisted on, that the conductors between
which considerable difference of potential exists should be carried in separate
tubes of stoneware, or other incombustible material, set in cement. The
possibility of percolation of water through these, and so saturating the
wood casing, must also be duly guarded against.
The Insulated Tube system employs iron pipes with an inner coating of
a special paper, prepared with an insulating material, and rendered fire and
water proof. The paper lining is made smaller than the iron tube, and after
being placed inside it, is expanded to it by pneumatic pressure. It is made
in ten-foot lengths, and elbows and bends are supplied for making joints.
The lengths are joined by insulating couplings. These tubes are built into
the walls, and the insulated conductoi'S drawn in subsequently. The advan-
tages claimed over the use of ordinary iron pipes are : Firstly, additional
insulation which can receive no mechanical injury ; secondly, the conductors
can be drawn in or out without fear of abrasion of the insulation by burrs
on the metal ; thirdly, a short circuit between the conductors in the pipe is
not accompanied by an ''earth," and the fireproof qualities of the inner
covering prevents injury to the pipe, as shown by the folloving test : A
length of bare iron pipe and another of insulating tubing were taken,
and two insulated wires run through both, under exactly the same con-
ditions. These wires were short-circuited, with the result that a hole was
B
1 8 FUSIBLE CUT-OUTa
blown in the iron pipe, whilst the insulated iron pipe stood the test
perfectly. A i5o-amp6re fuse was arranged in the circuit employed for this
experiment.
The principal objection to the Insulated Tube system is unquestionably
its expense, which must compare unfavourably as to fini cost with wooden
casing.
The use of concentric conductors for internal wiring presents many
advantages, but has not been widely favoured owing to the demand of
supply companies that both conductors should be highly insulated. For
private installations with a separate power supply no system could be more
safe. The central conductor is highly insulated from the outer, which is
made up of finer wires, forming a complete metallic sheath. If the outer
conductor be maintained at the same potential as the earth at one point,
for example let the corresponding dynamo terminal be connected to earth,
the maximum potential at any point will be that of the fall of potential
along the main, and the thinnest insulation will prevent any appreciable
leakage. The outer conductor may, in fact, be earthed throughout its
whole length without any risk of fire, and if 'the fall of potential along it
does not exceed i^ volts no appreciable electrolytic action will ensue. It
has sometimes been the practice to reduce the fall along the outer main by
making the conductor of greater section than the central conductor, so that
the fall of potential may be almost entirely due to the former, and to leave
the outer conductor uninsulated, but this is less satisfactory than the use of
a thin insulating covering which also protects the copper from corrosion.
The only possible leakage with the full potential of the circuit is between
the inner and outer conductors, and the only possible break-down is a short
circuit resulting in the blowing of a fuse. The lead covered concentric
conductors occupy much less space than any system of separate oonductorSy
and may be buried in plaster, etc., without fear of injuiy. The chief objec-
tion is the difficulty of making satisfactory joints for branch circuits, but
with suitably designed junction boxes the difficulty may be got over, and by
the complete elimination of joints, which are always the weakest points of
a system, a much safer result can be obtained. It has been suggested that
concentric conductors might be employed with advantage for the branches
of a three- wire system, the outer conductor being always oonnected to the
middle main, which is to be earthed at some point, and the central conductor
to one or other of the outside mains.
Fusible "Cut-outs.**
Having settled the maximum current which it is desirable to allow to
pass through any main or branch conductor, the next point is to arrange
some automatic device or " cut-out '* by which the current can be prevented
from increasing much beyond this limit. There are two classes of automatic
cut-outs in use, namely — those depending on the magnetic efifects, and tho<e
which depend on the heating effects of the current. The former consist of
electromagnets through the coils of which the current passes, and when the
current exceeds the strength for which the instrument is adjusted, the
electromagnet pulls over an armature and breaks the circuit. The con-
nection is in general restored by returning the armature by hand. These
are not much used for protecting cables, as they have neither the certainty
of action nor simplicity of the fusible cut-out. They are invaluable for the
protection of secondary batteries, and several examples will be treated under
that heading.
The latter class of cut-outs consists of a short length of thin wire,
generally of a material that fuvses at a low temperature, which will melt and
FUSIBLE CUT-OUTS. 1 9
break the circuit when the current rises to 50 or 100 per cent, above that
which the conductor is intended to carry. These short wires are termed
" fuses."
It must be noted that fuses are intended to protect the cables and
connections in a system, and are as a rule incompetent to protect incandes-
cent lamps from injury. Incandescent lamps will not bear an excess current
of more tban 10 or 15 per cent, for long, and will speedily be destroyed if
an excess of 25 per cent, were reached. Even if the fuses were made so
light as to melt with this small excess of current, which would involve their
remaining when all the lamps were burning at a temperature near melting-
point, they would be no protection whatever when only a few of the lamps
were alight. The kind of danger against which a fuse can provide a safe-
guard is an accidental short circuit between the flow and return leads such
as is very likely to occur in a fitting, or between twin wires, during the
removal and replacement of some kinds of lamp, or in the carrying out of
any repairs or additions to the circuit, should such work have to be performed
while the current is flowing. The conditions to be secured for a good fuse
are as follows :
(i) That it should melt with the desired excess of current. This excess
is frequently specified as 50 per cent., but there is much to be said in favour
of allowing a wider margin. From the nature of the events which call for
the action of the fuse, it will be seen that little harm is likely to be done by
strengthening the fuse, so that it melts with a 100 per cent, excess. Exact
calculation is not possible with regard to fusible wire, as so much depends
OQ the soundness of the contacts, and the conduction of heat from the wire.
Great inconvenience is produced by the frequent melting of fuses in which
too small a margin is allowed,
(2) A sufficient length of break. One inch is generally considered ample
on low tension circuits, up to 100 volts, and it is a safe rule to increase the
length in proportion to the electromotive force, so that in 2000-volt circuits
a break of nearly two feet should be allowed. It is convenient also to vary
the length with the current, less than an inch being necessary for small
currents, such as that of a 16 c.p. lamp. Three or four inches break is
advisable for main fuses (from 50 amperes upwards).
(3) The avoidance of unnecessary resistance. This is of no great
impoitance if the fuses are short and the number small, the additional
reaistanoe being insignificant.
(4) The fuse should melt suddenly in case of a short circuit. To obtain
this there should be as tittle metal ae possible in the fuse. Several strands
of fine wire will carry more current than a large wire of the same sectional
area, owing to the greater proportionate area for radiation, and therefore
for a fuse of the same capacity much less metal need be used. This gives
an additional advantage that the molten metal is not so likely to cause
damage.
(5) The fuse should be sufficiently protected from mechanical injury,
and from the possibility of the melted wire setting fire to combustible
material.
Fuses to carry small currents are commonly made of lead, or tin, or an
alloy of both metals. The resistance of tin is more than eight times, of
lead tban twelve times that of pure copper, snd of alloys of lead and tin
still greater. Moreover, the melting-point of lead is 612*^ F., of tin 442 ^'F.,
and of alloys of the two still lower. Fuse wire melting with currents of
from one ampere can therefore be constructed of these metals of sufficient
cross-section to be roughly handled.
For currents of over fifteen amperes, the use of fine wires of high con-
ductivity are much to be preferred. For small currents, copper wire would
20
BIZE OF FUSE-WIRE.
need to be so fine that most careful handling would be necessafy, and con-
tacts could not be depended on. But with large currents the quantity of metal
in lead or tin fuses would have to be very great ; they have therefore con-
siderable capacity for heat, and melt t')0 slowly when there is a rush of
current due to a short current. Moreover, being always warm with the
current, they are liable to deteriorate, and need to be replaced periodically.
Thin copper wires carrying the same current will, on the other hand, melt
nearly instantaneously with a large current. The resistance rising rapidly
with the temperature will cause the change from a cool state to melting-
point to be very abrupt.
Copper wire of No. 30 S.W.G. preferably tinned to prevent oxidisa-
tion when exposed to high temperatures, will melt when exposed to
free radiation in still air with about fourteen amperes. In short lengths
the free conduction to the terminals of the fuse will cause it to bear a
much larger current, but to compensate for this the radiation in a
closed box is much reduced. One or more strands of this size of copper
wire makes an effective fuse, and the quantity of metal is only about one-
tenth of the corroHponding tin or lead fuse. The currents with which wires
of different materials and sectional areas fuse have been very carefully
determined by Mr. W. H. Preece, but the results must be considerably
modified as above when the wire is of shoi-t length and enclosed in a fuse
box. In the course of his investigations, he showed experimentally that,
when the wire was not very fine, the current required to raise a wire of
given material, and of diameter (2, to a given teipperature varied as d . We
have seen that for a small increase of temperature 7\ it is reasonable to
expect that the current will vary as d T^, but this law would clearly not
hold for high temperatures, because the rate of cooling is no longer propor-
tional to the excess of temperature above that of the surrounding air. For
the stime material and the same temperature, the law of cooling does not
enter the calculation, and we may write C ■> ad^, the constant factor a
to be determined by experiment, which is the relation found by Mr.
Preece. The most important case is when the temperature is the fusing-
point of the particular metal, and the values of a for the fusing-point are
given by Mr. Preece as follows : —
MaterUL
a (d given in
a'(<lgivpnin
inches).
GentimetresX
Copper • • • •
10^244
2530
Aluminium • ,
7.585
1873
Platinum . ,
5»'72
1277
German silver ,
5.230
1292
Platinoid . ,
4»750
I173
Iron .
3»i4S
777.4
Tin . . ,
1,642
405.5
Alloy (lead 2, tin z)
1,318
325.5
Lead
X.379
340.6
It should be noticed that, for wires of the same diameter, but of different
materials, the currents which produce fusion are proportional to the corre-
sponding vidues of a or a*. The currents which will produce the same rise
of temperature in diflferent wires are not in any way deducible from the
values of a or of.
STBEET MAINa 21
A s an example to find the current which will fuse a copper wire of No.
i8 S.W.G.
C = 10244 ( 04S)' = 107.73,
80 that the fusing current is just under 108 amperes.
Mr. Preece's experiments were carried out with wires 6 inches in length.
In tine wires, the len^h, if over the minimum of one inch, will not aflfect
materially the magnitude of the fusing current. With thick wires,
however, a considerable modification would have to be made, were it not for
the fact that it is convenient to use a longer fuse for large currents. A
fuse generally will melb near the centre with a moderate exce><s of current,
showing the efifcct of heat conduction in cooling the ends of the wire. Care
must be taken to ensure good contact at the terminals, for it is a most
common event for fuses to melt through the additional heating of a bad
contact. Specially constructed fuses, having the ends of the wire soldered
to flat contact pieces^ are a great convenience, and ensure certainty of
action.
Much valuable space might be occupied by a description of the
many forms of fuses and fuse boxes designed for safety and convenience,
but the whole subject of electrical fittings is Ido extensive to be included in
such a work as this. It may be noted, however, that much convenience
arises from the commendable practice of concentrating all the switch and
fuse fittings for a floor or section of a building at one accessible point, and
the mse of a well-designed switchboard to unite them all. This not only
enables higher insulation to be obtained, but allows facile inspection of the
work at all times. The fuses may be labelled so as to be found immediately,
and, moreover, there can be adequate protection without porcelain covers for
each fuse, which, in the screw design lately universal, invariably broke after
being i-emoved or replaced several times. With this system of small switch-
boards, joints on branch circuits may be almost, if not entirely avoided. It
is generally insisted on by fire insurance companies that fuses should be
inserted in each main at points where the main decreases in size and upon
each branch conductor. But it is possible to take too many precautions as
to safety and needlessly multiply the number of fuses. It must not be
forgotten that these safety devices only guard aerainst certain contingencies,
and cannot replace the necessity of good workmanship which is the true
safeguard. Smgle-pole fuses upon the minor branches are quite suflicient,
provided they be all placed upon branches of the same main ; for a large
cnrreiit can only arise from a short circuit, or two faults upon branches
from dififerent mains, in which case there is always a fuse to break the
drcuit. Light single-pole fuses are necessary in every ceiling rose from
which a flexible connection to a pendant lamp is taken, and this should
generally be upon the opposite branch conductor to that of the fuse which
protects the group to which the lamp belongs.
Street Mains.
The first experiments in Electric Power distribution were made with
overhead wires, either bare and supported by insulators, after the manner
of telegraph wires, or with continuous rubber insulation. This method is
only retained in towns under severe restrictions, and except for certain
systems of electric traction is practically superseded by tlie use of under-
ground mains. The various systems of underground distribution are
commonly classed under two headings: (i) The ** fixed " systems, under
which the conductors when once buried are inaccessible for inspection,
renewal, addition, or connections, except by re-excavation, wbich, when the
mains are, as is most usual, laid under the pavement of the footpaths, is
22 CONDUITS.
generally an e2q>ensive matter. These systems include those using lead
covered or armoured cables, and others such as the Callender '* Solid
Bitumen" System, where the cables are laid in troughs and additional
insulation to that of the cables secured by a filling of solidifying bitumen,
run in in a fluid state. (2) The *^ dra wing-in " systems, under which the
cables can be more or less conveniently replaced or supplemented without
re-ezcavation.
The latter systems present many advantages, which render them prefer-
able except in special cases, though as a rule they are more expensive at
first than the former. A lead or armoured cable can be laid in a very
narrow trench, and can be bent with great ease so as to avoid the network of
gas, water or other pipes which is to be found under the pavement of
citios. Therefore much expense is saved in the excavation and laying
of the cables, and the lead covering or iron armouring is less expensive
than any knocvn form of conduit.
The most common form of dra wing-in system is that in which cables,
generally with vulcanised rubber insulation, are drawn into cast-iron pipes
previously laid under the pavement. For smaller branch-connections, and
where many bends are necessary, it is often cheaper to use wroughtiron
pipes. When many conductors have to be carried in the same street, and
on the same side of it, it is of great advantage to have several conduits for
the sake of dra wing-in cables without injury, and when both high and low
tension mains are used separate conduits are insisted upon by the Board of
Trade. The go-cmd -return mains must however always lie in the Fame
conduit, or the influence of any change in the current may be felt in
neighbouring telephone circuits.
Now, although for a single conduit the ordinary cast-iron pipe, with lead
or cement socket joints, can scarcely be improved upon for cheapness and
efficiency, the use of multiple conduits has given rise to several methods of
construction which are often cheaper and more compact. The " Johnston "
system employs iron castings in the shape of double, triple, or multiple
troughs, the conduits being of nearly square section, made in lengths of
five feet. These are covered continuously by flat covers of the same length,
but jointed together alternately with the joints of the troughs. The method
of coupling is somewhat intricate, and performed as the conduits are being
laid. Continuous joints are made between the troughs and covers with
putty, but it cannot be expected that they should be perfectly watertight,
nor is this at all necessary when under street pavement, and well-insulated
cables used. A great convenience in the system lies in the fact that access
can easily be had to the cables at any point, by simply uncoupling and
lifting a five- foot cover.
Doulton and Co. have recently introduced a very cheap system of
multiple conduits made of earthenware, three-foot lengths being cemented
together. This system may be made quite watertight, but the conduits
are of somewhat large external section, which may be at times an
inconvenience.
The Callender Company construct conduits of bitumen which afford
additional insulation in themselves to that of the cables. There are two
different systems, one a drawing-in system much used with either rubber or
bitumen insulated cables for low tension distribution ; but for high tension
work the additional insulation of the bitumen could not be considered of
any value unless the fixed system is employed.
The fixed system employs initially a trough of cast iron or sound
timber supplied in six-foot lengths. As soon as the troughs are placed in
position and connected in lengths a small quantity of refined bitumen, in a
molteu state, is run in, and before setting, spacing bridges of bituujinised
CONDUITS. 23
wood are placed in it, at inteirals of 18 inches. The insulated cables are
then paid into position, and held in place by these bridges, so that they
are clear of the sides and bottom of the trough, and of each other. More
bitumen is carefully run in, so that all the space remaining around the
cables and between them and the sides is filled solid by it to within half an
inch of the top of the trough, and, on its setting, the msin is finished off by
a covering of Portland cement concrete, about i inch thick. In alternating
current work, strong cast iron lids are substituted for this concrete. All the
bitumen employed is genuine natural Trinidad bitumen, free from admixture
of gas-tar or pitch.
For the drawing-in system two modifications are used. In the Cullender
Wehber system the conduit is formed by cases or blocks of bituminous con-
crete made in lengths of 6 feet, and pierced by varying numbers of ways.
The standard sizes of these cases are for 2, 3, 4, or 6 of such ways, of either
i|, 2, 2f , or 3 inches in diameter.
The bitumen concrote is composed of natural bitumen, sand, and wood
fibre, specially treated. The resiulting material is tough and strong, with
great power of re^iistance to crushing, breaking, and tennile strains. It is
impervious to water, is not affected by the gas or acids found in the
ground, and is a non-conductor of electricity. It does not expand or con-
tract. It is capable of withstanding the weights of heavy traffic. It can
be made in any shape, and the cases can, by suitable treatment, be bent on
the job. This type of conduit can be easily and rapidly constructed.
The best method of jointing the cases is by bringing them together,
placing specially made iron mandrils in each of the ways, running in
bituminous concrete between the two, and ramming home by jointing tools.
This quickly sets, and the mandrils are withdrawn, leaving a perfect joint
as strong as the main itself. Mandrils and jointing tools are supplied with
the cases when required.
The ways are quite smooth throughout their enture length, and there is
no projection likely to damage the cable whilst being drawn in, especially
where mandrils are used in making joints. The cables can be removed at
any time, as they do not adhere to the surface of the ways.
The Call(*nder Raworth system combines the great mechanical strength
of the solid bitumen system with the facilities for extension of the Cal lender
Webber. It consL^^ts of a conduit of cast-iron troughs having fianges suit-
able for joining the pieces into continuous length. When laid in the
trenches, melted bitumen is run in, and spacing bridges are fixed at short
intervals. Tubes of specially made paper impregnated with bituminous
compound are placed in position, refuting on the inverted arcs of the
bridges; these tubes are jointed together by sleeve-pieces, and are then
completely surrounded by bitumen. The conduit is completed by a
cast-iron lid, having fianges fitting over the main trough. A series of
wajs is thus formed having great mechanical strength and possessing con-
siderable insulating properties, and into these ways the insulated cables are
drawn in.
At various points throughout any system of conduits it is necessary
that access may be had to the conductors for connections, testing, and
generally for drawing in or out. For this purpose the arrangements are
manifold, and the generic term *' box " is commonly used for any accessible
p()rtion of the conduits. Sometimes these boxes are breaks in the line of
conduits, built in with brickwork or concrete with a removable iron lid ;
sometimes specially constructed of iron continuous with the conduits.
Where the insulation of the cable is continuous, the boxes need not be
made watertight, and the bottom of the box is left open if upon gravel or
other porous soil, so that the box and conduits may thus be drained.
24 CONDUITS.
Other boxes are made absolutely watertight, the cables entering through
glands, and attached to bare terminals resting upon insulators. These
bare terminals afford a means of disconnecting rapidly any section for test-
ing, and interconnecting an elaborate network without joints. For better
insulation they may be filled with insulating oil or paraffin wax. The
design and arrangement of the boxes must vary very greatly with local
conditions, and are generally left to the choice of the consulting engineer
rather than made an integral part of any of the various conduit systems.
The different kinds of boxes may be classed as follows i^-(i) Draw boxes,
inserted about every hundred yards, and at all points where sudden changes
in direction of the conduit occurs. The most important quality
requisite is that the length of the boxes should be sufficient that the cables
may be drawn in or out without great bending, and of sufficient volume
that a small amount of " slack " may be left in each box for convenience in
connecting, etc. (2) Service boxes, small boxes inserted at every party wall
for house connections, not necessarily with a removable cover upon the
surface of the pavement, as access without a few cubic feet of excavation is
not generally requisite. In many systems these boxes are mere '* hand-
holes," giving access to the cables by lifting a small cover or trap-door in
the conduit. It is important to note that all branch connections should be
made by V- joints, and not simply T-joints, so that the joint may be pulled
back into the conduit for a few feet in one uniform direction (say towards
the generating station). By systematic working in this way the amount of
slack necessary in the mains for conveniently making the joints may be
much reduced, it being possible to draw the mains backwards and forwards
for a short distance in the conduit. House connections are made from the
service boxes by armoured cables, or cables drawing tlirough wrought-iron
pipes. (3) Junction or network boxes, for detachable connections made by
bare terminals upon insulators. These are of great convenience for testing,
separation and rearrangement of circuits, insertion of fuses, etc. In the
very elaborate and extensive systems of supply from a central station these
junction boxes are now considered a primary necessity. With a never-
ceasing supply, day and night, alteration and additions can only be safely
made by disconnecting a section in which the jointer may work. Wherever
possible the main conductors should be " looped," that every point may be
reached by more than one route, and any section temporarily cut out at
junction boxes without breaking continuity to more distant points.
To replace vulcanised rubber various less expensive insulating materials
are used, of which a few will now be described.
The Callender Company use bitumen, for cables as well as conduits, pre-
pared in two different ways, claiming a greater durability for this mineral
substance than for rubber, at least when exposed to damp. The specifio
insulation is however much lower, necessitating an extra thickness, but even
thus the expense is much less than in rubber for equal insulation resistance.
The vulcanised bitumen cables are covered witl^ a solid sheath of bitumen,
vulcanised and put on under heavy pressure at one operation. It is then
taped and compounded, served with jute yarn or heavy tapes, and then
braided with hemp yarn, and passed through a bath of asphalte compound,
thus providing sufficient mechanical protection to stand the inevitable rough
usage whilst being handled on the streets. In other cables bituminised
fibre is used, and a solid drawn lead tube put on under great hydraulic
pressure. Such a cable may be laid directly in the earth, but when thus
used it is preferably protected by a serving of compounded yarn, over which
a steel armouring is placed, consisting of two wide tapes or ribbons of mild
steel wound spirally one over the other, and finally protected by a double
serving of jute yarn, well impregnated with preservative compound.
BARE-STRIP SYSTEMS. 2$
Nearly all the large low tension systems in London and the provinces
have been carried out for the most part with the use of bare copper strip
carried on underground insulators for the street mains. This method is no
doubt the cheapest possible when the conductors need to be exceedingly
large, but in its extensive application two great difficulties have arisen.
Firhtlj, in London and all the large cities of England, there already exists
beneath the streets a wonderful maze of gas-pipos, watei^pipes, and often
telegraphy telephone, and pneumatic tube conduits, so that there is con-
siderable difficulty in finding sufficient space for the large '* culverts"
required for the bare copper systems. In traversing the congested parts it
is therefore frequently necessary to abandon the culvert, and lay armoured
cables, or iron or bitumen conduits. The effect is to produce a patchwork
of many different systems. Secondly, a number of explosions have recently
occurred in connection with these systems, which, although not unknown
with the insulated cable systems, are much more difficult to prevent in the
large culverts. These have probably been due almost entirely to the leakage
of gas through the earth from neighbouring gas mains, and could have been
prevented by some effective system of ventilation, but it is certain that the
danger is greater in the presence of bare conductors with possible electro-
lytic action upon damp insulators.
In the Crompton system, now abandoned in favour of insulated cables,
bare copper atrip is carried beneath the surface of the footway in ctd-
verts with concrete walls, supported at intervals by glass or* porcelain
insulators, and maintained under great tension so as to be carried over
considerable spans without much sagging between the insulating supports.
It is employed by several of the principal West London and Provincial
Low-tension Supply Companies, most of them working on the three- wire
system of distribution. Three mains are therefore commonly required, but
in many parts of the system one or two pairs of feeder mains have to be
carried, so that five or seven-wire culverts are rendered necessary. In the
latter cases the feeder mains are usually carried on the side supports, the
distribution mains in the middle.
The minimum excavation in the footway for a three-wire culvert is about
2 ft. 5 in. broad by i ft. lo in. deep; for a five-wii'e culvert the breadth is
3 ft. 2 in. The walls and floor of the culvert are then built of concrete, six
inches thick, leaving an interior channel 17 inches (or 26 inches) wide by
13^ inches deep. The roof of the culvert consists of York flagstones
throughout, except that at every alternate party wall, or about every 15
yards, a removable iron cover is placed, at which points the interior is thus
rendered easily accessible, and house connections may be made. Under
these covers are placed the insulating supports. The culverts are bridged
across by stout oak baulks (4 by 3 inches) built into the concrete walls, and
upon these are mounted the glass or porcelain insulatoi'S. Glass insulators
were at first employed for this system, but porcelain insulators carrying a
gun-metal fork, in which the copper strip is placed, seem to be preferred.
The glass insulators were five inches in height, shaped with five deep
grooves in their sides, and painted with hot copal varnish to prevent the
creeping of damp, which is the chief cause of low insulation in bare copper
systems.
The copper strip is supplied in considerable lengths, having a section of
I inch by ^ inch. The requisite number of these strips to give sufficient
sectional area for each main are drawn into the culvert, and piled one upon
the other in the forks or grooves of the insulators ; into which they fit
easily with their greater breadth placed horizontally. A tension has now to
be applied to the strip so as to reduce the sag to not more than 2^ inches.
For this purpose ''straining girders" are built into the concrete walls
26 OIL INSULATION.
at conyenient distances, not more than a hundred yardn apart, and
at all places where bends in the culvert are necessary. These strain-
ing girders consist either of two heavy oak baulks, the one above the
other, or of a single cast-iron girder. A gun-metal bridge piece spans
vertically between the two oak baulks, or the upper and lower divisions of
the cast-iron girder, and presses against them through very strong insulating
blocks in such a way as to throw upon them the tensional stress which
is to be applied to the strip. This tension is applied by a hydraulic jack, or
for short lengths simply by a crowbar lever, and the gun-metal grip screwed
down to hold the strip. Two such straining girders are placed a few feet
apart, tensional stress applied to the strip in each direction, and the pro-
jecting strips clamped together by a gun-metal grip-box.
Occasional ly, where the culvert system has to be departed from, on
account of the paucity of space available, cast-iron casing of the section
illustrated is adopted with insulated cables. The branch connections for
house service, etc., are also made with insulated cable by means of specially
designed attachments.
The Kennedy system, also used by the Westminster Supply Company, is
similar to the Crompton save that the necessity of applying tension is done
away with by placing insulators every six feet. The culverts are made
much shallower (7^ inches internally), and hollow earthenware insulators
rest on the bottom of the culverts. As access to each insulator cannot be
had when there are so many, special provision has to be made for keeping
the strip in the insulator grooves when drawing in. The difficulty is
admirably met by the use of a trolley, which can be drawn through the
conduit with the copper strip attached. The trolley spans the conduit just
under the roof, and runs with wheels supported upon shelves made in the
concrete walls when building.
A third variation of the bare copper system is used by the St. James'
and Pall Mall Company. This district is unique, an enormous number of
lamps being supplied within a very small distance from the generating
station, and the load-factor, as may be expected in " Club-land," the highest
in the world. Cast-iron culverts, 11 by 7 inches, are used, the troughs
being supplied in lengths of three or six feet, and covered continuously with
iron lids. The insulators are in the form of porcelain bridges, and the
strip, which is of section 2 inches by -j^^ inch, resting edgeways in the
insulators, can be carried across spans between them as great as in the
Crompton system without similar tension of the conductors. The district
is conveniently supplied by one large ring of mains, fed by six sets of
feeder mains at convenient points, with minor branches. The three- wire
system is used, the largest conductors consisting of 8 strips, giving a
sectional area of 1.6 square inches, the middle wire being one half the
sectional area of the other two.
Brooks' system of oil insulation has been used for several years for
Electric Lighting mains, more especially in high-tension distribution, with
great success. The cables are drawn into iron pipes, and consist of stranded
copper conductors covered with raw jute, hemp strand, and braiding, several
conductors being generally combined in one cable. The cables thus covered
are first heated to about 300° Fahr. in a tank of insulating oil, generally a
thick, very viscous compound made of the waste products of rosin oil, which
impregnates the fibrous covering, driving out all moisture, and renders the
insulation extremely high. The iron pipes are carefully laid so that there
may be little or no leakage, and are filled with the same oil, a tank being
connected to the conduits and filled to a level above the highast point of the
system, so that the oil is under some pressure. Similar systems of insu-
lating cables with fibre impregnated with oil have been largely used for
ELECTRICAL TESTING. 27
telegraph and telephone communication, but it is also excellently adapted
for high tension distribution of power, on account of the valuable property
that the fluid insulation is self-remedied after the passage of an arc between
the cables, or to the pipes, caused by abnormal raising of the electromotive
force. Connections are made when the mains are laid by lead -covered cables
with junction boxes and connection boxes, the cables entering the pipes
through glands, but subsequent access to the mains themselves can only be
had after drawing ^ff the oil. On account, therefore, of the comparative
inaccessibility of the^ conductors for branch connection, it is best suited for
high tension distrij^ntion to sub-stations, with a parallel low-tension network
upon some more accessible system.
Further details as to the main conductors and conduits adapted to
certain special systems of transmission will be given in conjunction with
those systems. The present chapter has included a fairly wide summary of
those generally applicable, or at least applicable to the interior wiring or
low-tension street distribution, which invariably is the final stage in all
systems, except the series system of distribution for public lighting with
arc lamps.
Electrical Testing.
The methods of testing the conductivity and insulation of electric
iight mains, and conductors used in machinery for generation and distribu-
tion, will be found in the numerous laboratory text-books covering the
subject, and it is only intended here to touch on certain modifications of
the methods there described which commend themselves to practical engi-
neers. The aim of the laboratory experimentalist is to produce results of
the highest degree of accuracy, with the minimum use of artificial standards
for comparison, the time involved in calculations and allowance for errors
being of little consequence. The practical engineer demands methods which
shall be of sufficient accuracy, but would prefer such as may be rendered
direct reading and efifected rapidly by unskiUed assistants, after the standards
and connections have been verified by an expert. For these purposes the
Wheatstone Bridge and allied methods, though scientifically perfect with
due corrections for certain errors due to connections, etc., are of little value
except for the primary calibrations. The engineer, also, lays the greatest
on the importance, real or imaginary, of approximating in test conditions
to something of the actual conditions of working. The testing of resistance,
which is part of the everyday work of the Electric Light engineer, commonly
lies at the extreme ends of the scale of magnitude : either the minute
resistance of short thick copper conductors, and joints on the same, or
high insulation resistance. It is important that the former should be mea-
sured by methods involving large currents through the conductors, and the
latter with high electromotive forces.
The development to high perfection of the moving-coil or d' Arson val
type of galvanometer has placed in the hands of the engineer an instrument
highly convenient for arranging direct reading tests. It has the following
advantages over the older form of moving needle galvanometers : constancy of
calibration and freedom from external influences; low internal resistance,
combined with sensibility; uniformity of scale attainable for large angles
of deflection ; a complete dead-beatness of action. With reflecting types a
oon^t;ant per readable division of scale of the order of magnitude of one
hundreth of a micro-ampere is readily obtainable, with internal resistance of
only one hundred ohms; with portable pointer types a constant of about a
miei^ ampere with similar resistance, but uniformity of scaled, can only be
28
ELECTRICAL TESTING.
depended upon when the fixed magnet is specially designed for the purpose,
at the expense of sensibility.
The method of procedure which will commend itself to practical engi-
neers will be to priDiarily construct four or more resistances, by means of
which the same galvanometer may be arranged to read directly as ampere-
meter and voltmeter of widely varying calibration :
(a) A thick manganio strip with intermediate terminals for galvanometer
connections, such that the reading may be directly in amperes.
(b) A fine wire sliunt, giving similarly readings directly in micro-amperes
(or hundreths of a micro-ampere).
(c) A fine wire coil of the highest possible resistance, such that by
Fig. I.
t
.^^5^«<5N5«?^
X'^XV
Diagram of connecLioDS for conductivity test (a joint).
(C 6 E, two-pole change-over switch for galvanometer, to read amperes
or microvolts.)
Ftg. 2.
7b Earth,
■\
09690
o
€t C
O
Diagram of connections for insulation resistance.
(£ G C, two-pole change-over switch for galvanometer, to read volts
or micro-amperes. )
bridging a portion with the galvanometer direct reading in volts may be
obtained.
(d) A more moderate resistance, reducing the calibration similarly to
microvolts.
The calibration of these resistances is simply efiected and easily verified.
Once constructed, the measurement of conductivity and insulation resistances
is simply a matter of connecting and taking the ratio of pairs of readings,
the current and electromotive force or difference of potential, and may be
left in the hands of an unskilled assistant. It may be noted that any
moderate variation of the constant of the galvanometer will not afifect the
meiisurement of the resistance, as the ratio of the two readings remains
unaltered, and the methods are free from all errors due to resistances of
contact, " Peltier *' effects, etc. The exact current and electromotive force
employed are separately observed and recorded.
PARALLEL DISTRIBUTION. 29
The accompanying diagrams (Figs, i and 2) illustrate the methods applied
to testing the conductivity of a joint and the insulation of a cahle. A large
current is supplied by a dynamo or accumulator in the former test, and a
high electromotive force by a battery of small cells in the latter ; a two-pole
two-way switch for connecting the ^vanometer to the two resistances used
in succession.
A direct-reading ohmmeter, giving the ratio of electromotive force to
current, is sometimes used, with a small generator to supply the current or
electromotive force. This may also be adapted to high or low measurements,
but the method neither ensures the certainty or accuracy of separate read-
ings nor are the exact conditions of the tcbt t$pecified«
CHAPTER III.
Parallel Distribution.
Thb Parallel System of distribution of electric power has been compared
in the Introduction to the supply of water for a town, with this important
modification, that a collection return of the water to the source of supply by
an exactly similar or ** parallel " system of water mains is arranged for. The
lamps are compared to taps permitting small currents of water to pass from
the supply to the return main, thus subdividing the main outflow of water
from the source according to the section of the opening through the various
taps.
The ordinary conditions of water supply, in which the escaping water is
not again collected, but allowed to flow into a drain or is removed in some
similar way, and new water collected from an unpolluted source, may be
compared with a parallel system employing an '^ earth-return," not unknown
in the early days of Electric Lighting, but now considered impracticable and
dangerous. The only difference will then be that the water supply of the
world may be considered as an inexhaustible reservoir, whereas the electrical
capacity of the earth must, for the storage of electricity to supply lighting,
be considered practically infinitesimal.
The analogy might be further extended. The flow through any tap is
proportional to the sectional area of the opening and to the head of the
water at that point on the distributing main ; if a charge be made for the
water, according to the sectional area of the tap or feed-pipe leading into a
house, an equal head of water throughout the system ought to be insisted
on, but owing to vaiying heights of the points of supply, and the reduction
of head by friction of the water current in the mains, considerable
variation is unavoidable. This question, of comparatively little importance
in water supply as compared with that of the purity of the water, is of vital
importance in the analogous case of the supply of power by electric currents.
For it is the electromotive force or difference of potential between the mains
at various points of the system thdkt the accurate subdivision and regulation
of the light depends. As the friction of the water in the pipes causes a loss
of head at distant points varying with the current flow, so the resistance of
the electric conductors causes a fall or reduction of electromotive force
at distant points likewise varying with the electric current, and therefore
subject to considerable variations as the demand for light alters during the
day or night.
Incandescent lamps are almost invariably supplied from a parallel systtim
of electric conductors. This system may be directly connected to a generator,
or supplied as a branch from some system of transformation such as will be
30 CHOICE OF ELECTROMOTIVE FORCE.
described later. In either case the lamps require to be so constructed that, in
accordance with Ohm*8 law, the resistance of the filament allows a current to
pass through suitable to its surface for radiation of heat and light, when an
appropriate difference of potential for which they are designed is maintained
between their terminals. The light radiated varies verj greatly in reply to
comparatively small variations of the current, and a slight excess of current
Will speedily destroy the filament, so that it is requisite that the variation of
electromotive force from that specified should never be more than a small
percentage. We have thei*efore to arrange that a constant electromotive
force, or difference of potential, should exist at any point on the parallel
system at all times, and, as further regulation save that at points of
generation or transformation of electric power seems at present imprac-
ticable, the only solution is that of uniform and constant electromotive force
at all points throughout the system.
We have already observed that the possibilities of commercial efficiency
in any system of distribution depend on making the electromotive force
great in order that the current representing a given output of electric
power may be correspondingly small. The magnitude of the electromotive
force permissible is limited by two considerations. Firstly, that of safety,
both to persons and property. The rules of the Board of Trade permit the
use of the electric current for lighting of the interior of buildings subject to
the restriction that the maximum difference of potential between any two
conductors, or between any conductor and earth, shall in no case exceed
250 volts, except with the "express approval" of the Board of Trade.
(Rules issued February 1896.) With such pressure an accidental shock
can scarcely inflict any serious personal injury, and with reasonable pre-
cautions the danger of fire is undeniably less than with any other means of
illumination.
Secondly, the magnitude of the electromotive force is limited still further
by the demands of lamp construction. The most convenient sizes of lamp
for the lighting of the small rooms of dwelling-houses are those of from 8 to
16 candle-power, that is to say the equivalent of the common gas jet, or
perhaps somewhat greater. Incandescent lamps of the small size cannot
yet be constructed satisfactorily to suit an electromotive force of more than
about 120 volts on account of the necessary fineness of the filament. For
the lighting of ships, and smaller land installations, where the maximum
distance of distribution is so small that the efficiency may be kept high and
the cost of mains low without serious difficulty, it has been considered
convenient, in order that lamps of low candle-power may be used, to work
with electromotive forces of 50 to 70 volts. The more common practice in
larger installations, and the secondary circuits of transformer systems (to
which the discussions of this chapter apply as well as to those directly
supplied from a dynamo), is the use of 100 volts. In others a slight
advance on this, to no or 115 volts is preferred, partly on account of the
additional efficiency, but also with a view to convenience in supplying arc
lamps from the same system. Arc lamps requiring 45 to 50 volts and a
current of 10 amperes may conveniently be placed two in series between
constant potential mains, but for satisfactory regulation a resistance of from
one to two ohms must be placed in series with them, rendering necessary
an additional 10 to 20 volts.
Quite recently incandescent lamps of from 16 candle-power have been
manufactured with two filaments in series contained in the same bulb, each
giving 8 candle-power, and thus single lamps of 16 candle-power adapted
for an electromotive force of 200 volts can be obtained. Their use has not
yet been extensive owing to the present arrangement of the circuits for
100 volts, and the methods hitherto adopted for the direct use of higher
CHOICE OP ELECTROMOTIVE FORCE. 3 1
electromotive forces has required the partial combination of a series with
the parallel arrangement of lamps, in the form of the multiple wire systems
to be presently described.
It is still questionable whether any advantage can be gained by employing
lamps suited to high electromotive force beyond that at present in common
use. The increased fineness of the carbon filament causes greater fragility
and a shorter *^ life/' rendering a lower efficiency, or more watts per candle-
power, necessary. In short, the question of efficiency in distribution is
simply that of the relative lengths and sectional areas of the incandescent
kmp filaments and the conducting wires leading to them. By simultaneously
reducing the sectional areas of both the same efficiency may be obtained
with a lower initial expenditure in the installation, but with a greater
annual expenditure in lamp renewals. As the latter expenditure naturally
falls on the consumer, and the former mainly on a public Supply Company,
the latter having the power of choice are indined to choose an electromotive
force exceeding the golden mean which would give the best results. The
two interests are, however, ultimately, if not immeliately, identical, and
there will be no great difficulty in modifying present systems to suit the
electromotive force ultimately found to be most convenient. The improve-
ment in the manufacture of lamps is likely to cause a higher electromotive
force to be demanded ; but, on the other hand, the value of small lamps in
more convenient positions deserves more recognition than it has received
in the past, and may encourage a reduction in the electromotive force
supplied.
The distribution of electric power by the parallel system, using loo to
no volt lamps, is a sufficiently simple matter when the distance between
the furthest lamp and the generator is not greater than two or three
hundred yards. The points which require attention are :
(i) To reduce as far as possible the waste of power due to resistance in
the mains.
(2) To maintain the electromotive force between the terminals of each
lamp, as nearly constant as possible, whatever number of lamps may be
burning.
The two conditions are intimately associated, but the second is frequently
more difficult to secure than the first. If all the lamps be at nearly the
same die>tauoe from the dynamo, and supplied by the same mains, it is very
easy so to vary the difference of potential between the dynamo terminals
that a constant difference of potential shall be obtained at the end of the
mainfli whatever be the output of current. This may be secured in various
ways. The dynamo may be *' compounded " in such a way that the electro-
motive force between its terminals is raised by an amount CB, where is
the current and B the resistance of the pair of mains to the lamps, above
the electromotive force at no load, which will secure the desired effect. Or
a pair of ^* pilot " wires may be brought back from the extremities of the
mains, which, when connected with a voltmeter in the dynamo room, will
indicate to an attendant the electromotive force between the terminals of
the lamp, and thus guide the hand regulation, effected very simply by a
rheoetatic resistance in series with the coils of a shunt- wound dynamo. This
method of regulation is adopted in many low-tension central stations, but
the regulation might, if desired, be made automatic by causing an electro-
magnet to vary either the cut-off of the engine and thus the speed of the
dynamo, or adjust the rheostatic resistance in the same way as the attendant
would.
When the lamps have to be supplied from the mains throughout the
greater part of their lengths these methods are not sufficient, as, unless the
mains, are unlimited aa to size, it is impossible to maintain a oonstajct
32 FALL OF POTENTIAI..
potential at more than one point whatever the current in the main may he ;
the lamps nearer to the dynamo than this point will have a hij<)ier, and
those from it a lower, electromotive force, the variation increasing with the
number of lamps.
The difficulty may be grappled with, if not overcome, by subdividing
the mains into a number of pairs of conductors, each supplying a group of
lamps. The current density in the conductors supplying the lamps which
are near the generator may be kept high, and that in the conductors
supplying the most distant lamps as low as possible, so that as nearly as
passible a uniform fall of electromotive force may be secured in the mains
throughout the whole system. In this case, if the lamps in use at any
moment are distributed over the system with a moderate degree of
uniformity, a properly compounded dynamo may be made to keep the
electromotive force very nearly constant for all the lamps. The objection
to this system is that the highest degree of efficiency possible is not secured*
A b^^tter method, commonly adopted, is to approximate to uniformity by
the following device. Let us assume that the maximum variation in
electromotive force permissible is e, so that, E being the correct electro-
motive force for which the lamps are designed, we can allow a yariation
from E-^e to E + ^e. Six volts is about the greatest that should be
allowed on a normal hundred-volt circuit. As then the variation is between
97 and 103 volts, and these represent the extreme limits of yariation, which
will only be reached by the extreme lamps, and that only for short periods of
time in actual practice, it will not be excessive in large private installations,
although perhaps somewhat unfair in public supply to a number of
different consumers. The Board of Trade rules for public supply only allow
a yariation of four per cent, at any consumer's terminals from the declared
constant pressure. In this case there will be an additional fall along the
conductors within the building to be allowed for.
Suppose, now, the dynamo be so regulated or compounded that the
electromotive force at no load is E, and when all the lamps are burning is
E + ^e at the nearest lamp, the intermediate mains may be of such size that
a fall of pressure e is allowed between the nearest and furthest lamp. The
size of the main is frequently made proportional to the maximum current it
is intended to carry, so that the fall in pressure is • proportional to the
distance throughout the whole system. There are, however, advantages in
decreasing the current density as the mains become smaller and the more
distant points of the area of distribution are approached, so that, without
adding greatly to the weight of copper used, the electromotive force may be
more uniform throughout.
If we are prepared to submit to the inconvenience of supplying lamps
adapted to different electromotive forces for different parts of the system,
we may, by a suitable choice of lamps, reduce the variation allowed by one-
half. By obtaining lamps for the positions nearest the dynamo adapted to an
electromotive force of E + Je, for the furthest of E-^e, and a graduated
standard throughout, the maximum variation from the standard electro-
motive force will only be ^e, provided the regulation of the dynamo is
perfect.
As a simple example, suppose we have to supply 400 sixty-watt (or
nearly sixteen-candle power lamps), each requiring 0.6 amperes at 100
volts, uniformly distributed along the mains, that the branches to the most
distant lamps are 300 yards from the dynamo, and suppose, for the sake of
simplicity, that the area of the cross-section of the mains is such as to
maintain uniform current density throughout. The total length of
main is 600 yards, so that a maidmum loss of 6 volts corresponds with a loss
of one volt per hundred yards, or a current density of 400 amperes per
KELVIN'S LAW. 33
square inch. Then the sectional area of the mains at the dynamo end will
be 0.6 square incb, corresponding to about 61/11 ; its average sectional area
about 0.3 square incb, or rather less than 37/12 ; and the cost of the cable
with the standaid insulation resistance of 600 megohms per mile will be
about ;^200y or at the rate of los. per lamp.
Returning to the question of the efficiency of distribution, aFsistBuce in
the choice of the size of mains to attain the highest commercial efficiency is
given by an economic law propounded by Sir William Thompson (Lord
Kelvin). Setting aside the quebtion of regulation, if the co8t of the con-
ductor may be taken as proportional to its sectidnal area (which is nearly,
but not quite, the case), the most economical airaugement is secured when
the annual interest on the cost and depreciation of the conductor is equal to
the cost of the power annually wasted by it. In estimating this, of course
the cost of laying the conductor must be left out of consideration, except
such small portion as may be proportional to the weight or sectional area.
Let us apply this to the case considered above. When all the lamps are
burning the loss of power in the mains is 240 x 3 = 720 watts, the jvenige
drop in electromotive force being 3 volts. When half the lamps are in
actioD, the loss is only one quarter of this, and so on. The average loss
will be proportional to the square root of the mean square of the number of
lamps running at all hours throughout the year, and this will larg<«ly
depend on the purpose for which the building is designed. The average
time of using the lamps in a provincial town is about 700 hours per annum,
but in a city club the average may be twice as great. If the installation
considered is a building devoted to the latter purpose, we may reasonably
estimate the power lost in the mains as 720,000 watt hours, or 720 units,
the calculation being analogous to that used in the chapter on alternating
currents. Taking interest at 5 per cent, and depreciation at 10 per cent,
on ;^2oo we have ;^ 30 per annum, so that the size of mains chosen will be
the most economical if 720 units cost j;^ 30, which is at the rate of tenpence
per unit. This is several times greater than the probable cost of power, so
that in this case we are certainly limited rather by considerations of
effective regulation than by commercial efficiency, otherwise it would pay to
use lighter conductors and waste more power.
If a lower electromotive force than 100 volts were used, the current
would have to be increased in the inverse ratio of the electromotive force, in
order that the same power might be supplied, and if the limit of percentage
variation chosen is to be maintained, the loss in electromotive force must be
reduced in proportion to the decrease of the standard electromotive force.
The resistance of the conductor must therefore vary directly, and the
sectional area inversely, as the square of the standard electromotive force
chosen. Hence if 50 volts had been chosen as the standard in the above
installation the mains must have been of four times the size. The ccjst
would have been four times as great; the current 480 amperes instead of
240 ; the difference of electromotive force between the extreme lamps 3
volts instead of 6 ; and the waste of power would then be the same. On
the other hand, if we could use 200 volts as the standard electromotive
force (and it will be shown shortly how this is possible with certain qualified
conditions with the use of incandescent lamps adapted to an electromotive
force of 100 volts), the current would be reduced to one half of that with
100 volts ; and the weight of copper to one quarter for the same loss of
power in the mains, twice as much fall in potential only representing the
lame percent<age loss, and thus 12 volts being permissible instead of 6.
34 NETWORK AND FEEDEB MAINS.
Network and Feeder Mains.
We have so far dealt with the problem of efficient distribution on the
supposition that the power is supplied by mains proceeding directly from the
generator to groups of lamps, or by branch circuits from a larger pair of
mains. When the lighting is distributed over an area such as that dealt
with in central station supply, where the mains have to be laid in streets
running in various directions, and branches are taken off at various
points into the buildings, the system of conductors generally becomes very
elaboi*ate, being advantageously connected together so as to form a network
of complete loops, the positive and negative mains (and branches therefrom)
being, of course, kept separate throughout, and only connected through
the lamps. The current may then proceed to or from any lamp J)y many
different routes, and the calculation of the drop in pressure to any lamp is a
complicated matter. Such a network is generally connected to the generator
by what are called ^* feeder mains," to which it is intended that no lamps
on branch conductors should be connected, but which carry the current
direct to convenient points on the network, preferably to those near which
there is a heavy demand. The current density in these feeder mains may
be much greater than in the network, being only limited by considerations
of safety and efficiency ; for the loss of electromotive force in them can be
easily compensated for, and thus the potential difference between the mains
at the various points of connection maintained uniform. To secure this
compensation the large conductors intended for feeder mains are frequently
constructed so as to include two pilot wires among the strands of the con-
ductor, insulated from the other strands, and connected to a voltmeter (of
the electrostatic, or of the extremely high resistance type) in the generating
station. Where bare copper strip is used separate pilot wires must be
carried back in the culverts. This voltmeter will give information to the
attendant or " switchman " as to the potential difference at the extremity
of the feeder. If, owing to a heavy demand in the region of the network
near the point of connection, this potential difference is lowered, this pair of
feeder mains may, when several dynamos are used, be connected to one
having higher electromotive force, or one or more secondary cells may be
added to raise the potential difference of that pair of mains alone. The
cells used for this pui-pose may be charged by periodically shifting them to
feeders where lower electromotive forces are required, and at the same time
reversing their connections. If the network be carefully arran':ed, and the
current density fairly low, approximate uniformity of potential difference is
maintained throughout the system if it be kept uniform at the points of
connection of the feeders. The feeder mains will generally form radiating
branches, interconnected by a network of conductors in the cross streets.
There is scope for ingenuity in arranging this network so as to maintain
uniformity of electromotive force under all conditions. For example, in a
street crossing between the routes of two pairs of feeder mains, the supply
mains in them may be taken the positive from one pair of feeder mains, and
the negative from the other only. In this case the current from any lamp
in the street must necessarily pass the whole length of the street on one
main or the other, and if it be arranged that the current density is uniform
throughout, and likt^ly to remain approximately so with the varying
demand, unifori^aity in potential difference between the terminals of all
lamps will be secured. This device will, however, not produce the highest
efficiency possible, especially if the heavier deniand is at the extremities of
the street. As long as the current is fairly constiint in magnitude, varying
only as lamps are switched on or off, no appreciable inductive effect should
MULTIPLE-WIRE SYSTEMS.
35
result in neighbour! og telephone circuits ; but if it should happen that uses
of the current other than for lighting cause rapid though minute variations
the method described above could not be tolerated.
For the interior wiring of buildings in which the supply of power is
obtained from a central station, either as a branch from the street mains, or
the secondary circuit of a transforming system, it will be impossible to
further compen«^te for the fall of potential in the interior conductors,
except so far as may be effected by increasing the electromotive force over
the whole system by one or two volts at the time of maximum demand, and
thus giving an average compensation for the fall in the premises of the
various consumers. It is necessary to stipulate, for the protection of con-
siunei's and the reputation of supply companies, that firms contracting for
the wiring should allow for sufficient sizes of conductors that the maximum
fall to ).he furthest lamp should not exceed a certain amount when all the
lamps in the installation are in use. For this two volts is generally
specified, and the use of a maximum current density of looo amperes per
square inch throughout will then allow a distance of transmission of forty
yards, sufficient for most buildings. For a few isolated lamps in attics, etc.,
there would of course be no objection to allowing a greater fall of electro-
motive force. Adopting a imiform current density, the requisite sizes of
the leads are easily found from tables supplied by the manufacturers of the
cables.
Multiple-Wire Systems.
When the area of distribution extends to distances of more than about
a quarter of a mile from the generating station, the devices detailed above
will be insufficient to grapple with the difficulties of maintaining simul-
taneously high efficiency, commercial economy, and uniform potential with
the simple parallel system as long as small incandescent lamps cannot be
satisfactorily constructed to suit electromotive forces greater than 120 volts.
We are still well within the limiting electromotive force made advisable by
considerations of safety, and it would therefore be permissible to make use
of electromotive forces of upwards of 200 volts, placing lamps suited to
100 vults or more in pairs, connected in series, between the mains, and thus
enable us to supply to four times the distance with the same loss of power
and variation from uniformity.
This system would be attended with the inconvenience that two lamps
must always burn simultaneously, or an equivalent dissipative resistance be
inserted in place of one of them. The three- wire system, invented by Dr.
John HopkinRon, enables us to avoid this difficulty with large installations,
and renders direct supply of electrical power at 200 volts satisfactory to
distances at least of upwaixls of half a mile.
Similar generating plant is employed, the requisite 200 volts being
obtained most conveniently by placing two dynamos, each giving 100 volts,
in series ; sometimes one dynamo having three terminals and a double
armature is used to fulfil the same conditions. It is necessary of course
that the dynamos should be able to exceed this minimum voltage by 10 per
cent, at times of heavy load to allow for the fall of E.M.F. permitted in
the mains, being either compounded or hand regulated to maintain the
requisite electromotive force in the network.
A third main is taken from the intermediate terminal, and lamps are
connected throughout the whole system between the branches from this
main and the extreme mains, thus being supplied with the requisite hundred
volts. The system is thus like two combined parallel systems with a common
main. If the two systems were completely separate they would be like two
circuits of a simple parallel system; but the common main, which is the
36 MULTIPLE-WIRE SYSTEMa
positive main of one system and the nej^fative of the other, carries to or
from the generating station only the difference of the cucrents in the two
extreme mains, and thus if the two sections of the combined system be
carefully arranged that the number of lamps, or rather the power required
in each is as nearly as possible the same at full load, and likely to remain
approximately the name at different hours of the day or night, the third
main need only be very small, and will, in fact, only be used to any extent
in carrying the current from a group of lamps in one section to a group in
the other. The same current supplies the power for both sections of the
system, and is therefore only one half as great as it would be in a two- wire
or simple parallel system ; and with the same curreot density in the extreme
mains the percentage fall of potential is only half as great, provided the
lamps are so arranged that the current traverses very short distances in the
intermediate wire, and the corresponding fall can be neglected.
The maintenance of uniformity depends largely on the equality of the
demand in the two sections, and is therefore only applicable to extensive
systems, where equal averages can be depended on. To save great expen-
diture of copper in the third wire, it is to be recommended that the two
sections be combined as closely as possible, branches from all three mains
being taken into every building, and the sections as nearly equalised as
possible in each. The calculation of the necessary siase of the third wire is
often an intricate matter. It has been suggested that lamps should be
always connected permanently to branches from the intermediate wire, and
that it be arranged by means of a two-way switch that the same lamp
could be placed on either section. The lamp would always be brightest
when placed in the section which contained the fewest lamps, so that equality
might be maintained by the natural disposition of a consumer to obtain the
better light.
The system of feeder mains and other devices to secure uniformity of
electromotive force previously described are equally applicable to the three-
wire system. Owing to the cost of the third wire, and additional possi-
bilities of variation in electromotive force due to the switching on or off of
groups of lamps, the full theoretical increase in efficiency of this system,
that of four times that of the two-wire, is not secured, so that the limit of
the economical distance of distribution has been put at between a half and
three-quarters of a mile with loo-volt, or one mile with 200-volt lamps.
The power is correctly measured in watts in this system if ampere
meters be placed on the external mains, and the sum of their readings be
multiplied by 100, or the measure of the electromotive in volts in either
section.
For large areas of distribution we may maintain a direct supply for 100-
volt lamps without transformation by the use of four or five wire systems,
with electromotive forces of 300 and 400 volts. Thes*^ systems are only
extensions of the three- wire principle, having three or four separate sections
with intermediate mains to carry the current from lamp to lamp.
Multiple-wire systems will extend the limit of the economical area
to distances of a mile to a mile and a half. Further combination of the
series principle with the parallel is not permissible for interior lighting,
although permissible, with proper precautions, for street lighting. More-
over, with the five- wire system, it is not permissible to take branches from
all the mains into the same building, in accordance with the law insisting on
a limiting difference of potential of 250 volts; and it is necessary that the
middle wire should be *' earthed," preferably at the generating station, in
order that the potential of the extreme mains should not exceed the required
limit.
In these somewhat complicated systems there is considerable difficulty in
ELECTROMAGNETISM. 37
maintaining approximate equality in the demand in all the sections at all
times, and so preserving uniformity in the potential differences without the
necessity of large intermediate mains. To compensate for this inequality a
machine called the ''potential equaliser'' has been used with considerable
success at Paris and other places. This consists of several dynamo armatures
rigidly coupled, or built on the same shaft, each of which is connected across
an adjacent pair of mains. These armatures are identical in design, and
rotate in magnetic fields of exactly equal intensity. They are of as low
rpsistance as it is possible to construct them, consistent with the generation
of the requisite E.M.F. in each to oppose that of the circuits to which they
are connected.
If the E.M.F. in each of the sections of the supply system (between
adjacent mains) is the same, all the armatures will be motor-driven with
very small currents, sufficient power being absorbed to overcome the friction,
each giving an E.M.F. equal to that of the section to which it is con-
nected. If the E.M.F. in any section rises, owing to the reduction of the
number of lamps in it, a larger current will pass through the corresponding
armature of the equaliser, tending to increase the speed. The E.M.F. in the
other armatures will then rise, increasing that in the corresponding sections,
which had begun to fall owing to the larger current required in them. In
other words, the armatures connected to the sections which have a reduced
number of lamps become motors driving the remaining armatures, which
ac*^ as generators. A secondary battery may be made to fulfil the same
function as the equaliser, but requires much greater space and more atten-
tion. One or the other of these methods for equalisation requires to be
adopted when the supply is carried to a great distance, whether by the
three-wire or the more elaborate multiple-wire systems.
CHAPTER IV.
Electromagnetisni.
Tm term " dynamo," taken in its most general signification, denotes a
machine for the conversion of Power from the mechanical to the electrical
form. In other words it is a machine by means of which the uncreatable
and indestructible physical quantity which we know as '* power," and measure
as the product of its two factors of velocity and mechanical force, is trans-
ferred to the medium in which electrical phenomena are reproduced, and
measured by its two factors of electric current and electromotive force.
Dynamos vary very widely in design according to the relative magni-
tudes, and other conditions, of the two factors, electric current and electro-
motive force, whose product represents the power when in the electric form.
These conditions are determined by the system of distribution which it is
intended to employ for the electric power. We shall have to consider the
design of classes of dynamo suited to each system which is described in these
pages ; and, with the limited space at our disposal, we must confine our-
selves very rigidly to such parts of the theory as will have a direct bearing
on the efficiency and practical working of the system in question. It will
be advisable to give a summary description, limited in scope, but complete
as far as it goes, which shall neither intrench on the province of the
Scientific Text-book, nor the Technical Hand-book of the Dynamo
Designer, appealing rather to the user than the builder of machinery.
A dynamo consists essentially of two parts. Firstly, a magnetic field,
produced by permanent magnets or elect roniagnets ; and secondly, an electric
circuity in which an electromotive force and an electric cuiTent are produced
38 StAONETIC FIELDS.
bj its motion relatively to the magnetic field in which it is placed, &nd
which is continuouB with an external circuit through which the power is
distributed. The latter part of the dynamo b termed the armature.
Electromotive force in the armature is produced either by the motion of
the armature in the magnetic field ; or by the motion of the magnetic field
in which the Bxmature remains stationary; or by the variation of the
intensity or arrangement of the magnetic field in the neighbourhood of the
armature by the motion of masses of iron. A rotary motion is alone suit-
able to the ordinary practice of engineering, though a reciprocating motion
might conceivably ofl'er some advantages, and has, in fact, been uaed with
good effect for the generation of electromotive forces for certain purpoeee
Fig. 3.
outt>ide the scope of this article. For the production of continuous currenta
the motion of the armature ia almost exclusively employed, but for alternating
currents each of the other methods present numerous advantages.
Confining ourselves at first to the description of the magnetic field of a
<lynamo, the theory of which is for the most part the same for all classes of
dynamos, we will assume that the reader is fairly well acquainted with the
nature of permanent magnets, and with the representation of the magnetic
tield in its neighbourhood by the graphical method of drawing " lines of
force." All elementary text-books deal at some length with the theory of
lines of force, as produced by permanent magnets and by coils of wire
carrying electric currents, and with the methods of tracing them by scattered
irun tilings or by a com pass- needle ; it would serve no good purpose to fill
these pages with one more repetition of the same story. It will be sufficient
to indicate summarily a few points to be especially remembered, and take
up the theory at the point where it is generally left by the t«st-book on
Elementary Magnetism so as to apply the results specially to the m&gnetic
field of the dynamo.
Two typical examples of magnetic fields traced by lines of force are
given in the accompanying illustrations. Fig. 3 represents the magnetic
field produced by a permanent steel bar magnet in its immediate neighbour-
hood. Fig. 4 represents a uniform magnetic field (in which the lines are
parallel and the magnetic force constant at all points) distorted by the
presence of a bar of iron, which is converted into a magnet by being placed in
MAaNETIO FORCE. 39
the field. In all cases the lines begin and end in iron or stretch beyond the
field represent<?d. Points on the iron towards which the lines of force seem
to converge are termed poles ; areas of the surface of the iron where streams
of lines enter are said to have distributed polarity.
The following facts should be carefully noted : —
(i) lines of forces due to magnets begin and end at the surface of the
iron at points of opposite polarity, ''north" and "south." They are no
longer traceable within the iron by the methods described. An apparent
exception of lines passing away from the limits of the diagram is obviously
explained by supposing that they proceed to the magnetic poles of the
earth.
(2) lines of force due to electiic currents in wires form closed loops
passing round one or more wires.
(3) The magnetic force at any point of the field is in a direction
tangential to a line of force passing through that point. All the mechanical
forces of attraction and repulsion between magnets or coils in the magnetic
field may be at once determined by supposing a tension to exist along
the lines of force, and an outward pressure or thrust at right angles to the
lines.
(4) The relative intensity of the magnetic force in different parts of the
field is accurately indicated by the relative density of the lines of force in
that part, provided that the lines be drawn without discontinuity, and with
even distribution wherever the force is uniform. Just in proportion to the
spreading out or ditiwing together of the lines, when conceived drawn in
space and not on a flat surface as shown in the illustrations, the magnetic
force is correspondingly weaker or stronger.
(5) The magnetic force at any point, eociemal to iron or other magnet-
isable substance, may be represented quantitatively, just as it is direction,
by the density or nvmiber of lines 0/ force per aqtuire oeiUimetre, By this
we understand that lines are supposed drawn in the direction of the
magnetic force at the rate of one per square centimetre when the magnetic
force is of unit strength, and more in proportion where the force is greater.
The observation (4) made above, wluch may be mathematically demon-
strated, shows that it is possible with continuous lines so to represent the
magnetic force at all points of the field. This number is a measure of the
Magnetic Force, and may be fractional ; for the earth's magnetic field in
the neighbourhood of London it is somewhat less than one-half. The
graphic representation of the field by lines drawn on paper, or imagined
drawn in space, has suggested the unit, one '^ line of force per square centi-
metre," and some students are inclined to confuse the measurement and the
graphic representation so as to find a difiiculty in the expression half a
line per square centimetre, or to conceive spaces existing between the lines
drawn, in which no magnetic force acts. The more thoughtful are well
satisfied with a term for the unit which indicates direction as well as magni-
tude. Faraday's original term, the "unit tube of force," avoids the
suggested confusion, and a shorter term, '' Gauss,'' has been suggested for
the name of the unit of Magnetic Force, but it is seldom adopted. For the
definition of the unit on the centimetre-gram-second system we may refer
to scientific works, as the engineer is generally satisfied for his magnetic
measurement with secondary standards, and to work with formulse which we
shall give later without mathematical proof.
The part of the magnetic theory which is generally least understood,
and frequently misunderstood, is the continuation of the magnetic field
into the interior of a piece of magnetised iron. We can suppose the system
of lines of force to be continued, though untraceable, into the interior from
a pole, or distributed polarity, of one kind, and to emerge at another pole,
40 MAGNETIC INDUCTION.
or distributed polarity, of the other kind. As in any piece of iron the
total north and south polarity are equal, we shall find as many lines
emerging as entering when the field is correctly mapped out externally to
the iron, and so may imagine the continuity to exist through the interior.
Lines of force will thus in all cases, with magnets as well as coils of wire
carrying electric currents^ form closed loops. But it is all-important to
notice that in the interior of the iron these continued lines can no longer
represent the Magnetic Force. For, when passing from the exterior to the
interior of iron, the Magnetic Force due to the polarity is immediately
reversed, but the density, or number of lines of force per square centimetre,
is not correspondingly reduced, but as a rule rather increased.
We must here make a distinction between the number of lines of force
(supposed continued from the external field) which is called the Mctgnetie
Induction (and universally denoted by the letter B), and the calciSdable
Magnetic Force due to all poles, both of the piece
V^G. 5. of iron itself and external magnets, and to coils
' of wire carrying currents. The measure of the
Magnetic Force is denoted by the letter H, and
while in non-magnetisable material H = B always^
in the interior of iron H is generally very much
less than B. Consider, for example, a permanent
magnet of the form of an incomplete ring, Fig. 5,
a short air-gap intervening between the two poles.
The magnetic field between the two poles will be
very strong, and lines of force of very great density
must be supposed drawn across the gap to repre-
Iron riog with air-g»p. sent the field. If these lines be continued into
the iron the density will be the same, if not
greater, within the iron right round the ring ; but the Magnetic Force
due to the poles will in reality be very small on the other side of the
ring, as the two poles, being close together, will almost neutralise one
another. Further, a moment's consideration will show that the Magnetic
Force in the iron throughout the whole ring is in the opposite directior to
that expressed by continuing the lines of the gap round the ring. Therefore
neither in magnitude nor in direction can the continued lines represent the
Magnetic Force within the iron. In all magnets the Magnetic Force due to
its own poles is in opposition to the Magnetic Induction.
If we wish to explain the physical meaning of Magnetic Induction
within any piece of magnetised iron we may suppose ift small cavity made at
any point in the shape of a flat disc, having its axis in the direction of the
lines of force. If the thickness of this disc is very small in comparison with
its diameter, we may suppose the lines of force to pass across it as we have
supposed them to pass in the previous example of the ring with small gap,
maintaining the same density as in the adjacent iron. The Magnetic Force
measured in this disc-shaped gap will be the Magnetic Induction, and will
be that produced by all external poles and coils in the neighbourhood com-
bined with the distributed polarity over the flat surfaces of the disc-shaped
gap. The Litter will generally be much the larger term, so that the
Magnetic Induction will be immensely greater than the Magnetic Force,
which only includes the former term — the calculable forces of poles and
coils in the neighbourhood. If the cavity be of the shape of a long, thin
tunnel along the lines of force, the inner polarity at the ends of the tunnel
will be insignificant in efiect, and the Magnetic Force in the interior will be
correctly measured in such a cavity.
When an unmagnetised piece of iron is placed in a magnetic field, it
becomes of itself a magnet. It may or may not have magnetic poles of
FIELD PRODUCED BY CURRENTS. 4 1
its own. If it has not, as in the case of a complete iron ring magnetised by
the current flowiog through a solenoid ooil wound upon it, the Magnetic
Force, or Mofftietising Force as it may now be called, is unaltered, and is
simply that given by the ooil alone. But the iron is strongly magnetised,
the strength depending upon the quality of the iron and the strength of the
Magnetising I*orce, The effect of this magnetism of the iron could be
recognised by the extra strength of the magnetic field in a short air-gap,
made by splitting the ring at one point, but the poles thus formed would
inevitably weaken the Magnetising Force in the iron of the ring ; supposing
this weakening allowed for, we see that the Magnetic Force in the gap,
which is that above defined as the Mag^ietio Indiusliony is different from the
Magnetising Force due to the coii alone, and may be looked upon as a com-
bined field due to the magnetisation of the iron added to the original field of
the ooil.
It is possible, however, to recognise and measure the Magnetic Induction
without forming an air-gap, and thus modifying the Magnetising Force.
Any change in the Magnetic Induction produces electromotive force in
another coil wound round the ring during the change, and proportional to
the rate of change ; this gives an indirect method of measurement which
saves calculation of the weakening, or demagnetising force, of the poles
formed by splitting the iron ring. The details of this measurement are to
be found in any text- book for the electrical laboratory, and reference may
be made to the Standard Text-book by Prof. Ewing on " Magnetic Induc-
tion in Iron and other Metals.** We shall confine ourselves to the state-
ment of the general results^ and their application to the iron magnets of
electro-magnetic machinery.
The Magnetising Force at the centre of a flat coil, as used in the tangent
galvonometer, is deduced at once from the definition of the unit of current
and may be written
H. 2n»0
lor
where n is the number of turns ; the current in amperes ; r the radius of
the coQ in centimetres. The calculation for other shapes of coil for the
Magnetic Force at various points of the field speedily leads to complicated
mathematics.
At a distance d from a straight wire of infinite extent the Magnetic
Force is given by the f oi-mula
lod
This case is interesting from the fact that the lines of force forms circles
round the wire carrying the current, at every point of which the Magnetic
Force is the same. If we multiply the Magnetic Force by the length of a
circular line of force, 2ird, we get a quantity, - — 0, which is independent of
ihe distance from the wire, {.0., the name for any line. This result is a
particular case of a general theorem which is of incalculable value, and which
we will assume without further proof. If we have any closed circuit what-
ever, and divide this circuit into a large number of minute lengths, multiply
each length by the resolved value of H tangentially to the circuit, and add
all the products, the total sum, when a sufficient number of parts is taken
that H may be approximately constant throughout any part, is always
10
• . "m-uG mrougb the curve traced. This infimt«
f ..i:« Integral of the Magnetic Fon*." The
.-.f vL-'^riL-^ its expressioD as/Hils round the
1 <i^:i liii^ineDtar]^ dynaniics will see that it is
■ d * uuc pule Cftrried by the Magnetic Force round
'!> iivurom. Msily proved in the general case, lie« in
.,.,. 't» »a approximation of sufficient accuracy for
. uf BUipioved for electromagnets, giving us the
• 'viiic in the interior. If the length be ^;rv«t in
t.i.rr ot Che solenoid, the Line Integnil of the Mag-
t .uie of force from one end through the coil and
If 4iid by an external curve is approximatelv, IH,
-iiti st'ieiLoid. For in the inteinal part of the cii-cuit
ii throughout ; and in the external part, though the
<. tht^ value of H is so reduced by the (spreading of
lUuciod to the Line Integral is but emalL We shall
HI = ^nC.
^■,i*j , iK'i»iv'<'* df[>ending on the nuTnber of ampire-tumt per eenti-
,". vv.iuiu.ji tilt- coil to be very long in comparison with its diameter.
,., ,„-..!* fc >ii'l uioiv approximately true when applied to the interior of
... ...L.i .-11 « I'lii^ i>f uon-magnetic material so as to cover the ring with
\^ >^ .1 t vvtii;'"'!^ ring of iron in used as the core of the ring solenoid, or
< .v. 1 \-f)i «UM» is placed within the long solenoid, the Magnetic Force
, i-.ivi ^I'i uii»pi>»m'iahly altered except near the ends of the wirej, but
\,;^..viio lii.hu-tion is very much greater. This may bo shovm by
, i ,. ,>t 1 lio outivnt, which reverses both Magnetic Force and Induction,
- • — 1 current in a secondary coil, such as described
[h a Ballistic Galvanometer. The amplitude of
galvanometer measures the change, in this case
raed Magnetic Induction, and is many times
in a similar arrangement wiih non-magnetic
the Magnetic Force. The ratio of increase is
the iron, denoted by the Greek letter ft, so that
hus measured IB found to be independent of
relative dinmet«r and radius of the core ring,
netic circuit is maintiiined, or as far as the
hold. In the case of the long soft iron wire,
nly if the length of the wire is over 400
rmula for U applicable to short bars of iron,
which is illustrated in Fl. I. The esternal
by ft heavy A yoke of soft wrought iron, into
e tested are carefully fitted. The contribution
persed magnetic field in the yoke is practically
correction for the ends, the bar may he con-
length equal to the length of the bar C within
dug Solenoid ; D the secondary ooil in which
42 LTNE-INTEGBAL,
where C is the total flow of current through the curve traced. This infinite
summation is termed the ''Line Integral of the Magnetic Force." The
mathematician will at once recognise its expression as /Hds round the
curve; any one acquainted with elementary dynamics will see that it is
really the work done upon a unit pole carried hy the Magnetic Force round
the closed curve.
The importance of this theorem, easily proved in the general case, lies in
the fact that it will give us an approximation of sufficient accuracy for
solenoid coils such as are employed for electromagnets^ giving us the
Magnetic Force at any point in the interior. If the length he great in
proportion to the diameter of the solenoid, the Line Integral of the Mag-
netic Force following a line of force from one end through the coil and
returning to the same end hy an external curve is approximately, HI,
where 1 is the length of the solenoid. For in the internal part of the circuit
H is practically constant throughout ; and in the external part, though the
circuit is slightly longer, the value of H is so reduced hy the spreading of
the field that its contribution to the Line Integral is but small. We shall
have therefore
HI = l^nO,
lo
where n is the number of turns, or
H=l!!:.5[.0,
lo 1
the force therefore depending on the ntmiber of amph'e-tums per centi-
metres t, assuming the coil to be very long in comparison with its diameter.
The formula is still more approximately true when applied to the interior of
a coil wound on a ring of non-magnetic material so as to cover the ring with
fair uniformity.
When a complete ring of iron is used as the core of the ring solenoid, or
when a very long wire is placed within the long solenoid, the Magnetic Force
is unaltered (or unappreciably altered except near the ends of the wire), but
the Magnetic Induction is very much greater. This may be shown by
reversal of the current, which reverses both Magnetic Force and Induction,
and produces an instantaneous current in a secondary coil, such as described
above, when completed through a Ballistic Galvanometer. The ampUtude of
the '' throw '' of the ballistic galvanometer measures the change, in this case
the double value of the reversed Magnetic Induction, and is many times
greater than that produced in a similar arrangement with non-magnetic
core, which is proportional to the Magnetic Force. The ratio of increase is
called the ** Permeability " of the iron, denoted by the Greek letter /i, so that
B = /i H. The Permeability thus measured is found to be independent of
the shape, that is to say, the relative diameter and radius of the core ring,
as long as the complete magnetic circuit is maintained, or as far as the
approximations described will hold. In the case of the long soft iron wire,
the error is inappreciable only if the length of the wire is over 400
diameters. To render the formula for H applicable to short bars of iron,
Hopkinson adopted a device which is illustrated in PI. I. The external
magnetic circuit is completed by a heavy A yoke of soft wrought iron, into
which the ends of the bar to be tested are carefully fitted. The contribution
to the line Integral of the dispersed magnetic field in the yoke is practically
negligible, and with a small correction for the ends, the bar may be con-
sidered equivalent to a ring of length equal to the length of the bar within
the yoke. B is the Magnetising Solenoid ; D the secondary coil in which
MAGNETISATION. 43
the mstantaneous currents produced hy .reversal measure the Magnetic
Induction.
The Permeability of iron is not, however, a constant quantity, even for
any given sample of iron. Tested by the reversal method just indicated it is
found to increase at first with the Magnetising Force, so that the Magnetic
Induction increases at a greater rate than the Magnetising Force. The
Permeability reaches a maximum value, somewhat over 2000 for soft
wrought iron, and subsequently decreases, the ultimate limit being probably
unity. In very intense magnetic fields it appears, according to experiments
detailed by Professor Ewing in his standard work, that the difierence B - H,
the excess field due to the magnetisation of the iron, advances to a final
limit. In the purest iron this limit is the greatest and amounts to about
22,000.
If the magnetising coil could be removed, the magnetic condition of the
iron remaining intact, the density of the lines of force would be B - H. The
Magnetic Induction may, in fact, be considered as the total strength of two
fields, the Magnetic Force added to the magnetic field created by the ii-on
itself. At any point where the lines of force emerge from the iron the
strength of the polarity per square centimetre in c.g.s. units may be shown
B — H
to be , which is termed the Magnetisation and generally denoted by the
4ir
i^mbol I. For if these lines of force be allowed to spread out equally in all
directions from the end of a long wire (so as to be unaffected by neighbour-
ing poles) the density at a distance r from the end will be obtained by
dividing the total number, 49rl if the section of the iron be one square
centimetre, by 4irr^ the area of the surface of the sphere. This gives -= ^
r*
so that I is a measure of the polar strength per square centimetre. The
maximum value of I for the purest samples of iron is about 1750.
We have so far been able to avoid consideration of the reactionary effect
of the Magnetic Force due to polarity of the iron magnetised by employing
the complete magnetic circuit of the ring or Hopkinson's Yoke, or the very
long wire. In the dynamo the magnetic circuit in iron must unavoidably be
broken for the insertion of the armature, and, however short the air-gap, a
weakening of the Magnetic Force within the iron, and a strengthening in
the air-gap between the poles must ensue.
Consider for a moment the case whei e the gap is extremely short com-
pared with the section of the ring, so that the spreading of the lines of force
on the edges of the gap may be of little coobequence, and the Magnetic
Induction may be taken as uniform throughout the whole circuit. In the
air-gap the Magnetic Induction and Magnetic Force are equal, the latter
being immensely increased by the proximity of the poles on either side. On
entering the iron the Magnetic Force due to the poles is reversed, the
attraction of the nearer being in opposition to the Magnetic Force of the
ooils; and on the whole the Magnetic Force Is in the same direction as
before, but inversely reduced so that, with the same Magnetic Induction as
in the gap, the equation B^/iH still remains. Though it is hard to measure
the reduced value of the Magnetic Force, it may be confidently assumed that
the relation to the Magnetic Induction is the same as would be given by a
test of the sample where the value of the Magnetic Force is given by the
measurable effect of a coil carrying an electric current.
But although i^e Magnetic Force is increased in one part of the circuit
and decreased in another, the line Integral through the complete magnetic
circle is unaltered, and equal to ~nC, as before. It is sufficient to
10
observe that the Line Integral in any closed circuit is really the work done
44 MAGNETIC CIRCUITS.
on a Unit Bole in traversing the circuit, and that it would he in defiance of
the principle of the Conservation of Energy to suppose that fixed magnets
could supply continuously energy to a moving, pole, as would be the case if
the Line Integral for a closed curve were in any way altered. Dividing the
Line Integral into two parts, for the air-gap and iron ring respectively,
calling the lengths 1, and ],, and the value of the Magnetic force H^, H^ that
of B being the same throughout, we have
and the Line Integral equation
Hili+Haljrzl'nO
lO
becomes
I ilJ lo
It will be noticed that, for a sample of wrought iron in which the permea-
bility is 2000, an air-gap of txtuit ^^ ^^^ length of the ring core is sufficient
to call for twice the strength of magnetising current in order to produce the
same Magnetic Induction as was required for the complete iron ring.
The most general case, or at least the most complicated that is amenable
to mathematioEd calculation, of a magnetic field is one in which the magnetic
circuit consists of magnetic materials of various qualities and sectional area,
broken here and there by air-gaps of various breadth. It must be assumed
that the lines of force are kept very approximately within a prescribed area,
where we can make an average estimate of the length traversed in the various
materials. The magnetic circuit of the dynamo, for example, will include
the ^* limbs " upon which the magnetising coils are wound, an iron ring and
cylinder upon which the armature is wound, air-gaps, pole- pieces, and a
'* yoke " or bed-plate. It will generally happen that a certain amount of
leakage, or wandering of the lines of force outside the prescribed area, will
occur, and the method by which an approximate allowance is made for this
leakage will be described later. Dealing at present with the ideal case, we
may assume a certain total number of lines of force to traverse the whole
magnetic circuit. This total number is a measure of the whole strength of
the electromagnet ; it is termed the '* Flux of Magnetic Induction," and will
be denoted here by the letter N.
In any part of the magnetic circuit where the sectional area, taken over
a place perpendicular to the direction of the lines of force, is A square centi-
metres, we shall have
B--.
If the lines of force may be considered to traverse with this density a length
I of uniform section in a material of permeability /it, the contribution of
this part of the magnetic circuit to the J2ine Integral will be given by
H.1 = N.-L.
Supposing N to persist uniformly throughout the whole circuit we shall
then have, on addition to the Line Integral for all parts,
N C A- + -^3-+ etc. "j = l^na
\AiMi Aa^ / 10
EELUCTANCE. 45
On the right hand side of the equation the quantity nC is supposed to
represent the total number ol am p^i'e- turns in all ooilb wound round the
magnetic circuit ; the position and arrangement of such coils is of no momeixt,
except in so far as it affects the leakage of the lines of force. The above
equation gives a means of determining N from the known measurements and
magnetic properties of the materials employed for the electromagnet for any
number of amp^re-tums in the magnetising coils. Unfortunately the
variable value of the permeability according to the Magnetic Induction in
the various parts complicates the problem. It will be shown that a crraphio
solution is possible; but this is not absolutely necessary to the dynamo
designer, who will more frequently have to deal with the inverse problem,
which is simpler, to determine the number of amp^re-tums required to
produce a given Flux of Magnetic Induction. Each part of the Line
Integral may then be taken separately ; the requisite Magnetic Force in
each part of the circuit being determined from tests of samples of the
materials used.
The equation giving the Flux of Magnetic Induction in a magnetic
circuit affords a vei*y striking and instructive analogy to Ohm's Law, giving
the current in an electric circuit
O X R » E.
Comparing the Flux with the electric current, the quantity v^nC
creating the magnetic field may be termed the Magneto-mctive force in
analogy to the term Electromotive force. The sum of the values — is
exactly analogous to the summation of the resistance of the different parts
of an electric circuit, if only the permeability be compared with the electric
conductivity of materials employed. The summation of -r- might he
termed the Magnetic Resistance of the circuit, but the single word Hehtc-
tanoe has been genei-ally adopted to avoid confusion. The word is simply
a convenience, and it will serve us well in general discussions concerning
the magnetic circuit, but it must always be remembered that its value
depends upon the intensity of magnetisation of the iron, and the true
relation between the Magneto-motive Force and the Flux of Magnetic
Induction can only be represented by a curve.
We have defined the permeability of a sample of iron as the ratio of
the Magnetic Induction to the Magnetising Force producing it, noting that
this ratio is not a constant quantity, but that it depends upon the intensity
of the Magnetising Force. The permeability is also found to be modified
by the previous magnetic history of the iron, that is to say, whether brought
into its present state by an increase or a reduction of the Magnetising
Force. In applying the formulsB already given to the magnetic circuit of a
dynamo, it is sufficiently accurate to employ such a value for the permea-
bility as is given by tests involving a reversal of magnetism, or magnetisation
from an initially neutral state; for, with the constant vibration to which
the field magnets of a dynamo is subject, the effect of previous condition is
neutralieed, and the state attained is very nearly that given by the reversing
tests of the ballistic method.
For the sake of certain calculations with respect to the dynamo and
other electro-magnetic machinery, it is necessary to state briefly the con-
ditions which efiect the relation between Magnetic Induction and Magnet-
ising Force. On applying the Magnetising Force the permeability at first
increases to a maximum and subse<]uent1y diminishes ; on gradually
removing the Magnetic Force the Magnetising Induction decreases very slowly.
46
RETENTIVENESS AND COERCIVE FORCBL
and on complete removal a certain proportion of the Magnetisation remainB.
This is termed the '* Residual," generally expressed by a percentage of the
maximum. It depends mainly upon the purity of iron, and is therefore
greatest in wrought iron, least in cast iron, containing a large proportion of
carbon and other impurities. The corresponding quality is termed
" retentiveness " or " retentivity," but it must be carefully noted that this
quality must be measured under conditions where there is net cause to
remove the magnetism. The specimen must be of the complete ring form,
as the slightest magnetic polarity causes a demagnetising force which very
greatly reduces the residual magnetism in wrought iron. Mechanicsil
vibration and eddy currents due to sudden decrease of the magnetism from
the maximum have a similar efifoct.
The quality which may be called " permanentness " of a specimen, since it
represents its suitability for the construction of permanent magnets, is quite
difterent. This quality is best measured by the reversed Magnetising Force
required to first i*edu^e the specimen to a neutral state. The term *' Coercive
Force " applied to the measure of this Demagnetising force has generally
been adopted.
On increasing the Magnetising Force in the reverse direction to that
used for iirst magnetisation, the permeability at first assumes somewhat
smaller values than it did in the first magnetisation from the neutral state.
The difference gradually reduces till a little beyond the maximum
value, when the iron is said to be *' saturated," the difference is piuctically
negligible.
The foUovdng table will give a summary of the relative magnetic quali-
ties of several classes of iron and steel, when the Magnetising Force has
been raised to the high value of 240 c.g.s. units.
Msgnetin iDductioD.
B.
Residual Magnetio
Induction.
Coercive Force.
Wrought iron annealed . .
Grey cast iron
Malleable cast iron . . .
Whitworth mild steeH
(annealed) . . . . /
Whitworth mild steeH
(oil hardened) . • . j
Tungsten steel
Manganese steel ....
18,251
10,783
12,408
18,936
18,796
14,480
310
7,248
3,928
7.479
9,840
11,040
6,818
2.30
3.80
8.80
6.73
11.00
51.20
It will be seen from these results (due to Dr. John Hopkinson) that
very great differences exist, especially in the coercive force, it is clear
that the impurities of the carbon in grey cast iron considerably reduces both
the saturation limit of the Magnetic Induction and the residual Magneti-
sation. The purer forms of steel, although of much lass permeability with
low Magnetising Forces, approach the same saturation limit as wrought
iron, and the residual magnetism, when satisfactorily measured, is about
the same. The coercive force of steel, however, is much greater than that of
either wrought or cast iron, and is increased by hardening, and also by the
addition of tungsten. On the other hand, alloys of steel with manganese
tend to become almost entirely non-magnetic. One specimen, containing
12 per cent, of manganese and i per cent, of carbon, had a permeability of
1.3 to 1.5, varying very little with the strength of the field. An alloy of
steel containing 25 per cent, of nickel, which is also a magnetic metal, had
1
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HYSTERESIS. 47
magnetic properties similar to those of manganese steel, the permeability
being constant and equal to 1.4. When cooled below freezing-point, how-
ever, this alloy became strongly magnetic, and remained so on returning to
the ordinary temperature.
Plates II., III., IV., v., VI., VII., are the cyclic curves of Magnetic
Induction given by Dr. Hopkinson for various specimens of iron and steel
from which the measures of Ketentiveness and Coercive Force were taken.
The curves marked "residual,*' gives the amount of residual magnetism
(by its ordinates) corresponding to a maximum Induction represented
by the corresponding ordinates of the rising branch of the curve of
Magnetic Induction.
A magnetised piece of iron possesses, in virtue of its magnetisation, a
certain quantity of potential energy ; this is shown by the fact that when
the Magnetising Force is removed, the decrease in the Magnetisation can
generate currents of electricity in surrounding conductors, these representing
the expenditure of energy.
When a specimen is made to pass through a complete cycle of magnet-
isation, energy is alternately supplied to the system in magnetisation, and
removed in demagnetisation. In accordance with the principle of conserva-
tion of energy, all the energy applied to the system must be capable of
being subsequently traced, either still in the form of potential kinetic
energy, or in the degenerated form of heat. Now, part of this energy
supplied is removed from the iron and dissipated by currents in the
magnetising coil or other conductors, but part goes to heat the iron itself by
some process which cannot be explained until we know something of the
nature of magnetism. At present we must consider this process as akin to
some kind of molecular friction to which the name Hi/steresU has been
given.
The following investigation will show what is the excess of energy
supplied to the iron over that removed from it during a complete cycle of
magnetisation.
To avoid the mathematical difficulties of the general problem let us take
the case of a uniform magnetic circuit of length l^ and cross section a, as
we have it in an iron ring. On this ring, suppose a magnetising coil of n
turns, the the current being G absolute units.
Then
XT 4irnO
As the Magnetic Induction is increased, a back electromotive force is pro-
dB
duced in the magnetising coil of T^tf^r absolute units, requiring an expenditure
dB
of an additional power of z^^^-jT- ^ ^^ maintain the current. This represents
the power delivered to the magnetic field. When the Magnetic Induction is
decreasing, and -r— of the opposite sign to C, power is being returned to the
magnetising coil, an electromotive force being required from the generator
flR
to maintain the current, smaller by n<r-_ than that determined by the
dt
resistance of the coil.
The total energy absorbed by the magnetic field is therefore
nff/^C^dt = rnrfCdB = — . uJ^lUli ergs.
48
HYSTERESIS.
Now, La is the volume of the iron ; and/HdB, in a complete cycle, is
the area of the Magnetic Induction curve. Thus we may write that the
energy dissipated in a complete cycle, per cubic centimetre of iron, is
— X area of the Mcignetic IndiLction curve.
4ir ^
The proof given above with the notation of the calculus might be
rendered more simple and lucid by the substitution of a graphical con-
struction as shown in Fig. 6. The closed curve LBAL'B'A represents a
complete cycle of magnetic induction. The area QPpq may be shown to
FiQ. 6.
Hysteresis Carve.
be proportional to the work done in increasing the Magnetic Induction from
the valve Op to Oq, the coefficient — lo-, as before shown, being neces-
sary to give the measure of the work done in ergs. The energy returned in
the process of demagnetisation through the same range is similarly repre-
sented by the area P'Q'pq^ and the difference, the ai-ea PQQ'F^ represents
the energy dissipated in Hysteresis. This area is an element of the closed
curve, and the sum of all such elements, or the whole area of the closed
curve, represents the energy dissipated in a complete cycle, the coefficient
47r
giving the measure in ergs per cubic centimetre of iron.
The result obtained is equally applicable at any point of an irregularly
magnetised mass of iron. For hysteresis exhibits itself in the heating of
the iron, and the heat thus generated at any point of the iron can only
depend on the magnetic qualities of the iron and the range of Magnetisa-
tion. The complete curves are exactly analogous to the indicator diagrams
HYSTBRESia 49
of engines, the area, mnltiplied by a certain constant, giving the energy that
mutst be expended to produce the complete cycle of magnetisation.
The expenditure of energy depends on the variation between the values
of the Magnetic Induction when the Magnetising Force is increasing or
decreasing. If there is no iron in the field BaH, the Magnetic Induction
is always single- valued in terms of the Magnetising Force, the curve is
reduced to a straight line, and all the potential energy supplied to the field
is returned.
When there is iron in the field the Magnetic Induction has two valnes
for any given Magnetising Force, just as in the steam engine the pressure
has two valnes for any given volume, and HyHteresis, or expenditure of
energy takes place. It will be seen that in the different classes of iron and
steel the Hysteresis depends mainly on two factors, namely, the limits
of the Magnetic Induction (or Magnetisation), and the Coercive Force,
for the curves, in all cases, are very similar in shape, and may be made
apprx>ximately identical by choosing different scales for H. It will be
observed, moreover, that the horizontal breadth of the curve is very
approximately the same throughout, so that the area mny be taken as that
of a rectangle of the same height and breadth. Thus the Hysteresis is very
nearly in ergs per cubic centimetre,
Coercive Force x Maxironm Induction
The multiplier 0.00971 (or rather less than y^) reduces this to foot-
pounds per ton of iron.
In passing through a complete cycle, in which the Magnetic Induction
is carried above saturation point, the Hysteresis in different specimens will
depend chiefly on the Coercive Force, and will be approximately propor-
tional thereto. Dr. Hopkinson has conducted experiments on a number of
standard specimens to determine the Hysteresis from the measured area of
the curves It appears that in wrought iron the work done per cubic
centimetre in producing a complete cycle of strong magnetiom varies from
10,000 to 17,000 ergs; in carefully annealed wrought iron plates it should
not exceed 13,000 ergs.
In one sample of soft grey cast-iron an expenditure of only 13,000 ergs
was thus measured, but the usual qualities gave 30,000 to 40.000 ergH.
Whitworth mild steel, well annealed, gave from 40,000 ergs; hardened,
from 60,000 to 100,000 ergs; pianoforte steel wire gave 116,000 to 1x7,000;
whilst a specimen of tungsten steel gave as much as 216,800. In these
measurements the Magnetic Induction was raised to saturation, that is, to
aboat B a 15,000. The importance of using the softest annealed iron in
those portions of electro-magnetic machinery which are subject to constant
reversals of magnetism is evident; a still more important question for
investigation is the variation of the hysteresis-loss of energy with the
magnitude of the maximum value of B, since this will greatly affect the
design of such machinery.
The following table gives approximately the number of ergs dissipated
per cubic centimetre per cycle in the magnetisation of very Koft iron for
iqa^Timnm values of B givou in the first column.
B.
ErgB
per CO. per oydaw
tooo
206
i 000
466
3»ooo
830
4,0^0
1.340
5.000
i,7J2
6,000
'
2,256
50
HYSTERESIS.
K
Erg8 per ca per oyole.
7,000
8.000
•••
•••
2,842
•••
•••
3.464
9,000
•••
•••
4,162
10,000
•••
•••
4,937
11,000
•••
•M
5.710
12,000
•M
•••
6,675
13.000
mm
•M
7.600
14,000
•••
•••
8.596
15,000
•••
•»
9,560
16,000
—
•••
10,630
17,000
•••
•••
11,750
18,000
•W*
•••
12,940
19,000
•••
•••
14,250
20,000
•••
•••
15.500
These measurements are exhibited graphically in Plate VIII., taken
from the original paper of Dr. J. Hopkinson {Fhil. Trans, 1885). It will
be seen that the number of ergs dissipated increases with the maximum
induction, but at a higher rate ; for values of B between 2000 and 5000
the variation is almost exactly proportional to B*^; and for values of
B between 5000 and 10,000 to B'-^ Prof. Ewing gives, as the result
of further investigations, the following approximations covering a wider
range:
From B = 200 to 500 Hysteresis-lom Taries as B*^
From B = 500 to 1000 « varies as B
From B == 1000 to 2000 m Taries as B
From B = 2000 to 8000 „ varies as B
From B = 8000 to 14,000 „ varies as B**',
Thus the rate of variation is that of a power of B which varies from 2 to
1.47 (or as above to 1.4) and back to 2.
From B » 2000 to 8000 hysteresis-loss « '01 B'*^, and the nearest
approximation throughout the whole range is given as .0034 B*^.
The measurement of the hysteresis-loss of energy in samples of soft iron
is a matter of practical importance to buildei*s of dynamos and transformers.
The method involving the tracing of the Magnetic Induction curve is
tedious. A direct reading instrument, shown in Fig. 7, giving fairly
approximate readings, has been devised by Prof. Ewing. Specimens are
prepared in the form of thin oblong strips of exact lengths, a number of
which are clamped together, and pivoted so as to be free to turn about a
vertical :vxis. A pointer is attached moving over a scale, and a slight
adjustable weight keeps the specimens and pointer normally in a vertical
position. A permanent horse-shoe magnet is placed so that in one position
its magnetic circuit is completed through the specimen slips. This magnet
supplies a Magnetising Force of fixed intensity, and can be rotated about
the same axis so as to produce rapid reversals in Magnetic Induction through
the bundle of specimens.
The result of Hysteresis will be to give the specimens a torque in the
direction of the rotation of the magnet, which will produce a permanent
deflection proportional to the expenditure of energy per cycle. Fop it is
evident that to move a mass of iron in a fixed magnetic field so as to change
the Magnetisation will require the application of that amount of power
that iti dissipated in Hysteresis ; and in the same way in a rotating magnetic
field it will require a certain amount of torque to keep the iron stationary,
for in this case the iron still moves relatively to the magnetic field. The
power dissipated in Hysteresis would be measured by the product of the
torque multiplied by the relative speed of rotation (that is their measures
^
a
5*
b
o
Ergs per Gubio Ccntimjetr& per CydLet.
o
o
o
9-
hi
Oi
o
o
o
YfaJX£ p9r CuJfiC' Centurvetre' widv 100 Cycljes per secjonxij.
I 1 1
5^
t
«
MEASUREMENT OF HYSTEEESIS. $1
in certain unitR). Now the speed of rotation is proportional to the number
of reversals of magnetisation in the Bpecimena above mentioned ; and
therefore the torque, or the deflection of the pointer, is propoitional to the
Hysterewa-losa per rycle. It is also, within certain limits, fcnnd to be
sufficiently independent of the number of specimens clamped together, and
their thickness, on account of the variation of magnetic induction through
the specimnn, for little error to be thus introduced. The method is simply
one of comparison with standard specimens, in which the Hysteresis-loflB
has been measured by eiact methods, and only at a standard intensity of
magnetisation. The Hysteresis- loss with diflerent intensities of Magiutio
KiG. 7.
Induction can be calculated from that with the standard intensity, since
the law of variation is similar in all cases.
It is in the heating of the iron that the entrgy dissipated in Hysteresis
is to be traced, so that it would seem that the surei-t method of measurement
would be through the raising of the temperature of the iron. But since
the 10,000 ergs dissipated per cycle of strong Jiagnetisation in soft iron
only produce a rise in temperature of 0.000285° Centigrade, it will require
a large number of reversals to produce a measurable change. And if the«e
revenwla are made with great rapidity another source of heating is involved,
which is difficult to separate from that of Hysteresis. This is due to the
eddy currents set up in the iron by the magnetic changes. We shall see,
when studying the construction of dynamos and transformers, bow these
eddy currents are, to a large extent, eliminated by con.structing the iron of
thin plates or wiiee (technically called lamination) instead of a solid cnstiag
or forging. There does not appear to be any great variation in Hysteresis
owing to the rapidity of the magnetic changes, at uny rate with strong
52 THE CLOSED-COIL DYNAMO.
fields. The " time-lag " in magnetisation that has been observed seems to
be mainly due to eddy currents. Further experiments are needed, but at
present we must assume that the dissipation of power by Hysteresis is
proportional to the rapidity of the magnetio reversals. On the other
hand, if the eddy currents are proportional to the rate of changes in
Magnetic Induction (as is the electromotive force producing them), the
energy dissipated by them is proportional to the square of this rate of
change, and, therefore, to the square of the number of reveisala per
second.
CHAPTER v.
The Closed-Coil Dynamo (General Theory).
Electromotive Force (E.M.F.) is produced in a turn of wire while the
flux of magnetic induction through it is being increased or diminished,
and the magnitude of the E.M.F. is proportional to the rate of increase or
diminution. Measuring the flux, as explained above, by the number of unit
tubes of Magnetic Induction, or lines of force, it in found that the E.M.F,
is measured in c.g.s. units by the number of unit tubes, or lines of force,
added to or subtracted to the total flux through the turn of wire per second.
To obtain the practical unit of E.M.F., the volt, a rate of change of
a hundred million, lo^ lines of force per second is required. The
E.M.F. will be produced in such a direction in the turn of wire as will
cause a current to flow, whose electromagnetic eflfect opposes the changa In
other words, taking the positive direction along a line of force to be that in
which a north -seeking pole would be drawn, a decrease of the magnetic flux
through the turn of wire will cause an E.M.F. tending to make a current
flow through it round the lines of force in the direction of revolution of a
right-handed screw advancing in the positive direction of the lines of force
and vice-versa. This is commonly known as Lenz' Law, and will be shown
directly to be a necessary consequence of the Law of the Conservation of
Energy. It follows that a decrease of the flux of Magnetic Induction in
one direction is equivalent to an increase when the lines of force are in the
opposite direcion.
The E.M.F. may be calculated when the wire is wound in a coil of many
turns by multiplying that in each turn by the number of turns, provided
an identical flux of Magnetic Induction is included in all the turns. This is
only equivalent to saying that, the various turns being in series, the E.M.F.
produced in the whole coil is obtained by the addition of that in all the
turns.
The simplest conceivable form of dynamo armature will consist of a flat
coil, of one or more turns in the same or in parallel planes, rotating in a
uniform magnetic field about any axis in the plane of winding. In such a
coil, when rotating with uniform speed, the flux of Induction will reach
a maximum twice in every revolution, but alternately in one direction and
then in the other. The change must therefore be alternating in direction^
reversing at every maximum value of the flux of Induction. The flux of
Induction at any moment will be proportional to the sine of the angle
of inclination of the plane of the coil to the direction of the lines of force,
and therefore the rate of change, or the E.M.F. produced in the coil, will be
proportional to the cosine of the same angle. It will follow that the greatest
E.M.F. will be found when this angle is zero, that is to say, when the plane
of the coil is parallel to the lines of force, and therefore when the flux of
Induction through the coil is zero.
The circuit of the armature may be completed in the external circuit by
APPLICATION OF LENZ' LAW. 53
sliding contacts, or brushes, touching two separate insulated brass rings
on the axle to which the ends of the coils are attached. The E.M.F. thus
produced would be what is known as an alternating E.M.F., reversing in
direction every half-revolution. If, on the other hand, the extremities
of the coil be connected respectively to two semi-circular segments of the
same ring, insulated from one another, and the sliding contacts, or brushes,
which lead to the external circuit make contacts one with each segment, and
in such positions that the contacts interchange segments when the E.M.F.
changes in direction (every half-revolution at the moments at which the
magnetic flux through the coil reaches its maximum values), the E.M.F.,
though still alternating in the armature coil itself, becomes uniform in
direction in the external circuit. The E.M.F. will stiU, however, be
variable in magnitude, and is known as a reotified osctUatory or a puUating
E.M.F.
Considering for the moment only the case where the external circuit is
fi*eefrom self-inductance, let us say a bank of incandescent lamps, the current
at any moment will be determined by the E.M.F. at that moment and the
resistance of the whole circuit, and will follow exactl)' the same cycle of
changes as the former. It will be zero at the moment of the interchange
of the contacts on the divided ring, or commutator, but in any other position
a current will be flowing which will react upon the magnetic field. For
example, in any position where the flux thi*ough the coil is decreasing, the
current in the coil will, according to Lenz' Law, tend to maintain the flux
II gainst the decrease, and the effect will be, not so much to create new lines
of force, as to deflect or distort the neighbouring lines of the uniform field
s<) that they pass through the coil, weakening the surrounding field in the
same way as a piece of wrought iron would if placed in the field. The
appearance of the lines of force would be as if they were dragged forward by
the conductors of the armature, as threads when about to be cut with a blunt
knife. This distortion of the field will give rise to a force reacting on the
coil, of which the effect may be easily traced by supposing a tension along
the lines of force, and a lateral pressure. In this case the force will act to
oppose the motion of the coil towards the position of the zero flux, and it is
easy to see that in all cases a force-resisting motion will result.
The resistmg force has to be overcome by the driving force applied to the
dvnamo pulley. An alteration of the current by increase or reduction of
the resistance of the external resistance would result in a corresponding
alteration of the distortion, and therefore of the driving force, to maintain
the speed. An alteration of the speed would produce a corresponding
alteration of the E.M.F., and as the force, or torque on the pulley, would
still be proportional solely to the current (being quite independent of the
speed or E.M.F. except so far as they affect the current) the mechanical
power required will be proportional to the pioduct of the measures of the
E.M.F. and the current, or to the number of watts.
Let the E.M.F. be altered in another way, by increasing the number of
turns in the coil. The distortion produced by the same current will now be
increased in proportion to tho increased number of turns, that is, to the
increased E.M.F., so that the mechanical power employed is still pro-
portional to the number of watts, or the electrical power produced,
In Siemens "magneto-electric machine," which first appeared in 1856, we
have the nearest approach to this ideal simple dynamo that is practically
possible. The coil is wound on a shuttle-shaped piece of iron, and revolves
in the cylindrical interspace between the poles of a permanent magnet. The
coil is in fact wound in two deep horizontal grooves, on either side of a
cylindrical core rotating about its axis, the conductors crossing at each end
on one side or other of the shaft. Two collector rings, or a split ring, are
54 THE CLOSED COIL.
mounted upon the shaft to give alternating or pulsating E.M.F. This was
the first practical dynamo, or magneto-electric machine, for supplying con-
tinuous or alternating currents, but considerable modifications were nfrcesesry
before the dynamo was available for the efficient conversion of considersible
mechanical into electrical power.
The term '* continuous current " is generally understood to mean one that
is continuous or invariable, in magnitude as well as in direction, or rather, free
from rapid periodic variations such as must inevitably be produced in an
armature consisting of a single coil wound in one plane, or, as in Siemens'
shuttle-wound armature, wound with many turns in parallel planes. To
produce such an E.M.F. and current it is necessary that, though the
numerous turns of wire with which it is wound may be each passing
through different positions or phases of periodic change, the armature as a
whole should exhibit uniform conditions in any position during its rotation,
owing to its perfect symmetry in construction and winding about the axis of
rotation. Such an arrangement is closely approximated to by what is known
as the cloaed-ooil armature.
On a cyliudrical iron framework a single coil of many turns is wound,
every turn of which is in a plane passing through the axis of rotation, and
as far as possible in all such planes, so that the surface of the cylindrical
core is covered uniformly with conductors lying on its surface parallel to the
axis of rotation. In winding, the successive turns advance from plane
to plane with as small changes of inclination as possible, till the plane of the
first turn is again reached, after the plane of winding has made a complete
revolution, and then the two ends of the conductor are joined together,
forming a single closed or endless coil.
Fig. 8 illustrates the principle, a diagram showing one of the methods
of winding the ''closed" coil, known as ''ring'' winding. It represents a
case in which the successive turns contain only somewhat less than half
the total flux of Induction passing between the poles. Other types will be
described later, but this is most convenient for illustrating the principle by
diagram, as the separate windings do not cross one another, but form
an endless spiral round a ring core. The arrangement of the lines of force
are shown in Fig. 9.
In such a closed or endless coil the total E.M.F. will be nil, and no
current will tend to flow round the complete circuit thus formed. For the
total flux of Magnetic Induction through the coil is zero in all positions,
the turn in any one plane being counter- wound to that reached after
traversing half the winding from it. Or otherwise, observing that the succes-
sive turns exhibit at any moment similar conditions to those of any one
turn in successive positions during a complete revolution, the change of the
flux of Magnetic Induction produced in the closed coil, when it is so rotated
that each turn passes to the position of that consecutive to it, is the same as
the change in a single turn taken through a complete revolution, and there-
fore obviously zero.
In a uniform magnetic field, or when the armature is between two poles
of a magnet, two of the turns, those most nearly including the maximum
flux of Magnetic Induction at the moment, will themselves have zero
E.M.F* ; and since in rotation from the plane of one of these turns to that
of the other the flux changes continuously from a maximum in one direction
to a maximum in the other direction, the E.M.F. in all the turns at any
moment between those in these two planes (half the turns of the closed coil)
is in the same direction, thus creating a considerable difference of potential
between the two turns, which in themselves have zero E.M.F. The remain-
ing half of the closed coil is composed of an equal number of turns whose
E.M.F. is in opposition to that of the turns of the former half, in the sense
THEORY OF CLOSED COIL. SS
rf ooDtmnity round tlie clused coil, but tends to create the same difference
of potential between tbn turns o( zero E.M.F. The cloeed coil, in fact,
reeembles two exactly similar batteries of cells, the turns corresponding to
cells of different E.M.F. connected in seiiea in each battery: the two
batteries being then connected together by joining like poles, so that in the
cmnplete or closed circuit thus forujed the E.M.F. of one battery is in
opposition to that of the other. But if the tenninale of an outside circuit
were joined one to each of the connected poles of the batteries, the batteries
would be connected in parallel with i-e.-<pect to the outside circuit. We
should thus get only the same E.M.F. between the torminalB that one
Diagram of King- Winding.
FlO. 9.
Lines of Force throagh ArmBtnre Core.
battery alone would give, but the internal resistance of the combination will
be only one-half of that of a single battery.
In the closed-coil armature the points corresponding to the connected
poles of the batteries are not fixed points on the coil, but hold fixed pasitions
in space as the armature revolves, the highest and ]ou'».t points in the
diagram. We require to mske connections between the terminals of the
external circuit and points on the closed coil in cloKe proximitv to the turns
having at any momeut maximum magnetic flux, or zero E.M.F., and to be
perpetually altering these contacts as the armature rotates. The exact
positions of these contacts are not of extreme importance, as the coils in the
neighbourhood of the point of contact have very little E.M.F. ; and the
best positions in practice will not be always those indicated in this pre-
liminary explanation, but will need to be modified when a current is flowing
in the armature.
A method of making theee contacts with sufficient correctness for all
practical purposes and in such a way that they remain unchanged as the
armature revolves, is to connect every turn, or, if they are very numei'ous, to
make a sufficient number of branch connections from the closed coil at
$6 THE ARMATUKE-CORE.
equal intervals, to a number of metal bars or stri()6| insulated from one
another, but built up bo as to form a cylinder rotating with the shaft.
Sliding contacts, or brushes, pressing against the surface of this cylinder
may thereby make contact with the required points, and by sliding from
one bar to the next, change the points of contact with the closed coil with
sufficient frequency to be never far from the points where the E.M.F. in
the turns of the coil reverses, and thus to maintain a practically constant
difference of potential between the terminals of the external circuit. Such an
arrangement is known as the commutator.
The value of using an iron core on which to wind the armature coil is
that the lines of force prefer to follow a path as far as possible in iron, so
that even with permanent field- magnets a much greater flux of Magnetic
Induction passes through the armature owing to the concentration of the
lield ; and when electro-magnets are used, the magnetic circuit is decreased
greatly in resistance or reluctance, so that less Magnetising Force isrequii-ed
to obtain the requisite strength of field. The core or framework for a
closed-coil armature must be cylindrical in shape, so that it may fill while
revolving as nearly as possible the cylindrical interspace between the poles
of the field-magnets. The cylinder is generally hollow, the shaft of the.
dynamo having to pass through the central space, and an air-space being
advisaole between the steel shaft and the iron core, in order that the former
may not form part of the magnetic circuit. Moreover, since it is advisable
to have a large periphery to the cylinder, and wide-embracing pole-pieces,
in order that greater breadth and shorter length of gap (in which the
necessary conductors are wound) may be obtained between the poles of the
field magnets and the iron core, ample section of iron is given in the
interior of the core when the internal diameter is from one-half to three-
quarters of the external. The cross-section, therefore, of the armature, and
the disposition of the lines of force in the neighbourhood, is for a two-pole
dynamo similar to that shown in Fig. 9, the m»gnetic circuit dividing in the
interior of the core into two branches as indicated.
The effect of the iron core in concentrating the field through the arma-
ture is well shown by Figs. 10 and 11, representing the disposition of the lines
of force before and after the insertion of the core, as traced by the cohesion
of iron filings along them.
The first known application of the principle of the closed-coil armature
was in a model designed by Prof. Pacinotti, of Pisa, in 1864. The model
was not, however, improved upon so as to become of practical utility, and
quite independently, some seven years later, the same principle was re-
invented by Gramme. Pacinotti's armature was wound on a simple iron ring,
into which sixteen iron wedges were driven at equal intervals between the
windings, so as to form projecting teeth, which coming successively opposite
the pole pieces, enabled the air gap to be reduced to the minimum required
for clearance. Gramme improved on this by building a ring-shaped core of
iron wire, thereby avoiding the eddy currents which are produced in a solid
conducting mass rotating in a magnetic field, but in omitting the teeth or
projections his invention missed what most modern designers look upon as a
great advantage.
The modern armature-core is built up of ring-shaped stampings from
plates of the softest charcoal iron, of from one-quarter to one-half of a
millimetre in thicknes-^. These rings or discs are electrically insulated from
one another by thin sheets of paper, or a coating of varnish, and bolted
together in a framework so as to form the required ring or cylinder (the
former term being most appropriate when the diameter greatly exceeds the
length, and the latter when the length exceeds the diameter). The driving
is efiecLed from the shaft by a gun-metal " spider," or some equivalent
COSCEN'TRATION OF FIELD. 57
means. Observing that the eddy currents tend to flow, aa in the armatnre
coils, in planes perpendicular to the lines of force, it will be seen that the
core built up of plates slightly insulated from one another, <tf sufficient
thinness, will effectively eliminate this source of waste, while permitting
better continuity in the paths of the lines of force through the iron than
the iron-wire core of Gramme. By cutting deep grooves along the surface
(rf the core, in which the conductors are afterwards laid, the air-gap may be
Fin. lo.
Linea of Force with Armatare-core removed.
';?:'>r:s'
4u
L aea t>f Foice w th Armature-co e
e ted
reduced to the minimum requisite for clearance. This being the perfected
method of utilising an advantat;e first reuliseil in Faciuotti's model, such an
armature-core is generally called a Pacinotti core, and the projections, or
ridges, between the grooves, I'acinoUi teeth. The advantage.s over smooth-
cored armatures are reduced rel uctance in the magnetic circuit, and superior
mechanical strength, as the conductors cannot be displaced on the surface of
the core by the bhearing force to which they ai-e subjected in driving. It
is, in fact, exceedingly probable that very little shearing force on the con-
ductor)) exists at all when Facinotti armatures are employed ; the mit^netic
5 8 SMOOTH AND SLOTTED CORES.
lines of force confine themselves almost entirely to the iron teeth, avoiding
the gaps in which the armature conductors lie, and the bhearing force resist-
ing the revolution of the armature acts upon the teeth and not on the con-
ductors, thus relieving the insulation of the pressure that would otherwise
exist. In large machines a further distinct advantage is gained by the
lines of force avoiding the conductors, in that eddy currents are not produced
in the large copper bars employed, which need not therefore be laminated,
or built up of thin copper strip or wire. The objections to Facinotti teeth
are (i) An extra ditficulty in obtaining high insulation between the con-
ductors and the iron oore in which they are sunk ; (a) an increased dis-
tortion of the magnetic field by the current flowing in the armature, with
attendant difficulties in preventing sparking at the commutator, the causes
of which we shall deal with shortly ; (3) the production of eddy currents,
and consequent heating and waste of power in the poles of the field magnets,
owing to the variation of the distribution of the lines of force as the teeth
or ridges pass in front of them, unless certain precautions be taken as to
the size and shape of the teeth.
The excessive effect of armature reaction in distorting the magnetic
field and rendering sparkless commutation difficult that is found with Faci-
notti cores, and will be presently deK^ribed, caused the smooth core to be
preferred by manufacturers until recent years. Now that effective means
of compensating for or eliminating this effect have been devised, the intrinsic
merit of the Facinotti core is causing it rapidly to supersede the smooth
oore, which will in all probability be soon rendered obsolete, except for very
small and cheap dynamos. Considerable choice still remains as to the
shape of the teeth. Rectangular grooves and ridges are permissible only if
the former be of small width, not greatly exceeding the actual clearance
or distance of the outer surface of the teeth or ridges from the polar faces.
It will be shown later that under these conditions the Magnetic Induction
at the surfaces of the pole-faces will be practically uniform, and eddy
currents will not be formed within the pole-faces as the armature revolves.
This is a convenient type of core for large dynamo armatures, employing
large bars or copper strips as conductors, fitting into grooves somewhat less
than one centimetre in width, and allowing a reasonable clearance. Such
armatures are very easily wound.
If larger teeth are preferred, so that many conductors may be carried
along each groove, we must either build up the pole -faces of laminated iron
to prevent eddy currents (a common device with altematon$), or narrow the
groove on the external surface of the core, forming T-sbaped teeth or
ridges. The conductors are then nearly enclosed in the iron, and a further
advantage is gained in preventing the conductors from fiying outwards by
centrifugal force without the employment of bending wire, but the winding
is more di£cult and expensive. The narrow neck of the grooves is gener-
ally closed by a wooden plug. It is only a step further to completely enclose
the conductors in iron, pushing them through tunnels just beneath the
surface formed by stamping boles in the plates.
The armatures of Facinotti and of Gramme were both ring- wound, that
is, all the turns of the closed coil pass through as well as over the external
surface of the ring-shaped core, as indicated in Fig. 8. Every turn in
this method of winding successively includes, as a maximum, one-half of the
total flux through the armature- core. Another method of winding is that
in which every turn crosses at each end of the cylinder from one generating
line on its surface to that diametrically opposite (or nearly so) so that the
turn includes, as a maximum, the total magnetic flux. This method is
known as rfrwm- winding, and a partial diagram is given in Fig. 12. It is
inadvisable to trace all iliu turns in the diagram, owing to the confusion
DEUM-WINDINa. 59
through overlappiug. The dotted line repreneots a complete turn round
the cylinder, from ^' to ^, consisting of two surface conductors and a cross
connection at the back of the cylinder. Another cross connection (in the
middle of which the commutator connection is made) brings us to B^ where
the next turn begins in a plane slightly inclined to the preceding. The
winding of the closed coil is only complete when the starting-points of the
turns A' B (T^ etc., as well as the returning points A^ B, C, etc., symmetri-
cally cover the periphery of the cylinder.
The ring-winding is ob\dously the best adapted* to ring-shaped arma-
tures, that in, those whose diameter is great in proportion to their length :
far though the maximum magnetic flux through any turn is only one-half
of that through a turn in drum- winding, and therefore its contribution to
the £.M.F. of the dynamo only one-half, two turns jDassing through the
ring will be less in length and I'esistance than one turn of drum -winding.
On the other hand, when the length of the armature is to be great, drum-
FlG. 12.
Diagram of Dnim-windiDg.
winding is to be preferred in spite of many difficulties in its construction,
and is generally adopted for dynamos of large size.
The difficulties of drum-winding arise from the crossing of the turns
one over the other, and the complicated system of end connections necessi-
tated. In the larger sizes of dynamo, where the number of turns is com-
paratively small, the surface conductors are bars of square section, or copper
strip of moderate breadth, and the end connections of thin copper strip of
considerable breadth, and by suitable systematic methods which will be
briefly described later, the difficulties may be effectively coped with. A
fui-ther objection to drum- winding, of less importance for large sizes, lies in
the fact that conductors between which the greatest difference of potential
occurs are brought into close proximity, and special care has to be taken
with the insulation between them.
The E.M.F. of a dynamo with closed -coil armature may be calculated
thus. Taking first the case of the two-pole ring-winding, and the total
magnetic flux through the armature as N units, or lines of force, the total
change in any turn during a half revolution, in which it passes from one
brush contact to the other, is that of N lines, the maximum flux of ^N
being reversed. The time-average of the E.M.F. during this half revolution
is -T- — cg.s. units, or 2 ^ — « volts, where v is the number of revolutions
00 00 X lO**
of the armature per minute (the customary measurement of the speed of
machinery). The E.M.F. of the armature is obtained by multiplying this
result by one- half the number of the turns, for this number in series forms
6o CALCULATION OF E.M.F.
one of the internal circuits from brush to brush ; and since the turns are
wound at equal angular intervals, we are justified in taking the time-
average of the E.M.F. in one turn as the average of the E.M.F. in all the
coils at any moment. Hence if the armature be wound with n turns in all,
the formula giving the E.M.F. of the dynamo will be
^ N. n. V
~ 60 X lO^'
The same formula, without modification, will be applicable to the drum-
wound armature^ provided we take n to denote the number of surface
conductors. For in drum-winding there will be two of these for every turn,
and thus the double magnetic fiux through every turn will be allowed for.
Kapp has suggested as the unity of Magnetic Induction an Induction
of 6000 c.g.s. lines per square inch, taking a square inch as equal to 6.4
square inches. This is equivalent to a Magnetic Induction, B, of 937.5
c.g.s. units.
This method of reckoning Magnetic Induction has the disadvantage of
combining the inch and the centimetre as units of length and of applying a
system of measurements based entirely on the centimetre to measurements
mtide in inches. The method has the advantage of being directly applicable
for dynamos the dimensions of which are known in inches, and it is, of
course, possible to ignore entirely the derivation of the c.g.s. units of Induc-
tion, and regard it as a purely arbitrary unity, of which the Kapp unity is
equal to 937.5. There is another advantage since speeds are genei-ally
measured in revolutions per minute, for if the total number of Kapp lines
of Induction passing through the armature be multiplied by the number of
revolutions per minute, and the number of turns of wire on the armature in
the case of a cylinder armature, or twice that number in the use of a drum
armature, the result will be the E.M.F. in the armature in micro-volts.
The modification necessary in the case of dynamos with four or more poles
will be at once apparent.
The bars, or segments, of the commutator are made of gun-metal,
phosphor-bronze, or hardened copper, cast or rolled into the required shape,
and firmly held in a suitable frame, from which, as well as one another,
they are carefully insulated with mica. This substance alone seems to fill
the requirements as the insulating material, being of high specific resist-
ance, unafiected by heat and impervious to moisture, oil and its carbonised
products, copper dust worn from the segments, etc., and of considerable
mechanical strength. The conducting segments might with equnl propriety
be called sectors, since, owing to the wear to which they are subject, they
are made of considerable depth, and require very careful shaping in order
that the symmetry of the commutator may be preserved as the diameter
decreases by wearing out and re-turning,
The brushes are, in British practice, most commonly constructed of
copper wire gauze : carbon brushes are more common in American practice.
In both cases, but especially in the latter, considerable breadth of contact
must be given. Carbon brushes should allow a surface of not less than one
square inch for every 50 amperes. The contact must be of sufficient
breadth to bridge over the intervening insulation (about one-eighth of an
inch) between two successive segments, and establish a sound connection
with the second before receding from the first, since the current is to be
maintained without variation during the change of connection. While a
double contact is thus maintained for a short period, the intervening turns
of the closed coil connecting the consecutive bars from a short circuit of very
little resistance. According to the preceding elementary theory the E.M.F.
CROSS-MAGNETJSATION. 6l
in these turns must be zero, or at least very small, ami no great c
woulil be produced ; but the operation which in practice does take plnce in
the Rhort-circuited turns needs further discussion, modifying the previous
theory by considering the effect of the current in the aimtiture coil on the
magnetic field.
The arrangement of the lines of force in armature core when no current
is flowing in the <^0Bed coil has been illustrated in Fig. 9, The mere rotation
of the armature will have little or no effect on this arrangement, as may be
prored experimentally. When the armature circuit is completed through an
external resistance, a current fiows in the divided circuit from brush to
brush, which, it is easily seen, supplied a Magnetic Force tending to convert
the core into a magnet having poles, or rather distributed polarity, in the
neighbourhood of the coils of zero E.M.F. In other words, the lines of
fcare du9 to this Magnetising Force, if it existed alone, would, like the
original field, form a double magnetic circuit in the iron core, and leaving in
or near the plane of the brush contacte, return either by the field magnet
poles, or the interior of the core. The lines of force in thin field, when the
brashee are in the positione previously indicated, would be at right angles
to the original field, and hence the effect of the armature Magnetising
Foroe, of rtiuuion on the field, is commonly known as t^-oss-moffnetisation. The
resultant effect is that of a distorted
field such as that indicated by the yw. 13.
lines in Fig, 13 (a very eitreme case
for a closed-coil dynamo), the direc-
tion of rotation being clockwise, as
indicated by the arrow. The lines of
force appear as if dragged round by
the rotation of the iron, the distor-
tion being such as one would expect
if there were a great lag or delay in
the magnetisation and demagnetisa-
tion of the iron. With a laminated
tuinatureMX>re the distortion is almost
or entirely inappreciable till an arma-
ture current flows, though with a
Bolid core eddy currents would pro-
duce a considerable distortion similar
to that produced by the armature
current, and in a similar way. It is
evident from Lenz' Law that the distortion must be as shown, the current
tending to prevent the change of flux through the various turns of the coil ;
and supposing a tension to exist along the lines of force, the mechanical
force resisting the rotation, and necessitating the supply of mechanical
power, is accounted for.
The cross- magnetisation has no direct tendency to decrease the total
magnetic flux, through the armature, as long as the brush-contacts are with
segments of the commutator connected to turns at right angles to the lines
of force in the undistort«d field. But indirectly the increased length of
the lines, and the saturation of the polar tips, or horns, may have an
equivalent effect by increasing the magnetic reluctance.
But now the poeitiona of maximum flux, or zero E.M.F. are removed
from their previous position, which we may call the plane of geometric
symmetry {OA in Fig, 13) and advanced to the plane OB. If the
brush contacts are advanced so as to make contact with the segments
connected to turns in that plane, we shall divide the turns of the closed coil
correctly according to the direction of their E.M.F. The efiect of the
62 COMMUTATION.
advance, or lead, thus given to the brushes will be to modifj the distortion,
and, as will be shown later^ to further weaken the field. But neglecting
this effect, as inconsiderable in a well-designed dynamo, it will be when the
brushes make contact with the coils in the plane OB that the highest
E.M.F. will be obtained, since the E.M.F. in all the turns is used to the
best advantage, and for any other position of the brushes some will be in
opposition to others. But opposition of the E.M.F. in different turns does
not necessarily mean a waste of power, and a consideration of far more
importance than high E.M.F. necessitates a still further advance or lead, to
be given to the brush contacts, namely the consideration of sparklessness
on the surface of the commutator.
An electric current cannot be stopped or started in a circuit possessing
self-inductance absolutely instantaneously. Now the turns connecting two
successive segments of the commutator, being wound round laminated iron,
have very considerable self-inductance, and the current flowing in them, that
of half the output of the dynamo, has to be reversed during the time in which
the brush contact passes from one terminating segment to the other. We
have seen that the necessity of sufficient contact with the armature coil
being maintained throughout the process of commutation requires a short>
circuiting of the commutated turns for a finite time, and it is requisite that
during this time the current, which had, for the previous half-revolution,
been flowing without change in these turns, (-hould be reduced to zero, and
again grow to an equal magnitude in the opposite direction just at the
moment of the recession of the brush contact from the leading segment.
Should the cuixent in these turns fail to reach, or have grown in excess of
this current, which has to flow in the turns without change for the next
half-revolution, the short-circuit will be maintained for an instant by the
passage of a spark between the brush and the receding segment, a very high
E.M.F. being produced in the turns as the flux through them changes,
almost but not quite instantaneously, to suit the normal current. This
spark is more or less of the nature of an arc, resulting in the burning of
the segments of the commutator, the insulating strips, and the brushes, and
in their speedy destruction.
A simple way of explaining how the exact reversal, and consequent
sparklessness, is obtained, is to say that during the short period of short-
circuiting, a small E.M.F. must be produced by the motion of the turns in
the direction in which the current will subsequently flow, an E.M.F. just
sufficient during that period to reverse the current in the turns, the self-
inductance and the small resistance determining the requisite E.M.F. and
time for reversal. Hence the turns must be freed from short-circuit when
in a position somewhat advanced from that of zero E.M.F., to a position
indicated by 00 Fig. 13. This statement is not perfectly satisfactory, as the
magnetisation due to the short-circuited coils is part of the distortion of the
field, and the term self-inductance may be misleading. It would, perhaps, be
more correct to say that the magnetic field within the armature needs re-
arrangement during the time of short-circuit to suit the reversal of the
current in the short-circuited turns, and this requires the reduction of the
flux through the latter sufficient to allow for the reversal of their Magnet-
ising Force, and an additional rate of change at the end to sustain the
requisite current against their resistance.
The angle of lead should be very small in well-designed dynamos. It
depends in part on the breadth of the brush-contact, and can only be found
by actual trial. It increases with the current. The law of increase being
roughly proportional to the square of the current. The output of some
dynamos may be limited by the possibility of finding a sparkless plane at all,
for if it be not reached on an inclination to within the horns of the field-
ARMATURE REACTION.
63
Fio. 14.
magnet poles, further advance generally fails, as the E.M.F. in the turns
does not increase. When carhon brushes are used, less difficulty is generally
experienced as the resistance of the material is very much greater, and the
decrease of the area of contact on the receding segment, with the increase
on the advancing, assists the transfer of the current wholly to the latter from
both circuits of the armature. Also the possibility of an excessive current
flowing round the short-circuited turns at the commencement of the double
contact, which is possible with metal brushes, is avoided.
It will be seen that while high resistance in the brushes is a gain in
respect to the reduction of sparking and short circuit currents in the oom-
mutated turns of the armature, it has the disadvantage of adding to the
armature resistance. To secure the advantages while avoiding the disadvan-
tages it has lately been suggested to add to the thickness of the brush by
using strips of high resistance metal on either side, with low resistance
(pure copper) strips in the middle; the
latter always carries the main current, and
the high resistance metal causes a diminu-
tion of the current from the receding seg-
ment of the commutator, prevents an excess
short-circuit current if the brush be too far
back. A still more promising suggestion is
the adoption of laminated brushes, of thin
copper strips separated by thin insulation at
the extremity touching the commutator and
for some portion of the length, but without
insulating films where it is held in the brush
holder ; this is equivalent to low resistance
in the direction of the length of the brush,
but high transversal resistance.
A want of symmetry in the winding of
the armature, or in the magnetic field, is
certain to produce injurious s[>arking. The
former is solely a question of care in con-
8truction« the latter may also be caused by a
faulty design. If, for example, with a single
magnetic circuit, the pole-pieces be of in-
sufficient size, the extra reluctance offered
to the lines of force proceeding to the
further end of the poles may be sufficient to cause a difference in the density
on the two sides of the armature-core. Especially probable is this inequality
with cast-iron pole-pieces, and short magnetic air-gaps. Mather and Piatt
introduced the custom of boring the polar gap slightly larger than necessary,
and subsequently closing them so as to have a shorter gap and a concen-
trated field in the middle, but a larger magnetic gap and less concentrated
field near the horns : thereby reducing the possible inequality, and preventing
the saturation of the horns by the distortion due to armature reaction.
The question of the reduction in the E.M.F. when a lead is given to the
brushes may be treated as follows. In Fig. 14, suppose POQ to be the
plane of the turns when commutated, A OB the plane of geometric symmetry,
and call the angle POA, X. Draw FOQ so that P'OA is equal to POA,
that is to X. Then the armature currents in the sf^ctions POQ\P'OQ balance
one another as far as regards the Magneto-motive Force in the magnetic circuit
is concerned, and the net demagnetising turns are those in the sections
POF and QOQ^. These turns include practically all the lines of force in a
drum armature, or one-half in the ring armature, provided the plane of
commutation is within the polar gap, and it caiinot be far outside if spark-
DetDtfgnetisiiig Turns of Armature.
64
EFFECT OF ARMATURE REACTION.
lessness is to be obtained at all. If m be the total number of armature turns
the number within this space will be m_. The demagnetising effect of the
armature current is therefore that of im~ or im— turns, according to the
V IT
winding, carrying the whole current and wound in opposition to the field-
magnet coils. This calculation does not, however, make allowance for the
additional magnetic reluctance introduced by the field-distortion, nor the
increase of the magnetic leakage past the armature, nor the opposition of
the E.M.F. in certain turns of the armature. These can only be allowed for
by supposing them equivalent to an additional resistance added to that of
the armature, proportional to the lead, and determinable by experience.
In testing the effect of armature reaction with a small bipolar dynamo
■— '—
Fig.
15-
no
111
•0
'*\;
*
\
\
k.
"IT"
—
TO
60
1—
>,
^
M,
f
\
\
i
\
\
\
o
\
OUT^M
r IN AMF
CRtt
8
n
1(
a
^
»
4
M
Fall of Difference of Potential of Dynamo dae to Resistance
and Reaction of Armature Current.
Prof. Ryan measured carefully the actual drop in E.M.F. produced with
different currents and compared it with that due to the resistance alone,
calculated by multiplying the number of amperes by .34, the resistance of
the armature in ohms. The armature core was of the Pacinotti type, with
short air-gap, and the field coils supplied with current from an external source,
giving 2600 ampere- turns, and thus 100 volts at zero load. The carbon
brushes were led to the spai^kless position when possible, but it seems that
from the full normal load of 25 amperes to the double load to which the
experiments were carried sparklessness was not obtainable. The results are
plotted in Fig. 15, the curve I showing the E.M.F. actually given with
different currents, the former represented by the ordinates and the latter by
the abscissse, and the curve II the E.M.F. calculated as it should be if the
drop were due to the armature resistance alone. It will be seen that at
half -load the actual drop is not greatly in excess of the calculated, rising
to nearly double (19 volts) at the full load of 25 amperes, and increasing
with ever increasing rapidity till at 50 amperes the difference of potential
between the brushes is only 41 volts.
Thus we may conclude that the drop in E.M.F. due to armature reaction
NEUTBAUSATION OF ABMATOfiE BEACTION.
65
varies as the square or even a higher power of the current, and while in a well-
designed dynamo with an air-gap not too greatly reduced, its effect will be
small in proportion to that of the resistance of the armature at low loads, it
may become very great indeed with overloads, and restrict the possible
current output.
By giving the brushes a backward instead of a forward lead, the field
may be strengthened instead of weakened by the reaction of the armature.
Fig. 16.
to such au extent that it is even possible for the armature current to main*
tain its own field, but this on the commutator as commonly constructed, will
be at the expense of such destructive sparking as to be entirely non-
permissible. With certain modifications in design, the strengthening of
the magnetic field by '^forward'' armature induction has been utilised
to compensate for the fall of E.M.F. due to the resistance of the armature,
as will now be described.
A method of counterbalancing, or at least of reducing, the distortion of
the tield produced by armature reaction was suggested by Fischer-Hinnen
(see Thd ElecVneian for May 26, 1893). By means of small coils round the
necks, or narrowest section of the pole-
pieces, a counterflow of induction was
set up which opposed that of the arma-
ture current, at least so far as to pre-
vent the saturation of the pole-tips.
In a later design a single coil was
wound in longitudinal c^rooves along
the middle of the polar faces, this coU
being joined in series with the arma-
ture current, and, surrounding the
armature, tended to oppose the effect
of cross-magnetic induction. A moro
perfect method of counterbalance is
that due to Prof. Ryan, in which
** balancing " coils are wound in deep
grooves along the polar faces, and
round the ends of the armature, so as to produce a cross-induction
exactly similar and in opposition to that of the armature current. Figs.
16 and 17 show sections of the dynamo and the mode of winding.
The number of "balancing" turns was half of that in the (drum
Ryan's BalaDcin^!^ Coils for eliminating
Armature Reaction.
66 DEVICES TO SECURE SPARKLESSNESa
wound) armature, and joined in series with the external circuit. The
effect was entirely to remove the distortion of the field, and with the
broad carbon brushes used, commutation could be effected in the symmetrical
plane without spark. By giving the brushes a small backward lead, the
sparking' still being insignificant, it was found possible to strengthen the
field in proportion to the current, and so to "compound" the dynamo,
or maintain a constant difference of potential at all loads by compensating
for the drop due to resistance. The principal objection to Hyan's device
seems to be the expense involved in slotting and winding the pole faces.
Elihu Thompson obtained sparkless commutation at heavy loads by
winding the series turns necessary for compounding in two flat coils
embracing the armature itself, instead of round the field magnets, and tilt-
ing them backward slightly in a direction counter to revolution. This
proves sufficient to maintain the sparkless plane in one position, reducing
the tendency to saturation of the pole -tips, and is far less expensive than
the more complete method of preventing armature reaction devised by Ryan.
Swinburne and others have used small auxiliary poles facing the armature
in the polar gaps, thereby generating a supplementary E.M.F. in the plane
of commutation which gives the requisite sparkless reversal, and introducing
a cross-magnetising force opposing though not counterbalancing that of the
armature current. To reduce or prevent sparking on the commutator,
without in any way compensating for cross-magnetisation, a device called the
<* ammortisseur," invented by Hutin and Lebraun, has proved effective.
This consists of a " squirrel-cage " of copper rods lying beneath and along
the polar faces, terminating in two copper rings, the induced currents in
which tend to damp the slight oscillation of the magnetic field consequent
upon commutation.
The latest device for securing a constant sparkless plane of commutation
has just been introduced by Holmes and Co. The pole-pieces are divided by
a radial slot parallel to the axis of rotation of the armature into two parts^
forming parallel magnetic circuits. By an extension of the leading pole-tip,
and thus, by increasing the breadth of the air gap, decreasing the magnetic
reluctance for the lines of force passing through the forward half of the
pole-piece, this part of the magnetic circuit is practically saturated at all
loads ; the additional magnetic flux due to the compounding turns at heavy
loads is therefore almost entirely confined to the less strongly magnetised
trailing half of the pole-piece. The plane of maximum E.M.F. is thus kept
in a constant position, in spite of the cross-magnetisation, or may be thrown
back to any required degree, so that the plane of sparkless commutation
may remain in a stationary position.
In 1893 Sayers introduced a method of utilising the armature reaction
for strengthening the field with a backward lead to the brushes, while pre-
venting sparking by the use of what he termed '^ commutator " coils. The
connections between the closed coil and the commutator segments are no
longer direct, but of comparatively thin wire on the surface of the armature
core in the same grooves as the closed coil itself, so as to form a loop in
which a small E.M.F. is produced. The commutator coils need only be of
small section since they only carry a current when the corresponding seg-
ment is in contact with the brush, and do not add appreciably to the
resistance of the armature (that of all the turns in two parallel series). The
short circuit between two segments in contact with a brush consists of two
commutator coils and the intervening section of the closed coil, and the
difference of the E.M.F. in the former is sufiicient to produce the requisite
reversal of current, even when that in the intervening section is in favour
of the original diiection, as is the case with a backward lead. The preferred
method of winding is to carry the connecting wire along a groove in the
SATERS' COMMUTATOR COIL&
67
Pacinotti core somewhat behind the bar to which it is connected , so that
with a slight backward lead it may be passing across the strong field under
the pole-tip when brought into action, and return to the commutator end by
a groove equally in advance. It has been found possible with commutator
coils to allow the armature to create its own field, the magnetising coils
Fig. 18.
Sayers' Araiature.
Fio. 19.
Fig. 20.
Beversible Action of '* Ck>mmutator Coils."
being rendered unnecessary, and using the field magnets simply as return
circuits for the lines of force, or, as they have been termed, *' keepers."
The method of winding the commutator coils adopted by Sayers is
shown in Figs. 18 and 19. In order to increase the E.M.F. effecting the
reversal in the commutator coils a polar extension P is added, so that the
forward half of the coil is re-entering a detached part of the magnetic field as
the backward half is leaving the pole-tip. Figs. 19 and 20 show the winding of
two successive turns Jp A^^ of the drum armature, with the intermediate
commutator coil b connecting with the commutator C, and indicate the
68 ABMATURE WINDING.
principle aooording to which the dynamo is reversible, the same direction of
winding for the commutator coils being effective in whichever direction the
dynamo is driven.
The closed-coil dynamo is used for parallel, or low-tension, systems of
electric lighting, supplying most commonly an E.M.F. regulated for con-
stancy at from 50 to 500 volts, and therefore, for large power, currents of
considerable magnitude. The armature conductors, to carry large currents,
need to be of great sectional area, 2000 amperes per square inch being the
practical limit of current-density, and in order that no space may be wasted,
consist of bars of square or oblong section. In such conductors the avoid-
ance of eddy-currents is of great importance, more especially with smooth-
cored armatures. The bars are commonly made of stranded copper cable, or
thin strip, separately insulated with varnish, and forced into a rectangular
section by hydraulic pressure or drawing through a die. Those compounded
of strip should be placed so that their lamination is in a radial plane, and
even then are subject to undesirable currents flowing forward in one strip,
and backwards in another when the ends are soldered together and to the
cross-connectors, the bar being often of sufficient breadth for the E.M.F. in
one strip considerably to exceed that in another. To eliminate these a half
twist is given to the bar in the middle, bringing the strips into the reverse
position as regards lead on the armature, the shape being restored by
hammering in a mould.
Multiple winding supplies a means of reducing the section of the con-
ductors for dynamos supplying large currents. Double winding, for
example, consists in constructing two separate closed coils connected to
alternate sections of the commutator, placed in parallel by using brush con-
tacts of sufficient breadth always to touch two successive segments, and
bridge over the insulation to a third before leaving the first. The two
closed coils may be entirely separate in their winding, or by using an odd
number of commutator segments, the two may form a single coil, re-entering
after two complete revolutions of the plane of winding. Triple winding
consists of three coils, with brush-contacts capable of touching tour succes-
sive segments, and so on.
In drum- winding for bipolar dynamos the cros««-connectorR commonly
connect bars, or surface conductors nearly, but not quite, diametrically
opposed on the armature core a slight advance of the plane for every turn
requiring, for an even number of commutator bars, that the plane should
not be exactly radial. With an odd number of segments, the plane of each
turn may be exactly radial, and cross-connections between diametrically
opposite bars be made at the pulley end, but not at the commutator end, of
the armature. Swinburne introduced a system of ** chord- winding," con-
necting bars far removed from opposition. This much simplifies the
connections, allows of better ventilation of the interior of the armature, and
brings conductors having less difference of potential into proximity. But the
dynamo has a reduced E.M.F. owing to the turns not including the whole of
the flux. It is claimed, but difficult to see by what process, that the cross-
magnetisation of the armature is reduced by chord-winding.
The shape and arrangement of the cross- or end -connections in drum-
winding have afforded scope for much ingenuity. We can only briefly
summarise the most common methods adopted to preserve symmetry and
uniformity. Edison invented one of the earliest devices, which consisted in,
first of all, jointing all the bars on the drum-surface to flat radial plates,
eveiy alternate plate on the commutator side being also jointed to a segment
of the commutator. Each plate was then joined to that nearly opposite (as
explained above) by a semicircular connecting bar, the radii of these
connectors being varied so that they may lie one inside the otlier in the
AEMATUEB WINDING. 69
same plane perpendicular to the axis. The objection to this method is that
the semicircular connectors, being of different radii, are ako of diflferent
length and resistance. A later method of Edison is to use a number of
insulated copper discs, similar and similarly placed to the iron stampings for
the armature core, the number at each end being half the number of the
surface conductors. Two opposing bars are then jointed to lugs projecting
from the periphery of a disc
Spiral or volute cross-connection is effected with broad copper strip,
bent into a spiral shape, so as to connect a bar of the commutator, or a
corresponding bar at the pulley end, with a surface-bar diametrically (or
nearly so) opposed, or with one in a radial plane at right angles to its own.
The former, combined with direct radial connectors (lying) of course, in a
vertical plane parallel to that of all the spirals) constitutes what is known
as Crompton and Swinburne's winding. The latter, known as the Hefner yon
Alteneck, is that shown in the elementary diagram (Fig. 5), each cross-section
consisting of two shorter spirals, with curvatures, in opposite directions, and
arranged in two parallel planes. Eickemeyer's winding is similar in principle
to that of Hefner von Alteneck, save that a whole turn or several turns forming
a section of the closed coil between two commutator segments, is made without
joints, and wound on a mould of the right shape previously to being slipped
on the armature-core. It is wound so that any faulty section can easily be
taken off and replaced without interference with the rest.
Helical connectors were first used by Kapp. These are plates or strips
with their greatest breadth in vertical planes (ending in lugs for jointing to
the surface conductors) the commutator connections being radial. They
advance slightly along the direction of the shaft, as the threads of a screw,
the alternate bars projecting some distance beyond the core to meet them.
Helical conductors possess the advantage of all the joints being easily
accessible for repair.
In multipolar dynamos the arrangement of the field in the core-ring, or
c>'linder, consists in lines of force entering and leaving by consecutive poles,
which must be of differing kind, so as to form as many loops as there are
poles. Any turn includes a maximum flux when midwny between two poles,
and in this position, neglecting the distortion, of which the effect is to
necessitate a lead as in bipolar dynamos, requires to be shoi't-circuited by a
brush contact, and reverse its position relatively to the external circuit. If
the armature be wound, as ring or drum, similarly to that of a bipolar
dynamo, we should require a brush at each polar gap, and connect alternate
brushes together, and to one of the terminals of the external circuit. This
is sometimes done with dynamos supplying hea\y currents, but is open to
the objection that in the numerous circuits into which the armature is thus
divided, the currents are liable to be unequal owing to inequality of the
resistances of the brush-contacts, etc. It is more common to interconnect
turns in similar positions, so that only one pair of brushas need be used.
Two-cirouit winding for multipolar armatures consists in connecting all
turns or sections of several turns, in similar positions with regard to
magnetic flux in series, resuming the winding with those in consecutive
planes in similar order, every turn or section being connected to a com-
mutator segment in the same radial plane. There are then only two
circuits, as in bipolar dynamos, through the closed-coil armature, and one
pair of brushes only used. In drum- winding the circuit proceeds by a
chord-connector from one bar to that advanced by a distance equal to that
between two pole-centres, and again, still forward by a similar chord-
connector at the other end of the drum to a bar in a similar position to the
first, and so round the circumference to a consecutive set of bars, the
advance, ov pitch, being alwa^'s forward till the clo^sed coil re-euters.
70 FIELD-MAGNETS
It is in the design of tbe field- ran gat: ts of a dynamo that the greatest
freedom of choice is allowed, and upon the choice of shape depends the
characteristic esternal appearance. For the material a choice liee between
wrought iron, cast iron, and steel. Cast iron was till recently employed
for the pole pieces and other parts of the magnetic circuit where the shape
renders wrought-iron forgings inconvenient and expensive. A much lower
Magnetic Induction must then be employed in these parts, necesutating a
multiplication of the Fectional area two or three times. This is generally of
little importance when weight is of minor consequence, or even a distinct
advantage on account oE mechanical stability ; but the employment of cast
iron for the limbe upon which the magnetising coila are to be wound necessi-
tates a much greater length of conductor for each turn, with extra sectional
area, and thus the weight of copper must be multiplied many timee. In
multipolar Beld-magnets cast iron is more commonly employed than in
bipolar field-magnets. Annealed mild steel has recently met with much
favour, as it has been found possihie to produce steel castings with a
permeability little inferior to that of the best wrought-iron forgings except
with low magnetising forces. The expense of the steel castings counter-
balances the diminution in the labour of working.
The choice of shape is induenced by both magnetic and mechanical con-
siderations, and also by the type of armature adopted. Generally the
multipolar field is preferable for dynamos of very large size, and for slow-
speed armatures of considerable diameter ; there is generally less weight of
BIPOLAR MAGNETS. 71
iron, but greater weight of copper io the magnetising coils ia multipolar
field- magnets, than in bipoliu* field-mngnetn for ilynamoN of similar output.
The winding of multipolar armatures introduces greater complication and
t^zpenae, and for this reason the bipolar has had the pi'efei«nce in British
manufacture. With the bipolar field there is still a choice between eingle
aod double magnetic circuit external to the armature.
The leading types of field-magnets adopted for cloeed-coil armatures will
be illustrated by a selection of prominent examples.
Fig. 3 1 illustrates a large two-pole single magnetic circuit dynamo by
the Brush Electrical Engineering Company, suitable to an elongated drum-
wound armature of small diameter, to be driven at high speed. The field-
magnets are entirely of wrought iron, the long rectangular section of the
limbs requiring considerable e^ccess length per turn above what would
72 SHAPE OP FIELD-MAGNETS.
be necessary for a circular section of the same area. With this shape it is
somewhat difficult to maintain equal distribution of the induction across the
polar gaps, especially with Pacinotti armature cores, but this is of minor
consequence with drum- winding. The low position of the armature
renders the dynamo comparatively free from vibi-ation, and a similar type
inverted, in which the armature is at the top, and for this reason the field-
magnet yoke is formed by the cast-iron bed-plate, is only convenient with
dynamos of smaller size. There is in all cases considerable magnetic leakage
past the armature in all single magnetic circuit dynamos^ more particularly
in the type shown, in which a thick zinc plate is necessary between
the field-magnet poles and the bed plate to prevent a magnetic short
circuit.
The double bipolar magnetic circuit is illustrated by the Brush dynamo
in Eig. 22, and for this type heavy cast-iron pole-pieces and wrought- iron
limbs of circular section are most convenient. It is adapted for armatures
of shorter length and greater diameter than the single-circuit types, and
the symmetry of the magnetic field renders ring-wound armatures con-
venient. The large area for the radiation of heat, and the ready accessibility
to the armature by removing the upper pole-piece, renders this type most
advantageous. The magnetic leakage is, however, very large when the coils
are wound as shown, the limbs being far removed in the magnetic circuit
from the armature. A leakage coefficient (the ratio of the total flux of
magnetic induction in the limbs to that through the armature-core) of about
1.4 must be allowed, whereas 1.3 is sufficient with the single magnetic
circuit with a similar armature-core. The division of the magnetic circuit
also necessitates double the number of turns in the magnetising coils for
the same total flux, the length of conductor being greater than for a single
circuit of the same area, though the employment of circular section will
partly compensate for this.
The multipolar design is illustrated in Fig. 23. The armature is a
ring or Gramme-wound two-circuit type designed by Kapp for an out-
put of 500 amperes at 260 volts (130 kilowatts). The magnetising coils
are wound on the eight poles which project inwards from a surrounding
ring, and the leakage coefficient is extremely small. The arrangement
is known as the " ironclad '' type of field-magnets. The ironclad type
possesses the advantages of producing practically no external magnetic
influence, and having the coils in the interior and well protected fix)m
injury. It is not as a rule possible to extend the area of the pole faces in
proportion to the sectional area of the magnetic circuit in the iron to the
same extent with multipolar dynamos as with bipolar dynamos, and hence
the Pacinotti or tunnel-wound armatures are more favoured as afibrding a
shorter air-gap.
The above are typical forms of field magnets such as are now most
commonly adopted for closed-coil armatures. Others, less commonly used
for this type of dynamo, but quite permissible, will be illustrated in our
descriptions of open-coil and alternating current dynamos for which they
are better suited. The following general considerations should guide our
choice and design :
(i) It is better to reduce the length of the magnetic circuit or circuits
as far as possible, to save weight and magnetic reluctance ; but with iron of
high permeability this is of less consequence. On the other hand it is not
advisable so to shorten the length that the magnetising coils are crowded
into too short a length, necessitating great depth of winding with extra
length of each turn, and incurring difficulties in obtaining sufficient surface
for the radiation of heat. Snell gives as the suitable length of the limbs
available for the winding of the coils as .75 times the diameter of the
SHAPE OF riELD-MAGKETS. 73
armature with the siDgl»«ircmt bipolar field, .9 times thn diameter with the
double circuit bipolar, and .4 to .5 times the diameter with the multipdar
lidds.
(3) Sharp angles and turns in the magnetic circuit should be avoided as
favouring leakage. The maguetiaisg coils should, for the same reason, be
pr^erabl^ wound near the poles.
(3) Joints in the iron should be tralj' surfaced, and not too tightly
Fig. 23.
bolted, as this decreases the permeability' without inducing the magnetic
reluctance of the joint.
(4) A circular section for the limbs on which the magnetising coils are
to be wound is theoretically the most eOicient, but this will be modified by
practical consider^ttious.
With some Urge Central Station dynamos it ia found convenient to
supply the current for the magnetising coils from an additiooal small
dynamo; the field coils are then wound with a oompai'atively few turns o'
copper atrip or cable of considerable sectional area, and a larjje current i«
sent through tb«m with a low E.M.F., thus esjiending little energy in
magnetisation, while the current in regulated by bund with a rheostatii.
74 CHABACTERISTIC CURVES.
resistance to vary or keep constant the E.M.F. of the large dynamo as may
be required. The more common arrangement is to obtain the current from
the dynamo armature itt^elf, and there are two ways in which the magne-
tising coil or coils may be connected.
In series winding the magnetising coils are wound with a comparatively
few turns, capable of carrying the whole current output of the armature,
and connected in series with it, so that the magnetising force is proportional
to the whole current in the armature. In shtmt winding the magnetising
coils are of much finer wire, and many turns, giving considerable resistance,
and connected between the brushes or terminals of the dynamo, thus forming
a circuit independent of the external circuit of the dynamo. The current
in the shunt-coils is determined by the E.M.F. between the brushes or
terminals and its own resistance.
Series winding alone is generally employed with dynamos of high
E.M.F., more frequently with open-coil dynamos than with closed-coil, on
account of the difficulty of obtaining sufficient resistance in the shunt-coils
without an enormous number of turns, or considerable waste of power.
Shunt winding alone is employed for dynamos intended for charging
secondary batteries, and large Central Station dynamos, where hand regu-
lation is convenient. The combination of shunt and series winding, or
compound winding, having certain regulative properties which wUl be dealt
with in the next chapter, is adopted in by far the greater nmnber of
dynamos for Electric Lighting upon the parallel or multiple-wire systems.
CHAPTER VI.
The Closed-ooil Dynamo (Design and Befirulation).
Ths behaviour of a dynamo under all conditions of load, provided it be run
at a constant specified speed, may be determined by tracing a curve which
shows the relation between the electromotive force and current, or the
number of volts generated in and the number of amperes flowing in the
armature, when various external resistances are to be found between the
dynamo terminals.
This graphic method was first used by Dr. Hopkinson, and applied to a
Siemens' dynamo (series-wound), the results being given in a paper pub-
lished by the Institution of Mechanical Engineers (Proceedings Inst, M,E,
1879, p. 246, and 1880, p. 266). The expressive term ''characteristic" was
applied to Hopkinson's curves in 1881 by Marcel Deprez.
It^ias been shown how the total electromotive force of the dynamo can
be calculated in terms of the speed, number of turns in the armature, and
the total flux of Magnetic Induction through the armature-core. The latter
will depend upon the number of amp^re-tums in the magnetising coils,
which will in its turn depend upon the current flowing in the armature and
external circuit in the case of a series-wound dynamo, and upon the differ-
ence of potential between the terminals in a shunt-wound dynamo.
The difference of potential between the terminals of the dynamo will
not be the same as the total electromotive force obtained from tbe calcula-
tion made in the last chapter, but will be less than the total electromotive
force by an amount equal to the products of the measures of the current in
and resistance of the armature coils.
We, therefore, have all materials for the prediction of the characteristio
oorve by means of the principles of electromagnetism when we are given all
the measurements of the dynamo. The first step will be to find the relation
between the flux of magnetic induction and the number of amp^re-tums in
CALCULATION OF CHARACTERISTIC. 75
the magnetising field ooilB, and this relation will also be best represented by
a curve. Such a curve must depend solely upon the properties of the
magnetic circuit, and not upon the speed at which the dynamo is driven.
This curve is commonly called the narmcU characteristic of the dynamo, and
from this the characteristic curves predicting the action of the dynamo
nnder various conditions may be traced.
The relation between the flux of Magnetic Induction through the
armature and the cunent in the field coils in several different types of
dynamos were very fully discussed in a paper by Drs. J. and E. Hopkinson
presented to the Royal Society in 1886 {PhU. Trans. 1886, Part I. p. 331).
We may follow some of the investigations recorded in this paper.
As a first approximation let us assume that there is no leakage of
Magnetic Induction through the air, and that all the lines of force in the
field magnet pa.s8 through the iron core of the armature. Let N denote the
total flux of induction ; A the sectional area of the armature transversal to
the lines of force ; ]| the mean length of the lines of force within it ; A,
the area of the air spaces midway between the pole-pieces and the core of
the armature; 1, the distance between the pole-pieces and core; A, the
area of the cores of the field magnets ; 1, the total length of the field
magnets. Then assumiug in each portion a uniform distribution of Magnetic
N N
Induction, we have in the armature core B « --, in the air spaces B » .^.^
Aj Aj
N
and in the field magnets B«-— . InairB-^H, and in the iron
«
the relation between B and H is given by one of the curves described in
the chapter on Electromagnetism. From these curves we may suppose H
expressed as a function of B, say H s f (B). Then in the armature we
H . ( (|), i. th. «ri»^ dmpl, H . », „d k, the Md m.pH»
Now the line integral of H through the whole magnetic circuit Is due
to the current C in the coils of the field magnets. Suppose the coils to
contain n turns of wire, or that nC is the total numbei* of amp^re-tums in
the coils, this line Integral is equal to ^^^ , being measured in amp^es,
10
and, remembering that there are two air spaces, we have the equation^
which maybe taken as the equation to the normal ohai'icteristiOi giving
the relation between N and the magnetomotive force ^ —
10
This curve may be traced by drawing the three curves, in the common
notation of Cartesian co-ordinates
(A) X - M (i) (B) X - »!. (i) (C) X - W (i)
(which are really the curves giving the relation between H and B for the
dass of iron used, the straight line representing the quality of H and B in
air, the ordinates being multiplied by A,, A,, A,, and the abscissas by
1|, 1^ 1, respectively), and then forming a fourth curve whose absoiBsa
correspunding to any ordinate is the sum of the corresponding abscissflB in
the former curves.
y6 LEAKAGE COEFFICIENT.
There are several sources of error in this first approximation :
(i) In any actual dynamo the yoke and pole- pieces should be treated
separately from the cores or limbs of the tield magnet, owing to the
difference of area of the magnetic circuity and generally to the different
material used.
(2) The magnetic reluctance of joints already spoken of may not be
negligible.
(3) The lines of Magnetic Induction in passing from the pole-pieces to
the armature core will at the edges of the pole-pieces spread out laterally, so
that the sectional area A, will be greater than the area of the portion of the
cylindrical surface which lies midway between the pole-pieces and the arma-
ture-core. To correct for this Hopkinson added to the portion of the
cylindrical surface a strip on each side whose breadth was .8 of the distance
between the core and pole-piece.
(4) There will be considerable magnetic leakage through the air from
the pole-pieces and limbs of the field magnets, so that the whole of the
Magnetic Induction passing through the field magnets will not pass through
tne armature. If K be the induction through the armature, to be used in
calculating the electromotive force of the dynamo, vN may be taken to
denote the Induction through the yoke and limbs of the field magnets, y being
a quantity to be determined experimentally for any particular dynamo, or
estimated in designing. As N increases the magnetic permeability of the iron
diminishes, and therefore y increases. The value of B in the limbs or the field
magnets will then be — instead of t!. as before.
-Aj A^
Employing A^ to represent the sectional area of the yoke, and 1^ for the
mean length of the line of Induction through it, A^ and 1^ for the corre-
sponding quantities for each pole- piece, the equation for the characteristic
becomes
V© *'--l* '•' (t) * •-• (© * "■' (") - '^
when A, is now corrected for the spreading of the lines of Induction as
described above. If different kinds of iron, such as cast iron for the pole-
pieces and wrought iron for the remainder of the magnetic circuit, are used,
a different algebraic function, or a different shape of curve must be used for
the pole-pieces and limbs, &c.
The above equation is built up to suit the case of the single magnetic
circuit as used in the Edison- Hopkinson dynamo to which it was first applied
by Dr. Hopkinson. The variations in the case of multiple magnetic circuits
can be easily understood, it being only necessary to take the sum of the areas
of corresponding parts of the multiple circuits.
It will be seen that the measurement of some of the quantities, especially
A^, I4 and A^, 1^, must necessarily involve considerable uncertainty, since an
average has to be taken, and the exact disposition of the lines of Induction
cannot be determined. The errors thus introduced will not, however, be of
very great importance owing to the preponderating magnetic reluctance of
the air-gap.
In Plates IX., X., we have given reduced copies of curves published
by Dr. Hopkinson in connection with the paper above mentioned, from
which the characteristic curves of two typical dynamos were predicted, and
the . accuracy of the method verified by subsequent trial. It is not con-
sidered advisable to insert all the detailed measurements upon which the
calculations for these curves was founded, as they involve a large amount of
estimation which must be made in a different manner for any type of
dynamo which may be similarly deaU with, and the principles upon which
Oh
CO
i
i
s
t
j:*^S2Ss«s«
o c*
EXTERNAL CHARACTERISTIO. ^^
the calculations are to be made has been given above. A few detaOs may,
however, be found useful.
Plate IX. deals with a bi-polar, single magnetic circuit dynamo of
the Edison-Hopkinson type. The limbs of the field-magnets were rect-
angular in section, each limb and pole-piece one solid forging of hammered
scrap, to which the yoke was attached by bolts. The shunt-wound field -
magnet coils consisted of 3260 turns of No. 13 B.W.O., having a resistance
of 16.93 ohms at 13.5° Cent. The armature was drum wound (Hefner Von
Alteneok type) with 40 turns in two layers. The normal output of the
djmamo was 320 amperes at 105 volts with a speed of 750 revolutions per
minute.
The figures estimated from most careful measurements, and used for the
calculation of the Lme-Integral for various parts of the dynamo, were :
1. For armature core A^ s= 810 square centimetres ; li >■ 13 cms.
2. For air-gap (129'' of surface of mean cylinder between armature and pole-pieces
plus 4.8 cms. for spreading)
A) ss 1600 square cms. Ig » 1.5 cms.
3. For limbs h% k 980 square cms. Ig = 91.4 cms.
4. For yoke A4 s 1120 square cms. I4 = 49 cms.
3. For pole-pieces A^ = 1230 square cms. ls=ii cms.
The quantities ]|, A^, and 1, could only be roughly estimated. The ratio
of leakage yBi.32 for the armature was obtained by comparison of the
inducted instantaneous currents produced in turns of wire round the limbs
and the armature respectively when the field magnets were short-circuited.
This was found to vary according to the intensity of the Induction, and
corresponding corrections made.
The observations made of the electromotive force produced when running,
from which the total flux of Induction may be calculated, verify the extreme
accuracy of the method employed.
Plate X. represents the application of the method to a bi-polar,
double magnetic circuit dynamo made subsequently by Drs. J. and
£. Hopkinson. In this type, Mather and Piatt's '* Manchester " dynamo, the
opportunities for leakage were much greater, and thou^th the results proved
fairly accurate up to the point where the iron approached saturation, a rapid
falling off of the observed characteristic from the calculated was noticeable
from this point onwards. Owing to the comparatively excessive reluctance
of the air-gap with low magnetisations of the iron, the illustration scarcely
does justice to the accuracy of the calculations, but a figure showing the
variation of the observed and calculated curves from a straight line would
indicate a fairly close agreement.
The exUfmal characteristio of a dynamo is a curve drawn to indicate all
the possible relations between the £.M.F. or difference of potential between
the terminals, and the current flowing in the external circuit, as determined
by the magnetic and electrical pro|>erties of the dynamo itself, when driven
at a uniform speed. This curve will be practically useful to determine the
values of the output in current and E.M.F. when the dynamo is subjected
to various loads, by the alteration of the external resistance, as by the
switching on or off of lamps. A second relation between the current and
E.M.F. will be given by the conditions of the external circuit, either simply
by Ohm's Law, or a modification due to the variable resistance of the
filaments of incandescent lamps, and the regulating mechanism of arc lamps.
The latter relation may also be represented graphically by a straight line or
curve, whose intersection with the external characteristic will determine the
actual output in current and E.M.F. under the given conditions, the two
curves being equivalent to simultaneous algebraic equations for determining
the two variables, and their iuteisection to the solution.
78
SERIES CHARACTERISTICS.
In the series-wound dynamo, which will first be considered, the current
flowing in the external circuit is the same as that in the field -magnet coils,
there being but one circuit outside the armature, and the external character-
istic may be deduced immediately from the curve showing the relation
between the flux of Induction through the armature and the Line-Integral of
the Magnetising Force as predicted by the methods described above. A
change of the scales for both ordinates and abscissae will reduce the latter to
measurements of the current in amperes, and the former, by the formula
given in the preceding chapter, to those of the whole £.M.F. produced in
the armature in volts. Such a curve may be termed the " total character-
istic " of the dynamo. The available E.M.F., or the difference of potential
Fia. 24.
Characteristic of Series- woand Dynamos.
between the terminals of the dynamo, will be less than that produced in the
armature, by the E.M.F. required to maintain the current through it and
the series coils. There will also be a reduction due to the lead given to the
brushes, sensible only with heavy loads, and considered negligible in the
following discussion. Subtracting this reduction from the ordinates of
the total characteristic we shall obtain the external characteristic of the
dynamo.
Let AP in Fig. 24 represent the total characteristic curve obtained as
described, the ordinates representing the E.M.F. generated in the armature,
the corresponding abscissae the current C in the field coils and external
circuit. Draw the straight line OQ so that its ordinate at any point may
represent the corresponding value of r.C, where r is the resistance of the
armature and field maffnet coils combined, construct the curve AR so that
its ordinates are the difference of the corresponding ordinates oi AP and
OQ. Then AP is the external characteristic An previously shown, the
slope of the AP rapidly dimini^^hes when the induction reaches a certain
point, on account of diminishing permeability. Generally it will be possible
to find a point on AP where the tangent is parallel to OQ. The corresponding
point on OR will be the highest point on the curve, and its ordinate will
represent the maximum E.M.F. in the external circuit possible for the
dynamo with the speed employed. With greater or smaller currents the
external E.M.F. will diminish, but the current may vary over a considerable
POWER CURVES.
79
range with a fairly oonstant E.M.F. If OQ be produced to meet OP the
abficissffi of the point of intersection will represent the maximum current
obtainable from the dynamo, which will be when short-circuited. With
most closed-coil armatures a short circuit would result in the speedy destruc-
tion of the commutator ; but with open-coil armatures, to be described in a
future chapter, this limitation of the possible current, much hastened by the
armature reaction, is of great utility.
The product of the current in amperes and the E.M.F. in volts gives
the total power absorbed in watts, and this product divided by 746 gives
the power absorbed in horse-power. If, as in Fig. 25, a series of curves be
Fig. 25.
Mh <
90
eo
•/A \ \ \ \-^
^^;^-
rr%
^^^-
^^^^
J.
X
10 20 50 40 50 60 70 QO
AMPLRCS
Power carTOB for Series-wound Dynamo
drawn for each of which the product of the ordinate and corresponding
abscissae is constant at any point, any point on a curve will correspond to
the same absorption of power. Such curves would be rectangular hyperbolse,
if the scales for the current and E.M.F. are similar, having the axes of
co-ordinates as asymptotes. The curves shown are the characteristics of a
series- wound Siemens dynamo, with 720 revolutions per minute, tested by
Hopkinson in 1879, the following being the exact figures obtained for
the total characteristic, the resistance including the internal resistance of
.6 ohms :
Currant. KesisUnce. E.M.F.
a 0027
0.48
I.4S
16.8
18.2
24.8
26.8
32.2
34.5
42.0
1025
8.3
5-33
4.07
3«8
3 205
3.025
2.62
2.43
2.28
2.08
2.72
3-95
7-73
68.4
70.6
79.5
Si. I
84.4
83.8
84.6
87.4
If any point p be taken on cne characteristic curve AP Fig. 26 and Op,
pn be drawn, the resistance of the circuit corresponding to the point p is
8o
CRITICAL POINT OF SERIES DYNAMO.
given by the ratio of pn to On, which is the taogent of the angle pOn.
If this resistance, or the angle pOn be increased, the current will decrease
till p reaches the point where the line Op becomes very nearly coincident
with the curve. An exceedingly small increase in the resistance will cause
a very great falling off in the current and E.M.F., and if it were not for
the residual magnetism corresponding to OA would lead to the complete
demagnetisation of the dynamo. The point ff is thus a critical point on
the curve, an increased resistance causing the conditions to be exceedingly
unstable. An increase in speed of the dynamo will not alter the value of
Fig. 26.
O
Critical point of CharacteriBtic
the critical current represented by OL, though the resistance in the circuit
to which it corresponds will be raised by such an increase, and in practice it
is not possible to use the dynamo to supply a smaller current. The difficulty
presents itself frequently in series arc-lighting, for which series-wound
dynamos are commonly used ; for should the initial resistance of the circuit
be too great, as when a pair of carbons are not in contact and the circuit
is only complete through a shunt-coil, the residual magnetism will be
insufficient to start a current such as will draw the carbons together.
In a shunt-wound dynamo the normal characteristic, or the curve
expressing the relations between the Induction through the armature and
the Line-Integral of the Magnetising Force due to the current in the field-
magnet coils, i.e,y — times the number of ampere-turns, will be the same as
for a series-wound dynamo with the same magnets and armature, since it
depends solely on the reluctance of the magnetic circuit, but in this case
the magnetising current is no longer the current in the external circuit, but
dependent on the E.M.F., or difference of potential between the brushes
and the resistance of the shunt-coil.
The relation between the current c in the field-magnet coils and the
eicternal E.M.F. £', between the terminals of the dynamo is given by Ohm's
E' • •
law to be — where s is the resistance of the shunt-coils. Hence the relation
8
between the E.M.F. and the magnetising current will be represented by a
straight line drawn from with an inclination to the horizontal whose
tangent is s. Let this line be OQ in Fig. 27, and let ^P be the normal
SHUNT CHARACTERISTICS.
8l
characteristic. If theoe intersect at K, this point will correspond to a con-
dition in which the whole £.M.F. is found in the external circuit. But
since the current must meet with resistance in the armature this condition
cannot be realised. If r denote the resistance of the armature, and a
straight line OS be drawn inclined to the horizontal at an angle whobO
tangent is r + 8, and cutting OP at L, we shall have represented the con-
ditions requisite to maintain the magnetising current in the single circuit,
including the armature and field magnet-coils, and no external current.
Draw any ordinate peqn cutting OS in s, and OQ in q. Then for the
magnetising current e represented by On, the whole E.M.F. in the circuit
will be represented by pn, and the external E.M.F. by qn. The intercept
sp represents the E.M.F. required to maintain the magnetising current
(On) in the armature^ and therefore an amount represented by ps will
Fio. 27.
OB I 7^ IV X
Formation of Characteristic of Shunt-wouDd Djnamo.
remain to maintain the additioncU current through the armaiure which that
represented by qn sends through the external circuit. The current in the
external circuit corresponding to the E.M.F. represented by qn will there-
fore be proportional to pa, or exactly represented by pe divided by the
refiiBtanoe of the armature in ohms (the latter is graphically represented by
^\ If therefore we draw a horizontal line qm to the vertical axis, and
Onj
cut off m/r on a suitable scale proportional to pa^ so as to represent the
current flowing in the external circuit, r will be a point on the external
circuit. As the resistance of the armature is in general extremely small,
it will be necessary to make the scale of the abscissae in this new curve,
representing the currents in the external circuit, very much smaller than in
the normal charaeteristic first drawn, where they represent the currents in
the field-magnet coils. By taking a number of ordinates similar to pn^ we
may find any number of points on the external characteristic, and trace out
the curve BRM. The point M where the curve meets the vertical axis,
corresponds to zero current and maximum E.M.F. , and is on a level with z^
the point of intersection of OQ with the ordinate LN,
Let H be the point on AP where the tangentis parallel to OL. This
will correspond to a point on the external characteristic where the tangent
p
82
CRITICAL POINT OF SHUNT DYNAMO.
is vertical, say at E, for in the nei^^hbourhood of iT the intercepts pe have
a stationary value. Near this point the E.M.F. of the dynamo may vary
through a considerable range without very great change in the current, a
fact sometimes made use of in charging secondary batteries, for which an
unvarying current is advisable. It would seem as if it were possible to
have two values of the E.M.F. corresponding to the same current, though
with different resistances in the external circuit, but it may be shown that
the lower value, corresponding to the lower resistance is unstable, and that
it is impossible to maintain the conditions represented by the curve below
Ef the dynamo tending to demagnetise itself when the resistance of the
external circuit is further reduced. For example, in Fig. 28 the two values of
the E.M.F. represented by rj and r7 seem to be possible with the current
Fig. 29.
Fio. 28.
Critical point of Charac-
teristic of Shont-woand
Dynamo.
30 40
AMPERES
Power curves of Shunt-wound Dynamo.
represented by 0/, the corresponding resistances being represented by tne
tangents of the angles rO'Ji^ and rOx, Suppose the former resistance to
be slightly reduced so as to be represented by the tangent of the angle KOx^
the E.M.F. required to maintain the current 01 in the external circuit is
represented by /?, and, the E.M.F. being initially greater than this, the
current will begin to increase and the E.M.F. to fall to the values indicated
by the point K, Also an increase of the resistance will produce a falling
current and a rising E.M.F., so that in both cases there are the conditions
for stability. On the other hand, with the condition represented by r^, the
maintenance of the characteristic demands that an increase of the external
resistance should accompany an immediate increase in the current, and vios-
veraa, which is the reverse variation to that which will initially be produced
by the E.M.F., and thus the condition will be unstable, and cannot be
practically maintained. There is a critical resistance, represented by the
tangent of the angle EOx, below which the dynamo will not work with the
speed for which this characteristic is calculated, but an increase in speed
will lower the value of this critical resistance.
The abscissiB of the point B where the characteristic cuts the horizontal
EFFECT OF SPEED-VARIATION.
83
axis represents the current maintained by the residual niagnetism when the
dynamo is short-cirouited, and therefore no current passes through the field-
magnet coils. Fig. 29 represents the actual characteristic traced for a small
Siemens shunt-wound dynamo as far as the vertical tangent, but the
remainder of the curve is impracticable, if not incorrect. In Fig. 27 the
resistance of the armature was made absurdly great in comparison with
that of the field-magnet coils, as was necessary to produce a figure to
illustrate the theory with clearness, so that the downward slope of the
characteristic became intensified. The dotted curve represents the total
characteristic giving the relations between the whole E.M.F. and the whole
current produced in the armature, and is obtained by cutting off lengths
Fig. 3a
Effect of Speed on Characteristics
M
from the horizontal lines p/y proportional to pq (divided by the resistance
of the armature in ohms).
The effect of a change in speed upon the characteristics of a series or
shunt-wound dynamo may be studied by a reference to Fig. 30. The normal
characteristics will be similar curves OH^K^P^, OH^J^^ OH^KJP^^ as
shown, the ordinates alone being increased or diminished in proportion
to the speed. In the case of series-winding a higher total £.M.F. will be
obtained with a higher speed and a given resistance in circuit ; the increase
of E.M.F. will not be merely proportional to the increase in speed, but in an
excess ratio owing to a higher degree of saturation in the iron, the line
OHyKJP^ coxTesponding to the face resistance meeting the upper curves at
more distant points. Moreover, the fall in E.M.F. due to armature resistance
will remain constant, and proportionally less must be subtracted to obtain
the difference of potential between the terminals of the dynamo. The
maximum E.M.F. and the critical E.M.F. will however be found with the
same currerUa for different speeds, though of course with higher values of
E.M.F. and resistance at higher speeds, since the tangents at points on the
various curves having the same ordinates are parallel. There is evidently no
critical speed independent of the external resistance.
In (diunt- wound dynamos the higher speeds will give an increased
84 COMPOUND WINDING.
E.M.F. at zero load in a somewhat greater proportion, with a higher degree
of saturation in the field-magnets and armature-core. The current output
will be increased in a much greater degree, as is evident if the preceding
method of construction for the total and exterjial characteristics be applied
to the portions of three curves OH^, ORJC^^ OH^KJP^^ the line OH^KJP^
representing by its inclination the resistance of the shunt-coils and armature.
The maximum and critical current, though of coui^se much greater with the
higher speeds, will correspond to those points on the normal characteristic
curve shown in the figure which are parallel to OH^K^P^, and therefore to
points having the same ordinate. Referring to the preceding construction
for the external characteristic of the shunt-wound dynamo, it will be seen
that the critical currents correspond to the same external E.M.F., and there-
fore to a much lower resistance in the external circuit. There will also be a
critical speed, corresponding approximately to that which produces the
normal characteristic OH^K^P^^ below which no appreciable £.M.F. can be
obtained even qm open circuit.
Shunt-wound dynamos are frequently employed for supplying constant
E.M.F. to a parallel lighting system, driven at constant speed, the drop in
the E.M.F. as the load increases being compensated for by removal of some
additional resistance in the circuit of the shunt-coil. This method ' of
regulation, commonly effected by the attendant though automatic regulators
have been designed, is specially suitable in central stations where a consider-
able drop of E.M.F. in the mains has to be compensated for by an increase
in the E.Af.F. of the dynamo, so that it must be thoroughly under control,
and the satisfactory running of the dynamos in parallel may be effected
without complications. But when it is simply required that the E.M.F.
should be kept absolutely constant, or rise a small definite amount in pro-
portion to the current output, the method universally employed is to use a
self- regulating or *' compound-wound " dynamo adapted to the conditions.
Compound winding is a combination of both shunt and series windings
for the field-magnet coils. We have seen that under ordinary conditions the
E.M.F. of a series dynamo increases, and that of a shunt-wound dynamo
decreases as the resistance of the external circuit is reduced. By a suitable
combination the two variations may be made to counterbalance one another,
or give, as commonly desired, a slight preponderance to the former, so as to
compensate for the reduction in speed of the driving engine under heavier
loads, and the drop in E.M.F. along the leads to the lamps.
When the dynamo is running upon open circuit, the magnetic field
is maintained by the current in the shunt-coils alone, and the calculation to
obtain the number of turns, and the resistance of the shunt coils, to give the
required E.M.F. with the waste of energy permitted, is similar to that
described above for the dynamo with shunt-coils only. A curve is drawn
(the normal characteristic) showing the relations between the Induction
through the armature, and the Line- Integral of the Magnetising Foix^e
— — ; the requisite value of the former to give the E.M.F. being calculated,
the latter, and therefore the number of ampere-turns is deduced from the
curve ; the current permissible being decided upon, the number of turns in
the shunt coils is known. The di'op of potential in the armature due to this
current should be quite negligible.
To determine the number of series turns, let the conditions be considered
when the maximum current for which the dynamo is to be designed is
flowing in the external circuit. The E.M.F. required to maintain this
current through the armature and series coils (of combined resistance r) will
be Or volts, and if the Magnetic Induction through the armature be
increased by an amount suificient to produce this extra E.M.F.| the
RESIDUAL MAGNETISM. 85
difference of potential between the brnslies will remain nnnltered, provided
the speed of the dynamo is maintained. A still further increase may be
allowed for if desired.
There are two distinct methods of winding tbe shunt-coils to which
attention must be called. They are most commonly connected as a single
branch circuit to the brushes, in which case the difference of potential
between the terminals is not identical with but slightly greater than that
between the terminals of the dynamo, owing to the resibtance of the series
coils and consequent fall of the E.M.F. The latter should however be
insignificant, and the current in the shunt-coils may be looked upon as
invariable if the dynamo be regulated for constant E.M.F. (except for the
temperature changes in their resistance). This is termed the *^ short " shunt
winding. In the '' long " shunt winding the shunt circuit is a branch
between the terminals of the dynamo, and the number of seiies turns as
calculated below must be subtracted from the larger number in the shunt-
winding.
Supposing the current in the shunt-coils to be invariable, the extra
Magnetic Induction must be produced by the Magnetising Force due to the
current in the series coils. By the help of the normal characteribtic we may
calculate the extra number of ampere-turns required to raise the whole
E.M.F. in the armature by the amount desired, and thence the number of
series turns to give this number of ampere-turns with the maximum
current.
From consideration of the normal characteristic, predictable from actual
tests of samples of the iron used in the construction of the field-magnets, or
of the external characterifttic experimentally traced of an actual machine,
we have shown how it is possible to determine not only the stable conditions
under which the dynamo will work under given conditions of speed and
resistance in the external circuit, but to some extent the degree of stability
of thoee conditions. In another chapter we shall also show how the
characteristic curves may be used to predict the manner in which dynamos
will act when used conjointly, joined in series or parallel, to divide the
power supplied to the same external circuit. Yet two other considerations,
of the greatest importance to the engineer in charge of a supply station, can
be determined, or at least explained, by a reference to these curves. These
two considerations are the constancy of the polarity of field-magnets, and
therefore of the direction of the electric current, after periods of inaction
and practical demagnetisation ; and the degree of certainty or rapidity with
which the dynamo, on being started, will assume the calculated working
magnetic conditions determined by the characteristic curves.
The difference between the ascending and descending curves traced as
the external characteristic of any dynamo is found in actual practice to be
inconsiderable for any moderate degree of magnetisation, the vibration inevit-
ably removing any difference due to the property of retentiveness which
would appear in the predicted curve from tOvSts of iron samples. The
difference is more considerable near the origin, and even with soft iron field
magnets and smooth-cored armatures the magnetic circuit is generally
sufficiently complete to retain sufficient residual magnetisation for re-start-
ing, in spite of vibration for re-starting with little delay when the condi-
tions which have been specified above as producing a stable magnetic
condition near saturation are resumed. AVith cast iron or steel field-
magnets, or countersunk armature -cores, the residual ma^'netism is fairly
strong, and in all cases the reveisal of the magnetism of the dynamo after
inaction is a remote possibility, though not unknown. An increase of
speed with shunt-wound dynamos, or a temporary short circuiting, or
*' flashing/' of series or compound wound dynamos has occasionally to be
86 BE-HAGNETISATION.
resorted to to hasten the process of magnetisation. To such a degree are
the conditions of residual magnetism modified by the vibration, the elevated
temperature, and even the manner in which the current is switched ofi*
from the dynamo in the process of '' shutting down/' that the conditions in
resuming work after a period of inaction are most variable, and not amen-
able to any process of calculation.
Dynamos which are calculated to work with a low degree of magnetic
saturatioD, and therefore, though generally of the highest efficiency, near to
the oonditions of instability, often present considerable difficulties in re-
starting owing to a cause more difficult to combat than that of total
demagnetisation when at rest. The working conditions of such dynamos
are liable to excessive variations when their temperature is raised, and
though their high efficiency may make them cooler than dynamos in which
the magnetic saturation is higher, both on account of the reduced energy
spent in the magnetising coils, and reduced hysteresis in the armature-core,
the surrounding temperature in a small eugine-room is subject to suffioient
variations to cause difficulties.
The writer is responsible for the plant supplying an extensive institution,
consisting of a 57^K.W. steam-driven com pound* wound dynamo, supplying
550 amperes at 105 volts, having as a sole reserve 70 ampere secondary
battery. The field-magnets of the dynamo are of soft iron, and the dyuamo
is compounded at a fairly low degree of saturation, apparently close to the
** knee " of the magnetising curve. The dynamo is generally run from dusk
till 10.30 P.M., and loaded to the maximum for the last three hours. As
the dynamo is considerably over-compounded, that is to say, the number of
series turns is in excess of that required to give uniform E.M.F. at all
loads, the gradually increasing current in the early hours of the evening
automatically raises the electro-motive force slightly, as required to com-
pensate for the fall in the leads, and little hand-regulation is required. In
spite of the fact that the dynamo is rising all the time in temperature, it is
only when the load has been excessive, or the ventilation of the engine-room
poor, that the regulating resistance added to the shunt-coil has to be much
reduced to maintain the full electro-motive force. As, however, the demand
decreases, and the magnetising force due to the series coils is removed, it
frequently becomes painfully evident that most of the magnetisation was
due to the series turns, and the hot shunt-coil is now so much increased in
resistance that the electro-motive force of the machine is impossible to
maintain, except by increasing the speed of the engine. The dynamo begins
to work below the '^knee " of the magnetisation curve, becomes exceedingly
unstable, and the electro-motive force may easily fall twenty or thirty volts.
After shutting down, the dynamo will not remagnetise itself if started
before time has been allowed for cooling, nor even will it maintain its
magnetisation at the normal speed unless loaded. The reason for this is
that, working with the shunt coil only, the increased resibtanee when hot
takes it below the critical point on the characteristic. Even if connected to
a circuit of low resistance the hot dynamo will not self -magnetise, and it is
necessary to send a current through the shunt-coils from the secondary
battery, and switching the armature into parallel, give it a moderate load
before the battery is removed. Then the dynamo will maintain its mag-
netism with a much lower output than that necessary to start it.
We have now to illustrate the methods upon which the calculations
necessary to the design of dynamo machinery are made. From the pre-
ceding discussions it must be evident that a large amount of consideration
and experience must precede calculation, since there are many great difier-
ences in the general type, and a multitude of differences in detail, between
which it is impossible to decide as to superiority, and ai e left to the choice
EELATION OF SIZE TO CAPACITY. 87
and iDgenuity of the designer. The purpose of this discussion is not to
enter farther into these points, nor to give a detailed description of a model
type, but, taking one of the simplest forms, to show how the electric and
magnetic measurements to produce required results may be calculated, or
when exact calculations are impossible, may be estimated with sufficient
exactness to be a guide in their construction.
In making the calculations necessary with the design of a new type of
dynamo, when the general shape of certain parts, which is chiefly a matter
of choice and experience, has been decided upon, and the intention is to
produce a spedfled output in E.M.F. and current, it is most convenient to
fix arbitrarily upon certain dimensions, as near as experience has indicated
will be sufficient for the purpose, and calculate as nearly as possible, the
E.M.F. and current that can be produced with the requisite efficiency with
these dimensions. Then, if a relation between the output and calculated
dimensions of similar dynamos of various sizes can be determined, expressing
the result of multiplying or dividing the dimensions arbitrarily assumed
by certain factors, it will be easy to choose the factors which will modify
the first design to suit specified conditions. This relation between different
sizes will, moreover, be most convenient when a number of similar dynamos
are required, for example, giving the same E.M.F. but adapted for difierent
current-output. And the corrections which have to be made in the first
design when carried out in practice will indicate the corrections in the
similar designs with difierent dimensions.
Suppose, for example, we wish to find the effect of multiplying all the
linear dimensions of the field-magnets, armature, etc., of a dynamo by a
factor X but obtain the same E.M.F. ; as this will frequently necessitate,
owing to mechanical limitations, a change in the speed with which it can be
driven, we will suppose this also multiplied by a factor s. The sectional area
of the magnetic circuit is multiplied by x^, and if the density of the
magnetic induction is to be kept unchanged, the total flux will be multiplied
by x^. The number of turns in the armature winding to produce the same
E.M.F. will be multiplied by ^ • ^® space for winding the armature
conductors will be multiplied by z' (allowing a proportionate increase in the
magnetic gap between iron and iron), and therefore their sectional aiea by
s.x^. Tbe length of each turn will be multiplied by x, but since the
number is multiplied by — , the length of the armature circuits from
brush to brush will be multiplied by — . The resistance of the armature
will thus be multiplied by '^—i^
It follows that if the current output is limited by the current density in
the conductors of the armature, it will be increased by the factor 8.x^ ; but
if by the electrical efficiency, or drop of potential in the armature at full
load, by the factor s^ . x^ In general the peripheral speed of the armature
is pushed up to the maximum that is safe, being limited by the danger of
disruption by " centrifugal force," so that s ■» — . In this case the current
output, for the same density in the conductors and same electrical efficiency,
is multiplied by the factor x^, or is proportional to the volume and weight
of the dynamo.
But the larger dynamos have an advantage which does not appear in the
above calculation. The space occupied by the insulation may be less owing
to the reduced number of conductors, and very much less in proportion to
88 ARMATUEB CALCULATIONS.
the space available. And oonsideriDg the commercial consideration of out-
put in proportion to the cost, there will be a decrease in the relative cost of
labour and waste of material in the larger dynamos. To set against this it
must be noticed that the surface for the radiation of heat is only propor-
tional to z', so that a higher efficiency, or a reduced current density in the
armature current, is expected, if the rise in temperature above the surround-
ings is to be the same as for smaller dynamos.
Suppose we wish to design a large dynamo to supply a constant E.M.F.
of no volts, with an efficiency at full load about 95 per cent. Following
closely the design of a dynamo which has been constructed with the dimen-
sions here laid down, we will presuppose a cylindrical Facinotti armature-
core, built up of stampings 35 cm. in external diameter, with an internal
diameter of 1 2 cm. ; the grooves between the Facinotti *^ teeth " to be a
centimetre deep, aud the length of the cylinder 34.5 cm. Such an arma-
ture may be run safely at 1000 revolutions per minute, giving a peripheral
speed of about 40 miles per hour.
The minimum cross-section of the magnetic field in the armature will
have a breadth of 21 cm. (doubling the minimum depth of the cylindrical
core) and a length of 34- 5 cm., giving a total section of 724.5 square
cm., or, allowing 10 per cent, for thin paper insulation between the disoSy
of 6.52 square cm. of solid iron. Acix)ss this section there will be the
maximum density of Magnetic Induction, and it is advisable that this
density should never exceed 12,000 c.g.s. units, or lines of force. As the
E.M.F. developed in the armature must rise to about 115 volts at the
maximum load, we shall calculate the requisite number of turns on the
supposition that the Induction reaches its full value of 1 2,000 units with
this E.M.F. If n be the number of surface conductors
_, 652 X 120 00 X p X 1000
^~ 10^x60
whence n = 88 very nearly.
The circumference of the armatui'e-core will be ir.35 or about 105 cms.,
giving, with 88 surface conductors, a circumferential breadth of 1.2 cms.
for each conductor. Supposing the Facinotti ** teeth " to be of equal
breadth to the intervening grooves, and one conductor only to be laid in
each groove, the space left for each conductor, with insulation, will be .6 cm.
by I cm. Copper strip .9 x .5 or .45 square cm. (.07 square inch) may ,
be used, and this at 2000 amperes per square inch will carry 140 amperes.
There will be no objection to exceeding this current density to give 150
amperes, or 300 amperes for the total output. As the Magnetic Induction
will pass almost entirely through the teeth, avoiding the intervening gaps
in which the conductors are wound, lamination of the copper strip need not
be resorted to, as eddy currents will not be formed to any extent. It will
be advisable perhaps to employ two strips, .9 x .25 centimetre, for ease in
manipulation. To estimate the resistance of the armature : the length of a
turn in ring-winding will be about 100 cms., and the total length from
brush to brush 4400 cms., with a total section of .9 square cm. This, if
of high conductivity copper, should give a resistance of about .008 ohm
when cold, rising to about .01 ohm when raised to a temperature of 50°
Cent. With drum-winding a slightly lower resistance could be secured,
but with the relative length and diameter of the core here employed the
reduction would be small.
With the low value of the Magnetic Induction assumed the leakage of
the field past the Facinotti armature cannot be great, and may be estimated
at 25 per cent, of the flux through the armature. If the same density is to
be maintained in wrought-iron field-magnets (it might perhaps with advan-
KELUCTANCB. 89
tage be increased) a sectional area of 815 square cms. will be required. In
the limbs, on which the coils are to be wound, the section may be made
rectangular, measuring 33 x 25 cms. with comers slightly rounded off.
The centres of the limbs would be about 40 cms. apart, and the yoke
slightly wider and thinner.
We will suppose that the length of each limb of the field-magnets is
50 cms., the length of the mean path of Induction in the yoke 43 cms.,
and in each pole-piece 20 cms. The Induction is nearly uniform through-
out, except that it is somewhat less in the pole-pieces than in the rest of the
circuit. We may thus take the total length in the field-magnets as 183 cms.
It is not possible to determine the mean length of the lines of Magnetic
Induction in the armature-core with any great degree of accuracy.
Supposing the widtH of the gaps between the horns of the poles to be 12.5
cms. (corresponding to an angle of about 40° at the centre of the shaft), the
mean length may be taken as double, or 25 cms. There will thus be a
total length of 208 ems. in iron, and if the joints be duly surfaced and
tightly bolted together, we may safely neglect their magnetic reluctance.
The number of amp^re-tums in the shunt-coils necessary to maintain
the Magnetic Induction in the iron portion of the circuit only may now be
calculated. At zero load, the E.M.F. generated in the armature being no
volts, we may take the Induction to be 11,500 cg.s. units. For this
density the value of fi may be taken as 1550, and the value of H as 7.42.
Multiplying this by 208 we have for the Line- Integral of the Magnetising
Force /^or4^nC^ the value 1543, giving 12 18 amp^re-tums.
The magnetic reluctance of the air-gap will, in this dynamo, be the more
important term. The surfaces of each pole- piece will subtend an angle at
the centre of the shaft of 140°, and, allowing a little for the spreading of
the lines at the horns, we may take the width of the air-space as 44 cms.,
the total area over which the lines are spread as 1524 square cms., giving a
density of 4920 c.g.s. units.
The opposing surfaces of iron on the two sides of the gaps are by no
means uniformly distant. If a clearance between the extremities of the
teeth be not greatly less than the width of the teeth, say half a centimetre,
the arrangement of the lines of force in the gap
will be somewhat similar to that shown in Fig. 31, pj^ ^^
and it will be seen that on entering the surface of
the pole-pieces the field is fairly uniform, and the
wasteful eddy currents and heating of the pole-
pieces by the rotation of an irtegular field will Ik)
eliminated. When larger teeth are preferred it
will be necessary to stamp the discs so as to broaden
the extremities of the teeth, and leave only a
narrow neck to the grooves on the outer circum-
ference. The clearance may then be reduced to that necessary for free
rotation ; but the cost of stamping and winding will be greater, and the
reaction of the armature current on the field will make sparklessness on
the commutator difficult to secure unless a method of compensation such as
that previously described is employed. In our design we may fairly estimate
the average length of lines in the gap as .65 cm., increasing the minimum
of 5 cm. to allow for those which enter the teeth at the side.
The length of the double gap between the iron surfaces being thus taken
as 1.3 cms., the intensity of the Induction is 4920 c.g.s. units, and the
permeability being unity the requisite Line- Integral of Magnetising Force
will be 6396 so that the number of additional amjiere-turns to maintain the
Induction through the gap will be 5090, giving a total of 6318.
90 COMPOUNDING.
If we allow a current of 4 amperes, slightly over i per cent, of the total
output, to flow through the Bhunt-coils, we shall require 1580 turns to
obtain the required number of ampere-turns, or 790 on each limb, and a
total resistance of 27.5 ohms. The circumference of each limb will be 116
cms., but allowing space for insulation and, if desired, ventilation between
the bobbin and the field-magnet limbs, and for some depth to the
winding, the average length of a turn can hardly be less than 140 culs.,
giving a total length of 221,200 cms. For this length a resistance
of about 24 ohms will be given by a single No. 17 6.w.g. wire at
15° Cent., rising to the required 27.5 with a rise in temperature of about
35° Cent.
It will require but 3 volts to force the full current of 300 amperes
through the armature (.01 ohm). The high magnetic reluctance of the
air-gap compared with that of the field-magnets and armature indicate that
a very small lead of the brashes will be sufficient to obtain sparkless com-
mutation, and the fall of E.M.F. thereby (jccasioned will be small in
comparison to that due to the armature resistance. To allow for this, and
for a slight decrease in in the speed, we shall consider the number of series
necessary to raise the £.M.F. in the armature to 115 volts at full load.
For the air space the increase in the necessary ampere-turns is proportional
to the increase of the Induction, giving an increase of 231. In the iron
part of the circuit we have to allow for a decrease in the permeability of
the iron, which, according to the measuiements of Swing, reduce for a value
of the Induction of 12,000 units to 141 2. The necessary number of ampere-
turns will therefore be 1228 x ■ ^^ x — -,ot 1406, giving an increase
1412 120
of 178 ampere-turns. The total increase will therefore be 409 turns,
making no allowance for additional leakage. ' Two turns of series winding,
carrying the whole current of 300 amperes, should be sufficient to slightly
over-compound the dynamo, maintaining constant potential between the
mains at some little distance from the dynamo, or compensating for a dimi-
nution of speed at full load.
The total weight of wrought iron in the field-magnets and armature will
come to about 33 cwt. The weight of copper in the armature conductors
about 80 lbs., and in the shunt-coils 60 lbs. Wrought iron has been
supposed used throughout, regardless of expense. If cast iron be used for
any part, as is frequently considered advisable for the pole-pieces or the yoke,
the sectional area should be increased at least 50 per cent., with a corre-
sponding increase in weight, reducing the maximum induction to 8000 units.
Even thus the permeability will be reduced to about 100, and if cast iron
with this sectional area were used for the field-magnets throughout, the
length of the magnetic circuit being 190 cms., the number of ampere-turns
required for this part only, will work out as 10,580, or for the whole circuit
15,826, increasing that calculated above about 2.5 times. Moreover, the
length of each turn of shunt- winding must be increased 1.24 times, and
therefore the total length 3.3 times, and the weight of copper giving the
same resistance 1 1 times. Even thus no allowance has been made for the
increased leakage of the Magnetic Induction. Cast steel is a better substi-
tute for wrought iron, giving higher permeability when saturated, and its
employment is increasing in favour. Wrought iron will require a similar
Magnetising Force to that of cast iron if the sectional area of the field-
magnets be decreased so that the Induction rises to 18,000 units, but owing
to the smaller circumference the weight of copper will only be increased
about five times, while the weight of iron is reduced by about 10 cwt. The
number of ampere-turns in the series coils works out as 151 1, or five tiu^na
for correct compounding.
ESCAP£ OF HEAT. 91
The energy absorbed owing to hysteresis and eddy currents in the arm-
ature-oore may be calculated as follows. The total volume of the oore is,
allowing 10 per cent, for paper insulation between the discs, nearly 22,700
cubic cms. The hysteresis loss in a complete cycle of Magnetisation reach-
ing X2,ooo units, should be about 6675 ^S^ P^ cubic cm., or nearly 150
million ergs in the armature-core per revolution. Multiplying by the
number of revolutions per second and dividing by 10^ we find the power
wasted to be 250 watts, or about one-third of a horse-power, and .75 per
cent, of the total output. If the core plates or stampings are a quarter of a
millimetre thick, the eddy current loss will be a little more than half this
amount, and the total core looses will be about x.2 per cent, of the total
output of the dynamo.
The permanent temperature to which any part of the dynamo will
rise when working will be that at which the heat escapes by conduction
and radiation at the same rate as it is being generated. It is commonly
specified that the temperature of the dynamo should not, for any part,
rise beyond a certain number of degrees Oentigrade above the surround-
ings in continuous working. The Admiralty specify that the tempera-
ture of any accessible part of the dynamo should not rise more than
30* Fahr. (16® 0.), nor in the armature more than 70° Fahr. (39° C),
above the surroundings. These limits are unnecessarily low, or would
be for dynamos to be used in well- ventilated engine rooms and
temperate climates. Limits of 40^ and 70^ Centigrade respectively are
in general quite satisfactory. Exact predictions of the rise of tempera-
ture are dilUcult to make, but the following estimations may serve as a
guide.
From the armature the heat escapes largely by convection, a consideiv
able current of air being drawn between the polar faces and the armature
by the rotation of the latter; and escape at a higher temperature. In our
design we may assume that the air would cling preferably to the rougher
sui-face of the armature, but assuming a velocity of the air in the interspace
of one-half the peripheiul speed, or about 30 feet per second, a current of
air of about .6 cubic feet per second would be drawn past each polar face,
and thrown off by ''centrifugal force "at the horns. To raise 1.2 cubic
feet by one degree Centigrade every second woujd absorb the heat generated
by 1 2 watts. The total power absorbed by the armature conductors due to
resistance, and the core due to hysteresis is 900 H- 250 or 1150 watts. This
method of dissipation of heat would requiie a rise in temperature of nearly
100 degrees, but conduction to the field magnets and radiation and oon*
vecriou from them and from the ends of the armature will probably reduce
the limit to less than half. The rise of 20 per cent, in the resistance of the
armature conductors allowed for above corresponds to a rise in temperature
of about 50 degrees. The escape of heat by radiation depends upon the
colour and brightness of the surface and the rise of temperature above the
surroundings. The mean between bright and dark metal would give a rate
of radiation of .00025 calorie per second, or .001 watt, per square centi-
metre for every degree of the excess temperature. With an excess tempera-
ture of 50 degrees the heat developed 575 watts, or half the power absorbed
in the armature could be radiated from an area of 2875 cms., and this will
not greatly exceed the area available at the ends of the armature. In the
shunt-coils the power absorbed is 440 watti^, and to obtain a permanent
excess of temperature of 35 degrees a surface of 12,600 sq. cms. would be
required for radiation. The heat will be partly conducted to the field
magnets, and radiated thence, but the tot^al surface will not diflfer much
from that required in the above calculation. Bi'fore the shunt-coils reach
the temperature .vhich i^ thus allowt:d ior, their re.si>tance will fall short of
92 STEESS ON CONDUCTORS.
that requisite by several ohms, allowing a larger current to pass, and a
higher induction and E.M.F. in the dynamo. The normal value will, how-
ever, be reached after a few minutes running.
With a bright coloured or polished surface to the metal^ the radiation will
be much slower for the same rise in temperature, and ei&ciency will be
sacrificed to appearance. Esson found the radiation from a varnished cotton
covered coil to be sufficient to remove the heat absorbed by i watt from a
surface of 355 sq. cms. with an excess temperature of 1° Centigrade. An
area of 7.3 sq. cms. per watt dissipated gave a rise of 35° Centigrade. For
the armature the rise in temperature was given approximately in degrees
Oenti£crade by the expression ^55 ^ w being the watts absorbed ; 8
° "^ s(i + .ooo6.v)' °
the area of the surface in square centimeties, and v the peripheral speed in
feet per minute.
One of the most important advantages inherent in the Pacinotti form of
armature-core is that the driving of the conductors is effected by the teeth,
and there is no possibility of their displacement under the heavy stress. It
is, in facb, more than probable that the stress falls on the teeth, and not on
the conductors at all. In designing smooth-cored armatures provision to
meet the shearing stress has to be made by means of plugs or stops here and
there on the surface, and binding wire tightly wound round the armature.
It may bo useful to calculate the driving force on each conductor. Assuming
the Intensity of Magnetic Induction to be practically uniform in the air-
space, the driving force will be applied equally to all those conductors within
the polar horns, and be very small indeed on those outside, or opposite the
gaps between the horns. Assume, then, that the driving force is divided
equally among the foimer, in number about seventy-two, the total electrical
horse-power developed in the armature is at full load 3^4 x 115 ^^
746
forty-seven nearly. At 1000 revolutions the peripheral speed of the con-
ductors is about 3600 feet per minute, so that to correspond to 47 h.-p. a
force of 53 :47 qj. almost exactly 6 pounds weight is applied to each
1800 X 72 f »^ o M:t^
conductor.
A further stress to be considered is the tendency of the conductors to
fly outwards owing to " centrifugal force.*' The mass of a surface conductor
is about 177 grammes, the distance from the axis of rotation 17 cms.
Multiplying the product by the square of the angular velocity, 2jr.iJJ^, we
get a centrifugal force of nearly 33 million dynes, or 75 pounds weight for
each conductor. If the conductors be kept from flying off, or bulging in
the middle, by a lapping of binding wire, the problem of finding the necessary
tension is similar to that of finding the stress on the plates of a circular
boiler, or a steam pipe. There being 75 lbs. outward pressure per 1.2 cms.
of circumference, the radius being 17 cms., the total tension necessary is
Z5 Z or nearly 1063 lbs. Many turns of fine steel piano- wire may be
1.2
used, the low permeability making it practically non-magnetic, but a high
safety factor will be advisable.
The other sources of waste of power in the dynamo are mechanical
friction, hysteresis in the iron core of the armature, and eddy currents in
the iron core, pole- pieces, and conductors. The question of hystet*esis and
eddy currents in laminated iron will be treated with some care in dealing
with the subject of transformers, where it is of the grejitest importance.
It will be shown that the eddy current loss varies as the square of the speed,
the square of the intensity of Magnetic Induction, and the square of the
VARIOUS LOSSES. 93
thickness of the plates employed, the loss per cubic centimetre of iron being
given in ergs per second bj
6a-
where v is the number of revolutions per second in a bipolar field ; h half
the thickness of the plate in millimetres ; 9 the specific resistance of the iron
in electromagnetic units (about 10,000). This formula only applies to thin
plates.
Taking v •■ 1000, B » 12,000, h — — (or the thickness of the core-
40
plates one>half millimetre), the eddy-current loss becomes 16,300 ergs per
cubic centimetre of iron per second, or .00163 watt. The loss due to
hysteresis with this intensity of Magnetic Induction (12,000 units) should
be about 7000 ergs per second ; to reduce the loss due to eddy-currents
below this, the thickness of the core plates should be reduced to one-
quarter millimetre, giving a loss of nearly 4000 ergs per cubic centimeti-e
of iron per second. But with slower speeds a greater thickness will be
permissibla
The absorption of power by mechanical friction and by hysteresis will
be proportional to the speed at which the dynamo is driven. That absorbed
by eddy-currents in the iron core and the copper conductors will, on the
other hand, be proportional to the square of the speed, except that there
will be a slight falling off at high speeds due to magnetic screening of the
interior of the iron or copper. These laws of variation give a means of
separating the various losses in a practical test of a dynamo when the power
required to drive the dynamo at zero output has been measured with a
transmission dynamometer at various speeds. Mordey has applied such a
method of testing to a four-pole Victoria dynamo, intended for a maximum
output of 18,000 watts when driven at a speed of 1200 revolutions per
minute. The armature-core was built up of iron strip .3 millimetres thick
(.012 inch), and the density of magnetic induction in the core was 12,000
units. As there will be two complete cycles of magnetisation per revolution,
or 40 per second, the hysteresis loss should be about 14,000 ergs per cubic
centimetre of iron per second when run at full speed ; while, according to
the above formula, we may expect a Iops through eddy-currents in the iron
core of about 36,000 ergs per cubic centimetre per second. . Further eddy-
current loss is. however, likely to be added in the core framework, etc.,
which will be difficult to separate from that in the stampings.
The horse-power absorbed at different speeds was measured before and
after the armature conductors were wound on the core,, and the calculated
losses are shown by the ordinates of the curves in Fig. 32. Those due to
mechanical fiiction were first measured at different speeds, the dynamo
being run without magnetisation, and shown by the downward-drawn line
OM. The dynamo was then run at various speeds, the horse-power absorbed
measured, and plotted as shown by the curve ON, The armature was then
wound, and similar readings taken giving the curve ON'y the armature
being upon open circuit. The difference of the ordinates of ON and ON'
is due to eddy-currents produced in the copper conductors. The ordinates
of Oi\r representing the combined expenditure of power due to hysteiesis
and eddy-currents in the iron core may be analysed into its two components
by drawing a tangent line OF to this curve from the zero point. The
hysteresis expenditure of power will be represented by the ordinates of
OF^ except, perhaps, at the higher speeds, being proportional to the speed.
The expenditure of power due to eddy-currents in the iron is, on the other
hand, proportional at moderate speeds to the square of the speed, as its
representation by the difference of the ordinates of OF and ON approxi-
94
HOPKINSON'S TEST FOR EFFICIENCY.
mately indicates. The falling off from this square law at higher speeds
indicated by the variation of ON from the dotted line, is to be accounted
for by the magnetic screening of the middle part of the wrought-iron
stamping, the eddy-currents being confined to a moderate depth on either
side (see chapter on Alternating Current Theory).
Dr. Hopkinson devised a means whereby actual tests might be made of
the commercial efficiency of dynamos at all loads without the expenditure
Fig. 32.
of more than the wasted power during the test. The device consists in a
simultaneous test of two exactly similar d3mamos, of equal size and power,
which are coupled together. It appears from theoretical considerations
that the wasted power is the same for the same armature current whether a
machine is used as a dynamo or motor. If the two dynamos be coupled
together, one used as a dynamo and the other as a motor, the efficiency
might be measured by comparing the electrical power produced by the
former with that supplied to the latter. The square root of the ratio of
these measurements would give the efficiency of either machine expressed as
a decimal. An improvement consists in using the dynamo to supply the
STOKAGB BATTERIES. 95
extra current to the motor, the former being magnetised so as to supply a
slightly higher E.M.F. at the same speed; practically this amounts to a
cycle of interchanges of power between the machines, by mechanical force
Uirongh the shaft returning as electrical force through the armatures, and
any armature current may be established in the two machines without expen-
diture of the extra power above that wasted in the two machines. The
extra electrical power that must be supplied owing to the various expenditure
with various armature currents is given by an ampere-meter and volt-meter.
The principle of the Hopkinson test has been modified to meet the require-
ments of many types of continuous and alternating machines, it being
possible with the latter to subdivide the armature into two parts, so as to
perform an efficient test with a single machine.
CHAPTER VII.
Storage Batteries.
Ths primary cell has long been entirely superseded by the d3rnamo bs the
source of electric power for lighting, traction, and similar purposes, it having
as yet been found impossible to generate electric power by direct chemical
action so a6 to compete commercially with its indirect production from the
heat-energy of coal, converted into mechanical energy by the steam- or gas-
engine, and again into electric energy by the dynamo. The primary cell
still holds its own as the source of electric power in small quantities, for
telegraphy and laboratory purposes, and occasionally for lighting and motive
power on a very small scale, when convenience is of more importance than
the actual cost of generation of power. If the chemical action of a primary
cell could be reversed by the reversion of the current through it, the original
conditions of the electrodes and electrolytes being thus restored, the decom-
position of the same material could be used again and again as the source of
power in discharging. For the reversal of the current power must be sup-
plied from some other source, say by a dynamo whose E.M.F. is superior to
that of the cell, this power being measured by the product of the measures
of the E.M.F. and the reversed current, and absorbed or stored up by the
rfistoration of the chemical conditions, so that it may reappear (in part)
as electric energy when the cell is once more supplying a current in the
usual way.
A cell capable of being itsed in this way is termed a Beoondary or storcige
cell, or an cueumulator. Many of the common types of primary cell can to
some extent be used as secondary cells. For instance, by the reversion of
the current in the Daniel 1 cell the copper may recombine to form copper
sulphate, and the zinc be deposited on the zinc electrode. But this can only
go on very slowly and to a very limited extent, and a very small proportion
of the electric energy absorbed in the reversion can be recovered. Much of
the energy wOl be absorbed in heat owing to the high resistance of the
electrolyte, and other chemical actions ensuing in charging the cell than
the reverse of those in the discharge. The methods adopted in primary cells
to de-polarise or remove the hydrogen, mechanical or chemical, prevent its
reappearance when and where required for the reversal of the chemical
action.
Grove's gas battery is the most elementary form of a reversible or
secondary cell. It consists simply of two platinum plates, or electrodes^
immersed in slightly acidulated water, and arranged so that the gases
liberated on the platinum plates by the electrolysis of the water may be
collected in receivers placed above them. To separate the constituents of
g6 PLATTE'S ACCUMULATOR.
water, hydrogen and oxygen, requires an E.M.F. of not less than 1.46 volts.
If the platinum electrodes be connected to the terminals of a Daniell cell,
whose E.M.F. should be about 1.07 volts, no current will pass, except a very
small transitory current, and a subsequent current of almost infinitesimal
magnitude. If the Daniell be replaced by a Bunsen or Grove cell, giving an
E.M.F. of about 2 volts, a moderate current will pass and the oxygen will
be collected in the reservoir over the negative electrode and hydrogen in the
reservoir over the positive electi ode. When the source of power is removed,
and a galvanometer is connected in its place, a current will pass through the
latter from the platinum in the oxygen reservoir to that in the hydrogen,
the gases being slowly recombined to form water. The current can of
course only be exceedingly minute, the internal resistance of this secondary
cell being very great.
In 1869 Plants produced the first practical storage cell by using lead
instead of platinum plates as the electrodes. In the first type large plates
were used laid upon one another with strips of gutta-percha to keep them
from contact, and these were rolled together so as to form a cylindrical spiral.
Thus large surfaces of metal at a short distance apart are obtained without
occupying much space. The roll was then immer^ed in dilute sulphuric
acid (one in ten) and an E.M.F. of several volts applied. The oxygen
liberated on the lead plate connected to the positive terminal of the
generator combined with the lead to form peroxide of lead (PbO,), while
hydrogen was liberated on that connected to the negative, and escaped in
bubbles, the lead being unaffected. After the current had flowed for some
time the coating of peroxide of lead formed on the positive plate effectually
protected the lead against further action. Oxygen gas was then liberated
from the surface, and it was useless to carry the process any further.
On connecting the lead plates through an external resistance a current
in the reverse direction through the cell was produced for some time, at
first fairly constant, and then rapidly falling off to zero. During this
operation, that of '^ discharge," one-half of the oxygen was carried over to
the negative plate, so that on each plate lead monoxide (PbO) was formed,
combining as soon as formed with the sulphuric acid to form lead sulphate
(PbSOJ (which was deposited so as to coat either plate), and water (H,0) so
that the acid solution was weakened.
After the first charge and discharge Plants proceeded to charge the cell
again, but this time the connections weie reversed so that the plate which
was formerly the negative now became the positive. Lead peroxide was now
formed on the latter, while the plate which was now the negative was coated
with pure lead. On again discharging both plates were once more coated
with lead sulphate, and the connections were once more reversed for a third
charge. It was found that the charging could now proceed for a longer time
before the oxygen was liberated as gas, and the discharge with the same
current was correspondingly lengthened. The process of charge and discharge
with reversals of connections for each operation was repeated a great
number of times, the first charging occupying about a quarter of an hour, and
the time of subsequent charges increasing to an hour after six or eight
operations. The cell was then left charged all night. On the second day it
was discharged and then recharged in the opposite way during two hours;
again dischai'ged, recharged afresh in the opposite direction, and finally was
allowed to stand charged for eight days, After eight days it was again
charged during some hours without being reversed, and was then allowed to
stand charged for fourteen days, and so on. In this way the capacity of the
element was more and more increased." About two months were required
for the proper " formation " of Plant^'s cell.
The reason for the increased capacity, or total energy stored np by the
"FORMED" AND "PASTED" PLATES. 97
cnarge after these prolonged operations, was at once made clear by an
ezaiuination of the plates. The negative plate, when the cell was charged,
was found to be reduced to some depth to a porous, spongy condition, and
the peroxide ooating on the positive extended to a corresponding depth. In
other words, a much larger quantity of lead was reached by the dilute acid
electrolyte, and thus accessible to the chemical action. The reduction of the
lead to this state is probably effected by local action in the positive plate
when charged, the peroxide losing an atom of oxygen which oxidins the
lead further within the plate, causing it to become chemically active on the
next charge.
Plants suggested and experimented with other methods for obtaining a
similar result to that produced by the first tedious and expensive process.
A galvanic deposit of lead on the plates, and raising the tempei-ature to
hasten the formation by reversals, were tried. In 1882 a rapid formntion
was effected by previously immersing the lead plate.4 in nitric acid, diluted
with one half its volume of water. The plates were immersed for from 24
to 48 hours, and then taken out and thoroughly washed till all traces of the
acid were removed. The plates were then found to be already rendered
porous to some depth, and ooated with lead salts which were easily acted
upon, so that a much larger capacity was found at the first charge, and after
a few reversals the formation was as advanced as the earlier method had
effected in many weeks.
Meanwhile, however, a totally different method of manufacture had
been discovered by Faure, by which a much larger capacity in proportion to
the weight of the plate was obtained, and which first made the storage of
electric power a commercial success. Faure's method consisted in a separate
manufacture of ^he active material by a chemical process, and subsequently
attaching it to the lead plates. A paste of lead peroxide and load sulphate
was made by mixing red lead, or minium, with dilute sulphuric acid, the
chemical reaction ensuing being represented by the formula,
PbjO^ + 2H2SO4 = 2PbS04 + PbO, + 2HaO.
This paste was then spread over the plates, overlaid with slips of parchment
to prevent it falling off. The plates were separated with strips of felt,
rolled together, and immersed in dilute sulphuric acid as in Plante's first
cell. The plate was then immediately ready for use, giving a large capacity
on fii*8t charging. The loid sulphate on the positive plate was converted
into peroxide, that on the negative, as well as the peroxide, reduced to pure
spongy lead. In the modern developments of the Faure cell, or " pasted
plate " type, it is customary to use a paste called litharge (PbO) mixed with
dilute sulphuric acid for the negative plate, thus forming lead sulphate only,
avoiding the necessity of the reduction of peroxide by the first charge.
The main difficulty to be met in the manufacture of pasted plates is to
secure the adhesion of the paste, or active material, to the lead plate, or
framework, which supports it. The active material is subject to considerable
stress owing to its expansion and contraction during the chemical changes
of charge and discharge, which tend to make it break away from its support.
Faure secured adhesion by scoring the plates with deep grooves so that the
paste, which is itself fairly coherent, might be secured to the plates in the
same way as is found effective in plastering. This method was not found
sufficient when large flat plates were introduced, and Elwell and Parker
introduced the system of casting a lead framework in the form of a ** grid,"
or lead network with square perforations, into which the paste was forced.
The paste speedily hardens into pellets, and it was found advantageous to
make the perforations somewhat smaller in the middle of the plate than on
G
98 GENERAL FEATUREa
the surface, giving them an " hour- glass" form, so that the pellets keyed
themselves into the grid. For further security Drake and Gorham intro-
duced the practice of burring over the outer edges of the perforations, by
hammering or rolling, so as to enclose the pellets in a barrel-shaped enclosure.
The pellets expand when the sulphate is formed, keying themselves the more
firmly, but subjecting the positive grid to considerable stress. Pure lead
has but little elasticity, and it is advisable to use an alloy with a small
amount of antimony to give greater elasticity as well as mechanical strength
to the positive plate. A ten per cent, alloy is frequently used for the
positive, and pure lead for the negative grid.
Secondary cells are commonly dassitied as belonging to one or other of
these two classes, the Plants or " formed ** cell, and the Faure or '^ pasted
plate " cell. In the former class is included all those types of cell whei'e the
active material is formed out of the lead plates themselves, which ai*e
reduced to porosity, or the actual surface exposed to the electrolyte is
increased, by mechanical or chemical means. Those in which an electrolytic
deposit of porous lead is obtained from lead salts will also be included, as
the method was first suggested by Plants. The latter class, or Faure type,
includes all those in which the active material is separately made by chemical
means, whether the same as or difierent from that used by Faure, and
subsequently supported by a lead framework. The difference is solely in
the mode of manufacture, the chemical action being exactly the same in
the two classes, and the plates in each case consisting of a oeiiain quantity
of chemically alctive material supported by a head plate or framework.
The chemical action of charge and discharge is summed up in the formula.
Discharged State.
Positive plate. Negative plate.
PbSO^ + HjO • . H,0 + PbS04
Charged State.
Positive plate. NegatiTe plate.
PbO, + HjS04 . . H^O^ + Pb.
As a general rule the Plants type of cells are capable of withstanding
rougher usage, of being more completely discharged without injury, and of
being used for rapid discharges without buckling. But the capacity is
generally smaller for the same weight, and the cost of manufacture for the
same capacity somewhat greater.
Before discussing the details of design and manufacture of some of the
principal types of modern manufacture, a general sketch of the features
common to all or most of them will be convenient at this point.
Large flat plates are used, generally of a standard size, a number of
positive and of negative plates being separately connected in parallel to
increase the capacity and the current that can be used in charging and
discharging. Nearly all manufacturers adopt as the standard size of their
plates, except for small and portable batteries, about eight by nine inches,
giving a total area, for both sides of the plate, of one square foot. For
this area a current of four amperes in charging is found to give the best
results, though it may generally be exceeded, and even doubled, without
injury to the plates, but at the expense of a decrease in the capacity owing
to the rapid formation of the peroxide on the outer surface of the positive
plate preventing access of the action to the interior. With some forms of
cells, where the surface has deep grooves, the actual surface freely exposed
to the electrolyte is much greater, allowing a proportionally rapid charge
and discharge, but at some expense in efficiency.
In order to minimise the internal resistance the plates are brought as
near together as possible without running risk of contact, or short-circuiting
by particles of material falling from thp surfaces, or irregularity of action
owing to the surfaces being nearer at one point than another. The current
will prefer to pass at the points where the plates are close togeth'^r on account
/
DENSITY OP ELECTROLYTE. 99
of tbe reduced resistance, and the slight irregularity in the surface will be
of more consequence as the average distance is reduced. A distance of
from one-quarter to one-half an inch is commonly preferred. In order to
dqualise the action over both sides of the positive plates, it is found abso-
lutely necessary that all of these should have negative plates facing them
on either side. Otherwise the expansion and contraction of the active
material in charging will cause it to break away, and the plates to bend or
<' buckle." The negative plates are not subject to similar strain, so that it
18 found possible, by using one more negative plate than positive, placing
them alternately, with a negative plate at each end of the series, to avoid
injury from this cause, unless an excessive charging or discharging cuirent
is employed.
The E.M.F. of a cell with electrodes of lead peroxide and pure lead, with
pure sulphuric acid as the electrolyte, may be calculated theoretically to be
2.627 ^olt» ; with pure water it should be 1.35. Actual measurements by
Gladstone and Hibbert gave 2.601 and 1.36 respectively. It would be
imposedble to use pure sulphuric acid, owing to the violent chemical action
on the negative plate. The following measurements of the E.M.F. with
dilute acid of various strengths are also given by Gladstone and Hibberti
the measurements being taken with two fully charged pasted plates :
DeDsitj of Electrolyte, Percentage of Pure Acid. E.M.F.
1.045
1.065
1.080
I.I 15
I- 157
I.217
I.2S4
X.333
I- 530
1-750
The specific resistance of dilute sulphuric acid varies very greatly with
the density of the solution, and with the temperature. The resistance is a
minimum with a solution of about 30 per cent, pure acid, for which the
specific resistance is about i ohm at 6° Cent., increases rapidly with either
a stronger or weaker solution, and decreases with a rise of temperature.
The following comparative resistances, representing the minimum by unityi
were given by Kolrausch for solutions of diiferent strengths :
Feroentftge of Sulpbnric Add. Belative BenBtanoei
6.73
1.33
i.oo
I.3S
3-79
7- 29
The density of the electrolyte used in secondary cells is a little under
that which gives the minimum resistance. The chemical action in charging
the cell strengthens the solution by the decomposition of the lead sulphate,
producing additional acid, which is again absorbed in discharge. It is
commonly arranged that the density of the electrolyte should vary from
1. 117 to 1. 121, the E.M.F. of tbe cell thus being about 2 volts, but slightly
higher when the cell is fully charged than when nearly diBcharged. Owing.
however, to the slow difi'usion of the acid liberated during charging the
variation of E.M.F. will be greater than appears from the above tables, and
will need further discussion later on.
•••
6.5
1.887
9.5
1.898
1 1.5
1-915
16.2
1-943
21.7
1.978
29.2
2.048
33-7
2.088
430
2.17
63.0
-^
81.0
•iM.
2.5
•••
15.0
•••
30
•••
50
•••
71
•••
95
...
lOO
INTERNAL BESISTANCEL
The following table will give the specific resistance of sulphorio acid
solutions at different densities and temperatures :
BFSCIFIO RBSI8TAN0E OF SULFHURIO ACID SOLUTIONS.
Specific
Grmvity.
Specific Resifltanoe in Ohms at Temperatni-es
in degree!
1 Oeatigrftde of
o*
4*
8'
I2»
i6'
90»
24*
28"
I. ID
1-37
I.17
1.04
.925
.845
.786
.737
.709
1. 20
1.33
I.II
.926
.792
.666
.567
486
.411
1.25
I-3I
1.09
.896
•743
.624
.509
434
.358
1.30
1.36
1.13
•94
.79
.662
.561
.472
.394
1.40
1.69
1.47
1.30
1.16
1.05
.964
.890
.839
1.50
2.74
2.41
^•I3
1.89
1.72
1. 61
1.52
1.43
1.60
4.82
4.16
3.62
3-"
2.75
2.46
2.21
2.02
1.70
9.41
7.67
6.25
5.12
4.23
3-57
307
2.71
Taking the specific resistance of the electrolyte as an ohm, the dearanoe
between the plates one-half an inch, we may estimate roughly the resistance
between a positive plate and the two adjacent negative plates. Taking the
area i square foot, the resistance works out as .0014 ohm, so that with a
current of 4 amperes this would account for a reduction of the difference of
potential of only .0056 volt in discharging below the proper E.M.F. of the cell,
and a corresponding extra E.M.F. would be required to drive the current
through in charging. A difference of about .011 volt would thus be required
between the E.M.F. in charging and discharging. The actual difference
observed between the E.M.F. required for charging at the normal rate and
the E.M.F. of discharge, is never much less than one-tenth of a volt, showing
that the actual distance between the plates only accounts for a small part of
the internal resistance of the cell, and little is gained by attempts to reduce
it. The main resistance is to be found in reaching the interior active parts,
through the pores of the lead or peroxide, where the strength of the electro-
lyte, owing to the slow diffusion as the acid is formed or absorbed, is stronger
or weaker than that between the plates, either of which causes will increase
the resistance. This increase of the internal resistance through slow diffusion
will be intensified with a rapid charge or discharge. Ayrton found the
resistance of an E.P.S. pasted cell to average .011 ohm per positive plate
during a charge with 3.787 amperes; and .009 ohm during a charge with
4.205 amperes, rising, in either case, towards the end of the operation.
A secondary cell is fully charged when all the lead sulphate on the positive
plate to which the electrolyte has access is converted into lead peroxide, and
the negative plate is reduced to pure lead. A continuation of the charging
current liberates oxygen and hydrogen from the positive and negative plates
respectively, the former no longer combining with the lead, the gases rising
as bubbles, the surface of the electrolyte assumes a milky appearance, and
the cell is said to '^ boil." This boiling was formerly thought to be harmful
to the plates, as "well as a waste of energy, but experience has shown that it
is not only harmless but distinctly beneficial occasionally to continue the
charging current for some time after the plates are fully charged, so as to
reduce or remove certain insoluble coai pounds which are formed during
discharge, and are injurious to the plates. Boiling should be continued for
some time once a week when the cells are in constant use, and will be found
well worth the expenditure of power ; but the cui'rent should be only about
half the normal charging current.
The decomposition of lead peroxide is chemically equivalent to the passage
of .227 ampere-hour per gramme reduced to lead monoxide, or to nearly
CAPACITY AND EFFICIENCY. lOI
loo ampere-hours per pouDd. It appears that in practice a capacity of
on!}* six to twelve ampere-hours per pound of positive piate is obtained with
cells used for electric lighting, though in cells specially designed for port-
ability a somewhat higher capacity can be obtained. It follows that the
proportion of really active material is very small, and considerable improve-
ment is theoretically possible. But it may be remarked that capRcity and
efBciency are totally different matters, and except where portability or the
saving of space is a paramount consideration, the increase of weight due to
the addition of a considemble quantity of such a cheap material as lead is
subsidiary to the question of efficiency and durability of the plates.
According to chemical tests made by Robertson with pellets forced out
of the grids of a pasted plate, only about a half of the peroxide was decom-
posed in a complete discharge at the rate of four amperes per positive plate,
the lead sulphate forming an impervious coating round the remainder that
protects it from further action. In a non -pasted plate (formed by the Plants
process) it is probable that a more complete decomposition takes place, but
the nominally active material generally forms a smaller proportion of the
total mass. A more rapid discharge results in still less complete decom-
position, and the capfiusity of the cell is greatly reduced. At the normal
rate of charging, four amperes per positive plate of one square foot total
area of surface, the time of charging is most commonly arranged to be about
twelve hours. Higher rates, up to even twenty amperes, may be employed,
when by grooving or other means the actual surface, that is to say, the free
external surface in contact with the electrolyte, is increased. Discharged at
the same rate, the cell will maintain the current from nine to ten hours ;
doubling the discharging current will reduce the time of discharge to about
three hours, thus returning only about seventy-five per cent, of the current
integral, or the number of ampere-hours discharged at the slower rate.
Some forms of cells are adapted to give very high rates of discharge without
injury, and may be discharged in one hour with nearly five times the
normal current of charging, but the capacity (of discharge) is thus only
about one-half of that when discharged slowly. It does not follow that the
efficiency of storage is in the latter case reduced fifty per cent., since the
quantity of peroxide remaining undecomposed will shorten the time of
renewed charging.
Owing to the variation of the extent of the chemical action according to
the rate of discharge the efficiency of the cell can only be determined
correctly by taking the average in actual practice after many operations of
charging and discharging. With a unifoim slow rate of charge and dis-
charge there will be a difference of potential in discharge below that of
charging of upwards of .i volt, or about five per cent., corresponding to the
lo88 of energy owing to the internal resistance of the cell. With lai'ger
currents this loss will be greatly increased, rather more than in proportion
to the square of the current, as the resistance also will be increased by the
concenti-ation of acid on the plates in charge, and weakening of the acid
solution in discharge. A further loss will be realised in the reduction of
the number of ampere-hours, or the current integral, obtained in discharge
below that absorbed in the charge. The corresponding energy will be
waited in irregular chemical actions, the evolution of gas, &c. In practical
work a total efficiency of about eighty per cent, may be consiilered good,
and is obtained where the cells are under careful supervision, and kept in
good order. The efficiency often falls much below this when rapid charging
and discharging is frequently called for.
The extent of the charge remaining in the cell at any time may be esti-
mated in three different ways by the attendant. In the first place the
colour of the plates will, to an experienced eye, give a rough indication.
I02
ESTIMATION OF CHARGE.
When the cell is fully charged the positive plate assumes a deep black, and
the negative a grey slate-coloured surface. In dischargiog the positive
tones down to a dark plum colour, and then into a brown as the whole
surface is reduced to lead sulphate, the colour of both plates beiug the same
when the cell is discharged. Another more satisfactory criterion is the
density of the acid solution, which should be of specific gravity, about 1.2 1
when the cell is charged, and should never fall below 1.17. A handy form
of hydrometer to measure the density of the electrolyte is a necessary
instrument in the second battery room. Fig. 33 shows three forms supplied
by Drake and Gorham for this purpose. The ordinary floating hydrometer
has the objection that an unskilled attendant is not always to be trusted to
read a finely graduated scale correctly, and it is only available for reading
the density on the surface of the electrol3rte, which is often through the
slow diffusion very different from that at some depth, or between the plates.
A more suitable form is a narrow tube which can be lowered down between
the plates and filled with a sample of the liquid from any desired depth. In
this tube small differently-coloured glass balls are sunk, carefully adjusted
Fig. 33.
Storage-cell Hydrometers.
to rise with different densities of the sample liquid (viz., 1.105, i 170, 1.190,
and 1.200), which is a convenient and unmistakable, if not a very accurate,
method of measurement.
The third means of indication of the state of charge is the E.M.F. of the
cell. This will be fairly cf)nstant during the greater part of the time in
charging or discharging at moderate rates, rising or falling uniformly
between the limits of 2 and 2.1 volts, the actual difi'erence of potential being
in excess of this during the charging by about .05 volt or so according to
the internal resistance ; and a similar decrease when discharging. Towards
the end of the charge the E.M.F. rises rapidly, accompanied by more and
more vigorous "boiling" until an E.M.F. as high as 2.58 or 2.60 may be
reached. At the beginning of the discharge the E.M.F. falls to its more
moderate value in a few minutes, and then decreases very slowly for many
hours. After reaching two volts the fall is more rapid, falling a further
tenth of a volt in an hour or so, and then completely dischargii g in a few
minutes. It is likely to prove most injurious to the plates if the diccharge
is made complete, and it is generally considered inadvisable to allow the
E.M.F. of the cell to fall below 1.9 volts, or the actual difference of potential
between the terminals below 1.85 volts in slow discharging (1.8 may be
allowed in a rapid discharge). Very little of the capacity is lost by this
restriction, and the deleterious action that will follow further discharge will
appear from the following discussion of the chemical action.
The chemistry of secondary cells was investigated with some care by
Gladstone and Tribe in 1882, their results being published in some papers
in Nat/ure ; more complete investigations by Gladstone and Hibbert are
recorded in a paper read before the Institution of Electrical Engineers in
CHEMICAL ACTIONS. IO3
1892, to which we shall be iDdebted for much of what follows. In this
paper it was most conclusively proved that the variations in the E.M.F. of
the secondary cell must be attributed entirely to the varying quantity of
acid in the electrolyte during charge and discharge, greatly intensified by
the slowness of diffusion which causes great variations of the density of the
electrolyte close to the surface, and within the pores of the plates.
At the termination of the charge, especially if rapid and carried up to
the highest limit, a film of the strongest^ almost pure, add covers the
surface of the positive plate, and may actually be seen descending by its
heavier weight and flowing round the bottom of the plate. After the
charging current has been stopped the evolution of oxygen stOl goes on ;
this is due to the presence of persulphuric acid and hydrogen dioxide. These
while liberating oxygen also react on the peroxide, reducing it and liberating
further oxygen. Local action with the lead framework also goes on to some
extent, and thus by diffusion, local action, and reduction of the lead peroxide,
the acid strength of the electrolyte within the pores of the positive plate is
weakened. On the negative plate the acid is also veiy strong, and a slow
action on the lead, liberating hydrogen and forming lead sulphate, proceeds.
Theee actions increase the capacity of the cell. If the discharge imme<
diately follows on the cessation of the charge, an E.M.F.y very nearly that of
a pure acid cell (a.6 volts), is obtained, but speedily falls as the add strength
is reduced, this occupying only a few minutes. The higher E.M.F. may,
however, be restored by changing the plates to a vessel containing a stronger
add solution. It was formerly thought that the higher E.M.F. obtained
after boiling was due to the hydrogen bubbles occluded in the negative plate,
but the length of time the E.M.F. is maintained in discharging shows this
explanation to be insuffident. It had also been suggested by Qladstone and
Tribe that the higher E.M.F. was caused by the active oxygen produced by
decomposition of the persulphuric add.
After a careful chemical analysis of pellets of active material forced
from the grids of a pasted plate cell at intervals during the charge and
discharge at the normal rate, and from different parts of the plates,
Kobertson summed up his results in the following statements :
" (a) The particles of the peroxide very soon get coated in the discharge
with a layer of lead sulphatOi which protects the peroxide from further
action.
'* (b) The analysis shows also that a large proportion of active iriaterial
is still remaining at the end of the discharge.
'* (c) The loose powdery surface of the positive plate seems to be thoroughly
converted into lead sulphate.
*' (d) When the peroxide on the surface of the positive plate falls to about
31 percent, the cell loses its E.M.F. very rapidly, owing to the inactive
layer of sulphate impeding the action of the sulphuric acid on the active
material behind it, and also to the formation of peroxide on the negative.
The Miffusivity' of the acid is also then increasing, while it has to
penetrate further into the plate to find active material. When the whole
of the paste appix)aches this composition of 31 per cent, peroxide, the cell
loses its E.M.F. entirely.
"(e) The action seems to take place most rapidly where the current
density is greatest; the plate gets hard there from sulphate soonest on
discharge, and oxidises quickest on charge."
During the discharge of the cell, especially if rapid, the electrolyte on
the surface and within the pores of both plates is much weakened by the
combination of the acid with the lead monoxide to form lead sulphate and
water,
HgS04 + PbO = PbS04 + H,0.
I04 " SULPHATING."
The acid strength must be restored by diffusion from the fluid between the
plates, but this will frequently be so slow that the internal resistance is
much increased. Furthermore a mischievous chemical action is likely to
follow, owing to the deficit of acid, in the formation of a white basic sulphate,
PbSO^PbO or PboSOg. This action is known technically as " sulphating/'
and also ensues if the cell is left standing for a long period in a dischai'ged
state, whether the add strength of the electrolyte be weak or strong. This
basic sulphate is insoluble, a bad conductor, and when formed between the
active material and the supporting lead is liable to detach the former. The
only effective way to remove it is to continue the charging for some timo
with a moderate current (about two-thirds the normal) after the cells begiu
to boil. The basic sulphate then falls off, or is loosened and can be scraped
away in white flakes. To avoid the formation it is advisable to maintain a
constant circulation of the electrolyte during rapid discharges, and on no
account to leave the cells in a discharged, or even a partially discharged
state for long periods. The formation of the basic sulphate may be checked
by the addition to the electrolyte of a small quantity of caustic soda ; Sir
David Salomons recommends the addition of one ounce of solid caustic
soda (previously dissolved) to every five gallons of the electrolyte. Other
remedies found effective are potassium sulphate, sodium sulphate, and
oxalic acid.
To illustrate the variation of the internal resistance during discharge
we may give the following measurements by Ayrton : After over-charging
for some time the internal resistance had risen to .0115 ohm, the actual
E.M.F. of the cell being 2.30 volts. After standing for some time to allow
fK)mplete diffusion of the acid the resistance fell to .0038 ohm, the E.M.F.
being then 2.06 volts. During discharge at the normal rate the resistance
remained fairly coustant. When towards the end of the discharge the
E.M.F. had fallen to 1.95 volts the resistance had risen to .0055 ohnu
Iieading Types of Acoumnlators.
In dealing with a selection of the principal types of secondary cell, those
of the Faure, or pasted plate type, will be considered first, as it was by this
method of manufacture that cells were first made of suflicient capacity, and
sufficiently cheaply to be of value for electric lighting. The manufacture of
pasted plates has been xintil recently a monopoly in Great Britain, held by
the Electric Power Storage (E.P.S.) Company. The most important of the
patents held by this company have recently lapsed, but it is doubted
whether the inventions will be made much use of by others, as the monopoly
has encouraged the development of many forms of the Plants type, so as to
bring them into vigorous competition, and it is still impossible to say which
type is intrinsically the better.
The " grid " lead framework, or lead plate with square perforations into
which the paste is forced so as to form " pellets," was adopted at a very
early period, and still remains the standard form of the E.P.S. cell. This
shape certainly will give the maximum proportion of active material to the
whole mass of the plate, and for cells under good management, charged and
discharged at moderate rates, can scarcely be bettered. But it is open to a
serious objection when the cell is to be subject to very rapid discharges.
The conductivity of the active material is very low, especially when reduced
to sulphate, and the current has to find its way by the lead grid from the
active material in different parts of the plate to the connections, this path
being of small sectional area, and much longer from some parts than it
would be in a non- perforated plate. Now it has of recent years been
recognised that one of the most valuable uses of secondary batteries in
E.P.S. CELLS. 105
central station eupplj is that it may occasionally be used as a "stand-by,"
to maintain the supply for a short time during the night and in the event
of a breakdown of the generating plant. For this purpose a type of cell
capable of being discharged at a very rapid rate is needed, high efficieni^
being a minor object. The oells designed for this purpura have for the
positive plate a massive lead casting with rows of horizontal ridges on
either face. These ridges, or shelves, are turned upwards, and the pai^te
Fig. 34.
Secondair Batteries— Electric Power Stora^ Cells.
packed between them. The actual surface, owing to the ridges, is much
larger than with the grid type, and the rettietance of the plate itself is
greatly reduced : the normal rate of discharge is 8 amperes per positive
plate (instead of 4 smpires, as with the flat-surface grid plutew), and this
may be greatly increased without injuring them. For tlie negative plate a
light grid is cast, with very large [lerforations in which the litharge paste
is further secured by projecting lead prongs. This cell is known as the K,
or central station type, it being cii.stomary among secondary cell manu-
facturers to denote their different types by diUinguisliing letters. The
L type, that adapted for etatiorary batteries to be charged at moderate and
efficient rates, has plates with the grid framework as previously desciibed.
Pig. 34 sho'mt three L type cells in gla.<n boxes connected in series, and
mounted in the customary manner on wooden shelves, the wood being
thoroughly coated with shellac varnish to protect it from tbe acid fumes
I06 E.P.3. CELLS.
which are mvariably prosent in a secondary battery room. The rims of the
glass boxes are also coated with shellac varnish to prevent creeping. The
cells are placed in wooden trays containing sawdust, and rest on " mush-
room " insulators. Fig. 35 shows a single cell of the C type, a modification
of the L adapted for portability ; tm for such purposes as train lighting, the
cell being completaly enclosed in a lead-lined wooden box, and occupying a
small apace, the plates are longer and narrower than in the K and L types,
and thus less subject to injury through vibration.
The method ci supporting the plates in the cell also differs oonsiderably
in the various types. In the L type the negative plates are cast with pro-
jectiog lugs at three of their comers. These are for all the negative pktes
in one cell secured by " lead burning " to strips of lead, one of which forms
the connecting piece for the next cell ; the other two form bases upon which
the system rests, supported about i^ inches above the bottom of the cells
upon blocks of parafSued wood. Two additional strips upon the sides of the
plates about half-way up secure additional rigidity, and also form shelves
Portable Storage Cell.
upon which the system of positive plates is Hupported. The positive plates
have lugs at one corner by which they are burnt on to the same connecting
strip, and have projections from their sides by which they are supjiorted,
these resting upon ebonite shoes supported by the above-mentioned shelves
attached to the system of negative plates.
The clearance between the plates is further secured by long ebonite forks,
or U-shaped pieces, two of which are pushed over each positive plate, and
prevent any possibility of contact. The tops of these can be seen in Fig. 34,
and the method of interconnection is illustrated. The cells must be
mounted so that the positive and negative connecting strips are alternately
reversed in position.
In this method of arranging the cells of the L type it will be seen that
the path of the current in the lead connecting strips is unnecessarily long.
An improvement is effected in the K type, in which the connections are
made directly over the adjacent edges on the cells. The positive plates are
here suspended from two masi^ive lead bars supported upon the negative
plates through the medium of insulating blocks. The negative plates
are connected to two similar bars underneath. I^aige currents can thus
be carried with safety and little lot's, the conducting bars being much
shorter as well as stouter. Another advantage is that the electrical action
is better distributed over the plates, owing to the current having necessarily
BATE OF CHARGE AND DISCHARQE. 10/
to pass from top to bottom over either the positive or negative plates, and
the length of the current circuit in the plates is thus the same at whatever
point it mav pass through the electrolyte.
The plates are made for the K and L types of a uniform size of 8^
by 9I inches, and the number is from 7 to 33, the maximum rate of dis'^
charge being 64 amperes in the L type, and 135 in the K type. For
central stations where high rates of discharge are called for a special cell
of the K type is supplied, giving discharges up to 1300 amperes. These
are supplied in lead boxes, and are really seversd cells placed in parallel, the
positive plates of each of which can be separately removed for inspection.
The largest size supplied has 145 plates arranged as 5 sets, and can supply
1300 amperes for one hour, or 370 amperes for 7 hours, thb formal
lute of diarging being 570 amperes. It may here be noted that if
single cells be connected in parallel a difficulty may arise if the plates
are in separate boxes, so that the acid Folution may become stronger
in one than the other. A consequent difference in electromotive force
may cause a discharge of one cell into the other. The practice of
connecting two separate cells in parallel, especially when the arrange-
ment is temporary, is reprehensible. In the Central Station E.P.S. cells
the arrangement is, of course, equivalent to an extra number of plates in
the same cell, the electrolyte being the same. The arrangement of two
complete batteries in parallel, the electromotive forces being measured and
found the same, is on the other hand, sometimes a source of safety. In
the event of the circuit of one battery being broken, as by the fracture of
a glass box and consequent escape of the electrolyte, the electromotive
force of the charging dynamo if shunt-wound may suddenly rise consider-
ably, and if lamps in parallel circuit are being supplied at the same time
the result may be disastrous. A second battery in parallel will check this.
Sir David Salamons mentions a case in which the lamps in a private
installation were thus saved from destruction. The electromotive foroe
would, except for the second battery, have risen about 20 per cent.,
sufficient to destroy the lamps speedily, but not to blow the fuses.
The capacity of a cell depends partly upon the rate of discharge, for a
rapid discharge causes the peroxide to be convei*ted into sulphate upon the
surface of the active material, forming a coating impervious to the acid, and
preventing access to the peroxide in the interior. Greater capacity as well
as efficiency is therefore obtained by slow discharge. But if the discharge
is too slow inconveniences may arise. The slow deposit of pure lead upon
the negative plate tends to form fine needles of lead which, even if they do
not short-circuit the plate themselves, endanger a short circuit by pre*
venting any basic sulphate or other material from falling freely to the bottom
of tbe cell. The most satisfactory rate of discharge is a little more than
half the best charging rate (4 or 8 amperes per positive plate). Taking
the capacity of a 7 hours discbarge as 100, that of a 5 hours discharge is
about 90, 3 hours 75, and one hour 50. At its maximum rate of discharge,
4 amperes per possitive plate, a ceil of tbe L type should maintain the
current for 10 hours. The total weight of the cell in glass box is about
10 lbs. per plate of both kinds, so that the capacity is a little over 2 ampere-
hours per pound of total weight, or about 80 lbs. per horse-power hour.
In the K type the maximum rate of discharge is given as a little over
8 amperes per positive plate, and at this rate the discharge should last
3^ hours. At one half the rate the discharge should last nearly 9 hours.
The total weight of the cell in glass box is about 15 per cent, greater than
that of a cell of the L type with tbe same number of plates. The width
of the cells in glass boxes in the K and L type is from 11^ to iif inches,
for the central station cells from 2 to 5 feet according to the number of sets
I08 OTHER TYPES OF PASTED PLATE.
of plates. The length varies from 5 to iQf inches according to the number
of plates. The height of the cells over all is i6f inches in glass boxes, and
about 30 inches in lead lined wood boxes. It is customary when mounting
them in tiers to leave a clearance of at least 14 inches between the top of
the connections of the lower and the wooden supports of the upper tier.
When this is done, and glass boxes are used, the cells being connected so
that the plates stand in planes at right angles to the line of the shelves and the
interconnections, constant inspection is easy, it being possible to watch the
surfaces of the plates from the side, and obtain access from above for
hydrometer tests, kc,
A few other designs of supporting framewort for the Faure paste may
now be mentioned. Reckenzaun moulded the paste into short pencils,
three-sixteenths of an inch in diameter, and a little over an inch in length,
these pencils being partially buried in a solid lead plate, and in a horizontal
position. Tending to expand, when the cell is charged, in the direction of
their length, they are said to be less likely to cause the buckling of the
plate, or to be wrenched out if the plate is buckled. The Pitkin accumu-
lator, intended only for small portable hand lamps, uses thin lead plates
studded with small lead pins, with flat heads, which are found sufficient to
retain the paste on a small plate, and gives very great capacity in proportion
to the weight of the cell. Others have used a porous covering material to
prevent the active material from falling off the plate, as was done by both
Plants and Faure in the first secondary cells with strips of felt. Carrie,
using rods in the place of plates, protected them by asbestos tubes. In the
Hatch accumulator corrugated lead plates are used, the corrugations being
filled in with paste, and the plates separated by porous earthenware, which
is permeated by the electrolyte, thus making a '' solid " ceU. The objection
to these methods is the comparatively high internal resistance of the cell,
especially when a rapid discharge is used, the diffusion of the acid being
extremely slow. For portable accumulatora they may be valuable, as the
ordinary types are generally injured by vibration.
The Chloride accumulator must be classed as of the Faure type since the
active material is made separately, and not out of the lead-supporting frame-
work, though it is made by a totally different process to that of Faure.
Fused Chloride of lead is poured into moulds so as to form discs, or buttons
with rounded edges, and these are set in lead plates. The chloiide of lead
is then reduced to pure spongv lead by using it as the positive plate of a
primary battery, with a zinc plate as the negative, and dilute chloride of
zinc as the electrolyte. This battery is short-circuited, and the hydrogen
formed on the positive plate combines with the chlorine, reducing the
buttons to a mass of spongy lead. A slow preliminary charging in dilute
nitric acid frees the pores, and great capacity is obtained. The ordinary (R)
type of cells for electric lighting supplied by the Chloride Electrical Storage
Syndicate gives about 6 ampere-hours discharge per pound of positive plate
in a 9-hour discharge, a little more than half this with a one-hour discharge.
The normal rate of charging is from 13 to 20 amperes per positive plate, of
the standard size.
Efforts have been made to do away with the supporting lead framework,
using the active material alone, and thus effecting a great saving in weight.
The difficulties are that the specific resistance of the active material is very
great, especially when reduced to sulphate, and that its coherency and
mechanical strength are not sufficient to stand the strains of expansion and
contraction in the processes of charging and discharging. The coherency
depends mainly on the presence of the sulphate, so that the positive plate
would easily break up when fully charged. Efforts have also been maae to
replace the lead framework with one of some metal of material of better
FLANTE TYPEa I09
oonductivity and mecbanical sti'eDgth — iron, copper or aluminium, of course
covered completely with lead to prevent access of the electrolyte and forma-
tion of a primary couple. This protecting coating of lead seems to be
difficult to maintain at all points, especially in the positive plates, and only
for the negative has the device in any way succeeded. In very small plateH
it has been found possible to combine the Faure paste with other materials
so as to obtain sufficient coherency for it to be used alone. In the Bristol
accumulator a binding material, such as animal hair, asbestos, fibre, &c., is
used. Fitzgerald mixod the paste with glycerine to give greater coherency,
subsequently using sulphate of ammonia. The latter method produced the
material known as Lithanode, which has been largely used for the positive
plates of small portable batteiies alone, and for large cells has been used in
the form of flat cakes, several being combined in a light lead framework to
form a plate of large size. Litharge (PbO) is mixed with a semi-saturated
solution of sulphate of ammonia, moulded into small plates under great
hydraulic pressure. When thoroughly set and hard it is coated with per-
oxide of lead in a semi-fluid condition, and subjected to a slow electrolytic
process of formation in a bath of sulphate of magnesia. The resulting
material is almost entirely peroxide of lead, and the electrical capacity is as
high as one ampere-hour per ounce. Lithanode is only used for the positive
plate, a spongy lead plate being used as the negative.
Another self-supporting paste plate was manufactured by Hering in the
following manner. A dry mixture of powdered peroxide, minium, and lead
carbonate or sulphate, is mixed with a solution of acetate of lead, and
kneaded to a stiff paste. This is pressed into a mould and dried in an oven ;
hardened by immersion in sulphuric acid, and formed by charging in a
solution of sodium or potassium sulphate. A further electrolytic deposit of
lead peroxide from nitrate of lead produced ax^ extremely hard surface.
In the *' Plants " types of cells the active material is formed out of the
lead plate itself, and either chemical or mechanical methods, or both, are
used to increase the effective area of the plate worked upon, and to hasten
the electrolytic process of *' formation."
The Epstein Co. manufacture a type of cell which is claimed to be
unequalled for mechanical strength and durability, and to requii*e no skilled
attention. Very thick lead plates are used, which are deeply grooved
horizontally, so that the actual area exposed to the fluid is multiplied by
^ve. To basten the process of formation by " reversals," as in the original
Plants cell, the plates are previously boiled in a one-per-cent. solution of
nitric acid, which renders the lead -plate porous to some depth. The plates
then assume a dull greyish appearance due to the formation of lead salts,
and are thoroughly washed to remove the traces of nitric acid. They
receive a very strong first charge, and the capacity is somewhat increased
by a few reversals. The active material thus formed tends on expansion to
key itself between the ledges or corrugations and then to adhere very firmly
to the plate. The resistance of the plates themselves is very low, and the
acid has free access to all parts of the active material, which forms a com-
paratively thin layer over the whole surface. The cell is capable of very
rapid charge or discharge without injury. Since the area of the corrugated
surface is 5 times that of a corre«iponding plane surface, a rate of charge of
20 amperes per positive plate (8" x 9") and a rate of discharge of 26 amperes
per positive plate ai-e specified as the normal rates. The positive plates are more
massive than the negative. The M type, suitable for stationary lighting
installations is made in sizes from 3 to 31 plates of the ordinary size. For
the smaller sizes (up to 9 plates) glass boxes are used, and for the larger
lead boxes, since constant inspection of the plate surface*^ is not considered
necessary. The capacity of a 31 -plate cell is 2880 ampere-hours in a 12-
no RIBBON PLATES.
hour discharKe, or 1800 ampere-hours in a 3-hour discharge. The weight
of the lead pTatee ia 575 lbs., and of the complete cell 1 1 23 lbs., eo that a
maximam capacity of about 6J ampere-horn's per pound of lead, or about 8^
ampere-hours per pound of total weight is attained.
The D.P. secondary cell, a design due to Dujardin, Drake, and Oorbam,
employs plates built up of strips of thin lead ribbon, placed horJEontally,
the ends burnt together, and held in a framework of lead and antimony
alloy. The strips of lead-ribbon have points or projections on their faces,
preventing contact except at these pointij and allowing the electroljrte to
CBOldPTON-HOWELL C£LLS. Ill
permeate the intermediate space. Thus a very large initial surface is pro-
vided for the electrolytic action, and the efTectiye area is increased by a few
reversals. The cell is capable of giving a very rapid discharge, and the
capacity per pound of lead used in the positive plate is Z2 ampere-hours,
' which 18 one of the highest obtained with plates of large size. The De
Kabbath accumulator is similar save that the lead ribbon is placed vertically,
and alternate strips are corrugated to allow the electrolyte to permeate the
plate.
The Crompton-Howell secondary cell illustrates another mechanical
method of increasing the effective area by producing a porous plate. The
special object desired by the manufacturers of these cells is to construct one
suitable to the requirements of central station practice, where durability,
freedom from possible failure, and capacity for rapid discharge in
an emergency is of more importance than high efficiency, or great
capacity in proportion to the weight. Lead plates, porous Viroughout^ but
yet of sufficient mechanical strength, are manufactured as follows. A
quantity of molten lead is allowed to cool slowly, and begins to solidify in
the form of crystals o£ lead at the bottom of the cauldron. These crystals
are removed as they are formed by means of perforated ladles. Large iron
moulds are then filled with the crystals, and the addition of a certain
quantity of molten lead causes the whole mass to solidify in the form of a
Bohd block of porous lead. These blocks are approximately 2 feet long by
10 inches square, and are out into slices of ^ inch thick, which are trimmed
down to 8^ inches square.
The plates are then about 10 lbs. in weight, and their capacity about
30 ampere-hours when discharged in 5 hours, or about 15 ampere-hours
when discharged in i hour. The cells are made in a gradation of size up to
121 plates, the latter giving a total capacity, with slow discharge, of 3630
ampere-hours, or supplying a current of 181 5 amperes for i hour, the total
weight of the cell being (in lead box) 2300 lbs. Fig. 36 shows a central
station battery room (Netting Hill) employing Grompton-Howell cells. As
constant inspection is not considered necessary, lead boxes are preferred, and
the cells are placed so that their plates stand lengthways along the line of
cell-connections, and mounted in the following simple manner, which gives
the minimum length of path in the lead-connections. « Combs of celluloid at
the top and bottom of the plates maintain the clearance between them
(about half an inch), the plates being let in between the teeth of the combs.
Very ponderous lugs are burnt on to the comers of the plates, and to large
bars which make the connection with the next cell over the adjacent
edges.
A secondary cell of higher E.M.F., about 2.5 volts, may be obtained by
the use of a zinc plate for the negative, and a peroxide of lead plate for the
positive. The zinc plate must of course be of pure zinc, or carefully
''amalgamated " with mercury to prevent local action. In the discharge
zinc sulphate is formed, which is soluble, and the zinc is re-deposited in
charging. The principal objection to this cell is the high internal resistance
obtained towards the end of the discharge owing to the zinc sulphate in
solution, and at present it has not come into extensive use, except in small
portable batteries.
Use of Seoondary Batteries for laightlng.
Where loo-volt lamps are used in parallel circuit, 50 cells connected in
series will, at the normal rate of discharge, maintain the requisite difference
of potential between the mains. But a surplus will in general be required
to raise the E.M.F. somewhat, in order to compensate for the fall of potential
112 THE CHARGING DYNAMO.
to distant mains, and if the cells are to be allowed to discharge to the point
at which their E.M.F. is 1.9 volts, 55 cells in series will be necessary to
keep the difference of potential at the terminals of the battery at 104.5 ▼ol^
at the end of the discharge. It will also be advisable to have two or three
extra calls to replace any found defective, and that must be removed from
the circuit for repairs. To charge the battery a dynamo will be required
whose E.M.F. can be made to rainge between 100 volts and 125 volts (the
latter giving 2.25 volts per cell) in ordinary working, and should be able to
give a still higher E.M.F. when required, so as to cause the cells to " boil "
occasionally, in order to get rid of the white sulphate.
A shunt-wound dynamo is the most suitable for charging a secondary
battery, for more than one reason. Firstly, the falling characteristic, giving
a lower E.M.F. should the current increase, evidently tends towards the
condition for a constant current in charging. Secondly, the shunt-coils now
being connected across the terminals of the battery as well as across the
terminals of the dynamo, the current in them wDl be supplied by the
battery, without much decrease, even should the E.M.F. produced in
the dynamo armature fall below the E.M.F. of the cells, owing to the rise of
the latter during charging and any slackening in the speed of the driving
engine. In this event a small current will also be driven from the battery
through the armature, which being in the opposite direction to the E.M.F.
produced by its rotation wiil drive it as a motor, supplementing the driving
power so as to maintain the speed. With anjrthing short of a complete
break-down of the engine this motor current will be small, and a slow dis-
charge of the battery will proceed until the defect is attended to (by speed-
ing the engine or reducing the resistance in circuit with the shunt-coils to
strengthen the magnetisation of the field-magnets) 'so that the E.M.F. in
the armature may rise above that of the battery and the charging proceed.
On the other hand, a series- wound dynamo will depend for its magnetisation
upon the maintenance of the charging current, and when this is reduced by
the rising E.M.F. of the battery, the E.M.F. of the dynamo also falls. The
E.M.F. and current thus reducing together the dynamo is speedily demag-
netised, and re-magnetised in the reverse direction by a discharge, which
will become more rapid owing to the reversed E.M.F. of the dynamo. The
plates would probably be seriously injured by the rapid discharge, even
before a fuse could blow, and it would be difficult to regulate the dynamo
for constant current to prevent this happening. A compound-wound dynamo
would have the same objection, especially if " over-compounded," though of
course in a less degree, and could only be used where constant attention is
supplied.
In some small installations, for lighting small country houses, for example,
the most convenient practice is to store the whole of the power, running the
engine only in the day-time to charge the battery, and supplying only from the
battery at night. No difficulty then arises from the variation of E.M.F. in
charging, which may be effected with all the cells in circuit till the E.M.F.
rises to 125 volts (and occasionally higher). If a few lamps be required to
bum during charging, they may be connected across part of the battery
(from 50 to 45 cells, reducing the number as the E.M.F. rises), this part of
the battery receiving a slightly reduced charge. In commencing the dis-
charge 50 cells, or less, will give the requisite E.M.F., and the extra cells
must be switched in one by one as the E.M.F. falls. It is well to arrange
the battery that these extra cells are interchanged with others daily, as they
will be only partially discharged, and on re-charging will begin to boil before
the others. By doing this systematically all the cells will undergo boiling
in turn. The dynamo may be placed in parallel with the battery, at the
time of the heaviest load, and the distribution of the output with a shunt-
TKAIN LIGHTINa. 113
wound dynamo will depend on the relation of the E.M.F. in the armature
to that in the cells : the load may therefore be divided in any desired pro-
portion by altering the speed of the dynamo or its magnetisation. It will
not, however, be possible to proceed with the charging of the whole battery
while supplying incandescent lamps directly from the dynamo terminalsi as
the charging will require an excess E.M.F. Part of the battery, 45 to 50
cells, could be charged while lighting if the dynamo be of sufficient capacity;
but this would entail frequent changing of the connections to ensure the
charging of alL The value of the secondary battery thus used is : firstly,
to increase the total output; secondly, to allow periods of rest for the
generating plant, and perhaps to take its place altogether in the event of a
breakdown ; and thirdly, it a^ords the simplest and best method of regulating
the E.M.F. in a small installation, where attendance is reduced to a mini-
mum ; gas and oil engines, which are frequently used in these small instal-
lations, are often irregular in speed, and for this irregularity the oompoundiog
of the dynamo will not compensate.
When it is desired to proceed with the charging of the battery while
supplying a moderate number of lamps directly from the dynamo, so as to
minimise the time during which the latter need be run, a conveni«'nt method
is to use a small dynamo in series with that supplying the lighting circuit,
its E.M.F. being about 25 per cent, of the larger so as to supply the extra
E.M.F. for the whole battery. A continuous-current transformer, having a
reducing ratio of about 4 to i, from 100 to 25 volts, is most convenient. The
dynamo armature of the transformer is connected in series with the generator,
while the motor armature is connected in parallel ; this is better than direct
multiplication in ratio 100 : 125 for obvious reasons.
In cases where the secondary battery is used as a reservoir of power,
never discharging for long perioos so as to vary greatly in E.M.F., it may
be worked in parallel with the dynamo when running so as to regulate the
E.M.F., being charged or discharging slowly without more than the per-
missible variation allowed for incandescent lamps, and so needing no dis-
connection for a separate charging. An example is given by Stone's system
of lighting for railway carriages. Each cairiage has its own generating
plant, complete in itself and automatic, a small dynamo being driven by a
belt from a pulley on one of the axles of the carriage, with a secondary
battery to act as a reservoir of power during stoppages, or with speeds too
slow to maintain the requisite E.M.F. in the dynamo. The dynamo is
suspended from the floor of the carriage by one corner of its frame, and by
means of an adjustable link in such a manner that the dynamo is free to move
towards or away from the driving pulley' on the axle, with which it is on a
leveL The suspending link is provided with an adjusting screw which
oUows the tension on the belt to be varied when required, and to be so
adjusted that the belt begins to slip when a certain load is thrown on the
dynamo. A centrifugal governor automatically switches the dynamo into
parallel with the battery when the speed exceeds a certain amount (corre-
sponding to a train speed of 15 miles an hour, or more or less as required).
At this speed the E.M.F. of the dynamo is calculated to be that required by
the lamps in the carriage (40 volts). A further increase in speed will cause
the S.M.F. to rise, the lamps burning somewhat brighter than the normal,
and a charging current being driven through the battery. When the E.M.F.
has risen as high as is permissible, it is arranged that the belt should begin
to slip^ and very little further power can be developed by the dynamo however
much the speed of the train may increase. According to certified tests, the
speed of the dynamo seems to be maintained with marvellous constancy
under the same conditions of load after attaining a certain limit. After
being switched in at a speed of 650 revs, (corresponding to about
H
114 TBAIN LIGHTINa.
15 miles an hour), the output in lighting and charging current rose steadily
from zero to 16 amperes as the speed was increased to 870 revs. (21 mileB
per hour), at which point slipping hegan. A speed of 915 revs, with an
output of 20 anip^reA was reached vrith a train speed of 24 miles per hour,
both remaining unchanged though the train speed rose to 72 miles per
hour. Of course the slipping of the belt means a waste of power, but this
is of comparatively little consequence as compared with the total power
absorbed in driving the train at high speeds. According to tests by Prof.
Capper, the power absorbed by the plant at " normal speeds '* (presumably
when the maximum output was first attained) was 0.6 horse-power, and
rose to I.I at full speed, the corresponding consumption of coal being about
the same per train mile in each case, and estimated at one-twentieth of a
pound. The battery will supply lighting power with a slightly reduced
E.M.F. for some time at rest or slow speeds, an extra large, or double
battery being supplied for underground railways, or when long stoppages
are frequent. External switches are provided to extinguish half or all the
lights when required.
An improvement on the ahove system, enabling more uniform E.M.F.
between the terminals of the lamps to be obtained in charging and dis-
charging, is obtained by the use of two similar batteries, one of which is, at
all times, connected across the terminals of the lamps, and the other, when
the train speed exceeds a certain number of miles per hour, to the terminals
of the dynamo. The two batteries are further connected together, by their
corresponding terminals, through a low resistance, enabling a somewhat
liigner difference of potential to be maintained between the terminals of the
dynamo and the battery to which it is directly connected, and thus charges,
than between the terminals of the Limps ; the latter are supplied, with a slight
waste of power, directly from the dynamo when the excess of its E.1M.F. is
sufficient. When the dynamo falls in E.M.F. or is cut out by the automatic
switch, the current through the lamps is supplied by the discharge of the
battery directly connected to them with an unreduced E.M.F. An arrange-
ment is supplied which interchanges the batteries every time the direction
of travel of the carriage is changed, so that each is alternately in the
positions where they are charged, and where they supply the lighting current
when the dynamo is not working.
Other systems of train lighting are supplied by a single generator in the
guea*d's van for the whole train. A train-lighting dynamo designed by
Holmes and Co. obtains a constant E.M.F. at all speeds (above a certain speed
at which it is switched into the circuit) by the use of a small additional
dynamo upon the same shaft as the generator, the E.M.F. of which is made
to cause a weakening of the field magnetism of the generator as the speed
is increased, and so maintain constant E.M.F. in the lighting circnit. For
this purpose the field-magnets of the generating dynamo are supplied with
two coils, each connected across the terminals of a secondary hattery, but in the
circuit of one of these coils the armature of the small dynamo is connected
so that its E.M.F. opposes the battery and reduces the magnetising current
in this coil as the speed is increased. A separate secondary battery is
used for storing the power under each carriage, and when the dynamo
is switched off all these batteries are in parallel with each other and
with all the lamps. When the dynamo is switched in, its E.M.F. rising
above that of all the batteries so that it can supply a charging current,
an automatic relay throws a small additional resistance (about ^ ohm) in
series with the lamps in each carriage, so that with the raised E.M.F. they
continue to burn with the same brightness, though supplied directly from
tne dynamo. 3 5- volt lamps are employed, and batteries of 18 cells, and
a further permanent resistance of ^ ohm is placed in series with each
CENTRAL STATION PRACTICE. 11$
battery to ensure uniform distribution of the current in the many parallel
circiiitR.
In central station practice the use of the secondary battery will be chiefly
to supply the light load during the daytime (particularly with newly erected
stations), thus saving the cost of labour in attendance on the running
machinery, and the extra cost per unit in running the machinery at light
and variable loads, at which its efficiency is often much lower than with
heavy loads, and the oil, &c., used is much the same. The secondnry battery
is also of value as a " stand-by " in the event of break-down, provided it can
be discharged at high rates. As a rule, however, the battery need be only
of sufficient size to supply about ten per cent, of the maximum output of the
station at the normal and efficient rate of discharge. As there will be a
number of separate dynamos available, the difficulty of the excess E.M.F.
of charging can be easily got over by using a separate dynamo for the pur-
pose. By proceeding with the charging during times of moderate demand,
and ceasing when the demand approaches the maximum, it will be possible to
fairly equalise the rate of steam generation during the night, thus using the
boilers and engines under their best conditions. Aa only a small proportion
of the total energy output from the station will then be stored, say lo per
cent., the high efficiency of the Secondary battery is not of paramount im-
portance, a loss of 20 per cent, meaning a loss of only 3 per cent, of the
total number of units supplied, and this will be saved many times over in
the cost of labour, &c»
In a central station or private supply system worked by water power,
especially where the supply of water is limited, and there is no convenient
means of storing the water in a reservoir of sufficient size, the secondary
battery is of incalculable value, enabling the total number of lamps that
may be supplied by the generating station to be multiplied several times,
be(»iuse the generation of the electrical energy may then proceed uniformly
throughoui* the day and night, except during the hours of heavy load, when
the battery may be discharged in parallel with the generators. Supposing
that the average length of the time of burning of the lamps on the shortest
and darkest day of the year is four hours, a proportion that will hardly be
exceeded except where there are many clubs, public-houses and the like to
be supplied, or the district is subject to fogs, sufficient energy may be stored
up during the hours of light load, say in about fifteen hours, to light three
times as many lamps as can be lighted by the generating plant at the
maximum output of the direct supply, or four times as many when the
battery is discharged in parallel with the generating plant. This allows for
a loss of 20 per cent, of the energy in storage, and a loss of time available
for charging as all the lamps will not be lighted at the same time, and,
therefore, more than four times the number of lamps that could be supplied
directly from the generating plant could be connected to the system without
fear of failure in the supply. Another case where the employment of
secondary batteries is of great value for increasing the maximum output is
where the supply of steam for the engines is obtained from boilers heated by
dust destructors, where the consumption of the fuel should proceed at a
slow and imiform rate throughout the whole day, and therefore, unless
some system of thermal storage is adopted, the generation of the electric
ener^ must be uniform. A uniform rate of generation may be employed
with the necessary irregular rate of output of energy in stations where
secondary batteries are established, so that even where the steam can be
generated as it is needed, the secondary battery increaises the maximum
possible output from the station, when limited by the number and sign of
the generators.
Other uses of seconrlary cells and batteries, to vary the E.M.F. in
1 1 6 SWITCHES.
varioTiB feeder maiDS from a central station, to effect continuous current
transformation from high to low potential, &c., are described in their proper
places. It remains only to complete this part of the subject by describing
some of the additional apparatus used in charging and discharging secondary
batteries. In charging with a shunt-wound dynamo, some means has to be
afforded to increase the E.M.F. of the dynamo as that of the cells rises, so
that a constant charging current may be maintained. This will be either
by an adjustment of the governor to increase the speetl, or more commonly
a rheostatic resistance in series with the shunt-coil, by meeuas of which the
current in it and the magnetisation of the dynamo may be increased by the
attendant as the charge proceeds. Automatic machinery for this purpose is
best avoided, as some degree of attention must necessarily be given to the
cells. A *' current alarm " calling the attention of the attendant by causing
a bell to ring when the current exceeds a safe amount is easily designed,
and a great convenience. Another device that is absolutely essential is an
automatic cut-out which will break the circuit if the current should reverse
flo as to drive the dynamo as a motor, this being a safeguard against the
rapid discharge which would follow upon the break -down of the engine. A
fusible cut-out would not effect the same purpose, though it might be used
to stop an exceedingly rapid discharge ; it could take no cognisance of the,
direction of the current, would generally be too slow in action to prevent
injury of the plates, and is troublesome to replace. A spring switch, held
in the position of contact by a solenoid attracting a core, so as to release
when the current falls before reversing, and to be replaced by ha^d, is often
oonsidered sufficient. A more elaborate automatic switch, the " Nevile "
patent, effects automatic replacement. A permanent magnet moves between
the poles of a horse-shoe electromagnet^ actuating a switch consisting of a
copper fork dipping into a pair of mercury cups. The electromagnet is
wound with a thick wire coil in series with the switch, and a fine wire coil
shunting the switch. When the E.M.F of the dynamo rises above that of
the battery, the current in the latter causes the permanent magnet to lift
the fork dipping into the mercury contacts ; the magnetism of the electro-
magnet,, and therefore the contact of the switch, is then maintained by the
current in the series coil. The switch is thus not only turned off by the
decrease or reversal of the charging current, but automatically replaced when
the E.M.F. of the dynamo has risen sufficiently above that of the battery.
In discharging the battery, the difference of potential between the
terminals of any cell varies from about 2.05 to 1.90 volts with a slow dis-
charge, or a somewhat lower value throughout with a rapid discharge (the
extra high E.M.F. after a ** boiling" charge not being considered). A
variation of .15 volt per cell is equivalent to that of about 8 volts in a
battery of 45 cells. To maintain a sufficiently constant E.M.F. in the
lighting circuit, convenient arrangements must be made for the removal of
at least four cells, preferably six or more, from the battery when the dis-
charge begins, and their restoration, one by one, as the E.M.F. of the
battery falls. This will produce variations of the E.M.F. by 2 volts at a
time, meaning only a variation of i volt above and below the declared
E.M.F., which is well within the permissible limits. A circular multiple-
way switch, such as shown in Fig. 37 (E.P S Co.), forms a convenient hand
regulator. The revolving handle is connected to one of the supply mains j
the successive contact pieces to the interconnections of the last few cells, of
the battery. In order that the circuit may not be broken even for an
instant in passing from one contact to the next, the moving contact is
double, one piece being connected directly to the central axis and supply
main, the other through a short thick spiral of german-silver wire, through
which a single cell may be short-circuited without injury for a moment.
EEQULATIOK WITH "BOOSTEsa." II7
TiM double contect bridges over the iuterreaing gap in passing, but ahould
not be left in tins position. Fig. 38 shows a Rouble regalAtor, intended for
Bimultaneoua cfaarging &nd lighting, cne handle being connected to the
dynamo, and the other to the supply main, bo that a diSerent number at
cells may be used.
The method of regulation by cutting out cells from a battery is distinctly
convenient, and not likely to be superseded for small installatioDs. It is,
however, almost as mischievous to overcharge oells, except with a reduced
currant, aa to allow them to discharge too far ; and with any system
involving a. Beparation of cells from the rest of the battery both events are
likely to occur except where careful supervision is exercised. For this
reason central station engineers have of recent years preferred to intro-
duce methods of working whereby the battery may always be used complete,
alike for diecharging and charging. This is effected by means of the
oontinuoos current transformer, or " booster," mentioned above, which may
Fio. 37 Via. 38.
Storage Battery reflating Switch.
be reversed in action for discharge. The detans are varied according to
circumstances, and automatic as well as hand regulation may be obtained.
For power .supply, electric tramways and railways, where sudden and
irregular changes of load are called for, batteries thus regulated are of the
greatest value ; but further deecription would extend too greatly the soope
of this work.
CHAPTER VIII.
Continuous Current Transformer Systems.
Thb multiple-wire Byatenm reach the limit of their commercial efficiency
when the average distance of trans mitksion is about one mile. For even if it
be considered safe to use electromotive forces exceeding 350 volts between
any conductor and the earth in internal wiring, the five-wire or three-wire
system, with 400 volts (or preferably 420 volts) between the external
mains, introduces the maximum complexity permissible. Further multipli-
cations of the number of wires cannot be seriously entertained. The exact
point at which it is advisable to drop the direct systems, and use oue of the
toansforming systems, is determioed by numerous local considerations.
In makmg any economic comparison we have to consider both the
Il8 TRANSFORMATION^
capital and annual expenditure. In critical cases it will generally be found
that the direct current systems demand the larger capital expenditure, but
that at lower consumption of fuel per unit sold will be possible. Consider-
ing the latter, care must be taken to allow for the efficiency of the system
at all loads, from the minimum' to the maximum, ill view of the varying
demand day and night, summer and winter.
We may here odl attention to two intrinsic advantages of transforming
sydtems which are often forgotten in considering their relative commercial
values. Firstly, the percentage loss of power in distribution is greatest
with direct current systems at full load, and therefore the real maximum
output of the former is often considerably less than the nominal output of
the generators. The deficiency is generally less with transforming bybtems,
and therefore the maximum output of the station may often be taken as
much as ten per cent, greater than that of a direct current station with
similar generating power.
Secondly, it cannot be but a great advantage to separate the house
circuits from the supply mains, so that there is no electrical connection
between them. For faulty circuits then can be easily detected, and the
existence of faults upon one minor circuit does not affect the distribution of
potential upon another. It is possible, by maintaining high insulation on
each of the parallel mains of each separate house circuit, to have a double
safeguard against the mischief and danger produced by leakage. In a
direct supply the whole system supplied from the central station, or at least
the whole section supplied by one large unit of plant, is certain to be faulty
in some one part, and all other parts are thereby affected so that a single
" fault" is nearly sure to have immediate consequences.
By the " transformation " or ** conversion " of electrical power is meant
the transfer of that power from one circuit to another, without contact of
the conductors, so that the power reappears as another electric current
flowing with a different electromotive force. The first circuit, in which the
electrical power is initially generated, is called the primary circuit; the
second, in which it is absorbed, or rather re-(^on verted into light^ heat, or
mechanical power, is called the secondary circuit.
It is less easy to transform electrical power in the form of continuous
currents than in the form of alternating currents, for with the former
mechanical motion or secondary batteries, and t herefore attendance at the
point of transformation, is necessary. Alternating current transformation
is effected by a purely magnetic linking, without mechanical 'motion, for
which no attention is necessary, much higher efficiency is possible,
and therefore its use is very much more extended. Certain advantages,
principally that of the possibility of storage, remain with continuous
curreuts, and they may yet offer the most efficient and reliable system
of distribution, when the distance of distribution is not excessively
great.
Continuous current power may be transformed by two methods, the
combination of which is necessary to enable the system to compete with
other systems of distribution. The first involves the use of storage
batteries, charged in series with high electromotive force, and discharged in
parallel with low. The second is the use of dynamotora^ or continuous
current transformers, which transfer the power directly from the primary
to the secondary circuits. The transformation must be effected in sub-
stations at various convenient points in the area of distribution, and dis-
tributed to consumers by a low-tension parallel two or three-wire network.
Substations are compact and inexpensive constructions, which may be
placed in cellars in the midst of crowded districts without fear of annoyance
to neighbours. Even with alternating current distribution the value of
BATTERY TRANSFORMATION. II9
8ub tations transforming upon a large scale is becoming more and more
recognised as the cheapest and most efficient sybtem.
Battery Transformation.
It was at Chelsea, in 1889, that the first extensive trial of continuous
current traiisfoi mation was made, at first usin^ only battery transformation,
sufficient Htorage capacity for 10,000 lamps bfing provided. The subsequent
adtiitionof trxnsformers, worked in pai-allel with the batteries at the time cif
extensive demand, has enormously increased the possible output of the station,
and now more than 50,000 lamps are connected, the capital expenditure in
generating and transforming plant and mains being one of the least in pro-
portion to the total number of lamps supplied of any central station in this
country. Four substations were built, and each stocked with 440 secondary
cells of the E.P.S. 31 L type, charging and discharging at 60 amperes, with
a capacity of 660 ampere-hours. These were divided into two half -batteries
of 220 cells each, which were connected respectively to the primary, or
charging mains, and to the secondary or discharging mains, the half-batteries
periodically changing places.
In the half- battery connected to the charging mains all the cells are
arranged in series, and at the rate of from 2.2 to 2.5 volts per cell, re(|uirea
total electromotive force of from 484 to 550 volts. The four substations
are connected in series by the charging mains, so that a maximum electro-
motive force of 2200 volts is required. The substations are supplied from
the generating station by four Victoria dynamos in series (one for each sub-
station), each capable of giving a current of 75 amperes with 550 volts.
The field magnets of all these are excited by currents from a single com-
pound-wound dynamo, driven by a separate engine, the coils being supplied
in parallel, with regulating resistances. A magnetic cut-out prevents a
back-discharge from the batteries through the armatures. Should the half-
battery of any substation be disconnected for any length of time, a short
circuit is inserted in its place, and the exciting current of one of the gene-
rators cut-off, the generator itself being then short-circuited and removed at
leisure ; the passage of the current through the armature meanwhile will do
no harm. But during the rapid interchange of the half-batteries the circuit
ie maintained through a large carbon resistance.
When discharging, the half -battery is divided into four sets of 55 cells,
which sets are connected in parallel, and are intended to supply 100 volt
lamps, with an excess electromotive force to allow for the fall in the mains.
To maintain regularity during the discharge without the complication of
catting out cells and thus discharging them unequally, fourteen *' back-
electromotive force *' in each substation are used to reduce or increase the
electromotive force of sections of the batteries at different periods of the
discharge.
The operation of changing over the batteries from charge to discharge
requires some explanation. The fully charged battery is first short-circuited
by a carbon resistance capable of carrying the full charging current, and
having a resistance of a few ohms. The half-battery may then be removed
from the charging mains, and split up into sections of 55 cells each, which
are connected in parallel. It should then be placed in parallel with the half-
battery at present discharging, a few " back electromotive force " cells being
inserted to reduce the excess E.M.F. in the fully charged half-battery. The
partially discharged battery may then be removed without cessation of
supply, and subsequently connected to the charging mains. Elaborate
automatic switches have been dehigned to efiect the requisite cljangea, and
variation of the back E.M.F. cells for regulation. The most ingenious of
120 THE CONTINUOUS OUEEENT TRANSFORMEE.
these is a switch which diaconneots from the charging mainfi, after previously
inserting the carbon resistance, when the cells are charged to *' boiling."
The gas from one of the negative plates is collected in a gasometer, and Uie
rising pressure moves the switch. Whatever contrivances be thus used,
skilled attendance in the substation Is of course necessary.
Batteries used with so high an E.M.F. must of course be mounted and
handled with peculiar care. At Chelsea the cells are arranged in five tiers,
on wooden shelves with cast iron supports, care being taken to separate the
cells with great diiference of potential as widely as possible.
Initially the whole of the output was thus stored in and discharged from
the batteries This means a large amount of loss, for with the best of
secondary cells an efficiency of So per cent, is considered good practiqe,
which with the loss in primary and secondary circuits will reduce the station
efficiency to a very poor figure ; but this loss is greatly compensated for by
the fact that the engines and dynamos may always be worked at their most
efficient load. The combination of the transformers with the battery trans-
formation, the power being run in parallel with the dischaiging batteries at
times of large demand, not only inct-eases the efficiency of the transformation,
but enormously increases the maximum output possible with the fame
generating plant. The principle of the continuous current transformer must
now be described.
The ContinuoiiB Current Transformer.
The earliest conception of a continuous ciurent transformer was simply
that of a high tension motor driving mechanically a low tension dynamo.
In this way by the medium of mechanical power transformation might be
effected, and a constant electromotive force might be obtained in the
secondary circuit if the motor be regulated for constant speed, and the
dynamo be com pound- wound. But it is difficult to regulate continuous
current motors for constant speed with the requisite exactitude under
varying loads. An alternating current motor, designed upon the principle of
synchronism, would follow exactly the speed of the generator, and thus by
using alternating currents in the primary circuit we may still retain th«
advantages of continuous current in the secondary, with storage batteries,
and avoid many difficulties in regulation, and in the use of high tension
commutators. This system may very probably be applied extensively in the
future.
In the developed form of dynamotor, or continuous current transformer,
the motor and dynamo armatures are mounted on the same shaft, between
the same field-magnets, and frequently wound on the same core. Not only
is space thus economised, and the power wasted in mechanical friction
reduced to that of two bearings, but a regulative property is secured whereby
the E.M.F. generated in the dynamo, or secondary, armature is caused to
bear a fixed ratio to the E.M.F. in the motor, or primary, armature. This
ratio is termed the ratio of transformation, and being independent of the
current output from the secondary armature, except for a slight reduction
due to the resistances of both armatures, which will be presently explained,
ensures that constant difference of potential shall be maintained between
the secondary conductors provided that the higher difference of potential
between the primary mains leading from the frenerator be regulated for
constancy, except for the aforesaid reduction or fall.
The subject of electric motors has been carefully avoided in this work,
in order that the scope may not be too extensive, and the briefest explan-
ation must here suffice. In the motor armature, the design of which is
exactly similar to that of the closed-coil dynamo armature, the current
THE CONTINUOUS CUBKENT IBANSFOKMEB. 121
flows in tbe opposite directioD to that of the E.M.F. generatod by its
rotation. If E be the difference of potential betwtjen the t^minab of
the motor, R, the resistance of the armature drcuit, tbe ouneut o whiob
vUl flow llirough the armature ivill be of nnch a mngnitnde that E.c
repreeente, in Tatta, the power required to drive the motor, overeomicg
the mechanical friction and otlier resistance to its rotntion, and includirf!
the power abtiorbed by the rpsistmice of the niinnttire, hysteresis, and
otlier loaaeB ; also the motor armatiu'e must rotate at bucli a speed that
122 BATIO OF TRANSFORMATION.
an E.M.F. is generated in opposition to that impressed by the generator
which is given by
Bj a B — oRj
This being simply Ohm's law npplied to the armature circuit, taking
account of the two sources of E.M.F.
When the dynamo armature is on open circuit, and no mechanical power
is taken from the motor, the driving 'cuixent c is extreaiely small, and the
*' back E.M.F.," or that generated in the motor armature, is very nearly
equal to that impressed by the generator, or between the motor terminals.
The back E.M.F. , E,, will be given by the usual formula for the closed -coil
armature depending on the product of the measures of the flux of magnetic
induction, the speed of rotation, and the number of turns in the winding
(n i). If the dynamo, or secondary, armature be wound exactly similarly
on the same armature-core, so that the flux of Magnetic Induction and the
speed of rotation are identical with those for the motor armature, but the
secondary armature be wound with n, turnff, the E.M.F. geoerated will be
given by a similar formula with n, in the place of n^ and we shall have
E, = -2 Ej =3 -IE approximately,
-^ is called the ratio of transformation.
If the circuit of the dynamo armature be closed through an external
resistance, such as a number of lamps, so that a current, 0^, is allowed to
flow, the result will be to throw a load on the dynamotor, slackening its
speed, so that the back E.M.F. of the motor armature is reduced, and a
larger current passes. The additional primary current, 0„ must be sufficient
to supply the power absorbed in the secondary circuit ; if the current Cg
produce a similar number of ampere-turns in the armature to that of the
secondary current G,, but flow in an opposite direction, the driving force
produced by the double winding will be thts same as before, overcoming the
f rictional and other losses ; therefore we must have
The back E.M.F. of the motor armature will now be given by
Bi = E - [c + C J R^.
And if K, be the resistance of the dynamo armaturep the diflferenoe of
potential between the secondary terminals will be
or
B,-0,Ra = "iE,-.CA
By reducing the frictional and other losses in the dynamotor the driving
current c, and therefore the second term, may be made very small ; the
third term represents the fall of potential in the secondary circuit, due to
the armature resistances of the dynamotor, when a secondaiy output of 0,
amperes is called for.
It is customary, as giving the highest efficiency for a fixed amount of
copper, to make the fall of potential due to the resistance of the motor and
dynamo aimature approximately the same, that is to bay.
0'^-^'
EFFICIKNCY OF TRANSFORMATION.
123
ThiR will mean that the same amount of copper is used in both armatures,
the conductors in the dynamo armature exceeding in sectional area of those
in the motor armature in the inverse ratio of transformation, and the total
length of the armature circuits being in the direct ratio, their resistances are
as the inverse square of the ratio of transformation.
The field-magnets are best excited by a shunt current from the secondary
circuit, but the self- regulating quality of the transformer independent of the
constancy of this excitation, provided it is the same for both dynamo and
motor. The surest way of securing equality in the magnetic field is to use
the same field-magnets for both armatures, if possible winding them upon
the same core. Great care must be taken to secure good insulation between
the primary and secondary windings, as a failure at any point would raise
the Kecondary mains to a high potential, and be dangerous. An ^* earthing
device" may be used to prevent injury being done to anything but the
tranformer itself, as will be described in connection with alternating current
distribution. A safer arrangement is to mount two armatures end to end
on the same shaft, and between prolonged poles of the same field-magnets,
inserting a disc of ebonite between them. A great advantage obtained by
using the same field-magnets, and winding upon the same core is that the
effect of armature reaction, which was not allowed for in the above calcula-
tion, is thereby almost entirely eliminated. For the currents in the two
windings are opposed to one another, and have the same number of ampere-
turns, except for the small excess current in the primary. The distortion is
therefore practically insignificant, and the sparkless commutation on the two
commutators can be efifected at all loads without varying the lead of either
brush. Furthermore, a Pacinotti, or short air-gap, armaj^ure may now be
used without allowing for reaction, so that an intense field is possible ; and
as, with a motion purely rotary, a high speed presents no difficulty, the
transformer is very much reduced in size as compared with a generator of
similar capacity.
The two commutators are always placed at opposite ends of the double
armature, in order to separate the low and high tension terminals as much
as possible.
The transformers in uae at Chelsea are made by the Electrical Construction
Corporation, and are of the two-pole inverted type, with two cylinder wind-
ings on the same core, the high tension primary windings being threaded
through ebonite tubes under the surface. Bunning at 1000 revs., the
transformation capacity is 33^ kilowatts, and the efficiency appears to be at
full load about 82 per cent. The transforming ratio is from 5 to i at zero
load.
The following tests at full load of the first dynamotor employed at
Chelsea have been published : —
Primnry.
Secondary.
Efflciency.
£
604
606
588
72.5
72.5
64
Watts
43»790
43.935
37,632
E
III. 5
III. 5
I10.5
320
327
280
Watts
35.334
3^313 ;
30,814
Per cent,
81.3
82.5
82.2
An improved type more recently introduced attains an efficiency of 92 per
cent, at the full load of 40 kilowatts, and 81 per cent, at one-third load.
The transformers are intended to be run in series on the charging mains, and
in parallel with the batteries upon the secondary, at times of heavy load
only. The electromotive force need not theiefore be regulated with very
great care, as it is controlled by the ceils. It is, however^ arranged to force
124 SERIES DISTRIBUTION.
the presflure somewhat in excess of the batteries so that the transformer
may be fully loaded, and only the excess of the demand drawn from the
batteries. The electromotive force also requires to be kept high at the time
of heavy demand.
A system of continuous current transfoiTnation differing very consider-
ably from the Chelsea system has been established at Oxford since 1892.
The transformation is here effected entirely by dynamotors, of which the
primary armatures are connected in parallel instead of in series. The
generating station is at Osney, on the outskirts of the city, where water is
obtainable in large quantities from the river for condensing purposes, and
where, owing to the proximity of the Great Western Railway, the coal can
be obtained with the minimum expense in carriage. Triple expansion
engines drive dynamos giving 80 ampdres at 1050 volts with 400 revolutions
per minute. A small exciting dynamo is driven from a pulley on the
shaft of each, for the high tension of the dynamo itself is inconvenient in
shunt winding. Three substations are erected in which transformation is
effected from 1000 to 108 volts by means of transformers, consumers being
supplied with a specified voltage of 105. The full load of each transformer
gives 40 amperes in the primary, and 360 in the secondary armature. The
speed of each is 550 revolutions per minute, and the efficiency at full load
as high as 92 per cent., at half-load about 86 p^r cent. In one substation
only is installed a secondary battery of 114 cells of the 31 L. E.P.S. type.
These are charged in the daytime off (he secondary mains in parallel with
the lamps, being connected for this purpose as three sets of 38 cells in
parallel. They are never connected to the high tension mains as at Chelsea.
The battery is used only to maintain the supply during the light load after
midnight, and as a stand-by in the event of a breakdown. For discharging
they are connected as two S'^ts of 57 in parallel. The secondary mains
form a complete network, and can be supplied from this single battery
when the load is light.
All the operations to be performed at the substations, the starting and
switching on and off the secondary mains of the transformers and batterieSi
are under the control of the engineer at the generating station. The dyna-
motors need only veiy occasional attention when running, so that constant
skOled attention in the substations is unnecessary. Pilot voltmeter lines are
carried back to the generating station, and by short-circuiting these the
switch that connects thn traubformer to the mains is turned by means of a
solenoid in which the pilot line is wound.
CHAPTER IX.
Series Distribution.
Series distribution^ involving the subdivision of the electromotive force
factor of electric power, is the system which offers the highest possibilities of
efficiency in working and economy in capital expenditure in distant trans-
mission. There are, however, several considerations which render series
distribution inconvenient, if not absolutely impossible, for interior lighting,
so that for such purposes it has been totally abandoned, except so far as the
multiple-wire systems may be considered a combination of series with
parallel distribution.
The high efficiency and economy of series distribution arises from the
high E.M.F. and small current which may then be employed. But with
high voltage lamps on the simple parallel system, or with the multiple-wire
systems, the limiting E M.F. determined by conditions of safety for interior
SERIES DISTKIBUTION. 1 25
lighting may be reaehed, with greater oonvenienoe in the subdivision, and bs
long aa incandescent lamps are employed of the present customary candle-
power these systems are not likely to be superseded. A higher efficiency in
the smaller sizes of incandescent lamps might, however, be secured if shorter
and stouter filaments were employed, with a much lower difference of
potential between their terminals, and such lamps would be preferably
employed on a series circuit. Lamps of larger candle power than are really
necessary are frequently employed, owing to the difficulty of obtaining 8mall
incandescent lamps suitable to the E.M.F. commonly used in parallel systems,
and the adoption of series connection in branch circuits, though involving
certain obvious difficulties, might effect considerable economy, provided these
lamps are intended to be lighted simultaneously.
This device would not, however, effect any reduction in the size of the
conductor, except so far as the current is reduced by the use of smaller
lamps, or lamps of higher efficiency ; and unless the lamps connected in
aeries form a range, and not a group fed in all directions from a centre, the
amount of copper required will be greatly increased.
Very extensive subdivision of the electric light, using lamps of small
eandle power, is evidently impossible with a system wh^-re all the lamps
supplied from one generator are connected in series. It is 8ui table to a
system where all the lamps require the same current, of moderately large
amount, a comparatively low E.M.F. ; also to circumstances where a high
electric pressure may be employed without danger and where the arrange-
ment of the lamps is more or less that of a line or range, instead of a group
supplied from a central point. These conditions are exactly those afforded
in the lighting of streets by arc lamps, for which purpose the series system
ponesses enormous advantages over any other. To a limited extent it is
possible to place single incandescent lamps or groups of lamps on the same
circuits bat as a rule the adoption of a series distribution for street lighting
requires that entirely separate generating plants and systems of distiibuting
mains should be installed for this and for the lighting of interiors.
Looking through the records of the Patent Office in this country for the
early days of electric lighting, from 1865 onwards, when the problem of
subdivision was being slowly solved, one finds a large number of applications
with regard to a series arrangement of lamps. The low voltage incandescent,
arc, and semi-incandescent lamps were then in their elementary forms
competing for favour, and the comparatively high E.M.F. lamps first
introduced by Edison were needed to settle the question finally in favour of
parallel distribution. Oonvenience has proved of more importance than the
high possibilities of efficiency, for even to-day it is unquestionable that much
higher efficiency could be obtained by using low voltage lamps, with short
thick filaments, with a partial adoption of the principle of series connection.
The arc lamp series system is therefore the oldest, and has survived
unchanged through all the modifications to which incandescent lighting has
passed in its evolution.
In addition to the high efficiency and economy in the distributing mains
effected by series distribution, other advantages are to be obtained. With
the regulation for constant current necessary for series distribution, better
regulation of arc lamps can be effected than with a supply at constant
E.M.F.; as shown in the chapter on arc lamps an additional steadying
resistance with continuous currents, or impedance coil with alternating
currents, is required for parallel working of arc lamps, in both cases, but
especially the former, involving some waste, and seldom affording equal
steadiness to that which may be obtained when the series arrangement is
adopted* The higher luminous efficiency that can be obtained with con-
tinuous current arc lamps than with alternating currents (though denied by
126 ARC-LAMP CIRCUITS.
some), is a strong argument in favour of the adoption of a separate series
c mtinuous current system when an alternating current transformer system
is used for the incandescent lighting of interiors. The separation of the
street lighting from the interior lighting plant also enables the labour of
switching on and oflfthe lamps to be dispensed with, these being lighted and
extinguished simultaneously when required. On the whole, though the
extra conduits for the conductors, and the additional generating plant for
street lighting, will in general cost more than the requisite addition to a
system supplying interior lighting, the separate system is unquestionably
preferable for the lighting of large towns and cities from a central
station.
The standard size of arc lamp preferred for street lighting is the ten-
ampere arc, adjusted for a length of arc gap of two to three millimetres,
this length requiring a difference of potential between the electrodes of a
little less than 45 volts, and between the terminals of the lamp of from 45
to 50 volts, the higher value being generally reached when the coils that
control the mechanism are hot, and the lamp trimmed with carbons of full
length. The energy thus absorbed is seldom less than 500 watts, or two-
thirds of a horse-power. Sixty such lamps connected in series will require
an E.M.F. of 3000 volts, which is about the highest that can safely be
employed, though this has been greatly exceeded with special precautions.
The electrical power expended in the circuit, in addition to that absorbed in
the mains, will be 40 E.H.P., a£fording a load for a unit of plant of sufficient
size to obtain high efficiency in generation. To all intents and purposes it
is sufficient to calculate that one indicated horse-power will be required for
every arc-lamp employed.
A conductor of 7/16 S.W.G., giving a sectional area of copper of .0229
square inches is most commonly employed for this system. The current
density when ten amperes are carried is 436 amperes per square inch. The
resistsmce per square mile, with copper of standard conductivity, should be
1.953 ohms, giving a fall of potential per mile of 19.53 volts, and an absorp-
tion of power of 195.3 ^&^to« I^ other words it requires 2^ miles of con-
ductor to absorb the power that would supply one lamp. Allowing for the
return conductor, which must in all cases be carried iMick over the same
route, and in the same conduit as that carrying the current to the lamps,
the efficiency in distribution to a distance of 2^ miles, the power for sixty
arc lamps being carried on this small conductor, will be 96.7 per cent. The
supply will be regulated for constancy of current, which may be effected to
any degree of accuracy that may be considered necessary, though with aro
lamps for street lighting a much larger percentage variation is permissible
from the specified current than is permitted with incandescent lamps from
the specified E.M.F. For while with incandescent lamps a variation of one
per cent, in the E.M.F. produces about six per cent, variation on the candle
power, and, a fact of still greater consequence, considerably alters the colour
and apparent brilliancy of the filament surface, the candle power of an arc
lamp is only affected to the same extent as the current is varied, and the
colour and apparent brilliancy being unaltered a variation of ten per cent.
will scarcely be noticed.
Furthermore, with series distribution no difficulty will arise to corre-
spond to the difficulty of obtaining uniformity of potential difference at all
parts of the system which is the most important consideration in parallel
distribution. The current must necessarily be the same throughout the
single circuit, provided it is effectively insulated throughout so that the leak-
age may be insignificant. But as the current is now much smaller than in
a parallel system conveying similar power, and leakage will directly effect
the uniformity of the lighting throughout the system, more care must be
AKC-LAMP CIRCUITS. 1 27
used to prevent leakage, which the high electric pressure and the expof^ed
position of the lamps tend to promt ^te.
The circuit needs, of course, to be insulated as highly as possible
throughout. Circurastanoes prevent oul* ensuring insulation in arc lamps to
any high degree in exposed situations and during wet weather, so that a
slij(ht leakage over the insulating supports is unavoidable. Happily the
heat of the lamps tends to prevent the creeping of damp and to uphold the
insulation during the time of supply. Supposing the whole circuit as well
as the dynamo supplying the current are equally well insulated, the poten-
tial throughout the 3000-volt circuit would vary from + 1500 volts at the
positive terminal to — 1500 volts at the ne^irative terminal. If, however,
there be a point of low insulation, or a " fault," at any part of the circuit,
that point will be lowered or raised to zero, the earth s potential, while
the potentials at the terminals of the dynamo will be positive and negative,
and proportional to the number of lamps between either terminal and the
fault. By finding these potentials with an eUctrostatic voltmeter we have a
fairly accurate method of locating the fault while the lamps are alight.
If there be two points of low insulation there will be a leakage current
between them through the earthy and may result in the extinction of, or at
least the reduction of, the current in the intervening lamps.
By connecting the central point of the circuit permanently to the earth,
we could prevent the }K)tential of any point upon the circuit becoming
greater in magnitude than 1500 volts positive or negative. This is not to
be recommended, as in that case leakage ensues when the insulation fails at
any one point on the circuit. A breakdown of the insulation of some one
lamp will inevitably occur, even in the most careful practice. But if this be
rectified as soon as possible, the simultaneous occurrence of two such faults,
which alone is of importance, may be made a remote contingency.
Arc lamps connected to a series circuit may be extinguished by effecting
a short-circuit past the lamp, which maintains the series circuit with practi-
cally no resistance at the point where the lamp is connected. It is advis-
able so to design the short-cirouiting switch as entirely to remove the lamp
from the circuit after the short-circuiting has been effected, so that it may
be handled without danger. Automatic short-circuiting devices are an
essential part of the arc lamp mechanism when intended for series distribu-
tion, and the principle of their action is described in the chapter on arc
lamps. The real object of these devices is to protect the arc lamp from
injury in the event of the arc breaking and the carbon electrodes failing to
come together. ISuch an event, by no means uncommon in the best forms
of arc lamp, would result in the speedy destruction of the shunt-coil, which
would carry the whole current of the series circuit, but little reduced from
the normal ten amperes by the additional resistance of the shunt-coil (from
100 to 150 ohms), until it is fused into a solid mass. There is little danger
of the circuit being broken by any such event, as a con.siderable break is
necessary to stop a current maintained by 3000 volts.
Aro-lighting Dynamos..
Olosod-coO dynamos are ill adapted for the supply of power in the form
suitable for series distribution, on account of the hi^h tension involved.
The difliculty lies principally with tlie com mutator. The form previously
described would need to be of a very large size to be safe with a diti'erence
of potential between the brushes of more than 500 volts. About 10 or 12
arc lamps of the ordinary 500-watt type would therefore be the grefitest
number conveniently supplied in series by a closed-coil dynamo, this repre-
senting an output of only seven or eight horse-power.
128 BRUSH ARCLIGHTINa DYNAMO.
At the St. Paccraa oentr&l statioQ moet of the power for arc Ughting is
supplied from a foar-pole Oramme nuchine, giving 50 EunpAreB at 500 voltfl,
aod thus supply log 55 lampe m 5 parallel circuits of 1 1 in series. In tbU
THE BRUSH ARC-LIGHTING DYNAMO. 1 29
csae, however, the maximum dixtance of distribution is eomparfttavely small,
and a moderate potential baa advantegas. For perfect regulation this com-
bined system of mnltiple parallel circuits requires tbe insertion c^ an equi-
vaJent rasistaDoe in Uie event of the extinction ot any lamp in one of the
eiroaita; in a sinftle circuit the regulation mav be effected by a oorreepoad-
ing reduction in the electi-omotive force, witb no loss in efficien'-y.
For electromotive forces greater ibaii 500 volts, it is adviHable to adopt
■ome tona of open-coil armature. There are only two types of this class in
common use — the Bruah and the Thompeon-Houston. Of these we shall
give a short description.
The Brush are-Ughting dynamo is illustrated in external appearance by
Fig. 40, a portion of the armature core-ring before winding in Fig- 41. The
disposition of the magnetic field io tbe core-ring is similar to that in the two-
polar ring in cloeed-ooit dynamoti, consisting of a double magnetic circuit
throu^ the upper and lower halves of the ring. The lines of force enter
and leave the ring at tbe sides from the flat surfaces of wide-extended poles.
FIO. 41.
Cnre Kng o( Braab Arc-lighting Df n&mo.
The field magnets consist of two horse-shoe magnets placed horizontally,
with similar poles facing each other acroHs the intervening armature-core.
The armature-core is built up on Ibe rim of a foundation ring A,
Fig. 41, by winding a strip of thin wrought iron B, .oaa inch in thickness,
in successive convolutions, binding in a number of exactly similar H-sbaped
stampings aa illustrated, so ns to lie radially one above the other in thu
positions shown, the central limbs of a bunch subsequently forming the core
on which a coil of the armature is to be wound, and tbe side projections
fulfilling tbe purpones of Pacinotti teeth. The core is then securely bolted
together witb radial boltf, and to tbe supporting rim A.
In the type illustrated, the " sixty-light " dynamo, intended fo supply a
current of 10 ajnpires at 3000 volts, the armature consists of 12 separate
bobbins, each of muny turnx, in each of which a high E.M.F. is generated.
The coiln are wound by hand, each with about 900 feet ot No. 14 S.W.G.
wire. The specified speed of rotation of the armature is 800 revolutions per
minute. For arc li^btin^r dynsmns of smaller siie, eipht coila nre employed
instead of twelve, and the mirdificfition of the following description to suit
this case will be obvious. The field-mapnet coils are serie.i wound, the
resistance of the coils being about 14 ohms, and thun abKorbing ne'iriy a
electrical horse-power.
To describe the method of connection of these coils to tbe commutator,
I30 CONNECTION OF ARMATURE COILS.
9
and to show how an E.M.F. continuous in direction, subject to slight
fluctuations as the armature revolves, is generated and the current oollected,
the description is much simplified by observing that the armature really con-
sists of three separate sets of four coils in each, each set having a separate
commutator, and the three sets entirely distinct save that by inter-connection
of the collecting brushes they are always joined in series.
The four coils forming a separate set are mutually at right angles on the
armature ring. Any two coils diametrically on opposite sides of the armature
ring have at any moment electromotive forces of equal magnitude generated
by the revolution of the armature ; these opposing coils are always joined
in series so as to obtain the sum of their electromotive forces, and the
extremities of the connected coils terminate in opposite segments of a four-
part commutator. The pair at right angles to these on the armature ring
are also connected in series, and the extremities to the remaining segments
of the same commutator. When the former pairs are in the position of
maximum E.M.F., the brushes will make contact with the corresponding
segments of the commutator, and these coils alone will form the armature
circuit for this set of coils, the other pair, which will be then in the position
of zero E.M. F., being thrown out of circuit. In a semi-revolution of the
armature the pairs will change places, each pair being rendered active, or
placed on open circu|t, in turn.
The current is thus transferred from one pair of coils to the other pair
of the same set of four alternately, and, unless the change is to be accom-
panied by violent sparking at the commutator, it is necessary that the
pairs of coils should be placed in parallel for a short period in order that
the interchange of the current may be effected gradually by the rise of
E.M.F. in the pair that are about to become active, and the fall of E.M.F.
in the pair about to be placed on open circuit. The operation is thus analo-
gous to the reversal of the current in the sections of a closed-coil armature
short-circuited by the brush contacts, and sparklessness at the commutator
will be secured if the terminating commutator segments of the pair of ooils
about to be placed on open circuit recede from the brush contacts at the
moment when the current in this pair has just fallen to zero.
A temporary connection of the pairs of coils forming a set in parallel
might be effected by a broad brush contact, as on the commutator of the
closed-coil armature. But, owing to the high self-inductance of the coils of
the firush armature, each consisting of many hundreds of turns, a somewhat
prolonged period during which each brush is in contact with two commutator
segments will be required for the complete interchange of the current, which
would be difficult to effect by a broad brush contact alone. In place of this
the commutator segments are T-shaped, so placed that they overlap to the
extent of nearly 45° of the commutator circumference, thus permitting, with
a narrow brush contact, the pairs of coils to be placed in parallel for periods
equal to those during which each pair in succession are on open circuit.
During the period of parallel connection (one-eighth of a revolution) the
current gradually shifts from one pair of coils to the other, the lower internal
resistance of the parallel arrangement partly compensating for the reduced
E.M.F. obtained from the set of four coils in this position. The difference
of potential between the brushes will, however, fluctuate considerably,
reaching a maximum value four times during a revolution.
The three sets of four coils which compose the whole armature are
practically equivalent to three dynamos connected in series by the inter-
connection of the brushes. But, since the fluctuations in E.M.F. in the
three sets pass through their corresponding periodic values at successive
intervals, the E.M.F. of the whole armature is rendered much more uniform
than that of the distinct sets, or between any pair of brushes touching the
SPARKING AT COMMUTATOR I31
same commutator. The self-inductasce of the field-magnet coils and the
series ooils of the arc lamps in the circuit will also tend to reduce the fluctua-
tiana in the current, so that, according to careful tests, a variation of only
1.5 per cent, in the current seems to result in atwelve^coil machine. Though
this variation might be objectionable for some purposes^ such as the charging
of secondary batteries, it is of little consequence, if not distinctly beneficial,
for arc lamp supply.
The commutator segments are separated by air-gaps about \ inch broad.
The brushes are of flexible elastic copper strip, of the same breadth as the
commutator, slit longitudinally for some distance from the points of contact.
These are mounted on a rocking support, so that any pair can be rotated
through a considerable angle, while preserving opposition of their contacts
on the commutator, and the holders are connected by flexible conductors to
terminals on a slate base, under which the interconnections are made, which
joins the three divisions of the armature in series.
A very intense Magnetic Induction (as high as 27,000 G.G.S. units) is
produced in the armature-core, while that in the field-magnets is exceed-
ingly low (4200 C.G.S. units). The result is that the distortion due to
armature reaction is exceedingly great, the lines of force being dropped
forward to the trailing pole-tips, while the large side projections, or
Pacinotti teeth, cause a considerable oscillation of the magnetic field as the
armature rotates. The best position for the generation of E.M.F. will
therefore be where the coils emerge from the polar interspace, and tlie
brushes should be placed so as to make contact with, the segments termina-
ting coils in these positions, while the pair at right angles is upon open
circuit. A slight additional lead would be necessary to obtain sparklehs-
ness at the commutator, but it is inadvisable to give the full lead required
for absolute sparklessness for an important reason which must be carefully
explained*
If a lead somewhat less than that required for sparkless commutation be
given to the brushes, the result will be that each pair of coils successively
will be disconnected from the circuit just before the current in them has
been reduced to zero, and sparks will be produced by the current leaping
from the retreating segments of the commutator towards the brushes.
These sparks represent the passage of a cuirent which is decreasing owing
to the falling E.M.F. in the open-circuited pair of coil$«, and are therefore
instantaneously extinguished. It is advisable to allow these sparks, which
appear as a continuous discharge, to remain about a quarter of an inch in
length, or even half an inch if there be any doubt about the steadiness of the
engine or of the arc lamps in circuit. Sparks of this length should not
injure the commutator surface, as there is no insulation between the seg-
ments to become carbonised. Oiling of the commutator must however, be
avoided, except to a minute degree, as carbonisation may result, followed by
burning of the copper brushes and commutatoi- segments. As long as the
sparks remain with a bluish tint they are quite innocuous, the first sign of
destructive sparking being a greenish appearance.
If, however, the brushes be pushed forward to the sparkless point, it
may happen, through a slight slackening in the speed of the engine or an
increase in the resistance of the external circuit, that the current will fall,
the distortion of the magnetic field will decrease, and the position of spark-
lepsness may fall back leaving the brushes with too great a lead. In this
event the current in the pair of coils about to be placed on open circuit will
have decreased to zero, and a reversed current started in them while in
parallel with the active pair. The ensuing spark will represent a current
generated by the higher E.M F. of the active pair, tending to still further
increase, and an arc will be formed between the commutator segments. The
13^
EFFICIENCY.
active pair of ooils will thus be short-circuited through *the inactive pair of
coils through two Bhoi*t arcs, and an enormous current will pass. Happily,
this will not result in the destruction of the commutator or the ooils, as the
heavy current is sufficient to demagnetise the field, and the only result
will be the extinction of the lights. This accident is commonly known as a
^' flash over,'' and for the dynamo to recover itself it is generally necessary
to slacken the speed, pulling the brushes back, and again advancing them as
the speed is restored.
Owing io the high intensity of Magnetic Induction in the armature core-
ring, a considerable amount of power is wasted in hysteresis. The large
size of the side-projections from the core, or Pacinotti teeth, causes con-
siderable fluctuations of the magnetic field in the wrought-iron pole faoes^
Fig. 42.
DIAGRAM OF CONNECTIONS OF BRUSH-
GEIPEL REGULATOR.
r
s
REGULATOR
RELAY
^ Jjyl ■^
£)MHt»
■Q BRUSH
O DYNAHO.
F2
^M-
and the consequent heating by eddy-currentn causes lurther loss, which
might be almost entirely eliminated by lamination of the pole faces. It
appears however, that an efficiency of 78 per cent, is to be attained by the
Brush aix;-lighting dynamo, and as it is intended to be used only at full load,
this is considered sufficiently satisfactory. With the high E.M.F. generated
and the purpose for which it is intended, durability and immunity from
breakdown is of more vital importance than hi^h efficiency, and in these
respects the success of this type of dynamo has been phenomenal.
It would be impossible to compound a dynamo intended for series
lighting so that a constant current is maintained, whatever be the resistance
of the external circuit. It would, in fact, be most inconvenient to employ
shunt-winding at all, since the high E.M.F. of the dynamo would render
necessary a shunt-coil of great resistance and an enormous number of turns
to give the required magnetisation, with a current small in proportion to the
output current. A series coil is alone practically possible. Owing to the
comparatively high resistance of the armature and field-magnet coils, and
the reaction of the armature current on the magnetic field, the external
characteristic of the Brush dynamo is such that the E M F. falls rapidly if
REGULATION.
133
the current is increased above the normal 10 amperes, and the limiting
magnitude of the current, to be obtained on short-circuit, is barely double
the normal, or about 20 amperes. As this current will not injure the arc
lamps or mains, unless maintained for a considerable period of time, a fuse
is entirely superfluous.
As the Brush dynamo will in general be employed with an unvaiying
load, there will really be no necessity whatever for automatic regidation of
the current provided the driving engine be reasonably steady, and properly
attended. The current may be initially and from time to time adjusted,
Fia 43.
RKOUCATOM
O V N A M O
either by alteration of the speed of the engine, or by a rheostatic resistance
arranged to shunt some of the current from the field-magnet coils. The
latter may, however, be effected automatically by the Brush-Geipel regu-
lator, the connections of which are illustrated in Figs. 42 and 43.
The shunt, or " teazer " circuit, by the variation of whose resistance
mora or less current is removed from the field-magnet coils, consisted in the
original form of this regulator of a number of carbon blocks, built up in
four columns through which the current passed in series, and whose resist-
ance was reduced by the application of greater pressure, and vice tfersd. In
the later type illubtrated an electrolytic resistance is substituted, varied
by the raising or lowering of suspended electrodes, and provided with a
reversing switch for occasional reversal of the direction of the current
through the electrolyte. The electrodes are raised or lowered on a lever by
the action of a double solenoid carrying the main current and attracting a
core attached to the lever. To further increase the range of variation in
E.M.F. through which the regulator will maintain approximate constancy
of current and the sensitiveness of regulation, a relay is provided, worked
134 THE THOMSON-HOUSTON ARC-LIGHTING DYNAMO.
by another double solenoid in series with the main current, and lifting a
core which effects alterations in the number of turns of wire in t he bolenoid
directly controlling the regulator resistance. Tracing the connect ioun shown
in the diagram, it will be seen that the regulating solenoid is wound with
three coils, through the lower of which the current can only pass in the
opposite direction round the solenoid to that in the two upper coils. The
current only passes through all three coils when the relay core is in the
middle position, balanced by the attraction of the relay solenoid and an
adjustable spring against its weight. If the relay core falls through too
small a current in the external circuit, the middle coil of the regulating
solenoid is short-circuited, its attractive force is reduced, and the core falls,
raising the electrodes and lowering the resistance of the shunt across the
field-magnets, and thus increasing the magnetisation and E.M.F. of the
dynamo. If the current in the external circuit be too great the relay core
is raised, short-circuiting the lower or opposition coil in the regulating
solenoid, increasing its attractive force, lowering the electrodes and the
resistance of the shunt across the field-magnet coils, and thus lowering the
magnetisation and E.M.F. of the dynamo. An automatic rocker for
adjusting the lead of the brushes as the current varies is also combined with
the regulator.
Thi Thomson- E[oiL9ton are-lighting dynamo.
The only other type of d3mamo that has been brought into extensive use
for the purpose of series distribution is an American design by Professors
Elihu Thomson and E. J. Houston. A general view of this machine is
given in Fig. 44, and it will be seen by what follows that the theoiy of its
action is as unique as its appearance.
The armature is approximately spherical, and revolves between two oup-
shaped poles which terminate tubular-shaped field-magnet limbs upon which
the field coils are wound, the return circuit consisting of a ^' squirrel-cage "
of wrought iron bars. There are only three coils in the armature, wound one
over the other, but carefully insulated from each other upon a specially
shaped iron frame-work. This frame-work consists of two concave iron discs
keyed into the shaft, between the circumferences of which are wrought iron
ribs, over-wound with a number of layers of soft iron wire.
The three conductors which are to form the coils axe first connected
together, and to no other conductor. They are then wound over the iron
core so as to form three coils whose planes make angles of 120° with one
another, the other ends terminating in segments of a three-part commutator.
In Uiese three coils alternating electromotive forces are produced whose
maxima occur successively at equal intervals during a revolution. Suppose
the segments are so placed that the radial plane bisecting each is the plane
in which the corresponding coil is wound, let us consider first what
would happen if there were a single pair of brushes making narrow contacts
in the plane of generation of maximum electromotive force. The brushes
would always be making contact with the two segments of the coils
that have the highest electromotive force, and there would be a single
armature circuit through them, while the third coil is idle on open circuit*
As soon as the electromotive force in the third coil exceeds that in
either of the others, it takes its place. So far the action would be
simple, but the narrow brush is impracticable owing to the self-icductance of
the coils. The operation of changing from one coil to another must be
performed gradually by placing the two coils in parallel for sufSdent time
for the rising electromotive force in one, and the falling electromotive force
in the other to effect the transfer of the current before the latter is elimi-
nated. This operation, identical in theory with that of the Brush dynamo,
is here effected by having two brushes on each holder, connected together
CMMMLTATIOS WITH DOUBLE BRUSHES. I3S
and makiog contacts at a short distance apart, bo as to be practically
equivaieDt to oae broad brush. There will thea be no sparking if a
commutator section leaves the more advanced brush just when the corre-
sponding coil has had its current reduced to zero.
The field-magnet coils are neries wound. One of the most distinctive
features in tliis dynamo is the exceedingly ingenious and efiective method
of regulation which is employed to maiotaiii constancy of current. Instead
of altering the current in the field coils as is the usual method, the armature
reaction is made to vary by separating or closing together thfl two brushes
<m each holder. By rotating the trailing brush backwards round the com-
FlO.44-
ThomMta-Uouston Arc-lie hUnj; Djnamo.
mutator the idle coil is thrown into parallel sooner. This may cause a
current to be sent primarily in the reverse direction through this coil, but
in all cases it will tend to lower the electromotive force by introducing the
idle ooil too soon. The armature reaction will be increased and the magnetic
field become more distorted, so chat it is found to be advisable at the same
time slightly to advance the leading brush by an amount equal to about one
third the retreat of the trailing brush. The brush-holderB somewhat resemble
a pair of scissors, one lever of which supports the two trailing brushes, and
is moved forward or backward by a solenoid in series with the main current
(assisted also by a relay as in the Brush-Geipel regulator). The other is
made by a system of small levers to move a proportionate distance in the
oppoeite direction.
By this arrangement the brushes are shifted without much effect upon
the sparking. Another ingenious arrangement completely eliminates the
difficulties due to sparking. A small nozzle placed just in front of the
136 RECTIFICATION OF ALTEENATING CUEBENTS.
leading brush is made by means of a specially designed blower to deliver a
blast of air just at the right moment to prevent the spark developing into
an aro, and ^us prevent the possibility of a ^ flash-over." For the design
of this blower we must refer to more detailed works upon this subject.
The largest size of dynamo of this type is intended to be run at 820
revolutions per minute, and to supply 50 arc lamps in series, the current
being 9^ amperes. This should be equivalent to 22^ kilowatts, or 30 elec-
trical horse-power; and it is stated that 38 horse-power is required for
driving, giving a commercial efficiency of nearly 80 per cent. The dynamo
is exceedingly well ventilated, great durability and perfect regulation with
very little attention is claimed as its principal features.
Until the last few years the two types of open-ooil dynamos already
described have practically held the monopoly of series arc lighting, and by
far the greater number of arc lamps in existence were supplied from one or
the other. These types are not adapted to be built as very large units,
such as are advisable for central station supply for large cities in order to
minimise the attendant labour and subsidiary expenses, and to economise
space and power. Considerations of safety in using high E.M.F. impose a
limit on the number of arc lamps which can be connected in a single series
circuit, and some, perhaps not insuperable, difficulties would arise in the
regulation for constant current if large units of these types were employed
to supply several such circuits in parallel. In some cases several open-ooil
dynamos haye been employed driven by the same large engine, thus partially
overcoming the difficulty. In other cases motor -driven open-ooil dynamos
hUve been installed, so that the large units supplying the incandescent
lighting for interiors may also be used as the primary generators of power
for the series arc-lighting circuits; this system must involve unnecessary
waste of power.
It is most frequently in oombinfltion with high pressure alternating
current systems, as these are adapted to the supply of extensive areas, that
series arc lighting will be advisable, and means whereby the two systems
may be supplied from the same generators, other than motor-driven open-
coil dynamos, have been introduced. To employ directly the power generated
by the single-phase alternator for series arc lighting we require that the
current should be ** rectified," or oommutated so as to flow in a uniform
direction, and the regulation in the series circuit modified from that of
constant E.M.F. given by the generator to that of constant current. The
*' rectifier " system designed by Ferranti fulfils both these requirements.
The current supplied by the alternator with constant E.M.F. (generally
of 1000 or of 2000 volts), is first transformed by a modified alternating current
transformer, which produces an approximately constant current in the
secondary circuit, whatever may be its resistance. The current in this
secondary circuit is rectified by a revolving commutator, driven by a small
synchronous motor, the current for driving which is obtained from a trans-
foimer of the common type, connected to the primary alternating circuit.
Fig. 45 shows the complete apparatus. The constant current transformer
forms the table upon which the motor commutator stands. The magnetic
circuit of this transformer includes two somewhat elongated horizontal limbs,
round the middle of which the secondary coils are wound. The primary
coils are wound in two equal coils on movable discs placed on either side of
the secondary, and so suspended that they are capable of moving freely in
opposite directions alon^ the limbs of the magnetic circuit, from or towards
the secondary coil. When currents are flowing in both primary and
secondary coils, a repulsion exists between them, causing the movable coils
to be repelled further apart when the current increases. The repelling force
is balanced by adjustable weights, forcing the two sections of the primary
THE FEKBANTI RECTIFIER. 137
ooU towards the secondary coil. If the currant in the secondary circuit
riaes above the normal amount, aotl the movahle coile are repelled further
apart, great maguetio leakage in the field created by the primary circuit
ennuee, and the E.M.F. in the secondary circuit falls below tliat given by th«
transforming ratio, as determined by the ratio of the number of turna
PiQ. ^5.
Fenanti B<!ctlfler.
It is sufficient to arrange that the current should not rise in the secondary
circuit more than about loo per cent, above the normal when all the
external resistance is removed. Regulation of the current may then be
effected by shifting the adjustable weights, so as to alter the positions nf the
movable coils, and the automatic action of the transformer will prevent
any dangerous increase of the secondary current.
The commutator is driven by a smitU syuclii'otioiis motor, which is run
up to the speed of synchronism with the alternutiiig curitnt g-.-uwiitor by a
138 PULSATING CURRENTS.
modification of the connections which converts it to a non-sjnchronous
motor, and thenceforth keeps in exact step with the alternations. It is
preferable that a low frequency be employed for the purpose of rectification ;
with a frequency of 50 alternations per second, or 3000 per minute, a four-
pole synchronous motor will rotate at a speed of 1500 revolutions per
minute, and a commutator of eight sections be required. This commutator
has the alternate segments connected together, one set being connected
through a sliding contact touching an insulated ring on the motor shaft, to
one terminal of the transformer secondary coil, the other set to the other
terminal. Brushes, leading to the series arc eircuit, are arranged to touch
successive segments; two such pairs to divide the duty of collecting the
current are generally employed. The commutator-segments are separated by
air-gaps, and the brushes must be advanced to such a position that sparks
of about a quarter of an inch in length appear to trail from the brush con-
tacts ; otherwise the commutator wiH be subject to '' flashing over," for
reasons that have been already explained in dealing with the Brush open-coil
dynamo, and which apply equally to the rectifying commutator. Aa many
as sixty arc lamps may be connected in series to the rectified circuit, the
tranformer being wound to give 3000 volts in the secondary coils when
there is no magnetic leakage. Owing to the heavy load thus thrown on the
alternating current generators, it is advisable to perform the operation of
switching on the current gradually ; for which purpose a specially designed
fluid resistance is supplied.
The direction of the current in the series circuit will now be constant,
but its magnitude will fluctuate between a zero and maximum value. It is
therefore termed a '* pulsating " or ^ rectified oscillatory '* current. The
efficiency of arc lamps supplied with a pulsating current appears to be little,
if any, inferior to those supplied with a continuous current, that is to say,
one of invariable magnitude as well as direction. It is, in fact, claimed that
the vibration produced in the arc lamp mechanism promotes regularity in
the feeding, and greater steadiness in the light. It has certainly been
demonstrated that the temperature, and light radiating from the pulsating
arc lamp varies greatly during the pulsations, being lower and therefore less
efficient at the moment of commutation and zero current ; but this variation
of temperature of the positive electrode with the pulsating current can be
nothing like so great as in the electrodes of the alternating current arc
lamp, in which each electrode in turn is radiating heat and light without
generation for more than half of each complete period. It may be that the
total illumination emitted from the pulsating arc lamp is subject to no less
fluctuation than that from the alternating arc ; this is however a different
matter to the fluctuation of temperature of the electrode ; the source of
radiation in the alternating arc is constantly shifting from one electrode to
the other, and the heat energy stored up during the time either is the
positive is radiated with a smaller proportion of light radiation during the
time it is cooling down as the negative. With th^ pulsating current
the period of cooling of the positive electrode is much shorter, and the
radiation proceeds from this electrode without much loss in efficiency.
Furthermore, the advantage of better distribution of light remains with the
pulsating as with the continuous current.
A more complete discussion of the subject of the luminous efficiency of
the arc lamp, complicated as it is by the variation of the radiation in
different directions, and the practical question as to the advantages or
disadvantages of greater uniformity, will be given in the chapter on aro
lamps. It is the opinion of many that the advantages of the continuous
current arc lamp over the alternating current for street lighting has been
much exaggerated, and at least does not compensate for the cost of the
ALTERNATING CURRENT SERIES SYSTEMS. 1 39
rectifying machinery, and the ooet in skilled attendance that it inevitably
demands. If this be so the rectifier itself may be omitted from the system,
and all the advantages of the high efficiency of distribution, and combinar
tdon of the series-arc with the alternating-current generating plant with
constant potential maintained by the use of the above-described constant
potential to constant current transformer, giving a series system with
alternating current.
A branch series circuit from an alternating-current system for a few
lamps only requires the employment of a simpler transformer adapted to
give a constant current in the t^econdary coil when constant E.M.F. is main-
tained in the primary. Such a transformer has been devised by the
Thomson-Houston Company, which regulates the constancy of the current
in the secondary circuit with sufficient exactness for the purpose without
moving coils, as in the transformer for Ferranti's rectifying system. The
primary and secondary coils are wound on separate limbs of the magnetic
circuity and the magnetic leakage is enormously increased by a large mag-
netic by-pass across a short air-gap. The passage of a large current through
the secondaiy coil increases the magnetic leakage, and thus lowers the E.M.F.
which is generated in it. The simple form of constant potential to constant
current transformer can be easily regulated by a sliding laminated slab
slipped into the air-gap, so as to vary the reluctance of the by-pass, and
therefore the leakage.
For alternating-current arc lamps a series arrangement of transformers
has been employed, the primary coils forming the series circuit, and the
secondary coil, similar to the primary in the number of turns, connected to
the single arc lamp. The current in the secondary coil is in general equal
to and cannot exceed that sent through the primary circuit, even if the arc
lamp be short-circuited. An automatic short-circuit is combined with the
arc lamp mechanism, and, in the event of the lamp failing, the corresponding
transformer will offer very little opposition to the passage of the primary
current, and will absorb very little power. By this means the advantages
of series distribution are secured without the difficulties arising from hi^h
electric pressure on the lamps. A small transformer is fixed in the base of
each lamp-post. The alternator to supply the series alternating current
circuit should have an armature with very great self-induction, in order
that the current output may be limited, and the alternator short-circuited
with impunity. The characteristic of the alternator may be made rapidly
^ falling,'' and a fairly constant current may be produced without further
regulation, the conditions being similar to that of the shunt-wound dynamo
near the critical point, but without the low efficiency and danger of demag-
netisation.
For the lighting of large thoroughfares the use of arc lamps taking
currents of from 9 to 12 amperes, with a series distribution, is eminently a
satisfactory and efficient system. For minor thoroughfares a further sub-
divisioii of the light is advisable, and the demand for this further subdivision
will increase as the employment of electric lighting extends. If the electric
arc be still used as the illuminant a reduction of the current is necessary,
for the electromotive force of each lamp cannot be reduced much below
fifty volte. We are therefore limited, in series distribution, with regard to
the number of lamps that can be placed upon the same circuit, the practical
limit being about 60 lamps, giving a total electromotive force of 3000 volts,
which cannot conveniently be exceeded. The only way to further subdivide
the electromotive force is to employ incandescent lamps, which are not
similarly limited in electromotive force, but which can be constructed with
short thick filaments of carbon so as to use a large current of, say, 10
amperes, with electromotive foi-ces of from 6 volts upwards. The use of
I40 INCANDESCENT LAMPS IN SERIEa
low voltage incandescent lamps in series has not as yet found mucb favoar in
this country, but proved very successful in America. The multiplication of
the lights necessitates a multiplication of cut-outs, or short-oircuiting
devices, in order to complete the circuit in the event of the failure of any
lamp, and for this reason some veiy simple automatic device is required.
The Thomson-Houston Company insert a very thin film of paper between
the terminals of the lamp, which is capable of resisting the normal electro-
motive force of the lamp, but of which the dielectric strength at once gives
way before the full electromotive force of the dynamo which is applied in
the event of the lamp circuit breaking. The arc produced fuses the ter-
minals together and maintains the circuit.
The Westinghouae Company use an alternating current, placing an
impedance coil in parallel with each lamp. Very little energy is absorbed
by the resistance of this coil and the hysteresis of the core, and the passage
of a small diverted current raises the electromotive force of the lamp to its
normal amount. If the lamp breaks all the current paf^ses through the
impedance coil, raising the difference of potential by only a little, owing to
the saturation of the core, but not absorbing energy in propoi-tion owing to
the phase difference.
The Farfitt system, recently adopted for street lighting in several minor
English towns, is at present the only extensively used series incandescent
system in England. The lamps are connected in pairs, each pair consisting
of two lamps in parallel, the pairs of lamps being connected up in a series
circuit. Several circuits may be connected in parallel to the same dynamo,
which may be regulated either for constant electromotive force or constant
current, the resistance of each circuit remaining constant. Each lamp has
a small electromagnet in series with it which, in the event of an excessive
rise in the current through that lamp, attracts an armature which throws
into parallel a resistance equal to that of the lamp. Now in the event of
the failure of any lamp, a double current passes through the lamp in
parallel with it, but no injury results, as the electromagnet instantly acts,
and the lamp that failed is replaced by an equivalent resistance. If the
second lamp of the pair should fail subsequently, the parallel resistance is
capable of carrying the whole current, though with some extra expenditure
of power.
Bernstein hsB designed a form of incandescent lamp intended for series
working, in which a straight carbon tube takes the place of the ordinary fine
filament. This tube is woven of silk, or some textile fabric, which is then
saturated with syrup and carbonised. It is supported by hardened iron
wires, bent so as to make contact in the middle of their leujoiihs, at which
point platinum contact-pieces are fused on to the wires. The carbon tube
is held horizontally between the extremities, and is of Kufiicient rigidity to
bend the wires apart and break their contact, but in the event of a fracture
a short-circuiting contact is at once made. The lamp was intended for
internal as well as external working, and made in small sizes of i6 to 50
candle power, with 4 to 15 volts. It was held in an ingeniously constructed
holder, from which it could not be removed without turning a short-
circuiting switch.
Bernstein also designed a chemical short-circuiting plug, consisting of an
oxide of mercury combined with carbon, which, placed in parallel with the
lamp, gave a resistance of about 200 ohms. But when, owing to the
failure of the lamp circuit, the full electromotive force of the dynimo was
applied to its terminals, the resistance was speedily reduced by electrolytic
action to a very small amount.
Goldston has designed another purely menhanical method of maintaining
the circuit in the event of the fracture of the filament, or gliiss bulb of an
INCANDESCENT LAMPS IH SERIES. I4I
inoaDdexcent lamp. Th« contact is made exterDolly instead of inteniall^,
M in the BernBt^n iMnp. Two contact pieces, with bmad Burfacee, are
supported on levers «hich give a parallel motion, and are forced into contact
by powerful Hprings. The lamp terminals are connected to these, and by
tBeoDB of a screw collar connecting the sealed end of the bulb to the frame
of the lamp, a tension is applied through the glaaa which draws the contact
[Hecefi apart. A. fracture of the glass will allow the short-circuiting contact
F10.46.
Qoldstoo SstieH looarideBoeDt Lamp.
to be madn ; and the fracture of the filamcDt will allow an arc to be formed
between the supporting iron wirw, which will either fuse tbem together, or
ciiuse molten metal to fall and fracture the gla»s, and thus indirectly cause
the fhort-circiiit. Gold^ton's lamp is adapted to use a current of 10 amperes
with an electromotive force of about n or 12 volts, and is intended to be
used in series with a io-amp*re series nrc system for paiitions where a
smaller light is required. The candlf [<on-er b about 50.
CHAPTER X.
Alternating Currents (Theory).
The continuous current transformer system, employing secondary batteries
and motor dynamos for the transference of power from a high tension
circuit to a network of distributing low tension circuits, is probably destined
to play a more important part in electric lighting in the future than it does at
present, for the principal objection to the system is one which will lose its
142 GENERATION OF ALTERNATING CURRENTS.
force as the electric light becomes more universally adopted. This oV»jection
is the necessity of constant attendance on the batteries and motor-d> namoe,
requiring that they should be concentrated at transforming centres, or sub-
stations, which feed the surrounding district through a low tension network.
Now in entering into competition with older established means of illumina-
tion, the first customers of an electric supply system were to be found here
and there over the whole area undertaken, and numerous small centres
of transformation were necessary to utilise the full advantage of the high
tension transmission. The cost of construction and attention at these small
centres or substations is relatively much greater than it would be if the
number of lamps supplied within the same area were large. Henoe it
followed that a system which does not render attention at the transforming
centres necessary, and allows indefinite subdivision of transformation,
obtained and still holds the preference ; a system fulfilling the requirement
was supplied by alternating current transformation.
Alternating current transformation is effected in a manner analogous-
or in fact almost identical, to that of continuous currents by the motor-
dynamo, the only difference in the principles involved being that mechanical
motion (the rotation of the double armature) is replaced by rapid variation,
in magnitude and direction of the E.M.F. and current. The result is a
more efficient and less expensive transformer, effecting the same purpose,
and requiring no attention when in use. And though the alternating-
current transformation system will certainly be most efficiently worked when
the centres of transformation are concentrated into large substations, and
under supervision, as is necessary with continuous current trausformater,
it has hitherto been applied most frequently with a neparate transformer on
the premises of each consumer, an expedient acknowledged to be temporary,
and destined to be replaced by a more Katisfactory arrangement, when the
<< density" of the supply, that is the number of consumers within a fixed
area, is sufficiently increased. The alternating system suffers under the
disadvantage that at present no means of storage of the power has been
devised, but it is hoped by its advocates that by the utilisation of the
generating plant and mains to supply motive power during the daytime, the
load on the former may be made more uniform, and the advantage of
storage thus rendered of little importance. If this hope be not realised, or
a method of storage discovered, the continuous current transformater
system will possess an advantage which may, e-^pf cially in dense districts,
more than compensate for the more costly and inefficient transformation.
A combined system, of recent introduction and possessing some of the advan-
tages of each, is already under trial ; the main principli^s of this will be
considered when we have described the systems which, after prolonged use,
have been acknowledged as effective.
The generation of alternating currents is more simple in theory than
that of continuous currents. The E.M.F. produced by the variation of the
flux of Magnetic Induction through any closed circuit is necessarily alterna-
ting, that is to say, it must change in direction periodically, and the E.M.F.
can only be maintained in a uniform direction in a part of the circuit by
some interchange of the connections. In a continuous current dynamo the
E.M.F. and current in any turn of the armature are alternating, and it is
only in the external circuit that they are made continuous by the constant
change of the connections at the commutator. Tiie fiux of Mngnetic Induc-
tion through a coil cannot increase or decrease indefinitely, so that when the
variation changes in sign, the E.M.F. also changes in direi;tion. In a single
coil rotating with uniform speed in a uniform magnetic field about an axis
in its own plane, and perpendicular to the direction of the lines of force,
the direction of the E.M.F. is reversed twice in every revolution, and the
SINE LAW AND FREQUENCY 1 43
magnitude of the E.M.F. being proportional to the cosine of the angle of
inclination of the plane of the coil to the direction of the lines of force,
varies gradually between its extreme values in either direction, repeating
the cycle of variation with every revolution of the coil.
The alternating E.M.F. produced by the rotation of a coil in a field
which is not uniform, may differ greatly as to the law of variation in
magnitude of the E M.F. produced. For some purposes a very different
law of variation is purposely ejected. In the well-known induction coil,
for example, an alteruHting E.M.F. of high intensity is proiiuced in the
secondary coil by the magnetisation and demagnetisation of the bundle of
iron wires which form the core. The magnetisation by the current in the
primary coil is relatively slow, resulting in a relatively small E.M.F. in one
direction : the demMgnetisation is effected by the self -demagnetising force of
the poles of the shoi-t core, and is much more rapid, producing an E.M.F.
of much higher intensity and shorter duration than that produced by
magnetisation. The current produced across the spark gap will be only in
one direction unless the electrodes be brought close together, the direction
being that given by the demagnetisation E.M.F. In a form of electro-
magnetic generator adapted to medical purposes a high E.M.F. is periodi-
cally produced in a circuit which bridges a gap suddenly opened in a self-
inductive circuit carrying a current, the E.M.F. being alternating in direc-
tion with intervals of rest ; attaining high values momentarily, such as may
be called electric impulses, but a very moderate average value.
For the generation and transformation of considerable electric power, it
is advisable that the variation of the alternating E.M.F. should be fairly
gradual, at least avoiding the sudden changes described above. That
obtained by the uniform revolution of a coil in a uniform magnetic field,
commonly known as the *' harmonic " or '* sine " law of variation, is most
commonly purposely secured by the design of generators. It is most con-
venient for mathematical analysis to assume the sine law, though, as will be
seen later the variable permeability of iron will much modify the law of
variation in actual practice. Whether the sine law gives the most efficient
results remains a matter for discussion. A cycle of variation, corresponding
to a single revolution of a coil in a uniform field is known as a complete
aUerwUian, or period, the number of complete alternations per second is
termed the frequency of alteration. It is the universal practice to use the
symbol ro for the frequency of an alternating E.M.F. or current ; in mathe-
matical investigations, however, this quantity is almost always combined
with a multiplier 2ir, and we shall use the letter p for the product 27r ru.
An alternating E.M.F. will produce an alternating current in any
complete circuit, and if the circuit be non-self -inductive, as for example, a
bank of incandescent lamps, and if its resistance remain practically constant
dui-ing the alternations, the current will at all times be proportional to the
E.M.F., being determinable by Ohm's law, and will therefore follow the
same law of variation, arriving at its maxima and zero values at the same
moment. In self-inductive circuits, however, that is to say whenever the
current can create a magnetic field of its own, the variations of the latter
field with the current will introduce a further E.M.F. into the circuit, which
will modify the relation between the current and the original or impressed
E.M.F. But before considering this modification we must understand the
principle upon which alternating currents and E.M.F. are to be measured.
With the frequency commonly employed for electric lighting (from 50 to
150 complete alternations per second), an alternating current would not
deflect a galvanometer, as the motion of the needle (or moving coil) would
be far too slow to follow the current through its rapid chan^jes, and a position
would be taken up measuring the average (algebraic) value of the current,
144 VIRTUAL MEASUREMENT.
which if produced by electromagnetic changes in a generator whose total or
average value is zero, must also be zero. With such instruments as the
Siemens Dynamometer, or Kelvin Current Balance, where the current is
measured by the attraction of two coils in each of which the current is made
to flow, the direction of the current is immaterial, and the instrument may
be used to measure either continuous or alternating currents. Now in sueh
instruments the attracting or deflecting force on the moving coil is neoes*
sari]y propoitional to the square of the current passing, and provision for
this is made in the calibration for continuous currents. When used to
measure alternating currents the attracting or deflecting force is proportaonaJ
to the average square of the current as it passes rapidly through its cycle of
values ; with the same calibration as was used for continuous currents the
measurement given is of the number of amperes whose square is the average
square of the number of amperes of current at all times.
Now the power expended in the generation of heat by a continuous
current of amperes, flowing in a circuit of resistance R ohms, is C^R watts.
An alternating current flowing in the same circuit would involve the
expenditure in heat generation of the power measured in watts by the
average value of the square of the number of amperes multiplied by R.
Hence, if the continuous current and the alternating current are to generate
heat at equal rates, and represent the absorption of an equal amount of
power in this way, the measure of the former in amperes should be the
eqitare root of the mean sqttare of the variable measurements of the latter,
an infinite' number of measurements being supposed taken at equal but
infinitesimal intervals so as to obtain a true time-average. This measure-
ment of the alternating current is given directly by the dynamometer or
current balance, calibrated for continuous currents, and is that of the con-
tinuous current that will have on equivalent effect in heating the conductor.
A hot-wire ampere-meter will also give a similar relation between its
measurements of continuous and alternating currents. For the sake of
distinction the unit of measurements of alternating cuixents taken in this
way, is termed the *^ virtual ampere."
Alternating E.M.F. is measured by the number of virtual amp&res of
the alternating current it would produce in a non-setf-inductive circuit of
resistance equal to one ohm, the unit being termed the '* virtual volt." The
same measurement will also be given with electrostatic instruments calibrated
for continuous electromotive forces, since in these instruments the electro-
static forces of attraction are proportionaUto the square of the difference of
potential between the fixed and moving parts. An equivalent term, less
frequently used, is that of '^ effective " in the place of '' virtual."
If the alternating current or E.M.F. follow the harmonic or sine law of
variation the relation between its measiirement in virtual amperes or volts
and the maximum values reached every alternation may be found as follows.
Let £ be the maximum E.M.F. attained ; the E.M.F. at any moment, e,
is given by e»E8in0, where to is given all possible values (in the case of
the generation of E.M.F. by the rotation at uniform speed of a coil in a
uniform magnetic field, is the angle between the axis of the coil and the
direction of the lines of force). Now the average value of sin ^9 is one-half,
since for every value of there is another value (the complementary angle)
such that the sum of the squares of their two sines is unity, and therefore
the sum of a large number of values of sin^d taken at small uniform intervals
is one-half of the number taken. Therefore the average value of e^ is ^E',
and the measure of the E.M.F. in virtual volts is— 7-E. A similar relation
connects the measurement of the current in virtual amperes with the
maximum current reached.
SELF-INDUCTANCE. I45
Subject to other laws of variation than the harmonic the relation between
the virtual and maximum measurements will be different. For example,
with a uniform rate of increase and decrease from a maximum in one
direction to a maximum in the other, the ratio of the maximum to the
virtual measurements is that of the square root of three to unity.
The power absorbed in heat generation by the conducting circuit may be
in all cases obtained in watts by multiplying the square of the measure of
the current in virtual amperes by the resistance of the circuit in ohms.
But Ohm's law will not be applicable to give the relation between the
measurements of the E.M.F. and current in virtual volts and amperes,
except for circmts free from self -inductance, and, therefore, it is only for the
]atter circuits that the power may be measured by the product of these
measurements. The actual number of watts supplied to the circuit from
the generating source will lie between C'.R and E.O. The former, being the
lower value, will be correct when the power is wholly spent in heating the
oonductor itself, as it will be in a coil of wire without an iron core which
does not induce currents in any other cofl. In general some of the power
supplied is transferied to another circuit in which ourients are induced,
or converted into mechanical power directly, and this transferred power is
less than the difference between E.0 and C'.B. In the armature of the
continuous cnirent motor, it will be remembered, we have a similar formula,
the difference between the above quantities being the product of the
measures of the current and the " back " E.M.F. of the motor, in this case
representing, without reduction, the power transferred from the armature
circuit, for the most part (less the minor electrical losses in hysteresis and
eddy-currents) reappearing as mechanical power.
We have now to consider the modification of Ohm's law mentioned
above, applicable to self-inductive circuits.
Suppose the electric circuit external to the generator of an alternating
E.M.F. to consist of a coil of wire wound round a laminated electromagnet,
which we will for simplicity suppose to have a closed magnetic circuit of
uniform section embraced by the coil. We will further suppose that the
mafmetism of the iron be never carried to saturation, and that the permea-
bility of the iron may be taken as approximately constant through the
range of magnetic variation that will ensue. If c be the current that at
any time passes through the coil, n the number of turns, and A and 1 the
cross-sectional area and lenfifth respectively of the magnetic circuit, the
magnetic flux of induction will be given in ag.s. units by,
i?nc
Am
The variation of the magnetic flux will produce an E.M.F. in the coil which
will oppose the impressed E.M.F. of the generator, this opposing or " back
RM.F." beingJi^ . ^ volts, so that if e be the value of the E.M.F. of the
10 dc
generator at any moment the current will be given by.
^•^""^ I^- di'"" — -dt
The expression — r-* in this equation is termed the " coefficient of self-
146 IMPEDANCE.
induction," or " self -inductance," or briefly the ^ inductance " of the circuit,
and is commonly denoted by the letter L. In any electric circuit we may
define the inductance by stating that the back E.M.F. in the circuit is
measured in volts by the rate of change of the current in amperes multiplied
by the inductance. The unit of inductance is that of a circuit in which the
change of one ampere per second results on the back E.M.F. of one volt, and
is termed the " henry," or •* secohm."
Every electric circuit, even a straight wire, possesses some inductance,
since a magnetic field is produced in the neighbourhood. For coils of wire
the inductance is proportional to the square of the number of turns, and
inversely proportional to the reluctance of the magnetic circuit. The
inductance may be predicted from the measurements of the circuit, as in the
simple case given, or may be measured by laboratory methods. When iron
is present in the magnetic field the inductance is not a constant quantity
bat varies according to the intensity of magnetisation at any moment, and
in general gives different values according to whether the current is increasing
or diminishing. We may, however, consider, at least for the elementary
investigation of the action ef alternating currents, that the inductance is
constant for the coils of electromagnets as long as the magnetism does not
approach saturation, postponing the modifications that will be introduced
by the considerations of variable permeability and coercive foroeu
We may now write the equation for the current^
B.c + L.^-e.
dt
If the variation of the E.M.F. follows the harmonic law^ we may write^
6 vBsinpt
where E is maximum E.M.F. attained during the alternations, and -£- it
the frequency, or number of complete alternations per second. The oolation
of the above differential equation may then be written,
OB08iD(pt - 9).
B
Where ■■ , ^ x and is the maximum current (in ampires) attained
during the alternations; and tanO ■> JL^ Obeing termed the ''lag^or difiTer-
ence in ''phase" between the current and the E.M.F. Thequantify j/R^+p*!^
is called the '' impedance " of the circuit, or sometimes the '' viitoal resistance,"
and may be denoted by the letter L So that mnB ■■ ^^ and oosO » ^
Denoting by E' and Cf the measurements of the E.M.F. and euirent in
virtual volts and virtual amperes respectively, the relation between them is
E*
the same as that between the maximnm values, namely CT » .y^
The energy stored up in the magnetic field when a current ii jtummg
through the coil is ^ Ixd* watt-seconds, or ^ L.<^. 10^ ergs.
Graphioal Methods.
Two distinct graphical methods are in vogue for the iUustration of the
phenomena of alternating currents. The first is known as the dook-diagramy
in which the maxima values of the current or E.M.F. are repreeenteid by
vectors, or lines of fixed length drawn radially from a central point. Sup*
posing the figure to revolve uniformly about the central point once daring
VECTORS AND CURVES.
147
a complete alternation, the actual values of the current or E.M.F. would be
given by the horizontal or vertical projection of these lines. The rotation
is generally supposed to be in the opposite direction to that of the hands of
the clock, and drawing one of the vectors in any direction such that the
vertical projection represents any instantaneous vulue, the direction of any
other vector, representing similarly another current Fig. 47.
or E.M.F. will be determined by the phase relation
between it and the former.
For example, in Fig. 47, OP may be drawn to
represent the maximum value of the E.M.F.inany
circuit, PN its instantaneous value. Let C be the
maximum value of resulting current, somewhat
lagging in phase (given by the angle POQ), If OQ
is drawn to represent OR, the product of the
number of amperes and numbers of ohms, it can be
shown that PQ is perpendicular to OQ^ and in
magnitude is equal to pLCy where je> is 2 tt times the
frequency. For, completing the parallelogram
OPQP', it will be seen that since OQ is the diagonal of the parallelogram,
by a well-known geometrical theorem familiar to students of mechanics,
its vertical projection is the algebraic sum (in the figure the projection of P'
is negative) of OP and OP*. Now OP* is a right angle, or a quarter-phase,
behind OQ, and represents the back E.M.F. due to Inductance in the circuit
at any moment. This subtracted from the instantaneous value of the E.M.F.
should give the value required by Ohm's law, which is the projection of OQ.
The second method is to represent the instantaneous values of any
quantity by ordinates drawn upwards or downwards from a horizontal zero
line. The time is here represented by the position on the zero line at which
the ordinate is drawn, any convenient length representing a complete
period. The extremities of these ordinates, representing a recurring
quantity, will be on an undulating curve repeating with each period. The
Fig. 48.
curves so drawn may represent coDvenieiitly all the values through which a
quantity such as alternating current or E.M.F. may pass.
For example, Fig. 48 represents the curves for the E.M.F. and current
in a circuit possessing high self- inductance v/ith very little resistance, the
difference in phase being practically a quarter- period.
The great advantage which these curve diagrams possess over the clock
diagrams, and, to some extent, over mathematical formulae, is that they are
capable of representing simply alternating quantities which do not follow
the harmonic law of variation.
Fig. 50 is drawn to indicate the deformation of the current curve in a
low-resistance coil of high self -inductance, where the magnetism of the iron-
core is raised to a moderate degree of magnetic saturation. This current
curve is traced from purely theoretic considerations from the cyclic curves of
Fig. 49. The E.M.F. in the circuit being represented by the harmonic
148 DISTORTION DUE TO IRON CORES.
carve haying the smaller ordinates, the curve for the Magnetic Induction
must be similar to that having the larger ordinates (the magnitudes of the
ordinates depending on the scales chosen for each). For the variations of
the Magnetic Induction, there being negligible resistance, are such that its
variations balance the E.M.F. in the circuit, and therefore foUowa harmonic
Fig. 49.
Fig. 50.
law, with a quarter period phase-retardation behind the E.M.F. The current
is at any moment such as will give the Magnetic Induction, whether rising
or falling, at the corresponding points on the cyclic cui*ve. The maxima
values of Current and Induction will occur simultaneously, but the current
curve will fall more rapidly to give the corresponding values of the Induction
which is " retained " when the magnetic circuit is complete, and will reach
the sero value at the point corresponding to residual Magnetic Induction.
CURVE TRACING. . 1 49
The current must also reach, when the Induction is zero, a value corresponding
to the Coercive Force. The shaded curve AA thus represents the actual
current curve, very much distorted from the true harmonic law, which is
only approximated to when very low magnetisation is given by the alter-
nating current, or when the magnetic circuit is incomplete.
The actual tracing of the curves for E.M.F. and current is exceedingly
interesting and instructive, and in some cases of definite practical utility^
This has been done directly for currents with an instrument termed the
osciUograph, which ia practically an instantaneous reflecting ammeteri
recording on a moving photographic film. The instrument is, however, far
too delicate when the errors of natural oscillation are sufficiently reduced
for the measurements to be exact. The more practical ** point-to-point **
method is now used in every experimental laboratory, in which readings are
taken upon an electrostatic instrument arranged to be switched on <mly at
certain definite instants taken in succession through a complete period. A
revolving switch upon the axle of the generator, or synchronous motor, is
used, and many practical forms have been devised. A condenser charged
through the revolving switch every revolution of the generator will maintain
the d&erence of potential the same as at the moment of contact throughout
the revolution, and this may be read with the electrostatic voltmeter,
quadrant electrometer. The writer has a preference for a delicate moving
coil galvanometer, with an immensely high resistance in series with it, to all
intents and purposes equivalent to an electrostatic voltmeter, but much wider
in range and quicker in reading. With the latter it is possible to trace the
curves for currents also, by introducing an additional resistance of very
small value, and without self -inductance^ into the circuit; the fractionid
differences of potential between its terminals are then readable, and give
the true instantaneous current values when multiplied by the resistance
in ohms.
Most alternating-current dynamos give curves of alternating E.M.F.
differing appreciably from the harmonic law. The current curve will in
general follow that of the E.M.F in non-inductive circuits. Ohm's law
being applicable for all instantaneous values, but self -inductance, and, still
more, the variable permeability of iron, causes modification. The assump-
tion of the harmonic law is generally sufident for all purposes of calculation
relative to design, but the deformation of curves has more effect on
eficiencies than is at present recognised. It appears that a modification of
the harmonic law for the electromotive force in the direction of a more
** peaked " curve, that is a more rapid rise at the maximum and less near the
aero value, would give less iron-core losses in transformers ; while, on the
other hand, arc lamps and motors would be more eficiently served by a
blunt or flattened curve. The former is accounted for by the fact that a
peaked curve for the E.M.F. results in a flattened curve for the Magnetic
Induction, avoiding the hysteresis loss which mainly depends on the
maximum value reached. Steinmetz has reported a test upon a 200-kilowatt
transformer where the loss was reduced 13 per cent, by the substitution of a
peaked E.M.F. curve for one following the harmonic law.
The power absorbed in the cii^ciiit by the resistance, and thus converted
into heat, is measured in watts by C'-R or ^ G'K. This may also be written
in either of the forms —
E'>, 5.K'«, ?E'0'. B'C'oos^.
The product E'C, multiplying the readings of voltmeter and amperemeter,
is often called the number of *^ apparent " watts, and the correcting factor.
ISO MUTUAL INDUOTANOR
the cosine of the angle of lag, is necessary to obtain the real value of the
power absorbed.
The expression E'C cos for the true number of watts supplied to a
coil from a generator remains true under all conditions, and even when the
harmonic law is not followed by the current or E.M.F. there still remains a
factor equivalent to cos 6, which may be measured experimentally and is
called the ''power factor." But the great value of alternating-current
employment consists in the fact that the power is not necessarily absorbed
as heat in the coil, but may be transferred to another coil in its neighbour-
hood by induced currents through the medium of a magnetic field common
to both. Generally the coils carrying the inducing and induced currents
are named primary and secondary, and the combination is termed a
** transformer " or '' converter." The current in the primary coil is largely
modified by that in the secondary, and it is necessary to enter into a short
theoretic investigation of this influence as the foundation of the practical
discussion of alternating-current systems of distribution.
Suppose, for example, the secondary coil to consist of n, turns, wound
80 as to include the whole magnetic fiux through the primary of n| turns.
Let the primary, as before, be wound roand a laminated iron ring, or be of
the foim of a long solenoid or complete ring, so that the magnetic field is
uniform. In this case it is possible to calculate the flux of magnetic
induction through the coils as before, and the E.M.F. produced in the
aeoondary coil by change of the current in the primaiy will be
The oo-effident of -7- is called the '^ co-efficient of mutual inductioiii* or
dt
^ mutual inductance " of the two coils. In all cases, for any pair of coilsy
the mutual inductance is a measurable quantity, and is the same whichever
ooil be taken as the primary, and which as secondary. As with self-
inductance, it is difficult to calculate mathematically from measurements
except in the example given above, nor is it a constant quantity when an
iron core is used and a high degree of magnetisation permitted. The letter
M is commonly employed to represent the measures of mutual inductance,
and the same units as for self -inductance, the '' henry " or '' secohmi" is used,
as the quantities are "of a similar nature.
The graphic representation of the power supplied to the circuit is indl«
cated in Fig. 51, where the thick line represents the curve for E.M.F., the
thin hne the current; and the shaded curve, whose ordinates are pro-
portional at each point to the product of the number of volts and amperes
at each instant, represents the curve of power supply. The shaded portions
represent by their area the total energy delivered to the circuit during any
oorresponding period, the part below the zero line indicating energy returned
from the magnetie circuit to the generator. It may be easily shown that
the power curve also follows the harmonic law, with twice the periodicity of
the E.M.F. and current curves, and preserves its own dimensions exactly
so long as the maxima values of the E.1V1.F. and current remain the same.
Its central line is, however, raised somewhat above the zero line of the
diagram, by an amount dependent upon the phase difference. When the
E.M.F. and current are co-phasal the power curve is raised wholly above the
zero line, touching it at every zero value of the former curves. As the phase-
difference is increased the power curve sinks down, indic^iting a smaller and
smaller amount of average power delivered to the circuit; until with a
QENERAL CASE 0? TWO COILS. 151
phMe-differenoe of & quarter period the power cnrre is equally above &nd
Delow the zero line, indicatiDg that the energy oecill&tes without loss, an
ideal case, between the generator and the circuit.
It will be noticed that Fig. 50 given above to illofitrate the detomu^
ticKi of the current-curve due to intense magnification shows that power is
really delivered to the circuit. For, on the whole, the curt«Dt-cnrve is
in^gti forward so as to give an equivalent lag somewhat lees than a qnarter-
penod. The power-curve may easily be traced to show this more dearly,
and obviously represeota the power absorbed by the bystereeie of the ir(m.
Calling the circuit connected to the generator the primary, and the
other the aeeondary ooil, we will suppose the latter to have a self-inductance
I^, and a resistance R^ and a current c, to flow in the secondary drcait,
while we shall now use the symbols c„ B„ L, for the current, ix., in the
primary drcait. The equations for c, and c, may be written :
R,.c, + Li
^+M.^ = E.di,pt
:-^=
The complete solution of these simultaneous differential equations give
the magnitudes and phases of the currents in the primary and aecondary
coils. It may be shown that
That is to say, the current flows as if in a circuit of equivalent reBistanoe
and inductance given by R/ and L,'.
For the secondary circuit the conditions may similarly be shown to be
fulfilled by Eupposing an equivalent electromotive force in the coil equal to
that in the primary circuit multiplied by
with a lag of
90- + »»■■'#.
152 PRACTICAL APPROXIMATION.
behind it; and with an equivalent resiBtanoe and inductance respectively
given hj
It has been thought advisable thus to write down the complete solution
without pursuiDg the intervening steps. For, though in the actual trans-
former as commonly constructed a simpler investigation would serve the
purpose, there are often cases in which a modification is necessary; for
example, when the coils are wound on opposite limbs of a laminated electro-
magnet, and the magnetic flux is by no means common to both. It is then
convenient to refer to the complete solution to explain practical variations
from the commonly accepted formulae, which suggest the explanation even
when the quantities involved are not known or measurable.
In the transformer as used for power distribution the problem is very
much simplified by the use of a high value of the frequency ; further, by
the use of transformers with little or no magnetic leakage between the coils
(except under 'conditions to be described) ; and thirdly, in systems intended
for electric lighting and not for motive power, by the i^uction of the
inductance of both primary and secondary to a very small amount above and
beyond that of the coils themselves. Except when these conditions are
fulfilled, the engineer must be very wary in the use of the simpler formula
DOW to be deduced.
If the frequency be very great, so that B^ may be neglected in comparison
with pl^, we shall have approximately,
Oa M
If, moreover, there be no self -inductance in the secondary circuit except
that of the coil which is wound with n, turns so as to include the identicad
flux of Magnetic Induction that passes through the primary coil of n, turns,
the ratio of the co- efficient of mutusd inductance to the self -inductance of
the secondary will be —^ so that C^n^ — G,n^
Further, if the primary circuit have no additional self-inductancci or if
by the alternating E.M.F. we mean the alternating difference of potential
between the terminals of the coil, we shall have,
L, : M : L9 as Di^ : DiD^ : nf
and, theref orei
R'j = R, + "_4Ra L'l-o
Dl
B',-B, + ?^Ri L',-o
The current in the primary coil will be cophasal with the E.M.F., that
in the secondary will lag 180° (since -g-* ia taken as infinity), or, in other
words, flow in exact opposition.
If E, cannot be neglected in comparison with pL^ that is, if the resist-
ance of the secondary circuit be very great, the current in both the primary
and secondary coils are extremely small, and no longer in a fixed ratio. If
the secondary circuit be broken, or B, infinity, and 0,bo, we shall have
B IS
0, s ^ -—-T = — approximately.
ALTEENATIKG OURBENT TRANSFORMERS. 153
This 18 called the magneiising current of the transformer, and were it
not for hysteresis and eddj-currents in the iron core would lag behind the
E.M.F. of the generator 90^ or a quarter-period. When the frequency and
the self-inductance of the primary coil are great, the magnetising current
becomes very small. The quantity pL,, being analogous to the resistance
in a non-inductive circuit, is sometimes termed the re-<icUince of the coil.
The above investigation has shown how an electric current may be pro-
duced in a secondary circuit, having no electric connection with the primary
circuit connected to the generator of alternating E.M.F., the sole require-
ment being that the coils forming part of each circuit should be wound so
as to embrace the same magnetic circuit. Such an arrangement is known
as an alternating current "transformer," or ''converter." The general
investigation shows that —
(i) The curi'ents in the primary and secondary coils are in the ratio of
the coefficient of mutual inductance to the impedance in the secondary coil.
(3) With a high frequency, and no further self-inductance in the
secondary circuit beyond that due to the magnetic circuit of the transformer,
the ratio between the currents in the primary and secondary circuits is
inversely as the number of turns in the respective coils (to a close approxi-
mation except in the case in which the resistance of the secondary circuit is
very high).
(3) If the resistances of the coils of the transformer are small, the
eorrent will be determined by the external resistance of the secondary
circuit, the difference of potential between the terminals of the secondary
ooQ being to that between the terminals of the primary coil in the direct
ratio of the numbers of turns (which is therefore called the ratio of trwrhB"
foTfiMiion). The effect of resistances R, and K, ^'^ ^^® primary and
secondary coils respectively of the transformer will be equivalent to an
additional resiBtance added to the external resistance of the secondary
of Bf + •^, Rif so that if E be the difference of potential (maximum)
n '
between the terminals of the primary coil, that between the terminals of
the secondary will be
(4) When the secondary circuit is broken, a small magnetUing current
flows in the primary coil, determined by the frequency and self-induction, if
its resLstance be comparatively small. This current lags something less than
quarter-period behind the E.M.F. of the generator. In the intermediate
case^ when the secondary circuit is complete and of *'pure" resiHtance,
the primary current becomes more and more nearly oo-phasal with the
EJI.F. of the generator as this resistance is reduced ; the secondary current
is given by Ohm's law, the E.M.F. being -^E, applied to the ''pure'' re-
■istaiioe circuit. The ratio of the cun*ents in the primary and secondary
dieuits approaches that of — » &8 the resistance of the secondary circuit is
reduced.
In the use of the alternating-current transformer for electric lighting^
whether for incandescent or arc lamps, there will be little or no self- induc-
tance in the secondary circuit, to which the lamps are connected. The
regulating solenoids of arc lamps introduce a small amoimt of self-inductance,
but insignificant in comparison with that of the coils of the transformers.
The purpose of the transforming system will be, as already described in the
ebapter on continuous-current transformation, to utilise a high constant
154 ANALOGY TO CONTINUOUS CURRENT TRANSFORMER,
E.M.V. in the distributing mainB, and obtain a lower E.M.F. in the various
circuits to which the lamps are connected in parallel. For regulation it is
required that the reducing ratio of transformation should be constant, that is
to say, independent of the currents flowing, so that the lamps may be con-
nected in parallel between conductors joined to the terminals of the
secondary coil, the primary coils of the various transformers in parallel
between conductors connected to the terminals of the generator of alternat*
ing E.M.F. ; constancy of E.M.F. between the lamp terminals may then be
secured by regulation of constant E.M.F. between the terminals of the
generator, and the currents in both primary and secondary circuits deter-
mined by the resistance of the secondary. The requisite conditions have
been shown to be, firstly, that the self-inductance of the transformer coils
and the frequency of alternation should be as high as possible, and secondly
that the resistance of the primary and secondary coils of the transformer
should be low in comparison. The resistance of the primary coil (and dis-
tributing mains) is, however, only equivalent to an addition to the secondary
resistance of its measure multiplied by the square of the transforming ratio
of reduction.
Analogy to Continuoiui Current Transformer,
The alternating current tran^tformer may be considered as analogous to
the continuous current transformer or dynamotor. The motor armature
corresponds to the primary, the dynamo armature to the secondary. The
driving current with zero load in the armature is analogous to the magnet-
ising current. Again we have a certain definite ratio between the number
of turns, producing an equal ratio between the primary and secondary
electromotive forces. Also this ratio is modified in a similar way by the
resistances of the armature and coils, the falling off of the potential between
the terminals of the dynamo armature or secondary coils being equivalent
to that given by the secondary current passing through a resistance
-■^a'"-
where B^ and Rj '^^ ^^^ respective resistances. The formuln for calculating
the dimensions and number of turns is also analogous in the two cases ; that
for the dynamo or motor being
while for either primary or secondary coil of the transformer we shall use
where N is the maximum flux of Induction throngh the coils; to the
frequency, and n the number of turns. The numerical factor ^2 iris
nearly 4*4, and to compare the formulae we may note that E is doubled
owing to each turn of the coil being equivalent to two bars on the arma-
ture ; again doubled because all the turns are in series, not forming two
parallel circuits as in the closed-coil armature ; and finaUy increased by
about ten per cent, owing to the necessity of taking the square root of the
average square of all values (following the harmonic law), instead of the
average values for all the bars, as in the thet-rv of the armature.
As soon as the secondary armature or coil, in either the continuous or
alternating current traubf ormer, is connected to an external circuit a current
INSULATION WITH ALTERNATING CURRENTa 1 55
sot only flows in the ftecondary, but an additional current in the primary
armature or ooil. Ag'iin the ratio of the two currents is in inverse pro-
portion to the number of turns in the windings, that is to say, io the E.M.F.
generated in them. But the analogy needs an important mcxlification
for the alternating current transformer. The ad<iitional current is not
generally co-phasal with the magnetising current, but in advance, being, if
the self-inductance of both circuits is practically confined to the transformer
coils, co-phasal with the E.M.F. The two currents are couibined, not by
direct addition, but more nearly by taking the square root of the sum of
their squared values. Using the clock diagram we may combine them by
taking the resultant of the two vectors, the diagonal of the parallelogram,
Bs wiUi vectors representing mechanical forces, reprcBenting in magnitude
and direction the combined alternating current. Algebraically, if c and
represent the two currents, with a phase difference ^, the resulting current
is the square root of
C* + 0* + 2Cccos^.
Experience is wanted to determine whether alternating or continuous
current requires higher insulHtion. It has been pointed out that it is not so
much the question of the leakage current, but the prevention of the disrup-
tive sparking, that has to be dealt with. An alternating electromotive force
of ICO virtual volts reaches a maximum of 14 1.4 volts when the sine law of
variation is followed, so that to avoid the possibility of a " disruptive dis-
cbarge" it would seem that the insulation tbicknes-s for alternating currents
should be about half as great again as for continuous currents of the same
standard electromotive force. 50 or even 100 per cent, extra is frequently
specified by insurance companies, etc., for there is no guarantee whatever
that the sine law will be maintained, in fact, it is most common for the
maximum value of the electromotive force to exceed the virtual by a great
deal more than 50 per cent.
Material subjected to rapid alterations of mechanical stress undergoes
** fatigue," and often gives way ultimately to stres8e8 far below the ultimate
strength or elastic limit of the material, through the degeneration of its
mechanical structure. By analogy it would be reasonable to expect insulat-
ing material, subjected for many years to electrical stresses which are
reversed from 100 to 300 times per second, would ultimately break down
though the stress may be far below that which they could bear if the stress
were uniform, so that a larger factor of safety should be required for alter-
Dating currents than continuous.
On the other hand it appears that a certain time is required to establish
the condition of electric strain in india-rubber, gntta-percha, or other insu-
lating material which exhibits the phenomenon of "residual discharge," and
it may be maintained that in the case of rapidly alternating currents the
stress is revei>ed before the ni iterial has had time to be appreciably strained,
and that consequently not greater, but less insulation is required for alter-
nating than for continuous currents.
Since, however, a safety factor of at least 10 is used in calculating the
necessary insulation, this important pant may still be left undecided, until
prolonged experience with high tension alternating currents has given
sufficient data for a satisfactory decision. At the time of writing the matter
is receiving great attention from leading engineers connected with electrical
distribution of power. In view of the excessively high E.M.F. demanded
by long di.«>tance transmission, and losses due to electrostatic hysteresis
which are no longer negligible with high E.M.F., considerable improve-
ments are to be looked for in cable manufacture in the near future.
I $6 PARSONS TURBO- ALTERNATOB,
CHAPTER XL
Alternating GurrentB (Machinery)*
Thb sbuttle-wound armature may be looked upon as the starting-point In
the evolution of the alternator, or alternating current dynamo, as it is of
the continuous current dynamo. Connecting the extremities of the single
coil to two separate insulated rings mounted on the shaft, with which two
brushes make sliding contacts, and form the terminals of the external
circuit, we shall have an alternating E.M.F. generated between these
terminals, giving one complete alternation per revolution of the armature.
But the variation of the magnetic field, owing to the revoli^tion of the
shuttle-shaped core, will give rise to wasteful eddy-currents in the poles of
the field-magnets^ which could not be tolerated in a large machine.
Parsons alternator is only a step removed from this simple form. A
cylindrical core or framework is adopted as for the closed-coil dynamo, and
long flat coils, wound round an oblong disc of wood fibre, are placed on the
surface, and firmly bound with steel wire wound spirally round the cylinder.
In a later form the coils are '' tunnel-wound " beneath the surface of the
cylindrical iron framework, so that the air-gap is reduced to the necessary
clearance for rotation. In the two-pole form each of these coils covers one-
half the surface of the cylinder, and they are connected in series or parallel
to the two connecting rings. The frequency of alternation is the number
of revolutions of the armature per second, and a very high speed of revo-
lution is required to give sufiiciently high frequency for alternating current
transformation, without making the transformers of excessive dimensions.
A frequency of 50 to 150 alternations per second is usually employed for
lighting purposes, the higher rate being preferable if the power is to be used
for lighting purposes only owing to the smaller size of the transformers, but
the lower being more suitable if alternating current motors are to be employed
in connection with the distributing system, as is very desirable for com-
ihercial economy. These frequencies will require a rate of revolution of
3000 to 9000 revolutions per minute with a two- pole alternator. Parsons
alternator is intended to be driven by the steam turbine designed by the
same inventor, and is therefore suited to the^e high speeds, which with the
ordinary reciprocating engine would be impossible, except with a high rate
of speed multiplication by belt or rope driving. Fig. 52 shows a four-pole
Parsons alternator directly driven by a steam turbine, the whole generating
plant occupying a very small space indeed. The field-magnets are of the
" ironclad " type, giving no external leakage field. Similar poles are opposed
to one another, and four armature coils are required, which may be con-
nected in series to give the high E.M.F., the connections being such that
the armature circuit reverses in direction with consecutive coils since
the E.M.F. is at any moment the same in magnitude, but in an opposite
direction. The frequency of alternation is that of twice the number of
revolutions per second. The field-magnets are excited by coils in which a
continuous current is caused to flow, generated by a small closed-coil dynamo
on the same shaft as the alternator itself.
The electrical efficiency of the alternator is very high, there being little
opportunity for waste of power by hysteresis, &c. Also the small amount
of friction in the whole combined plant renders a very high mechanical
efficiency possible at all loads. The steam turbine is however of high
efficiency only when condensation is available, and a good vacuum obtained
in the condenser. A very ingenious electrical governor is employed with
this combined plant to maintain constancy of E.M.F. at all loads between
158 ■ ELECTRIC REGULATION OF TURBINE.
the terminals of the alternator. The E.M.F. generated in the arm&tvire
with a constaot speed and cflastant ma^etic field would be constant, but a
reduction in proportion to the current flowing due to the resiBtance and
self-induction of the ai-mature would require to be subtracted to give the
alternating difference of potential between the termiDals, that absorbed in
sending the current through the armature being given in vertical volts by
the number of virtual amperes multiplied by the impedance of the
armature.
The speed of a steam turbine may be regulated by the opening and
closing of a throttle- valve, so as to control the steam admission with a
reduced pressure. A better efficiency is obtained by an intermittent
admissicoi at the full pressure, for which purpose the throttle-valve is opened
and closed every 14 revolutions of the turbine by a steam relay, the valvee
Fio. 53.
Siemena Alternator.
of which are operated by the raising and lowering of a lever which is jointed
to an eccentric rod moving on an axle pinion-geared to the main shaft of
the turbine. The duration of the opening of the throttle-valve is regulated
by a compound solenoid, or two solenoids, attracting a core attached to a
distant end of the lever controlling the st«am relay, A solenoid conveying
the magnetising current controls the speed of the turbine so as to maintain
this constant when the alternator is on open circuit ; this solenoid is
opposed by a solenoid carrying the output current of the alternator, so
as to give an increased speed and higher E.M.F, in the armature when
the load is heavy.
For alternators to be driven at the speeds more common id engineering
practice, it will be necessary, in order to obtain the required frequency, to
employ a multipolar fiald, the number of poles depending upon the speed at
which the alternator is to be driven. The armature wil cmsist of a number
d similar coils, so arranged that at any moment the mic of change of flux
MULTIPOLAR FIELDS. 159
of Magnetic Induction through all of them is the Bame in magnitude, and
these ooils will in general be connected in series, as a high E.M.F. is com-
monly desired for alternators, the connections of the coils being made so
that the E.M.F. produced by the variation of the flux of Magnetic Induction
is for all the coils in the same sense when considered as a single circuit
embracing the magnetic field, though not necessarily in the same sense
with regard to the symmetry of their position on the armature. The num-
ber of complete alternations per revolution of the E.M.F. generated in the
armature will be equal to the number of positions of maximum flux in the
same sense through which any coil passes. The E.M.F. generated in the
armature will be the sum of that in all the coils (if in series) ; or if m be
the number of coils, N the maximum flux through any coil, n the number
of turns in each, and the harmonic law of variation be followed, we shall
have for the E.M.F. at any moment,
6M ™'°I* •-- (N. sm.pt) the freqaencj beinfc —
And therefore the virtual and maximum E.M.F. will be given by E' and E|
where
VT Va lo"
One of the earliest forms of multipolar alternator was designed by
Siemens^ and is illustrated in Fig. 53. From a base plate there rise two
cdrcolar iron frames, which are steadied by tie-bars extending from one
frame to the other. On the inner faces of these are fixed two ranges of
magnet poles, poles of opposite oharacters being opposed to one another
ao that the magnetic induction passes across from one to the other in a
direction parallel to that of the shaft. The relative positions of the poles
are changed aa we pass to a consecutive pair, so that any two consecutive
pairs afford a complete magnetic circuit with a section of each supporting
frame and two polar gaps. Between the two ranges of poles pass a series
of armature coils of oval disc shape, the longest diameter being radial, each
wound on a wooden core. These coils are mounted on a framework of
german silver, and are generally connected in series, the direction of winding
following the armature circuit, being reversed as we pass to a consecutive
pair. The closed- coil dynamo supplying the current for the magnetising
ooils 18 driven by a belt or ropes from the shaft of the alternator.
The Orompton Alternator in Fig. 54 illustrates a more modem machine,
following the same lines of design. The improvements consist in more
flattened coils, reducing the magnetic reluctance, and easy accessibility to
the armature.
A more widely used type of alternator has been designed by Ferranti
with a similar arrangement of the magnetic field and armature coils, though
with considerable modification of the mechanical details. This type is
empWed at Deptford in very large units to generate electric power at an
E.M.F. of X 0,000 virtual volts directly, in order to transmit it to a distance
of upwards of seven miles for the lighting of London. It has been found
more convenient, however, to generate an E.M.F. of 2400 virtual volts,
and to multiply this by transformation to the higher value for transmission.
The following details concerning an alternator generating 625 electrical
horse-power, 35 ampere at 2400 volts may be of interest. The copper
ribbon forming the armature conductor is 12.5 millimetres in width, and
0.75 millimetre thick, each coil consisting of 25 turns wound over a brass
core (laminated at right angles to the direction of rotation, and insulated
with asbestos), the copper strip being bare, and the successive turns insulated
irom one another by means of a continuous strip of fibre, 0.5 millimetre
l6o FERBANTI ALTERNATOB.
thick, wound betweeo them. The ioner end of the coil is connected to the
brass core, which is also connected to a consecutive core, while the onter end
of the coiJ is couoected to that of the previous coil, so that the direction of
winding is reversed in passing from one coil to the next. The coils are not
all connected in series, as this would involve the maximum difference of
potential existing between the first and last coils, which would be adjacent ;
instead of this the armature is divided into two semi-circular ranges of coils in
series, the ranges being connected in parallel, and the connections to the
collecting rings taken from opposite sides of the armature. The number of
coils in the armature is 40, joined in two sets of 30, and the diameter of
Fia. 54.
Cromptoa Alutrnalor.
the armature 7 feet. The current density in the annature conductor is
thus nearly 1200 amperes per square inch, and the number of alternations
per revolution of the armature, 20, giving a frequency of 83 complete
alternations per second, with a speed of 264 revolutions per minute. The
peripheral velocity being nearly 6ooo feet per minute, special attention has
to be paid to prevent the coils flying off through the great strain due to
centrifugal force. Each of the laminated braes cores is solid at the inner
end, nearest the driving shaft, and an inKulatcd bolt passed through a hole
drilled in this solid portion parallel to the shaft secures the core and coil to
the revolving framework. The internal reaistance of the whole armature
is 0.176 ohm, the clearance between the pole-pieces is 0.875 ■■'^b, the power
absorbed in the magnetining coils about 1 2 electrical horse-power.
An external view of the alternator designed by Mordey, for the Brusb
HOBDET ALTERNATOR. t6l
Oomftaj, is given in I^. 55. This type preserves the same arnutgement
Aod connection of the umature coils as in the Siemens and Femmti ^pes,
but the armature is stationary, while the vahation of the fiuz of Magnetic
Induction through the coils is effected by the rotation of the field-magnets.
The armature is shown separately in Fig 56. It consists of a number of
flat pear-shaped roils nf copper rihhon, wound on cores of paraffined wood
or porcelain. The outer broad end of each coil is clamped between a pair
of gennan- silver plates, carefully insulated from the coil by strips of ebonite,
and through these plates and tlie core of the coil a bolt passes which fixes
the coil ^rmly in position on tlie interior of a (rnn-metal ring. The arma-
ture nng is constructed in two portions, bolted together at ibe top and
l62 MOBDEY ALTERNATOR.
bottom, eo that it can be divided, and either portion easily wittidrawn to
obtain access to the armature for reptirn; and any coil caa be removed and
replaced in a few minutes if found f&ulty.
In order to simplify tbe field-magneu, and use but a single ma^etising
coil, it is arranged to employ only north poles on one side of the armntur^
and only south poles on the other, so that tha flux of Magnetic Induction
throQgh the coils of the armature is always in one direction. The number
of poles on either side is therefore made only one-half of the number of
coils in the armature, so that the flux through any coil fluctuates between a
Eero and a maximum value, any coil reaching the zero position when the
adjacent coils are at a maximum value, the interconnections being made so
Fig, 56.
Aromture at Ucrdey Altamator.
as to reverse the direction of the winding in successive coils, as in the types
of alternator previously describtd. The tield-magnets are showu ^parated
from the armature in Fig. 57, They consist of two massive iron castings,
with a number of horns or claws, bent as shown, so an to face one another
at a short distunce upart, and form the poles which embrace the armature.
The magetic circuits are completed by a short cylindrical casting, through
which the shaft passes, and round whicii the single magnetising coil is wound
with numerous turns. These miissive iron castings can of course be rotated
at very high s|)eeds without fear of flying to pieces, and their great moment
of inertia produces the greater steadiness of running. A thrust bearing as
shown is necessary to prevent the slightest traverse of the rotating field-
magnets, which, since the minimum clearance is allowed for the coils in
order to reduce the reluctance of the magnetic circuit, might bring them
into contact with the coils. The magnetising current is generated by a
small " Victoria " closed-coil dynamo on the .same shaft as the alternator,
and conveyed to the magnetising coil through brushes touching insulated
IBON ARMATURE-COBEa 1 63
brasB ringa oa the shaft, which form the terminals o! the coi]. The number
of complete alteroAtionB per revoiutioD of the field-magnets is that of the
poles OD one side only, or oae-balf the number of the coils. A frequency
of 100 complete alternationn per second is generally employed with thu
type oi alternator.
In the typeii we have so far described, the coils are made as flat as
possible, in order to reduce the uecessary clearance between the field-magnet
poles. Iron cores <K>uld not be employed, as they would involve consider-
able variations of the strength of the magnetic field, causing heating of the
pole-pieces by eddy -currents, and objectionable noise and vibration. Owing
to their flatness the armature coils are with difliculty made sufficiently
strong to bear the great mechanical stress to which they are constantly sub-
jected. In order to increase the mechanical strength of the armature, a
different arrangement of the magnetic field is preferred by some, in which
Field-Uagtiets of Morde; Alleriuitor.
the coils can be wound on a laminated iron framework which will support
them and render buckling impoxeible. To compensate for the mechanical
superiority, a waste of power will in general ensue owing to hystereeis in
the core ; and with constant speed and magnetisation there will in general
be & greater fall of potential as the current output is increased, owing to the
greater self -inductance of the armature. The latter will not, however, imply
a waste of power, but will demand more attention to the regulation of con-
stant E.M.F., which is generally efiVcted by rheostatic adjustment of the
magnetising current under the control of the attendant.
Ad alternator designed by Kapp, and illustrated in Figs. 5S and 59, was
probably the earliest of the iron-cored armature types. The alternator
illustrated is one made at the Oerlikon works, nnd largely used in Con-
tinental Lighting and Power stations. The field-magnet system consists of
two sets of magnet poles, on either side of the armature as in the Siemens
alternator, but having the opposing poles of similar nature, with rectangular
pole-faces and circular yokes on which the magnetising coils are wound.
l64 KAPP ALTEENATOB.
The armature-cora is composed of charcoal iron strip soiled up with psper
iosnlation over a gan-metal supporting wheel, to fonu a narrow and
deep ring to which lateral strengtb is giveo b^ steel bolts inserted radially
Eapp AlLetiuktor.
Fio. 59.
as shown. The magnetic circuits are completed through sections of this
ring, passing from one pair of poles to the next adjacent pairs along the
ring. The armature coils are wound round the ring, carefully insisted
ELWELL-PABKER ALTEENATOB. 16$
from the ring with mica strips. In a. ]at«r form designed by K&pp, and .
manufactured by Johnson and Phillips, the field-magnet system is made
similar to that in the Mordey alternator, and forms the revolving part,
while the iron-cored armatere is retained as above. A few details of sn
alternator of the original form are given as follows : The pole-pieces are' of
wrought iron, 4^ inches in diameter, and 14 in number on each side.
The magnetising coils are each wound with 186 turns of wire, and the total
resistance in series is 1.76 ohms: the magnetising current is 31 ampires,
requiring an E.M.F. of 37 volts nearly, and the expenditure of power of a
little more than i E.H.F., or 1.3 per cent, of the maximnm output. The
Elwell-l'utker Allerntilor.
armature coils are also 14 in number, eai-h consisting of 80 turns of wire
I20 mil. diameter, wound in two layers on the iron core-ring, and connected
in series giving a total re.sLstAnoe of t.8 olims. At 600 revolutions the
armature gives 2000 virtual volts, with a frequency of 70 complete alter-
nations per Reconil, and the maximum current in 30 virtual amperes. The
loss of power due to armature resi.stance is thus 2.7 per cent, at full
load.
The Elwell-Parker Alternator (Fig. 60) of the Electric Construction Co.,
illustrates a different type of iron-core armature greatly favoured in England
and America. The revolving field-magnets have in this case numerous
poles radiating outward, the magnetic field being completed through an
external laminated iron ring. Against the inner face of this ring the coils
are laid flat, facing the poles and e<jual in number. In more recent practice
the coils are tunnel- or groove-wound to reduce the magnetic reluctance.
OTHER TYPES OF ALTERNATOR. 1 6/
The more common American practice is to invert this arrangement, the
pole-pieoes being attached to the external ring and pointing inwards, the
ooils groove- woand on an inner cylindrical armature.
Recognising that the use of rope or belt driving in order to obtain the
requisite multiplication of speed for the employment of slow-speed hori-
zontal engines in oonjunction with high speed alternators is a source of
considerable waste of power, as well as a frequent cause of accident or
faflure, it has lately been the endeavour of the manufacturers of large
alternators to adapt their designs to direct driving with slow speeds of under
ICO revolutions per minute, in order that the moving parts of the alternator
may be direct-driven, taking the place of the fly-wheel of a slow-speed
engine. This requires an alternator of very large size, though certainly
occupying less space than the combined plant with belt or rope driving, and
encourages the employment of the heavier field-magnets as the rotating
part rather than the armature of the alternator. For this purpose also a
radial clearance between the armature and field-magnet poles would seem
the most suitable design, as with the large diameter and the irregular
strains of the engine cranks it will be more difficult to maintain a small
horizontal clearance. In Fig. 6i the engine-room of the Portsmouth
Electric Lighting Station is illustrated, showing three direct-driven Ferranti
alternators, of design totally difierent to the Ferranti moving armature
alternator previously described. The engines are of the slow-speed hori-
sontal type, with Corliss valve-gear, making 95 revolutions per minute.
The fly-wheel of the engine carries the field-magnets, and is surrounded by
a laminated core-ring in which the armature coils are wound.
There are numerous other types of alternator of which we cannot afford
spaoe for more than a summary reference. The iron-cored armature
designs last described have been modified so as to use only one magnetising
ooil for the field-magnets. This has been effected by arranging two sets of
inward or outward pointing poles, facing two parallel rows of coils round
the ring, the north poles on one side and the south poles on the other. This
is equivalent to a number of horse-shoe magnets, and a single coil wound
round in a radial plane, between the two rows of poles suffices. Again the
poles have been bent in alternately so as to face a single row of ooils.
Another group of designs, termed Inductor alternators, follow the same
winding arrangement, but both the armature and field coils remain
stationary, while the poles alone rotate. The difficulties, however, of
collecting alternating currents, or the continuous currents for magnetisation,
by sliding contact have been so completely eliminated that Inductor
dynamos are in no way more efficient or economic. Other designers have
returned, mainly for small alternators, to a very similar arrangement to
that of ring-wound armatures for continuous currents, or to multipolar
drum-armatures with wave winding.
Alternating Current Transformers.
The starting-point in the evolution of the alternating current trans-
former was Faraday's anchor-ring, Fig. 62. Coils of thick and thin insu-
lated wire were wound round an anchor-ring of soft wrought iron, each
ooil occupying nearly one-half of the ring. The thick wire was connected
with a battery through a key, by which contact could be rapidly made or
broken. The long thin wire was connected with a galvanometer. On
starting the battery current there was a transitory current in the secondary
wire, and on stopping the battery current there was again a transitory
current in the secondary wire, but in an opposite direction to the previous
current. With a " reversing key " the primary current may be reversed,
1 68 EARLY TYPES OF TRANSFORMER.
and the secondary current is then found to be about double what it was
when the contact in the primary circuit was simply made, and considerably
more than double of that when it was broken. This was the first trans-
forms, and involved the principle of the closed magnetic circuit. It
shortly developed into the Ruhmkorff coil, or induction coil, an instrument
too well known to need description, in which a backward step was taken by
the omission of the closed magnetic circuit and its replacement by the
straight core. This incomplete iron circuit is essential when the induced
current is due simply to the make and bieak, and not to the reversal of the
primary current ; for in this case the residual magnetism of a closed mag-
netic circuit would be so great that the induced current due to the simple
make and break would be very much enfeebled. In the anchor-ring, as in
the Buhmkorff coil, the transformation was from low tension to high
tension. When the current is started or stopped on reversal by hand the
charges are so slow that the primary current is flowing steadily for a great
portion of the time, and while so flowing it is doing work in generating
heat, but producing no secondary current. The efficiency of such an
arrangement must necessarily be insignificantly low. and to obtain high
efficiency the charges must be so rapid that the current has not nearly time
to reach its maximum value. Afi an
Fio. 62. instrument for laboratory research,
and as an electro -medical appliance,
the Ruhmkorflf coil is still extremely
valuable.
The first tranformers for the dis-
tribution of power by means of alter-
nating currents for electric lighting
purposes were constructed by Messrs.
Gaulard and Gibbs^ and in principle
were precisely the same as Faraday's
anchor-ring. At first these trans-
formers were used in series, the high
tension alternating current being sent through the primary coils of a series
of transformers in succession, while the secondary coils of each provided
current for a group of lamps. This was the original arrangement on the
Metropolitan Railway. As the primary current traversed a number of
transformers in succession, the wire employed in winding them did not
require to be very fine ; in fact, the primary and secondary windings might
be made of a precisely equal number of turns, in which case, supposing the
efficiency perfect, the primary and secondary currents would be equal, and
the electromotive force in the primary circuit would be equal to that in the
secondary circuit, multiplied by the number of transformers, together with
that necessary to maintain the primary current against the resistance of the
circuit. The transformer arranged in series worked satisfactorily so long as
the load on each remained steady, but any alteration of the load on one
changed the electromotive force upon all the others. This method of using
the alternating current transformer is still employed occasionally for sup-
plying a system of arc lamps, each having ite own transformer at the base
of the standard. Such a system avoids many difficulties as to the use of
high tension, and sub-division of the power, but makes no use of the valu-
able regulative property of the transformer. Alternating current trans-
formers are now almost universally arranged in parallel circuit, so that
except for a minute loss in the mains, the primary of each transformer
takes the whole electromotive force of the alternator, but, as described
above, only such portion of the current as may be required by the secondary
load.
LAMINATION. 1 69
We baye seen that it is necessary that all iron which is subject to
rapid magnetic changes should be laminated, so that the eddy-currents
indaoed may be as far as possible eliminated. The object of lamination is
not solely the attainment of efficiency by the removal of a source of dissi-
pation of energy, but is an essential condition in order that the magnetic
qualities of the iron should be utilised. With the usual frequency of
alternation the magnetism will not penetrate into solid soft iron to the
depth of more than a fraction of a millimetre, the eddy-currents tending
to set up a counter magnetisation. In fact, the solid iron of an electro-
magnet will act similarly to a closed electric circuit, and render the self-
inductance less than it would be if the iron were removed and the magnetic
field existed in air. The following experiment once performed by the writer
forms a remarkable illustration of this fact. The armature of an Elwell-
Barker alternator was placed in series with twelve hundred-candle-power
lamps, the current being supplied at 100 volts from a transformer connected
to a similar alternator. The former alternator was at rest, and its field-
magnets unexcited, its armature simply acting as a choking coil, and causing
the lamps to bum at about half their normal voltage. Chi slowly rotating
the field-magnets it was found that the current rose and fell as the cores of
the field-magnets passed in front of the coils of the armature, the lamps vary-
ing veiy greatly in brightness. But it was observed that the largest
current and the greatest brightness were obtained when the cores were
opposite the coils, that is, when the magnetic circuit was completed through
tibe solid wrought-iron cores. These cores therefore acted as a closed
secondary, and their presence diminished the self-inductance of the arma-
ture. With small machines the reverse of this rasult is obtained, unless
the frequency of alternation be very high.
There are therefore two matters to be considered in dealing with the
requisite thickness of the plates, strips or wires of which the iron core of
transformers must be built, viz., the degree of lamination necessary that
the whole of the iron may be efficiently magnetised by the rapidly alter-
nating magnetising force, and the degree of lamination necessary that
the waste of energy of eddy-currents may be reduced to within permissible
limits.
A very complete mathematical investigation of the magnetisation of
iron plates by rapidly alternating magnetising forces following the harmonic
law was made by Professors J. J. Thomson and Ewing in 1892, which
served to confirm the already established practice of transformer builders
in using plates of about .014 inch or .35 millimetre thickness with the
common frequencies of alternation ; and also indicated how this thickness
should be varied according to the frequency employed, and other conditions
of the transformer. These investigations were published in papers in the
EUctrician for April 8 and 15, 1895, of which we give the following
summary :
First, with regard to the magnetic screening, it was shown that:
^ The magnetisation is substantially the same as if the plate consisted of
two skins, each a quarter of a millimetre thick, with an empty space
between. It is only when we make this total thickness less than the sum
of two such skins (one millimetre) that any marked falling off begins to be
seen in the total quantity of magnetisation of the plate.'' This result, which
insists on the reduction of the thickness of the plates to less than a milli-
metre if the diamagnetic properties of the whole of the iron are to be
brought into action, was calculated on the assumption of a frequency of
100 complete alternations per second, and a permeability of 2000. The
general formula for the '^ equivalent depth of uniform magnetisation " on
either side of a plate of thickness 2h centimetres is given as
170
where
EDDY-CURRENT, SCREENING AND LOSSEa
^ /cosh 2mh - COS. 2mh \
in ^2 \cosh 2inh + cos 2xnh/
j^,^2TMp
0"
|i and 0* being the permeability and specific electrical conductivity (c.g.8.
units — - ohm) of the iron, and ■£- the frequency of alternation*
By the '* equivalent depth of uniform magnetisation " is meant the depth
of an imaginary skin whi(£, if magnetised uniformly with the given magnet-
ising force, would contain the same total number of lines of force as the
actual plate, with its varying distribution, does contain at that instant of
each period in which the number is a maximum* For a thick plate this
-ir or 4>v — ^9 ^^^ therefore varies inversely as the square root
becomes
of the permeabflity, or of the frequency. The following table may be
deduced from the general formula giving the equivalent depth (in millimetres)
of uniform magnetisation for different frequencies and thicknesses of the
plate. We may take |ft«>20oo o- ""10,000 as approximate values in trans-
former iron.
ThickneBS of plates
in millimetres.
Freqaenoy 5a
Frequency xoa
Fieqaeney 250^
so
.356
•252
•205
a
•362
•250
•204
I
•399
•282
•217
.75
.355
•285
.233
.5
.2475
.233
•3l6
.25
•1343
.1245
.1245
Whence it will be seen that little is gained by reducing the thickness of
the plates below half a millimetre, except with high frequency, as far as the
magnetic screening of the interior of the plate is concerned. Consideration
of the loss of power owing to the eddy-currents, however, imposes a further
limitation on tiie thickness.
The energy absorbed in ergs per second per square centimetre of the plate
by the eddy-currents, when the plate is subjected to an alternating Magnet-
ising Force, whose variation follows the harmonic law, and whose maximum
value at the surface of the plate is H,, is given by the expression
^m - gtnh 2mh - sin 2mh
i6«* * ^ • cosh 2mh + cos2mh *
With thick plates this expression becomes (the last fraction being unity)
tfrm
With thin plates (expanding the last fraction)
i.MVh».H.«{l-JZ5(amh)«+....}
With a frequency of zoo complete alternations per second^ we may take
m*28.i; so that 2mh becomes unity, when the thickness of the plate is
EDDY-CURRENTS AND HYSTERESIS.
171
.356 millimetre. Therefore, as long as the plates do not greatly exceed
this thickness, which is about the value employed in common practice, we
may omit the second term in the bracket, and write for the energy absorbed
per square centimetre of the plate io ergs per second,
-L . MVh* Ho« or L p%» . B^
3*^ 3^^
And, therefore, per cubic centimetre of iron employed
JL m'^P^' Ho« or *- p«h« . B^
Off o<r
The energy absorbed in thin plates per cubic centimetre therefore varies
as the square of the thickness. Taking H^^a, corresponding to about
B i- 4000 in the iron, the energy absorbed per cubic centimetre when the
plates are one-quarter of a millimetre thick, and the frequency 100, is 16,500
ergs per second (.00165 watts). This loss will be quite insignificant in com-
parison with the power absorbed in hysteresis with this maximum value of
Magnetic Induction.
Ewing found the absorption of power by hysteresis by the iron, when
the maximum intensity of Magnetic Induction was between 2000 and 8000
units was given very approximately by the expression 1340 H* z6io, where
H is the maximum value of the Magnetising Force. Except in the thinner
plates, in which the magnetic screening is insignificant, there exists a con-
siderable difference between the maximum Magnetising Force at the sorfaoe
of the plate and the mean maximum value of the Magnetising Force from
one side of the plate to the other. Galling the former H^, and the latter Hj,
Ewing calculates the following values of these and for the absorption of
power by eddy-currents and hysteresis for different thicknesses of plates, to
produce a mean maximum Magnetic Induction BH4000 throughout the
plate:
Thickness of
plate,
lammetrea.
Ho
Hi
Power absorbed by
eddy-cnrrentfl.
Ergs per second.
Power absorbed by
hysteresis.
Ergs per second.
Total
2
I
.5
.25
8
5.87
3-55
2.15
2.01
2.74
2.46
2.23
2.02
2.00
569,000
427,000
241,000
fc6,ooo
16,500
206,000
169,000
138,000
110,000
107,000
775»000
596,000
378,000
176,000
123,500
Also for other values of the mean maximum magnetic induction in plates
of one-quarter and one-half a millimetre thick.
Thickness of
plate.
Millimetres.
B
Power absorbed by
eddyonrrents.
Ergs per second.
Power absorbed by
hysteresis.
Ergs per second.
TotaL
.25
.25
.25
4000
6000
8000
16,500
37,000
66,000
107,000
241,000
375*000
I23,5«)
278,000
441,000
•5
•5
.5
4000
6000
8000
66,000
147,000
262,000
110,000
245,000
379.000
176,000
392,000
641,000
172
DETERIORATION OF IRON.
It appears, therefore, that the redaction of the thickness of the plates to
one-quarter of a millimetre will reduce the absorption of power by the eddy-
currents in the iron to less than one-sixth of that due to hysteresis, when
the frequency of alternation does not exceed loo ; moreover, when the iron
grows hot owing to the conversion of this power into heat, the specific
resistance of the iron will rise, and so also will the permeability, so that the
absorption of power will be less. The common pt-actice of using plates of
about .35 millimetre thick, insulated from one anot!ier by the thinnest
possible paper, will therefore reduce the eddy-current loss to insignificance,
and justify calculations made on the assuihption of uniform Magnetising
Force and Magnetic Induction throughout the whole of the iron with the
xibubI formuiffi for the reluctance of the magnetic circuit.
Dr. Fleming gives the following formula for the absorption of power in
watts per cubio centimetre of iron
__ .0032 ^ -dI-ss . /fcnBX*
the first term referring to the hysteresis, and the second to the eddy-
current losses ; n being the frequency of alternation ; t the thickness of the
plates in mils, or thousandths of an inch. This is a ''hybrid" formulAi
utilising two different units of linear measurement, but has the merit of
simplicity. The first term assumes a law of variation of hysteresis loss with
the Magnetic Induction which is approximate through a wider range of the
latter than is common with transformer cores, for which a somewhat lower
power of B seems nearer the results obtained by tracing the complete cycles
of magnetisation.
While the hysteresis and eddy-current losses will both be diminished by
the rise in the temperature of the transformer iron when the transformer is
active, this rise in temperature exercises a deteriorating influence on the
iron which causes an increase of the hysteresis loss after the transformer has
been in use for some time. This deterioration has been carefully investi-
gated by Mordey, and the causes described in a paper read before the Royal
Society, December 19, 1894. He found that in six months' intermittent
use upon an alternating current the hysteresis absorption of energy rose
nearly 75 per cent. The magnetising current only rose about 50 per cent.,
the extra increase in hysteresis-loss being due to the increase of the power-
factor, or cosine of the equivalent lag, pointing to a considerable increase in
the Coercive Force of the iron. With a transformer in which the value of
the maximum Magnetic Induction was 2500 units, and the value of iron
49*7 ^* <^ii^M ^he following measurements were taken (amongst others) :
HAgnetiBing carrentu
Power factor.
Watts absorbed.
Date.
.41
.50
•53
.60
.62
-74
.87
.81
.85
16.54
20.76
20.65
26.71
26.96
Aug. 27. •
Sept. 2a
Sept. 27.
Nov. 27.
Feb. 7.
Mordey's experiments gave rise to the following conclusions :
(i) The effect is not fatigue of the iron caused directly by repeated mag-
netic reversals — it is not " progressive magnetic fatigue."
* Commeucement of test.
SIMPLIFICATION OF WINDING. 1 73
(2) Neither magnetic nor electric action is necessary to its production.
(3) It is a physical change resulting from a long-continued heating at a
▼ery moderate temperature.
(4) It appears to he greater if pressure is applied during heating.
(5) It is not produced when the iron is not allowed to rise mora than a
few degrees above the ordinary atmosphere.
(6) It is similar to the effect produced by hammering, rolling, or by heating
to leduess and cooling quickly.
(7) The iron returns to its original condition on re-annealing.
(8) It does not return to its original condition if kept unused and at
ordinary atmospheric temperatures, whether the periods of rest are short or
long.
The cost of transformers is a fairly large item in the capital expenditure
attendant upon a light tension system of electric supply. AJs a large number
are generally necessary, it is advisable that the labour required in their con-
struction should be as small as possible. Designers have attended nearly as
much to this as to the attainment of high efficiency. If the iron core forms
a complete magnetic circuit, as is almost universally the case, it is necessary
either that the coils should be wound tediously by hand, being parsed through
the core at each turn ; or else that the coils should first be constructed and
the core built up round it, or at least completed after the coils are in
position : the latter method is far preferable and generally adopted.
Another matter to be considered is economy in material ; much can be
saved if the iron stampings are of such a shape that they can be stamped
out of a large sheet without leaving much waste.
The early forms of Gaulard and Gibbs' transformer were constructed with
the intention of being employed in series on a constant current primary
circuit, and so to generate in the secondary currents exactly equal to that in
the primary. The primary and secondary windings were, therefore, exactly
similar. They were constructed of perforated copper disks slit along one
radius and soldered together so as to form a helix, the two helices being
interlaced like the threads on a double-threaded screw. The copper disks
were insulated by similar disks of varnished cardbeard. The iron wire core
was slipped through the perforations of the disks, being insulated from the
copper hy an ebonite tube.
In more recent forms of Gaulard and Gibbs transformers the iron core
forms a closed magnetic circuit resembling a nearly rectangular chain link,
around the two longer sides of which the conductors are wound, the primary
being wound in separate sections, each provided with its own terminals by
which the several sections can be coupled in series or parallel, according to
the electromotive force available in the primary circuit, and that required in
the secondary.
The firbt step towards the more convenient method of oorapleting the
iron circuits after the coils are placed in position was made by Zipemowsky
and D^ry, who wound the primary and secondary conductors in two large
flat coils, each of hemii^pherical section, so that when placed together their
section was circular, and then overwound them with iron wire. In this type
of transformer the copper conductors have taken the place of the anchor ring,
and the iron wire that of the copper conductors in Faraday's Transformer.
The closed magnetic circuit is, of course, preserved. The two transformers
are geometrically similar, but the magnetic and electric circuits are inter-
changed. Elihu Thomson's welding transformer is constructed in preciisely
the same way, but in this case the secondary conductor consists of less than a
turn (a horse-shoe) of very stout copper bar, in which enormous currents of
▼ery low tension are generated. The chief objection to this design is the
great labour expended in winding the iron wire around the copper, and the
H£Da£HOa AND FEBBAKTI TRANSFOBMEBS. 1 7$
necessity of unwinding it ail if a fault should appear in the insulation of
the conductor.
Swinburne employed a (^lindrical coil, with a core made up of iron wires
which were considerably longer than the coils. The ends of these wires as
they project from the coils are bent round in all directions, forming a bunch
at each end by which the lines of Magnetic Induction are distributed through
a great section of air, the external appearance suggesting the name
** Hedgehog," by which it is known.
It will be noticed that in the Hedgehog transformer the magnetic circuit
is not closed in iron, and in this respect it differs from all other types now
in common use. It however possesses a distinct advantage over the early
forms of Gaulard and Oibbs' open circuit tranformers in that the section of
the air through which the Magnetic Induction passes is always much greater
than that of the iron core. The spreading of the ends of the wires enables
the Magnetic Induction to remain in the iron until it can utilise an air
section of something like a hundred times as great as the section of the iron
core. Even this does not compensate for the great difference between the
magnetic permeability of iron and of air, the value of u in the softest iron
being 2000 or more with such low Induction as should be used in trans-
formers, the system appears to introduce unnecessary magnetic leakage.
An open-circuit transformer will generally not maintain its electromotive
force in the secondary as nearly uniform with great variations of load as is
the case with transformers possessing closed iron circuits, and much greater
mass of iron in their constitution. While possessing the advantage of
dissipating little energy by hysteresis, it unquestionably demands a very
great magnetising current, the self -inductance of the primary circuit being
low. Tins magnetising current, however, does not represent a proportionate
load upon the generator, as it will be in a different phase to the electro-
motive force.
In the Ferranti transformer the iron wires in the core are replaced by
hoop iron. Strips of hoop iron insulated by paper from one another are
laid one above the other, and six such bundles are placed side by sid& The
central portion, for about one-third of the length of the strips is overwound
with the secondary conductor, which consists of copper strip, cotton insulated ;
and over this the primary coils, wound in sections, and carefully insulated,
are slipped. One half of the strips of hoop iron are then bent upwards, and
the other half downwards, and their ends brought together above and below,
overlapping some distance. The apparatus is then placed in a cast iron
frame, the upper and lower parts of which are bolted together, the ends of
the coils being protected by suitable shields, which form part of the castings
of the frame. Spaces are left between the bundles of hoop iron which
greatly facilitate the escape of heat.
Fig. 63 shows one of the sub-stations of the London Electric Supply
Ck>rpoi'ation, in which a number of 150 h.-p. Ferranti Tiunsformers are
employed in parallel to transform the Electric Power transmitted from
Deptford at 10,000 volts, to 2400 volts suitable for the distributing mains
locally, whence it is again transformed to 100 volts by smaller transformers
on each consumer's premises.
The Lowrie-Hall transformer (Fig. 64) constructed by the Electrical
Ck>nsti*uction Corporation is somewhat similar. Both the primary and
secondary coils are divided into two sections, one of each being placed upon
a limb of a single magnetic circuit. Sheets of wrought iron are used which
are made to bend over and overlap at the top only, the whole being held in
a cast-iron frame.
A very large number of transformer designs employ flat wrought-iron
plates, stamped out in various shapes, but with suitable perforations through
176 PILED PLATE TFPES.
which the limbs of the tnuiafonner coils are threaded, so that each plate
forms two complete magnetic circuits in its owa pl&ae. The plates are
placed side hy side, separated by thin paper insulation, so that the
completed iron core tokea the shape of an elongated prism, twelve inches or
more in length, with two (generally rectangular) perforations extending
Fia. 64.
Lowiie-Hall Tranatormer.
throughout the whole length through which tbe limbn of the fxSe pass. It
would not, of course, be possible to thread the coiiductors through theee
loDg tunnels bo aa to form a compact coil, the genfral method of procedure
being to wind the two coils in an oblong shape on formers, and subsequently
to build up the magnetic circuits around and through them. Tbe following
are a few of the contrivances adopted for the purpose.
In the Mordey (Brush) transformer (Fig. 65) a rectangular plate, A,
is stomped out with a perforation in the middle, whose length is e^iial
to the breadth of the plate A, and whose breadth is half that of the
breadth of the plate, and equal to the difference of the lengths of the plnte
and the perforation. The portion H, Ktamped out of the middle of the
plate, ia preserved and subsequently laid across the plate so as to complete
the magnetic circuit through the coils as shown at A', ff, the return
THE BRUSH TRANSFORMER.
177
magnetic circait being doable, but of the same total sectional area tlin>ii|^-
oat. The primary and seeondaiy coils are of oblong shape, the limbs fittang
easily into the perforations, with ample insulation, in the positions shown.
The external iron rings are slipped over the coils, and the central pieces
through them alternately. When complete the prism of iron discs is
pressed together by the ends of the frame in which they are held, being
drawn together by screwed longitudinal stays and nuts. The transformer
is admirably ventilated by the air spaces between the external rings, and
Fia. 65.
m
A
The Brush Transformer.
the necessity of the lines of force having to pass from a central strip to the
adjacent external rings twice in a complete circuit, across the intervening
paper insulation, introduces little extra magnetic reluctance. But the
toted sectional area of the actual iron is less than half that of the available
space between the coils. The primary and secondary coils are wound with
approximately the same amount of copper, but owing to the extra insulation
required for the former it occupies somewhat the larger space.
The iron discs in the transformer, dasigned by Kapp and Snell, are
illustrated in Fig. 66. Each plate is made in four pieces, the portions
stamped out to make room for the coils being employed to complete the
iron circuit on one side.
In the Weston type of transformer (Fig 67) there is only one joint in
the magnetic circuit of each plate, but the form of the stamping involves a
further waste of material. The central portion, T, forms a tongue, which is
M
178 WESTON TRANSFORMER.
bent upwards bo as to allow of the disc beiog placed over the ooils, after
which the tongue is beol back into its podtiou, forming a central core
through the interior of the coils. By reversiog the direction of the
tonguea In tho alternate plates it ie rendered impossible for them to be
Fio. 66.
Espp and Ebiell Traosformer.
Fio. 67,
Weston Transformer.
deflected out of their respective planee, so as to fail to make contact with
their proper discs.
In any well-designed transformer it is essential that the reeistaDoe of
the coils should be so low that when the transformer is working " on open
circuit," the electromotive force required to maintain the current in tho
primary coil against the " ohmic " resistance may be quite negligible. That
TIIANSFOKMEB LOSSES. 1 79
is to say, tbe impedance must be almost entirely due to the inductance,
otherwise the transformer will fail to ''regulate/' and the electromotive
force upon the lamps will be much greater when the load is light than at
full load, the E.M.F. in the primary circuit being regulated for constancy,
but the transforming ratio not maintained.
Oonsider, for example, a transformer capable of transforming 2500 watts
from a pressure of 1000 volts to 100 volts, and suppose, as is the oommon
practice, that the same weight of copper is used in the primary and
secondary coils. Neglecting the small excess of current in the primary
required to produce the magnetisation, we may assume that the secondary
omrent ia exactly ten times that of the primary ; and with one tenth the
number of turns, the same weight of copper would make the resistance of
the secondary one hundredth of that of the primary coil, the watts dissi-
pated in heat will be the same, and the resistance of each coil wiQ produce
the same effect in lowering the electromotive force at the terminals of the
•eoQndary.
Suppose we fix a loss of 3 volts as the maximum permissible at full load.
This will mean a loss of 1.5 volts in the secondary, and with a current of
25 amperes will correspond to a resistance of .06 ohm. The resistance of
the primary will be 6 ohms, and with a current of 2.5 amperes there would
be a loss of 38.5 watts, the same as in the secondary, therefore necessarily
producing a siimlar loss of potential difference between the secondary ter-
minals, viz., 1.5 volts. At full load this transformer will give 97 volts
between the secondary terminal, while on open circuit it gives 100 volts,
the potential difference between the primary terminals being supposed to
be kept uniform at 1000 volts. This is certainly the greatest loss which
can be permitted, and consequently for a transformer with this output the
resistances should not exceed the figures mentioned.
In a transformer recently tested by the writer, the coils of which had
the above-mentioned resistances, the current in the primary when the
secondary circuit was left open was shown by the electro-dynamometer to
be .179 virtual ampere, corresponding to a copper loss (C^H) of .191 watts
only. Oompared with the current which would be produced in a circuit of
6 ohms resistance by an impressed electromotive force of 1000 virtual volts,
this magnetising current is barely -^ per cent. It follows, therefore, that
the impedance is almost entirely due to the self-inductance of the coil, and
the ohmio resistance may be safely neglected when considering the magnet-
ising current. It then follows that on open circuit the rate of change of
Magnetic Induction in the iron must balance the electromotive force
applied to the terminals of the primary coil.
The induced electromotive force in any circuit is measured by the rate of
change of Magnetic Induction through the circuit. If the electromotive
farce follows the harmonic or sine law (as it does very approximately in most
types of alternators, except with those of very self -inductive armatures when
heavily loaded), the rate of change of the Magnetic Induction must also
follow the harmonic law, and hence the Magnetic Induction itself must also
follow the harmonic law, but there must be a difference of a quarter period
between the phase of the Magnetic Induction and that of the electromotive
force, so that the Induction must be just changing its direction when the
electromotive force is at its maximum, and the magnetism will be at its
maximum intensity when the electromotive force is changing sign and has
its zero value.
Let n^ denote the number of turns of the primary conductor; A the
total croes-section of the iron core in square centimetres, B the magnetic
induction per square centimetre at any time, e the corresponding value of
the electromotive force at the terminals in volts. The total magnetic flux
l8o DESIGN OF TRANSFORMER.
through the oirouit is ]ijA.B, and the rate of charge of this quantity must
be equal to zoH^ or with the notation of the differential oalculus i
BiA— Bid^
"i"^ dt
If there are -^ complete alternations per second, and if B be the
2ir
maximum value of the electromotive force, the harmonic law being
followed,
e a B sin pt
Henoe
OiA ^ « iG^Xsinpt
at
and therefore
B - .i5^ co8pt - !5!B .in ^pt- L\
pujA *^ pni A V ^ J
The maximum value of B is ^ . and the variation of B Is repre-
pnjA
sented by a ''sine* onrve, the maximum crdinates of which will represent
'^ 1 o^.s lines of Induction per square centimetre.
pUjA
Take, for example, the transformer above referred to. .In this trans-
former A was 63.36, n, was 920^ the virtual electromotive force intended to
be used was 1000 volts, and therefore E was zooo ,J2 or 1414 ; the number
of alternations per second was 100, so that p was 2ir • 100, or 628, and henoe
the maximum value of the Magnetic Induction B should be
628x920x63.36 '^ ^
This intensity of magnetiRation would correspond to a loss of about isoo
ergs per cycle per cubic centimetre of iron through hysteresis if the best
charcoal iron plate most carefully annealed were used. It would, however,
be safer to take the loss at 1500 ergs per cycle per cubic centimetre. The
total value of iron in the core was about 4800 cubic centimetres, so that the
power dissipated by hysteresis when there was no load on the secondary
dzcuit was about
1560x4800x100^^
10'
If the permeability were constant, and the same for increasing and
decreasing magnetism, the current would be always strictly proportional to
the Magnetic Induction. It would therefore follow the harmonic law, and
be represented by a curve which would be a projection of the curve B. In
the transformer referred to, the mean lengt h of the magnetic circuit was 68
centimetres. Hence the current would be given in amperes by the
equation
'^ 10 X 68
If we suppose /a constant and equal to 1000, we have, since Uj ■■ 920^
_ 10x68 g
920 X 4a- X 1000
POWER FAGTOB. l8l
And sinoe the mazLmtiin value of B is 3860, it follows that the maximum
Talue of is .227 amp^nre, and therefore assuming the harmonic law, the
measure of the magnetising current in yertical amperes is .z6; a result
calculated upon data and assumptions that make no pretence to exactitude^
bat not Tery greatly different from the observed value of .179 ampere.
To obtain exact agreement between the observed and calculated values
of the current it will be necessary to refer back to Fig8.4p and 50. To illustrate
the deformation of the current curve we there employed the cyclic curve
traced for the iron used in our typical transformer, through the range of
magnetisation for which it was designed. (The horizontal scale there
indicates ampere turns per centimetre, the magnetising force being — of
4ir
the horizontal readings.) The sine curve is not followed, and therefore the
virtual amperage is not —j- ^^ ^^® maximum, but rather greater, as a glance
at the curve will show. The correction may accurately be made by plotting
a new curve whose ordinates are proportional to the squares on the ordi-
nates of the current curve, and finding the area contained between this
curve and the zero line. The ratio between this area and that enclosed
between the zero line and a parallel line at a distance equal to the maximum
ordinate of the curve will be the square of the reducing factor for obtaining
the number of virtual volts from the maximum.
In measuring the power supplied from a generator to one or more
transformers under the ordinary conditions of alternating current supply
for lighting purposes, the product of the measures in virtual volts and
amperes of the E.M.F. and current is generally taken as giving the correct
measure of the power in watts ; and this is sufficiently approximate except
where a very smiall output indicates that the magnetising current forms a
large proportion of the demand. In this case the retardation of the
current phase causes a considerable discrepancy to exist between the real
and the apparent output thus measured. Also when the load is taken up
to any considerable extent by alternating current motors, or in any other
way, the circuits either primary or secondary, are affected by self -inductance
or perceptible capacity, information concerning the relation between the
phases of the electromotive force and current are necessary before the out-
put of power can be correctly measured.
Let this be illustrated by our typical transformer. The magnetising
current was measured and found to be .179 virtual ampere, the electro-
motive force of the generator being 1000 volts, so that the apparent con-
sumption of power when the secondary circuit was open would be 179 watte.
X^ow the power dissipated in heat through the resistance of the primary
coil is quite insignificant, viz., .19 watt, and that dissipated by hysteresis
has been estimated at 72 watts. The calorimetric method of measurement^
to be described below, enabled us to trace a total expenditure of 91 watts,
the additional amount being due for the most part to eddy currents in the
iron. The real dissipation was therefore barely one haif the apparent,
corresponding to an equivalent lag, or retardation in phase, of about 60*.
The ratio of the real to the apparent number of watts dissipated in a
oirouit is commonly termed the poioer factor.
To measure directly the efficiency of a transformer when loaded, we may
use a wattmeter to measure the power delivered to the transformer, and
measure the power absorbed in the secondary, if the circuit outside the
transformer is non-inductive, by multiplying the readings of a voltmeter and
ammeter. The power delivered to the transformer by the generator, or
by any self-inductive circuit, may also be measured by the "three voltmeter"
method devised by Ayrton and Sumpner, and the corresponding *' three
l82
TRANSFORMER TESTING.
ammeter" method of Fleming. In the former a known non-inductiYe
resistance B is placed in series with the primary of the transformer, and an
alternating current made to flow. Three voltmeter measurements are
taken, Ej between the terminals of the known resistance ; E, between the
terminals of the transformer coils ; E between the extreme terminals. E is
found not to be the sum of Ej and E, as it would be with two non-inductive
resistances in series; nor by the formula E^ =» E/ + E,' as it would be if
the current in the transformer coils lagged a quarter phase behind the
electromotive force between its terminals, giving a '' wattless '^ current.
But it may be shown that the expression -i— — l is the power factor tor
the self -inductive circuit. In the *' three ammeter" method a similar non-
inductive resistance is placed in parallel, and measurements are made of the
Fio. 68.
""
,
/
^
-
— .
■
/
/
/
/
z
t- —
current sWpptdcUA.
Calorimetric test of Transformer.
currents in the transformer coil, in the resistance, and in the main from
the generator; then with similar notation the power-factor in the self-
inductive circuit is ■ > "^^ ; ~ — .
2C,C,
The above methods may be applied to test the transformer at any load.
But another method, which is most simple in theory, and with due care
most reliable in practice, is the calorimetiic method. It simply consists in
packing the transformer in some non-conducting material, and observing by
thermometers suitably placed the rise of temperature in a given time, while
the transformer is at work, or connected to the mains with open secondary
circuit. Allowance may easily be made for the heat that is conducted
through the packing, by observing the rate of cooling at different tem-
peratures after the current is cut off. If the temperature of the transformer
is found to rise at the rate of T^ Centigrade in one hour, after making the
suggested correction, the number of heat units supplied must be
I .113. Ml + .95 . Ma j
3600
where M^ is the mass of iron in grammes, M^ of copper ; and the watts
dissipated can be obtained by multiplying this by 4.2.
EPFICIENCY OF TRANSFORMEEa 1 83
The calorimetrio method applied to the small traDsformer mentioned
above gave a fairly uniform rise in temperature of about 10° Centigrade
per hour, as shown by the plotted curve in Fig. 68, whence the power
absorbed by hysteresis was calculated. Correction for the waste heat
escaping through the packing was made by observing the law of cooling.
Dr. Fleming has recently made a very laborious and complete series of
experiments with alternating current transformers constructed by the leading
manufacturers, with a view to a comparison of the relative efficiencies, <tc.,
of the various types. It will appear from these records that a drop of
potential difference between the terminals of the secondary coil, due to the
resistances of the coils
of from 2 to 2.5 volts is commonly permitted. An absorption of power on
open (secondary) circuit, owing to hysteresis and eddy currents in the iron
core, of from 1.3 to 2.8 per cent, of the specified maximum output of the
transformer, is permitted ; the lower percentage being obtained with large
transformers with a specified maximum output of about 20 electrical horse-
power.
At full load an efficiency of from 96 per cent, upwards is obtained with
transformers designed for a maximum output of from 6 E.n.P. upwards.
At half load this efficiency is but slightly reduced, the increased percentage
of the core losses compensating for the reduction in the loss due to the
resistance of the coils. At a load of 10 per cent, of the maximum the
efficiency varies from 80 to 85 per cent. The rise in temperature permitted,
when the transformer has> been working for some time at full load, is from
150 to 212 degrees Fahr.
To estimate the actual diurnal efficiency of a transformer, that is to say,
the ratio of the total amount of energy output in the secondary circuit to the
total energy absorbed from the generator, when employed in the customary
manner for domestic lightiog, the primary coil being permanently connected
across the high-tension distributing mains, and a varying demand being
supplied according to the number of lamps switched on to the secondary
conductors, we require a knowledge of the efficiency at all loads, and the
variation of the demand throughout the day. As a rough approximation
to common conditions of practice, suppose the demand for lighting to be
confined to four hours, during which the load is varying, but of sufficient
magnitude to give an average efficiency of 94 per cent. : and during the
remaining twenty hours of the day the output to be insignificant, while a
uniform core loss of 2 per cent, of the maximum proceeds. Suppose, also,
that the total secondary output of energy be equivalent to that given by a
uniform maximum output for three hours^ or 12.5 per cent, of the total
possible output for the twenty-four hours. The energy absorbed by the
transformer during the continuation of the supply will be six per cent, for
four hours, or one per cent, of the total possible output during the day ;
the energy absorbed during the remaining twenty hours will be two per cent.,
or 1.66 of the total possible ; the total loss will be thus 2.66 of the total
possible output, or 21.3 per cent, of the actual output. Thus the diurnal
efficiency of the transformer will be 78.7 per cent.
i84
EFFI0IENC7 OF TBANSFOBMEBS.
SUMMARY OF DIL FLBMINGP8 TRANSFORMER TEST&
15 E. W. Ferrantl • •
II E. W. Ferranti . •
6i E. W. WestinghoQfle •
4I K. W. Thomson Houston
6 K W. Brash (Mordey)
4 K. W. Eapp .
6 E. W. Hedgehog .
it
°
o o
1.3
1.46
1.6
1.82
2.8
3.6
EfBeUodes at Tarious loads.
i
65.4
79.0
75-9
76.6
67.6
56.5
05.2
1
1
o
86.5
88.1
85.7
78.8
67.6
72.3
79.0
t
96.1
96.0
96.0
93.8
93-7
91.9
94.8
I
I
&
96.8
96.1
96.8
95
94.9
93.8
96.1
ll
£
o
96.6
2.1
95-5
2.2
96.9
2.4
94-7
2.3
95.4
2.25
94.2
2
96.1
3
I
o
I
•a
a. 75
3-77
5-95
19.68
7.73
11.38
7-93
.0061
.0092
.0108
.019
.0163
.024
.01512
Experimenting with an open oiroait ** Hedgehog " transformer^ designed
for a maximum oatpnt of 3000 watts. Dr. fleming found that the magne*
tifling current was nearly half that of full load, instead of barely three
per oent., aa would be obtained with a closed circuit transformer designed
for a similar output. The power factor, or cosine of the equivalent angle
of lag, was, however, very small, reducing the measurement of the actual
loss to about 112 watts. Even the latter (3.7 per cent, of the maximum
output) was greatly in excess of that found with dosed magnetic circuit
transformers, but the excess was probably due to eddy currents in the
large secondary conductors, which were wound inside the primary coil.
The total volume of the iron was only 856 cubic centimetres (weighing 598a
grammes), consisting of wires 18 inches in length and .0225 in. in diameter;
thus, in spite of a somewhat high intensity of Magnetic Induction, amount*
ing to 10,000 lines per square centimetre in the middle of the core, Dr.
Fleming estimated Uiat the hysteresis loss could not be greater than 20 to
30 watts, and the eddy current 10 to 15 watts. In testing a larger six-
kilowatt transformer, transforming from 2400 to 100 volts, having a
laminated secondary conductor, the magnetiBing current was found to be
1. 194 amperes, whereas in a closed circuit transformer for corresponding
output .07 ampere would be enough, and the power absorbed on open
secondary circuit was reduced to 2.6 per cent. (156 watts), not greatly in
excess of that obtained with closed magnetic circuits. The excessive magne-
tising currents necessary with thut type of transformer, although not
representing a corresponding waste of power, still remain a great objection
to their employment as these currents mean a waste of power in the distri-
buting mains^ and no corresponding advantage, unless it be in the saving
of weight in the transformer, seems to be obtained. By the employment
of a condenser connected across the terminals of the primary coils, the
** wattless ** magnetising current can be maintained by an interchange of
potential energy between the magnetic field and the condenser which reduces
the necessary magnetising current flowing from the generator in the distri*
buting mains, but it is improbable that this device, if perfectly satisfactory,
will fdlow an open circuit transformer to compete in commercial efficiency
with the closed circuit, owing to the extra cost of the conednser.
Having designed a transformer for the conversion of a given electric
power from one electromotive force to another, it will be easily seen that
TABIAHON OF DIMElilSIONS. 1 85
the only alteration required to adapt the same design to different electro-
motive foroefl is a corresponding multiplication or division of the number of
turns in either coil, the magnetic circuit remaining the same, and the
capacity and efficiency remaining the same if the sectional area of the wire
18 only limited by the total sectional area allowed in the design. This
statement makes no allowance for the extra insulation which is required as
the electromotive force, and the number of turns are increased. In short,
the capacity of the transformer, that is the total power convertible with a
given percentage loss, is a function of the sise of the transformer, and
independent of the cdectromotive forces in the primary and secondary,
except for certain practical modifications.
We must now investigate the relation between the size and the capacity
for a given type of transformer; we shall suppose that the linear dimensions
of all the parts in a given type are multiplied in a given raUo, call it x, and
investigate the variation in capacity and efficiency ; from this investigation
we shall be able to decide whether it will be advisable to alter the ratio of
the parts in increasing or diminishuig the size.
The variations are identical with those previously discussed in the theory
of dynamo design, save that there need be no reservation corresponding to
the speed of the dynama The linear dimensions being multiphed by x, the
area of the magnetic circuit, and therefore with the same induction density,
the total number of lines of force is multiplied by x^. The number of turns
in either primary or secondary is therefore divided by x'. The total area
available for the coils is multiplied by x^, and therefore for each wire by x^.
The length of each turn is multiplied by x, and therefore the total length
of either coil is divided by x. Hence the resistance of either coil is divided
byx».
It follows that for the same percentage of loss in copper resistance, and
fall of pressure in secondary at full load, the capacity is multiplied by x^ ;
but if we are limited by the current density in the conductors the capacity
is only multiplied by x^. The hysteresis and eddy current losses are multi-
plied by x^, the increase in the mass of the iron, but the percentage loss is
divided by x' or by x in the two cases. It will follow that if the balance
between these losses is to be preserved a slightly smaller proportion of iron
would be used in larger transformers, and the density of Magnetic Induction
increased. But on the other hand, we must remember that the surface of
the transformer is only increased in the ratio of x^, so that an increase in
the quantity of iron instead of a decrease will be necessary, if the same
limiting temperature is to be reached.
An increase in the size of the transformer should therefore be designed
with a somewhat lower density of Magnetic Induction. The capacity may
be specified to increase at a greater rate than the weight, and the efficiency,
both in respect of the iron and copper losses, must necessarily be improved,
or the temperature of the transformer will be higher than in the smaller
sizes.
Ewing has given empirical formulae for the hysteresis loss in iron per
cubic centimetre during a complete alternation, from which it appears that
.oiBi*™
gives a very close approximation to the loss in ergs per cubic centimetre,
when B, the maximum value of the Induction, is between 2000 and 8000.
It appears that the rate of variation is the same as B^'^ between B»2000
and 5000 and B^** from 5000 to 10,000, so that we shall be very close to
the truth in taking £ wing's formula as representing the variation at the
1 86 VARIATION OP FREQUENCY.
intensity most commonly used in transformers. The eddy-carrent losses
should vary as B^, and these should be almost negligible in comparison ;
still the combined loss would be most approximately and simply repre-
sented by a variation in proportion to B^*^. Upon this assumption, let us
find the effect upon the efficiency of increasing all the linear dimensions of
a transformer in the ratio z, but decreasing the induction so that the same
power may be converted with the same loss in copper resistance.
If the number of turns in either the primary or secondary be multiplied
by y, the sectional area of the conductors will be multiplied by — , and the
If
length multiplied by zy ; the resistance of either coil must be multiplied by
^, and if this is to remain the same, y^ « z. The total number of lines of
z
force throughout the core must be divided by y, and since the area is multi-
plied by x' the intensity of induction, that is the maximum value of B,
must be divided by x^y or by x***. The loss by hysteresis per cubic centi-
metre will therefore be divided by z*''^ and since the mass of iron is
increased in the ratio x^, the total loss will be divided 1>y x*'^ Or if W be
the weight of the transformer, the iron losses will vary as W^. If we had
assumed that the loss varied as B'*^, they would have been found to vary
as W^. The efficiency, therefore, improves very shortly with size at
moderate intensities of Induction, but at very high or low Inductions, where
the loss varies as a higher power of B up to the second, the rate of improve-
ment with size is more rapid.
The effect of varying frequency on the iron core losses of a transformer
was investigated practically by Mordey for his own design by the calori-
metrio method. He found that the rate of rise in temperature reached a
minimum with a certain rate of alternation, about loo complete periods in
the transformer tested being greater with 75 and 125 periods. In any case
the best frequency will depend on many conditions, chiefly the quality of the
iron and proportion of eddy current to hysteresis losses ; but it will follow that
a wide range of frequency will be possible without much variation in the
loss, according to the universal law that the variation of any quantity near
its maximum or minimum values is slow. A simple test, measuring the
power with a wattmeter at various frequencies, is sufficient to indicate
without much error the best frequency to employ.
The reason for the small variation can be seen by consideration of the
fundamental equations. If the frequency be increased the maximum
Magnetic Induction is correspondingly reduced. The loss per alternation is
therefore reduced according to some power, about 1.5, of the frequency;
the loss per second is then reduced according to the .5 power, or square root,
of the frequency. The net advantage is therefore somewhat in favour of
the higher frequency if an effective lamination is secured. The loss by
eddy currents is, though it should be very small, much increased ; the
magnetic screening causes irregular density of Magnetic Induction in the
iron ; and other increased losses occur through eddy currents and skin
effects in the copper conductors ; these accumulate till the small gain owing
to reduced Magnetic Induction is more than neutralised.
LOXG-DlsTANCE TKA^'SMLS^iON. 187
CHAPTER Xn.
Altaraatins Cortent Dtatribatioiu
The most oommon Briti>h practice in Alt ema ting Current IH^tributioii
eiiiploY& a parallel constant potential system for the distributing mains, ai
2000 virtual volt^ with secundarr two- wire sx-stems at 100 voit& There
is much, however, to be said for the reduction of the distributii g E.M.F. to
1000 virtual volts, as adopt4rd in a few instances, the avoidance of the
difficulties and dangers introduced by the higher £.M.F. possibly more
than ooDipensating for the extra sectional area of the distributing mains.
The gradual concentration of the transforming centres may also tender a
three-wire secondary system more common in the immediate future. With
a current densitj of 1000 amperes per square inch, the fall of E.M.F. at the
distance of one mile from the generating station, considering the lesistanoe
of both the go and return conductors, will be about 90 volts, if the copper
be of standard conductivity. This is a drop of only 4^ per cent, of a
distributing E.M.F. of 2000 volts, with a corresponding percentage waste
of power. Within this distance, therefore, it will be possible to arrange
that the vaiiation of the pressure at different points of the system should
not exceed 2^ volts, and it would be easy to arrange for a stUl smaller
variation by compem-atiDg for the extra fall at the extreme di^tances by a
few extra turns of the secondary coils of the transformers there employed.
Further, by a system of feeder mains, in which a greater fall of potential is
permissible, it will be possible to reduce the variation at different points of
the distributing system, while allowing a somewhat greater percentage waste
of power, which will still be within very moderate limits. There will be a
still fuither drop of potential to be allowed for in the transformers and
secondary mains ; but since the demand aU over an extensive system will, as
a rule, rise and faU with approximate uniformity, a corresponding increase
and decrease of the KM.F. of the generators, according to the total demand,
may be made to partly compensate for this variation. We have therefore
to deal with three causes of fall of potential and corresponding sources of
waste power, the drop owing to the resistance of the high and low pressure
distributing mains, and the variation of the transforming ratios between
xero and full load. Each of these may conveniently be reduced within
2} per cent. The variation of the transforming ratios may be entirely
compensated for by the variation of the E.M.F. at the generating station
according to the total load, if the demand over the system is at all times
similarly proportioned to the maximum. The variation at different points
according to the length of the primary and secondary conducting mains
leading to them can similarly be reduced to 2^ volts in excess or deficit of
that specified. As a variation of 4 per cent, is permitted by the Board of
Trade regulations, this is well within the limit, allowing for further varia-
tion by imperfect regulation, <kc. ; but we may still further approach
uniformity by overwinding the secondary coils of the transformers where
the fall is great, using lower current density for the conductors leading to
more distant lamps, and other similar devices.
For widely distributed systems, extending upwards of a mile from the
generating station, it may be advisable to use a current density of less than
1000 amperes per square inch in the distributing mains. Bemembering
that, with an E.M.F. of 2000 volts, every ampere represents 2.77 electrical
horse power, and will supply 30 sixty- watt lamps (with a loss of ten per
1 88 TBANSFQBMEB SUB-STATIONS.
cent.), it will be seen that a conductor of -^ sq. inch section, carrying a
current of loo amperes, will supply 3000 sixty-watt lamps. The extra
expense of a further increase of the section of such a conductor, so as to
diminish the current density, and corresponding fall of E.M.F. and power
absorbed, will be far less in proportion to the cost of the generating and
transforming plant than a corresponding increa^^e in low pressure systems
for the same purpose. With a current density of 500 amperes per square
inch at 2000 volts, the system may be carried to upwards of two miles from
the generating station without experiencing &ny difficulty in maintaining
uniformity over the whole system.
The alternating current transformer system possesses this advantage
over the continuous current, in addition to the higher efficiency of trans-
formation possible, that the centres of transformation can be in practice
indefinitely sub-divided, and no attention whatever need be given to the
transforming plant. But, as has also been shown, the concentration of the
transforming plant in large units ofiers possibilities of greatly raising the
efficiency of distribution, owing to the fact that the larger sizes of trans-
formers may be at the same time much cheaper in first cost, and of much
higher efficiency than the small. Nor is thu the only advantage gained by
concentration, for it may be practicable, when large tran^ormers are
grouped together in a sub-station, greatly to reduce the daQy loss of power
in the magnetising current by adjusting the capacity of the plant to the
ever-varying demand, this being done either by a sub-station attendant or
by some automatic device which switches out some of the transformers at
times of light load.
It is now universally recognised that the concentration of the trans-
forming plant, and the use of a secondary network supplying many con-
sumers, is the best method of applying the alternating current system.
The method which has hitherto been most common, that of supplying each
consumer with a separate transformer, having been intended only as a
temporary expedient, necessary while consumers were only to be obtained
here and there over a wide area, and the minimising of capital expenditure
was advisable before the commercial value of the system had been thoroughly
demonstrated. In the place of this scattered system the sub-station or
concentrated system is being rapidly introduced, with a secondary network,
preferably using a three-wire system, or direct employment of 200 volts.
For the house transformer method branches are taken from street mains
to a transformer in the basement. Concentric cables are inconvenient with
this system, owing to the numerous joints that must be made. The branch
conductors leading into the house should not be of less size than 7/18, giving
ample mechanical strength, although the current could be carried on wire
of much smaller cross-section. The transformer should be contained in a
fire-proof chamber, or oast-iron case, and double pole fuses used on both
primary and secondary mains.
When the mains are drawn into conduits, a few feet of slack should be
left in each '* drawing-in '' box for convenience in making the joints for the
branch conductors. Moreover, the joints should be V- joints, made at such
points that they can be drawn back at the service boxes a few feet into the
conduits, always in a uniform direction, say towards the generating station.
This, when worked systematically, enables a large number of branches to be
made from a pair of cables with the minimum amount of slack. The Metro-
politan Company, which is the largest supply company in London, have
improved upon the branch conductor system by drawing in the cables in
short lengths from one consumer to the next, so that the main circuit can
be broken in the transformer chamber of each consumer, and joints in the
street culverts are thus almost entirely avoided. King-mains are completed
TRANSFOBMEB SUB-STATIONS. 1 89
wherever poauble, so that each consumer may be supplied from two direc-
tions, and any small section of the mains separated for testing without
stopping the supply.
As the number of consumers increases, or to use an expressive phrase,
the lamp *' density " in any area increases, the advantage of the conoentrap
tion of transformer plant becomes obvious. At present the tendency is
towards transformer-pits sunk under the pavement, feeding a secondary net-
work laid parallel, but of course in separate conduits, to the primary or
high-tension mains. It is, of coorsOi vitally necessary that such transformer
pits should be well drained, or else hermetically sealed. Moisture is fatal
in any high tension system, and far greater care must be taken than with
low tension work. On the cables themselves, with high rubber insulation
and impermeable covering, the presence of water in the culverts is probably
rather beneficial than otherwise. The transformer coils are proof against
creeping moisture when worked continuously, and therefore always at a
moderately high temperature — ^the danger comes in restarting after a period
of Test. It has been attempted to protect transformers from moisture by
completely sinking them in a thick oil, of greater density than water,
supposing that the water would not sink to the fibrous insulation of the
coUs. At the cost of dear experience this device has been proved worse
than useless. For underground cables a fibrous insulation impregnated with
oil is the safest of all systems, owing to the self -restoring properties of the
insulation ; but where there is (circulation and oil-surface exposed to deposit
of moisture the insulation sooner or later breaks down. In the oil -sunk
transformer a circulation in the heavy oil is at once set up by the heat
generated, and a thin film of the water floating on the surface is carried
round with it. This moisture is at once taken up by the cotton or other
fibrous insulation, actually taking the place of the oil in it, and refusing to
be expelled by heat, until the cotton is absolutely saturated, takes a leakage
corrent, carbonises, and a short circuit results. Hermetic sealing will alone
prevent this happening, and then the oil is probably superfluous.
In the Depdord *' extra high tension " system, the transformation from
10,000 volts to the intermediate 2400 volts is effected in several sub-stations.
The transformers have a capacity of 1 50 h.p., and a number are placed in
parallel. These are kept under constant supervision, and the number in use
is varied according to the demand. By this means the immense loss due to
the magnetising current is greatly reduced. In the transformer pits the
same efficiency could be secured by an automatic switch, or the employment
of an attendant to visit the transformer at stated periods. Several attempts,
more or less successful, have been made to design an efiective automatic
switch. But the action must be absolutely certain, and the switch-break
very sudden fof such to be efiective. A design by Ferranti employs a small
motor, which attains a certain speed before the switch acts, so that it may be
thrown over suddenly when the load reaches or falls below a certain amount.
The primary and secondary are each wound as two separate and similar coils,
whidi are placed in series for small loads and parallel for large. The capa-
city, when the coils are in parallel, is four times as great as when they are
in series, but the loss in hysteresis would be between three and four times
as great, as the Magnetic Induction in the transformer core is carried to twice
the maximum value.
The seoondary network may be arranged so as to be fed from several
transforming centres. A fairly uniform distribution of electromotive force
in the secondary may thus be secured, but unless a number of transformers
be thus connected, and the fuses be very heavy, a breakdown in one trans-
former will cause the fuses of the others to be blown. If, however, a suffi-
cient excess of current can be drawn from the remaining transformers to
I90 TEANSFORMERS IN PARALLEL.
blow the fu8e of the one that has broken down, the lighting will be con-
tinued, though perhaps with an undue fall of pressure at certain points on
the network. In the borougb of Portsmouth the following sy^tem was
initiated, and since adopted elsewhere. A secondary network of uniform size
is used throughout the whole system. This secondary network consists of
three pairs of No. 19/18 cables, or a total sectional area of only just over a
tenth of a square inch for the three conductors, connected to one pole.
Running parallel to the high tension mains in adjacent conduits, it is feid by
transformers at convenient intervals, the positions of the transformers being
preferentially chosen so that the conductors for a large installation may be
led off at once from the junction -box. It is initially arranged that the trans-
forming centres should not be more than about 300 yards apart. In the
event of a heavy demand along the intervening gap, the secondary mains are
not increased, but an intermediate transforming centre is laid down and
connected to the same secondary network. Concentric cables are universally
preferred to pairs of cables for sub-station transformer systems, the advan-
tages being the slightly reduced cost, the smaller space occupied, the greater
immunity from risk of injury, and the total absence of inductive effects upon
telephone wiras, <&x!. There are now no longer numerous branch joints to make,
as the transforming centres are few in number, and some convenient form of
junction-box, in which the branch of the transformer may be disconnected,
takes the place of the joint. The insulation between the inner and outer
mains is alone of vital importance ; the external insulation need only be
sufficient to prevent a leakage which, by joining a partial earth return, may
affect neighbouring telegraph and telephone lines. It is customary to earth
the conductor at the distributing station, so that all parts of this conductor
may only vary slightly from the zero potential. Thus, if the output be such
that the maximum fall of potential at any point of the system be 4 per cent.,
in a 2000 volt distribution, or a total of 80 volts, the difference of potential
between the outer conductor and the earth at that point is only 40 voltSi so
that the electrical strain is insignificant.
In connecting alternating current transformers in parallel so as to supply
the same secondary system, dividing the load between them, care has of course
to be taken that the transforming ratios are identical, or at least very
approximately identical ; and also that the terminals are so connected that
the secondary coik are in parallel, and not in series, with regard to the phase
of the E.M.F. generated in them. When joined in parallel, the division of
the load will depend on the fall of potentiai in the transformers, owing to
the primary and secondary resistances, each taking such currents as will lower
the transforming ratio so aa to bring the difference of potential between the
secondary terminals down to that between the main conductors to whidh they
are connected. In other words, if the transforming ratios tit sero load ans
identical, the proportion of the current supplied by each transformer will be
inversely proportional to the resistances either of the secondary or of the
primary coils ; for the ratio of the resistance of the secondary to that of the
primary coil in any well designed transformer should be the square of the
transforming ratio, and therefore the same in each transformer (otherwise
the division of the current will be inversely proportional to K| + -^fia
in each transformer). Provided, therefore, that the transformers are designed
for the same transforming ratio, and the same fall of secondary E.M.F. at
full load, transformers of different sizes, joined in parallel, will divide any
load in the proportions of their specified maximum output ; but if any trans-
former give a higher secondary E.M.F. at zero load than another, it will take
a larger proportion of the load when they are joined in parallel, so that an
••skin effect." 191
extra fall Id the former may equalise the difference of potential between the
secondary terminals.
To supply a secondary three-wire network two transformers, op sets of
transformers, may be joined with their primary coils in parallel across the
high pressure distributing mains and secondary coils in series. Again,
however, care must be taken that the secondary coils are correctly in series,
their E.M.F. at any moment being in the same direction through the series
circuit, and not in opposition ; otherwise the middle wire will take the sum,
instead of the difference, of the currents in the two sections of the system.
Multiple wire systems have been employed by the Thompson-Houston Oom-
pany in America with alternating currents for incandescent street lighting,
employing a " compensator," or '' equaliser'' to maintain equality of potential
difference between the different sections of the system. The principle of this
apparatus is similar to that of the equaliser employed for continuous currents,
save that mechanical motion is now no longer necessary. In place of the
motor-dynamo armatures, similar coils are wound round the same laminated
magnetic circuit. The self -inductance of these coils prevents anything more
than the minute magnetising current from passing through them ; but in the
event of the difference of potential between the terminals of one coil rising
above that of another, a current will pass through the former conditional
upon a current in the reverse direction passing through the latter. Thus, if
the current through the lamps be reduced in any section, tending to raise the
E.M.F. between the corresponding conductors, and lower than in the other
sections, a current will flow in the corresponding coil of the compensator,
and acting as the primary of a transformer, generate higher E.M.F. in the
remaining coils, and once more equalising the difference of potential between
each adjacent pair of conductors.
In dealing with the size of conductors required to carry alternating cur-
rents, a modification is introduced by the fact that, since a magnetic field is
created even by a straight wire carrying a current, the alternations of this
magnetic field will introduce further impedance, and increase the fall of
potential along the conductor above that given by the application of Ohm's
law. This modification will be partly equivalent to a co-efiicient of self-
inductance, which, as previously bhown, modifies Ohm's law so that the
relation between the virtual amperes and volts is given by
E
where B is the resistance of the conductor, — the frequency, causing the
— * oil
current to lag in phase tan -^y and the power dissipated to remain C'B watts
as before, except for hysteresis loss, etc. ; but in addition to this, the alter-
nating magnetic field created within the conductor itself will cause the self-
inductance of the interior of the conductor to be greater than that on the
surface, causing a varying distribution of current density throughout the
conductor, larger upon the surface and diminishing towards the interior, the
conductor being thus equivalent to one of smaller section and greater resist-
ance, with further dissipation of energy than would be given by a continuous
current of the same number of virtual amperes flowing in the same conductor.
This phenomenon, known as the ** skin effect/' was first observed by Prof.
Hughes in 1883.
The fall of potential due to self-inductance owing to the alternations of
the magnetic field external to the conductor itself, so far as it does not affect
the distribution of current density, would be very great in the case of a single
conductor drawn into an iron conduit, but is rendered very small if the return
192 "SKIN EFFECT.*
oonducfcor is drawn into the same conduit and lie in clcNse proximity, thus neu-
tralising its magnetic field. With a concentric cable the effect would be still
less. Taking the precaution mentioned, we may neglect, except for very
long mains, consideration of the fall of potential and retardation due to self -
inductance alone, but with conductors of large section the varyiog current
density, which also increases the absorption of power, becomes of great
importance.
Lord Bayleigh has shown (Phil, Mag, May 1886 '' On the Self-induction
and Resistance of Straight Conductors ") that the '* virtual resistance " R'
and co-efficient of self-induction L of a long straight wire of length 1 and
permeability ^ may be given in absolute units by the formula
_, _ r I p«lV I ^/** T
R = R [^x + £5 -gr - jg; R4 + • • • • J
A being some constant depending upon the position of the return wire, and
B the ohmic resistance of the conductor, measured in absolute electro-
magnetio unite \ — , ohm ). With a copper wire or rod one centimetre in
radius, carrying an alternating current of frequency 100 (the permeability
being unity), we have
and therefore
r» r» r ^ tO,000 "fc
SZ.I28R.
Or the virtual resistance is about one eighth more that the actual for a
copper wire of one centimetre in radius. For an iron wire the resistance
would be enormously increased, the increment being multiplied by 11^, which,
even for the low magnetisation involved, would probably be several tnousands.
The increment to the resistance varies as the square of the frequency, and
the fourth power of the diameter or radius. The building up of the con-
ductor with stranded wires of small section, at least by the common method
in which the internal and external wires maintain their relative positions
throughout the length of the cable, does not prevent this augmentation of
virtual resistance. What is needed is a method of stranding in which all
the strands are brought in succession to the exterior of the cable every few
feet, so that the self -inductance of each strand may, in a reasonable length,
be the same, and the current uniformly distributed.
The following table, calculated by Mordey, will give the relation of virtual
resistances to the actual resistance of solid round wires or rods of oopper of
various diameters, carrying currents of various frequencies of alternation.
For stranded cables the increment may approximately be taken as the same
as for a solid conductor of equal diameter.
UNDESaSOUNO MAINS.
>93
Diameter of
Copper.
*l
10
15
20
25
40
100
1000
9
134
18
22.4
I
a
0.3937
0.5907
0.7874
0.98^2
1.575
3.3937
39.37
0.3543
0.5280
0.7086
08826
7.75
0.3013
I2.6f
0.4570
15-5
0.6102
19.36
a 7622
Area of Cron>
Beotion.
S^B
78.54
176.7
314. «6
490.8
1,256
7.854
785,400
63.62
MI. 3
2544
394.0
47.2
106.0
189.0
294.0
0.12
0.274
0.4S7
0.760
1-95
12.17
1,217
0.098
0.218
0.394
0.61 1
0.071
0.164
0.292
0.456
Inereiae over
Ordinary
Besiiitanoe.
M
leM than x^ %
8
17*
68
3.8 timos
35
ft
less than x^(f
8
I7i
/O
If
«f
t>
less than y^^ %
24
8
I7i
PI
^1
5^«
55
133
220
45
98.5
178
32
74
131.4
to
r
^
110,000
266,000
444,000
90,000
197,000
356,000
64,000
148,000
263,000
8
H
m o
5,500
13.300
22,000
4,500
9,850
17,800
3,200
7,400
I3,»40
^ 80
100
133
The Board of Trade Regulations insist that *' A high pressure electrio
line shall not be used for the transmission of more than 300,000 watts, or
in the case of an aerial line 50,000 watts/' so that we see from the above
table that, as long as this Regulation is adhered to, the increment of the
resistance owing to the skin-effect will not be a serious matter, amounting to .
only a little over 8 per cent, with the maximum size and high frequency of '
alternation. With the low pressure mains lighting a large building, the
skin-effect becomes more serious, especially as the fall in E.M.F. is generally
as great as is permissible. A subdivision of these large conductors, limiting
the carrying capacity to about 100 amperes for each cable, is advisable from
other considerations ; for example, to diminish the rise in temperature, by
which means the skin-effect is rendered inconsiderable.'
Some of the typical systems of underground mains have been described
in a previous chapter, and, except the bare copper systems, these are all ,
applicable to high tension distribution with some extra care as to insulation.
The concentric vulcanised-rubber insulated cable drawn into some form of .
conduit is likely to remain the favourite method, especially where a secondary
network is combined, though the buried armoured concentric main is less
expensive. For the latter the Ferranti main is the most famous example.
The inner conductor is a copper tube ^ inch thick, having an internal
diameter of ^ inch, and an external diameter of ^ inch, and therefore a
sectional area of slightly over \ square inch. The external conductor is
another tube of -^^ inch thickness, with an outer diameter of i-^ inch, and
the same sectional area. The insulation between the tubes, as nearly as
possible ^ inch thick, is made up of layers of brown paper soaked in black
mineral wax or ozokerit, and the whole is drawn through a taper die which
compresses the insulating material into a solid mass. The external insula-
tion is of similar material but only J inch in thickness. Over the whole is
drawn a thin wrought-iron tube. The conductor is made in lengths of
20 feet, which are laid in wooden troughs filled with pitch. The joints are
194 SAFETY DEVICES.
made in position, and in spite of their number add very little to the resist*
ance of the main. The copper tubes are tamed at one end into a long cone,
which fits exactly a hollow cone in the next length. These are hammered
together and soldered. Four trunk mains thus constructed carry the power
from a generating station at Deptford to the distributing stations in the
Metropolis. The capacity of these cables is great, about .367 microfarads
per mile, and with a frequency of 67 alternations per second, and an electro-
motive force of 10,000 volts, the condenser current is nearly i^ amperes
per mile of cable. This great capacity has the effect of raising the electio-
laotive force at the sub-stations above that of the generating station at
light load, owing to the oscillating interchange of potential energy between
the magnetism of the transformers and the static charge of the cables, an
effect which has to be allowed for at the generating station. A full account
of the theory and experimental investigation of this phenomenon will be
found in Dr. Fleming's Alternate Current Transformer, vol. ii.
A word should here be said concerning the fuses to be used on high-
tension circuits. It has been stated elsewhere that the fuse break should
not be less than one inch for every 100 volts, and this length should be
increased for veiy large currents. This rule gives a great length of fuse for
high-tension work, and ingenuity has been brought to bear upon designing
a fuse of shorter length with some means of extinguishing the arc. A
design of Ferranti is most effective, the fuse being only about two inches in
length, consisting of several strands of No. 30 copper wire. An earthen-
ware vessel is used, divided by a diaphragm into two troughs, which are
filled with thick resin oil. The ends of the fuse terminate in two ringSi
which are slipped over the ends of two spring rods, one in each trough^
which keep it in tension over the diaphragm. These rods are connected to
brass blocks, which pass through the earthenware and form contact pieces
with the main terminals. In the event of a fuse melting, the spring roda
plunge into the oil and extinguish the arc.
An important safety Regulation issued by the Board of Trade iaaa
follows (Regulation 12^ :
^' In every case wnere a high pressure supply is transformed for the
purpofle of supply to one or more consumers, some suitable automatic and
quick-acting means shall be provided to protect the consumer's wires from
any accidental contact with, or leakage from, the high pressure system,
either within or without the transforming apparatus."
The sole point of danger in a carefully supervised transforming system
is in the tnmsformer itself, where the primary and secondary coils are
necessarily in dose proximity, and the most perfect insulation is liable to
break down in the presence of damp, or through mechanical injury. The
most effective safeguard is probably the separation of the coils by strips of
metal, effectively earthed, which, in the event of any considerable leakage
from the primary coil, would cause the protecting fuses on the primary
circuit to cut out the transformer immediately. This arrangement is,
however, inconvenient, and an automatic device to connect the secondary
conductors, or at least one of them, close to the transformer to earth,
immediately the potential rises above a permissible limit (400 volts) is
commonly preferred. This may, or may not, result in the blowing of the
high-tension fuse protecting the transformer, according to the resistance of
the leak ; but the occurrence of such an event will be recognised at the
Supply Station by the fall in the insulation resistance of the distributing
mains. A thin film separating a point on the secondary conductor from an
earthed conductor, the insulation of which will be destroyed by a disruptive
spark when the potential rises, has been employed for the purpose. An
'* earthing device '* designed by Major Cardew, and approved by the Board
POLYPHASE CUBEENTS. IQS
of Trade, dependBon the principle of electroetatic attraction. This apparatas
is ahown in Fig. 69, with and without the protecting cover. It oondstB of
two metal discs inaulafced from one another, the lower eupportisg a thin
strip of aluminium foil fw aa to bring it very cloee to the upper diao. The
upper disc is connected to the secondary or house circuit, dose to one of the
terminala of the transformer, and the lower, by a wire not less than No. iS
8.W.O. to the nearest gas or water pipe. Should the insulation of tht
primaiy ooil of the transformer fail at any time, causing a leakage into tbr
Fig. 69.
Cftrdew Earthing Device.
seoondary, and thus the potential of the latter rise above 400 volts, the
static charge developed on the upper disc attracts one end of the strip of
aluminium foil and thus puts the circuit to earth.
Polyphase Currents.
Instituting a general comparison between alternating current and con-
tinuous current dynamos with respect to efficiency and output, it will be
Been at once that a considerable advantage must inevitably lie with the
latter, from which the complication of winding and commutator iliSiculties
will subtract but little. The hif;her iiite of magnetic variation, owing to the
rapid alternation generally demanded has already been noted as requiring a
196 TWO-PHASE ALTEENATOE.
lower induction and therefore liu-ger eUe or lower e£BcieDC7 in altematoT&
Another disadvantage is the lees economic use of the interpolar space, or
surface of armature-core, which is only partly filled in by conductors ; so
that for a given sise of machine, and involving given Tnechanical and mag-
netic loseea, approximately only one half the space can be utilised, and the
output for the same efficiency correspondingly reduced.
This immense wa«te of space and consequent inefficiency can immediately
be removed by utilising the spaoe devoted to the oore of tbe coils for the
Fio. 70.
Oerlikon Thiee-phase Alteraator.
winding of other coils, and forming a second armature of an equal number
overlapping the former and utilising the same magnetic field. In this
second armature the pha^e of the electromotive force is a quarter-period, or
right-angle, in advance or in retard of that in the former armature; aud
except for the heat to be removed, we may consiiler the output of the
alternator to have been doubled by this means, vith the slight inconvenience
that two separate circuits mu^t be employed for the distribution of power.
An alternator such as described is known ae a " two-phasd " alternator,
and for lighting purposes alone amy be looked upon as a generator suitable
to systems already descril)ed, simply requiring a division into two circuits.
THREE-rHASB ALTERNATOR 1 97
For some purposes It is advisable to go even further and construct a
triple armature of symmetrically overlapping coils, generating electromotive
forces with phases at equal intervals, sixth periods, or 60 degrees, in differ-
ence. Three independent circuits could in this way be supplied, but if, as,
with the two- phase system, the circuits be totally independent with six
conductors, we shall only succeed in increasing the elaboration, and secure
no further advantage beyond that obtained by the two-phase system.
Should it however be found possible to arrange that the demand in
three different circuits should be approximately equal, and fairly evenly
distributed, the three-phase system presents a considerable advantage. A
moment's consideration will show that at any moment whatever the sum of
the electromotive force in the three different circuits is zero, or in other
words, the greatest of the three is in opposition and equal in magnitude to
the sum of the other two electromotive forces. It is easy to show this
from either of the graphical methods in vogue, or from the simple
trigonometrical identity
sin # + sic (# + 60*) + sin (0 + lao*) =a
If the resistances, then also the currents will in non-inductive circuits
follow a similar law, and it will be possible to reduce the number of con-
ductors in the system from six to three, the circuits being between any
pair. At any instant one of the three conductors may be looked upon as the
return wire for the currents flowing in or out in the other two.
The '' Three-phase " System, as the arrangement thus briefly explained
is termed, presents certain difficulties in direct application to lighting which
has prevented its adoption except to a very limited extent. Theoretically
it may be worked in a similar way to the three- wire system with continuous
currents, except that lamps must be connected with approximate equality in
load between each pair of wires, the effective difference of potential being
the same for any pair. The weight of copper will then be about 25 per
cent, less than for a single-phase system, but the difliculty in maintaining
a sufficient approximation to equality in load in the three circuits, and the
variation of potential difference arising from irregularity, is almost pro-
hibitive in practical work. Incandescent lamps have been constructed with
triple filaments so that the load for each is equally partitioned, but this is
scarcely likely to prove a practical solution of the difficulty. It is more
likely that when the three-phase system is used for distribution a trana-
formation to direct current will be employed for lighting purposes.
We have shown that the two- and three-phase, or to use an inclusive
term, the polyphaae systems, have a distinct advantage over the single-phase
system in the larger output and higher efficiency of generators having the
same dimensions. Still greater advantages are to be found in the fact that
self-starting efficient motors can be employed in either system, and that
transformation to continuous currents can be effected with great facility.
Motor supply is very closely connected with the economic question
of electrio lighting, but a complete account of the "rotary field" or
•* polyphase " motor would involve a long chapter on the general theory of
polyphase systems, and for this, not directly bearing on the subject to which
we have carefully limited ourselves, we must refer to one of the many
specialist text-books recently published. It must be noted, however, that
the polyphase motor is better supplied where possible by a separate system
of mains to those used for lighting, thou|]rh the same generator may be used.
For the large currents with low power-factor employed in starting them is
likely to cause a heavy drop of potential along the mains, temporarily
affecting the lighting supplied from the same mains; the effect on the
generator is less marked with separate circuits for liehting and motor
198 KOTARY CONVERTERa
supply as the real load at starting is comparatively small. The polyphase
motor is oonveuiently used to drive a separate arc lighting plant, a con-
tinuous current dynamo of the open-coil type supplying a long series
circuit, or a closed-coil dynamo supplying several shorter eeries drcoits in
parallel, as the conditions may determine.
For continuous currents of lower potential as required for incandescent
lamps a similar arrangement may be used, but a much more e£5cient and
satisfactory means of transformation to continuous currents is supplied by
the '' rotary converter." This really amounts to a synchronous polyphase
motor and closed-coil armature combined in one machine. An ordinary
closed-coil armature is supplied with '* slip-rings " and brushes by means of
which connections may be made permanently to fixed points of the armature.
For the transformation of two-phase currents four slip-rings are provided,
one pair connected to two points on the closed coil in simultaneous contact
with the commutator brushes, the other pair to similar points which reach
the positions of commutation at intervals midway between those of the
former pair. Driven by an engine, this machine can supply either continuous
currents from the commutator, or two-phase alternating currents from the
slip-rings ; it can also act as a transformer from continuous to two-phase, or
vice verad. In transforming to continuous currents it is, however, necessary
to rotate the armature until synchronism is obtained, before connecting to
the two-phase mains. This is easily effected when a secondary battery is
worked upon the continuous current side, which may be employed by dis-
charge to start the converter, and be subsequently charged. A system
worked out on these lines goes some way to solving the question of the
storage of power hitherto impossible with alternating current systems.
The rotary converter for three-phase currents is exactly similar, save that
the three slip- ring contGUsts are made to points on the closed coil at equal
intervals (120* apart on bipolar dynamos). The great advantage of the
converter over a motor-driven dynamo lies in the compactness and reduced
cost owing to the same armature and magnets being used for both alternating
and continuous currents, and still more to the fact that for the most part
the current passes directly from one circuit to the other through a portion
only of the armature, with little loss due to resistance of the coils. The
current is in fact only redistributed, and not regenerated in the closed coil,
the greater part passing to the nearer brush ; the distribution in the armature
at any moment of the alternating cuiTcnts is such as to produce the noxmal
distribution in the closed coiL
Supposing the E.M.F. produced in any turn of the dosed coil during
rotation followed the sine law there would be a definite ratio between the
effective E.M.F. in the alternating circuits and in the continuous current
I 2/3
circuit. This would be -7- or .707 for the two-phase and — ¥- or .613 for
y/^ ^/3
the three-phase. The practical divergence from the theoretic ratio is seldom
more than 5 per cent., and we may assume that to produce i^ volts between
the commutator brushes will require 71 volts between either pair of mains
in the two-phase system, and 61 volts between either of the three mains of
the three-phase. It will be necessary to transform the alternating currents
to these voltages, or other necessary values, before conversion to continuous
currents.
With polyphase systems it has been found advisable to reduce the value
of the frequency to 50 or even a smaller number of periods. This low
frequency is to suit the convenience of motor construction ; and where the
use of large motors for mechanical power is the principal purpose of the
system, a frequency as low as 25 has found favour, as in the two-phase system
CONVERSION FROM ONE SYSTEM TO ANOTHER. I99
supplied by the generators at the Niagara Falls. The rotary oonvertera
must run at a corresponding speed, being synchronous motors: with a
frequency of 50 complete periods a speed of 3000 revoiutioDS per minute
would be necessary with a two-poiar machine, and 1500 revolutions with a
four-polar machine. The latter is of course the more convenient, and within
the practical limits for a converter of very large output, since, owing to the
reason explained above, the size of the converter is very small as compared
with a dynamo of similar output.
Without altering the £.M.F. between the mains of the alternating
current circuits it is possible by varied excitation to modify to some small
extent the E.M.F. in the continuous current circuit, sufficient at least to com-
pound the machine so as to compensate for the fall of potential at heavy
loads. The speed being simply dependent upon the frequency, the E.M.F.
in the continuous current circuit must necessarily be raised, when the
excitation is increased by hand regulation in a shunt circuit, or automatically
by series coils, the magnetisation being of course supplied by the continuous
current. The result in the alternating current circuits is to produce a
higher E.M.F. between the slip-rings than between the terminals of the
generator, but a reference to the theory of the parallel running of alternators
will show that this is not impossible, but only involves a variation in the
phase relation of the generator and converter. Power is stiU transferred
from the former to the latter, bat the power-factor in the alternating current
drooit is reduced.
Two-phase and three-phase circuits may be with facility connected
together for the conversion of power from the one to the other. Suppose
two transformers be wound on similar cores with an equal number of toma
in the primaiieS| which are to be connected to the two-phase drcaitSy but
2
with the secondaries in the ratio cxf — -^ or 100 : 86.7. Let the latter
secondary coil have one terminal connected to the middle point of the former
secondary coil, its other terminal to one of the three-phase mains, the two
free terminals of the first secondary to the other three-phase mains. In
this way the transformation may be effected from two- to three-phase or
ffiee verad.
The theory of this transformation is as follows: Galling the effective
electromotive forces in the secondaries 100 and 86.7 volts respectively, it
will be noticed that the difference of potential between the free terminal of
the latter and either of the former is the resultant of 86.7 volts, and
50 volts with a [)hase difference of 90°. The resultant is in magnitude
sJ{S6,yy + (507 or 100 volts, and its phase difference from that of the
lOO-volt transformer tan' — ^ or 60^. In actual practice the ratio of the
secondary windings is made 10 : 9 to compensate for the somewhat longer
transformer circuit. The length of wire through which the three-phase
current has to pass in one of the coils is only about ^ of that in the other
two, and the lower resistance would give this a higher difference of
potential.
With somewhat less practical efficiency, three-phase currents may be
directly converted to sinp^le phase. For this purpose three separate trans-
formers are used, the primaries connected to the three-phase mains, the
secondaries in series, in such a way, by the reversion of the normal sequence
of one coil, that their electromotive forces do not neutralise one another,
bat give an electromotive force in the phase of the reversed coil. By this
means, while equal currents are still obtained in each of the three-phase
200 COUPLINO OP GENERATOKS.
circuits, a single-phase alternating current represents the whole power.
Owing, however, to the partial opposition of two of the secondaries, the
efficiency is not as good as is to be desired ; unfortunately the conversion is
not reversible, from single to three-phase.
On the whole the two-phase system is likely to be preferred, as more
" flexible," for systems mainly devoted to electric lighting. The two circuits
are practically independent, and may be used in e^tctly the same way as in
the single-phase distribution already detaOed. But the groat value will lie
in the manner it will link with the many systems now in use, and probably
solve the problem of concentrating the source of generation when various
lighting and power stations now established under different systems in one
city desire to nnite. This problem must inevitably be dealt with within a
short time in London. In dealing with futiure possibilities it must not be
forgotten that we are dealing wim a youthful science which is advancing
even more rapidly than the industry which it controls ; another decade of
similar advance as the last may as completely upset our predictions as those
of ten years ago.
Present conditions would be best suited by the establishment of one or
more large generating stations at some distance from the centre of a city,
where every advantage of natural power, or condensing water, cheapness of
land, and carriage of fuel, might be obtained. A three-phase, or two-phase
with three wires, system of transmitting mains at extreme high pressure,
would connect the generating station with many sub-stations, for which the
present generating stations could conveniently be modified. A transforma-
tion to lower tension, single or two-phase could then be effected, the latter
where motive power is required. Motor driven arc-light generators would
supply the public lighting locally on the series, or multiple series system*
Botary converters might be combined with large secondary batteries, the
continuous current with a three-wire distribution and storage for safety
and uniformity of load still being unsurpassed for the denser areas of
supply. More scattered areas would still prefer alternating current, with
grouped or separate transformers, according to the density of supply. Large
rotaj^ converter sub-stations would be established for the working of electric
traction, railways and tramways, since at present alternating motors cannot
compare with continuous current motors for variable speed.
OHAPTBR XIIL
The Conpling Together of Gtoiierators.
In the supply of electric power from central stations it is very often abso-
lutely necessary, and always a matter of great convenience, if the various
units of plant, that is to say, combinations of dynamo and engine, can be
made to assist one another by dividing up among themselves the load that
is demanded. If no arrangement is made for such coupling it will be
necessary to divide the supply mains into a number of different circuits, in
each of which the maximum load must not rise above the output of one of
the units of plant ; and a great complication of switches will be necessary in
order that any one of the dynamos may be readily switched on or off from
any one of the circuits. In addition to this complication of switches, it will
generally be found difficult to arrange the working so that the minimum
number of units are at work, and some loss in efficiency results. Also, and
this is most important of all, a momentaty extinction of the h'ghts accompanies
switching over of circuits from one dynamo to another, and even if the
CONTINUOUS CURRENT GENERATORS. 201
switching be done so rapidly as not to be noticeable, yet the sudden diminu-
tion or accession of load to any unit of plant will, unless the government of
the engine and regulation of the dynamo be exceptionally good, result in an
oscillation or *^ hunting " of governor or regulator, and consequent variation
in the lights for several seconds.
There will be a vital difference between the conditions necessary for the
connection of various classes of dynamo in series and in parallel, especially
between the conditions for alternating and continuous current dynamos.
The safety and smoothness of running will depend at least as much upon
the government of the source of mechanical power, steam-engine, turbine, or
other, as upon the dynamo itself. In this chapter we shall discuss the various
classes of dynamo Mrtottm, and in most cases assume that they are driven
by a steam-engine with the ordinary type of centrifugal governor, being
thereby maintained at a speed which is approximately constant.
Two series wound continuous-current dynamos, giving of course the same
durent^ may be connected in s&rtes without any further precautions, each
supplying the same electromotive force as it would if run separately, and the
two dynamos together will give the sum of the two electromotive forces.
There is, however, seldom a demand for such connection in practical working.
Nor should any great difficulty arise in connecting shunt or compound dynamos
in series ; this arrangement is frequently required for systems of parallel
distribution with multiple mains. In such cases secondary batteries are
frequently connected between the mains to ensure equality of potential
difference when the number of lamps in the different sections varies. But
in the case of two dynamos connected in series, of the same design and size,
equality in the division of the electromotive force between the two may be
further secured by connecting the shunt coils in series as a separate circuit
between the external terminals, and removing their contacts with the
intermediate terminals, so that the same current must inevitably flow
through each shunt coil. The strengths of the magnetic fields being thus
made identical, equality of electromotive force is secured in the armatures if
the speeds are identical.
The coupling of continuous current dynamos in parallel is a matter of
much greater practical importance, and more consideration and care is
required. Two dynamos giving the same electromotive force may of course
be thrown into parallel upon the same circuit without interference as long
as equality of electromotive force is maintained. But the variation of the
current in the armatures causes variations in the electromotive force, and in
certain cases the equilibrium is unstable so that the slightest variation may
oause a complete upsetting of the conditions.
The criterion of satisfactory stability is simply this : thai the eharacter-
iatio curves of both units of jilami, shouM be falling a^ the points a^ which they
are iDorkihg. By the characteristic curve is meant, not merely the curve
giving the relation between the current output by the dynamo and the
difference of potential between the terminals at a constant speedy but the
curve giving the actual relations between these when driven by its steam-
engine, or other source of power, the government of which inevitably causes
a slight decrease in speed as the load increases. By the characteristic
falling is meant that an increase in the current-output causes a decrease in
the difference of potential between the terminals.
A very little consideration will demonstrate the truth of this simple
criterion. For the two dynamos will so determine and divide the current
in the external ra«istance that the difference of potential between their
terminals may be the same ; and then, in the case of both characteristic curves
being ** falling/' if a variation of this balance occurs, the dynamo which
obtains an increase of current will also obtain a decrease of electromotive
202 DYNAMOS IN PARALLEL.
force, and vice versd, so that the combination will tend at once to return to
its former condition of equilibrium. Even with dynamos compounded for
constant eUctromotive force at constant speed, satisfactory stability may be
possible owing to the dt^crease of speed as the current in the armature and
therefore the load of either unit is increased ; thus the characteristics may
be really falling. If one characteristic is falling and the other rising
stahility probably depends upon whether the rise in one is more rapid than
the fall in the other or not ; but in practice it is necessary that the stability
should be unquestionable and maintained through a wide range of load, so
that it is advisable to remove any uncertainty by the precaution which
will be explained directly. Series- wound dynamos cannot be run in parallel
except when both are supplying a larger current than that which gives the
maximum electromotive force. Should an attempt be made to couple them
in parallel with smaller currents the result of a decrease in the current
in either dynamo will be a fall in its E.M.F., a stUl further decrease
ensuing, and ultimate reversal of its magnetisation and of E.M.F., the two
dynamos becoming really arranged in series; and unless the armature-
resistance or armature reaction on the magnetisation be very great, the fuses
will be blown, the engines be pulled up, or the armatures destroyed by
a huge current. It is erroneous to state that one will be driven as a motor
by the other, the converse being true as long as the dynamos continue to
run in the same direction, for the current and E.M.F. will be reversed
together in one of them, its engine being still loaded, and the electromotive
forces will ashist one another to send a current through the two armatures.
Shunt- wound dynamos work without difficulty in parallel, for their charac-
teristics are necessarily falling. It is also very easy to divide at pleasure
the load by an adjustable resistance in the shunt circuit, or by increasing or
reducing the speed of the engine, provided, of course, that the dynamos are
designed to give similar differences of potential between the terminals with
their suitable loads. As with the charging of secondary batteries, the
worst that can happen is that one will be driven as a motor by the other to
maintain the requisite speed.
With compound dynamos there is generally an uncertainty as to whether
parallel running is possible. With over-oompounded dynamos particularly
it is necessary to adopt the following precaution which cannot fail to secure
perfect stability. When exactly similar compound-wound dynamos are to
be coupled those brushes which are not connected to the mains directly,
bat from which the series coils proceed, should be also joined together by
a short thick piece of cable of negligible resistance. This wUl ensure
that the whole outgoing current shall in all cases be divided equally between
the two series coils, whatever currents flow in the armatures of the dynamos
respectively. With d3mamo8 of different sizes the resistances of the series
coils should be carefully balanced so that the current may be divided in
the right proportion. It is customary to arrange that the aforesaid brushes
should be invariably the negative brushes of the dynamos, and then the
greatest care should be taken that the positive terminal should be the last
to be connected to the positive " omnibus bar '* or massive copper bar upon
the switch board to which the positive terminals of all the dynamos are
switched. This precaution tends to safety because the previous division of
the current in the series coils when the negative switch is on prevents the
possibility of the dynamo which is to be switched into parallel attaining
with the normal speed its full electromotive force in the opposite direction
to that of the other dynamo or dynamos, an event which might occur if
the initial magnetisation were reversed before starting. Before a dynamo
is switched into parallel its electromotive force should be tested and made
equal to a volt or so in excees of that between the " omnibus barsy" so that
ALTERNATORS IN PARALLEL 203
it will immediately take a small portion of the load, and not be driven as
a motor by the d^numo or dynamos with which it is placed in paralleL
Coupling of Alternators.
The tbpory of the coupling together of alternating current dynamos is
entirely different from that of continuous current dynamos. For it is
evident that if two alternators are to work together upon the same
circuit so as to divide the power by combining their cun-ents or electro-
motive forces, the frequency of the alternations in the two alternators must
be identical, and therefore the speed of the alternators must be the same
in each if the machines are similar, or if dissimilar their speeds must bear
a certain fixed relation to one another.
]^ow supposing two alternators are driven by engines each governed to
maintain, a constant speed, it is evident that either the speeds for which the
engines are governed must be absolutely identical (or in a fixed ratio giving
the same frequency for the alternators), or else subject to slight variations
within such limits that the speeds may be made identical by some electrical
control tending to keep the speeds identical when the armatures are con-
nected. We shall show that the alternators themselves supply this control,
but it must first be noted that with an engine governed in the usual manner
to maintain constant speed, the factor which determines the power supplied
by that engine can be nothing else than the variation of its speed, or rather,
.the decrease in speed below that which it will acquire when running
freely with no load. For example, in the ordinary steam-engine the power
depends upon the admission of the steam, and this dependis simply upon
the position of the governor, and therefore upon the speed of the engine.
Thus, when two alternators driven by separately governed steam-engines
are connected together so as to divide an electricfd load the combination
runs as a rigid system as if mechanically coupled, whatever be the speed of
the system the governor balls take up a relative position according to their
adjustment, so that the amount of steam admitted to each, and therefore
the power supplied by each is fixed, and quite independent of the electrical
conditions of the alternators. Of course, it is not meant that the ratio of
the horse-power of the two engines remains the same for different total
loads, for neither is the power necessarily proportional to, though determined
by, the steam admission, nor is the steam admission in any way proportional
to, though determined by, the variation from any speed ; also the slipping
of belts and ropes may affect the relative speed of the engines at different
loads. But this all-important statement is unquestionable : given a fixed
output in power from the combined plant, no alteration of the excitation
of the field-magnets or other interference with the alternators themselves,
so long as it does not alter the total load, and their combined speed remains
the same, can afiect the division of the load between them ; for this is
entirely determined by the positions taken up by the governors at the
speed with which the engines are running.
The property which renders the parallel running of alternators possible
was discovered by Wilde, and his experimental results published in the Phil*
Mag, for January 1869. The importance of the discovery was not recog-
nised, as the demand for its application did not exist, until in 1883 it was
re-discovered by Dr. Hopkinson, who also gave the theoretical explanation,
and the results of practical experiments upon the large DeMeritens alternators
used to supply the arc lamp in the lighthouse at South Foreland.
The property, possessed by all types of alternator in a greater or less
degree, is simply this : that when alternators, running at approximately
the same speed, and excited to give approximately the same electromotive
204 MUTUAL CONTROL.
force, nave their terminals oonnected in pairs, a current passes through
both armatures which tends constantly to pull them into such a position
that the total electromotive force in the circuit through both armatures
and therefore the current is a minimum ; and therefore into such a position
that the al^rnating electromotive forces are in opposition as regards
the circuit through the armatures, and in parallel as regards any external
circuit joining the pairs of terminals. Unless, therefore, a difference of
speed be enforced by a power greater than this control, the alternators will
be constrained to maintain the same frequency, and to remain in a position
favourable for parallel working. The alternators are then said to remain ^' in
step " or '* in synchronism."
In following out the line of argument used by Dr. Hopkinson, by which
he deduced the existence of this control, before applying his theory to
practice, we prefer to adopt the clock-diagram method, in preference to the
current curve or the algebraic methods ; and in the first place we shall deal
with two alternators with their terminals connected together, so that there is
no external circuit, but only one circuit through the armatures ; then we shall
show that the alternators will be constrained by the current to move
relatively towards such a position that their electromotive forces are in
opposition.
In Fig. 71 let the vectors OAj OB represent in magnitude and relative
phase the (maximum) electromotive force of the two alternators at any
moment. Their resultant OP (the diagonal of
Pjq -i^ a parallelogram) represents the resultant elec-
tromotive force in the circuit of the armatures.
There will be considerable self -inductance in
this circuit, and the current will lag behind OP
in phase, so that in the right-angled triangle
OPQ, where tan POQ = ~-t OQ may represent
it
X R. Supposing the electromotive forces OA^ OB to be very nearly equal,
the products of these and the current (the '^ apparent watts ") are the same
for each alternator, and OP bisects AOB, But the true load is obtained by
multiplying the products respectively by ^ cos AOQ and \ cos BOQ^ and the
latter angle being the smaller the load upon the corresponding alternator is
greater ; in many cases the angle AOQ will be obtuse, and then the alternator
whose electromotive force is OA will be motor driven.
In any relative position of OA, OB^ the load will always be heaviest
upon the alternator which is in advance of the phase of opposition to the
other, and lightest upon that behind. And unless the separating force is
great, or the charge of relative phase is so rapid that the retardation or
propulsion changes from one to the other too rapidly to have any effect in
bringing the alternators into step, they will take up a position of relative
equilibrium not far from that of exact opposition.
The existence of a controlling force tending to bring and to keep the
alternators in step, and in the correct relative position for parallel working,
being thus demonstrated under the simplest conditions, let us suppose the
opposite terminals connected through an external resistance. For the sake
of simplicity, let us suppose the alternators similar in design, and that the
external resistance is not self -inductive, and also, as will generally be the case,
that the resistance of the armatures of the alternators is negligible in com-
parison with their self -inductance (that is, the number of ohms much fewer
than the number of secohms L multiplied by the periodicity p). Let ON \vl,
the clock-diagram, Fig. 72, represent the product of the amperes C (maxi-
mum) and ohms K in the external circuit, and therefore the maximum value
(in volts) of the difference of potential between the terminals. The division
ADJUSTMENT OF EXCITATION.
20S
of the power between the alternators \b determined solely by the balance of
the governors of the driving engines. If OA represents the impressed elec-
tromotive force in the armature of one alternator, which must be slightly in
advance of the phase O^j AN represents the back electromotive force in the
armature jpLC, where G^ is the maximum current, and the area OAN is equal
to pL times the true watts supplied by the alternator. Similarly with regard
to OBy the electromotive force in the second alternator. The areas OANy
OBN being thus determined when a certain electromotive force ON is called
for between the terminals, it will follow that if the currents G| and C, in the
armatures are to be co-phasal with the external current, OA and OB must be
made of different magnitudes according to the division of the load, so that
ABNkcq in one straight line perpendicular to ON^ and the alternators take
up an equilibrium position with a difference of phase (in the impressed
electromotive force) represented by the angle AOB, Then we have simply
2
C = O, + C„ or is represented by NG multiplied by -y.
But if the excitation of the alternators be altered, one may be increased
and the other diminished, but yet the difference of potential between the
Fig. 72.
terminals maintained the same. The condition is then represented by the
clock diagram of Fig. 73. The areas OAN, OBN msj be maintained by
moving the positions of A and B along lines parallel to ON. Then ANemd NB,
which give the back electromotive forces in the armatures, must represent
in magnitude f>LCj and />LC,, and the phases of the currents by vectors per-
pendicular to the.se from the point 0, The resultant of the armature
currents muht be of course the current in the external circuit, and repre-
2 •
sented in magnitude and direction by ^ . NC (or rather a vector perpen-
dicular to and equal in magnitude to -j-NC), and therefore the line
joining N to the middle point C of AB must be perpendicular to ON. It
will follow that the horizontal displacements NF,NQ of A and B must be
equal, the phase difference of the alternators altered (increased in the special
case of Fig. 73, but sometimes decreased), and the current increased in
both armatures ; for their algebraic sum is now greater than that in the
external circuit, even as the sum of NA and NB is greater than their resultant
2NC. It may be easily shown that the control still exists since a relative
advance in phase of either throws a heavier load upon it.
We have simplified this discussion as much as possible, considering it
already sufficiently intricate for the scope of this work. The more complete
problem in which the resistance of the armature, and the self-inductance of
the external circuit is taken into account may be easily treated by inclining
the lines CN, NO, &c., in the figure at certain fixed angles, and the laws
governing the parallel running of different sizes of alternators, (S:c., may be
deduced. Our chief object is to point out the method of obtaining the
most efficient results in practical working. The load can only be adjusted
206 " SYNOHEONISING.*
by attention to the engine governors, '' speeding " the engine to obtain a
greater share of the load. And in order to obtain the minimum current in
either armaturoi which is obviously the most efficient condition, the excita-
tion should be increased as a heavier load is given to it, its impressed
electromotive force being the same as it should have if it were supplying its
portion of the load upon a separate circuit, with the specified difference of
potential between the terminals. The same argument that shows that
parallel working of alternators is possible, shows that series working is
impossible except with a mechanical coupling, but happily the demand for
this practice does not exist. Parallel running, on the other band, is now
the rule in nearly all stations using alternating currents.
The mistake has often been made of supposing that, since the mutual
control of alternators depends upon the existence of self -inductance in the
armature, those which have no iron core, and therefore small self-inductance,
such as the Mordey or Ferranti (Deptford type), are unsuitable for this
purpose. As a matter of fact, some of the first and best results in parallel
running were obtained by these types, the control being more and not less
powerful. The reason is not far to seek. Although with the same con-
trolling current the synchronising force is smaller, yet with the same varia-
tion from the true phase difference, the current changes are so much the
greater. Furthermore, with iron core armatures lines of force may be
created by the armature current which cause self-inductance, but do not
pass through field-magnets and have no motor power to produce controL
So that in reality the control is more rigid in alternators that have no
armature-core. This is partly an advantage and partly a disadvantage. An
advantage in respect of more powerful electrical coupling, a disadvantage
in respect of danger from the huge current that will pass in the event of a
breakdown of the existing current, or the engine driving one of the alter-
nktors, and the inferior strength of the armatures themselves. The excess
of readings of the sum of the alternator ammeters over the circuit ammeters,
Oj + 0, — 0, is sometimes incorrectly called the '' synchronising " current. A
better name is the *' equalising " current, since it compensates for defects in
the regulation of the excitation.
For any advantage to acci*ue from parallel working it is necessary that
the range of speed through which the governors work should overlap.
Otherwise it must either happen that the slower alternator and engine wOl
be 'driven as a motor at the speed of the faster, or if the load is too heavy
for this, the two will work together within the range of the slower and the
faster will be ungovemed and take the maximum amount of steam possible.
Some adjustabUity is advisable, but many prefer to work with an equality
of normal speed, and to vary the distribution of the load in switching on or
off by closing the stop- valve. By one means or the other the load may be
removed previous to switching one of the alternators off the supply circuit ;
or the speed of an engine reduced to equality with a loaded engine just
before switching in. But not only is it necessary that the speeds of two
alternators should be almost exactly identical before switching on in order
to prevent the sudden strain on the armatures which drags them into
synchronism, but it is advisable to switch on at the moment when the two
alternators are exactly in phase. The first condition, of equal speed, might
be told with considerable accuracy by the musical notes emitted by the two
alternators, the well-known effect of ''beats" or surgings of tJie sound
indicating approximate equality in frequency of alternation. But to tell
when the alternators are in the same phase requires what is known as a
'' synchronising" arrangement. The following, or some modification of the
same principle, is employed. A small transformer is connected to each
alternator, transforming the potential down to 50 volts when normally
INCANDESCENT LAMPS. 20/
excited. If the secondary coils of these transformers are connected in series
and sapply a loo-volt lamp, it is easily seen that this lamp will only light
up with its full brilliancy when the alternators are exactly in phase, or to
epeak more correctly, if the diiSerences of potential between the terminals
of the two alternators is the same, which is the correct relative position for
parallel running. Otherwise the difference of potential between the lamp
terminals will be loo volte multiplied by the cosine of the phase-difference.
Full brilliancy will then indicate the correct moment for switching on. A
dead-beat voltmeter may be used instead of the lamp; and the transformer
secondaries may, in either cose, be connected in opposition and the moment
for switching on determined by darkness or zero reading. There is, however,
more liability to accident, for a lamp filament may br^k, or other deceptive
event happen. On approaching equality of speed the synchronising lamp
begins to oscillate slowly between full brightness and darkness, as the relative
phase difference slowly changes. Seizing the opportunity of full brightness,
the alternators may be placed in parallel when exactly *' in step," a load can
then be transferred to the incoming alternator by ^ speeding " its engine ;
the exciting current should at the same time be increased, whUe that of the
alternator or alternators that were previously supplying the lighting should
be reduced to maintain the correct difference of potential between the out-
going mains, and at the same time to retain the minimum current in the
armatures*
CHAPTER XIV.
IncandeBcent Ijamps,
The generation of heat in a conductor by the passage of an electric
current owing to its resistance was one of the first observed phenomena in
connection with voltaic electricity, and therefore the possibility of using a
conductor of high resistance as a source of illumination, by thus nusing
it to a temperature at which it emits radiation of light as well as of
heat, must have been obvious long before it was thought possible that
electrical energy could be produced at a cost that would enable it to com-
pete with other sources of illumination. The invention of the incandescent
lamp followed naturally upon the development of the dynamo and steam-
engine.
The rate at which the temperature of a conductor begins to rise when an
electric current is started in it depends upon the rate of production of heat
units in it, and upon the capacity for heat of the conductor. The capacity
for heat depends upon the mass and specific heat of the material of which
the conductor is made. The rate of production of heat is .24C'B calories
per second, a calorie being the quantity of heat required to raise one
gramme of water one degree Centigrade. Hence if M be the mass, h the
specific heat of the conductor, the temperature will begin to rise at the rate
.24C'R
of ^1 1 degrees Centigrade per second. The ultimate temperature
reached with a steady current will be independent of the capacity for heat,
and will depend upon the facility with which the heat-energy is removed by
conduction and radiation from the conductor. Equilibrium will be estab-
lished when the excess temperature of the conductor over surroundiDg
bodies causes the rate of removal of the heat to be equal to the rate of
production.
In the incandescent lamp every means is taken to reduce to a minimum
208 BADIATION.
the removal of heat from the electrio oonduetor by heat-conduction, and
practically the whole of the heat-energy generated is radiixted in the form of
light or ludiant heat. The mazimam temperature is then attained when
the rate of heat generation is equal to the rate of heat dissipation from the
surfaoe. The latter depends upon the temperature, the area of the surface,
and some property determining the rate of energy radiation per unit of
surface which is called the emissivity. l^ow for the purpose for which the
irtcandescent lamp is primarily intended, that of illumination, the energy
which is radiated in ^e form of the non-luminous heat rays is entirely
wasted, and may be considered in most cases positively objectionable, as
well as wasteful. One of the greatest benefits attending the use of the
electric light is that for a given amount of illumination there is less heat
radiated by the lamps than with any other source of illumination, and hence
the interior of buildings may, when desired, be kept cooler. But even with
the highly efficient arc lamp it appears that of the total energy converted
into heat as much as 90 per cent, is dissipated in non- luminous radiation,
and with the incandescent lamp a still larger proportion. An enormous
improvement in efficiency is therefore theoretically possible. The proportion
of bimintms radiation to the total radiation increases with the temperature
so that it cannot be expected that the efficiency of the incandescent lamp
can be nearly as high as that of the arc lamp as long as the same material,
pure carbon, is used for the filament of the one and the electrodes of the
other ; since in the latter case the material is raised to the highest tempera-
ture possible, that of volatilisation, and in the former a much lower tempera-
ture must be maintained if the filament is to remain intact for any length
of time. Furthermore, even if we could discover and utilise a body much
more refractory than carbon, or by some method prevent the volatilisation
and so use a much higher temperature, we could increase the proportion of
luminous radiation, but it does not follow that the efficiency of illumination
will be proportionately increased. The value of the illumination in enabling
the eye to distinguish details of shape and colours is not in simple propor-
tion to the energy of the luminous radiation. The more refrangible rays
at the '* upper " or violet end of the spectrum require a very much larger
amount of power to produce a given intensity of illumination measured by
its efifect on vision than equivalent rays of less refrangibility. The arc
lamp, and the " overrun" incandescent lamps are disproportionately rich in
the former rays. The ideally efficient source of illumination, if the distin-
guishing of shape were the only object, would only emit rays of a certain
refrangibility and colour. Probably the best source, for all purposes, would
be one which would follow the proportion of radiation in the dififerent parts
of the visible spectrum exhibited by day light, to which the eye is accustomed^
but omit the heat radiation as far as possible.
The proportion of the various rays of different refrangibilities emitted
by different materials at the same temperature depends upon the nature of
their surfaces. This property, called '^ selective radiation," must not be con-
fused with the emissivity, which is a measurement of the total rate of energy
dissipation at any temperature. Certain metallic oxides — ^magnesia, lime,
^j^ — will at ahigh temperature emit a{very much largerproportion of luminous
radiation than will carbon at the same temperature. The whole question is
too wide to be dealt with in this place ; it is largely physiological, depending
not only on th^ relative sensibility of the eye to illumination of different
colours, largely modified by the colour of the object to be observed, but also
on the adjustability of the pupil of the eye, whereby the efficiency of illumi-
nation is by no means proportionate to the intensity. Attention is called to
this in order to qualify apparently dogmatic assertions which will follow in this
chapter, and the main question, affecting all sources of illumination alike, left
SEMI-INCANBESCENT LAMPS. 20g
to be dealt with in another section of the series, of which this article forms
a part.
The first recorded application of the thin filament in a vacuum as a source
of light dates as far back as 1840, when Moleyns, of Oheltenham, employed
a thin platinum wire in the very partial vacuum which was the best that
could then be obtained. In 1847 Petrie employed iridium in the same way.
In 1845 ^^® ^^ ^^ ^ carbon filament was patented by Starr and King, of
America. Their invention was exhibited in England, and is said to have been
seen and admired by Faraday. But the prinoiples of electro-magnetism
discovered by the latter philosopher had not yet evolved the dynamo, and
electrical power could only be obtained from primary batteries at a totally
prohibitive cost.
For nearly thirty years the history of the incandescent lamp is a blank,
but in 1873 Lodyguine, a Russian physician, revived the carbon filameut in
the form of straight needles between blocks of carbon in the best obtainable
vacuum.
By this time several forms of passably efficient dynamos had been
designed, and the arc lamp had advanced to a practical success. Inventors
were striving to find some means of further subdividing the electric light, so
as to make the lamps independent of one another, and suitable for interior
lighting. Their efforts gave rise to a class of Ifunps which are sometimes
called " semi-incandescent)" and may be considered as holding an interme-
diate position, in principle and history, between the arc and incandescent
lamps. Their appearance was partly previous to and partly contemporaneous
with the first successful incandescent lamps, and though this dass of lamp
is practically obsolete, the principles may yet be revived in the future, so
that a short review of some of the types may not be amiss.
Semi-incandescent lamps are of two types : The first is the Lampe-Soleil,
in which the electrical arc between two carbons is made to heat to incandescence
a block of highly refractory material interposed between them. This type
seems to have been in its origin a development of the Jablochkoff candle,
for the principle is formulated in the specification of a patent taken out by
Jablochkoff in May 1877. ^* The passage of a spark (arc) through a slab of
kaolin renders the substflince of greater conducting power at all points where
it is touched by the spark ; in a few seconds the current passes readily, and
the kaolin becomes incandescent along the entire pnth of the cuirent." The
invention was worked out by Clerc (engineer to the Jablochkoff Company)
and Bureau, and an English patent was taken out in April 1880. '' The elec-
trodes are surrounded by a guide-block of refractory material, for instance,
marble, which protects the poles from the air, compels the arc to take a pre-
scribed line, and may give a special tint to the light. The block hides the
poles from view, and has orifices which serve as a guide for the voltaic arc.
It is cut out like a vault for distributing the light in any desired direction.
The guide-block is enclosed in a casing (of cast iron). The carbons advance,
as they are consumed, by their own weight, or by counter weights and springs.
To light the lamp, rods of plumbago, for instance, connect the carbons
through the orifices at the top of the vault which communicate with the
points of the carbons. One of the carbons may be replaced by a metallic
electrode." The carbons were large and burnt extremely slowly. The marble
glowed with a golden colour, and became conducting, so that it would relight
even if the current should be extinguished for a minute ; the intensity of
illumination was very steady, the great heat capacity checking irregularities
in the supply. These qualities must have been of great value with the
generating machinery of 1 88o.
In the semi-incandescent lamps of Reynier and Werdermann intense
heating is produced at a bad contact between a thin carbon pencil and a
o
210 SEMI-INCANDESCENT LAMPS.
large block. The local heating raises the extremity and sometimeA a con-
siderable length of the former to incandescence. By this means a large
current of about 50 amperes with an electromotive force of only about
6 volts will give about 300 candle power, and thus, with a series system of
distribution, the question of subdivision of the electric light was first
partiaUy solved.
The first English patent is that of Beynier in June 1877. The idea
was probably suggested in working out an invention of the preceding year
in which an arc was maintained between the edges of two rotating discs,
separately driven by small motors, with axes in the* same plane inclined at
an angle of from 20^ to 120*'. In the 1878 patent : *' A long vertical rod
or stick of carbon is guided by an insulator at its lower part and by a
'pivot' or bracket at the top: this bracket, by its weight, presses the
carbon downwards. A rack on the bracket rotates a carbon disc by means
of a train of gearing of rapidly-increasing speed. The edge of the disc
supports the carbon rod, which descends as it consumes by the passage of
the electric current through it and through the disc. At the same time
the rod acts as a brake to the disc and thus, by friction, regulates its own
descent. The conductor to the rod presses laterally upon it, the rod simply
gUding past it. The contact is kept constant by the pressure of a spring.
" In another instance the rotation of the disc is obtained by the tangential
force of the weight of the rod and its attachments upon the disc, the rod
being in a position not normal to the disc. Instead of a disc, a flat or
rounded surface roller, or a sliding surface may be used. This apparatus
prevents the accumulation of cinder, and presents fresh surfaces to the
descending rod."
Werdermann's patent is almost simultaneous. Instead of the rotating
disc there is a slightly convex block of carbon, for which copper was after-
wards substituted by Joel, and the lamp was inverted so that the pencil
was pressed upwards against the block (which was connected to the negative
conductor). The pressure of the pencil on the block was maintained by a
weight acting through cords which passed over pulleys, and drew upwards
the saddle in which the carbon pencil rested. The invention includes an
arrangement in parallel circuit, the resistance of the lamps being sufficiently
constant, though low. The lamp which is first with regard to its position
on the positive main is the last on the negative main, and so on ; thus the
fall of electromotive force from the dynamo to each lamp is the same. The
loss of energy would of course prevent transmission to any distance on this
system with the large currents used. Ducretet (Jan. 1879) floated the
carbon pencil in a tube of mercury, the electrical connection being made
through the mercury; but this was associated with the production of
mercury vapour to an extent which could not be permitted indoors, and the
pressure between the electrodes alters as the carbon is consumed. Sawyer
placed the lamp in an atmosphere of nitrogen and thus greatly lengthened
the duration of the carbon pencil. Further development of the semi-
incandescent was checked by the appearance of the true incandescent lamp.
In October 1878 Lane-Fox filed a patent for "obtaining light by
electricity ; and for conveying, distributing, measuring and regulating the
electric current for the same." The light was produced by the incan-
descence of a continuous conductor, made of an alloy of platinum and
iridium, surrounded by an atmosphere of nitrogen gas. The lamps were to
be arranged in parallel with a network of single conductors, and an " earth "
return. The engines driving the generators were to be regulated electrically,
and the regulation assisted by placing a battery of Plante secondaiy cells
in parallel with the lamps. A further invention provides for the covering
of the metal filaments with finely-divided asbestos or some other refractory
CARBON FILAMENTS. 211
material. And again in March 1879, it is proposed to use a mixture of
plumbago with some refractory non-conducting substance for the conducting
** bridge."
In November 1878, Sawyer revived the use of carbon rods, again in an
atmosphere of nitrogen, preparing the carbons by '* flashing/' that is
heating electrically in a hydrocarbon gas or liquid, as will shortly be
described. Edison in America was meanwhile working with filaments of
platinum, ruthenium, and other metals. In Oct. 1878, several arrangements
were patented by which the metal may be prevented from fusing by an
excess of current. " The heat evolved in the light expands a wire or other
body, either by the passage of electricity through it, or by its proximity
to the source of light : the expansion acts through a lever, or similar
device, to shunt or short circuit more or less of the current, or throw a
resistance into the circuit." Later the wire is " pyro-insulated " by a
coating of some refractory metallic oxide such as lime or magnesia.
The next year 1879, Edison used a carbon filament in a nearly perfect
vacuum. The filament was made of carbonised cotton or paper covered
with a plastic compound of lamp black and tar. Carbonised paper without
further treatment was subsequently used, and in the following year
(Sept. 1880) fibres of bamboo, stamped out and shaved down to the proper
thickness by special machinery, were used. The inventor aimed at pre-
serving the structural character of the bamboo fibres, and obtaining a
filament of high resistance in order to use a parallel distribution. The
preservation of organic structures in the filaments is now believed to be a
mistake, but still this bamboo fibre lamp must be looked upon as the first
really successful form of incandescent lamp.
Various patents were taken out in 1880 by Swan, of Newcastle-on-Tyne,
for obtaining greater durability in the carbon filament. He appears to have
made attempts as early as i860 to produce an efficient lamp, but the vacuum
that could then be obtained was insufficient. The development of the
Sprengel mercury pump had now made a high state of exhaustion possible,
and by raising the filament to incandescence during the exhaustion, the gas
occluded in the carbon was driven out. Cotton thread was the carbonising
material preferred, the organic structure being destroyed byparchmentising.
Swan exhibited his lamps before the Newcastle Literary and Philosophic
Society in October 1880. The resistance of the lamps for the same candle-
power was lower than that of Edison's lamp, and it appears to have been
intended to use a series system of distribution.
The exhibition of the incandescent lamps of Swan at Newcastle was an
anticipation of the master -patent of Edison, but only carried the right of
manufacture of lamps with filaments of a greater diameter than |^ in.
We shall not attempt to follow further the history of the incandescent
lamp ; it is by no means easy to give due credit to the multitude of inventors
who have contributed to its development. The amalgamation of the Edison
and Swan Companies in 1883, gave the '^Ediswan" Company control of
sufficient patents to monopolise the manufacture of incandescent lamps in
this country up to the autumn of 1894.
The qualities of carbon which render it entirely without competitor as
the material for the construction of the incandescent filament are as follows:
(i) Its resistance is suitable, requiring for the consumption of power
required for small lamps a conducting filament of sufficient thickness to give
good mechanical strength with the electromotive forces suitable for parallel
distribution. Metal filaments, of platinum or iridium, require to be very
long and thin to give the right resistance and radiating surface. The specific
resistance of pure carbon is sufficienti though it might with advantage for
this purpose, be higher.
212 CARBON FILAMENTa
(2) Its emissivity is very much superior to that of metals, so that at the
same temperature the caniile power per square millimetre is much higher.
With regard to '^ selective radiation/' it is as stated above much inferior
to certain '* earths/' or metallic oxides, such as ziroonia, &c, which emit at
the same temperature a larger proportion of luminous rays. These oxides
are, however, very partial conductors oi.'y at high temperatures, and non-
conducting when cold. Efforts have been made to combine these substances
with carbon, or to cover the carbon filament with them, but have failed
owing to the variation of expansion when heated, which causes the oxide to
break away. It seems preferable to strive to obtain the purest carbon and
increase the efficiency of radiation by the higher temperature which can thus
be permitted.
(3) Oarbon is much more refractory than any of the metals. At the
temperature of incandescence it retains its rigidity, and is still far from its
melting or volatilisation point. Platinum or iridium are near their melting-
point when incandescent, and become very soft.
(4) The conductivity for heat is very low, and therefore very little of the
heat-energy escapes by conduction through the terminal.
Foullet has given the following summary of the relative incandesoenoe of
a carbon filament at different temperatureS| showing the colour of the
luminous radiation from the surface :
Degrees OentlgTftdti
525 •••••• Indigent ledneak
700 Dull red.
800 •••••• Incipient cherry redDeob
1000 •••••• Full cherry redneBS.
1100 TellowUh-green.
X200 Bright yellow.
1300 White heat.
Z400 •••••• White heat (strong).
1500 •••••• Dazzling whiteneas.
The first carbonising subetanoe used by Edison in 1879 "^^^ parch-
mentised paper, which he replaced the following year by fine strips of
. bamboo, stamped out by special machinery. Swan also used both parch-
mentised paper and cotton. The latter substances, paper and cotton, still
remain the favourite raw material from which the carbon filament is manu-
factured. Ootton is almost exclusively used in England except for special
<< focusing " lamps, where filaments with a fiat, or oblong, croesFsection are
required, but there are various methods of preparing the filament for
carbonisation. The older established process consists in " parchmentising "
by the action of sulphuric acid, and then drawing through die-plates till it is
shaved down to perfect uniformity of the required section. The parch-
mentising process destroys the organic structure of the thread, reducing it
to a semi-transparent gelatinous, or "amyloid," state, but retaining the
same chemical composition (Cfi^fi^. The thread (of loose knitting or
crochet cotton) is drawn slowly through an acid solution (of specific gravity
1.64), and thence through a large basin of water. The thread thus remains
soaked in the strong acid for from 4 to 5 seconds only, and the acid must
be entirely removed in the water bath.
After drying the thread is found to be in a tough, homy state, and can
be shaved down to the required uniform section by drawing through succes-
sive jeweUed wire draw-plates with very sharp cutting edges.
The latter operation is expensive and difficult, but most essential in
order to obtain the perfect uniformity requii*ed« Other less expensive
methods, giving a similar result from the same raw material have lately
come into extensive use. The cotton material may be completely dissolved
MANUFACTITBE OF FILAMENTS. 213
in chloride of Einc, &Qd the reflulting viacoun fluid squirted throngh a small
hole into a veasel containing alcohol, which cansee it to set and harden
immediately. A difficulty arises, however, in ohtaining uniform visconty,
and preventing the formation of air hubbies, so that tbti squirted thread
may be perfectly uniform. There are other methods of converting cotton
into a fluid mass, which may then be either squirted or squeezed into a flat
sheet between glass plates, dried, and cut into stripe.
Next comes the carbonising process. As this will destroy the flexibility
of the thread, it is necessary previously to wind it upon carbon frames,
which will maintain the shape which is ultimately required, before plaoiDg
in the carbonising furnace. For low voltage lamps the short filament
required may be of a simple horse-shoe shape, but for the longer filaments
required for higher voltage a looped form is more convenient. Two carbon
Fio. 74. Fio. 75.
Hig;h and Low Voltage Lamps.
cylinders held apart by wooden struts will constitute a framework upon
which the thread can be wound continuously, so that it can be out after
carbonisation into either required shape. For the horse-shoe form the
thread must be wound continuously round the two cylinders together; for
the looped form an intermediate turn must be taken round the cylinders
separately. The thread shrinks considerably in carbonising, and for this
aluiwance may be made by separating the carbon cylinders with wooden
struts, which in also carbonising shrink proportionally. For looped fila-
ments it will be necessary to split the cylinders in the direction of their
length and separate by similar short wooden struts. The frames are then
pla^d in plumbago crucibles, and raised gradually to an exceedingly hif^h
temperature in a reverberatory furnace. A carbon box containing about ten
of these frames is placed in the crucible and the intervening space between
the box and the crucible filled up with powdered charooaL The small
214 MOUNTING.
amount of air left in the box cannot cause the thread to burn while car-
bonising. The temperature must be both raised and lowered very gradually,
and the subsequent duration of the filament will depend largely upon the
highest temperature reached. If the carbonisation is not complete, the
resistance of the filament is high, and the subsequent carbonisation when
incandescent will cause rapid disintegration by the gases produced in the
interior of the filament. Tne carbonisation process, formerly a matter of
several days, is now abridged to a few hours, a temperature of something
like 2000 degrees Centigrade being attained for a short period at about
half-time.
The next process is that of " mounting," or connection to the " leading
in" wires. These wires have to pass through the glass bulb into which the
filament is subsequently inserted. The wires must be sealed into the bulb,
and must be capable of withstanding the many changes of temperature to
which they will be subject, both during the sealing with the blow pipe, and
subsequent incandescence of the lamp, without either melting or cracking
the glass. We need therefore a metal having a high fusing- point, and a
coefficient of expansion identical, or nearly identical, with that of glass.
Platinum fulfils the conditions excellently, the only objection being its
expense, but this is after all a very small proportion of the cost of the lamp.
Certain alloys of silicon and iron or nickel have been designed to meet the
requirements, but these are liable to injury in " sealing in," and the extra
skill in manipulation costs as much as the extra expense of platinum. The
coefficient of expansion of platinum is .0000088, which is sufficiently near
the average for various kinds of glass. In lamps of high candle power, where
the stouter filaments would allow of a peroeptible conduction of heat to the
wires, and injury as well as a slight inefficiency might result, F^hort lengths
of iron wire, which is a worse conductor of heat as well as cheaper than
platinum are sometimes interposed between the leading in wires and the
filament itself.
A good joint, both electrically and mechanically sound, is of course
essential. Edison in his first lamps made a socket for the carbon filament,
by twisting the platinum wire into a spiral ; the joint was then made secure
and electrically perfect by an electro-deposit of copper. The objection to this
joint is that with the high temperature of incandescence some of the copper
is inevitably volatilised and forms a deposit upon the interior of the glass
bulb. Swan and Lane Fox used small jointing tubes of carbon, into which
the wires and the filament ends were pushed, and secured by a little carbon
paste. Maxim flattened the ends of the filament, and used a miniature
platinum bolt and nut.
The best joints were till recently after Edison's method, but with a dense
oarbon deposit produced by the decomposition of a hydrocarbon gas or fluid
when the joint is heated to a high temperature, instead of the electro deposit.
The socket is made with less waste of platinum by flattening the ends of the
wire for about one eighth of an inch and bending round so as to forma tube.
The filament-ends are then inserted in these sockets and placed in the hydro-
carbon gas or fluid. A current many times larger than that which will
subsequently pass through the joints is now made to flow through them, the
rest of the filament being meanwhile short-circuited. The heating at the
joints decreases as carbon is deposited, and the joint becomes sound.
Ordinary coal gas may be used, but the deposit from fluids is more rapid.
The mineral oils, kerosene or pure petroleum, are the best, but care must be
taken to prevent ignition. A mixture of four parts of kerosene to one of
turpentine is recommended by Ram as giving a rapid and hard deposit.
More recently, with the keen competition in price arising from lapse of
the master patents, this somewhat difficult process has been abridged by the
FLASHINO. 215
use of a suitable cemeDt, applied directly. The composition of the various
cements is geD«»rally a secret, and a weak joint is a common fault in some of
the cheaper lamps, resulting in a speedy fracture.
The process of carbon deposition mentioned above for the purpose of
perfecting the joint must not be confused with the next operation which is
similar, but introduced for the purpose of improving the filament itself.
The deposition by ''flashing" takes place over the whole surface of the
filament, but must be conducted with much greater care. The objects of
fli^shing are threefold: (i) To secure perfect uniformity in the filament.
This was the primary object for which it was introduced by Weston. Any
parts of the original filament which happen to be of reduced sectional area
are heated to a higher temperature, and the deposit there proceeds more
rapidly till the incandescence is uniform. Filaments can now be made by the
process of drawing through die-plates with a uniformity which is practically
perfect ; but even thus there will generally be a slight difierence in incan-
descence owing to the slightly freer radiation from the limbs than from the
loop. The loop requires^ for uniform incandescence, to be of slightly lower
resistance than the limbs, and the requisite variation in dififerent parts is
effected by the flashing process.
(2) The deposited carbon may be made far more durable, and capable of
bearing a higher temperature of incandescence than the original filament.
The surface, has, after a slow deposit, a silver-grey appearance, and the
emissivity is considerably reduced. It does not follow, however, that the
efficiency, which depends upon the ratio of the luminous to the total radiation,
is correspondingly lowered, but simply that a larger surface is required for
the same amount of illumination . Higher efficiency may certainly be attained
with flashed filaments since a higher temperature of incandescence may be
used.
(3) Flashing gives a method of exactly adjusting filaments for different
lamps so as to give the same temperature of incandescence with the same
electromotive force, or of using the same size of unflashed filament for
several slightly varying candle powers, by giving deposits of vaiious thickness.
The adjustment of the filament so as to obtain the required temperature
of incandescence with the appropriate electromotive force is very important.
From a commercial point of view this equality of the temperature of incan-
descence is more important than equality of candle power in different lamps
which are to be sold as similar ; for the eye is a very good judge of the colour
of the light (its comparative whiteness), which»depends upon the intensity
of illumination, but is less critical of the actual candle power of the lamp.
Now an error of i per cent, in the length of the filament will make consider-
able difference to the temperature attained by the filament and therefore to
the intensity of illumination, but will affect the total candle power to a far
less degree. By the process of flashing any such error may be corrected very
simply and effectively.
The deposit may be obtained either from a hydro-carbon gas or fluid. The
process is really the same in each case, since even in the fluid the filament,
when raised to incandescence by the electric current, is surrounded by a
gaseous envelope. Ordinary coal gas, or vapour of benzine, ether, or other
volatile hydro-carbons may be used at ordinary atmospheric pressure. But
the deposit will be rapid, and a rapid depasit cannot be very dense, nor does
it allow time for the careful regulation of the resistance and temperature of
the filament, which is the chief advantage to be attained by the process. It
is advisable to use a gas whose consistency is exactly known, and to perform
the flashing in a closed vessel in which the gas is kept at a fixed pressure
consi<1erably below that of the atmosphere. Pentane, or purified gasolene,
as used for the Harcourt standard gas flame, is recommended.
2l6 FLASHING.
Having thoB airangsd a syatem b; whi'<b the deposit may be dow sad
denae, and at the same time the conditions of preaeuie and composition of
the hydro-carbon gas the same, the flashing current may be applied. This
current may be many timee larger than that whicb will be subsequently carried
by tbe filament in its exhausted bulb, owing to the rapid convectiou of beat
by the surrouodLDg Taponr. The resistance of tbe filament rapidly decreases
as the deposit thi^ens, since the specific reeistanoe of the deposited carbon
is often about one tenth of that of the original carbonised thi^^.
Flashing is generally effected by using an electromotive force about twice
that for which the lamp is ultimatel; intended. At first the current is small,
as the reeistanoe of the unflaabed filament is about twice as great as it will
be after the deposit, and is oold. As the deposit proceeds tbe resistance
decreases, both by the rise in temperature and increasing section, until the
current has reached about double the amount intended for the lamp, with
which current only about the normal incandescence is reached, owing to
Fia. 76.
Seal for l^igo Lamp (" Bottom Loop "). Short Seal for Small I^mp.
convection currents in tbe gas. An automatic switch at this [toint breaks
tite circuit, and the titsment is taken out ready for insertion In tbe tclaes
bulb.
The familiar pear-shaped glass bulbs are blown with tbe best dint glass
containing a large quantity of lead, either direct from the crucible, or from
glass tubing. The construction and the sealing in with the blowpipe of tbe
platinum wires need not be described here, though it requires considerable
mechanical skill. Owing to the difficulty in obtnining efficient glass-workers,
it was necessary for the hrnt lamp manufacturers to make very long seal,
with a great waste in platinum wire. A short seal can now be made very
dependable, hut care must be taken to cool the glass verj- slowly, or cracking
may ensue. A long, tbin tube is left projecting at tbe further end through
which tbe air baa to be exhausted.
It is not only in order to prevent the consumption of the carbon by com-
bination with the oxygen of tbe air at the high temperature of incandescence
EXHAUSTIOIT.
ai7
that the TMniuin needs to be as perfect as possible. The prermtioii of con-
duction of heat away from the filameDta by convectioD currents of any air
or gaa contained in the bulb b a matter of scarcely less importance. Sawyer
in 1S79 employed a bulb filled with nitrogen, the oxygen of the air being
Fio. 78.
p with Farallel Filametita ("Sbip Side-ligbt ") for Bajooet Sooketi,
I'lo. 79.
Details of BajoDet Socket.
removed by burning phosphorus in the bulb ; but even with nitrogen the
carbon can, at the high temperature used, combine to form cyanogen. Further
patests have been taken out to avoid the necessity of high exhaustion, the
most expensive process in incandescent lamp manufacture, by previously
removing the oxygen with burning phosphorus, and leaving a very attenuated
2l8 CANDLE POWER AND EFFICIENCY.
atmosphere of nitrogen. A vacuum of at least 3^^^^ of atmospheric pressure
is the minimum necessary to make the incandescent lamp serviceable. The
convection currents decrease the efficiency of the lamp, the beat removed
being so much wasted energy, and moreover they cause the glass bulbs to
become heated unpleasantly, if not dangerously.
The development of the mercury pump has made the incandescent lamp
a practical possibility. The original designs of Sprengel and Geissler have
been enormously improved by Swinburne, Stearne and others, so as to
produce a pump which will exhaust the air with considerable rapidity, and
attain a vacuum which is, practically perfect. A difficulty arises in removing
the thin film of air which tends to cling to the inner surface of the glass
bulb and the occluded iSair in the filament itself. The glass bulb requires
to be gently heated with a blow-pipe, or other convenient means, and the
filament should be raised to incandescence by the passage of the full current
for which it is designed during the last stages of the exhaustion.
Space cannot be affi^rded for descriptions of the many forms of mercury
pumps used. Fairly complete descriptions may be found in Eam's ** In-
candescent Lamp and its Manufacture^" or Slingo and Brooker's " Electrical
Engineering.'' Until quite recently the mercury pump was believed in-
dispensable to the incandescent lamp, the discovery of the former having
made the latter possible. At the same time it was known to be, at its best,
defective, slow in its action, and imperfect in its results since mercury
vapour must be left behind, and it has been strongly suspected that the
latter is mischievous in more than one way. Attempts to construct an effi-
cient mechanical pump have been made continuously since the incandescent
lamp has been in use. Kecently by perfect fitting of the cylinders, and the
device of distributing the vacuum through a gradation of chambers, Ber-
renberg has succeeded in producing an exceedingly good vacuum, but mer-
cury exhausted lamps are still generally considered the highest class. The
vacuum should certainly not be less than about -^jj^jj^ of atmospheric pressure
and as much nearer perfection as can be obtained. It may be tested by the
glow produced in it by an induction coil, but perhaps the most practical as
well as simple criterion of the vacuum would be the temperature of the
glass bulb when the lamp has been burning for a shoH time.
Candle Power and Effloienoy.
In the modern incandescent lamp with long fine filament, and its
vacuum to all intents and purposes perfect, the heat generated by the
electric current is almost entirely removed by radiation ; the only two
possible sources of conduction, by the leading in wires and by convection
currents in any enclosed gas are now almost entirely eliminated.
The filament therefore attains the temperature at which the radiation
from the surface is equal to the rate of production of heat in the mass of
the filament. The former depends upon the temperature, the area of the
surface, and the emissivity. The latter upon the current and resistance.
Heat is generated in the filament at the rate of .24EC calories per
E»
second ; or we may write this .24-^ or .24 0^.11 calories, according as the
known factor of the power is E or With constant electromotive force
the rate of heat production varies inversely as the resistance of the filament.
The total capacity for heat of the filament is of no importance, only affecting
the rapidity with which the lamp answers to the switch, and perhaps
tending to compensate for rapid variations in the current or electromotive
force, acting as a sort of " thermal fly-wheel."
The absolute efficiency of the lamp is the ratio of the useful luminous
CANDLE POWER AND EFFICIENCY. 219
radiation to the total radiation from the surface, and this is found to
increase rapidly with the temperature which the filament maintains. The
efficiency depends upon, and is commonly expressed as, the number of watts
ilissipated per candle power, since it is most difficult to determine the
absolute efficiency, that is the ratio of the energy radiated as light, to the
total energy absorbed. This expression '* watts per candle power " should
be more correctly termed the inefficiency of the lamp, as the number of
watts per candle power should decrease as the efficiency improves. The
measure of the candle power per watt dissipated would be a more correct
method of measuring the efficiency, but would be subject to the disadvantage
of being in almost all cases a fraction.
Accepting, however, the expression established by custom, it is necessary
to define further the meaning of " candle power.'' The illumination of the
incandescent lamp, though more uniformly distributed than that of the arc
lamp, is by no means uniform in all directions. Suppose the lamp suspended,
as is most usual, with the exhausting seal downwards, the best illumination
is to be found in a horizontal plane through the lamps. But even in this
plane it is far from uniform, especially in the looped filament types^ but
varies with the visible area of the filament. At right angles to the plane
of the filament the visible area is practically given by the product of the
length and diameter ; but from a direction at right angles to this, that is in
the plane of the filament, the apparent area is distinctly less, that of the looped
part being reduced in the ratio of 2 : x. Underneath the lamp the apparent
area is much reduced, especially with " horse-shoe" filaments. In fact the sur-
face of which the radii vectores from the lamp express the illumination in any
direction would be approximately an ellipsoid of three unequal axes. By
the candle power of the lamp is to be understood, not the Qiaximum, but
the ^ mean horizontal " candle power, that is, the average ratio of the
illumination in all directions in the horizontal plane through the suspended
lamp to that of the standard candle.
The variation of the illumination in different directions might well be
considered somewhat more than it is in arranging incandescent lamps to
obtain the best effect. The " mean spherical " candle power, that is to say,
the average candle power in all directions, would be in some cases a more
satisfactory method of measurement. More especially would this be the
case with large lamps, which are commonly suspended at a considerable
height in halls or large rooms. With such the '* horse-shoe " shaped fila-
ment is preferable, in order that the light may be better distributed, there
being then no bright patch underneath the lamp. The light under the
lamp is also weakened by the dispersion due to the sealed end of the glass
globe.
The efficiency of the incandescent lamp must necessarily be lower than
the nominal efficiency of the arc lamp, about ^ watt per candle power.
The efficiency may, however, be raised to very nearly this amount by an
increase of electromotive force, producing a temperature which almost
immediately volatilises the carbon and so destroys the filament. Even at
a temperature which gives an efficiency of i watt per candle power, the
carbon filament rapidly disintegrates, and is destroyed in a few minutes.
A giudual decay of the filaipents takes place with greater or less rapidity,
according to the temperature of incandescence, and finally results in the
rupture of the filament at its weakest point. The time which a lamp may
be allowed to burn with a given electromotive force before this rupture is
called its life. The life will be shortened as the electromotive force and
therefore the temperature and efficiency are increased. In improving the
efficiency of radiation we therefore require more frequently to replace the
lamp by a new one. The electromotive force which, for a given type of
220 DECAY OF FILAMENTS,
lamp, gives the beet coiitmercia.1 efficiencjr, that is, the minimum total cost
per candle power per hour will be called the be«t electromotive force (or
that type of lamp, and will be said to give the bttt effidencj and beat life.
It will, of course, depend upon the relation between the costs of power and
the prime cost of the lamp, as well as the law of dependence of the life upon
the electivmotive force, Ac.
But with large lamps having coaner filaments it will be found eome-
timee advisable to replace a lamp before its " life " is ended, that is before
the filament breaks. For the disintegrated particles of carbon are deposited
upon the interior surface of the globe, and cause great obecaratton of the
light, and hence decreasing efficiency as time goes on. The time of burning
Screw Tenainal Holder for I^rgfe Lamp (Bdlawan Sunlight).
with a given electromotive force before replacement is rendered advisable
will be called the ecmwrnie life-
The physical cause of the decay of the carbon filament is not yet fully
understood. The fact that the rate of decay seems to be idei.tical with
continuous and alternating currents of the same virtual magnitude sssures
us that the effect is entirely due to heat, and not to any extraneous electrical
effect. It may be, as some have supposed, mainly due to minute quantities
of occluded gas in the filament, or gas formed by some chemical reaction at
high temperature, which causes minute volcanic projections of the carbon
particles. If so it should be partially curable by obtaining perfectly pure
carbon. Or it may be premature voUtilisation, which may proceed slowly
in the vacuum before the temperature of volatilisation at atmospheric
pressure is reached. It is at once evident that the "best" efficiency will
depend upon the size of the lamp, since the prime ccet does not increase at
DIMENSIONS OF FILAMENTS. 221
anything like the same rate as the oandle power, and for this reason alone
it will be more economical to use lamps of higher efficiency allowing more
frequent renewals. For example, the prices of lamps up to 25 candle power
are generally the same, the cost of manufacture being probably even
greater for the smaller, fine filament, lamps. While the price of a 1000
oandle power lamp is only between 16 and 17 times as great instead of
upwards of 40 times. Moreover, supposing the decay of the filament to
proceed at the same rate per square millimetre of surface, as it may be
reasonably supposed to do at the same temperature, the life of the larger
lamp will be longer in proporijon to the increase in diameter of the filament.
It is therefore possible to use a higher temperature and efficiency, and retain
the same length of life. The economic life may be somewhat shortened,
since the surface area of the bulb will not increase in proportion to the
candle power, and the lamp will blacken faster, but it is still commercially
advisable to allow a higher efficiency. Postponing further discussion on
this point, we may state that at the present cost of power, and of lamp
manufacture^ the efficiency considered most advisable is that of 3 to 4 watts
per oandle, according to the voltage for lamps of less than 16 candle power ;
from 2.5 to 3.5 for lamps from 16 to 100 oandle power ; and for larger
lamps the efficiency may be improved to the limit of 3 watts per candle
power.
Having decided the efficiency which is advisable for a lamp of a certain
candle power, the next thing is to calculate the requisite dimensions of the
filament. For this calculation we shall have two simultaneous equations
for the length and diameter of the filament given by the following
conditions :
(i) The resistance of the filament must be such that the power dissipated
with the given electromotive force or current must be the product of the
candle power and the watts per candle.
(2) The area of the surface must be such that the requisite radiation is
obtained at the temperature that corresponds to the specified effidenoy.
For the first formula we need experimental data concerning the specific
resistance of the carbon ; for the second experimental data concerning the
relation between the candle power per square millimetre (c), and the watts
per oandle power. We shall treat solely of the case of a filament of
circular cross-section. Let P be the total candle power required (" mean
horizontal **) ; the candle power calculated on the assumption of a straight
filament would be somewhat greater than this acoordmg to the shape of
the filament, say r P. Let E be the candle power per square millimetre of
the surface. Then since Id is the apparent area viewed from a point at
which it is a maximum, the second condition gives us
IT P = < Id.
If p be the specific resistance ; W the watts per candle power, the first
condition gives us
B 4I
Whence for lamps of various candle power intended for the same electro-
motive force we shall have
docP* locP*»
and for lamps of the same candle power intended for various electromotive
forces we shall have
docB"* 1«B*
222 DIMENSIONS OF FILAMENTS.
Exact determination of the dimenBions of the filament can pcarcely be
made owing to the diflScultj of determining the properties of the cai bon
denoted above by the symbols e and p. And even if such determinations
could be made it would be hard to carry them into practice with any
certainty in the manufacture of fine filaments. The flashing process how-
ever gives us a means of correcting small variations, and obtaining lamps
giving exactly the required candle power. It is, however, far more important
to ensure that the lamps when placed upon the same circuit should glow
with equal brilliancy, reaching the bame temperature of incandescence, than
that their actual candle power should be identical. The eye soon detects this
variation in brilliancy, while it judges badly of the actual candle power.
Such equality can easily be attained by carefully regulating the time and
conditions of flashing.
The preliminary determinations must be based upon careful measure-
ments with the material used in any lamp factory. In order to get some
idea of the requisite sizes of filament, we shall give some calculations based
upon certain average properties of carbon filaments. With regard to the
quantity e, we may assume that with a temperature of 4 watts per candle,
the candle power per square millimetre is about ^ for unflashed filaments ;
but according to Bam the radiation is reduced in the ratio of 10 : 14 by the
denser deposit after flashing.
The specific resistance of the unflashed amyloid carbon filament is about
•035 ohms per cubic millimetre at the temperature of incandescence, and of
the deposited carbon about -^ of this. The resistance does not appear to
vary greatly with the temperature throughout the range of incandescence
from 2 to 6 watta per candle power ; but when cold the resistance of an un-
flashed filament is about i^ times as great; and the specific resistance of
the deposited carbon about 2^ times as great. Hence the resistance of a
lamp may be in general taken roughly as about twice as much cold as hot.
Suppose we require the dimensions of a 16 candle power lamp intended
for an electromotive force of 100 volts, with an efficiency of 4 watts per
candle power (somewhat low in view of recent developments), and therefore
requiring 64 watts. The ratio of the maximum to the mean horizontal
candle power will depend upon the shape of the filament ; with a looped
filament of the common shape the maximum would be about 18 — ^that is
the coiled filament will give a mean horizontal intensity of illumination of
about |- that it would give when straightened out*
Then for an unflashed amyloid filament
Id = 36
.03s >< iFd* = ^ = i56.aS
whence
4. 36 X. 03s
IT 156.25
giving approximately d « .217 mm. ; la 160 mm.
The effect of flashing is to reduce e in the ratio of zo : 14, so that for a
veiy thin coating
1 d = sa4
and
T 156.25
whence d a» .243 mm. ; 1 = 207 mm.
When, however, the deposited coating is of appreciable thickneos, the
mean specific resistance of the filament is considerably reduced, owing to
the fact that the specific resistance of the deposited carbon is only about
VARIATION OF EFFICIENCY, 223
•^^tb of the amyloid variety, so that a smaller diameter, and greater length,
are required for a lamp of the same candle power. For example, if the
flashing be continued until the surrounding tube of deposited cai-boa is
one tenth of the sectional area of the unflashed filament, so that the resist-
ance is reduced by flashing to one half, the mean specific resistance will be
ih (-035) ohms per cubic millimetre ; hence the diameter will have to be
increased in the ratio of (.55)^ : i» and the lenglh correspondingly increased,
giving (appioximately)
d = .2 Dim. 1 = 250 mm.
For lamps of other sizes intended for the same electromotive force, and
the same efficiency (4 watts per caudle), we have d x P2, 1 x P^. Thus for
a 100 candle power lamp the diameter will be increased in the ratio 3.399
and the length in the ratio 1.84.
But for lamps of this size a higher temperature and efficiency may be
used with advantage. Now we shall give directly the results of experiments
showing that the candle power of the same lamp varies approximately,
taking the average through a considerable range of variation, as the 5.5
power of the electromotive force. This will give us a means of calculating
how the radiation per square millimetre wiU vary with the efficiency, by
considering the variations in the same lamp.
The resistance varies but slightly with the temperature when the filament
is incandescent, and therefore the total radiation, being the number of watts
absorbed, varies as the square of the electromotive force. Hence W, the
number of watts per candle power, varies inversely as the 3.5 power of the
electromotive force. For
PocK"
focP.W ocB?
whence
and therefore
That is to say, the candle power per square millimetre varies inversely as the
•^th power of the watts per candle power.
The same law must apply to lamps of various sizes. Therefore, since
P d*
1 d X — and -y x PW for lamps intended for the 8ame electromotive force
and
Hoc JL ooPW*^ or 1 oc P* W*
For a lamp of 100 candle power, with an efficiency of 2.5 watts per
candle power, we should therefore require to multiply the dimensions of the
filament given above for a 16 candle power lamp, with an efficiency of 4 watts
per candle power, by 2.65 and 1.80 for the diameter and length respectively.
It is inevitable that the glass globe should arrest a certain amount of the
radiation from the filament, but when it is clean the loss in the luminous
radiation must be very small. Any defect in the vacuum speedily shows
itself in the conduction of heat to the glass, but a properly exhausted lamp
of small candle power should remain quite cool, and may be safely placed
amidst highly inflammable material. In the event of the lamp breaking,
the filament of such a lamp will be consumed almost instantly.
Not so, however, with large lamps ; they must be suspended at a con-
siderable di-itance from combustible material. For the mass of incandescent
224 VARIATION OF ELECTROMOTIVE FORCE.
filament is greater in proportion to the surface, and a perceptible time must
elapse before the filament is consumed after the fracture of the bulb. Also
the normal temperature of the bulb is higher, since the area of the globe
must be smaller in proportion to the total radiation from the lamp.
To see this clearly, notice that with the same temperature of incan-
descence the length of the filament must be proportional (with the same
E.M.F.) only to the cube-root of the candle power. And therefore if the
other linear dimensions follow the same law of variation, the area of the
surface of the globe varies as the f power of the candle power of the lamp,
that is to say, increases in a smaller proportion. But more than this ; if the
efficiency is improved in the larger lamps, the length of the filament and the
area of the glass will be increased still less. The pear-shaped globe of an 8
or i6 candle power Ediswan lamp measures about 3^in. by 2^in«,and of a loo
candle power 5 in. by s^in. (not including the sealing ends), so that the areas of
the surfaces are about in tne ratio of i to 2.2, while the candle powers are
in the ratio of i : 12.5 or i : 6.2. The large multifilament lamps will be
constructed upon a still more deficient scale, and will be raised to a very high
temperature.
Jamieson in 1882 first gave the sixth power of the electromotive foroe
as the law of variation of the candle power of a lamp. Some tests by
Ayrton in 1892 gave, with 100 volt 8 candle power lamps, a somewhat more
rapid variation through a short range when the lamps were new, approaching
more nearly the seventh power. But with lamps which had burnt for some
time (two or three hundred hours) the following formulse are given for the
variations of candle power F, total watts EC, and watts per candle power W.
5.n 24
F Qc E cc (BC)
and therefore ^
.SB8
BocW"
G. S. Bam has observed the law of variation through a much wider rangOi
testing two lamps intended for very low electromotive force, which was more
than doubled before the filament gave way. Expressions of the form
aE*^ gave an exceedingly close approximation to the observed candle power
up to the point where the efficiency is about one watt per candle. The values
of a and n for the first lamp were 7.76 x 10"* and 5.35 respectively, and the
expression aE'^ gave a very close approximation up to .65 watts per candle.
The discrepancy from this point was no doubt due to the rapid disintegration
of the filament, which broke down at .6 watts per candle power. The other
lamp gave a ■> 1*695 ^ io~*;n -> 5.51, the expression giving the coirect
candle power approximately up to 1.25 watts per candle power. The filament
gave way at 8.5 watts per candle power.
The measurements on the opposite page were taken of the two lamps, and
are placed side by side with the calculateid value of the candle power by the
formula.
The nominal efficiencies of the arc lamp and incandescent lamp are for
the former about ^ to ^ watts per candle power in the best direction and for
the latter 2 to 4 watts. The real illuminating efficiencies would, however,
be more satisfactorily compared by taking the number of watts to the " mean
spherical candle power " which would reduce the former to about ^ watt per
candle power. The efficiency of the incandescent lamp may, as shown above,
be improved to nearly the same limit, ^ watt per candle power, but the
filament will then be raised to a temperature that it will be destroyed in a
few seconds.
Still more recently Dujon, in investigating the law of variation of the
candle power with the E.M.F. adopted an empiric formula of the form
P s K (E - a)n
COMMERCIAL EFFICIENCY.
225
which he found to express the law of variation very closely through an
extensive range, K depending upon the resistance of the filament when
cold, a and n constants dependiug upon the physical conditions of the
carbon, ^c. For lamps manufactured by three leading companies the
following values of a and n were found to agree with the experimental
results:
Ediswan (no volt lamps) «
Kohtinsky (105 volt lamps) ,
Cie. Francaise (no volt lamps)
a= 9.75
a = 46
a = 9.8
n
n
n
=x6
3-5
5.7
This law of variation shows a more rapid increase of illumination with
the E.M.F. than was given by the experiments of Bam, which were con-
ducted with lamps suited to a much lower E.M.F.
a-p.
Volte.
Amp.
Watts.
Obma.
aEn.
c..p.
Volts.
Amp.
Watts.
Ohms.
aEB.
5.3
29
I.I3
32.6
25.7
5.15
2
30
1.03
30.9
29.1
2.33
6.2
30
I.15
34. 5
26.1
6.2
3
32
I.I
35-2
29.1
3.22
8.8
32.5
1-23
4a
26.4
9.52
4
33
I.15
37-9
28.7
3.92
Z0.6
33
1.28
42.2
25.8
10.3
5
34-5
1.2
41.4
28.8
5.05
X2.5
34
1.31
44.5
26.0
12.07
6
36
1.24
44.7
29.0
6.34
14.2
35
1.34
46.9
26.1
14- 18
7
37
1.27
47
29.2
7.4
16.0
3^
1.36
48.9
26.5
16.4
8
37.5
1.3
48.7
28.9
7-97
17.7
36.5
1.39
50.7
26.3
17.65
9
38
1-33
50.5
28.6
8.5
22.2
38
1.45
5";. I
26.2
22.0
10
39
1.36
530
28.7
10.0
26.5
39-5
1.49
58.8
26.5
26.8
12,5
40.2
14
56.3
28.7
11.68
35-4
42
1.58
66.4
26.6
37.4
15
41.8
1.45
60.5
28.6
H.5
44.0
43-5
1.66
72.1
26.2
45.0
20
43.7
1.51
66.0
29.0
18.4
530
45
1-7
76.5
26.4
51.4
25
45-5
1.58
72.0
28.8
24.0
62.0
-»^o
^•75
80.5
26.4
61.3
30
47
1.62
76.3
29.0
27.6
71.0
46.8
1.78
!l-5
26.4
66.8
35
48.5
1.68
81.5
28.9
32.6
80.0
47.8
1.81
86.5
26.4
74.6
45
51.2
1.77
90.5
28.9
44.2
106.0
50.5
1.9
96.0
26.6
loao
50
52.2
1.85
94.5
28.2
48.9
124.0
52
1.94
lOI.O
26.8
117.8
60
54.2
1.88
102.0
27.8
60.7
142.0
53
1.98
105.0
26.8
130-0
70
55.4
1.92
106.2
28.8
68.0
X59.0
54
2.05
112.8
26.8
159.0
80
57^
1.98
1 1 30
28.8
80.0
177.0
55
2.07
1 16.0
27.0
1745
90
57.8
2.01
1 16.3
28.8
86.5
195.0
58
2.13
123.6
27.2
210.0
100
59.6
2.08
124.0
28.8
102
212.0
60
2.2
132.0
27.3
252.0
120
62
2.17
134.5
28.6
127
230.0
61.8
2.22
137.2
27.8
296.0
140
64.8
2.25
1450
28.8
166
248.0
65
2.3
^95
28.3
389.0
160
68
2.35
160.0
28.4
211
180
69
2.38
164.0
28.9
230
200
72.5
2.49
181.0
29.1
300
250
80
2.7
216.0
29.6
S18
295
90
2.8
252.0
32.2
1000
The commercial efficiency (or rather inefficiency) may he ezpr^^ssed as the
cost per candle power per hour. This will depend upon the costs of the
energy per unit, the prime cost of the lamp, and the relation hetween the
life of the lamp and the illuminating efficiency (watts per candle power).
Let p he the prime cast of the lamp ; h the cost of one watt hour ; L the life
in hours ; P the candle power ; W the watts per candle power ; the cost per
candle power per hour may be written
I^ + hW
This self-evident formula was employed hy Professor Perry in 1885 to de-
tennine the electromotive force, which reduced the cost to a minimum with
certain types of lamps then manufactured, L, P and W being determined
P
22d
DECAY OF CANDLE POWER
empiricallj as functions of the electromotive force E. The expression found
for certain small lamps was reduced to
p. ,0 07545 B - ".697 + h {3.7 + io««^ " ^^^ "}
and bj either an algebraic or graphical method the value of E giving a
minimum cost per candle hour was thus determined.
Shortly afterwards Professor Fleming expressed the relation between
L, Py Wy and E, by the formuhe
T « A B I
From which it follows that for maximum economy
h
p
For certain Edison lamps it was found that 0^6^, /3=°4i9 from which it
follows that of the whole cost of lighting the cost of renewals should be about
17 per cent. This result is of course dependent simply upon the durability
and efficiency of the lamps, and not on the costs of lamps and energy, and
is probably approximately true for small lamps as at present constructed.
But with larger sizes of lamps the deterioration of the efficiency due to the
blackening of the globes greatly affects the average cost per candle hour.
The blackening is practically insignificant in really good lamps of 16 candle
power or less, and it has been found that the candle power of the lamp will
actually increase for the first hundred hours. This increase may be due
partly to an improvement in the vacuum, and partly to a decrease in the
resistance of the filament itself, allowing a larger current to pate.
Perry's expression for the cost per candle hour must now be corrected
by P and W, their mean valuer, and for L the economic life, or the number
of hours of burning up to the time at which it is advisable to renew the
lamp. It may sometimes be necessary to renew a lamp owing to the
Table shovoing average Candle power (per cent) and Efficiency of Incandeaeent
Lamps at various periods of their Lives,
Class I. having initial efficiency of 2 to 2. 5 watts per candle power ; Class 11., 2.5
to 3 ; Class III., 3 to 3.5 ; Class IV., 3.5 to 4 ; Class V., more than 4.
Time in
Hours.
L
n.
III.
rv.
v.
P.
w.
p.
W.
P.
W.
p.
W.
P.
w.
•
.
100 .
200 .
300 .
400 .
500 .
600 .
700 .
800 •
900 .
1000 .
1 100 •
1200 .
100
84
70
59
H
48
45
4>
39
38
37
36
35
2.4
2.8
3.3
3.7
4.2
4.6
4.8
5.2
5-3
5-5
5-7
57
5.8
100
93
81
76
71
69
64
62
59
56
53
50
2.9
3.0
3.3
3-5
3.8
4.0
4.2
4.4
4.7
•5.0
5.3
6.0
6.3
100
95
91
88
84
79
76
72
69
69
64
62
59
3-3
3-4
3-5
36
3.7
3.9
41
4.2
4.4
4-7
5.0
54
5.6
100
96
91
86
81
77
73
69
66
63
60
58
56
3.8
4.1
4.3
4.5
4.7
50
5-3
5.6
5-9
6.1
6.3
6.7
100
96
92
87
82
75
72
68
65
62
60
58
56
4.5
4.7
4.9
5-3
H
i:f
6.4
6.8
6.9
7.0
7.1
7.x
DETERIORATION. 227
f*ll of ita candle power, even before such a coiirae may be rendered advisable
by the lessening efficiency. The same lamp may then still be used with
advantage in a position where lesb candle power is required.
One of the moat complete sets of experiments were made by Feldmann,
giving the mean of the results for 500 lamps made in twenty-eight different
fbctories, American and European. The table on previous page gives the
law of deterioration of P ana W, with electromotive forces wMch gave
various initial efficifQcies.
Vta. 81.
Variation In Candle power daring Life of Incaodescent I^unpa.
Pio. 8z.
Variation of Efficiena; (Watt« per OandleJ during Life of IncaodesceDt Lamps.
The same results are shown graphically in Fiirs. 81 and 82.
It will be seen that the deterioration appears to increase less rapidly in
proportion to the i-fficiencv thnn might have been expected, except perhaps
with the lamps of very high efficiency. In fact, the lamps of the class T,,
giving low efficiency, are evidently of inferior make and deteriorate more
lupidly than those of II.. III., IV.
For the life ot the iiicnndescent lamp up to " smashing " point (the
average time of binning till the filament breaks) the tests require to be
applied to a very laige number of lamps, in order to obtain a satisfactory
228 VARIATION OF "LIFE."
avernge, and the electromotive force must be maintainedf with sorapulous
exactness. The relation between the life and the efficiency candle power, or
electromotive force, will of course vary very greatly according to the size of
the lamp, that is, the sectional area of the filament ; but the law of variation
of length of life with the same size of lamp subjected to different electro-
motive forces, and therefore giving different initial candle power and efficiency,
should be determinable and approximately constant.
The following table gives the average life of Ediswan lamps of a normal
1 6 candle power, when the electromotive force is varied, for various initial
•andle powers.
Candle power.
Life tn Honn.
CSuidle power.
Life In HoQiiL
lO
5.550
19
534
ZI
3,963
20
443
12
2.857
21
371
13
2.134
32
312
14
1,628
23
266
15
1,292
24
228
i6
z,ooo
25
196
17
802
30
163
x8
651
From which we find that the law L x P expresses the variation very dosdj
indeed.
Now, taking the law of variation of the candle power with the electro-
motive force as P ac £ , and remembering P.W x E' when the resistance
of the filament is constant, we get L oc E ^ and W oc E"^ and therefore
Let us apply this law to the case of a lamp which, with an efficiency of
3.5 watts per candle power, is found to have a life averaging 1000 hours.
Watte per Candle power* LUe « 1000 (rr)
1.5 8.6
2 43
2.5 151
3 421
3-5 1000
4 2120
Supposing that the lamp is to be used until the filament brealn, It Is now
easy to construct a curve giving the relation between the total cost per candle
hour and the '* life,'' the latter depending as above on the initial efficiency.
Perry's expression for the cost per hour may be written
. Kt—. + b average watts per candle^
total candle hours
p being the prime cost of the lamp ; h the cost of power per watt hour.
Fig. 83 shows three curves traced by Prof. Ayrton from experiments
with 8 candle power Ediswan lamps, to show the total cost per candle hour
with three different prices of energy and of lamps, according to the probable
lives that are determined by the efficiency chosen. The data for the respective
curves are
L Cost price of lamp i«. energy .ooqd, per hour, or 9eK. per unit,
n. „ 2J. „ .0045^. „ 4ld. „
IIL n i<* M .oo4Sd. „ 4i(L „
Whence for these three places, taking lowest point on curve to determine the
most economic conditions, we determine
THE KEEN3T LAMP. 229
L Best Ute, 400 honra, miDimnm coBt .04$d. per candle boar.
II. „ 650 „ „ ,02sd. „
III. „ 600 „ „ .o3isd. „
Frofwsor Ayrton also finds for these lamps that it is not adyisable to
replace till the filament breaks, and no marked economjr is gained by over-
running even at these low prices. To determine at what point it is advisable
to replace a large lamp owing to the decrraaing efficiency a curve might be
tneed/or any given initiai effietenet/, showing the cost per candle hoar upon
Fia. 83.
Ciurea showing Total Coat per Candle houi for Lampe of various "Lives."
the assumption that it is regularly mplaced after a certain length of time,
the ordinatee of the curve giving the cost, and the abscissie the life allowed.
This curve would begin to rise at the point determining the "economic"
life of the lamp.
The ITemat Iiamp.
Only recently a new form of lamp has entered into competition vnth the
carbon filament, which may still be classed as an incandescent lamp, though
the term " electrolytic " proposed by the inventor, Pi-of. Walther Nemat of
Qottingen, suggestu a di^tinguiHhing characteristic. In some respects it is a
revival of the extinct JablochkofT candle, anil the " Sun " light, in which the
kaolin or marble becanie fused by the arc, and subsequently self-conducting
and luminous.
The f;lower is composed of sticks or tubes of what are commonly called
rare earths, the principal one being zirconia. The tube or stick of zirconia
is a non-conductor of electricity when cold, but when it is heated it allows
the current of electricity to pass. The lamp, therefore, contains the necessary
arrangement for the preliminary heating of the glower. In the lamp as
now made by the Kernst Electric Light, Limited, Fig. S4, the heater a
230 THE NERNST LAMP.
of a porcelain spiral upon which is wound fine platinum wire. As soon as
the current is switched oo to the lamp the heater becomes red-hot, and in a
space of time varyii^g from lo seconds to 40 seconds, according to the size,
the glower lights up. In circuit with the glower is the magnetising coil of
a cut out in the heater circuit. As soon, therefore, as the current passes
through the glower the heater circuit is broken, so that the heater is only in
use for the short. time while the lamp is lighting. As the resistance of the
^ower falls rapidly as the current passing through it and the tempera-
ture increase, it is necessary to have a
FiQ. 84. balancing resistance in the glower circuit.
Thie renistaace is preferably made of iron
wire, owing to its very high temperature
coefficient. The iron wire is protected
atfainot oxidation by being eoclosed ia a
ftlass bulb containing hydrogen. Reeistancee
of platinum wire may also be used under
cei'tain conditions.
The lamp ia arranged with a detachable
replacement piece containing the heater and
the glower, which can be renewed at a slight
cost if either the heater or glower faib. The
out-out and balancing resistance are contained
in the case of the lamp. The large lamps
are made to hang up like small arc lamps ;
the smaller sizes are mode to fit into ordinary
lamp-holders like the usual incandescent
lamps. The glowers are made in sizes of ^,
^, and I ampere. These are cut in lengths
according to the voltage required, and the
candle power consequently varies according
to the voltage. The i ampere lamp, for in-
stance, at zoo volts, is double length of
glower and double the candle power c^ a
lamp at 100 volts.
Owing to the high temperature at which
/ the glower can be worked, the efficien(^ of
the lamp ia very great, and may be token
B8 from 1.5 to 1.75 watts per candle.
The light ia of an exceedingly white
colour, with a large proportion of ultra-
violet rays. The efficiency doee not seem
The Nernst Lamp. ^ improve much with an increase in the
electromotive force, or " overrunning," as
with the carbon filament, while the life is greatly phortened thereby. There
appt^rs inevitably to be a slight falling off in the quantity and efficiency
during the first few hours of burning, but subsequently, at least with the
lamps by the best makers, the efficiency is maintained well foi about 400
hours, when the falling off is rapid, and ibe glower should be replaced.
It does not appear possible to enclose the Nernst glowers in a vacuum,
though unquestionably great gain would ensue if convection currents of aii
did not remove the heat-energy. Happily the glowers are so small that the
escape of heat is far less than it would be from a carbon filament giving an
equal amount of light. The access of air appears to be an essential con-
dition for the electrolytic conduction in the glower. Another curious fact,
of which at present no adequal'.e explanation can be given, is the polarisation
of the glower adapted for continuous currents, which renders it essential
ABC LAMPa 231
that tbe direction of the current should remain unchanged throughout the
Hf e of a glower, or speedy destruction will ensue. The glower can be adapted
to alternating currents. The details of the composition are at present kept
as trade secrets by the various manufacturers who have acquired rights, some
being more successful with continuous and others with alternating currents.
There appears also to be a difficulty in constructing glowers for currents
larger than i ampdre, which gives upwards of 60 candle power with 100
volts, and 130 candle power with 200 volts. Unless, therefore, multiple
glowers are to be employed, the introduction of the Nemst Inmp will favour
the use of high electromotive forces when large lamps are to be used.
CHAPTER XV.
Arc Lamps.
TTnbbb ordinary conditions of atmospheric pressure, and with electromotive
forces such as are commonly used for electric lighting, air and all other
gaseous media may be considered as of infinite specific resistance. Absolute
contact of the metallic or other solid or fluid conductors to complete any
circuit is necessary before any current will flow, and the circuit is efiectively
insulated by an extremely thin surrounding of air, no appreciable current
escajfing over a large area by conduction through the gaseous envelope, and
no further insulation would be necessary if we oould ensure that the
separation from other conductors at all points was complete. There are,
however, certain abnormal conditions under which an electric current can
cross a gap in an incomplete metallic circuit. These conditions will now be
very briefly described. The manner in which the conduction takes place,
and the attendant phenomena, differ very widely under the difiTerent
conditions, and from the manner of conduction by metals and fluids.
With very high electromotive forces, such as are obtained by influence
machines or induction coils, air or gas at ordinary temperatures and pressures
will allow the passage of a small current of electricity. The terminals of
the metallic circuit, or *' electrodes," are observed to glow with a faint
bluish light ; this electric conduction, which is known as the *^ glow dis-
charge," is in all probability of the nature of convection by charged particles
of metal, extremely minute, disintegrated and repelled from the surface of
the electrodes, conduction by the gas itself taking no part in the phenomenon.
The ^* spark," or '' disruptive discharge," takes place when the electrodes
are brought within a certain distance dependent on the nature, pressure, and
temperature of the gas, and the shape of the electrodes. The property of a
gas by which it resists the passage of a disruptive spark has been termed by
Maxwell the " electric strength " of the gas. It appears that in air at atmo-
spheric pressure there is a minimum electromotive force of between 300 and
400 volts, with which a disruptive spark can be obtained between two elec-
trodes of any shape or material, and however closely approached short of
absolute contact. This minimum electromotive force for a disruptive dis-
charge is not reduced by decreasing the pressure of the air, but a longer
spark may be obtained with the same electromotive force. To produce a
spark of a millimetre in length at atmospheric pressure requires an electro-
motive force of about 4000 volts, one centimetre about 20,000 volts, the
electrodes being planes or spheres of moderate curvature.
Hot gases appear to conduct electricity with varying degrees of facility.
According to J. J. Thomson, " Gases, such as air, nitrogen, or hydrogen,
which do not experience any chemical change when heated conduct electricity
232 THE ELEOTEIO ARa
only to a very small extent when hot, and in this case the conduction appears
to be, as Blondlot supposed, oonvective. Gases, however, which dissociate
at high temperatures, that is, gases such as iodine, hydriodic g€LS, &c., whose
molecules split up into atoms, conduct with very much greater facility, and
the conduction does not exhibit that dependence on the material of which
the electrodes are made, which is found when the electricity is transmitted
by convection.''
With the non-diBsociable gases are to be included those whose dissocia-
tion consists in the splitting up of the molecules of the gas into simpler mole-
cules, but not into atomsy as when a molecule of steam splits up into molecules of
hydrogen and oxygen, these gases giving very low conductivity. Thomson
concludes that '' the molecules even of a hot gas do not get charged, it is the
atoTna and not the molecules which are instrumental in carrying ^e discharge
. . • The small amount of conductivity which hot gases, which are not decom-
posed by heat, possess, seems to be due to a convective discharge carried
perhaps by dust produced by the decomposition of the electrodes ; in some
cases perhaps the electricity may be carried by atoms produced by the chemical
action of the electrodes on the adjacent gas."
Gases in a state of high rarefaction become very passable conductors of
electricity, and induced currents may be produced with electromotive forces
of a few volts. The conduction is in this case also due to dissociation of the
atoms and not to convection by the molecules. The electromotive force
required when the current passes from the gas to metallic electrodes is much
higher, and is always accompanied by disintegration of the latter. Th^ phe-
nomena produced by conduction through '< vacuum tubes " are too elab<mte
for discussion in this article, since up to the present time the luminous radia*
tion obtained in this way has not approached the conditions necessary for it
to become a practical means of illumination.
With differences of potential between the electrodes of less than 300
volts, a disruptive discharge is probably impossible in air at ordinary tempe-
ratures and pressure. If, however, a current be once started between the
electrodes, as when they are separated from contact, or an initial disruptive
discharge is caused with a higher difference of potential, the current can be
maintained across a gap of considerable length with a comparatively low
difference of potential between the electrodes. This phenomenon is loiown
as the electric '' arc."
A current of considerable magnitude, and therefore a considerable
absorption of power and production of heat, are necessary accompaniments
of the electric arc. The material of which the electrodes are composed is
rapidly consumed, being melted, vaporised, and oxidised owing to the high
temperature. It is still uncertain by what operation the conduction takes
place, whether by the statical charges of minute particles thrown off from the
electrodes and traversing the gas, or direct conduction by the intensely
heated gases, accompanied by molecular dissociation ; it is almost certain that
both operations take place to some extent, but in proportions hitherto un-
determined. ,
A certain minimum difference of potential between the electrodes is
necessary to maintain an electric arc, even of infinitesimal length, the differ-
ence depending on the material of which the electrodes are composed ; also
a certain minimum current seems to be necessary, and the difference of
potential may be somewhat, but very slightly, decreased when a larger current
is allowed to pass. An excess difference of potential above the minimum
proportional to the length of arc-gap between the electrodes will be neces-
sary, the excess difference being less when a large current passes. In other
words, if E be the difference of potential between the electrodes, L the
length of the arc-gap (in millimetres), we may write
TEMPERATUBE OF AEa 233
Baa + b. L
where a is a number depending on the material of the electrodes, very slightly,
if at all, modified by the current passing ; b is a function of the current, and
does not seem to be dependent on the material of the electrodes to anything
like the same extent as a.
Lecher gave the following results of measurements of a and b with dif-
ferent materials, the variation of which with the current in the particular
of carbon electrodes will be considered later at some length :
Horizontal carbon electrodes
Vertical „
Platinum (5 mm. diameter)
Iron (5.5 mm. diameter) •
Silver (4.9 mm. diameter)
B = 33 + 4-5 I«.
B = 35-5 + 5-7 L.
S = 28 + 4.1 L.
E =s 20 + s Li
B a 8 +6 L.
There seems to be some relation between the first term, the minimum
of potential that can maintain an arc with the given material for
the electrodes, and the temperature at which that material meltB, these
being:
For platinum •••••• 3»o8o deg. Fahr*
For iron ••••••• 2,786 i^
For silver • • 1,873 «
that of carbon being too high to be satisfactorily determined. As the aro^
or at least the extremity of the positive electrode which is the point where
the greatest heat is always found, seems always to attain a certain fixed
temperature depending only on the material of which it is made, it has
generally been assumed that this temperature is that of the boiHng'^int
of the material. The necessity of a minimum difference of potential is
then very simply explained by supposing that this difference corresponds to
the power that must be supplied to evaporate the material at the rate
required for one ampere of current, supplying the latent heat of evaporation,
assuming that the rate at which the material is volatilised is proportional
to the current. It has been objected that, taking the case of carbon elec-
trodes for example, it would be necessary to assume an enormous value of
the latent heat of evaporation as compared with metals which boil at a
lower temperature (such as mercury), if the energy supplied is to be thus
accounted for by the volatilisation. But it must be understood that by far
the greater portion of the material volatilised is re-condensed as it cools on
leaving the positive electrode, and has to be re-evaporated ; the actual con-
sumption of material bearing no relation to the volatilisation in the arc
itself, but depending upon oxidisation or other chemical combination at
the high temperature. YioUe succeeded in approximating to the tempera-
ture of the extremity of a positive carbon electrode in the following manner :
A 400 ampere arc was used, and the end of the positive carbon was isolated
from the rest by a narrow isthmus. When the arc had caused this to be
heated throughout to a nearly uniform temperature, the luminous button
was struck off into a calorimeter, consisting of a copper tube containing a
number of pieces of graphite, and sunk in a suspended water vessel. It
was found that a gramme of carbon thus heated in the arc possessed 1300
units of heat (gramme-degrees Centigrade) above what it possessed at
zero Centigrade. Now it requires 300 units to raise the temperature
of a gramme of carbon 1000 degrees, so that there remained 1300 to be
accounted for, Violle estimated the specific heat of the carbon throughout
the higher ranges of temperature leading up to that of the arc as .52, this
giving a rise of an additional 2500 degrees, so that the probable ultimate
temperature was about 3500 degrees Centigrade.
234 CAKBON ELECTRODES.
In using the electric arc as a means of illumination, electro'^es made of
some form of carbon are alone practically possible, for the following lequire-
ments are essential :
(i) In extremely high temperature of volatilisation. The high tem-
perature is demanded by the consideration of efficiency, since the proportion
of li^ht radiation to the total radiation increases rapidly with the tem-
perature.
(2) A moderately good conductivity for electric currents.
(3) A low heat conductivity, in order that the heat may not be con-
ducted away, but escape, as far as possible, solely by radiation.
(4) The oxide of the mateiial must be a gas, otherwise it will be
deposited in the neighbourhood of the electrodes, with obvious attendant
inconveniences.
(5) A high selective emissivity for luminous radiation (see the discussion
in the chapter on Incandescent Lamps).
The discovery of the electric arc is generally attributed to Davy, who in
1 8 10 first obtained the phenomenon in experimenting with 2000 large
primary cells connected in series. Two pieces of wood charcoal were used
as electrodes, and after contact the electrodes were separated horizontally
to a distance of over four inches ; a brilliant *' arch '' of light was formed,
the upward current of heated air causing the shape which gave rise to the
term "arc." Davy's apparatus was, of course, most inefficient both as
regards generation of power, and its conversion into light, and no expecta-
tion was entertained at the time that the electric arc could become a
practical source of illumination. From the arc itself a very small propor-
tion of the heat generated is radiated as light, the heated electrodes them-
selves radiating light infinitely better, and the high E.M.F. required for a
long arc corresponds to a large expenditure of power which is converted
mainly into heat radiation and not luminous ladiation, or heat carried
away by currents of air.
As soon as the development of the dynamo had caused it to supersede
the primary battery, coal replacing zinc as fuel, and the cost of the pro-
duction of electrical energy immensely reduced, the possibilities of the
electric arc for lighting purposes were recognised, and it soon became a
practical success. The first carbons used as electrodes were made of hard
gas-retort carbon cut into suitable shapes, round, straight rods placed in
the same line replacing all other forms, though revolving discs, and rods
inclined to one another were fully considered. The electrical resistance of
gas-retort carbon is very much higher than that of modern artificial
carbons, and the necessary uniformity in its composition would not be
obtainable. Finely pulverised coke is generaUy the basis of the improved
material, this being mixed with pure carbon powder to improve the con-
ductivity, and some adhesive paste, such as S3rrup of cane sugar or gum,
cements the combination together. This is either squeezed into a mould,
or through a die of the proper size, and cut into the requisite lengths (from
8 to 14 inches). The materials and method of manufacture are very varied,
and in many cases are held as trade secrets. The chief requirements are
that it should be perfectly homogeneous, of the lowest resistance possible,
free from extraneous matter of a less refractory nature, and as mechanically
strong and dense as possible to diminish the rate of consumption. The
carbons intended for the positive electrode are somewhat longer or else of
larger diameter as the rate of consumption is generally twice as great, and
should be " cored." In moulding, or as it passes through the die, a central
hole is made throughout its length, which is subsequently filled with a core
of carbon in the soft graphitic form, which, being of higher conductivity
and more easily volatilised than the remainder of the rod, causes the arc
APrEARANCE OF AUG.
235
to spring from the centre of the rod, and assists the formation of the
" crater " as will be described.
The diametei' of the carbon rods vary according to the current they have
to carry : for the most common current employed, that of ten amperes,
they require to be from twelve to eighteen millimetres. It used to be a
common practice to cover the surface, especially of the negative carbon, with
a thin layer of copper or nickel, electrolytically deposited, in order to improve
the conductivity, and ensure good contact with the carbon-holders ; but this
is now generally abandoned, as the volatilised metal entering the arc reduces
and varies the intensity of the light, and befouls the globe and regulating
mechanism.
The following is a list of the various diameters of the carbon rods supplied
by three leading arc lamp manufacturing firms in England, the diameters
being given in millimetres :
Nominal
Candle power.
Current.
Electric
ConBtmction Co.
Brash.
Crompton.
lOOO
1500
2000
40CO
10
30
+ -
13 8
16 9
18 12
+
II 8
13 "
15 12
15 15
+ . —
13 8
16 9
18 12
25 18
Viewed through deeply tinted glasses, or better still by projecting with
a lens a much enlarged image upon a white screen, the general appearance
of the arc between two carbon electrodes may be studied. Owing to the
great changes in appearance according to the length of the arc, the magnitude
of the current, &c., and the utter failure of any uncoloured illustration to
represent anything except the shape of the electrodes (which is subject to
extreme variation), it is preferable to adhere to a purely verbal description.
The illumination proceeds almost entirely from the ends of the carbon
electrodes, a small area on each of which is raised to an intense white heat.
On the positive electrode the area of this bright surface is far the greater,
and somewhat the more brilliant, and is responsible for at least 80 per cent,
of the light. The area of the bright surface of the negative carbon may be
responsible for about 10 per cent, of the light, with short arcs, while not
more than 5 per cent, comes from the intermediate gap, or the arc itself.
The latter appears a bright violet ball, shading off into green, and sur-
rounded by a golden aureole. The bright surfaces of the electrodes are
likewise surrounded by yellow and yellowish-red belts, darkening rapidly
into black.
With a long arc, upwards of about 5 millimetres, or about half the
diameter of the carbon rods, th6 carbon ends are both flattened, becoming
more and more rounded as they are brought nearer. With less than
5 millimetres gap the positive begins to hollow out into a ^* crater," and the
negative to become pointed. Shortening the arc still further, to less than
a millimetres, the point of the negative carbon develops into a knob, or
mushroom, especially when a cored positive carbon is used, the crater be-
coming very deep. The knob seems to be formed by a deposit of graphitic
carbon thrown off from the positive, piling itself up on the negative point,
finally breaking off and causing *^ hissing." On the fringe of the bright
surfaces there generally appear a number of bright balls or nodules, which
are more easily seen on the negative than on the positive electrode owing to
the light thrown on them by the former from the crater, and which are
236 THE "CBATER."
probably boiling syrup or the adhesive paste which is used in the manufac-
ture of the carbon.
The shapes a^umed by the carbons confirm the statement already made
that the consumption of the material is due less to the volatilisation in the
arc, than to the oxidisation, or burning, of the material at the high tempera-
ture, and this takes place from the heated surface surrounding the bright
parts. The conical or pointed shape of the negative carbon, which is more
marked as the arc is shortened, is to be explained by supposing that it
receives its heat entirely by radiation and conduction from the crater and
arc, thus being consumed more rapidly with the shorter arc ; except at the
centre, when it is in an atmosphere of volatilised carbon, and carbon gas
80 that the oxygen has no access to permit burning. With long arcs the
consumption of the negative carbon is greatly reduced. The positive
carbon, on the other hand, is heated in the neighbourhood of the bright
surface by conduction, and the shortening of the arc allows this to be better
protected from the oxygen by the carbon gas, and thus avoiding being
consumed, forms the rim of the crater. With long arcs the brilliant spot of
intense heat wanders uncontrolled over a flat area of positive carbon,
volatilising and burning it away uniformly ; with shorter arcs it becomes
stationary, throws up a surrounding ridge where burning is prevented by
absence of oxygen, and bums by conduction a surrounding cone.
When the current is increased beyond a certain strength, depending
upon the sectional area of the carbons and the length of the arc, the difier-
ence of potential between the electrodes falls considerably, and the arc
assumes an unstable state, the carbon breaking off in lumps, accompanied
by a hissing sound. This hissing is probably due to a state of the arc which
is similar to that known as *' priming " in a boiler, the bright surface being
too small for steady and complete volatilisation of the requisite amount of
carbon, and is a sign of too short an arc, or a too heavy current for carbon
electrodes of the section used*
The highest temperature, as well as the source of by far the larger
amount of light radiation, is to be found at the bright surface of the positive
carbon. It may therefore be expected that the energy supplied will be
largely absorbed at this point, and converted into heat. Dr. Fleming has
shown this experimentally by introducing the extremity of a well insulated
third carbon into the arc, and measuring with an electrostatic voltmeter
the difference of potential between it and the electrodes. It will be found
by this method that a considerable difference of potential is obtained between
the positive electrode and a point in the arc very close to its surface, in fact
probably that minimum electromotive foroe 33 to 39 volts which is necee-
sary to produce the shortest arc. The bright surface of the positive carbon
is therefore the place where the energy corresponding to this E.M.F. is
absorbed and partly radiated as light. An additional E.M.F. of about
30 volts or so per centimetre (this additional E.M.F. varying greatly accord-
ing to the current) is necessary to overcome the resistance of the arc-gap,
but of the energy absorbed to which it corresponds a very much smaller
proportion is radiated as light. The highest eficiency of the arc as a means
of illumination will be obtained by reducing the length of the arc as far as
is consistent with steady burning, avoidance of '' hissing " through incom-
plete volatilisation, and with free radiation from the positive electrode
without eclipse from the negative. The use of a smaller diameter for the
negative carbon, now the almost universal practice, allows much freer
radiation from the positive carbon extremity, and it also has the advantage
of equalising the rates of consumption in the electrodes, thus simplifying
some arrangements in their regulating mechanism. A ratio of diameters of
about 3 to 2 secures this equality of rate of consumption with ordinary
DISTRIBUTION OF LIGHT. 237
lengths of arc ; with longer Arcs the negative ma; be further reduced in
diameter.
The lumiDOua intenrity of the bright eurface of the poeitive carbon
reaches according to the beet determinations, about 170 candle power per
square millimetre. Thie luminous intensity is obtained only when bard
and fairly pure carbon is used, and reduces very considerably, to about
130 candle power per square millimetre, when a soft graphite core, more
easily volatilised, aUows a lower temperature. As, however, the latter ia
soon consumed to some depth in forming the crater, the luminous intend^
is restored to the higher value after a short time of burning. The luminoua
intensity of 170 candle power per square millimetre to a temperature of
about 3500 degrees Centigrade, which is maintained, without much varia-
tion, whatever current flows through the arc. The area of the bright surface
is, roughly, proportional to the current, but measurement of this area is
not easily or satisfactorily made owing to its irregular shape, and the gradual
shading off of the luminosity round the edge. With a ten-amp^ arc the
Fro. 85.
□Imnfnatian of CoDtinnoiu Current Aic Lamp in differently inclined Directions.
area of the brightest surface is about twelve square millimetres, so that in
a direction normal to the surface the luminous radiation should be about
3000 candle power.
The luminous radiation in different directions from an electric arc of
the common type will be found to vary very greatly. The greatest illu-
mination will be found in the direction in which the whole of the positive
bright surface is visible, as near as possible normal to the surface, but just
avoiding eclipse by the negative electrode. This maximum value is known
ts the nominal candle power of the arc lamp. With the common arrange-
ment, vertical carbons, the upper being the positive, the maximum illu-
mination will be found in a direction inclined at an angle of about sixty
d^^rees to the vertical : with shorter area the inclination to the vertic^
giving maximum illumination will be greater, with longer arcs less. In
other directions the illumination will vary in proportion to the apparent
area of the bright positive surface viewed from that direction, scmewhat
modi&ed by the illumination from the negative carbon end and the arc
it«elf. The distribution will depend very greatly on the shape of the
electrodes and the length of the arc, but Fig. 85 will illastrate a typical
238 CANDLE POWER.
relative distribution in various directions in one vertical plane (given by an
arc of 10,000 nominal candle power). The illumination in any direction is
shown by the corresponding radius vector of a curve.
The total illumination emitted by the arc may be calculated from this
curve, and compared with the total illumination from a standard candle
emitting light equally in all directions. To perform the calculation the
mean candle power for every five degrees or so may be multiplied by the
solid angle generated by its revolution about the middle line of the carbons,
and the sum of the products divided by 4x. This will give what is known
as the mecm spherical candle power, and will generally be about one-third
of the nominal. The mean spherical candle power of the arc lamp would
be a very fair measure of its value as a means of illumination if the light
in all directions were equally valuable. Ab a rule, however, the arc lamp
is placed in an elevated position, and only the light projected downwards
is of any use. When a hood reflector is placed over the arc lamp, or, as is
often preferred, a white screen, eighty per cent, or more of the light thrown
upwards is reflected, and the mean spherical candle power may be con-
sidered the best criterion of the illumination. The same may be said of an
arc lamp used for interior lighting of large halls or factories when the
ceiling and walls are white ; and even for exterior lighting when the light
is thoroughly diffused by an opal or dioptric globe. One of the most
effective methods of interior lighting with arc lamps is to invert the
positions of the carbons, and place a reflector under the arc to throw all the
light upwards towards a white ceiling. A perfectly diffused light is thus
obtained without shadows, and as the diffusing globe, which would otherwise
be necessary and absorb probably half the light, is dispensed with, the
system is highly efficient.
When the light thrown upwards is waited, the mean hemispherical candle
power, calculated as before but taking account only of the light thrown
downwards in the lower hemisphere, is often preferred as the measure of
illumination. This is generally about half the nominal candle power. It
may be noted however that when using a few widely scattered lights, in
elevated positions, the difficulty of maintaining uniform illumination over
the ground may be partly met if the increased illumination in directions
considerably inclined to the vertical be made to compensate for the increased
distance. With a short arc and thinly opalescent globes it may be arranged
that the illumination on the ground level given by any lamp should be
fairly constant within a ditstance from the foot of the support equal
to the height of the arc, and beyond this decreases slowly till the
point intermediate between this lamp and the next is reached. The
best criterion of the illuminating power would be the measurement of the
illumination in the direction of this intermediate or darkest spot. With
closely set lamps this criterion may not be far different from Uie nominal
candle power.
When an opal or other diffusing globe is used, it is advisable to place
the arc somewhat above its central point, so that the light thrown down-
wards from the arc may illuminate the globe with a broad equatorial belt of
light, and not the lower hemisphere alone. The light is more evenly
diffused, or in other words the mean spherical is more nearly equal to the
mean hemispherical candle power, than with the naked arc. This even
diffusion, lessening the proportion thrown downwards, is a disadvantage in
itself with the usual methods of outdoor lighting, but an unavoidable neces-
sity if we wish to eliminate the dazzling effect, and heavy shadows cast by
the naked arc. The following figures were given by Guthrie and Redhead
showing the relative value of the two measurements when different diffusing
globes were used :
EPflCIENCY.
239
—
Naked Arc
Clear GUms.
Rough GlflM.
OpaL
Mean spherical candle power
Mean hemispherical candle power
319
450
235
326
160
215
144
138
From the arc itself a large proportion of the heat generated is carried
away by currents of air, but that generated at the bright surface of the
positive carbon is almost entirely removed by radiation. The best determi-
nations of the energy thus radiated give from 27 to 30 watts per square
millimetre. Of this the greater amount is heat radiation, probably about
90 per cent., but even with this great loss the proportion of the luminous to
the total radiation is far higher than can be obtained by any other artificial
means. Many attempts have been made to incorporate with carbon some
other substance which, by its properties of selective radiation, should give a
higher proportion of luminous rays. But the lower temperature resulting
from any impurity in the carbons has prevented success in this direction, as
it has with incandescent lamps.
As with the incandescent lamp, we may express the efficiency (or ineffi-
ciency) by the number of watts absorbed per candle power, using for the
latter either the nominal, mean spherical, or mean hemispherical. From
every square mm. of the bright surface of the positive carbon the luminous
radiation in the direction normal to the surface is 170 candle power, and the
total radiation the equivalent of 27 to 30 watts, the efficiency is that of one
sixth of a watt per candle power. This may becompared with the corresponding
figures for the filament-surface of an incandescent lamp; for example, at a tem-
perature commonly used for small lamps, the luminous radiation may be ^
candle power in a direction normal to the surface, the total radiation ^ watt
per square mm., and thus the luminous efficiency i watt per candle power.
ABj howeveri with a filament of circular section the apparent area viewed
from any point is, as a maximum, — of the actual surface, the illumination
in that direction will be — candle power for every millimetre of the whole
surface, and thus the luminous efficiency is 3.14 watts per candle power in the
direction of maximum illumination, which will be reduced to about 4 watts
per candle power if we took the mean spherical value, or about 3^ watts per
candle power if the mean horizontal.
The luminous efficiency of the bright surface of the positive electrode
thus seems to be about 6 times as great as that of the incandescent lamp fila-
ment, a comparison which need not be greatly modified whether we compare
the normal radiation from a square millimetre of bright surface, that is, the
radiation in the direction of the greatest illumination, or the mean radiation
in all directions. This comparison is, however, considerably modified in favour
of the incandescent lamp when we take into account the energy was'ted in
the arc itself, from which very little light is radiated, and the waste in pro-
ducing the necessary diffusion of the light, when we desire to compare
the relative efficiencies of the arc and incaudescent lamps as means of illu-
mination.
The luminous intensity of the surface of the sun is, according to calcu-
lations made by Prof. Young, about 1000 candle power per square millimetre,
or about 6 times that of the bright surface of the positive electrode in the
electric arc, and about 2000 times that of the incandescent lamp filament at
the most common temperature employed. This corresponds to a temperature
of about 8000 degrees Centigrade. The power dissipated by radiation of
240
ALTERNATING QUERENT ARCa
heat and light is about loo watts per square millimetre, giving an efficiency
of about one tenth watt per candle power.
The electric arc may also be maintained bjan alternating current. The illu-
mination is emitted equally from both electrodes, and each is flattened or hol-
lowed out slightly into a crater. The electromotive force, that is to say, the
"virtual" electromotive force, measured as will be described in the proper
place, is less than that required for the same length of arc with continuous
currents, since the maximum E.M.F. attained during the alternations con-
siderably exceeds the virtual (generally by 40 to 50 per cent.). A virtual
E.M.F. of about 30 volts is most commonly used, so that three arc lamps may
be connected in series across mains having a constant difierence of potential
of 100 volts, a small regulating resistance, or choking ooil, absorbing the
remaining zo volts. With solid carbons the arc is liable to. become very un-
steady, wandering round the edges, but with both carbons cored this wander-
iny may be prevented. Any regulating mechanism suitable to continuous
currents can be used for alternating currents, save that the solenoid cores
require to be laminated, and a different adjustment made.
The luminous intensity varies very greatly during an alternation as the
E.M.F. and current vary in magnitude. This variation will be quite imper-
ceptible to the eye, owing to the extreme rapidity, but it may be shown and
measured by causing a screen to revolve synchronously with the alternator
supplying the arc, in such a manner as to cut off the light except at regular
periods, corresponding to various phases of the alternations. The heat of
the arc is, however, not radiated with sufficient rapidity for it to cool and
break during the short periods while the E.M.F. is lower than that required
to maintain it.
Dr. Fleming, in the course of an investigation of the effect of the shape
of the alternating current curve on the variation of luminosity and efficiency
of the arc gave the following figures, which will show the relative efficiencies
of the continuous and alternating current arcs.
I. Contimums Cwrrent ArOm
rx-_w„__ /posItlTO 15 mm. dl»m. oored.
v;»rijuiM -(^neratlTe 9 mm. dlam, solid.
both Z5 mm. cored.
Power in watts
Mean spherical o.-p.
B.M.J.
Current
Length of arc (mm.)
582
675
56
10.6
7.15
380
455
44
8.7
2.5
299
372
8.1
7.
181
26
8.2
0.
607
562
60
10.0
8.07
313
344
40
8
.61
II. Alternating Current Are,
Carbonn 15 mm. dlam. both oored.
Freqaency 83 alternations per seo.
50 altematioiML
Power in watts
Mean spherical
c-p.
E.M.F.
Current
Length of arc
601
307
355
16.2
6.2S
501
274
34
15.1
404
256
28
15. 1
1.25
305
250
21
.16
233
144
15.3
15.4
.01
596
526
39
16
.7
459
322
31
15.1
307
254
22
14.6
From these figures it may be deduced that the ^ciency of the alternating
EESISTAKCE OF Alta 24 1
current htc is much lower thnn that of ibe contiououe current. 'With lone
arcs the mean spherical candle power for the same number of watte absorbed
is only about half as great in the former, with a high rate of alternation, but
with a shorter arc the diHerenoe is not so great. With a slower rate of alter-
nation the effimency appears much higher, and compares well with that of
continuous currcnis.
The distribution of the light is much more uniform, as sbown in Fig. 86
the light sent in a horizontal direction being not far from the maximum.
For this reason alternating currents are commonly preferred {<« search-
lights, lighthouses, and like purptiees.
The relations between the E.M.F., current, and length of the electric arc,
or in other words the modification of Ohm's law applicable to the electric
arc, are naturally oomplicated by the fact that the conducting belt of
volatilised carbon is created and maintained by the current itself, and
therefore varies in sectional area according to the current. The equivalent
reeiatance of the arc gap depends on the material ot which the cartxm elec-
DiBtribntion of IDainlnatloii with an Alternating Cmrent Arc.
trodee are made, their size and shape, and also to some extent on the cnrrent
already established in the arc. Exact measurements of the simultaneous
values of the E.M.F. current and arc-length require to be made with extreme
care in order to obtain consistent reaults, and properly to investigate the laws
of their variation.
A comprehensive series of such measurements have been published and
discussed by Mrs. Ayrton in the £leciricUiti, vol. xxziv. (January 11, 1S95,
and succeeding numbers), to which reference may be made for more com-
plete information on thiu part of the subject. The current was sopphed
from a secondary battery, and the current and length of arc kept constant
for a considerable period before measurements were taken, in order that the
carbon ends might have time to assume a shape appropriate to the length of
arc. Without this precaution the results were most inconsistent, and
different simultaneous values of tbe E.M.F. and current could be obtained
for the same length of arc immediately after being changed in value. The
difference of potential was measured between carbon contacts touching the
carboik electrodes close to the arc, and the length of the arc itself measured
242 CONSTANT CUBEENT AND POTENTIAL-DIFFERENOHL
bj means of the magnified image thrown on a white screen. In meRSuring
the length of aro the vertical distance between the lowest point of the
positive and the peak of the negative carbon was taken, so that, owing to
the formation of the crater in the former, the true length of the arc-gap
slightly exceeded the measurement, and an aro of sero length represented,
not contact of the carbons, but the condition that the peak of the negative
was on a level with the edge of the crater in the positive carbon.
Different sizes of carbons such as are used in common practice were
tested in this manner, and the results illustrated by curves drawn to show
the relation between the current and E.M.F. for fixed lengths of the arc.
It would appear that the law of variation differs in some important respects
according as a cored or solid carbon is used as the positive electrode. In the
latter case a comparatively simple algebraic formula of the form
_, . - + dli
B-ia-f bL + — Q—
may be made to represent to a close degree of approximation the relations
between E the potential difference between the electrodes measured in volts,
C the current in amperes, and L the length of the arc -gap in millimetres
(measured as described above), a, b, c and d being constants determined by
experiment. For example with carbons 1 1 and 9 millimetres in diameter
respectively, both solid, the following formula is given to express the triple
variation,
B - 38.88 + a.«>74.L + '-i:^^^^5^i^
Under these conditions 8uppo8ing the current to be maintained constant,
say at 10 amperes, the difference of potential will rise uniformly as the
length of the arc increaseS| the formula reducing to
B 8840.54 + 3.128.L
or an increase of over 3 volts per millimetre with this current, and a greater
rate of increase with a smaller current. This will be seen later to be the
required condition for regulation with a shunt solenoid.
If the conditions of supply be that a constant difference of potential
be maintained between the electrodes, say 50 volts, the current will be
given by
+ d.L 11.66 + 10.54.L
=
E— a-b.L 8.12 - 2.047.L
a length of a little over 3 millimetres giving a current of 10 amp6re8, and in
all cases the current will increase when the arc increases in length. This is
the very opposite to the condition required for regulation by means of a
solenoid in series with the arc, a decrease of the current as the carbons are
consumed being necessary in practice. A. constant difference of potential
between the electrodes is therefore inconvenient, but with a supply at
constant E.M.F. we can still obtain the required condition of a falling
current as the arc inci*eases in length by the insertion of a small *^ regu-
lating " resistance in series with the arc. This additional resistance would
be necessary to prevent the rush of current which would take place when
the lamp was first switched on while the carbons are still in contact, and is
partly supplied by the series solenoid and the carbon rods themselves.
Suppose that some additional resistance be added, making the total added to
that of the arc itself about one ohm. The E.M.F. in the circuit will bo
required to be raised about 10 volts (for a lo-amp^re lamp), with a corre-
sponding waste of power, to allow for the fall of potential in the added
resistance, but an increase in the current can now no longer follow upon an
VARIATION OF ABO LENGTH. 243
increased length of arc, for the former would involve a greater fall of
potential in the added resistance, and therefore a lower difference between
the electrodes.
To see that the inverse effect^ a fall of the current as the length
of the arc-gap increases, is actually produced, we must notice that the
difference of potential between the electrodes is E - O.R, where E is the
E.M.F. of the circuit, and R the added reeistanca The current will be
given by the quadratic equation
C?R - 0(B - a - b.L) + o + d.L =
giving two possible values for the current, the greater of which will be
estabEshed when the carbons are separated from contact. Taking the same
values of the constants as above, and the E.M.F. in the circuit as 60 volts,
the quadratic becomes (when R » i)
0* - C(2i.ia - 8.074.L) + 11.66 + 10.54.L ss o
which gives us for La 2 the values 14.7 and 3.3, for L = 2.5 the values 13
and 3, and for L — 3 the Values zi and 4 very nearly. Since the higher
values in each case will be assumed by the current, as the arc burns away
the carbons the current decreases under these conditions. With thui
E.M.F. in circuit, and resistance of one ohm added, the maximum
length of arc possible is about 3^ millimetres.
The effect of using a cored carbon for the positive electrode is to reduce
the difference of potential between the electrodes by 5 or 6 volts, at any rate
for the longer arcs. With a constant current the difference of i)otent]al
between the electrodes increases with the length of arc, and for arcs longer
than about 2 millimetres, .or about a tenth of the diameter of the carbons, a
smaller difference is, as with solid carbons, necessary with a larger current.
For shorter arcs the difference of potential seems to rise with the current,
but this is very probably due to the greater hollowing out of the crater in
the soft core, which makes the true length of the arc somewhat longer than
is given by the method of measurement employed. With difference of
potential less than about 42 or 43 volts, and an arc length always less than
the critical value of about 2 millimetres, it seems that the current will
decrease as the arc lengthens, but these lengths would be inadvisable in
practice, and the arc subject to extinction on a slight fall in the E.M.F.
supplied, or over extension of the arc For greater length the conditions
are as before, and an added *' regulating '' resistance is necessary for parallel
working.
From measurements taken with carbons 13 and zz millimetres in
diameter respectively, the former, or positive, being cored, the following values
of the current are deduced. With an E.M.F. of 55 volts in the circuit, and
a resistance of one ohm added to that of the arc, a length of 2 millimetres
should give a current of about 14 amperes, of 2.6 a current of zo amperes ;
and the maximum possible length of arc should be about 3 millimetres.
With an E.M.F. of 60 volts, and the same added resistance, the currents
should be 15 amperes with a length of 3 millimetres, Z2.5 amperes with 4
millimetres, 10 amperes with 4.7 millimetres; and the maximum length of
aro about 5 millimetres.
Begulating Meohaziisni.
Before the regulating mechanism of arc lamps had been developed so as
to give satisfactory results, efforts were made to do away altogether with
the necessity for such mechanism, or at least to simplify the motion of the
electrodes, by placing the carbons otherwise than in the same line. The
244 EARLY FOUMS OF AUG LAMP.
first of these was known as the Jahlochkoff " candle,** invented by M. Jab-
lochkoff in 1876. The two carbon rods were placed side hy side, and
separated by a thin layer of insulating material which burnt away with the
carbons. For the latter purpose kaolin was at first employed, but this was
found to melt and form a liquid conductor between the two carbon points,
which carried the current so that a true electric arc was not maintained ;
the luminosity of the kaolin under these conditions suggested the semi-in-
candescent Lampe-Soleil which has been described in the chapter on Incan-
descent Lamps. Subsequently a mixture of equal parts of sulphate of
calcium and sulphate of barium, was employed, which was found to volatilise
more completely at the high temperature of the arc A fine strip of plum-
bago connected the extremities of the carbon, allowing an initial current to
be started, the arc being formed as soon as this was burnt away ; subsequent
lighting in the event of extinction of the arc being effected by touching the
extremity of the candle with a piece of carbon or wire. When continuous
currents were used it was necessary to use a larger carbon rod for the positive
electrode to equalise the rate of consumption, but more satisfactory results
were obtained by the use of alternating currents with carbons of equal
Motion. The diameter of the carbon rods employed was four millimetres, the
length twenty-five to thirty centimetres, and thickness of the separating
material three millimetres. Several candles were arranged in the same
lamp with an automatic or hand switch so that they might be consumed in
taocession, each lasting from an hour and a half to two hours.
Wilde produced a more efficient electric candle by doing away with the
intervening strip which separated the carbons, placing them side by side at m
distance of three millimetres, but balancing the holder of one of the carbons
on a pivot so that it became inclined till in contact with the other carbon,
unless held apart by the attraction of an electroxnagnet with a magnetising
ooil in series with the arc. The necessary initial contact of the carbons was
thus automatic, and as the carbogos were vertical, the heat of the arc as well
A8 the magnetic field created by the current flowing up and down the
carbons prevented the arc from descending from the upper extremity of the
candle. In order to invert tha candle, obtaining the arc at the lower
extremity of the carbon rods, Jamin surrounded the whole lamp with a ooil
wound in a vertical plane, the magnetic field of which caused the arc to be
impelled downwards, according to the principle that a conductor carrying a
current tends to move transversally to the lines of force. Debrun improved
on the starting device of Wilde, that of inclining one of the carlK)ns to
obtain initial contact, by making an automatic short cii*cuit between the
carbons near the holders, thus starting an arc which was immediately
carried to the extremity by heat and magnetic impulsion.
Bapieff in 1878 devised an extremely simple means of maintaining the
requisite length of arc by duplicating both positive and negative carbons,
and inclining the two positive, and also the two negative, at a slight angle
to one another. Each carbon rod being guided in its own line, two similai-
carbon rods are forced by springs or by their weight to advance towards the
point where these directions intersect, and can advance no further, except
as they are consumed at this point of intersection. The intersections of
the lines of motion of the positive and negative carbons respectively are
separated by such a distance as will give the requisite length of arc. The
initial contact is made by the descent of the holder of the two positive
carbons, and subsequent separation by a series- wound electromagnet. In
Bapiefifs lamp one pair of carbons was placed vertically above the other
pair ; in a subsequent improvement by Gerard each pair pointed down-
wards, being inclined to the vertical, and meeting at a similar angle to that
betweeiii^he two positive, or two negative rods, thus allowing more freedom
EEQULATma FACTORa 245
for the diffusion of the light. Alternating cuirents were used for both of
these types of lamp.
It will be clear from the preceding discusBions that the functions of the
automatic mechanism of an arc lamp are these : (x) To allow or cause the
carbon electrodes to come into contact, either, immediately the current
through the arc ceases, or directly a difference of potential between is pro-
duced; (2) to separate the electrodes from contact a short distance im-
mediately a current passes, or, as it is termed, to *' strike ** the arc; (3) to
maintain the required distance of two or three millimetres between the
electrodes by allowing them to slowly approach, or '' feed " as their ends are
consumed, in general arranging that each electrode should move in propor-
tion to the rate at which it is consumed so that the arc may remain in the
same position at all times.
The number of devices employed for securing these ends have been
innumerable. Those which liave survived and been most extensively used
seem to have excelled, not so much in their intrinsic value and ingenuity,
as in the care that has been exhibited in their construction. The chief
merit of any device is its relative simplicity, which will enable it to be
attended to for cleaning or adjustment by unskilled persons, and be worked
for long periods with the minimum of attention. Often placed in exposed
situations, the works should be such that they are little affected by the
inevitable entry of damp and dust, durable, and free from wear in the
delicate parta Perfection in adjustment is in most cases of less importance
than certainty of action at all times in spite of unskilled handling.
Time-regulated clockwork might regulate the feed of the electrodes with
absolute uniformity, and might be efficient if the exact rate of consumption
of the carbon were known. But as the latter would vary with slight
variations of the current, some further control would be necessary, and the
striking and primary adjustment of the aro-length otherwise effected.
Moreover, all dockwork, unless of the most elementary nature, is objection-
able owing to the impossibility of excluding dust. Purely mechanical
regulation of the feed has been used with arc lamps in lanterns, the striking
being effected by hand. For this purpose the carbons are restrained from
feeding by steel pointed stops pressing against them near the arc, past which
they slip when the carbon has worn away sufficiently owing to the proximity
of the arc.
The only satisfactory methods of automatic regulation are those in which
the length of the arc affects the motion of the electrodes indirectly through
the consecjuent values of the difference of potential between them and the
current through the arc. A coil of thick wire and few turns placed in
series with the arc, thus carrying the whole current that passes through it,
produces a magnetic field of proportional strength. This magnetic field
can be made to produce mechanical effects, the tendency of which must be,
directly or indirectly, to separate the electrodes or strike the arc, and to
check the motion of feeding. A coil of fine wire, with many turns and a
high resistance, connected as a shunt circuit between the electrodes, will
carry a current proportional to difference of potential between them, and
the mechanical effects produced by its magnetic field must be made to tend,
directly or indirectly, to oppose those of the series coil, limiting the striking
of the arc and permitting or causing the feeding of the carbons together.
One of these regulating factors, the difference of potential or the current,
must be kept constant by the conditions of the supply of power while the
lamp is burning. The most satisfactory condition is that of constant current,
under which the series coil has no longer any regulating power, its action
being unaffected by the length of the arc, and is useful solely as an agency
for ijiiikiug the arc, and sustaining it until the cessation of the current
246 BEQULATINQ MECHANISM.
demandfl a renewed contact. TJpon tlie shunt coil we must depend for the
regulation of the feed, and as the difference of potential will rise uniformly
with the increasing length of the arc, its action will be effectual if it cause
the feeding to commence when the current in it exceeds a certain value.
If the condition of supply be that of constant E.M.F., and the shunt coil
were thus connected between mains maintained at a constant difference of
potential, its regulative properties would cease to exist, and we must depend
solely on the series coil. The latter will be available for striking the arc
when the lamp is switched on, but if it be also required to cause or permit the
feeding when the arc exceeds the permitted length, the feeding must be con-
sequent upon a decrease in the current. Now it has been shown that with
a constant difference of potential between the electrodes, a decrease in the
current does not ensue when the length of the arc increases, in fact, the
reverse is generally the case ; but if a small resistance (of an ohm or so) be
placed in series with the arc, and constant E.M.F. maintained in the com-
bined circuit, the current will decrease as required. A shunt coil, though
not absolutely necessary, may cow be used to assist regulation if connected
between the electrodes, that is, not including the additional resistance, for
the decrease in the main current will cause a smaller fall of potential in the
additional resistance, and therefore higher difference of potential between
the electrodes, giving a larger current in the shunt coil, which may thus
assist the series coil, if opposed to it, in promoting the feeding.
The simplest method of producing mechanical motion through a limited
range by an electric current is to cause it to create a magnetic field varying
in intensity at different points, so that a piece of soft wrought iron placed in
the field is magnetised by induction, and tends to move towards the stronger
part of the field. This is more suitable to our purpose than the attraction
of permanently magnetised steel, as is used, for example, in the galvanometer,
owing to the possibility of demagnetisation. A coil wound in the form of a
solenoid will attract a wrought iron cylinder, sucking it into the interior of
the coil. As the cylinder, or ** core," enters the coil, the attracting force
will increase till it reaches a maximum at some point when the core is half
in and half out of the interior of the coil, and thence will decrease until
eqilibrium is attained when the core is wholly within the coil. The magni-
tude of the pull varies roughly as the square of the strength of the current
in the coil when it is not sufficient to give saturation of the magnetism of the
core, the relation reducing to variation directly as the strength of the current
when the magnetism of the core approaches saturation.
Another method adopted in some designs is to use a fixed iron core for
the solenoid, which becoming magnetised by induction, attracts a small
" armature " or block of iron. This allows more attractive force to be exerted
for the same number of ampere turns than the former methodybut a smaller
range of motion.
A simple form of electric motor has also been used, arranged to drive in
one <iirection when the main current exceeds a normal amount, separating
the electrodes, and in the reverse direction with an excess current in the
shunt circuit, causing the electrodes to approach or feed. Also reversing
gears controlled so as to produce a similar result when driven by a unidi-
rectional motor or clockwork. But these methods involve a departure
from simplicity, and therefore certainty and durability, which has pievented
any great success.
The simplest form of solenoid -controlled action for the regulation of the
length of arc is that in which a movable core is rigidly attached to €he upper
or positive carbon-holder, and arranged to be lifted by the attraction of the
series coil, and depressed by that of the shunt coil. The core and CArbon-
holder will then be lifted, striking the arc and lengthening it until a balance
KLSEN AKO LAMP. 24/
is obtained betwi^en the attractions, except for an excess of the attraction by
the series coil sufficient to support the weight lifted. The condition of equi-
librium will then be that there should be a certain relation between the
currents in the series and the shunt coils, which may be adjusted to corre-
spond to that given by the current and difference of potential with the
required length of arc. The difficulty with tbis simple arrangement is that
the position of the core relatively to the solenoids must vary as the carbons
are consumed, so that the same currents do not produce the same pull, a
different relation between them will be required for equilibrium, and a dif-
ferent length of arc result.
In the Pilsen arc lamp the difficulty is overcome by the use of conical
instead of cylindrical cores. The atti-active force on a cone of iron entering
a solenoid may be made constant throughout a considerable range of motion
if the current be unaltered. The two coils may be counterwound on the
same bobbin, so that the current in the shunt coil reduces the attraction of
that in the series until it is just sufficient to support the weight lifted. Or
a double cone (torpedo-shaped) may be used, balanced between an upper series
coil and a lower shunt ooil,the upward pull of the former being resisted by the
downward pull of the latter, equilibrium being qbtained by the same currents
in all positions of the core, which must necessarily descend if these currents
are to remain unaltered as the carbons are consumed^ these currents depending
on the length of the arc.
To keep the position of the arc constant as the carbons are consumed, it
is necessary to permit the lower or negative carbon to ascend as the upper
or positive des^^nds. If the sectional area of the latter be half that of the
former the rates of consumption will be equal, and the simple device of con-
necting the holders by a oord passing over a pulley, and allowing both free
vertical motions with this sole constraint, will be sufficient, but we must take
precaution that the weight of the descending parts should exceed that of the
ascending, so that the carbons may fall into contact when the lamp is switched
off ready for striking the arc when the supply is renewed. When carbons
of equal sectional area are used, some arrangement of pulleys or gearing
must be employed giving only one half the rate of movement to the negative
carbon-holder that it does to the positive.
In other arc lamp mechanisms the attracted core (or cores) is not con-
nected rigidly to either of the carbon-holders, but by means of a clutch,
brttke, or other such device, obtains a temporary grip, which it releases and
renews at frequent intervals at a slightly different point. Thus, while the
series an(} shunt solenoids attract a core, or pair of cores connected together,
so as to permit of only a very limited range of motion, the latter controls
the motion of the electrodes through a considerable range. As before, the
attraction of the shunt solenoid must oppose or weaken that of the series ;
an excess of current in the latter must cause the core system to grip the
holder system and further separate the electrodes : an excess of current in
the latter to cause the electrodes to approach, and a still further excess to
release the grip. By their own weight, or a spring, or like means, the elec-
trodes must of themselves tend to approach or feed when the grip is released,
a renewed restraining grip being taken by the core system when the length
of arc has decreased somewhat during the short period of release. These
are the essential principles common to a very great percentage of arc lamp
mechanisms, of which a few typical examples will now be selected for
description.
One of the simplest as well as the oldest surviving types is the
Brush arc lamp, the mechanism of which is illustrated in Figs. 87, 88.
The shunt and series coils are counterwound so as to form a pair of
solenoids C* These attract a pair of cores united by a cross bar B, which
248 BRUSH ARC LAMP.
can more Tortically bo as to raise or lower a lever to the end of which it is
attached. The motion of the lever is steadied by a dash-pot A| and the
weight of the oores balaDced bj a spring which adjusts the force of attrac-
tion Qeceasary to lift the lever. Near the fulcrum of this lever a liok is
attached which supports and actuates the clutch, l^e type shown is the
double carbon lamp, having two pairs of carbons, which are consumed in
succession, the arc being formed between one pair only at first until they
are consumed, and then transferred to the other pair in the manner shown
below. The positive carbon-holders alone move, so that the arc is " non-
focusiug,'' that is, changes its position as the carbons are consumed. These
holders are attached to long vertical rods E E', which are pushed op into
chimneys at the top of the framework when the carbons are inaert«a, and
KlG. B7.
descend as they are consumed. The current is conveyed to them by fine
wire brushes, but as it would be difficult to insulate them from the frame-
work of the lamp, it ia customary in this and nearly all types of arc lamps
to allow the whole framework of the lamp to remnin at the potential of the
positive carbon- holder, insulating the terminals and nestitive holder from
them, and to suspend the whole lamp from an insulating fiupport. The
negative carbons are supported by the central pillar, of which only the
upper part is shown in the figure.
The clutch arrangement consists simply of a fork engaging with a loose
ring or washer upon the carbon rod, fairly clearly .shown in Fig. 87. The
raising of the regulating lever lifts the fork, tilting the washer, and so
getting a grip, raises the positive carbon rod and strikes the arc. As the arc
lengthens and the piiU of the solenoids weakens, the wa.sher is lowered until
it comes into <v>ntiii't with the framework, nnd thereby the tilt is lessened
till the rod slides throjgh. The fork clutch is again raised to grip the rod
BRUSH ARC LAMP. 249
when the &rc length h&B decreased very sli^^htly, and if the action be very
quick, there will be no need for the wauier to be again raised, as when the
carbODB are first separated from contact, but with slight Tariations of the
tilt while still in contact with the frame, the rod will be allowed to slip
throngh the washer by frequent and almost imperceptible steps, the conse-
quent T&riations in the arc length being only detected by close observation.
In the double carbon lamp here illijBtrat«d the washer on one of the rods is
made a trifle looser than the other, the grip on the latter being in conse-
quence effected somewhat earlier than on the framer, and thus the corre-
Eraonding carbons are separated first without ftomiug an arc, and held apart
till the other pair are consumed. It is arranged that the positive and
negative carbons should be of such relative length and 8ecti<mal area that
Fig. 88.
The lirusb Arc Lnnip.
they should be consumed simuHaTieously to witliin about an inch of the
holder, and then a stop should prevent further descent of the rod, so that
the arc lengthens and finally breaks. The Hecond pair then come into con-
tact and an arc is struck between them, which continues until this pair is
also consumed. If the arc lamp is burning on a parallel circuit a stop on
the second positive carbon rod extinguishes the lamp by breaking the
circuit, and the lamp requires "trimming" or insertion of new carbons.
But if it be in series with other lamps the circuit must be maintained, or
the other lamps will also be extinguished. The circuit is still complete
through the shunt-coil, but this being wound with fine wire only calculated
to carry a small current of about one-thiril of an ampere, it will be speedily
burnt up. To prevent this happening we require an automatic device
which will short-circuit the lamp when the current through the usual path
ceases, or if the regulation of the supply would be interfered with by a
short-circuit, an equivalent resistance (ot about 5 ohms) may be substituted.
250 BBOCKIE-FELL ABO LAMP.
Upon the cessation of the current in the arc there will be a rush of oarrent
through the shunt-coil, this reversing the magnetism in the cores so that
they are pulled up with some violence, and this violent lifting of the cores
may be made to strike up the spring-balanced lever shown in Fig. 88, estab-
lishiDg a contact which short-circuits the arc. It is advisable that this short-
circuit should be automatically released when the supply is cut off^ so that
it need not be replaced by hand when the lamp is trimmed. As the current
in the shunt coil will cease directly the short-circuit is established, the
contact is maintained by the attraction of a small solenoid D in series with
the short-circuiting lever, acting on a core attached to its extremity, which
allows it to fall away directly the supply ceases. Some arrangement similar
to this is necessary for all types of lamps used in series distribution,
preventing a break in the circuity or injury to the lamp, in the event
of the arc breaking through the consumption of the carbons, or failure to
feed.
The washer-clutch is an exceedingly simple device which works very well
as long as the rods are clean, but the rods are rather liable to stick when
foul, and when released to slip too rapidly through the washers and " over-
feed." Various improved forms of clutch have been designed, but the
motion of the rods is so very gradual (at most an inch and a half per hour),
that perfect steadiness can scarcely be expected when the clutch is applied
direct to the rods. Brockie has succeeded, however, in preventing the
possibility of over-feeding by the use of a slightly conical rod, of which the
ever-largening diameter at the point where it is giipped requires a still
further descent of the cores before the rod can slip more than an extremely
minute distance.
In most other forms of arc lamp regulating mechanism the motion of
the rods is greatly multiplied by some simple gearing so as to be much
more rapid at the point at which the clutch or brake is applied. The
favourite form is a large brake-wheel revolving in conjunction with a small
pinion which is in gear with a rack rod which is the positive carbon-holder ;
or else the rods are supported by flexible cords passing round a small pulley
revolving with the brake- wheel. The main difference between these types
will lie in the different methods adopted to reverse the motion, so as to
strike the arc ; for this is the main difficulty, the feeding being governed
with any degree of steadiness that may be desired by sufficient multiplicsr-
tion of the motion, and the balance between the descending and ascending
parts being arranged to ensure feeding, without the possibility of sticking,
when the clutch or brake is released.
The latest design of the Brock ie-Pell arc lamp mechanism, the same
general principle having been employed with numerous changes of detail, is
illustrated in Figs. 89 and 90. The coils are wound as separate solenoids,
attracting cores on opposite sides of a rocking lever. The carbon rods are
suspended by separate flexible cords, wound round pulleys rotating with a
large brake-wheel. The brake-wheel has a broad flange on one side, against
the inner surface of which a small leathern pad is pressed to check its
rotation and thus regulate the feed. This pad is mounted on a short
weighty lever, which is pivoted on the extremity of a second longer lover,
the other extremity of which is screwed to a flat horizontal spring, which
supports it, taking the place of a fulcrum. The weight of the short lever
causes the pad to press against the interior of the flange of the brake- wheel
when the long lever is slightly raised, but on the descent of the fulcrum
the pad is lifted off by an adjustable stop which tilts the short lever. The
levers are raised or lowered by a long rod attached to the rocking lever
near the fulcrum on the same side as the series coil. Wh6n the current is
first switched on the carbons being in contact, the connecting rod is lifted
BROCKIE-PELL ABC LAMP.
2S2 E.C.a ARC LAMP
untQ the brahe-pad touches the brake-nhee), and then od further ri»ng the
short lever is lifted off the stop and carriee the brake-wheel round for a
flfaort distance till the carbons are separated sufficientl]r. Only a small
motion in this direction would be possible, owing to the limited motion of
the solenoid oores and the levers. The motion in the opposite direction,
that of feeding, consiatB in the descent of the levers with the brake-wheel
till the stop causes the release of the latter, which slips round allowing
the rods to approach by a minute amount before the brake is a^jain
applied.
Fig. 91. Fio 92.
E.C.C. (Electric ConatructioD Co.) Arc Lamp.
CROMPTON-POCHIN ABC LAMP. 253
The lamp manufactured by the Electrical Oonatruction Oompany (KC.C.)
IB showD in Figs. 91, 92. The solenoids are here inverted, bo that the
mechanism of the lamp is very compact. The positive carbon-holder is sus-
pended by a double copper tape coiled round a drum D. The latter is geared
by cogwheels to a more rapidly rotating brake-wheel. Two brake-blocks are
applied to the circumference of this, attached to
a band, one end of which is supported by a spring Tlo- 93-
and the other by the rocking lever L near the
fulcrum. The extension of the spring allows a
slight rotation to be given after the brake-
blocks are applied, thus striking the arc. The
subsequent release of the brake allows the rota-
tion of the brake-wheel, and the de=c6nt of the
positive carbon rod, the negative rod ascending
proportionally. The spring also adds to the
Benaitiveness of the feed. An inverted dashpot A
secures perfect steadiness.
The Crompton-Pochin arc lamp (Fig, 93)
mechanism resembles the preceding in the
arrangement of the solenoids and rocking lever,
difiering mainly in the manner of striking the
arc. The positive carbon rod is a rack-rod gear-
ing with a small pinion rotating with the brake-
wheel, to the circumference of which a small
brake-pad on the rocking lever is applied from
underneath. To strike the arc this brake-pad
lifts the brake-wheel, pinion, and rod bodily,
rotation of' the first-named being of course
impo«<aibIe until it descends once more to the
" feeding " pin which supports the axle, and the
release of the brake-pad from its periphery
allows it to rotate and the carbons to feed. To
balance evenly the parts thus lifted, a double
brake-wheel is used, as shown in the illustra-
tion. In the double-carbon lamp, the " feeding-
pin " of the pair intended to be consumed first
is set rather higher than that of the other pair,
so tliat the latter may be the first to separate.
The " Phcenis " arc lamp mechanism (Patter-
son and Cooper) is illustrated in Fig. 94. The
shunt and series coils are counter- wound, so as
to form one solenoid A, and have a fixed soft
iron core P, which attracts an armature K upon
the brake-lever H- The brake-wheel B is
pivoted upon this lever, and connected to a small
cogwheel G, which gears into the large wheel E,
which acts as an intermediate gear wheel between the brake-wheel and rack-
rod, giving a high rate of multiplication for the motion of the former. The
revolution of the brake-wheel is checked by the lever N, which applies
a brake to its circumference when the armature- lever is raised. The brake
is released when the armature -lever pulls so that the brake-lever is supported
by the screw S- The further raising of the armature- lever after the brake
is applied strike.s the arc ; the feeding subsequently proceeds as the brake is
released by the fall of the armature-lever. The failure of the circuit through
the arc cause.s the lever H to fall shiirply upon the block M, and short-cir-
cuits the lamp tliiough the resisiaLce R, which mayor may not be sufficient
254
"PHCENIX" ARC LAMP.
to form an equivalent resistance (5 ohms) to that of the arc, but must be
sufficient to allow a large current to pass in preference through the carbons
when closed together, so that the lever may be initially lifted and the arc
struck when the lamp is trimmed.
Fig. 94.
ne Phceniz Arc Lamp.
The Siemens '' Band " arc lamp is illustrated in Figs, 95, 96 and
differs from all of the preceding in employing a shunt-wound solenoid only,
having a fixed core and attracted armature. The frame i which carries the
SIEMENS "BAND" ARC LAMP.
255
upper positive carboo of the lamp is suspended b^ a conducting metal
"band" i, wound round the circumference of tbe bturel b mounted on a
horizontal axi£ d, and containing a volute or coiled spring so arranged that
tbe weight of the poBitive carbon and its frame tends to rotate the barrel
Fio. 95.
FIQ. 96
"Baud " Arc Lamp.
in opposition to tbe spring. The barrel carries a toothed wheel which is
connected by a train of wheels to an escapement and pendulum g, whereby
the rotiition of tbe barrel and consequently the descent of the positive
cttrbon due to gravity is allowed to take place at a Blow regulated speed.
The barrel b with its wbeelwork and escapfment is mounted in a fi'ame r
pivoted at its lower end at a point a (near the axis of the barrel d) to
256 SIEMENS "BAND" ARO LAMP.
standaxdB projecting up from the lamp -base. The upper end of the frame
carries an alrmature e facing the pole-piece m of an electro-magnet which is
connected as a shunt across the terminals of the lamp. When the electro-
magnet attracts the armature it draws the frame r downwards about its
pivots, and thus lowers the barrel carried by it, and also the positive carbon
with its frame t. The frame r is held back at its upper end by a spring f
(adjustable by a setting-screw A), the tension of which is regulated to with-
stand the puU of the electro-magnet with more or less force, and which
when no current is passing through the electro- magnet, holds the frame r in
such a raised position that the movement of the escapement is arrested by
the stop U When the frame r is in this position, and the positive carbon
is thus raised to such a distance from the lower or negative carbon that no
arc is formed, the relatively strong current passing through the coils of the
electro-magnet attracts the armature 0, thus lowering the positive carbon
until the escapement is freed from the stop A, whereupon the positive
carbon and its frame t will be free to descend by gravity until it comes in
contact with the negative carbon. Owing to the passage of the current
through the carbons the current through the electro-magnet will be weakened
to such an extent as to allow the spring /to raise the frame r again so as to
arrest the escapement and raise the positive carbon sufficiently to strike the
arc. A position of equilibrium is thus established by the increase of the
resistance of the arc, and sufficient current passes through the shunt to
cause the electro-magnet to balance the pull of the spring /l
As the positive carbon bums away, and the resistance of the arc in*
creases beyond this point, the attraction of the electro-magnet overcomes
the force of the spring /and the frame r is attracted, whereby the escape-
ment g is released from the fixed stop I, and the frame % is lowered partly
by the descent of the axis of the barrel h and partly by the rotation of
the barrel on its axis d. When the resistance of the arc is thus lessened
the electro-magnet becomes correspondingly weakened, and the frame r is
raised again by the spring/ raising the positive carbon frame % and engage .
ing the escapment with l£e stop I, In this manner the normal resistance
of the arc is re-established , and the regulation of the positive carbon i«
governed according to the variation in the resistance of the arc itself.
For introducing fresh carbons the upper carbon frame % is raised by hand,
whereupon the volute spring in the barrel is enabled by uncoiling to turn
the baml so as to wind up the suspension band again, the train of wheels
being so arranged that this can be done more or less rapidly without
actuating the escapement. To compensate for the variation in weight of
the positive carbon in burning away, a helical spring % strains a cord which
is led over a pulley on the frame r and becomes wound on the axis d of
the barrel as the barrel revolves, lowering the positive carbon so as to
exert more and more downward strain on the frame r the more the carbon
is consumed. Thus the spring being strained to the least extent when the
carbon has been freshly introduced, its tension and consequently its down-
ward pull upon the frame r will increase as the weight of the carbon
decreases.
The shunt is permanently connected up to the terminals of the lamp,
and, as already described, the regulation is effected by the variation in the
strength of the current in it. Messrs. Siomens Brothers and Oo. supply
lamps of this pattern to bum with currents ranging from three to twenty-
five amperes, and can adapt them for working in series of more than two
or for alternating currents. The negative carbon is fixed in a holder at
the bottom of the lamp framing ; as this has no automatic adjustment these
lamps have not a fixed fqcus.
The Luna arc lamp (Figs. 97, 98) is another example of the use of a
LUNA ABC LAMP. 257
peudtdnm eecapemeot to regulate the rapidity of feeding. Two series and
two Hhunt solenoids are wound on separate wrought-iron cores with large
square pole-piecee. The armatures are rocking levers mounted on the
same spindle at right angles to one another, so that each is normally
inclined to the Wrizontal, but the attraction of the seriee solenoids and
(X»«fl tends to bring one armature to the horisontal position, and that of
the shunt solenoids and cores the other, thus rotating the spindle in
opposite directioDs. To the aforesaid spindle ia fixed a biBse lever, which
in striking the arc lifts the pendulum escapement and the positive carbon
rack-rod with which it is geared. The movement of the escapement is
regulated by a fork, an extension from which is free to move upwards, but
Fia. 97. Fitt 98.
i
The Lnna Arc Lamp,
whose downward motion ia arrested on its coming into contact with an
adjustable stop. In striking the arc the pendulum is locked by being
pulled against the movable fork, but released when the latter descends to
the regulating stop ; the weight of the rack-rod causes the pendulum to
swing, and the escapement allows a feed of about o.z millimetres for each
swing of the pendulum. The coonection to the positive carbon rod is made
by means of a zig-zag copper strip as shown in the illustration.
There is no doubt whatever that continuous current arc lamps are more
efficient than alternating current arc lamps, in whatever way the candle
power be measured, whether spherical, hemispherical, or maximum, and
that the dicpoflition of the radiation in various directions is more conveni-
ent for most purposes. But for detached arc lights on circuits intended
258 ENCLOSED ARCS.
primarily for incandescent lighting, the user of alternating currents is met
with fewer difficulties. First of all, since the alternating current trans*
former is much simpler and more efficient than the continuous current trans-
former, especially with small sizes, it is easy to establish a local circuit
with any voltage desired, and use the lamps in series or parallel. The
transformer suitable for this purpose will be one allowing a large amount
of magnetic leakage, or designed with a comparatively small amount of
iron ; in transforming from constant potential the transforming ratio will be
modified so that that in the arc lamp circuit will fall with an increase of
current, and the regulative effect equivalent to the additional resistance
required with the constant-potential continuous-current arc lamp.
For a single arc lamp a little over thirty virtual volts is required, and
three will work well on a hundred-volt circuit. If incandescent lamps are
to be uSed on the same circuit, instead of the transformer with magnetic
leakage, the regulative conditions may be obtained by the use of a choking
coil, which is simply a coil wound on an open circuit laminated electro-
magnet. The self -inductance of this coil takes the place of the regulating
resistance with continuous currents, and possesses the advantage that the
waste of energy does not correspond to the number of volts " choked-off,^'
as a low power factor may be obtained in a well-designed choking-coil. It
is possible to use a single thirty- volt lamp on a hundred- volt circuit,
the choking-coil absorbing the remaining volts with little loss; but it
must be noted that a supply meter, unless it is an integrating watt meter,
will record the apparent, not the actual amount of energy required, and a
consumer supplied from public mains may thereby be defrauded.
It will be better for a consumer supplied with, say, 100 volts for incan«
descent lighting, who wishes to employ one or more independent arc lights,
to re-transform in the ratio of 3:1. The transformer for this purpose
need be very small, as the transformer losses need only exist while the lamp
is burning, being designed for a semi -saturated and therefore leaky magnetic
circuit. Moreover, there need be but one transformer coil — it will be
sufficient to bridge one-third of this coil for the arc lamp connections to
obtain the thirty-three volts, and calculate the size of wire for the coil to
carry one-third of the current for the arc lamp. This coil may, if desired,
be subdivided at two points so as to supply three arc lamps, which will be
independent of each other in every way.
Enclosed Arcs.
The cost of the carbons consumed in the electric arc form no incon-
siderable part of the total expenditure in producing the illumination.
Adding to this the labour and inconvenience of frequent re-trimming, it
will be seen that a means of prolonging the life of the carbons is a great
desideratum. From the earliest times in the history of electric lighting
attempts have been made in this direction by enclosing the arc to keep it
from the oxygen of the air, so as to prevent combination and allow the
volatilised carbon to re-condense. In 1879 Andr6 employed an air-tight
globe for this purpose, allowing the volatilised carbon to combine with the
oxygen to form 00 and CO,, and thus being enclosed in an atmosphere of
nitrogen and carbon gases, the life of the electrodes was greatly prolonged.
With a short arc produced by E.M.F. the carbon is thrown off from the
arc, and the globe speedily blackens and obstructs the light. With an arc
of considerable length, maintained by a high E.M.F., very little blackening
ensues, the carbon being consumed with extreme slowness and combining
with the limited amount of oxygen admitted. The high potential enclosed
arc was developed of recent years independently by Marks and Jandus.
ENCLOSED ARCS. 259
Marks, after experimenting with an air-tight enclosing glohe, in which
the oxygen combined to form carbon gas, and the heat of the arc caused
the pressure to rise within the globe, invented what he called the '* venti-
lated " arc lamp, in which air was admitted only in very limited quantity,
thereby prolonging the life of the carbons four or five times. Using
E.M.F. of eighty to eighty-five volts, a current of four to five amperes,
and a length of arc about one centimetre, he obtained 431 mean hemi-
spherical candle power with 368 watts, or an efficiency of 1.17 watts per
candle power. The corresponding maximum candle power was 595, thus
the distribution in a downwards direction proved, owing to the free radia-
tion with the long arc, to be much more uniform than with the ordinary
arc lamp, in which the mean hemispherical is only about one-half the
maximum candle power. If this be a gain, which is doubtful for some
purposes, it will partly make up for the waste of power owing to the
excessive E.M.F. For the same power the ordinary arc lamp would give
a sliehtly greater value for the mean hemispherical candle power, but
would throw much more light in a direction considerably ioclined to the
vertical, where it is the more needed owing to the greater distance of the
objects illuminated.
In the Jandus arc lamp the arc is enclosed in an inner opal globe, and
this again within a larger external globe. The former is close-fitting, but
not air-tight, admitting air in limited quantity from the surrounding
space within the latter, which is closed by a non-aximission valve. Thus
whUe the pressui*e within the globes cannot rise much above the atmo-
spheric pressure, the oxygen within the inner globe combines with the
carbon to form carbon monoxide and dioxide, which difiuses slowly into the
external space, and is replaced by minute quantities of oxygen necessary to
combine with and carry off as gas the carbon as it is slowly consumed.
The negative carbon is fixed, the bottom of the inner globe being thus
hermetically sealed ; the positive carbon pisses through a steel cap which
covers the inner globe, the whole being just large enough for it to slide
freely, and to be controlled by a series, or series and shunt, solenoid, working
through a simple form of clutch. Owing to the great length of the arc,
about nine to twelve millimetres, the regulation need be far less delicate
than with the open arc lamp. The carbons are both solid, one-half inch
in diameter, and lengths of 10^ and 6 inches respectively are used.
According to tests of the lamp by Houston and Kenelly, the consumption
of the carbons was at the rate of .057 inch for the positive, and .015
inch for the negative per hour, the current being 5.6 amperes, and the
difference of potential between the electrodes eighty volts. At this rate
the positive carbon would be consumed in about 150 hours, only 2^ inches
of the negative being consumed in the same time. The use of a non-
focusing type having a fixed negative carbon is thus justified, since the
position of the arc undergoes little change. The carbon ends become fiat,
and the light is fairly uniform in the lower hemisphere, the maximum
value in the lamp mentioned above being 1295 candle power. The lamp is
used with a resistance of about five ohms in series with it, so as to be placed
across a iio-volt circuit.
Search Lights.
For the large alternating current arc lamps employed for search lights
hand regulation of the length of the arc is commonly preferred ; for light-
houses, &c.y automatic regulation is of course required, but the conditions are
somewhat different from those of the smaller arc lamps for ordinary lighting
purposes, in that the lamp must necessarily be under constant supervision for
260 SEARCH LIGHTS.
cleaning and correction of adjustment, and the cost of an elaborate regulating
mechanism for a single large arc lamp is of comparatively little importance.
Clockwork or an electromotor, with a gearing giving a great reduction in the
speed of motion, and controlled by a shunt coil, is therefore suitable for the
regulation of the feeding; the striking is effected by hand or by the reversal
of the motor or gearing effected by the current in a series solenoid.
In the South Foreland lighthouse an alternate current arc is employed
with a difference of potential between the electrodes of from 35 to 38 virtual
volts. The carbon electrodes are fluted, or of star-shaped section, both cored,
a pair having 60 millimetres external diameter being used in stormy weather
wir.h a current of 300 virtual amperes ; in fine weather a pair having 50
millimetres externid diameter are employed with a current of 180 ampdres.
The object of the star- shaped section is to give a larger surface for the radia-
tion of heat from the carbons, which otherwise are heated throughout a
considerable length from the extremities, as well as to centralise the ai«. A
short arc of one-eighth to a sixteenth of an inch is found to give the best
light in a horizontal direction, though in other directions much of the light
is screened by the carbons. The rate of consumption of the carbon jods is
from i^ to 2^ inches per hour. With a current of 240 virtual amperes tho
illumination in a horizontal direction is found to be about 16,000 candlo
power.
For search lights, or, as they are often called, projeoton or holophotee,
reflectors are now almost universally employed, having superseded the Fresnel
lens, which was at one time largely adopted, the reason for this being that
the latter is so liable to be injured by the heat from the are, and is not so
well suited for rough usage as the reflector ; and in addition to this, the
Fresnel lens could not be protected from external injury, as is the case with
the mirror. A serious drawback to the use of the reflector in the first
instance was that the parabolic surface, which theoretically gives a perfect
result, was so difficult and expensive to manufacture. Colonel Mangin in-
vented a mirror which got over this difficulty by having the two surf aces
portions of spheres of different radii, thus making the glass much thicker
at the circumference than at the centre. The rays from the arc on striking
the inner spherical surface are refracted by the glass, and finally emerge in
parallel directions, which renders it possible to direct the beam upon objects
at a much greater distance from the light than is the case when a simple
spherical surface is employed.
CHAPTER XVI.
Central Station Economy.
In this chapter we shall deal with the various sources of power available for
conversion into Electrical Power for lighting purposes, briefly noting their
suitability for the purpose and their economic possibilities. The various
causes of waste and inefficiency attending the employment of the various
sources, and the conversion into the Electrical Power will be investigated,
and the directions in which these may be reduced. Thus we shall be able to
compare the actual cost in practice of producing the electric light with that
theoretically possible.
That the greatest economy in the generation of power is to be obtained
where that power is generated upon the largest scale may be considered an
axiom. It is true of nearly all undertakings that the larger the scale upon
which they are conducted the smaller the percentage in expenses of manage-
CENTRAL STATIONS. 26l
ment, &c. and the higher the possibilities of efficiency. This is peculiarly
the case in the supply of power for electiic lighting, where attendant labour
forms a very lar^e proportion of the current expenditure, and within a
certain limit, larger plant gives higher efficiency and entails a great decrease
in the proportion of cost in labour and materials. It is therefore quite
certain that the highest economy will be attained by concentration of the
generating source, and the supply of power from a ** central supply station.^'
The last few years have been a time of great activity for electrical engi-
neers and capitalists in the establishment of such stations. The possibility
of supplying energy for electric lighting at a cost which at present competes
with and offers possibilities, when conducted upon a larger scale and with
improved systems, of eventually underbidding other sources of illumination,
has at length been satisfactorily demonstrated. Where such undertakings
have been organised by companies, with privately subscribed capital, a few
years have, except in a few cases, placed them on a sound dividend-paying
basis, and the disasters that arose from premature speculation are now things
of the past. The progressive tendency of local governing bodies in the
British Isles has, however, fortunately caused them to undertake the supply
of electric lighting for the benefit of their respective municipalities in a very
large number of oases, and the results have been almost uniformly satis-
factory to the consumer and the ratepayer.
In seeking the best system for the supply of electric power over an
extensive area^ it is the duty of a consulting engineer first to seek for the
peculiar advantages, if any, which may be secured by properly choosing
the position of the generating-station. It ia obvious that for economy in
distribution the most central position is the most advantageous. But this
advantage will in many cases be counterbalanced by the cost of land,
carriage of fuel, and the inconvenience and bad appearance of a lofty
chimney in the centre of a town or city. If water-power or any other
source of energy be available which does not demand the consumption of
fuel, it would be most foolish to neglect its use. Where the distance of the
fall from the area of distribution is great, or considerable capital expendi-
ture is demanded for its utilisation, it will be necessary to consider whether
the interest on the excess in the capital expenditure over that of a fuel-
consuming station would be greater than the corresponding cost of fuel.
Even a very limited and irregular source of water-power may be worth
securing, to be supplemented by steam-power at such times as it is unable
to sustain the required load. Next, if water can be obtained in suffi-
cient quantities as to be available for condensing purposes, such an advan-
tage is worth considerable capital expenditure. With engines running at
variable loads as is almost unavoidable in central station practice, the
saving in fuel by efficient condensing is found to be considerably greater
than in marine practice where a uniform load may be calculated upon, and
may rise to as much as thirty or forty per cent, with some steam-engines.
When power is to be generated by the consumption of fuel, proximity to
a railway or canal is likely to save considerable expense in the carriage
of the fuel.
Ab the subject of fuel has been very fully treated in the first volume of
this series, to that volume we must refer for the scientific discussion of the
relative values of various classes of fuels, and of their utilisation for
evaporation, &c. This discussion must be supplemented by a few remarks
concerning certain modifications that arise in the employment of fuel for
the generation of electric power as demanded for electric lighting, and then
we shall enter into a discussion of the efficiency of the various stages by
which the energy of combustion of the fuel is transformed into energy
iu the form of the electric current. In the term Jud we here include all
262 FUEL.
substances, the chemical decomposition of which is used as a source of energy.
This is perhaps an extension of the ordinary meaning of the term, as it
does not confine us to those which demand the intermediary production of
heat in the process of power generation.
There are three methods known to us by which fuel may be consumed so
as to produce energy :
(i) The dii-ect method of producing electrical energy by the chemical
action in a primary battery.
(2) The utilisation of combustion in a boiler furnace to evaporate water,
and employment of the steam-engine.
(3) The direct expansion of the fuel in the form of gas after explosion
when mixed with air in the cylinder of a gas or oil engine.
The first method is capable of producing the highest efficiency ; that ia
to say, the electrical energy obtained may be made very nearly equal to
the total amount theoretically possible. It is at present, however, only
applicable to the chemical action of acids upon metals, such as zinc and iron.
The cost of metals as fuel is very much greater than coal or oil, and the
energy obtainable per pound of fuel considerably less, in spite of the more
efficient method of utilisation. So that the cost is enormously greater, and
primary batteries have only been used for electric lighting where expense
has been a minor consideration to that of convenience.
Of the second and third methods the former has up to the present been
employed universally wherever electric power has been generated upon a
large scale ; the latter has been considered more convenient in small isolated
plants, where gas has been obtainable direct from some neighbouring gas
works, or the compactness of oil-enginee and the little attention demanded
is a great boon. Quite recently the high efficiency of the gas-engine has
been more thoroughly recognised, and hopes are entertained of attaining far
higher efficiency than is possible with the steam-engina Several large
central stations have been erected having their own gas-generating plant and
using large gas-engines in place of the hitherto universal st^m-engine
plane. A few years experience will show whether, under the conditions
of central station supply, the gas-engine will compete with, or excel, the
steam-engine.
In considering the financial economy of the production of electrical
energy from fuel we have four items to take into account :
(i) The local cost of the fuel per ton.
(2) The calorific value, or the total amount of energy theoretically
obtainable per ton of this fuel.
(3) The ratio of the total amount of energy which can actually be
obtained in the form of electrical energy to that theoretically obtainable in
accordance with the principle of the conservation of energy.
(4) The capital and maintenance costs of the plant and cost of labour
necessary for the conversion of the energy from one form to another.
The subject of the calorific value of various kinds of fuels has been
treated fully in the first volume of this series, and the results of the most
careful experiments given. The total quantity of heat obtainable by com-
bustion may be determined in several ways. Either by direct experiment
in boiler furnaces with a careful calculation of the various sources of
waste; or by complete combustion on an experimental scale in a oalori-
nieter ; or having made careful measurements of the heat developed by the
complete combustion of the elements, which will be chiefly carbon and
hydrogen, we may make a calculation of the calorific value of the composite
cuels from their chemical analyses. Combustion in a furnace when there is
.sufficient draught of air will always proceed with such a chemical combina-
tion that the greatest heat is produced. Having therefore measured the
EFFICIENCY OF BOILERS.
263
heat obtained by tbe ooinbustion of bydrogen so as to produce water, and
carbon so as to produce carbonic dioxide, these being the combinations
which produce the greatest amount of heat, we can find the number of
units of heat obtained by the complete combustion of one pound of a com-
pound of these, which may be taken as
8080 C + 34,462 (h - g j
C, H and O being the quantities of carbon, hydrogen, and oxygren in one
pound of the fuel, 8080 units being that developed by the combustion of
carbon with oxygen to form carbonic dioxide and 34,462 by the combustion
of hydrogen to form water, the second term allowing for the oxygen which
is already in combination with hydrogen in the form of water. The
numerous defects of the above methods of measurement are discussed in
the first volume of this series.
There are various ways of expressing the result. The heat unit used
above is that required to raise a pound of water from o^ to 1°
Centigrade. We can also express the calorific value by the number of
pounds of water evaporated from and at 100^ Centigrade, by one pound of
the fuel, or by the number of foot pounds of energy which could theoreti-
cally be developed.
The following table gives the mean of some of the best results obtained
for various elements and fuels.
FaeL
Hydrogen
Carbon (wood charcoal)
>*
Silicon •
Salphnr •
Phosphoms .
Zinc (primary battery)
Welsh coal
Newcastle coal •
Derbyshire coal •
Wood (dried) . .
Crude Petroleum •
Prodaot
Heat Units
of Combus-
per Ponnd
tton.
o< Fuel.
HgO
34,462
Ct)g
8,080
CO
2,474
SiOa
7,830
SOg
2,140
F^Ob
5,747
ZLS04
1,670
—
8,241
8,220
—
7,733
—
3,547
^"^■^
10,190
Poanclsof
Water
Bvaporated
at zoo" a
62.66
14.69
4.5
14.24
4.09
10.45
15.37
1533
14.42
6.61
18.53
Foot Ponnds
of Energy
Theoretically
obtainable.
47.900,000
II. 2 X 10^
3.44 X !©•
ia9 X 10^
2.97 X 10*
7.98 X lO*
2.32x10^
11.45 X i^
11.42 X 10^
10.77 X 10^
4.93x10^
14.16X 10^
From ih\s table we gather that it is theoretically possible to evaporate
more than 15 lbs. of water per lb. of the best steam coal, the evaporation
tiiking place at atmospheric pressure from water already at boiling-point.
Of course in practice the water has to be heated from its normal tempera-
ture, and for the highest efficiency it is necessary that the water should be
evapoiated at high pressure. Under these conditions a much smaller
quantity can be evaporated with the same quantity of heat. But we are at
present simply using this measure of evaporation as the measure of the total
quantity of heat developed, and hereafter when we are measuring the
efficiency of the engines by the quantity of steam used to generate a horse-
power hour it will be advisable, for the sake of comparison with engines
which use a different boiler pressure, in the calculation of the fuel consump-
tion to reduce this measurement to that of the equivalent amount of water
evaporated by the same quantity of heat at atmospheric pressure, though it
is better practically to use the same quantity of heat to evaporate a smaller
264 EFFICIENCY OF BOILERS.
quantity of water at a high pressure. To raise water from 15^ Centigrade to
boiling-pointy and evaporate at a pressure of 150 lbs. per square inch,
requires about 20 per cent, more heat than to evaporate the same quantity
at atmospherio pressure from 100^ Centigrade.
The energy which is the theoretical equivalent of the heat produced by
the complete combustion of 1 lb. of Welsh coal is given as 11,450,000 foot
lbs., or about 6 horse-power hours, or about 4^ Board of Trade units of
electrical energy. The generation of this unit would therefore cost, if
perfect ofiiciency were possible, and coal were at 209. per ton, something less
than -^jgd, for fuel.
In the London electric supply-stations the cost of fuel varies from |<2. to
2d. per unit supplied to consumers, and the price charged from 6d. to Sd,
The various losses in the generation, or rather successive transformations of
energy from its condition of storage in the coal to the electrical energy
delivered to consumers, that give rise to this enormous disparity between
the ideal and the actual, must now engage our attention.
From the trials of Lancashire and Galloway boilers, of which the results
are given in the first volume of this series, pp. 724-725, it will appear that
under test conditions, when the feed-water is supplied from an economiser
or condenser at nearly boiling-point, the quantity of water evaporated at
atmospheric pressure is from 11 to 12 lbs. per lb. of the best coal consumed.
This means that an efficiency of 75 per cent, is about the maximum attain-
able. In ordinary working conditions it cannot be expected that the
efficiency will be maintained as high as this. Even if the boiler be always
worked at or near its most efficient rate of steaming, an evaporative
efficiency of 9 lbs. of water per lb. of coal, or 60 per cent, of the theoretical
maximum will be considered fairly good practice. The skill of the stoker
may make considerable difference in the quantity of fuel consumed to
produce a given quantity of steam.
But in the steam generation for an electric supply-station a new source
of inefficiency arises from the. constant variation of the demand, which
compels us frequently to employ boilers to generate steam at a rate at which
the efficiency of evaporation is very much less than it is under test con-
ditions. As we shall meet with an analogous source of inefficiency in nearly
every successive stage of the generation of electric power^ it will be well to
say a few words upon the effect of this variation in the demand in the case
of boilers.
It is not unfrequently maintained that the experience of the marine
engineer, who is the largest steam user in the world, is the best guide to the
selection of engines and boilers for electric light stations, but there is one
very important feature in which the two classes of work differ very essen-
tially. The marine engineer has to deal with a continuous and nearly
uniform load, and in the mercantile marine arranges to run at the speed
which, taking all expenses and returns into consideration, will prove most
profitable. In the Boyal Navy the question of economy is of secondary
importance, but even there the load is usually steady for long periods except
during special manoBuvres. But in an electric lighting station the load is
exceedingly variable, and is liable to be suddenly increased by very large
amounts. A theatre with 1000 lights may be switched on when quite
unexpected, perhaps in the middle of the day, and solely for the purpose of
exhibiting the lights to a visitor, and thus an extra load of 100 horse-power
may be thrown upon the generating -station. Hence it is necessary to
maintain an ample margin of power in the running machinery, especially
when working at light load, and experience shows that the attendants in
charge always like to have plenty of power available at any moment. But
even where no single installations exLst which, from their magnit'jde, are
IRREGULARITY OF DEMAND. 265
liable to put a greatly increased load suddenly upon the generating plant,
the demand at different hours of the day is very different. In a provincial
town the load may be increased five or six times or even more within an hour
of sunset, and three hours afterwards it may again become insignificant. A
very large portion of the plant may thus be required for three hours only
out of twenty-four, and during the remaining twenty-one hours the plant
which is in operation may be loculed with only a small fraction of its most
economical load.
Under these circumstances a number of boUers may have to be kept
with fires banked for twenty-one hours in order that they may supply steam
for three hours, and during a considerable portion of this period they may be
working considerably below their best load. It is not, therefore, sufficient
to consider what boiler will show the greatest economy when steaming at its
best rate, but we must take into account its economy when steaming at a
small fraction of its full power, and special attention must be paid to the
loss of heat from the boiler when the fires are banked. In this latter
relation the amount of surface exposed by the boiler to the air, and the
character of its seating, are important considerations.
Suppose, for example, that during the three hours it is at work the boiler
consumes 1200 lbs. of coal and produces 10,200 lbs. of steam, and that during
the twenty-one hours that the fires are banked it is necessary to bum 30 lbs.
of coal per hour to maintain the steam pressure. The total coal consumed
during the twenty-four hours is 1830 lbs., while the steam produced is
10,200 lbs., giving an evaporative efficiency for the twenty -four hours of only
5.57 lbs. of steam per lb. of coal, while during the three hours of steaming
the evaporative efficiency was 8.5. If another boiler had an evaporative
efficiency of only 8 lbs. of steam per pound of coal, but could maintain its
temperature with a consumption of only 10 lbs. of coal per hour when no
steam was drawn, the total consumption of coal for the production of
10,200 lbs. of steam, under the same conditions as before, would be 1275 +
210, or 1485 lbs. instead of 1830 lbs., and the evaporative efficiency for the
twenty-four hours 6.8 instead of 5.57.
It is easy to find results of carefully conducted experiments on the evapo-
rative efficiency of different types of boiler when perfectly clean and steaming
under the most favourable conditions, but these are not the average con-
ditions of working in an electric light station, and what is wanted is carefully
conducted experiments on the coal consumption in a boiler for every 5 per
cent, additional load from zero {i.e., just maintaining constant pressure with-
out any steam being drawn) up to the full power of the boiler, and such
results are not usually published.
In a twelve and a half hours test of a Lancashire boiler at an electric
lighting station for ten hours the average rate of steaming was about 33 per
cent, of the full power of the boiler. For the remaining two and a half
hours it varied between 60 and 85 per cent. It was found that with the
feed- water at 50^ F. the average evaporation throughout the whole trial was
7.78 lbs. of water per pound of coal, the average steam pressure being about
no lbs. per square inch. This corresponds to the evaporation of 9.4 lbs.
of water from and at 2 1 2^ F. per pound of coaL The boiler was in its ordinary
working condition, the flues having been swept a week before the test was
made.
To raise the efficiency of steam generation under varying load, as well
as to reduce the excessive number of boilers necessary when a heavy load has
to be dealt with only for a few hours in the day, various methods of storage
have been suggested. A station which uses secondary batteries has a great
advantage in this respect, provided the efficiency of the batteries is so high
that the loss in them is not greater than the loss through the inefficiency of
266 REFUSE DESTRUCTORS.
boilers and engines due to variation in load. A system of thermal storage
has been proposed, in which the heat generated is stored in the form of water
heated considerably above atmospheric boiling-point, and retained in well-
packed steel cylinders, equivalent, in fact, to largely increasing the water
capacity of the boilers, and thus enabling them to supply a heavier demand
than the normal rate of steaming could supply, for a few hours, without
greatly lowering the pressure. According to calculations, the correctness of
which is undisputed, this ought to effect a large saving in f uel, and a possible
reduction in capital expenditure.
Another method of reducing the loss is to go to the other extreme and
use boilers with a very small water capacity, and therefore very rapid in
'* getting up steam." This obviates the necessity of keeping the boilers
banked ready for an emergency, as a new boiler can be brought into action in
a short time. The Babcock and Wilcox water-tube boiler is very popular
among Electric Central Station engineers, and almost exclusively used in
London, both on account of the small ground space occupied and its rapidity
of steaming, in view of the sudden demands for power that may occur at
unexpected times owing to a fog, which would otherwise require a large number
of boilers to be kept banked. Of course this rapidity of steaming has no
connection with the efficiency ; and though water-tube boilers, owing to the
large heating surface in proportion to the grate area, can generally, if the
draught be good, be pressed to supply steam in an emergency at a rate much
greater than the efficient rate of steaming, yet they have the disadvantage of
possessing very little reserve power in the form of water heated above atmo-
spheric boiling-point. A suitable combination of the two types of boilers,
say the Lancashire and Babcock and Wilcox, would be the most satisfactory
arrangement both as regards prime cost, economy of fuel, and readiness for
all emergencies.
A most promising means of reducing the cost of generation of Electric
Power has lately been undertaken by several London vestries and provincial
Corporations — namely, the combination of refuse destructors with central
supply-stations. The heat generated in the furnaces where the town refuse
is consumed may be employed wholly or partly in the place of coal. In the
attempt to wholly replace coal a difficulty arises, even if the quantity and
calorific value of the refuse be sufficient to provide the necessary heat ; it is
most inexpediant to vary the rate of consumption of refuse, as would be
regarded by the variable demand, on account of the enormous size of the
furnaces to meet the steam demand at heavy load, and the difficulty in
securing perfect consumption of the refuse if the rate be varied. Only by
some extensive system of thermal storage, or large secondary batteries, would
it be practicable to avoid the employment of coal as supplementary fuel.
But a uniform supply of heat, obtained with very little additional
capital expenditure in its application above that which is incurred in effec-
tively disposing of refuse, has a value for the economic generation of electric
lighting power which can scarcely be over-estimated, though the actual
amount of electrical energy obtained from this heat forms but a small propor-
tion of the total output. It prevents the slow but steady consumption of
coal in the banked or slow-steaming boilers during some twenty hours
of the day ; enables a large number of boilers to be kept ready for action at
all times without waste, and thus relieves of all anxiety of failing to meet a
sudden increased demand ; and renders the low efficiency of the generating
plant at reduced load, and the never-ceasing absorption of power by trans-
formers, of little moment. For large towns and cities wasteful transforming
systems are rendered necessary by the extensive area of distribution, and
dust destructors, more especially needful in large towns, seem the natural
antidote.
EFFICIENCY OF MACHINERY. 267
In the Oentral Station of the St. Pancras Vestry, eighteen furnaces for
dust destruction are placed round the main building, and connected to
refuse tanks by iron hoppers, by which the refuse is carried from the tanks
to the back of the furnaces. Thence it is gradually carried forward to the
front of the furnaces by the motion of the fire-bars. Every alternate bar
moves on a cam shaft at the back end, and slides on a dead plate at the
other end. Into these bars are coupled, by knuckle-joints, Sorter bars
at a steeper inclination, and placed at the bottom of the hoppers. The
intermediate bars of both the front and back grate do not move. The
moving bars, thus effecting mechanical stoking, are operated by shafting
from the engine-room, while two small vertical engines operate blowers to
effect a heavy forced draught from the ash-pits. Lancashire boilers are used
for steam generation, 30 feet in length and 7 feet 6 inches in diameter, with
two flues 3 feet in diameter, with six tapering cross tubes in each, designed
for a working pressure of 125 lbs. per square inch. They are supported on
cast-iron saddles resting on brickwork near the front, and on cross joists at
the other, in such a manner that the whole of the outside shell below the
water-line is exposed to one large flue, affording considerable heating sur-
face. The large flue carrying the destructor gases to the chimney runs along
the back of the row of boilers, and the gases can be diverted into the large
outside boiler flue by opening valves at the back end, and through the
internal boiler flues, passing out of the furnaces to 15 inches diameter down
pipes leading to the chimney. The boiler furnaces and ash-pits are pro-
vided with air-tight doors, to be closed when the destructor gases are being
used, and the down pipes have valves to be closed when the coal fires are in
use. Grid dampers are also provided at the back ends of the internal flues
for regulating the draught to the ooal fires while the destructor gases are
allowed to pass under the boiler only, and another damper to the outlet of
the main flue into the chimney. Thus the destructor gases can be employed
partly or wholly for steam generation. A Green's economiser is also em-
ployed, heated entirely by the destructor gases.
The average efficiency attained in steam generation throughout the whole
year in a oentral electric supply-station will vary considerably according to
circumstances. A station using storage batteries, and thus having a fairly
equable load upon the engines, should attain a far higher efficiency than can
be attained in a station which supplies steam according to the demand.
Statistics bearing upon this are not published, though measurements might
easily be taken by a water-meter in the supply-main for the feed-water, and
would be of great value. In an alternating current station using boilers
which gave upon test an evaporative efficiency of nearly zo Ihs. of steam per
lb. of coal, the writer found the average efficiency for the winter months
alone to be less than 6 lbs. of steam per lb. of coal. Probably the efficiency
of steam generation in most central stations is between 30 and 40 per cent.
Low as this may seem at first sight, yet as compared with the efficiency of
the next process, that of conversion of the heat energy into mechanical
energy, it will appear highly creditable.
Efflcienoy of Machinery.
Under the very best conditions with triple expansion condensing-engines
working at their mo^t efficient load the production of a brake horse-power
requires per hour at the very least 13 lbs. of steam at high pressure, which
is equivalent to about 1 5 lbs. evaporated at atmospheric pressure. Even
this very exceptional piuctice only represents an engine efficiency of 18 per
cent., the horse power hour being represented in heat energy by the evapora-
tion of as nearly as possible 2§ lbs. of steam from and at 100° Cent. What
270 TYPES OF STEAM-ENGINES.
rates of steam consumption were registered at approximate full-load, half-,
quarter-, and zero-loads :
I. NdP-oondensing.
KilowattB. Total Weight of Steam per hour. Steam per E.H.P. hoar.
Iba. ItM.
319.2 9466 32.22
98.7 SM 44.18
54.5 4330 59.30
o 2092 —
II. Non-condensing^ but aiiperheating 30° F,
203 8429 30.97
106.1 5287 37.17
o 1402 —
II L Condensing Vacuwm 25" ; no supei'heathig.
ao8 5443 19.51
1084 3037 2a90
o 531 —
Combined with a 150 KW. alternator, employing helical spur-gearing
with a ratio 2:1, with a steam pressure of only 70 lbs., superheated, and a
condenser vacuum varying from 26^ to 26} inches, the following rates of
consumption were registered :
Kilowatts. Total Weight of Steam per hoar. Steam per K.H.P. hour*
lbs. Ibe.
17.28
20
22.01
It will be interesting to compare these figures with those for other high-
speed engines which will be given later on. They will show, however, that
effective steam condensation offers possibilities of reducing steam, and
therefore fuel, consumption by nearly 50 per cent., but with reciprocating
engines the practical saving will be somewhat less, and the subsidiary waste
in the condenser air-pump will lessen the advantage. At reduced loads the
gain appears to be still greater than it is at full load, rendering condensa-
tion peculiarly desirable for central station practice. In cases where the
water available for condensing purposes is limited, it has often been thought
advisable to incur considerable expense in cooling tanks and similar means
in order to secure these advantages.
As stated above, when deaUng with the choice of boilers, the experience
of marine and mill engineers, though of the greatest value in connection
with an industry of more recent growth, must be greatly qualified if we
wish to make the best choice for the special conditions of electric lighting.
The peculiarities that must be considered are chiefly the high speed advis-
able, and the demand for high efficiency at variable loads. To these may be
added possibly a natural preference to minimise the prime cost of the
machinery at the expense of its durability, in fear lest the advance of the
science should render the existing machinery antiquated in a short time.
And, moreover, in supply-stations which are most advantageously placed in
the midst of congested districts, the saving of space is a matter of at least
as much importance as in marine engineering, and, as a rule, of far more
importance than in mill engineering.
In industries to which the latter expression applies, almost universal
experience has pronounced in favour of large slow-moving engines, generally
150.33
3484
72.84
1950
38.97
1 150
.175
437
WILLANS ENGINE. 271
•
of the horizontal pattern, with Corliss or other trip-gear, whereby a variable
expansion may be obtained with high-pressure steam and efficient governing
through a wide range of load. And this experience has been largely followed
in alternate current systems, to which some of the above-mentioned pecu-
liarities do not apply so forcibly as with continuous-current systems. The
tendency has been lately to construct alternators of very large size to meet
the requirements of the slow-moving engine, and thus avoid the inefficient,
and sometimes dangerous, use of ropes and belts which were at first, and
are still very largely, employed in this class of work.
For continuous-current low-tension systems the necessity of high speed,
and the limited space generally at the disposal of the engineer, have caused a
strong preference for high-speed vertical engines, and the demand has given
an impulse to the design of these types, so that several manufacturers are
now producing high-speed engines which not only give great satisfaction
when combined with continuous-current dynamos, but endanger the former
monopoly of the low-speed mill-engine.
Of these designs for high-speed engines, Willans' central valve, single-
acting type is the most popular. It appears that at the beginning of
1895 the total capacity of the engine in electric light stations in Great
Britain was 101,390 indicated horse-power, and of this 53»34o horse-power,
or more than half, was supplied by Willans' engines, and little more than
4000 by other types of high-speed engines. The chief difficulties in the
running of high-speed engines are to secure satisfactory lubrication, the
working parts being inaccessible when in motion, and to get rid of the
vibration to which high-speed engines are peculiarly subject. Vibration is
not only fatal to durability, but cannot be permitted when the station is
located in a crowded district on account of the annoyance to neighbours.
In the Willans engine the crank chamber is an enclosed oil- bath, the lubri-
cation is perfect when good oil is used, and the engine requires very little
attention during running. The vibration is reduced to a minimum by the
constant thrust principle.
As to the steam consumption of the Willans engine, it appears that upon
full-load, non-condensing, test efficiencies of 18.45 ^^^* ^^ steam per indicated
horse-power per hour have been obtained with an initial pressure of 170 lbs.
per square inch (triple expansion being used), and 19.45 lbs. of steam with a
pressure of 150 lbs. (compound). And condensing, with a triple-expansion
engine of 500 horse-power, 12.48 lbs. of steam per I.H.P. hour has been
recorded. Directly coupled with a Orompton dynamo, the following tests
are given to show the commercial efficiency of the combination :
Terminal output of dynamo • • • . . • 15? E.H.P.
Indicated H.P. of engine 178 H. P.
L088 in dynamo 7-5 •>
n engine friction 17.8 „
Total efficiency 85.8 %
Efficiency of dynamo 95.3 „
„ engine* . • . • • • 90.0 „
Tests of steam consumption with two such combinations, non-condensing,
are given as 19.2 and 18.9 lbs. of steam per I.H.P. hour, and 24.35 *^^
24.2 lbs. of steam per E.H.P. hour, or 32.63 and 32.44 lbs. of steam per unit
respectively.
It appears, however, that the engine losses are practically the same at all
loads, owing probably to the fact that the cylinder friction represents much
the greatest part of the loss, the lubrication of the bearings being all but
perfect. For this reason the efficiency at low loads is comparatively poor,
and distinctly inferior to some double-acting types which cannot compete
WILLANS ENGINE.
Jho. 99
WillanH Canltal-Valve Engine.
GAS-ENGINES. 2/3
with it at heavy loads. This fact must be taken into consideration whenever
it is intended to run the engine with great variations of load, but is of
small importance when an accumulating system is used. With a constant
frictional loss a full-load efficiency of the engine of 90 per cent, will give an
efficiency of only 81.8 at half- and 69.2 at one-quarter load.
The Bellis double-acting high-speed engine obtains very perfect lubrica-
tion by forcing the oil through channels into all working parts. This is
effected by a simple pump, without valves -or packing, working off the
eccentric from a well in the framing, and discharging at a pressure of about
10 lbs. The durability and freedom from breakdown of this engine has
caused it to be used largely by the Admiralty for driving ship dynamos.
The efficiency claimed at full load is very high, and at low loads is unques-
tionably maintained better than with single-acting engines, owing to the
diminished cylinder friction. Tests of a Bellis-Compton 200 horse-power
combination at St. Pancras Central Station, for 6 hours at full load, with a
mean steam pressure of 121.75 lbs., non-condensing, ^ive a consumption of
17.58 lbs. of steam per l.U.P. hour, 19.89 lbs. per E.H.P. hour, or a total
efficiency of 88.39 per cent
In the gas-engine which has made marvellous strides lately towards high
efficiency, we are not limited to the same extent as in the steam-engine, and
there are possibilities of attaining a much higher efficiency. Already it is
possible with large gas-engines to rival and even improve upon the steam-
engine in efficiency, and if the regulation of speed can be made equally good
it is possible that the latter may fail to maintain its pre-eminence. For
central electric lighting station work especially the possibilities of the gas-
engine are great; for gas can be st>ored ready for immediate use in the
engine without any loss, while, as explained above, it is impossible to do so
with steam, and an immense loss results through the constant variation of
demand. With gas-engines the only cause of discrepancy between the
efficiency under working conditions and under test conditions is the remedi-
able one due to the working of the engines at variable or inefficient loads.
The production of gas may be carried on uniformly, and stored without
loss.
Gfas-engines are open to the following objections for electric light work.
They occupy a somewhat larger space than steam-engines of corresponding
capacity ; they commonly prodiice greater vibration, and often considerable
fluctuations in speed between the explosions ; their regulation is seldom as
perfect as can be attained with the steam-engine ; and the starting of large
engines presents certain difficulties.
As to the variation of efficiency at reduced loads the following measure-
ments given by Prof. Unwin as to the gas consumption per brake horse-
power per hour may probably be taken as typical of the present practice :
Brake H.P. Cable Feet of Gas per B.H.P. honr.
100 21.65
75 23.78
50 28.05
25 40.85
12.5 66.48
For small private installations a gas-engine and dynamo, supplemented
by a secondary battery of sufficient capacity to store the whole of the energy
required for daily supply, is the most satisfactory arrangement ; but a direct
supply, employing a smaller secondary battery to restrain the fluctuations of
E.M.F., and discharging in parallel with the dynamo at the time of full
demand, is often employed. The use of the dynamo as a motor, driven by a
discharge current from the secondary battery, afibrds a convenient means of
8
274 EFFICIENCY OF DYNAMOS.
starting the gas-engine, obtaining the necessary initial compression of the
explosive mixture.
For central stations large gas-engines may be built with several cylinders,
and steadiness obtained by a suitable arrangement of the order of the
explosions, together with great inertia in the fly-wheels and rotating parts
of the generators. In the Belfast Central Station, employing a low tension
' three-wire system with storage, tandem cylinders are employed for the 120
indicated horse-power gas-engines. The explosive impulses occur at each
end of either cylinder, following one another in succet^sion — first an explo-
sion in the back end of the back cylinder, then one in the front end of the
front cylinder, then one in the back end of the front cylinder, and, finally,
one in the front end of the back cylinder. Or, in other words, the four
operations of the '* Otto cycle " take place in the four cylinder ends succes-
sively, admission and compression in one cylinder at the time of expansion,
and exhaustion in the other. A uniform speed of 160 revolutions is main-
tained at all loads, requiring 320 explosions, the speed being regulated by
variable admission of explosive gas; this method producing the greatest
steadiness in running, but lower possibilities of eflicienoy at reduced loads
than the method of missing explosions. With rope-driven Siemens (60
kilowatt) dynamos, a consumption of 26 cubic feet per E.Q.P. is obtained at
full load, the gas being supplied from the Corporation gas-works, and
consisting of coal-gas admixed with a considerable proportion of enriched
water-gas.
It is for alternating current transformer systems that gas-engines
possess a special advantage, since, storage batteries being impossible without
transformation to continuous currents, and thermal storage dificult, much
is lost by irregular steam generation. The great inertia possessed by the
moving parts of most types of alternator will help to modify the tendency to
fluctuation of speed in gas-engines. In the few central stations where gas-
engines have been adopted, some difficulty seems to have been experienced
in the parallel running of alternators, and direct coupling has had to be
resorted to.
The efficiency of the dynamo, or machinery by which the mechanical
power is converted into electrical, although under test conditions at a
suitable load much higher than the boiler and the steam-engine, suffisrs
to an equal, and often to a still greater extent from the variation of
output. While 95 per cent, is easily attained at full load with closed-
coil continuous current dynamos, and 90 per cent, with alternators, some
of the causes of the wasted energy, those of mechanical friction, hysteresis,
and eddy-currents, and expenditure in the magnetising coils remain con-
stant at all loads, while that due to armature resistance decreases rapidly
at low loads (in proportion to the square of the current output). Thus,
while attaining the highest efficiency commonly at somewhat less than the
maximum output, that at low loads is veiy poor indeed. The obvious
remedy for this source of inefficiency is a proper graduated series of units,
engine and dynamo, of various sizes to be employed according to the
demand, and adaptability to parallel running of the larger units. A com-
bination of dynamo and engine is much more rapidly brought into action
than a boiler, so that the elimination of the loss due to variable demand
is far more possible.
*
Efficiency of Distribution.
The final question, that of efficient distribution, has been the subject
of consideration throughout the greater part of this work, only second in
importance to that of uniform regulation througliout the area of supply.
EFFICIENCY OF DISTRIBUTION. 2/5
To sum up our results we may note that with direct distribution, the
simple parallel and multiple wire systems, the wasted energy cannot exceed
the proportion to the total output of the maximum di-op in potential
permitted to the whole E.M.F. With systems of feeder mains it may be
possible to extend the supply to such distances that the fall of potential,
and consequent absorption of power in transmission, will reach 12 per
cent., while the extreme variation with different lamps is kept within half
this amount. The total percentage waste (U the time of maximwin, demand
will thus be less than 12 per cent., and will diminish greatly as the
demand reduces, being proportional to the square of the current output.
The losses by leakage should of course be insignificant.
When storage is adopted a further loss of at least 20 per cent, most
be allowed for, but this, as stated above, is generally far more than com-
pensated for by the increased efficiency of the generating plant supplying a
variable demand, and the reduction of labour and other expenses.
With an alternating current transforming system in which the trans-
formers are permanently connected to the distributing mains, a transforming
loss of 3 to 4 per cent., and a simUar loss in the distributing mains,
high and low pressure, may be expected at heavy load. At moderate loads
the former percentage remains fairly constant, while the latter rapidly
reduces, as with direct supply. With very small or zero loads the efficiency
of the transformer becomes exceedingly low, and in cases where the lighting
is confined to a few hours in the day, an average diurnal efficiency of barely
70 per cent, may result. In many of the central stations employing
small transformers on the consumer's premises the number of units regis-
tered by the meters scarcely exceeds, and is often less than, this proportion
of the total output of energy during the year. Concentration of the centres
of transformation, with secondary distributing systems, so that the trans-
forming capacity may be adjusted to the demand, will reduce this great
source of waste to less than 10 per cent., though an increase in the low
tension distribution waste may be expected.
With continuous current transforming systems the efficiency of distri-
bution will certainly be lower than with alternating currents, on account of
the lower efficiency of rotary transformers ; but as concentration of the
transforming centres is necessary, it should be higher than a distributed
alternating current system, and the possibility of storage will enable a
higher efficiency of generation to be attained.
Series distribution being applied almost exclusively in practice to street
lighting with arc lamps, is not subject to variable demand, nor need the
generating plant be run otherwise than at full load and during the hours of
darkness only. An efficiercy of 80 per cent, is considered ample with
the dynamos employed, and decreased efficiency at light loads does not enter
into consideration. The loss in distribution is also extremely small when
high E.M.F. is employed; for example, with 3000 volts terminal E.M.F.,
and a current density of 400 amperes per square inch., the loss is less than
I per cent, per mile of conductor.
Considerable economy may be effected by a reduction of the street
illumination after midnight when the heavy traffic has ceased, and the
brilliant light of the arc lamps is somewhat superfluous. Por this purpose
several supply stations in this country have lately adopted an arrange-
ment whereby the arc lamp is replaced by a pair of incandescent lamps
(commonly thirty-two candle power) about midnight, supplied from the dis-
tributing mains for interior lighting, these being started by an automatic
switch upon the extinction of the arc lamp. A secondary advantage to be
obtained from this arrangement is that in the event of an accident to the
arc lamp supply generators, or individual lamps, the street is not wholly
2/6 REQUISITE CAPACITY OF PLANT.
thrown into darkness, universally or locally. The automatic switch of the
incandescent lamps has heen also designed to switch off the lamps when
required, the incandescent supply heing uninterrupted, this being effected
by the sending for an instant a current through the series arc system in the
direction reverse to the normal direction, the switch gear being of the
nature of a polarised relay.
An important economic question arises as to the necessary capacity of
the generating and distributing plant for any area of supply, that is to say,
its relation to the total number of lamps connected to the system. When
the demand is mainly from public halls and theatres, shops and clubs,
it may be expected that at times nearly all the lamps will be burning
simultaneously, and provision must be made to meet the full demand, or
nearly so. Generally, however, this full demand will last for but a short
period every evening, and the plant may be overloaded for some time,
with probably a somewhat low efficiency and some extra fall at the distant
lamps; so that a provision for some 80 per cent, will be ample, running
no danger of failure. In the early history of an installation a larger pro-
portion of the maximum demand possible may be expected than will be
the case when it is more extensively used in private houses. The latter
are generally slower to adopt a new illuminant, for obvious reasons, and
the conditions are somewhat different, the demand seldom exceeding half
the maximum possible ; a much larger number of lamps in private houses
may therefore be connected to the supply system than is provided for by
the limiting capacity of the plant. This pro(K)rtion can only be decided by
judgment and experience being determined by the habits and customs of
the consumers in the district.
A further important question is concerning the extent of reserve gene-
rating plant required to prevent failure through accident. In deciding
this much will depend on the nature of the plant, the rapidity with which
repairs can be executed, &c. Of late, designers of machinery have expended
much thought on provision for rapid repair of the parts most subject to
injury, such as armature coils, &c., reserve parts being kept in stock. Still
it is considered absolutely necessary by most engineers that the capacity of
the total generating plant should exceed by that of one of the largest units
the total output that must be provided for. A large secondary battery can
take the place of this reserve unit only when repairs can be speedily
executed. For boilers similar reserve is required.
Since the interest on the capital outlay in generating plant and distri-
buting mains forms in general a large, if not the greater, part of the cost of
production, it is evident that the cost per unit of the energy suppUed
depends largely upon the time during which the lamps are kept alight, and
the charge ought to be made to correspond. Generally, the longer the
average time of burning, or the larger the total consumption of energy by a
lamp of fixed size, the cheaper the total cost of supply, including the neces-
sary capital expenditure. The following methods of charging for electrical
energy have been put into practice in place of uniform charges, each of
which presents certain advantages :
(i) A fixed charge according to the number of lamps installed, with a
small additional charge per unit consumed.
(2) A fixed charge according to the maximum demand with additional
charge per unit. The maximum demand at any time is measured by a
special indicator. The most successful type has been devised by Mr. Arthur
Wright, and is practically a differential air thermometer causing an over-
flow of liquid, registering the maximum current used at any time. The
indicator is slow in action, taking some ten minutes to cause the full over-
flow, so that an accidental excess current for a few minutes is not registered.
VARIABLE CHARGE TO CONSUMERS. 2//
(3) Sliding-scale charges according to load of full installation, or maxi*
mum demand. This has the usual objection that applies to all sliding-scale
charges.
(4) The use of double circuits, with separate meters, with a two-way
switch arranged that one circuit only can be used at a time. On' one circuit
are connected only such lamps, or electric motors, as are likely to be employed
in the daytime ; on the other circuit such as will be necessary ddring even-
ing hours. A smaller charge is made for units registered upon the former
circuit.
(5) Meters have been designed with two coils, giving different itites of
registration with the same consumption of energy. The coils are alterna-
tivcy and brought into action, the one during hours of light and the other of
heavy load, either by clockwork or a switching gear controlled electrically
from the central station.
PHOTOMETRY.
The increase, during the last few years, of the commercial as well as the
scientific importance of an exact method of measuring the luminous energy
of artificial light has resulted in the establishment of the science of photo-
metry on a definite and precise basis.
This science has for its object the accurate measurement or comparison'
of all light sources and the determination of their power to produce a certain
amount of illumination on a given object.
The fundamental law of photometry is that the intensity of illumina-
tion on any point varies inversely as the square of the distance of the
source of light from that point. Hence it is evident that the measurement
oi a source of light must be made by comparison with another source taken
as a standard, so that the two essential requirements are (a) a photometer,
or the means of comparing lights or degrees of illumination, and {b) a light
which shall act as a standard of comparison.
An adequate explanation of the law of inverse squares will materially
assist in the solution of many problems which will occur, and will show the
basis upon which different photometers are constructed.
Light is emitted from a luminous body in all directions in straight lines,
and it is therefore evident that if any two rays are taken which radiate
from a point in this body, they will diverge in proportion to their distance
from that body, and their power of illumination will decrease in the
same ratio. The following diagram affords a simple explanation of this
point :
Fig. 100.
A
From a point on the radiant A a number of rays are emitted which
diverge proportionately to their distance from the source.
Suppose at a distance of i foot they illuminate a small screen, at 2 feet —
i.e., twice the distance — they will illuminate a screen four times the size, and
at 3 feet — i.e., three times the distance — ^a screen nine times the original
size, therefore the light which at one foot was spread over a screen of x
area, is at 2 feet spread over 4X area, and at 3 feet, gx area ; therefore it is
280 LAW OF SQUARES.
evident that the illumination at any point on the screen 2 feet distant can
only be one quarter of that on the screen i foot distant, and in the
same way the light at 3 feet is spread over a screen nine times as
large, and therefore can only be at any point on this surface one-ninth the
intensity.
This explanation gives a good idea of the meaning of the law of inverse
squares, but it is not a mathematical exposition, since it neglects the fact
that every point on the screen is not the same distance from the source of
light. To obtain such a result the screen would have to be a perfectly
circular hollow globe in the centre of which the light source was placed, then
the mathematical proof of the law would be as follows :
Let two hoUow spheres be represented in section, One inside the other
(Fig. loi), and P be the luminous point in the centre; this point will illumi-
nate the whole of the interior of the hollow shell C^, 0^, C^, of the radius
R^ equally. If the shell C^ Cp C| be removed, the light P will illuminate
Fio. loi.
the whole of the interior of the shell 0„ C„ C, equally, but as this shell
presents a larger surface to the light P, it is obvious that the intensity per
unit surface must be less.
If I represents the intensity of illumination in a sphere, I = ^
^^r
where Q = quantity of light and 4irT^ = area of the sphere. This applies
equally for any sphere, consequently it follows that for Cj, Oj, Cj of nuiius
^i» ^i = p Q > and for C„ C„ 0, of radius R„ I, = ^ ', then.
Li_ Q /
Q . I, R,'
3
4irlV ' ' h~ V
Thus suppose the shell C,, Cp C^ has a radius Rj of 1 2 inches, then the area of
the surface of the sphere Op Cp C, will be 1809.5 square inches, taking the
area of the surface as equal to four times the square of the radius multiplied
by 3. 141 59, and the shell C,, C,, 0, with a radius R, of 24 inches, will have an
area of 7238 square inches. Now since this area (7238 square inches) is four
times the smaller area (1809.5 square inches), it follows that the illumination
EARLY PHOTOMETEES. 28 1
on every unit area of tbe interior surface of the larger shell C,, 0,, 0, is
one-fourth of that on the same area of the interior surface of the smaller
shell Oi- • ■
For the purpose of demonstrating this law the source of light has been
considered as a point, whilst in practice the source of light is usually a
surface of several square inches, and in some cases, such as that of an
electric arc, covered with an opal globe, considerably more ; this, however,
does not vitiate the proof, as a body of light is made up of an innumerable
quantity of incandescent particles, each of which may for a theoretical proof
be considered as a source of light.
From the law of inverse squares it follows as a corollary that the
intensity of the source of light varies directly as the square of the
distance.
On these two laws, which are the basis of the science of photometry, the
following photometers have been constructed :
Bougv/er' 8 Photometer (a.d. 1729) consists of a semi-transparent screen of
white tissue paper, ground glass, or thin white porcelain, divided into two
parts by an opaque partition at right angles to it, with the two lights under
comparison placed one on each side of this partition, so that each illuminates
one-half of the transparent screen. The distance of the two lights is
adjusted Until the two portions of the screen, as seen from the back, appear
equally bright. The distances are then measured and the intensity of the
two lights calculated in comparison with each other.
FowsaulCMPhotomeUT (aj). 1850), much used in France, is a modification
of Bouguer's. In place of the fixed partition between the two illuminated
halves of the screen, which casts a shadow and renders a comparison some-
what difficult, Foucault introduced a movable partition with an adjusting
screw, so that when the partition was moved a certain distance from the
screen the two illuminated portions of the screen were contiguous and could
be more readily compared.
Rwmfardds Photometer (a.d. 1792). — In this photometer, as in Bouguer's,
the two lights to be compared are placed on the same side of the screen, but
instead of comparing the illumination of the two halves, the intensities of
the shadows cast by an object placed in the path of the rays of light are
used as the means of comparison. On two tables, one 1 2 feet and the other
20 feet long, by 10 inches broad and 35 inches high, the ends of which are
fixed at an angle of 60 degrees to each other, the two sources of light, fitted
to movable platforms, are placed. By means of endless bands attached to
the movable platform the lights are capable of adjustment by the observer
whilst taking his observations. The screen on which the shadows are cast is
fixed in the back of an open-fronted box, thus excluding the extraneous
light as much as possible, and in order to avoid the inconvenience arising
from comparing two shadows projected by the same object, shadows which
are either too far from each other to be compared with certainty or when
they are close enough together are probably obscured by the object itself,
two objects are used, and in such a position that the shadows which they
cast can be seen between them. The objects used to produce the shadows
are narrow cylinders, movable about their axis, each fitted with a vertical
wing of the same length as tbe cylinder and about one half -inch broad ; this
is necesvsary to enable the operator to adjust the shadows to the same breadth,
as the lights are seldom equi-distant from the screen.
The Bunsen Photometer (a.d. 1843). — Bunsen adopted an entirely different
principle for his photometer. A sheet of paper, on which a large grease
spot was made, was fixed on one side of a box in which a small gas flame was
kept burning. The reverse side of the disc was turned towards one of the
lights under comparison, which was then adjusted until the grease spot
282 LETHEBY'S PHOTOMETER
apparently disappeitred, when the distance from the disc to the light was
noted and the same procedure adopted with regard to the other light. Thus
he obtained the distance at which the two lighta gave equal illumination and
from it calculated their relative luminosity.
This arrangement had an unfortunate defect : the gas flame in the box
was not a constant, so that, although the principle was admirable in every
way, it met with but little favour. The idea, however, was communicated by
Lyon Playfair to Alfred King, who constructed the modified an-angement
now known as the Bunsea open-bar photometer. In this modification King
plac^ the two lights one at each end of a bar loo inches long, and the greaae
spot disc in the centre, and by altering the distances of the light from the
disc until it was equally illuminated on both sides, obtained a direct com-
parison.
LeUieby considerably improved King's modification by shortening the
loo-inch bar to 60 inches, constructing a sighting box for the disc with
liethebj'B Photometer.
mirrors bo arranged that refiections of both sides of the disc could be seen
simultaneously, and so surrounding the two sources of light with screens
that the eyes of the observer were protected from direct tight and the flames
themselves from draughts. Unfortunately the very trouble that Letheby
sought to prevent by boxing up the flames he cultivated. It was found
that gas and candle flames in such a photometer were seldom steady, and
when a sluggish flauie, such as that of the one-candle pentene flame, was used
in the photometer, the unequal draughts induced seriously afiected the
results.
This defect was so marked that it was found necessary during an
inquiry on standards of Ught to somewhat modify Letheby's arrangement in
order to obtain a steady flame. This was done by enclosing the top of
Letheby's boi-ends with flat boards having ciroular apertures of about
6 inches diameter cut in them. By thus slowing the up-current of air a
sufficiently large and* steady volume was obtained to produce a practically
EVANS" PHOTOMETBB. 283
steady flame. With these modiflcatione this photometer is now largely in
tise ander the name of the Improved Letheby or Tootey Street pattern
Kio. 103.
Tooley Street Pattern Photometer.
photometer, in consequence of its having been first fitted up at Tooley
Street Oas-Teeting Station under the Oas B«feree'8 instructiona.
Evaus' Fbotometer.
Various other modifications of the original Letheby photometer, chiefly
s regards its outer casing, have been constructed, such as the Evans, the
284 TABLE FHOTOMETEB.
Tower, Canadian, Ac^ but in all of them the above essential featares are
carefully preserved.
The Evans Photomeler, introdaoed in 1858 for the purpose of testing the
illuminating power of street lamps, is a reversion to the 100-inch bar photo-
meter with several modifications. All the essential parts of the photometer
are enclosed in a huge rectangular box fitted with ventilating tops over the
two ends and a series of doors in front to enable the operator to attend to
the various parts. The Bunsen disc is clamped in the centre of the bar, with,
the gas flame fixed at one end 50 inches from it, whilst at the other the
standard candles are mounted in a travelling holder. Headings are taken
through a glass window in the central door ; a winch fixed immediatelj
below and attached to an endless cord enables the operator to alter the
position of the candles until the two sides of the disc axe illuminated
equally.
Barcaurfs TMe PholomeUr. — ^This modification of Foucault's photometer
has been recently introduced for the purpose of the official testing of the
London gas supply under the direction of the gas referees.
The following is the official description and instructions for its use:
The several parts of the apparatus stand upon a well-made and firm
table, 5 feet 6 inches by 3 feet 6 inches, and 2 feet 5 inches high. The
upper surface of this table is smooth, level, and dead black. Upon this are
placed or clamped in the positions shown in Fig. 105 :
i) The gas meter;
2^ The gas governor ;
3) The regulating tap;
[4^ The '' Sugg's London Aigand, No. i " burner ;
;)
(7) The photoped ;
i5) The connecting pipes ;
6) The Pentane ten-candle lamp ;
(8) The flBrorthometer ;
(9) The stop clock ;
(10) Dark screens : mirrors.
The Gob Meter. — The meter is a wet meter, constructed with a measur*
ing drum which allows one-twelfth of a cubic foot to pass for every revolu-
tion* A hand is fastened directly to the axle of this drumy and passes over
a dial divided into one hundred equal divisions.
The Oaa Ocvemor, — ^The gas governor must be such as will effectually do
away with any variation of pressure produced by the working of the meter
or other causes. A loose bkckened screen, eight and a half inches high by
six inches wide, should be placed upon the base of the governor near the
tank to prevent heat from the Argand burner from warming the water in
thetank^
The Regulating Tap, — ^This must have a large well-fitting conical plug
with a round hole on each side of such a size as to allow gas to pass at the
rate of about four cubic feet per hour, under the pressure at the outlet of
the governor. In addition there must be narrow saw cuts on opposite sides
of the two holes when viewed in plan, which will allow an additional passage
of about two cubic feet of gas per hour when the tap is so turned that the
holes and the saw cuts lire both opposite the orifices of the fixed part of the
tap. The construction of the tap is shown in Fig. 106. The index must
be secured to the conical plug without any play, and its pointed end most
pass over a scale graduated in degrees upon an arc of not less than eighty
millimeters radius. The arc is to extend over 90°, and the degrees are to
be numbered from o to 90. The arc is to be made of white enamel glasSy
and the divisions are to be etched upon it, and the marks filled in with
black. The tap is to be off when the pointer is at one extremity of the aro
TABLE PHOTOMETER.
28s
IM
Harcourt'a Table Fbotometer.
286
TABLE PHOTOMETER.
at o', and fully on when it is at the other ertremity at 90°. The small
bole should be fully open at about 20°, so that the action of the saw-cuts
may extend over the remaining portion of the arc.
The tap must be kept clean and sufficiently lubricated lo work easily.
The " •Sii'jys London Argand, No. i" Bu-nier. — Tliis is the burner
described in Appendix A. It is to be mounted upon a Iripod with flat pro-
jecting feet, HO that ite position upon the table can be udjusted at any time.
It may be clamped into position by three ^-shaped clnmps, each made to
pinch upon one foot by the action of a single caipentei-'s sci'ow. The con-
Htruction of the foot and clamp is shown in Fig. 108. The height of the
top of the steatite burner in 353 millimetres above the table. The axis of
the burner should be vertical. If it is found to lean in anv direction, paper
or canl should be inserted under one or more of the feet until it is found
to be vertical after being clamped in ixwition,
Tlw. Contiecliiiij Pipe^i. — These are to be made of half-inch (outside
measure) com])OKilion piping. They 1 ire to be connected with the different
pieces of apparatus by three-eighth -inch unions, except in the case of the
gas meters, where the unions bejoiigiug to the mi'ter may be i-etained. In
TABLE PHOTOMETER. 287
all cases the boas of the union U to be attached to the apparatus and the
cap a,nA lining to the ends of the connecting pipe. These pipes are to be
placed above the table. No grooves, recesses, or holes, other than the screw
holes for the screws referred to, are to be made in the table.
Tfis Pentane Ten-Candle Lavtp.- — Tliis is described under Standards of
Light. The lamp need only be placed in position upon the table, but for
permanent use clamps corresponding to those used to secure the feet of the
London Argand sliould be employed. The height of the top of the steatite
burner is 353 miilimetrea above the table. The construction of the screw,
swivel, plate, and clamp is shown in Fig, 107.
The }'koto]>i'd. — The photoped is represented in Fig, 108 ; it consists of the
following parto : a plate, 100 millimetres square, withacentralhole 21 milli-
metres square. This is held in a vertical position by an upright support so that
the centre of the square is 400 millimetres above the table. The upright is
carried by a tripod similar to that used for the London Argand and secured in
the'same way to the table. To one face of thesquareplateisfaatened, by two
binding screws, a damping-plate, 60 by 40 millimetres, also with a central
hole 21 millimetres square, so that the two openings are opposite one
another. A piece of suitable white paper is pinched between the two plates
BO as to cover the openings and project a Uttle way below the clamping-
Vio 107.
plate. The clam ping- plate carries centrally a borizontal tube about 35
millimetres in diameter and 30 in length. In this slides smoothly a smaller
tube containing a diaphragm in which a rectangular alit, 35 by 7 milli-
metres, has been cut. To the upper surface of the larger bra.ss plate, and
on the same side as the clam ping- plate, is fixed a strip of glaj«, so that the
lower edge is close to, and exactly parallel to, the plate, while the upper
edge is so much in advance a& will allow the reflection of the flames described
on pige 289 to be observeil.
The photoped should be plumbed vertical. If it is found to lean sensibly
towards or away from the lamps, jtajier or card should be inserted under one
or more of the feet until it is found to be vertical after being clamped in
Dark Screens, Mirrors, Meaaiirimj RcfU. — Five dark screens are provided
in order to prevent the inaccuracy and inconvenience to which stray light
would give rise.
The first is placed between the burners and the photoped in the position
shown in Fig, 105. This.icreen 1^500 millimetres square with two rectangular
openings. The oiiening to the left is 40 millimetres wide and 55 high, and
iCs lower edge is 350 millimetres above the table. The opening to the right
is 50 millimetres wide, its lower edge is 340 millimetres above the table,
288
TABLE PHOTOMETER.
and it extends to the top of the screen. The centre is carried by a wooden
foot about 500 by 100 millimetres and 30 thick. Gare must be taken that it
is so adjusted that the whole of the flame under the tube of the ten-
candle lamp and the whole of the chimney and burner of the Argand can
be seen through all parts of the slit of the photoped when the paper is
removed for that purpose. The foot may then be fastened to the table by
means of two hinges, so that the screen may be folded down when the
position of the lamp is being verified and may be easily replaced.
Fio. 108.
400
r
G
®
Ji
u l I T T 1 7 T JLU
Scalt of MiUinuttn
V
r
I ! r
^
:x
I
The second dark screen consists of a piece of black velvet or black doth
350 millimetres square, stretched on a frame and supported so that its lower
edge is 1 50 millimetres above the table. In this is cut a hole 50 millimetres
square with its lower edge 380 millimetres above the table. This screen is
placed close to the photoped, but on the opposite side to that facing the
lamps, and with the square hole opposite the square hole in the plate of the
photoped. To the right side of the frame is hinged a light frame 350 miUi-
metros high and 300 wide, with its lower edge 150 millimetres above the
TABLE PHOTOMETEB. 289
table. On this also is stretched hiack velvet or black cloth. This prevents
the illuminated dial of the meter or arc of the regulating tap from inter-
fering with the photometric observatiooa, while at the same time it can be
readily moved when these are to be observed.
The third dark screen is about 500 millimetres wide and 570 high. The
fourth is about 450 wide and 570 high. These may be card painted dead
black, or of thin wood, and may be placed approximately in the positions
shown in Fig. 106, and with their lower edge 180 millimetres above the
table.
Ihe fifth dark screen consigta of a piece of black velvet or cloth large
enough to form a black background to the Umps when viewed from the
FiQ. 109.
Uaicourt's Table Photometer.
photoped. It is best placed upon the wall, but if that is inconvenient, or
other objects intervene, it should be supported on a stand, but always so as
to be at least 300 millimetres behind the flames of the lamps.
Two email mirrors ajfl carried on light stands. One of these, made of
ordinary flat silvered gXass, is vertical, and is so placed as to enable the gas
examiner, when seated at the photoped end of the table, on moving his
head to the left of the second dark screen, to see by reflection the tip of
the flame of the ten-candle lamp through the mica window in the tube C,
The other, which should be about izo tnillimetres in diameter, is convex,
and should have a radius of curvature of about 400 millimetres. It is
placed on the observer's right, and is so inclined that it casts a diverging
290 KADIAL PH0TUMETEK6.
beam of subdued light upon the divided arc of the regulating tap, the
face of the meter, upon the aerorthometer, and upon the gas ezaminer^a
note-book.
All the apparatus on the table upon which light can fall, and which
might by reflection illuminate the pbotoped or catch the eye of the operator,
is to be painted dead black ; or, if of finished brass, it is to be bronzed before
being lacquered.
The correct position of the photoped and of the two burners is to be
verified as follows : Each burner is provided with a measuring-rod securely
fastened transversely to a cylindrical and shouldered plug which just fits
into the steatite ring and rests upon it. The rod belonging to the ten-candle
lamp is to be 1 .000 metre from the axis of the plug to the extreme point.
The rod belonging to the London Argand is to be 1.265 i^^^^i^ from the axis
of the plug to the extreme point. Each rod is to be balanced about the plug.
Each must be capable of being placed in its burner without disarranging
the burner, except in the removal of the glass chimney of the London
Argand or the conical shade of the ten-candle lamp. Each rod should
terminate in a rounded ivory point. When these rods are in position upon
their burners, and the long ends are moved gradually round towards the
photoped, they should just come in contact with the paper under the clamp-
ing plate at the middle point.
A third rod is provided with two plugs, one to fit each burner, and with
their centres exactly 0.522 metre apart. The two plugs should just drop
into the steatite rings oi the two burners. If any one of these tests shows
the burners to be incorrectly placed, their position is to be altered until all
the measurements are correct. After this process the burners are to be
lighted, the flames turned low, and the reflection of one is to be observed
over the other on the glass of the photoped. If the reflection appears
central, the photoped is symmetrically placed with respect to the two
burners ; if not, the nut on the standard is to be loosened and the plate
turned until the reflection is central. The two lights are then to be turned
up and the slit is to be moved in or out until the two rectangular spaces
illuminated by the two lights just meet but do not overlap.
Fig. 109 shows the complete apparatus as fitted on the table ready for use.
Badial Fhotometers.
The photometers above described are all arranged for testing the rays of
light emitted in a horizontal direction only, and afibrd no means of estimating
those directed either above or below that line.
The introduction of electric light and of high- power gas burners for the
improved illumination of open spaces and large areas, however, led to a
reconsideration of the methods for estimating the value of the various
systems then in use.
Before their introduction it was considered sufficient to estimate the
intensity of the luminous rays in a horizootal direction only, irrespective of
the value of those rays which are actually utilised in practice. Such a
system was doubtless useful in those cases where burners of similar primary
construction, such as flat flame and Argand burners, were employed; but
with the introduction of the various improved forms of burners and
lanterns such as are now in use, a modification of these methods became
necessary.
In order to ascertain the true value of a luminous agent, it is necessary
to determine the power of those rays falling below the horizontal line, and
indeed through the whole of the semicircle from the vertical line above to
the vertical line below the point of illumination.
HARTLEY'S PHOTOMETEB. 291
This will be evident when it is remembered that when two lights are
opposed to each other in a horizontal direction and a vertical screen is placed
between them, the rays that impinge thereon are in each case horizontal
rays and therefore comparable, and the intensity of the source of light can
be measured under the law of inverse squares. If one of the lights is
moved through the circumference of a circle of which the centre is coincident
with the centre of the disc, the number of rays impinging on any unit area
of the disc will, provided the disc is kept in its original vertical position,
decrease as the position of the light becomes more remote from the
horizontal, in ratio to the cosine of the angle formed by the horizontal line
and the line which joins the light and the centre of the disc. It is quite
evident, therefore, that when the light is raised through a quadrant the
number of rays impinging on the disc will be nil, whereas the burner may
really be emitting a considerable amount of light in this direction.
The loss of light due to reflection and absorption when the two lights
are not in the same plane with the disc is no inconsiderable amount, and
would of itself prohibit the use of the vertical disc
A series of experiments made upon lights of various powers and in
different positions, with the disc arranged, first, in the usual vertical
position, and secondly, so that the light from the two sources should
impinge upon it at equal angles of incidence, clearly demonstrated this
point.
After correcting the results obtained when working with a vertical disc
for the diminished number of rays impinging upon the disc when the light
is at different angles, the value obtained was deducted from that found by
estimation with the disc arranged for equal angles of incidence, and the
difference between the two results calculated into percentages. By this
means it was found that, when the burner is at an angle of 22.5° above
the horizontal, the average loss due to reflection from the vertical disc is
4.4 per cent. ; at 45° it is 12 per cent., and at 67.5° 69 per cent.'
It is obvious, therefore, that the method of estimating the illuminating
power of angular rays by means of a vertical disc is erroneous.
By arranging the disc so that the angle of incidence is equal on either
side, both the proportionate number of rays impinging thereon and the loss
due to reflection are equalised.
HcvrUei/a Universal Photometer, — This instrument consists of a narrow
table 1 1 inches wide, 2 feet 6 inches high and 5 feet 6 inches in length, in
the centre of the top of which a slot runs practi^ly the whole length. The
base of the standard burner passes through this slot and is connected by a wire
passing over pulleys to a winch- handle fixed in the centre of the table, as in
the Evans' photometer. The disc carrier is mounted on a stand which also
passes through the slot in the table, and is capable of adjustment. To the
base of this stand, in a vertical line with the disc, is fixed a pointer, main-
tained at the zero of a scale, which can be moved in a groove throughout the
length of the table. The burners under examination are supported on a
sliding pillar, the base of which, like the photometer, is fitted with levelling
screws and plumb-lines. For use with this photometer. Fig. no, Mr. Hartley
calculated a series of tables which greatly facilitated its use. When first
constructed the disc was rigidly fixed in the usual vertical position, but
later this was so altered as to be capable of adjustment to any angle
required, so that the lays from the standard and the burner under
examination, whatever its position, would impinge on the disc at equal
angles.
DihdirCa Eadial Photometer. — This photometer, which was designed in
1S83 to estimate the angular rays emitted from a light in any position,
consists of two vertical supports, one of which is permanently fixed to a
292 HARTLEY'S PHOTOMETEB.
base-board or foot, while the other OD the right hand (Fi^. iii) tmrels on
roUera on th« same base-board in such a position that it will run in front of
the fired anpport.
These two uprights are connected hy a bar, the ends of which work upon
trunnions or oues attached to blocks, which travel in grooves in the np-
rights, and can be clamped in any desired position. One end of the bar is
attached to the front of the fixed upright, while the other end is attached to
the travelling upright at the back, so that when the two uprights are in
juxtaposition the bar is perpendicular between them. The centres of the
trunnions oorrespond in poeition to the centres of the two graduated dial-
platee in front of the uprights, the distance between the centres of these
dial-plates being 50 inches. By this means, whatever position the bar may
be in, the dUtanee from the centre of one dial to that of the other is con-
stant. In front of the dial-plate on the travelling upright the screen or
disc-bolder is fixed, so that its centre is coincident with the centre of the
dial.
Attached to the block in the groove of the travelling upright support is
HarUer's Univenal Photometer fitted with Dlbdln's Botating Duo.
a horizontal bar canning the staodard, which is supported in front of the
borisontal bar by a travelling carriage, fitted with rollers and an endless
cord and winch conveniently placed on the right-hand side of the graduated
dial on the support, by which it con be moved in either direction horizon-
tally. To the block carrying the photometer-difc a brass rod, forming the
segment of a circle, is fixed, to carry a velvet curtain to screen o£f extra-
neous light when readings are being taken.
The two dial-plates are graduated, the larger one on the fixed support in
degrees and the smaller one on the travelling support in half degrees, Uj©
latter being numbered as whole degrees for the purpoae of facilitating the
setting of the disc for equal angles of incideuce, so that when the bar is set
(say) at 40°, the disc-pointer is to be set at the point marked 40°. It will
then be in the proper position — vij. 20°. The disc may be arranged to
work automatically with the movement of the bar by means of a simple
mechanical appliance, so that, whatever may be the position of the bar, the
diisc will be at the correct angle.
A brass rod is used for adjusting the position of the burner, &c., under
examination, and i& pushed through the centre of the block and trunnion on
DIBDIN'S RADIAL PHOTOMETER.
293
the fixed upright support ; it is then at right angles with the plane of the
dial, and projects exactly through its centre, by which means it is easy to
fix the correct position of the flame in front of the apparatus. The light to
be tested may be brought forward to the full extent that can be attained by
the disc and standard, which, obviously, can be regulated as desired, so that
the size of the burner or lantern to be tested by this apparatus is practically
unlimited, due regard being paid to the length of the bar and the power of
the light.
In order to estimate the illuminating value of any lamp it is fixed on the
support attached to the block of the fixed upright, and accurately centred
with the dial-plate, which is then lowered to the bottom of the groove in the
support. The block in the travelling support is next raised, thus bringing
it immediately over the burner, the travelling upright being in fi*ont of the
fixed support, and the pointer on the bar indicating 90° on the large dial-
plate. The photometer disc is arranged for equal angles of incidence by
Fio. III.
Dibdin'B Radial Photometer.
turning it until its pointer is at 90°, and a reading is taken. The clamp
holding the top block in position is then loosened, and the travelling
support moved away from the fixed support until the bar is at an angle
of 80°, when the block is again clamped, the disc adjusted to 80°, and
further readings taken, this adjustment and measurement being repeated for
each degree or 10 degrees as desired, until the horizontal rays are estimated.
The block suppoi-ting the light is then raised to the top position, the bar
adjusted for the desired angle below the horizontal, and a series of readings
taken until the downward vertical rays are estimated.
Comparative tests of various burners should be so conducted as to show
the actual work done by them, not only in one but in all directions. With
Argand and all other circular burners, this can be done by making one
series of tests from the vertical above to the vertical below, at every
10 degrees. But in the case of fiat-flame burners it is necessaiy that this
series should be made in duplicate, one with the flame flat, or at right angles,
294 DIBDIN'S EADIAL PHOTOMETEE.
Flai'flame Bumera — lUumincUing Power of Horizontal Bays.
Burner
Bamer
Burner
Fosltion of Flame.
No. I
No. 2
No. 3
Candlei.
Candles.
Gandlea.
Flat to Photoxneter Bar
B •
30.8
24.2
8.5
Flame turned
. 10*
30.8
24.2
8.5
f» »» •
. 20"
30.9
24.3
!-5
»i
• 30:
30.9
24.2
!-5
t
. 40
30.8
24.0
8.4
)
• 50'
30.2
24.0
8.3
f
. 60**
30-1
23.8
8.2
f
• 70'
30.2
23.5
8.2
»
80'*
29.8
22.4
8.2
Edge to Bar ,
. 90^
24.4
20.3
7.9
»
f
. 100"
28.7
21.6
8.2
11
I ID'
29.6
22.8
8.3
»i • •
120*
30.3
23.5
f-3
»i • "
130'
30. s
23.4
!-3
W »l • •
140"
30.5
23.2
!-3
M »l • •
'5°!
30.5
23.4
8.4
If »l • •
i6o'
30.1
23.5
8.4
»I »» • •
170*
30-4
23.2
8.3
Flat to Bar „ » • •
i8o'
30-3
23.1
8.4
i» »» • •
190:
30-4
22.8
8.4
» It • •
200
. 30.8
23.0
• 8.4
It • •
2I0'
30.8
22.7
8.4
1 • «
220'
30-7
22.9
8.3
>f »» • •
230;
31.0
22.9
f-3
>t It • «
240
30.6
22.8
8.3
ti It • •
250"
30.1
22.0
8.2
It tt • «
260"
29.5
21.2
8.1
Edge to Bar „ „ • .
270*
25.0
18.6
7.8
M ft • «
. 280"
28.5
20.6
7.8
«4 »« • <
. 290*
29.5
21.9
8.1
M It • '
. 300*
29.7
22.2
!-3
ft 11 • "
. 310^
29.8
23.0
8.3
ft tt •
. 320*
303
23.0
8.4
♦» ff •
. 330'
30.3
23.5
!-3
ft II • "
' 340'
30-5
23-4
8.4
. ft tt •
. 350'
30-7
235
8.4
Flat to Bar „ „ . .
. 360'
309
23.4
8.S
Flat-flaine Bumera — Illuminating Power of Angular Bays.
Direction of Raya.
Burner No. i
Candles.
Burner No. a
Candlei.
90** above horizontal
27.8
8.9
8o- „
29.2
9.0
70* „
29.0
9-3
60 „
30.5
9-3
50- M
30.8
Vr
40' t.
309
30; t.
30-3
9.4
20* „ „
30-4
9.3
10' „
29.4
9-3
Horizontal .
29.8
9-7
10* below horizontal
29.9
9.9
20'' „
30.2
10.0
3°:
)i II *
30.2
10. 1
40''
» It <
29.8
10.
50**
II II •
29.8
10.0
60'* ,
> ft ■
30.0
ia7
70** ,
1 It •
29.2
ia3
80"
» ft •
28.7
11.2
90-
)t It •
19.6
5.8
DIBDIN'S KADIAL PHOTOMETER.
295
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296
RADIAL PHOTOMETRY.
to the disc of the photometer, and the other with the edge of the flaine
towards the disc. An extensive series of experiments on this point has
Fia. 112.
Flat to Bar.
Fig. 113.
Flat to Bar.
Horizontal Rays.
Flat-flame Barner, No. i.
Aogular Rays.
shown that very considerable differences exist between the quantity of light
emitted from the flat surface and the edge of the flame in various burners
Fig. 114.
Flat to Bar.
i
bo
Flat to Bar.
Flat-flame Burner, No. 2. — Horizontal Rays.
(see page 294), this difierence varying from 10 to 35 per cent, of the light
emitted from the flat surface. It is very necessai-y, therefore, that two
RADIAL PHOTOMETRY.
297
such series of tests should be made and an average taken to obtain the repre-
sentative value of the burner.
Fio. 115.
Flat to Bar.
Fig. 116.
Flat to Bar.
Horizontal Rays.
Flat-flame Burner, No. 3.
Angular Rays.
This photometer is of special value in the examination of burners shaded
by globes, reflectors, <&c., which may be tested at every degree where neces-
Fio. 118.
With Compoand Reflector.
Without Reflector.
Union Burner, No. 4. — Angular Rays.
sary, and thus most valuable comparative results obtained. The table on
p. 295 contains a typical set of results, also put in diagrammatic form, which
were obtained from three Argand burners fitted with different shades.
298
RADIAL PHOTOMETRY.
6
M
9
2
S
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BADIAL PHOTOMETRY.
299
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RADIAL PHOTOMETRY,
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I
HAECOUET-S HULOl'HOrOMETEB.
llaremcTt's ffolopholometer.* — The hoi o photometer la an instrument
designed, like the " Kadial " Photometer, to measure the light emitted in
every direction by any luminous source. It is mounted upon a table capafaie
HarcoDit's Holopbotometer.
of beins moved nearer to, or farther from, a fixed table containing a graduated
bar witn movable disc, and having a standard lamp fixed at the zero of the bar.
The light to be meoaured is mounted upon, or is in rigid cannectiou with,
the movable table, and is therefore not moved during a Heries of readings.
• "Journal ot Gaa- Lighting," Julj 17, l883.
302 THE HOLOPHOTOMETER.
The inBlrumnnt conoiflto of an axis working friction-tight in a ooU&r
flupported by a vertical pillar. The axis is accurately fixed at the same
height, and in a line with the centre of the disc. At the end of the arm
ueoreet to the disc is placed a larger min-or, with its centre concentric with
the axis but so arranged that the plane of the mirror may be ' inclined and
clamped at any angle to the axis, and at the other end is fixed a telescopic
arm, carrying a small mirror, which is capable of being turned in anj
required position. The arms beiog rigidly fixed to the rotating axis of the
instrument, to which is also attached the larger mirror, it follows that
the rotatory motions of the mirrors about the axis are identical. The
angles of rotation are measured by indications upon a divided circle attached
to the moving axis, and are shown by a pointer fixed to the upright support.
Fig. 129.
The mirrors are adjusted in auch a way that the light from the lamp to
ITio. 130.
be measm^d falls upon the smaller mirror, thence is reflerted on to the larger
one, and finally along the axial line of the photometer disc. As both
mirrors rotate together, it follows that if a horizontal beam is reflected
correctly, all other beams will find their way along the axis of the photo-
meter. If, therefore, tlie arm carrying the small mirror be moved through
various angles, it will receive the light emitted from the lamp at those
angles, and the light will at every angle bo transmitted along the axis of
the photometer. The divided circle i>> made large enough to serve as a com-
plete ec-reen for all direct light ; and only the light falUng on the small
min-or can find its way to the disc. For the purpose of making an absolute
test an additional measurement must be made. The direct horizontal light
is measured without the interposition of the holophotometer (which is
mounted Ko as to be easily moved out of the direct line); then the mirrors
are interposed, and a new measurement made. The additional path travelled
by the light is allowed for in calculation ; and thus the absorption of the
THE HOLOPHOTOMETEE. 303
mirrors found. The absorption of the two mirrors used is stated usually to
be only about 1,8 per cent.
The employment of mirrors in photometry has sometimes led to serious
errors, but it will be seen from the fore^ing description t'nat inasmuch as
the relative angle of the mirrors is never changed, and as their absorption
is easily calculated and allowed for, the only objections to their use have
been guarded against and avoided.
Id order to eliminate the second source of error — viz., that arising
from the formation of a principal focus — it is only necessary to take a
Fio. 131.
series of readings with the table in one position, and then take another
series with the table at a greater ditttance. If a focus is formed at suffi-
cient distance to produce an appreciable error, it will clearly appear in the
difference between the readings at the two distances, when it is only
necessary to wheel the table to such a distance that the discrepancy is
inappreciable. In other words, this is equivalent to using a bar of sufficient
length to make it practically infinite, compared with the distance between
the focus and the real source of light.
The instrument was <1e!!i<nied specially for use in lighthouse work, where
it is of the highest importance to measure accurately the total light given
304
PREECE'S PHOTOMETER.
by any lamp, and not only that emitted in any one particular direcstiony
ifphich may or may not be the majdmum, but in every direction.
Fig. 129 is a view of this instrument from behind showing the divided
scale ; Fig. 130 is a view taken from the end of the photometer-bar, showing
how the horizontal light from the lamp is transmitted to the disc ; and
Rig. 131 is a view taken from the disc showing how the vertical light would
be transmitted to the disc.
Preeoe's lUumiruxtion Photometer J^ — This photometer, described by Mr.
(now Sir William) Preece to the Royal Society in 1883, is in principle
similar to the original Bunsen photometer. " A small glow-lamp (Fig. 132)
is fixed in a box, carefully blackened in the interior. Over the end is
stretched a Bunsen screen of paper, on the middle of which is a grease-spot.
At about twelve inches from the latter is another screen in which drawing-
paper is fixed. The grease-spot is so screened that no light falls upon it
beyond what is reflected from the screen. At the end of the box opposite
the Bunsen screen is an eye-piece, consisting of a plain tube. To make an
Fig. 132.
Preece*s Photometer.
observation, it is only necessary to place the instrument so that the reflecting
screen receives the illumination which it is desired to measure, and to alter
the electric current of the glow-lamp by means of an adjustable resistance,
or a rheostat, until the grease-spot becofiies invi&ible. Preece found by
experiments that the candle-power of the glow-lamp increased as the sixth
power of the current. The current in amperes thus gave, for a particular
lamp, a constant whose sixth power expressed the illumination measured.
Professor Kittier of Darmstadt and Captain Abney independently corrobo-
rated this function, and recent observations show that, in modern glow-
lamps of eight candle-power, the candle-power varies as the current, raised
to powers of from 5.3 to 6.9. By the use of a reflecting screen, Preece
avoids the difficulty met by Professor Massart ; but, for feeble illuminations,
a serious loss of the dim light to be measured is entailed.
Preece and Trotter Photometer. — A modification was designed by Trotter t
in conjunction with Preece in April 1884, with the view of obviating the
colour difficulty and the liability of error produced by different kinds of
Bunsen screens. The usual arrangement of screen with two mirrors was
• Trotter on the " Distribution and Measurement of Illumination/' Proc. Instii
C.E., vol. ex. part iv.
t Ihid.
PEEECE AKD TROTTER'S PHOTOMETERS.
305
employed, ftUowing both eidee of the spot to be Been Bimultaneously. Upon a
tripod, a cylindrical case, Fig. 133, is covered at the top by a horizontal Bunsen
Bcreen, and two observing mirrors are inclined at a suitable angle. A glow-
lamp slides on a vertical rod, and connection with external tenninala is
maintained byccaled wires. The lamp is moved by a lever which pushes it
in opposition to a spring. The lever is pressed against the cam, and bears
against the roller. This roller is mounted on a nut which traverses a
vertical screw. Its position may be read on a scale. At thu lower end of
Trotter's Photometer
the Bcrew is a handle and a graduated wheel. The cam is shaped to such a
curve that, when the nut moves through any given distance, the displace-
ment of the lamp is as the square of that distance ; the light of the lamp
being adjusted to any required power, balances a given illumination when
the scale reads a unit. A balance being effected for any other illumination,
its value in terms of the said unit may be read off on ihe scale. It is not
necessary to measure the distance of the filament of the lamp from the screen.
This instrument is easy to use, but the range is only from one to ten, and
Fio. 134.
Trott«r'i Photometer (Flan).
cannot easily be increased. A sliding rheostat and an ampere meter were
used for maintaining at a fixed value the current required to balance the
unit illumination.
Subsequently Trotter somewhat modified this photometer. He wished to
make the measurements as near the ground as possible ; therefore, avoiding
the tripod, he arranged a Sunken screen 6 inches from the ground. To see
both sides simultaneously would re<[uire three mirrors, and would necessitate
a very limited view. As it is very much easier to make observation:^ with-
306 TROTTEE'S PHOTOMETER.
out an eye-pieoe, using both eyes, he decided to return to the observation of
one side of the screen only and to make an empirical correction if necessary.
A 6-inch cube, covered by a Bunsen screen at the top and open at the side,
was used. Numerous experiments were made with different kinds of Bunsen
spots, with the result that a simple cardboard screen, with a star-shaped
hole cut in it, was found to be far more sensitive than any other arrange-
ment (Figs. 134 and 135).
Very little difficulty was found in matching the illumination from an
arc lamp even when the standard lamp gave a light of about the same
colour as that of a candle. Greater accuracy was possible between 0.6
candle-foot and 0.2 candle-foot than with higher or lower illuminations.
The electric lamp was mounted in a box, so that it stood 5 inches above
the ground. The object of raising it above the level of the middle of the
reflecting screen was to prevent any light from falling directly on the under-
side of the Bunsen screen, and many erroDCOUs measurements were made
before this precaution was taken. Telescopic tubes were used to shut out
stray light from the reflecting screen ; these tubes, when fully extended,
measured i foot 4 inches in length. The storage batteiy consisted of four
lithanode cells manufactured by the Mining and General Electric Lamp Co.
Two of these are quite capable of running a f-candle lamp, at a fair
Fig. 135.
TFRy
Trotter's Photometer (Section).
brightness, for ten hours, but in order to allow a good margin, two more
cells were connected in parallel. These proved to be sufficient for the pur-
pose, and no appreciable difference in the candle-power was observed be-
tween the preliminary calibration and the one which followed each evening's
work. A slightly higher power was given immediately after charging, but
a quarter of an hour's continuous discharge seemed to bring the batteries
into a very steady condition. The lamp was lighted for as short periods as
possible, about ten seconds being sufficient for each reading.
The instrument was calibrated by direct comparison with various lights
of known intensity.
As a typical example of an old-fashioned gas-lit street, Great George
Street, Westminster, was the scene of the first street-lighting measurements,
and the first observation was made 2 feet from the curb, immediately
opposite the entrance to the institution of Civil Engineers. The illumina-
tion was found to be 0.03 candle-foot. This was read off on a measuring-
tape graduated directly in candle-feet, the divisions being the reciprocals
of the square of the length in feet. In the case of feeble illuminations, it
was found that stray light, especially from distant lamps, fell on the reflecting
screen, and caused the readings to be too low in spite of the telescope tuhes
which were used. It is convenient, especially for feeble illumination, to
have the Bunsen screen at least 3^ inches square, and the star-shaped
TEOTTEB'S PHOTOMETER. 307
hole should not be less than 1^ inch across. Trotter finally adopted
the method of inclining the reflecting screen at difierent angles mounted
on Alt axis passing through its upper edge and arranged so that it
could fold up quite out of the beam. In order that a convenient scale
might be proviiJed, motion was given to the reflecting screen by a fine
chain wound upon a snail-cam. The cam was designed upon the assump-
tion that the illumination upon the screen would be proportional to thecoeine
of the angle of incidence of the light upon it. This is not strictly the case ;
especially as a convex lens was tued in many of the tests to increase the
available light from the electric lamp. The object of the snail-cam was
iDOrely to Kpread the divisions of the scale more evenly, and did not aim at
uniform division. The scale was empirically calibrated with a standard
candle.
TrotUr'a Ptuytovxeter . — Mi. A. P. Trotter, in a communication to the
Physical Society on June 9, 1893, described a new photometer based upon
his former suggestion of a horizontal screen of white cardboard having a
clear star-shaped hole in ths middle, below which and enclosed in a box whs
an inclined white screen illuminated by a small glow-lamp. In this com-
munication Mr. Trotter described his arraiigement of screens to ordinaiy
hght photometry (us distinguished from illumination
photometry). His first plan consisted of two screens
(Fig. 136) each inclined at 45° to the direction of the
lights and the eye. One sci-een was immediately be-
hind the other ; the first screen was perforated, and
mounted on a xliding- carriage on a photometer bar. '
The lights wei* placed, the one a little in front and
the other a little behind the plane of intersection of the screens. The back
of the perforated screen was blackened and was shaded from the light which
illuminated the bnck screen. The edge of the pei-forations was bevelled, to
assist the complete disappearance of the hole. The hole consisted of two
lozenge- shaped ajiertures one over the other, point to point, the object being
to concentrate attention on a vertical line. The screens were held in a
frame cipable of rotiition round a verticiil axis through a Rmall angle, for
the purpose of producing small and rapid variations. But although one
screen thus received more light and the other less, the cosine law of illumina-
308 OBSCURATION METHODS.
tion caused the former to increase but slightly in brightness, while the
latter diminished considerably. It should be observed that this arrange-
ment of screens, although developed from a Bunsen photometer, is a modi-
fication of the Thompson-Starling photometer, in which two screens at 45*^
to the lights and to the eye are used ; but side by side, however, instead of
one behind the other.
Mr. Trotter subsequently used a slotted screen or grid, as shown in
Fig. 137, for measuring rapidly fluctuating lights. At one end of the grid
the perforations appeared as dark strips on a light ground, and at the other
as light strips on a dark ground. At the point of balance the strips were
not distinguishable from each other.
The final form of this photometer, for ordinary work, is a pair of screens
arranged so that the light falls on them at an angle of 35^, the angle
included between the screens is therefore 125^. This angle is chosen because
oonsiderable variations may be made from it without appreciably afiecting the
result. A star-shaped hole is perhaps best, but the edges must be carefully
bevelled, and the easiest form of hole to make is a circular one, cut in a lathe.
The effect of such a photometer is precisely the same as that arrived at
by much more complicated means in the Lummer-Brodhun instrument —
viz. one screen is seen through a hole in the other. Being constructed of
ordinary white Bristol board, the screens may be very cheaply replaced
when soiled.
In addition to the foregoing methods of direct comparison of two lights
many other systems have been devised, amongst which may be mentioned :
Obacuration meihoda^ which are, however, mostly fitted for the estima-
tion of the irUenaity per unit area or brightness of a radiant, rather than
for the estimation of the total light volume ; as, for instance, the case of a
gas-flame produced by a flat-flame burner. The Bunsen disc is suitable for
estimating only the total quantity of light emitted from any source, and any
increase in the size of the flame will immediately affect the indications of the
disc ; but if this same flame is examined through an opaque glass ground
into a wedge shape so that at its thinnest part it is translucent, it will be
found that at a given point on the wedge the light of the flame will be
eclipsed. If the flame be then diminished or enlarged, the indication of the
obscuring wedge of glass will remain the same. Let the gas be next so
burnt that a greater intensity per unit area is obtained, but the volume of
gas be reduced so as to afford the same indications by the Bunsen disc as
against the same standard of comparison employed in the first experiment,
and then let it be re-examined by the wedge of glass. It will now be
found that the indications are decidedly higher, and thus, although the
light volume is the same, the brightness has increased, t.e., the obscuration
method indicates brightness, but not total illuminating value. It is, there-
fore, always necessary to carefully bear in mind the object of the investiga-
tion. If it is desired to ascertain the total volume of light emitted from
a given radiant, then one of the methods of direct comparison, such as the
Rumford or Eoucault shadow systems, or the Bunsen disc system, must be
employed. If, on the contrary, the intensity per unit area, or brightness,
is required, then an obscuration method must be used. It does not appear
that this material difference between sources of luminous energy and illu-
mination has received the careful attention it deserves. Majiy experi-
menters and writers use the term ** intensity " as indicating the total light
volume, or quantity of light, whereas the term " intensity " should clearly
be reserved to indicate the energy of the state of ignition — ^brightness or
incandescence.
Various suggestions for measuring the brightness of a source of light have
been made by different scientists, commencing with Bouguer, who judged
POLARISATION METHODS. 309
the relative brightness by counting the number of the pieces of glass that
it was necessary to interpose, or the number of reflectors required, respec-
tively for extinction. Huyghens used the system of diminution of aperture.
These two methods apparently form the foundation upon which nearly all
other obscuration methods are based. The most satisfactory of these is the
wedge of smoke-coloured glass first employed by Dawes in 185 1, and
recently developed and placed upon a scientific basis by Professor Pritchard.*
Chemical Photometers, — ^The American physicist, Draper, in 1 843 noticed
that light caused a mixture of the two gases hydrogen and chlorine to com-
bine and form hydrochloric acid, and estimated the intensity of the light
by noting the diminution in voJume of the mixed gases, when exposed to
a light, in a glass vessel over hydrochloric acid saturated with chlorine, a
solution which dissolves the acid as it is formed, but does not dissolve
either of the unchanged gases. Hunt, in 1884, employed the indications
afforded by the precipitation of carbonate of iron from a solution of sulphate
of iron in common water. Such a precipitation takes place slowly under
any circumstances, but if the solution is exposed to sunlight this precipita-
tion takes place rapidly, and the weight of the precipitate is, up to a certain
point, a measure of the light to which the solution has been exposed.
Angus Smith utilised the action of light on a solution of iodide of
potassium, acidulated with nitric acid, the quantity of iodine liberated
bein^ the indicator.
Various photographic methods have been tried, but none appear to have
been available for purposes other than the special object to which they were
applied.
The fact that, when light is absorbed by a black surface, heat is pro-
duced, has been utilised by several experimenters, the first of whom, Leslie,
in 1797, estimated the intensity of the light by the depression of the liquid
in the limb of a differential thermometer, one of the two bulbs of which he
blackened.
Fdarisation Methods. — In the various phenomena which take place when
a ray of light encounters the surface of a new medium, it has been supposed
that the direction and intensity of the several portions into which it is sub-
divided will continue the same, on whatever side of the ray the surface is
presented, provided that the angle and the place of incidence continue un-
changed. In other words, it was taken for granted that a ray of light had
no relation to space, with the exception of that dependent on its direction ;
that around that direction its properties were on all sides alike ; and that,
if the ray be made to revolve round that line as an axis, the resulting
phenomena would be unaltered.
Huyghens was the first to prove that this was not always the case. In
the course of his researches on the law of double refraction, he found that
when a ray of solar light is received upon a rhomb of Iceland spar, in any
but one direction, it is subdivided into two of equal intensity. But, on
transmitting these rays through a second rhomb, he observed that the two
portions into which each of them was subdivided were no longer equally
intense; that their relative brightness depended on the position of the
second rhomb with regard to the first ; and that there were two positions
in which one of the rays vanished altogether.
On analysing the phenomena, it was found that it depended on the
relative positions of the planes or principal sections passing through the
axes of the crystals, and perpendicular to the refracting surfaces. When these
sections are parallel, the ray which has undergone ordinary refraction by
the fii-st crystal will also be refracted ordinarily by the second ; and the ray
* " Memoirs of the Rojal Astronomical Society," vol. xlvii.
310 POLARISCOPIO METHODS.
which has been extraordinarily refracted by the first will also be extra-
ordinarily refracted by the second. On the other hand^ when the principal
sections of the two crystals are perpendicular, the ray which has suffered
ordinary refraction by the first crystal will undergo extraordinary re-
fraction by the second ; and the extraordinary of the first will be refracted
according to the ordinary law in the second. In the intermediate portions
of the two principal sections, each of the rays reflected by the first crystal
will be divided into two by the second, and these two pencils are generally
different in intensity ; their intensities being measured by the squares of
the cosines of the distances from the position of greatest intensity.*
This physical fact has been utilised by a number of experimenters ; it
will suffice, however, to shortly describe the following as illustrative of the
methods employed.
Becquerel^ in i860, constructed an instrument which consisted of two
small telescopes 35 centimetres long by 3 centimetres diameter, with their
axes at right angles, and having the same eye-piece. A right-angled
prism is fitted bo that the observer may see the two images side by side.
In order to reduce the stronger light, a Nicols prism is inserted in the tube
which is straight with the eye-piece, and another similar prism is inserted
near the eye-piece. The relative intensities are calculated from the angle
through which this second prism is rotated. f
In 1868 Crookes constructed the following instrument: A brass tube,
blacked inside, was fitted with two short side tubes which are near one ex-
tremity and opposite to one another. At the same end is slipped in a separate
piece with sloping sides, which are covered with white paper, or finely
ground porcelain, so that one slope is illuminated by the light which enters
through one side tube and the other slope by that entering through the
other tube. At the other end of the long tube is the eye-piece made up in
the following way : At the end nearer the illuminated surfaces is a lens ;
then, taking the internal fittings in order as they approach the observer,
we have first a series of thin plates of glass capable of moving round the
axis of the tube and furnished with a pointer and graduated arc ; next a
prism of Iceland spar, a film of selenite, and at an appropriate distance, a
second prism of Iceland spar. At the end nearer the eye is added a lens
adjusted to give a sharp image of the two discs produced by the second
prism. In comparing a flame with a standard light the former must be
moved until the two discs of light are nearly equal in tint. The final
adjustment is then effected by the eye-piece turning the polarimeter one
way or the other up to 45° until the images are seen without any trace of
.colour. The square of the number of inches between the flames and the
centre gives their approximate ratios, and the number of degrees the eye-
piece is rotated will give the number previously determined by comparing
equal lights to be added or subtracted to obtain the necessary accuracy .j:
In 1878 Heisch modified this arrangement in the following manner:
Two brass tubes were fastened together in the shape of a T, and where they
joined two reflecting prisms were placed which reflected the rays from the
two sources of light up the tube. Immediately over the faces of the prisms
a plate of tourmaline, or a Nicols prism, and then a plate of selenite were
fixed, and at the eye-end a double-image prism. Two images of each of the
reflecting faces were thus seen, and by proper an^ngement of the eye-piece
the ordinary image of one prism could be made to overlap the extraordinary
image of the other, and when the lights were of equal intensity this com-
♦ Lloyd, "Wave Theory of Light."
J "Am. Chem. Phys." (3) Ixii, 14, 1861.
" Chemical News," xviii. 28, 1868.
WEBER'S PHOTOMETER. 311
pound im&ge appeared white. The inventor, howpver, states that the disad-
vantages of this instrument are :
1. That m&n^ people are deficient in the perception of certain colours,
so that what to them would appear quite white might to others seem
distinctly coloured.
2. That no two artificial lights are of the same colour, so that if rod
predominates in one it will have the sume effect as an increase of intensity
if the red image of that light be used. The persontil errors of different
observers may generally be allowed for, but it is almost impossible to over-
come the error caused by the different colours of the lights.*
&uoy {Govt) compared two sources of light by allowing the rays to enter
at the opposite ends of a tube, in the centre of which were placed two total
Fia. 138.
reflection prisms. The rays which were thus brought side by tdde and
parallel to one HDOther were allowed to fall on the collimator slit of a spectro-
scope. By this means the eye, instead of having to judge between two
illuminated surfaces, had two spectra presented to it, and could thus detect
the differences of tint as well as differences of intensity in the two sources
of light. In this method the lights are equalised by varying their dis-
tances. It will be seen that this is not a polariscopic but a spectroscopic
instrument (nee pnge 316).
Z. Weber's Photometer (Fig. I38)t consists of a horizontnl tube with a
revolving tube at right angles to it, supported on a column which, aa the
carrier of the instrument, is firmly screwed on to a box. Tlie rigid tube
• "Lieht and Health." Dec. 21, 1878
t 'Journal ot Sue. ot Chemical Industry." 1885, p. 446.
312 WEBER'S PHOTOMETER.
carries on its middle part a millimetre scale ; on the right, fastened with a
bayonet-clasp, is the lamp-case with a benzine lamp ; on the left is a
graduated arc, on which an index travels and moves with the revolving
tube.
The rigid tube contains a ring fitted with an opal glass which, by means
of a rack and pinion, can be moved backwards and forwards. A pointer
always shows on the millimetre scale the distance of the opal glass from the
benzine lamp. In the case is a rigid hook for regulating the flame height^
as well as a scale fastened on to a miiror on which the flame height can be
read off. The movable tube can be revolved over about i8o° and fixed in
any position. It has (in the figure turned downwards) an eye-piece, and in
the centre a reflecting prism,* one of the cathet surfaces of which is turned
towards the central axis, and the other towards the eye-piece. The light
coming from the rigid tube is refracted 90^ by means of the prism, and so
made visible to the observer. The sheet metal box at the other end of the
tube, to which can be added a shading-off tube, serves for holding one or
more opal glasses. The light coming from here occupies the left-hand portion
of the field of vision, whereas on the right-hand half there can only be light
from the rigid tube.
On the eye-piece is a slide with red and green glass plates, with a free
opening ; so that adjustments can be made with natural white light, green,
or red light. Besides this, the eye-piece has a reflecting prism as well to
hinge over it, which can be used for greater comfort when the light falls
slantingly or perpendicularly.
When measuring point-shaped light sources with a colour equal to that
of the standard light (Benzine lamp^ the apparatus is set up in the
manner described ; the movable tube airected towards the flame and the
benzine lamp flame adjusted to 20 millimetres height. An opal glass is now
inserted into the box, its distance {R) from the light source measured and the
distance (r) of the movable glass plate from the benzine lamp altered until
both halves of the field of vision show with equal brilliancr^. If this should
not be possible with one plate several must be used. The influence of the
plate is determined by first directing it towards a standard lamp and the
R^
value of C found according to the formulas J=~^C where*/— i ; that is, by
first finding the constant value for the plates. This constant value is deter-
mined once for all, for all the plates. If, for instance, when testing a light
source, B « 100 cm. and r = 25.5 cm.; and G = 9.33, already determined,
the intensity of the flame tested J ■■ — x 0.33 = 5.07 candles.
25-5 ^ 25*5
With diffused light, of equal colour with the standard light, a white card
can be used, which is placed at the desired inclination at the place where the
measurement is to take place. The revolving tube is directed towards the
centre of the card, the distance of which is generally unimportant. An
adjustment is now made, as previously mentioned, and the strength of
illumination found, after reading off r, from the formula E ■» — -^ — (7^,
in which C^ is a constant coefficient to be determined once for all and has
different values according to whether the adjustment is made with or with-
out plates.
If in a given case C « 0.07157 and r ■= 18.5 cm., then £ ^ 2.21
metre candles. If in this, as well as in the preceding instance, it had
been found that the colour of the light source to be tested did not coincide
• Instead of this reflectine: prism the Lummer-Brodhun prisjm combination is
mostly used at the present time (see p. 317).
GROSSE'S PHOTOMETER,
313
with that of the benzine lamp, a case which renders an adjustment for
equal brilliancy of both halves of the field of vision difficult, the procedure
is somewhat different. Two readings must then be taken, viz., one with
green and one with red glass. The result obtained for red is then to be
multiplied by a factor k; k being less than i for redder flames than the
benzine lamp, and for whiter fljBimes, greater than i. It is dependent
entirely on the colour intensity of the light source. It has been found
that k alters directly with the number obtained by dividing the intensity
or brilliancy found with the green glass {Gr) by that of red glass (/?).
A table can thus be drawn up for each sort of light which gives the
Gr
value for k which the relation -5- represents.
Gro88e'8 Photometer, — The following description by Dr. Kruss* of a
polariscopic photometer recently devised by Grosse will be interesting as the
latest outcome of this branch of the subject of photometry :
F16. 139.
d
I
J^
^^
^
^^
^r
Mi
^^
^T ^
k/.. ,
: y^
jT
/
■ *«
By cutting a four-sided calcspar prism A (Fig. 139) diagonally, so that a
thin space of air remains between the two halves, a ray a b falling on it
will be split into ordinary and extraordinary broken rays, and with a correct
position for the division of the ordinary ray it will be reflected on the
surface of separation c dsct the point b, while the extraordinary ray b c will
pass undeflected through the entire calcspar body. By connecting this
prism with a second half prism B, as shown in Fig. 139, the VAj/g thrown
on it will also be split into two rays polarised at right angles to each other,
and the extraordinary ray will also, as in the prism A, pass through, but
the ordinary one will be reflected on the surface h k s.t the point g, and will
again be reflected on the surface c din the first prism A at the point 6, so
that this extraordinary polarised ray from prism B reappears in the same
direction with the extraordinarily polarised ray abc from the prism A. In
order to pa^ss light from two difi'erent sources J^ and «/, (Fig. 140) through
this combination of prisms in the way described by Dr. Grosse,' two simple
reflecting glass prisms, i and 2, are used in the position indicated in Fig. 140.
From the light source J^ a single bunch of rays passes into the combinar
* Schilling's Journal fur Gasbeleuchtung, &c.
314 GROSSES FHOTOMETEB.
tion, but from the eonrce •/, two bunches so pasa, one of which paaees
through the calcepar A only, while the other with repeated reflection posses
through both B and A and at the same time falls in with the one from
source J,. The sight surface is therefore divided into two halvee, the right
receiving light from J, only and the left from both J, and J,. In con-
sequence of the different Iors of light on the double course in the combina-
tion of prisma, the share of J^, appearing in the left half of the surface of
light, must be multiplied by a factor x, which Is easily detennined and
supplied with each instrument. Both halves being arranged at equal
intensity, and the distances of the sources •/, and J^ taken as i^, and L^
then
Jt J, J*
and therefore the proportion of intensity to be determined
A certain measurable portion of the stittnger source of light can be
admixed with the light of the weaker; wheieby, fiist, the difference ot
intensity between the two sources of light is diminished, and, secondly, the
difference of colour whicii makes phoiumetrical comparisons so extremely
difficult is diminished. But the instrument gives still further results by
tiLking advantage of the pwuliarity of the my which passes straight through
prism A_a.nd is polariseil nt right angles to those reflected by B and A. By
interposing in the way of the rays a calcspar prism similar to A and a so-
called Nicols prism X, all the light from the rays passing straight through
prism A will pass throiigh ,V, as maincut of prism i\' is parallel to that of
prism A, but with a deflection of go'" nothing of this light will pass through
prism y. In the case of the rays fi-om the prism B the proportion is
reversed, therefore either of the rays can be entirely extinguished; by
GBOSSE'S PHOTOMETER. 315
eztinguishiiie the raye paBsing thi-ough the prism B, this photometer can be
ut<ed as an ordioary oue, and iu this way the factor x can be easUy determioed.
By adjusting the Nicols prism at any other angle of inclination than the
one at wbicli one of the bundles of rays was extinguished, a share from each
of these bundles is admitted and its intensity calculated. Starting from
the position of Nicols prism in Fig. 140, by inclining the same the light
coming from prism A alone, it is gradunJly reduced, while that coming from
prism B (being the portion of scJ^) is increased. A table is provided with
the iuetrument and gives the influence of the angle of inclination on the
intensity.
By adding another small reflecting prism 3, Fig. 141, by which a second
PiQ 141.
^-H?.*
bundle of rays from light source J^, is direciH^i ihmugb the calcspar prism S,
the light vnhime J, + xJ,ia obtaineil in the field of view to the right, and
•', + xJi in the field to the left. By adjustment to equal intensity,
J^ xJ, _ J,
>r the proportion of intensity
the factor x is therefore entirely done away with, the calculation of the pro-
portion of inteiisicy is obtained simply from the proportion of the squares
of the diBtance as in the ordinary Butisen Photometer, and the usual sc&les
from which the intensity is read off direct are also applicable.
With this arrangement a complete mixture of the rays from the two
Kources of light J", and /, takes place, und in both lu.lves of the field of view
3l6 SPEOTROSCOPIO PHOTOMETRY.
we have the same colour mixture, whereby a photometrical comparison of
different-coloured lights is made possible with remarkable ease.
It is claimed that this photometer provides quite a new method for
the correct adjustment for equal intensity.
In the course of a series of experiments on Standards of Light the
writer employed the spectroscope for comparing the different colours of
the various units. The results were obtained by viewing the spectra of
two lights side by side, and thus qualitative comparisons were readily made.
Professor E. L. Nicols has used this method in the following manner, by which
quantitative results are readily and conveniently obtained. In this apparatus
the principle of the Bunsen photometer is applied successively to the various
regions of the visible spectra of the sources of light under comparison. He
employed a spectroscope, the optical axis of the collimator being horizontal,
and at right angles to the photometer bar. The slit is horizontal and lies
in a straight line joining the sources of light, which are set up in the
usual manner at 'the ends of the bar. The bar itself is preferably of con-
siderable lengthy and in the original inbtrument was 500 centimetres long.
In front of the spectroscope slit are placed two right-angled prisms of the
same size and made of the same glass. Their vertical edges bisect the slity
and the light coming from either end of the photometer-bar is totally
reflected by them, entering the right- or left-hand end of the slit in a direc-
tion parallel to the opti(^ axis of the collimator tube. The two sets of
rays thus gathered into the spectroscope from the lights at the end of the
bar are vertically dispersed by the prisms, and appear in the field of view
as two vertical spectra standing side by side. Equal wave-lengths are in
the same horizontal line, and any desired region may be brought into the
centre of the field by an angular movement of the ocular telescope.
When the instrument is placed at the middle of the bar between two
lights of identical amount and brightness, the two spectra are of equal
brilliancy throughout, from red to violet. If the two lights differ in
intensity but not in quality, their spectra will differ in brightness by the
same amount from end to end and the instrument may be used as a simple
photometer. When the lights to be compared differ both in intensity and
quality, ordinary photometric indications do not possess any peifectly
definite significance. In this case the brightness of different points of the
spectra must be compared. Professor Nicols used the new instrument to
compare the lights of a Welsbach incandescent burner with that of an
Argand, which, as determined by a Bunsen photometer, had a relative
brilliancy of 1.701 : 0.015. ^^® following was the result: •
Colour.
Wave Length.
Ratios.
Probable Error of a Simple
Obeervation.
Red .
Yellow .
i> • •
Green . ,
Blue .
Violet
702
5S9
558
500
466
439
0.709 : 0.017
1.476 : 0.017
1.760 : 0.023
2.395 : 0.047
2.738 : 0.036
3.090 : 0.073
i
1
2.45 per cent.
1. 14 „
1-34 ..
1-99 ».
t.30 »
2-35
Average 1.76 „
Laramer and Brodhun's Photometer Head. — For the purpose of the experi-
ments which the German Imperial Technical Physical IiistittUe carried out
at the instigation of the German Society of Gas and Water Experts on the
various light units the Bunsen photometer was first employed.
• "Journal of Gas-Lighting," vol. Ivi. p. 141.
LUMMER AND BRODHUN'S PHOTOMETER.
317
In order to find, if possible, a contrivance which would give more
sensitive readings Messrs. Lummer and Brodhun devised the following
apparatus :
Fio. 142.
/
In order to exemplify the principle used it is necessary to refer to
Fig. 142.
Let I and X be diffusely illuminating surfaces and A and B such a com-
bination of two rectangular glass prisms that at certain parts (pq and hi)
of the hypotenuse surface of the prism B, the light coming from X, is reflected
to 0, whereas at the remaining parts {gh) it passes through the prism and
goes to r ; the reverse being the case in regard to the hypotenuse surface of
the prism A with the rays emitted by /.
If an eye stationed at looks towards the surface pqhi it sees the por-
tion of it, qhy illuminated by the light from I, and the portion pq and hi by
the light from X. At a certain relative intensity of the fields I and X, pqhi
appears as a completely uniform bright surface.
Suitable prism combinations can be made in many ways, the following
being that generally used :
Fig. 144.
Fig. 143.
The spherically-shaped surface of the prism A is cut flat at cd (Fig. 143)
and pressed against the similarly flat hypotenuse surface of the prism B,
The elliptical-looking spot appearing in this case has perfectly sharp edges
and completely disappears with equality of the fields.
In order to make the *' photometer head " slide on a straight bench
like that of the Bunsen photometer, the arrangement sketched in Fig. 144
3l8 LUMMER AND BRODHDN'S PHOTOHETEB.
waa made. Perpendicular to the axis of tbe photometer bench U the opaque
screen ti, the aideB of which are illuminat«d by the light sources m and »
respectiveiy. The diffused rays of light reflected from the two aides of the
screen k and / fall on the mirrors e and^, which throw them perpendicularly
on to the cathet surfaces c6 and dp of the prisms B and A ; the observer
at looking through the magnifying-glass W perpendicularly to ao focuses
sharply on the surface ar«d.
Fig. 145 gives a perspective view of a photometer- head made according'
to arrangement for the experiments in the workshop of the German /m^Mrtof
ImlUuie. The vertical bra.ss column S carriea the metal cross-piece 6 into
which the small columns i)^ and S, are screwed. . In the upper part of these
ore the screws m, and m, with conical cups turned in their ends. These
cups form the bearing for the horizontal axis a of the photometer-case h.
On the case at w ie placed the tube r with a eliding magnifying-glass. In
the inside of the case lie the prifim combination A B ; the two mirrors, of
Via. 145-
which only the oaef is visible, and the photometer screen P. This latter
rests in the frame n, the foot-plate of which ia movable and adjustable on
the floor uf tbe case k ; for the purpose of renewal or turning round over
\%o" the screen can be remolded from the frame n. Each ot the mirrors
e and /can be revolved in a vertical as well as in a horizontal direction' from
the outiiide by means of two screws each passing through the floor of h.
The fastening q presses the piism A and B closely together and rests on a
plate which iu movable in a similar manner as the frame n. The case h
is closed by a lid, which in the figure has been removed, with slot for the
handle of ihe screen I', the light striking the paper of the screen /'through
tbe hide openings. In the position of the photometer-case represented, a
screw heiid k (not visible in Fig. 145) is pressed hard against the column S,
and acts as a stop-blo<'k, anil on revolving the axis of the case over 180° a
second screw head k .serve.s aa the other stcp-block. The column 5 placed
on a slide of the photometer bench can be moved up and down in a circular
direction on a vei'tical asi£.
DISCS. 319
The screen P is made up of double layers of paper, separated by a sheet
of tinfoil fixed between two metal sheets having circular openings; e and/
are selected, even mirrors, cut from the same piece and coated with silver
amalgam. In place of these mirrors, totally-reflecting prisms can be used.
In front of the magnifying glass a diaphragm rather larger than the pupil
is fixed, and by painting on the outer portion of the hypotenuse surface of
B with asphalt varnish a sharp definition of any desired form is givQn to
the field of vision.
Discs.
The Bunsen disc, most commonly used in England, is mnde by rotating
a circular piece of stout paper, the centre of which is clamped between two
metal blocks, in melted spermaceti, thus coating the outer half with wax
whilnt the centre remains opaque The excess of spermaceti is eliminated,
whilst still in a molten condition, by centrifugal force and the disc allowed
to dry in an atmosphere free from dust. In Germany the discs are made
of thinner paper and rectangular in shape. Instead of being partially
coated with wax, several bar-like marks, usually three, are made on the discs
with sperm oil. Both of these forms give delicate readings. When used,
however, for testing lights of different colours, such as electric lights and
recuperative gas-burners, considerable difiiculty is experienced in judging
whether the two sides of the disc are equally illuminated owing to the
diflferenoe in colour of the standard light and that under examination.
The Leeson star disc in a great measure eliminates this difiiculty. As
originally made it consisted of a stout piece of paper the centre of which
was perforated in the form of a star, with a number of short radiations,
the two sides having sheets of thin paper attached to them. Owing to the
thin paper not bein/sr contiguous a serious error at times crept in, and the
Metropolitan Gas Referees refused to sanction it as a substitute for the
Bunsen disc.
Recognising its value and utility for comparing difTerent-ooloured lights,
the writer modified it somewhat, so that it was impossible for either of the
papers to buckle individually, the three papers being moistened with starch
water, pressed together and dried under pressure. In this form it is now
considerably used for testing lights which have a different colour to the
standard employed, such as electric arcs and Welsbach mantles, and was
prescribed by the gas referees in their notification for use with the bar
photometers in the London testing-stations.
Various alternative discs have been suggested of which the Lummer &
Brodhun photometer head, already described, and Joly's disc, consisting of
a pair of rectangular spermaceti blocks used together or with a sheet of tin-
foil separating them, are the principal.
Standards of Light.
In England, the spermaceti candle is the only Parliamentary unit, although
in London the gas companies have at present agreed to accept, for use in
the London gas-testing stations, Harcourt's ten-candle pentane lamp. In
France the Carcel lamp is steadily adhered to, but in Germany the paraffin
candle has given way to the Heffner lamp, whilst in Holland a lamp
burning ether and benzine has been adopted as the standard.
This variation in standards would not be of so much importance if the
different methods for producing a working unit were all referable to an
agreed constant, such as the platinum unit of M. Violle. Unfortunately,
however, no such agreement exists. A solution of the problem would
320 STANDARDS OF LIGHT.
be found in an agreement between the various Governments ooncemed, in-
cluding that of America, to appoint a commission consisting of an indepen-
dent body of international representatives, to examine and compare the
various legal standards, and the substitutes proposed for them, which
commission should be empowered to agree to a definite international
standard unit of light, and suggest any method or methods by which this
standard might be conveniently duplicated for use in practical work. Until
some such course is pursued we shall have, imfortunately, to continue to
rely on the isolated exertions of individuals whose efforts must be always more
or less open to hostile criticism. There can be no doubt that the absence
of a generally accepted international standard is most unsatisfactory, and the
present position of the question is a reproach to the scientific and industrial
spirit of the age. It should be clearly and definitely agreed, however, that
before any commission such as that above suggested, be appointed, its decision
should be immediately registered in all the countries concerned as part of
the law of the land. There are many excellent standards which have been
proved to be sufficiently reliable for practical use, several of which are now
in use in different countries, and no possible harm could arise by the
adoption of any one of them.
In the Metropolis Gas Act of i860 (England) the standard of light is
deecribed as '* sperm candles of six to the pound, each burning 120
grains per hour.'' In France the standard is the *' Caroel" lamp, burning
refined colza oil at the rate of 42 grammes per hour. In Germany the
standard is a paraffin candle of which ten weigh 500 grammes, the flames of
which should have a height of 50 millimetres. While these are the three
recognised legal standards, many proposals have been made for substitutes
for them. It will be well, however, to deal with the present legal instru-
ments first, and then to describe the various systems suggested as improve-
ments upon them.
The Sperm Candle, — ^The only definition of this legal candle is that given
above, with the addition of the Gas Kef erees' instruction that, when the
sperm actually consumed falls short of 114 grains per hour, or rises above
126 grains per hour, they shall not be used for testing purposes. In prac-
tice these figures are determined by noting the time required by two candles
to consume 40 grains weight of sperm, which should fall within the limits
of nine and a half minutes and ten and a half minutes, or, calculated on the
weight consumed in ten minutes, 38 and 42 grains, the prescribed rate of
120 grains per hour being 40 grains for two candles in ten minutes. The
weight of a single candle as supplied by the maker should be 1167 grains
nearly. The extreme variations from this weight rarely exceed 20 grains,
and more generally fall within a few grains. The length of the candle
varies, with different makers, from 8^ inches to 9 inches, measured from the
shoulder. The diameter at the shoulder is very nearly 8/ioths of an inch,
and 8^/ioths to 9/ioths at the bottom. The writer is indebted to Messrs.
Miller and Go. for the following definition of what they understand to be
a ''sperm candle '' according to the Act of i860 :
^' We think that there can be no doubt that, at the time the Act was
passed, a sperm candle was understood to consist exclusively of spermaceti
(the product of the spermaceti whale), pure white and dry, having a melting-
point of as nearly as possible 109°, and to which was added just so much
air-bleached beeswax, having a melting-point of 140^, as would suffice to
break the crystals of the spermaceti; the rate of combustion, fixed at 120
grains an hour, being secured by a properly proportioned cotton plait serving
as the wick. With regard to the size of the candle to be used, we have
never attempted to make candles which should individually weigh i/6th lb.,
as we have understood the intention of the Act to be to indicate that the
ENGLISH STANDARD CANDLlL 32 1
candles to be nsed should be those knoini in the trade as * short sizes/ and
which do approximately weigh six to the pound."
Unfortunately considerable variations have taken place in the number
of threads to each strand in the wick. This has arisen from the endeavour
of the makers to free the spermaceti as much as possible from the '' sperm
oil/' and thus obtain a more solid product. Naturally a higher melting-
point has been thus obtained, which necessitates the employment of larger
wicks to effect the same rate of combustion, a remedy which unfortunately
reduces the light yielded per unit of sperm consumed. The Standards of
Light Committee of the British Association for the Advancement 0/ Science, in
their report presented in 1888, stated :
"Thus the effect of the improvements in spermaceti has been that
standard candles give less light than they gave ten years ago, and probably
still less light than they gave at earlier dates, when the average consumption
of candles of mx to the pound was 140 grains per hour."
On the other hand, there is on record a statement to the contrary effect,
made by two gentlemen of no little skill — viz. Messrs. Heisch and Hartley
— who in 1883 said :
" We may here mention what we are convinced to be a fact — ^namely,
that sperm candles generally now develop more light per grain of sperm
burned than they did several years ago."
Owing to the want of uniformity in the manufacture of candles and the
fact that they were gradually being altered from the form intended for use
when the Act was passed, the Gas Referees laid down the following regulations
for their manufacture :
" I. The wicks shall be made of three strands of cotton plaited together,
each strand consisting of eighteen threads. The strands shall be plaited
with such closeness, that when the wick is laid upon a rule and extended by
a pull of about i ounce, just sufficient to straighten it, the number of plailiS
in 4 inches shall not exceed thirty-four nor fall short of thirty-two.
" As it is found to conduce to the regular burning of candles that the
wicks should have been as far as possible cleaned and freed from mineral
matters, it is recommended that the candle-maker, before steeping the
wicks, shall wash them first in distilled water made alkaline with between
I and 2 per cent, of strong liquid ammonia, then soak them for several
hours in dilute nitric acid containing about 10 per cent, of strong acid, and
finally wash them in distilled water made alkaline with a few drops of
ammonia.
''Each wick shall be of suitable length, not less than 12 inches, and
looped ready for fixing in the mould. After having been bleached in the
usual manner and thoroughly washed, the wicks shall be steeped in a
liquid made by dissolving i ounce of sal ammoniac and half an ounce of
crystallised boracic acid in a gallon of distilled water; they are then to
be gently wrung or pressed till most of the liquid has been removed, and
dried at a moderate heat while lying horizontally.
'' Such a wick cut to a length of 1 2 inches when stretched as above shall
weigh not more than 7 nor less than 6 grains. The weight of the ash
remaining after the burning of ten wicks which have not been steeped
in boracic acid, or from which the boracic acid has been washed out, shall
be not more than 0.03 grain.
" Wicks made in accordance with this prescription shall be sent to the
office of the Gas Referees, by whom they will be examined and certified.
" The wicks so certified are to be used by the candle-maker in the condi-
tion in which they are returned to him.
" When the wicks are set in the mould they should be pulled with only
so much force as is necessary to straighten them.
322 ENGLISH STANDARD CANDLE.
^ 2. The spermaoeti of which the candles are made shall be genuine
spermaceti, extracted in the United Kingdom from crude sperm oil, the
product of the sperm-whale {Phf/seter Maerocep/uUus), It shall be so r^^ed
as to have a melting-point lying between 112° and 115° Fahr.
" As various methods are used by different refiners of spermaceti for
determining the melting-point, which lead to different results, it must be
noted that the temperatures here given as the limits within which the
melting-point of a sample of refined spermaceti should fall — ^viz. 112^ and
115^ Fahr. — have been found by the following method, which is known as
the capillary -tube method :
'* A small portion of the spermaceti is placed in a short test-tube, and
melted by plunging the lower end of the tube in hot water. A glass tube
drawn out at one end into a capillary tube about i millimetre in diameter
is dipped narrow end downwards into the liquid spermaceti, so that when
the tube is withdrawn 2 or 3 millimetres of its length are filled with sperma-
ceti, which immediately solidifies. The corresponding part of the exterior
of the tube also becomes coated with spermaceti, which must be removed.
'* The narrow part of the tube is then immersed in a large vessel of water
at a temperature not exceeding 110° Fahr. The lower end of the tube
which contains the spermaceti should be 3 or 4 inches below the surface
and dose to the bulb of a thermometer. The upper end of the tube must
be above the surface, and the interior of the tube must contain no water.
The water is then slowly heated, being at the same time briskly stirred so
that the temperature of the whole mass is as uniform as possible. When
the plug of spermaceti in the tube melts it will be forced up the tube by
the pressure of the water. The temperature at the moment when this
movement is observed is the melting-point.
*^ Since candles made with spermaceti alone are brittle, and the cup which
they form in burning has an uneven edge, it is necessary to add a small
proportion of beeswax or paraffin to remedy these defects. The best air-
bleached beeswax, melting at or about 144° Fahr., and no other material, shall
be used for this purpose, and the proportion of beeswax to spermaoeti
shall be not less than 3 per cent, nor more than 4^ per cent.
'^3. The candles made with the materials above prescribed shall each
weigh, as nearly as may be, one-sixth of a pound, and will be found to
answer to the following test : Immerse a candle taper-end downwards in
water of 60° Fahr. with a brass weight of 40 grains attached to the wick
by a small piece of thread ; when a further weight of 2 grains is laid on
the butt-end of the candle it will still float, but with a weight of 4 grains
it will sink."
The following is the procedure for the use of this standai'd : the candle
selected for the test must be a straight one, with the wicks central in the
longitudinal axis, and must not be too tapered from end to end. The
sloping top is to be cut off at the shoulder, and the candle then equally
divided in the centre. The two new ends thus obtained are to be trimmed,
so as to form new wicks, which, when lighted and burning, are to be set in
Kuch a position that the plane of the curvature of one wick is perpendicular
to the plane of the curvature of the other wick.
The candles should be mounted on the c^ui die-balance in the photometer
for ten minutes, or longer if necessary, before making a test, so that the
"cups" are " fairly dry," the wicks curved and the ends glowing. If the
candles are used while the wick is in a vertical position, the results obtained
are certain to be too high.
In Schedule A, Part II., of the Gasworks Clauses Act, 187 1, it is stated
that " candles are to be lighted at least ten minutes before beginning each
testing, so as to arrive at their normal rate of burning, which is shown
CANDLES. CORRECTION FOR CONSUMPTION.
323
Table for finding the Conswmption of Sperm by two Candles in 10 MintUes
from Observations of the Tir*!^ required to bum 40 Grains,
Time required
to burn
Consamption in
Time required
to bum
Coiisamption in
40 Gr^infl.
ID Minutes.
^0^^ m^%mm mm
43 Grains.
zo Minutes.
Min. Sec
Grains.
Kin. Sec
Grains.
9.0
44.44
10. 1
39.94
9.1
44-36
10.2
39.87
9.2
44.28
10.3
39.80
9.3
44.20
10.4
39.74
9.4
44.12
10.5
39.67
9.5
44.04
lao
39.60
9.6
43.96
10.7
3954
H
43.88
10.8
39-47
9.8
43.80
10.9
39.40
9-9
43-72
10.10
39.34
9.10
43.64
10. 1 1
39.28
9.11
43.56
10.12
39.21
9.12
43.48
10.13
39.15
913
43.40
10.14
39.09
9.14
43-32
10.15
39.02
915
43.24
10.16
38.96
9.16
43.16
10.17
38.90
9.17
43-08
10.18
38.83
9.18
43.01
10.19
38.77
9.19
42.93
10.20
38.71
9.20
42.85
10.21
38.65
9.21
42.78
10.22
38.59
9.22
42.70
10.23
38.52
923
42.63
10.24
38.46
9.24
42.55
10.25
38.40
925
42.48
10.26
38.34
9.26
42.40
10.27
38.28
9.27
42.33
10.28
3822
9.28
42. 2 ^
10.29
38.16
9.29
42.18
10.30
38.10
9.30
42.11
10.31
38.03
9.31
42.03
10.32
37.97
932
41.96
10.33
37.91
9-33
41.88
10.34
37.85
9.34
41.81
10.35
37.80
9.35
41.74
10.36
37.74
936
41.67
10.37
37.68
9-37
41.60
10.38
37.62
938
41.52
10.39
37.56
9-39
41.45
10.40
37.50
9.40
4i-3»
10. 41
37.44
3738
9.41
4«-3i
10.42
9.42
41.24
10.43
37.32
9.43
41.17
10.44
37.26
9.44
41.10
1045
37.21
9.45
41.03
10.46
37.15
9.46
40.96
10.47
37.09
9.47
4089
10.48
37.03
9.48
40. 82
10.49
36.98
949
40.75
10.50
36.92
9.50
40.68
10.51
36.87
951
40.61
1052
36.81
9.52
40.54
10.53
36.75
9.53
40.47
10.54
36.70
9.54
40.40
10.55
36.64
9.55
40.34
10.56
36.59
9.56
40.27
10.57
36.53
9-57
40 20
10.58
36.47
9.58
40.13
10.59
36.42
9-59
40.07
11.00
36.36
10.00
40.00
324 FRENCH STANDARD. CARCEL LAMP.
when the wick is slightly hont and the tip glowing." Although it does not
appear that this Act has heen overruled by any subsequent one, the in-
structions of the Gas Referees omit the latter part, and simply say that
the candles shall attain *' their normal rate of burning/' Probably they
presume that this expression relates as much to their condition of burning
as to the actual rate at which the sperm is volatilised ; otherwise it would
be oorrect to use a candle with the wick in such a position that a large pro-
portion of the sperm escapes as unconsumed carbon.
The candles, having thus been brought into readiness for the test, are
counterpoised upon the balance until the weight is slightly in excess of
that of the counterpoise. Ad experimental seconds-clock being in readiness,
with the hand pointing at zero, the candle-balance is ^sratched, and as soon
as the pointer passes the zero mark the clock is started. A 40-grain weight
is then carefully placed in the pan iinder the candles, which thus brings
them again to rest, and the comparison of the two lights proceeded with
one reading being taken each minute for ten minutes. When the tenth
reading has been taken the candle-balance is again watched, and as soon as
the pointer passes the zero mark the clock is stopped and the time noted.
It is now necessary to calculate the amount of sperm burnt in a given
time from the time it took to bum 40 grains — i.e, suppose it took 10 minutes
30 seconds to bum 40 grains ; then in 10 minutes it would have burnt :
10 mina. 30 sees. » 630 seconds : 40 : : 600 : x => 38. i grain ; or divide 24,000
by the time, in seconds, and the result will give the number of grains of
sperm per 10 mins.
The table on p. 324 will be found convenient for reference :
A very short experience with candles will suffice to convince a careful
operator that the only way to attain concordant results is to burn them in
such a manner that they are not overheated by exposure to an excessive
temperature in an insufficiently ventilated chamber, such as that of the
original " Evans " photometer. They give the best result in a perfectly
open room free from draughts. As this cannot always be obtained, good
results may, however, be ensured by surrounding them with a large box,
18 or 20 inches square, perfectly open for 3 or 4 inches at the bottom, and
closed at the top except for a circular aperture at least 6 inches in diameter
(see p. 283). This arrangement provides for a steady current of cool air,
free from side and top draughts, in which the combustion of the sperm will
be uniform and complete, provided, of course, the candles are pi-operly made.
" Carcel *' Lamp, — ^l^his lamp was devised by M. Carcel in 1800. It con-
sists of an annular wick as first arranged by Argand, fed with refined colza
oil by means of a small clockwork pump. M. Monuier, in his ''Etude sur
les Etalons Photom^triques," gives the following instructions for the use
of this standard : " The conditions to be observed when testing with the
Carcel lamp are by no means definite, ns each lamp must first be tested
before being used as a photometric standard. The rule is to arrange the
height of the wick and chimney so that the consumption of oil falls within
the limits of 38 and 46 grammes per hour ; but for exact experiments it is
preferable to restrict these limits and maintain the consumption between
40 and 44 grammes per hour. The light given by the lamp is corrected by
simple calculation on the assumption that 42 grammes of oil per hour yield
one * Carcel '" (see page 326).
The results of exj)eriments made by MM. Auduoin and Berard show,
first, that an increase in the height of the wick up to a certain point—
10 millimetres — increases the consumption of the oil as well as the intensity
the light, beyond which both the consumption of the oil and the intensity
diminish ; and, secondly, that the elevation of the constricted portion of the
glass chimney tends to augment the consumption of the oil in an increasing
GERMAS STASDABD. 3:5
ratio ; but that there is a point where, although the consumption continues
to increase, the intenaitj diminishes. Consequently there is a certain
position for the glass which oDrreepoDds to the maximum illuminating power
of the lamp.
For each experiment a new wick, which must be cut level with the
wick-holder, is neceettary. The height of the wick must be frcni 8 to 10
millimetree, the shoulder of the glaes being fixed about 7 millimetres above
the wick, and that of the flame about 36 millimetres. The lamp, replenished
with oil up to the level of the gallery, is allowed to burn for half an hour
before commencing the experiment. The calculations for coritction from
the observed weight of oil coneumed are facilitated by reference to the
table on the next page :
FiQ. 146.
The Oaroel Lamp and BaUiice.
The value of the " Carcel " in terms of English sperm candles was deter-
mined by Mr. Sugg in 1870 as 9.6 candles. A series of experimenta con-
ducted by the writer in 1885 gnve 9.4 as its mean value. It may, there-
fore, safely be inferred that it is equal to about 9.5 English candlex.
Ths Gm-man Standard. — Ab a renult of experiments carried out by the
German Society of Oaa and Water Experts in conjunction with the Imperial
Technical Institute it wsb found that :
If (a) The amyl-acetate flame 40 mm. in height = i.ooo,
' (6) Tbe German Society's Piiraffin Candle, flame 59 mm.,
'^ 1.224 amyl-acetate flame, or A. A. L.
Tu J W ^l"^ English Spermaceti Candle, flame (L), 45 mm. in height,
^'''"'1 =r.i3sA.A. L.
{d) Tbe SpermacetiCandle,flamB(K), 45mm. inheiglit, = 1.140
[ A. A. L. ;
(L) and (K) being candles obtained from different sources ; or oon-
vertely :
326
CAHGEL LAMPS.
Tahh shewing the Weight of Oil Bumed per Hour, caietdated from the
Time occupied in hwrning lo Orcmimee,
*^ % d
Mln. Sees.
3.0
3.1
3-2
3-3
3-4
3.5
3.6
3.7
3.8
3-9
3.10
3."
3.12
3.13
3.14
3.16
317
3.18
3-19
3.20
3-21
3.22
323
324
3-25
326
327
3.28
3.29
30
31
32
33
34
35
3-36
3-37
3.38
3-39
3.40
3.41
3-42
3-43
3-44
3.45
3-4^
3-47
3.48
3-49
350
3-Si
3.52
3-53
3-54
3.55
356
3-57
3.58
3-59
J
Gnns.
46.15
46.09
46.OJ
45.98
4592
45.86
45.80
45.74
45.68
45-62
45.57
45-5^
45.45
45-40
45.34
45.28
45-22
45.16
45.11
45-06
45.00
44-94
44.88
44.83
44.78
44.72
44.66
44.61
44-55
44-50
44-46
44-39
44-33
44.28
44-23
44.17
44.12
4.^.06
44.01
43-9t>
43-90
4385
43.80
43-74
43-69
43-64
4J.5«
43-53
4348
43-42
43-37
43-32
4327
43.22
43.16
43.11
43.06
43.01
42.96
42.91
16^0
.0989
.0975
.0961
.0947
.0933
.0919
.0905
.0891
.0877
.0864
.0850
.0836
.0823
.0809
.0795
.0782
.0768
.0755
.0741
.0728
.0714
.0701
.0687
.0674
.0661
.0648
.0635
.0621
.0608
.0595
.0582
.0569
.0556
-0543
.0530
.0517
.0504
.0491
.0479
.0466
•0453
.0440
.0428
.0415
.0402
.0390
.0377
.0364
.0352
•0339
.0327
.0315
.0302
.0290
.0277
.0265
.0253
.0241
.0228
.0216
M
In. Sect.
14.0
14.I
142
14.3
14.4
14.5
746
14.7
14.8
14.9
14.10
14. 1 1
14.12
14.13
14.14
14.15
14.16
14.17
14.18
14.19
14.20
14.21
14.22
14.23
14.24
14.25
14.26
14.27
14.28
14-29
14.30
1131
14.32
14-33
1434
14-35
14-36
14.37
14.38
1439
14.40
14.41
1442
1443
14.44
14-45
14.46
14-47
14.48
14.49
14.50
1451
14-52
14-53
14-54
14.55
14.56
14.57
14.58
14.59
I
d
o
Grma.
42.86
42.81
42.76
42.70
42.65
42.60
42.55
42.50
42.45
42.40
42.35
42.30
42.25
42.20
42.15
42.10
42.06
42.01
41.96
41.91
41.S6
41.81
41.76
41.71
41.66
41.61
41.57
41.52
41.47
41.42
41.37
41.33
41.28
41.23
41.19
41.14
41.09
41.04
41.00
40.96
40.91
40.86
40.81
40.77
40.72
4068
40.63
40.59
40.54
40.49
40.45
40.40
40.36
40.31
40.27
40.22
40.18
40.13
40.09
40.04
n
1.0204
1.0192
1.0180
Z.0168
1.0156
1.0144
1.0132
1.0120
1.0108
1.0096
1.0084
1.0072
1.0060
1.0049
X.0037
1.0025
1.0013
1.0002
0.9990
0.9978
0.9967
09955
0.9944
0.9932
0.9921
0.9909
a 9898
0.9886
0.9875
0.9864
0.9852
0.9841
0.9830
0.9818
0.9807
0.9796
0.9785
0.9774
0.9762
09751
0.9740
0.9729
0.9718
0.9707
o. 9696
0.9685
0.9674
0.9663
0.9653
0.9642
0.9631
0.9620
0.9609
0.9S08
0.9588
09577
0.9566
0.9556
0-9545
0.9534
t
i
Mln. Secg.
5.0
5. a
5.3
5-4
ii
ii
5.9
5-10
5.II
5.12
5.13
5.14
5-iS
5.16
5.17
5.18
5-19
5.20
5.21
5.22
5- 23
5.24
5.25
5.26
527
5.28
5.29
5-30
5-31
5.32
5.33
5.34
5.35
536
5 37
5.38
5 39
540
5-41
5.42
5.43
5.44
5-45
5.46
5-47
5.48
5-49
5-50
5.51
5-52
5-53
5-54
5-55
5-56
5-57
5-58
5-59
9 s-t^
Ormi.
40.00
39.96
39-91
39-87
39.82
39.78
39.74
.39.69
39.65
39.60
3956
39.52
39-47
39 43
39.39
39-34
39.30
39.26
39-22
39.17
39.13
3909
39.05
39.00
38.96
38.92
38.88
38.83
38.79
38.7s
38.71
38.67
3863
38.59
38.54
38.50
38.46
38.42
38.38
38.34
38.30
38.26
38.22
38.18
38.14
38.10
38.05
38.01
37.97
3793
37.89
37.85
37.82
37.78
37.74
37.79
37.66
37.62
37.58
37.54
!5 Am-^
0.9524
0.9513
0.9503
0.9492
a 9482
0.9471
a 9461
0.9450
0.9440
a9430
0.9419
0.9409
a 9308
0.9388
0.9378
0.9368
09357
0.9347
09337
0.9327
0.9317
0.9307
a 9297
a 9286
0.9276
0.9266
0.9256
0.9246
0^9236
0.9226
0.9217
0.9207
0.9197
0.9187
0.9177
0.9167
0.9158
0.9148
0.9138
0.9128
0.9119
0.9109
0.9099
0.9090
0.9080
0.9070
0.9061
0.9051
0.9042
0.9032
a9023
0.9013
0.9004
0.8994
0.8985
0.897s
0.8966
0.8957
0.8947
0.8938
THE QERMAN STANDARD.
327
t
(a) I amyl-acetate flame 40 mm. in height has an illuminating power
equal to :
(b) 0.808 Grerman Society's Paraffin Candle, flame 50 mm. in height.
Ic) 0.883 Siiglish Spermaceti Candle, flame (L), 45 mm. in height ; and
[d) 0.879 English Spermaceti Candle, flame (K), 45 mm. in height.
At the Munich meeting in 1890 the Commission reported as the result
of further tests that :
I German Society's Paraffin Candle >■ 1.22 amyl-acetate lamp.
I English Spermaceti Candle « 1.145 to 1.160 „ ,,
As a result of the work of the Photometrical Commission in conjunction
with the Imperial Technical .Physical Institute it w^as decided that :
(i) The amyl-acetate lamp, which in future is to he called the '* Hefner
Light," shall be accepted as the light Measure of the Society in place of
the Society's Paraffin Candle.
(2) The relation of the illuminating power of the Hefner lamp, con-
structed according to the description in Schilling's Journal of 6<m Lighting
and Water Supjily^ 1884, p. 74 et aeq,f with a flame 40 mm. in height com-
pared with the illuminating power of the Society's Paraffin Candle is
established as i to x.20, with a plus or minus variation up to 0.05.
At the request of the Society, the Imperial Institute undertook to
further verify the Hefner lamp.
At a consultation on March 15, '8979 at Berlin, between the Photo-
metrical Commission and the representatives of the Society of Electricians
a complete understanding was arrived at, based on the Geneva resolution on
the question of a light unit and certain measurements connected with it
which have to be considered in photometry. By this agreement the expres-
sion *' Hefner-candle " and the following table of names, symbols, units and
their abbreviations was agreed upon for recommendation to the Society of
Electricians and to the German Society of Gas and Water Experts :
QOANTITT.
UlVIT.
Name.
Symbol.
Name.
BymboL
Light Power
Light Onrrent . •
lUnmiDating Power •
Surface Illumination .
Light Supply
J
= Jw = p S
" = »-?
J
Oandle (Hefner Oandle)
Lumen • • • •
Lux (Meter Candle)
Oandle to i sq. m. • •
Lumen Hour. • •
HC
Lm.
Lx.
w, a solid angle.
6, a surface in sq. metres \ both perpendicular to the
s, a surface in sq. cm. J
r, a distance in metres.
T, time in seconds.
direction of the rays.
In addition to this table the following remarks were appended as a pai*t
of the agreement :
^ By light current is understood the whole mass of light, within a solid
angle, given off from a source of light ; or the whole quantity of light which
a surface S receives at a distance r from the source of light — e.g., suppose the
surface to be the inside surface of a sphere with a radius r, then the light
current reproftents the total quantity of light given off by the source of light.
328 GERMAN STANDARD.
The unit of light current is represented by that quantity of light given off
by a source of light with a light-power of J » i hK inside the solid angle
w s I, or on a surface S ^ i square metre at a distance r « i metre.
This unit of light-current is exprest^ed by ■■ i lumen.
'* The strength of illumination of a surface E is measured in Iakz (Lz) a
quantity which has the same magnitude and meaning as tho former metre-
candle already in use. It is represented by the magnitude of the light-
current in relation to the magnitude of the illuminated surface in square
metres, or by the magnitude of the light power in relation to the square of
the distance from the surface to the source of light.
" On the other hand, the surface illumination represents the brilliancy
of a surface expressed in candles per square centimetre. By one metre-
candle is expressed an illumination such as a surface receives from a candle
placed at a distance of one metre from it. This unit surface illumination
is formed by that brilliancy of a surface which is ao constituted that
I square centimetre of it sends out a brilliancy equal to one candle. The
surface illumination is, therefore, in case the surface receives its illomination
from outside, dependent not only on the brilliancy of the illuminating source
of light, and its distance from the surface of light, but also on the nature
of the surface. The surface illumination comes, in the first place, under
consideration with self -illuminating bodies, such as the carbon filament of
the electric glow-lamp, or the illuminating surface of the incandescent mantle
of an incandescent gas burner. For this reason i square metre could not
be used as surface unit, but i square centimetre had to be chosen.
'* The last rule on the light supply Q refers to the quantity of light sap-
plied by a source of light in a given time."
Herr von HefTier-AUeneck's Amyl-aoetate Lofmp. — The following is the
official description of the Hefner lamp :
The Hefner lamp with Hefner- Alteneck sis^ht gauge is shown in sectional
elevation in Fig. 147 and in plan in Figs. 148 and 149. Fig. 153 shows an
elevation and Fi$;. 154 a plan of the flame measure of Kruss. Figs. 15;,
156, and 157 bhow the check gauge to be supplied. All diagrams are
drawn full siza
The lamp itself consists of a vessel A^ a head B^ containing the wick
guide, and a wick tube 0.
The vessel A serves for the reception of the amyl-acetate, and is made
either of sheet or cast brass tinned inside.
The head B contains the wick-guiding tube A (Figs. 147 and 148) at the
bottom of which there are two right-angled slots opposite each other, and
an arrangement for altering the height of the wick. The latter consists of
two axles d and d (Fig. 148) on which are fixed two toothed rollers %d and to'
(Figs. 147 and 148) which intrench within the above-mentioned right-angled
slots. On one side of the rollers and rigidly connected with these arcs are
the toothed wheels d and e', which, by means of the two endless screws y and
f fastened to the same an^le' axle 6, can be turned in opposite directions.
The axle terminates in the head g by means of which the gearing is set in
motion with the hand. The axle h is prevented from sliding backwards or
forwards, first, by the strong spring /, and secondly, by a circular enlarge-
ment of the axle h between the screws /and f\ which runs in a metal fork m,
fixed to the top of the head B, The wick-guiding tube A projects above
the top plate of the head B, by about 4 millimetres and has on this project-
ing end an outside thread with which the shell D (Fig. 147) protecting the
wick tube can be screwed up. Close against the tube a there are in the
upper plate of the head B two verti^ openings of about i millimetre
diameter which serve to let in air to take the place of the consumed com«
GERMAN STANDARD.
329
bustible, and are so placed that they are covered by the shell D when this
is screwed up.
The wick tabe is made of german silver and jointless ; its length mu^t
be 35 millimetres, its inside diameter 8 millimetres, and its thickness of metal
0.15 millimetres. It is pushed into the tube a as far as it will go. The
projecting end of the wick tube must then be 25 millimetres in length. The
wick tube must be movable in its socket with Uttle friction, so that it can
Fig. 147.
IE
"
The Hefner Lamp.
be easily removed, but it must fit sufficiently tight for it not to be raised
with the motion of the wick.
The flame measure, which is used for ascertaining the proper flame height
(40 millimetres), is fastened to a movable revolving ring h (Figs. 147,
149, 153, and 154), which can be fixed at any position and rests on the
upper plate of the head B. The arrangement of the fixing appliance will
be seen from Figs. 150 and 152. The carrier % (Figs. 147 and 153) which
330
GERMAN STANDARD.
connects the ring with the measuring appliance proper, must be so rigid
that it is difficult to bend without mechanical appliaiices.
For measuring purposes either the Hefner- Alteneck sight-gauge or the
optical appliance of Dr. Kruss may be used. One lamp can be supplied with
both flame measures, but both may not be fastened to the same ring.
Fig. 148.
Fio. 149
The sight-gauge K consists of two tubes sliding one into the other with
horizontal axes passing through the axis of the wick tube. The inner tube
is cut along its entire length and carries a horizontal thin bright steel plate
q (Figs. 147 and 152), 0.2 millimetres in thickness and having a rectangular
opening ; the underside of this steel plate must lie 40 millimetres above the
top edge of the wick-tube.
The optical arrangement r (Figs. 153 and 156) consists of a tube about 30
millimetres long the axis of which is also horizontal and passes through the
GERMAN STANDARD.
331
axis of the wick-tube. This tube is closed up on the side turned towards
the wick-tube by a small lens of about 1 5 millimetre focus, and on the
opposite side by a piece of ground glass of fine grain with the ground side
turned towards the lens ; the ground glass having a horizontal black mark
across its centre not more than 0.2 millimetres in thickness. The picture
of the upper edge of this mark projected by the lens must be exactly 40
millimetres over the centre of the edge of the wick tube.
Fig. 15a
/^ff^
Fig. 151.
H
Fig. 152.
9
No part of the flame measure must be capable of being revolved or
unscrewed. Where screws are used in joining parts together their heads
must be filed off flush so that the slot disappears.
The gauge serves to check the proper position of the upper edge of the
wick tube as well as that of the flame measure. Its arrangement is shown
in Figs. 150, 151, and 152. When it is slipped over the wick tube eo that it
Fig. 153.
stands rigidly on the top of the head B, on looking through the slot S
(Figs. 151 and 152), about half way up the gauge, a fine line of light must
be visible, less than o. i millimetre wide between the top edge of the wick
tube and the horizontal top of the hollowed out space in the gauge, and
when using the sight gauge the edge of the upper part of the gauge must
be on the flame of the lower surface of the sheet plate. When using the
optical flame measure the edge of the gauge must be sharply reproduced on
332
GERMAN STANDARD.
the upper edge of the mark on the flame measure, by which means the
distance between the upper edge of the wick tube and the edge of the gauge
will be exactly 40 millimetres.
The upper part of the gauge has a diameter of rather less than 8 milli-
metres. It must be capable of being easily slipped over the wick tube and
serves to pull out the latter in case it requires cleansing.
Fig. 154.
V.
Fig. 155.
Fig. 156.
Fig. 157.
The gauge is of brass and is made iu one piece.
All metal parts of the lamp, excepting the wick-tube and the steel plate
of the sight gauge, must be coloured dull black.
The following are the official instructions for the use of the Hefner lamp :
The Wick. — The nature of the wick has generally no influence on the
light power. Care must, however, be taken that it completely fills the wick
tube, and also that it is not prassed too hard in it. It is, therefore, prefer-
able to use a sufficient number of thick cotton threads laid together. As
AMYL-ACETATE. 333
this kind of wick is sometimes improperly raised by gearing which is not
carefully finished, and as it easily forms loops in the inside of the vessel
which jam in the toothed wheels and rollers of the gearing, twisted wicks
are often used. There is no objection to the use of these so long as they
fulfil the condition previously mentioned — ^viz., that they completely fill the
wick tube without fitting too tightly in it.
The Amj/l-cusetate. — In order to lessen the difficulty of obtaining service-
able amyl-aoetate the '' German Society " of G&s and Water Experts has
undertaken to procure suitable amyl-acetate in sufficient quantity, to test it
as to its serviceability, and to supply it in a sealed bottle (of from i litre
upwards) through its place of business (Court Councillor Dr. Bunte, of
Karlsruhe).
If the amyl-acetate is not obtained from this source, it is advisable to
examine it as to its serviceability before use by tests given by Dr. Bannow,
who states that amyl-acetate is appHoable for photometry when the follow-
ing conditions are fulfilled :
(i) The specific gravity must be between 0.872 to 0.876 at 15^ Cent.
(2) When distilled (in glass vessels), at least ^ of the volume of amyl-
acetate must distil over between 137° and 145° Cent.
(3) Amyl-acetate must not turn blue litmus paper red strongly.
(4) If to the amyl-acetate is added an equal volume of benzine or carbon
disulphide, the two substances must mix without becoming cloudy.
(5) If in a graduated cylinder i c.c. of amyl-aoetate with 10 c.c. of a
mixture of 90 per cent, alcohol and 10 per cent, water are shaken together,
a clear solution should result.
(6) A drop of amyl-aoetate on white filter paper should evaporate without
leaving behind a permanent grease spot.
Amyl-acetate should be kept well corked, preferably in the dark.
The following details of manipulation should be carefully attended to.
Before MeaaitremerU. — ^When the lamp has been filled and the wick put
in, it must be left until the latter is completely saturated. Before use the
Limp must be examined to see that the gearing moves the wick up and down
properly without carrying the wick tube with it. The wick must then be
raised out of the tube, and the end projecting out of the wick tube cut off as
smooth as possible with a sharp pair of scissors, and the proper position of
the edge of the wick tube examined by means of the gauge supplied with
the lamp as well as the fiame measure, so that the following conditions are
fulfilled :
The gauge having been slipped over the wick tube so that it stands
rigidly on the head carrying the gearing, on looking through the slot about
half way up the gauge against a light, even background (white paper illu-
minated by the sky), a fine light streak, not more than o.i millimetre wide,
must be visible between the upper edge of the wick tube and the inner
hollowed out space of the gauge. The edge of the gauge, when using the
sight gauge, must be on a level with the under surface of the steel plate ;
when using the optical flame measure the edge of the gauge must be sharply
reproduced on the upper edge of the mark on the fiame measure.
The holes situated against the wick tube must not be stopped up.
The measurement must not be made until the lamp has been alight at
least ten minutes, and the temperature of the room in which the ol^rva-
tions are m^de must be between 15° and 20° Cent.
During the Measurement. — The lamp must be placed during the measure-
ment on a small horizontal table, free from vibration and in a pure air free
from draught. Vitiation of the air, especially by carbonic acid (through
the burning of open flames, breathing of several persons, etc.), lessens the
illuminating power of the Hefner lamp considerably. The photometer room
334 krOss flame measure.
must consequently be carefully ventilated before each measurement. In
very Bmall rooms or in closed photometrical apparatus th^ Hefner lamp
should not be used. Draughts of air prejudice to a very high degree the
steady burning of the flame, and render impossible a sufficiently exact
adjustment of the proper flame height.
'' Light measure " is represented by the illuminating power of the Hefner
lamp in a horizontal direction with a flame height of 40 millinietres measured
from the upper edge of the wick tube (Hefner light). This height is
adjusted by means of the flame measure supplied, and the following regula-
tions given by Herr von Hefner Alteneck hold good when using the Hefner
sight gauge :
When looking through the flame to the sight gauge the light core of the
lamp must appear to play on the under side of the light gauge, the less
intense end of the flame point then coincides with the thickness of the sight
gauge ; on a close observation a glimmer of light appears up to about 0.5
millimetre above the sight gauge. The edges of the sight gauge illuminated
by the flame must always be kept bright.
With the Krtiss flame measure the outer border of the flame is absorbed
by the ground glass, accordingly when using it the flame height must be
so regulated that the outermost visible point of the flame picture touches
the mark on the ground glass, and the observer must therefore look at the
ground glass in a direction as perpendicular as possible.
An error of i millimetre in the height of the flame causes a variation of
about three per cent, in the light power.
Care must be taken that, with the exception of the wick tube, the parts
of the lamp illuminated by the flame, particularly the flame measure, are
well blackened. If the fittings do not appear to be sufiiciently blackened
it is as well to place a black screen with an opening in it between the flame
and the photometer screen near the lamp, thus cutting off all reflection, care
must however be taken that portions of the flame are not cut off at the
same time.
After Measur&msni. — Whilst the light is burning a brown thick liquid
residue forms on the edge of the wick tube. This must be wiped off as
often as possible whilst it is still hot and always after using the lamp. If
the lamp has not been used some time the amyl-acetate as well as the wick
must be removed and the lamp thoroughly cleaned. If it is necessary to
remove the wick tube for this purpose this may be done by means of the
upper portion of the gauge.
The writer in the course of his investigations on different standards found
this flame to be very steady, but considerably less than i candle in value
when adjusted to the indicated height — viz., 40 millimetres, but when the
height of the flame was raised to 5 1 millimetres, or 2 inches, it gave results
agreeing with the Pentane and Methven standards. The extreme simplicity
and portability of the arrangement are strongly in its favour, but on the other
hand, the colour of the light, even when pure amyl-acetate is used and the
flame fixed at 40 millimetres in height, renders it extremely diflScult for
various operators to agree. When compared with the Pentane standard the
depth of its colour is most marked. This is an important point, as the
tendency at the present time is to insist as far as possible upon a white
light from all artificial sources, and the adoption of a yellow flame as a
standard of comparison would undoubtedly be a mistake.
Kriisa Flame Measure. — The measurement of the flame height by means
of a pair of compasses or by direct measure is most inexpedient, because on
the one hand the candle or lamp flame is disturbed in its normal combustion
by the close proximity of the observer, and on the other a disturbance is
brought about by the contact of the measure with the soft edge of the
KKUSS FLAME MEASURE. 335
candle or wick. In order to obviate these difScuIties Bindorff placed a
millimetre measure behind the candle and observed it at a distance with
a telescope.
Snmewbat more convenient, especially when dealing with a long series
of tests, is the ezperimental device of Ertiss (Fig. 158) in which an optical
flftme measure is used. On the front end of the tube A an achromatic
objective B is fixed, and on the back portion of a ground glass screen C with
a millimetre eemie. The distance of the optical centre H of the objective
from the ground gbss screen is equal to twice the focal length of the
objective. The complete tube A is adjustable in the shell D by means of
the button a and the ground glass screen, with the scale in a vertical direc-
tion, by means of the button (, the
complete apparatus being adjust- Fio. 158.
able in height by means of the
button C.
The use of this apparatus is
very simple. It is ptaoed at such
a distance from the candle that
the distance from the candle to
the objective is about equal to the
distance of the latter from the
ground glass screen. It is then
net, at the proper height by means
of the button C, and afterwards
by means of the button A, the
picture of the flame F is sbarply
focuHsed on the ground glass
When this sharp focus is ob-
tained, the distance of the optical
centre H of the objective is
exactly equal to the distance of
the optical centre from the ground
glass screen C, and the picture of
the flame is consequently exactly
the same size as the flame itself^ A millimetre division on the glass plate
represents therefore exactly a millimetre of the flame its«lf.
The scale is 100 miliimetres long, and when it is in its highest position
the 50 division is exactly in the axis of the objective. The height of the
complete apparatus being regulated by means of the buthm C, the picture
of the flame is symmetrical to the optical axis of the objective. As the
scale can be moved by means of the button b until the zero stroke just
touches the bluish root of the flame, its height can thus be read off direct
with the picture at this point.
As the candle burns down the zero stroke no longer coincides with
the beginning of the flame, and the height of the complete apparatus
must be altered by means of the button C, leaving the scale unaltered, so
that the picture of the flame remains symmetrical to the optical axis of
the apparatus, following the candle as it burns down. It is unnecessary
to be very particular about the symmetry of the picture with the axis,
but complete eqimlity between flame and picture can no longer be attained
when the flame apjieam in a very oblique position, owing to the properties
of the optical picture. This, however, is easily prevented by the construction
of the apparatus.
Dutch, Standard of Light. — A Committee of the Dutch Gas Association
in 1894 issued a report on photometry and standards of light, in which
336 DfTCH STAKDARD.
after reviewing the English, German, and French gtandanls they oon-
duded that no lamp io use meets all the requiremente, and they aoooniiDgly
proceeded to the conatmctioQ of a new one (Fig. 159). This was a lamp
dedgned upon the biksis of Yemon Uar-
Fia. 159. oourt'a screened one-candle lamp (No. 3)
but burning a mixture of ether and
benzol in the proportion of nine parts of
benzol to 100 of ether by weight, or at
15° Cent, 500 of ether by voliuae to
36.65 of benzene. Of this mixture the
lamp ccmfiumee about 30 c.c (about i
fluid ounce) per hour, and affords a
light equal to 1.48 English standard
candles, which is practically constant
when the proportion of benzene to ether
is between S.5 and 10.3 parts of the
former to 100 oF the latter. This mix-
ture of 9 of benzol to 100 of ether,
made up with ether of specific gravity
0.7215 and benzene of specific gravity
0.886, had a specific gravity of 0.7335,
all the gravities being taken at 15° Cent,
and it was found that after 33 per cent,
of the mixture had burned away the
residue in the reservoir retained exactly
the original specific gravity, i.e., a 7335,
so that the margin of variation which
had been found possible did not affect
the mixture, it having evaporated as a
whole. The ether and benzene used
were redistilled ethyl-ether and purified
benzene (free from thiophene) obtained
from Khalbaum, Berlin. The Commit-
tee recommended that this lamp should
be called the "ether benzol " or " A. B.
l«mp," that it should be adopted as the
standard, the results being stated in
terms of the English Parliamentary
Standard Ciindle as the unit of light.
The results of tests of various stand-
ards by the Committee were as follows :
Atnyl-acetate lamp — 0.9313 English
candle — 0.S333 German candle.
Engliish candle — 1.0854 amyl-acetato
Iam]> —0.9045 German candle.
Cerman candle — i .a amyl-aoetate
lamp — 1.1053 Eii^Iiish candle.
Carcel lamp— 9.631 English candles.
Proposed Substituttis for Candlea.
The most important of these are, in the order of their introduction :
Keates' lamp, Harcourt'a one-candle Pentane unit, Methven's screen,
Sugg's ten-candle screened Arfrand, Violle's molten platinum, Hefner-Alle-
necks amyl-ncetate lamp, Dibdiii's Pentane Argand, Haroourt's Pentane
lamp, and Blondel's etber-benzol lamp.
HARCOURT'S PENTANE AIB-GAa 337
KeaUa' Lamp, when first introduced in 1869, was arranged to yield &
light equal to ten candlen. Subeequently it was improved and made to
yield a light equal to sixteea candles.
The lamp is a modified form of the " Moderator " lamp which, when burn-
ing sperm oil at the rate of 925 graios per hour with a 2-iDch flame, gives a
light equal to sixteen candles. As with candles and the Oarcel lamp, the
consumption of oil must be weighed and a correctioa made. Mr. Sugg has
modified this lamp by placing a screen with an aperture in it in front of
the flame, so that the light may be regulated from two or ten candles,
without the necessity of weighing the oil consumed.
Sarcour£s PerUane Air-Gaa. — Mr, A. Vernon Harcourt, F.B.S., intro-
duced this form of proposed standard to the notice of the Physical and
Chemical Sections of the British Association at
their meeting held at Plymouth in August 1877. Fio. 160.
The following detailed description was pre-
sented by Mr. Harcourt to the Board of Trade
Committee in 1892 :
1. T/ie Burner. — This consists of a bnws tube
4 inches in length and i inch in diameter which
the gas enters near the bottom. The upper eud
of the tube is closed by a brass plug, half an inch
in thickness, in the middle of whitji is a round
hole a quarter of an inch in diameter. Around
the burner is placed a glass cylinder 6x2 inches,
the top of which is level with the top of the
burner, air entering through the gallery on
which the chimney stands. A piece of platinum
wire, about 0.6 millimetre diameter and from 2 to
3 inches in length, is supported at a height of 63.5
millimetres above the burner.
2. The Air-Gae.— This is made by bringing
together, in a gasholder, air and liquid pentane,
which evaporates and mixes with the air, in the
proportion of t cubic foot of air to 3 cubic inches
of pentane. The pentane used is a mixture of
pentane with some of the paraffins of lower and
higher boiling-points, which is prepared by dis-
tilling the light petroleum at 60° Cent., at 55°
Cent., and twice at 50" Cent. The pentane thus
prepared must satisfy the following tests : On
agitation with -jV''^ '^^ '^ bulk of fuming sul-
phuric acid for five minutes, it must impart to
the acid only a faint brown colour. The density
of the liquid must lie between 0.62 and 0.63 at
62° Fahr. The liquid must evaporate at the
ordinary temperature absolutely without residue
when the tension of its vapour is not less than Harcoorfa One-Candle Pen-
7.5 inches of mercury. The densityof the vapour tane Air-Oas Unit,
compared with that of air must be not less than
2-47, nor greater than 2.53.
3. MeagviremerU of the Gat and other Conditions fvr Obtaining a Light of
One Candle,~For the preparation of pentane air-gas it is convenient to
use a gasholder consisting of a cylindrical bell of about 7 cubic feet capacity,
suspended and counterpoised in the usual manner over a tank having an
annular space filled with water. A graduated scale attached to the bell
serves to measure the volume of air drawn in, and also the volume of
338 HARCOURT'S PENTANE AIR-GAS.
vapour formed from the measure of pentane which is discharged from a
pipette through a tap into the holder. Three cubic feet of air (corrected
tor the actual eonditions under which the air is measured) and 9 cubic
inches of pentane (measured at 62^ Fahr.) yield, after standing for some
hours, a volume of air-gas which, corrected to standard conditions, should
not he less than 4.02, nor more than 4.1 cuhic feet.
The air-gas should pass to the burner through a small meter delivering
at each revolution ^\j^th of a cubic foot: The flame having been set at
the standard height, the meter is I'ead two or three times at intervals of
two minutes. A test should be rejected in which the rate of the air-gas
has exceeded 0.52 cubic foot, or fallen short of 0.48 cuhic foot per hour.
After the meter, the air-gas siould pass through a small governor of
the usual construction, fitted to regulate a flow of half a cubic foot per
hour.
The height of the flame is to he adjusted, by means of a delicate stop-
cock, by gradually raising it until the top appears to touch, but not to
pass, a horizontal platinum wire, which must be placed exactly over the
flame, and extend not less than half an inch beyond it.
Since the apparent position of the tip of the flame varies slightly with
the sensitiveness of the eyes of the observer, and the principal variations
in sensitiveness are due to the greater or less exposure of the observer's
eyes to light, both during an observation and for some time previously, it
is necessary to deflne the conditions under which the height of the fliame
is to be judged.
For photometry in which the relative illumination of two adjacent
surfaces is compared, the observer must be in a darkened room and
screened from the two sources of light. In the screen which is between
the observer and the one-candle flame there should be an opening, the
horizontal length of which is about i inch, and its width about ^ inch, at the
same level as the top of the flame. Through this slot the ol»erver looks,
lowering his head until he sees only the tip of the fliime and the wire
extending above it. Behind the flame and wire the background should be
as uniform and dark as possible.
The variations of temperature and humidity, though affecting the absolute
light which the standard yields, and needing to be corrected for where absolute
measures are concerned, do not affect comparisons made between this standard
and other hydrocarbon flames.
Summary of Ohaervatioiia on the Amowni of Light given hy ike Pentane
Flame at Different ff eights. — A pentane lamp, adjusted to give the same
light as the standard pentane flame, was set at such distances from the
photoped (rectangle of paper illuminated as to part of its surface by light
from one source, and as to another adjoining part by light from another
source, see p. 287), as to give an illumination of 1.2, i.i, i, 0.9 o.i
times the normal illumination, given by a light of one candle at the distance
of one foot.
The light from the pentane standard fixed at a distance of 12.17 inches
was made such by so adjusting the height of the flame so as to give an equal
illumination. Thus the light of the pentfine flame was made successively
1.2 — o.i candle, and for each light the ht^ight of the flame was measured
two or three times by depressing a horizontal platinum wire by means of a
rack and pinion carrier. The mean of these measurements, and of several
settings of the flame to produce equal illumination, was taken as the height
corresponding to each fiaction of a candle.
Four such sets of measurements were made over different parts of the
scale.
The actual distance between the fixed pentane standard and the photoped
HARCOURT'S PENTANE AIE-QAS.
339
being 12.17 inches (hypotenuse of a triangle whose vertical is 12 and whose
base is 2 inches), the pentane lamp was set at the following distances to
produce such an illumination as would be produced from a constant distance
of 12.17 inches by a light having the values given in the second line under
the corresponding distances :
Distanoe
Light .
Distance
Light •
• ■
12.17
I
17-205
.5
12.82
•9
19.23
•4
13.6
.8
22.21
.3
14.54
.7
27.2
.2
15. 70s
.6
38.5
.1
I^ight.
Set I.
Beta.
Set 3.
Bet 4.
Mean.
DUt
X.2
_^
••^
70
71.3
70.6
^_
I.I
—
—
—
—
(ix6.4)
3-2
I.O
..
65
—
63.5
64.2
3.3
0.9
61
60.5
...
60
60.5
3.7
0.8
56.5
55.5
—
56.5
56.2
4.3
0.7
52
5;
51
5 J 5
5'-9
4.3
0.6
47.5
48
48
47.8
4.1
0.5
43
44
43.5
44
436
4.2
0.4
38.5
39
39
39-5
35.8
39
4.6
0.3
35
35
34
35
4.0
0.2
28.S
29
29
293
29
6.0
0.1
24
22.5
-
23.3
23-3
5-7
Even taking the average of several observations, the height of the flame
for a given light cannot be estimated within about 0.5 millimetres. If the
flame were to be used above the standard height, further observations must
be made to fix its value. The mean difference (^ height between i and 1.2
candles is probably too low ; both values were only observed in Set 4, and
these give 3.9 as the difference. Probably, the differences from 1.2 to 0.3
may be taken as equal, and as approximately of the value 4 . — ^^ 3'96*
If a difference of 4 miUimetree makes a difference in light of o.i candle,
a variation in the height of the flame of x millimetre makes a difference in
the light of 0.025 cai^cllo.
As will be seen from Mr. Harcourt's description, the method of pre-
paring the air-gas from the liquid pentane is very simple. For every cubic
toot of gas 3 cubic inches of the liquid pentane are required — ^the air bein^
measured at a pressure of jo inches of mercury, and a temperature of
0° Cent., the pentane at 60 Fahr. In practice, 3 cubic feet of air and
9 cubic inches of pentane are usually taken. Mr. Harcourt's method of
introducing the liquid pentane into the gasholder containing the measured
volume of air, is to pass the bent end of a pipette to the bottom of a syphon-
tube, sealed with water, the long limb of which passes through a cork fitted
tightly in the top of the bell of the holder. On opening the stopcock of the
pipette, the pentane passes down through the tube, and, taking the direction
of the bent end, rises upward through the column of water and over the
top bend of the syphon into the bell. Finding this system very liable to
accidents, the writer employed another of a more simple and reliable
character. The bent end of the pipette is- cut off, and the pipette ground
accurately into a second shorter and stouter tube, provided with a stopcock,
which is fitted into the crown of the bell of the holder by means of a
tightly-fitting indiarubber cork. The charging of the holder with pentane,
340 METHVEN'S SCREEN.
without either letting air into the holder or losiog peotaae, thus becomes a
■natter of absolute certainty. The pentane-pipetto is first filled in the usual
manner, and the ground end gently but firmly fitted into the open end of the
tube in the belt of the holder. When the two stopcocks are opened, the
pentane runs through the tube into the holder at once without possibility of
loss. When the pipette is nearly empty, it is closed at the top with the
finger, and the bulb warmed by clasping it with the hand, the pentane
"Methven'H" Staodard Light Doit or Screen, with double ilot,
vapour thus becoming expanded, drives the last portions of pentane into
the holiler. The stopcock in the short fixed tube, which is ground perfectly
gas-tigbt, is closed, and the pipette removed.
The adoption of Harcourfs pentane air-gas unit as a standard of reference
was recommended by a committee of the Board of Trade in 1881; by the
Metropolitan Board of Works in 1887; and by the Standards of Light
Committee of the British Association in 1888, and accepted by the Board of
Trade Committee in 1895 ns " a true representative of the average light
furnished by the sperm caudle constituting the present standard."
VIOLLE'S MOLTEN PLATINUM. 34 1
The Methven Screen was introduced in 1878 by the late Mr. Methven as
a means of obtaining a light of constant illuminating power. The apparatus
consists of an upright rectangular metallic plate or screen, with a horizontal
tiange or bracket, upon which a standard *' London *' Argand burner is fixed,
the latter being supplied with gas through a plug or uose piece projecting
(downwards. The upright plate has a slot or hole above the bracket holding
the Argand burner ; over which is fixed a thin silver plate, having a vertical
slot of such dimensions as *' will allow of the passage of as much light as
equals that afforded by two average standard sperm candles when the
Argand burner is delivering sufficient gas to give a flame 3 inches in height."
Finding that this arrangement was only to be depended on when gas of a
certain quality was used, Mr. Methven experimented with '' carburetted ''
gas — 1.0., gas enriched with certain hydrocarbons, adopting the same agent
as Mr. Harcourt — ^viz., pentane. From his experiments Methven found
that all carburetted gases were too rich to be burnt properly in a standard
Argand with a 6-inch by 2 -inch chimney, unless the flame was reduced to
2 inches in length, but with a flame of this length the amount of light
yielded was constant and altogether independent of the actual illuminating
power of the coal and cannel gases employed for enrichment. In order to
compensate for the greater luminosity of the flame so obtained, he employed
an aperture of smaller area than that used with the plain coal ga&
In 1883, Messrs. Heisch and Hartley repoil^d on this proposal to the
QdA Institute in the following terms :
'* The range in qualities of the gases with which the Methven plain gas
standard can be safely used is much wider than has been generally sup-
posed ; as in our experiments the extremes are 13.65 and 22.4 — a range of
8.75 candles. • • • The Methven standards are simple in construction ; not
liable to get out of order ; and extremely easy to use. They do best, like
candles, in an open photometer ; but can be readily used in a closed one, if
due care is taken to freely ventilate the photometer and avoid violent air
currents — conditions which are extremely difficult to fulfil with closed photo-
meters. • • The only conclusion which can be drawn from such a mass of
evidence is that the Methven units are not only perfectly reliable instru-
ments for ordinary gas testings, but are suitable for use in photometric
investigations of a much more refined character.''
Sugg*8 Ten- Candle Standard. — ^This standard consists of an Argand
burner, burning ordinary gas in a 3-inch flame ; the height being accurately
fixed by regulating the pressure. The top of the flame is cut off for photo-
metrical purposes by means of a screen, which leaves the whole of its width
visible, but reduces the height of the light to about i| inches. The standard
is mounted on a meter, which registers the amount of gas consumed, and
constitutes an indication of the highest value of the variations of the flame.
Thus the standard is not based on the assumed constancy of a portion of a
gas-flame, or on the height of flame and pressure of gas ; but is essentially
a variable standard, the variability of which is known, and can be allowed for.
ViclUs Molten FkUinnm^ Standard. — In this proposal the unit of light
suggested is that emitted from a square centimetre of platinum in a solidi-
fying condition. Professor YioUe states that ^ the principle of the standard
is the constancy of the point of fusion, and the constancy of the temperature
during the whole time of the change of state,'' when the maximum light is
given off, and the reading of the photometer taken. This system was
recommended by a Congress of Electricians held in Paris, as an international
standard; but in 1881 another Congress rejected it, on account of the
difficulty attending its application, and the colour of the light — at the same
time recommending the adoption of the Carcel lamp.
Fig. 162 illustrates the arrangement constructed by M. Carpentier at
342 VIOLLE'S MOLTEN PLATmUM.
the lostanoe of M. Violle for oonveuieDtly effecting the fusion of the
platiDum.
The crucible C coataioing the platinum which is heated by the blow-
pipe GO, is carried on a slide so that it can be moved rapidly by the rack-
work and pinion M, under the screen BD, which ia kept oool by the flow of
water through the pipes RR.
Thla^reen is pierced with a round hole D having an area of t sq. cm.
The rays passing through this hole are thrown on the photometer by reflec-
tion from the mirror M, the coefficient of absorption of which is determined.
The Britiah Association Committee on Standards of Light, in their
report presented in i88S, stated that, in their opinion, Professor Yiolle's
standard of molten platinum was not a pisctical standard of light, although
they were quite prepared to agree to the adoption of the light emitted by a
square centimetre of molten platinum as a unit, but not as a Btondard of
light.
Dibdin's Pentans Argand. — This proposed standard, Fig, 163, is a modi-
fication of Sugg's lo-candle test, and Hurcourt's pentane air-gas unit, used
in conjunction with Methven's carburetter. The burner is an Argand of
the type used by Sugg, but modified to burn air-gas, the height of the
flame being regulated to 3 inches, seven-tenths of which is cut off at the top
by a screen. The air-gas is obtained by passing air over liquid pentane
contained in an ordinary Methven carburetter. For practical purposes ooal
gas may be used equally with air, the difference in the lurainoaity of the
flame being negligible.
This standtuil is made to give a light equal to 10 candlee, and owing to
the nature of the flame and the fact that, unlike the Methven standard,
only the top portion is screened off, a variation of 1.5 inches in the heiehl
of the flame makes no difference to the luminosity of the unscreened part.
This is due to the whole of the light emitted from the lower and blue part
of the flame being included in that of the " titandard " portion, which admits
DIBDIN'S PENTANE AROAND.
of conflidentble variation in the height of the flame, because when the flame
18 lowered, although there is lees bod; beneath the screen there is also lees
W. J. Dibdin'B Pentane Argand and CarburetWr,
Heaaarementi ot Dibdin'B Ten-CaQdle Pentane Argand Bimer.
Number o( holes 41
Diameter of bolei 0.71 mm. — o.ozE icohf
Inside diameter of steatite .... 9.90
Oatside diameter of steatite . . . 19-05
Diameter of inside of metal cone at top 23.63
Chimney length 152-4
„ inside diameter . . . . jS.i
Height of cut-off 54.61
The centre of the flame to be immediately over the terminal of the photometer bar.
" blue." and inversely, when the flame is lenjithened, the " body " of white
light IB increased, and at the same time the amount of " blue " Is also in-
344
DIBDIN'S PENTANE AKGAND.
creased, and these two differenoes compensate for each other. As it might
be thought that temperature would influence the quality of the pentane gas
formed by merely driving air over liquid, the carburetter was first sur-
rounded with ice and water, and afterwards with water at 90° Fahr. After
adjusting the flame to the altered flow of gas thus caused, the luminosity
was found to be precisely the same in each case.
It was recommended by the Board of Trade Committee, 1895, as the
practical standard of light at the same time as Harcourt*s pentane air-gas
unit was recommended as the standard of reference. The following descrip-
tion is reproduced from the report of that Committee.
Dibdin^s 10-Candle Pentane Argand Air-Gas Standard, as Recommended hy
the Conimittee for Use in Gas Testing,
The apparatus used in producing this standard of light consists of two
separate portions ; viz., the burner and the carburetter.
Fig. 164. Fig. 165. * Fig. 166,
n
I
;
1
i
6ku
USm
Elevation and Section of Pentane Argand Burner.
The bumet-, Figs. 164, 165, 166, is a specially constructed tri-ourrent
Argand burner, the annular steatite ring being perforated with forty-two
holes, each hole being 0.71 millimetre in diameter. The three air currents
are : (i) The central current rising inside the steatite to the inner portion
of the flame; (2) a current rising outside the steatite, and caused to
impinge upon the flame by an inner metal perforated and incurved cone,
the top of which is level with the top of the steatite ; (3) an outer current
rising on the outside of the above cone, and between that cone and the
glass chimney.
The inner perforated cone, Fig. 169, is punctured with ten apertures
0.25 inch in diameter, which are provided for the purpose of equalising the
two outer currents of air as may be required to suit tbe height of the flame.
The glass chimney is carried in the groove provided on the outer conOi
dibdin's pentaxb aeqand.
345
which answers the purpose of a gallery ; the dimensions of the chimney
being 6 inches high and i^ inches inside diameter. The top of the flame
should be maintained as nearly as possible at three inches above the steatite ;
this point being indicated by the wires crossing the blue glass screens carried
on each side of the burner on the metal supports. The flame is steadied by
the small air-directing cone situated centrally beneath the steatite ; the apex
being 0.03 inch below the metal support carrying the steatite.
On the side of the burner to be presented to the photometer disc, a metal
screen, Fig. 170, S^ inches in height, is placed and screwed securely to the
base-plate. The middle portion of this screen is cut way so as to leave
Fig. 167.
FiQ. 168.
Fig. 169.
^"* •«*••*••• •}<Za • • »••• • A^Mc^lf
lU
t
a
t9i
CLtVATIOH
Dibdin's Ten-Candle Standard Pentane Argand Bamer Plan.
above the top of the steatite burner an opening 2.15 inches in height and
1.4 inches in width ; the lower portion of this opening being exactly level
with the top of the steatite. The light emitted horizontally through this
opening by the flame produced by the combustion of the gaseous mixture
of atmospheric air and pentane formed in the carburetter described below
is used as the standard of light. It is equal to the light emitted by 10
parliamentary sperm candles.
The lower portion of the screen has an opening i inch wide by 2.3 inches
in height, to allow free access of air to the under portion of the burner.
The various dimensions and arrangements of the parts are more particu-
larly described in the following table of measurements and accompanying
detailed plans.
The position of the burner in relation to the photometer disc is to be
fixed by the burner fitting gas-tight into a faced joint attached to the
346 DIBDIN'S PENTANE ARGAND.
photometer at the required point ; and the burner is to be set at such a
height that the centre of the illuminated disc and the bottom edge of the
cut-off shall be in the same horizontal plane. The length of the connection
between the burner and the carburetter may be varied, but should not be
more than 5 feet.
The centre of the flame is to be immediately over the terminal point of
the photometer-bar.
Fig. 170.
rffottr vicMf
SUmkfdHtiqkt
levt/ of Ibp
of Scnmit
otBurmt
Sf Of ¥I€W
OpVMtg Ihf
6t»u fiicu.
Wire bed tia.
^LMH •r Borrom nATt.
Framing and Screen of Dibdin's Ten-Candle Standard Pentane Aigand Barner.
The carburetter, Fig. 171, for the lo-candle pentane Argand consirts of
a circular vessel constructed of tinned plate —
Diameter
Depth .
203.2 mm. = 8 inches.
50.8 „ =2
>«
having a spiral division 25.4 mm. = i inch in width. This division is made
by soldering in a spiral strip of metal 4 feet 6 inches in length and 2 inches
wide, gas-ti^ht, to the under side of the top of the carburetter, so that when
the top is fixed on, the bottom of the strip comes close to the bottom of the
DIBDIN*S PENTANE ARGAND.
Fig. 171.
347
mmm CF BPttuu CMAmmu
rtAM.
Carboretter for nse with Dibdin*f Ten-Candle Standard Pentane Aigand Bamer.
Fio. 172.
Oarbnretter for use with Dibdin's Ten-Candle Standard Pentane Argand Bamer.
348 HARCOURT'S SCREENED LAMP.
vessel and is sealed by the pentane, so that the air has to pass over pentane
for a distance of about 4 feet 6 inches, and becomes thoroughly saturated.
At the end of the spiral division, near the side of the carburetter, a bird
fountain is fixed for charging the carburetter, Fig. 172, and keeping it
charged at a constant level with liquid pentane.
The lower end of the inlet fountain tube is closed, and rests upon the
bottom of the tank.
Through the side of the tube, which is y(yths of an inch (lo.i mm.) in
diameter, 16 holes i mm. in diameter are bored close to the bottom ; and
through these the pentane enters the carburetter. At one side of the inlet-
tube, and one inch from the lower end, a small tube 33 mm. in diameter,
and 20 mm. in length, is connected thereto, and turned upwards. The
fountain inlet-tube is carried up through the top of the carburetter, and
continued in the form of a bulb having a capacity of about 200 cc. Stop-
cocks are provided at the top and bottom of the bulb for convenience in
HI ling with pentane, and the portion above the upper stop-cock is opened
out in a funnel shape for the same purpose. When the carburetter is being
charged, the gas must be extinguished to avoid the risk of the vapour firing
and causing an explosion.
The inlet for gas or air is at the side of the carburetter, and at one
terminal of the spiral division. Fig. 171 ; tho outlet being placed in the
centre of the vessel, so that the air or gas may travel over the liquid pentane
throughout the whole length of the spiral division, and thus become fully
charged with the volatile vapour of the pentane.
When using this standard, the pentane must be visible in the fountain
bulb.
Uarcourta Screened Pentane Lamp (No. 2) is constructed as follows : The
vessel A , which may conveniently be of glass and of the form and dimen-
sions of an ordinary spirit-lamp, contains the oil or liquid which is
used in the lamp, preferably pentane obtained by purification and repeated
rectification from American petroleum. This liquid is so volatile that it is
converted into gas within the burner ; the wick serving only to bring the
liquid to a part of the tube where the heat is sufficient to cause it to evapoi-ate
at the required rate.
The two illustrations show — Fig. 173 sectional front view, and Fig. 174
side view of the lamp. The glass vessel A is mounted upon a stand 2?, pro-
vided with levelling-screws C. To the vessel is fitted a cap D surmounted
by a tube E, in which a wick is wound up and down by tho ordinary arrange-
ment of a double-spiked wheel F turned by a handle. Around the upper
part of this tube, the diameter of which may be about ^ inch, and its
length 6 or 7 inches, is a second tube G of about i inch in diameter and
4 inches in length, which serves as a jacket to keep more constant the tem-
perature of the inner tube, and to guide the air current, upon which the
steadiness and brightness of the flame depend. The two tubes are joined
by fiat plates ff above and below, and constitute the burner of the lamp.
Attached to the inner tube by brunches is a gallery J carry a metal
chimney K, which surrounds both the burner and the lower part of the flame.
Above the burner the part K^ of this chimney is reduced to a diameter
intermediate between that of the aforesaid outer tube G and inner tube B
of the burner, and terminates at a short distance above the burner. The
upper part of the flame is again enclosed by a continuation L of this metal
chimney, which is of the same diameter as the lower part, but is enlarged
in diameter towards its upper end.
This upper portion of the chimney is cnnneoted with the lower chimney
by curved metal bands J/, usually two in number, and sufiiciently removed
from the flame on either side as not to aft'ect it. Through the space thus
^ • 1
EAECOURT'S SCREENED LAMP.
349
left Wtwwn the upper wnd lower metal chimneys, the ceotral part of the
loug fluuie which the burner produces is alone visible. The attachment of
these baudti connecting the upper to the lower chimney is adjustable by set
Bcrens N, so that the opening through which the central part of the flame
is seen may be made longer ur smailer as desired. By any simple means,
such as an adjusting strew, or preferably by means of cylindrical gauges of
the same diameter as the tubes which they separale, this opening can be set
quickly and accurately to such Kizes as will give exactly ihe light of 0.5
candle, 1 candle, 1.5 candles, or values intermediate between the^e as
desired.
At opposite sides of the lower part of the upper chimney are two narrow
Fig, 171. Fio. 174.
Harconrt'a Pentane Lamp (No. 2).
slots 0, through either of which the tip of the flame may be seen ; and the
construction of the lamp is such that the light emitted through the opening
between the two chimneys is the same whenever the tip of the flameappesrs
opposite the plot, whether towards the lower or the upper end.
The bands M connecting the two chimneys are made half the width
of the tube that surrounds the flanie. When the lamp is vertical so that
these hands are in a plane perpendicular to the horizontal bar of the
photometer, a point in which the plane containing the edges of the bands
neareet to the photometric disc, midway between these edges, and at the
height of the centre of the apeiture through which the luminous flame is
visible, is to be taken as the point from which distances are to be measured.
This point represents the zero of the usual photometric scale.
In order to easily obtain the plane in which this point lies, two slots S
are cut in the bands M on the side nearest the disc, and into these slots a,
flat piece of metal fits of the same thickness as the depth of the slots. The
point from which distances are measured hes on the surface of this piece
nearest to the disc.
Suitable attachments are provided for carr3-ing a plumb-line P to serve
in setting the lamp vertical, and for carrying a small piece of coloured glass
350 HARCOUETS TEN-CANDLE LAMP.
fitting in the plumb-liDe socket T', ao as to stand oppoeite to the slot 0. Sy
rtillection from, or direct vision through this glass, it may oasOy be observed
whether tiie tip of the Same is within the slot or not The height of
gaugee which produce, when burning pentane, a light equivalent to 0.5, i,
Fio. I7S-
Hareourt'H Ten -Candle Pentane Lamp.
or 1.5 Btaodard English parliamentary candles, is respectively 7, 16, and
27.5. millimetres.
This Ump is a, great improvement upon the first one devised by the
same inventor. In addition to being very constant, it ia exceedingly simple
to manipulate. It is in reality an improved amyl-acetate lamp, burning
pentane. A3 in the Methven standard, only a portion of the centre d the
flame is taken for the standard.
HARCOUKT'S TEN-CANDLE LAMP. 35 1
Earcowris Ten-Candle PetUcme Lamp, — The following f"?oscription of
Harcourt's ten -candle pentane lamp is taken from the notiiicatiou of the
Gas Referees.
This lamp is one in which air is saturated with pentane vapour, the air
gas so formed descending by its gravity to a steatite ring burner. The
flame is drawn into a definite form, and the top of it is hidden from view,
by a long brass chimney above the steatite burner. The chimney is sur-
rounded by a larger brass tube, in which the air is warmed by the chimney,
and so tends to rise. This makes a current which, descending through
another tube, supplies air to the centre of the steatite ring. No glass
chimney is required, and no exterior means have to be employed to drive
the pentane vapour through the burner.
Fig. 175 shows the general appearance of the lamp. The saturator A is
at starting about two-thirds filled with pentane.* It should be replenished
from time to time so that the height of liquid as seen against the windows
may not fall below one-eighth of an inch. The saturator A is connected
with the burner B by means of a piece of wide indisrrubber tube. The rate
of flow of the gas can be regulated by the stop-cock S,, or by checking the
ingress of air at Sy For this latter purpose a metal cone, acting as a
damper, is suspended by its apex from one end of a lever, to the other end
of which is attached a thread for moving the cone up or down. The lever
is supported by an upright arm clamped to the upper end of the stop-cock
immediately beneath the cone. From the top of the lamp the thread
descends to a small pulley on the table, and thenca passes horizontally to
the end of a screw moving in a small block, by turning which the gas
examiner can regulate the lamp without leaving his seat. It is best so to
turn the stop-cock iS', as to allow the flame to be definitely too high, but not
to turn it full on, before letting down the regulating cone to its working
position. Both stop-cocks should be turned off when the lamp is not alight.
The chimney tube C should be turned or screened so that no light
passing through the mica window near its base can fall upon the photoped.
The low end of this tube should, when the lamp is cold, be set 47 millimetres
above the steatite ring burner. A cylindrical boxwood gauge, 47 milli-
metres in length and 32 in diameter, is provided with the lamp to facilitate
this adjustment. The exterior tube D communicates with the interior of
the ring-burner by means of the connecting box above the tube E and the
bracket F on which the burner B is supported. A conical shade G is pro-
vided. This should be placed so that the whole surface of the flame beneath
the tube G may be seen at the photoped through the opening.
The lamp should be adjusted by its levelling screws so that the tube E^
as tested with a plumb-line, is vertical, and so that the upper surface of the
steatite burner is 353 millimetres from the table. A gauge is provided to
facilitate this latter measurement. The tube G is brought centrally over
the burner by means of the three adjusting screws at the base of the tube 2>.
This adjustment is facilitated by means of the boxwood gauge.
When the lamp is in use the stop-cocks are to be regulated so that the
tip of the flame is about half-way between the bottom of the mica window
and the cross-bar. A variation of a quarter of an inch either way has no
material influence upon the light of the flame. The saturator A should be
placed upon the bracket as far from the central column as the stop at the
end will allow. If it is found, after the lamp has been lighted for a quarter
* Oaution. — Pentane is extremely Inflammable ; it gives off at ordinary tem-
p<»ratare8 a heavy vapour which is liable to ignite at a flame at a lower level thRn the
li(]uid. The saturator must never have pentane poured into it when in position, if the lamp
is aJifjht.
352 HAKCOURT'S TEN-CASDLE LAMP.
of an hour, that the teadency of the flame ia to become lower, the eaturator
may be placed a little nearer the central column.
To prevent a gradual accumulation of dtiat in either the burner or the
air-pasi^a^e, a email cover of the size of the top of B and i^baped like the lid
of a pill box should be kept upon the lamp when not in use.
The following are the more important dimensions on which the precision
of tbe lamp depends ; but no departure should be made from any of the
dimensions as shown by the working drawings. All dimensions are given
in millimetres.
Saturator J.— 184 x 184 x 38 deep, inside measurement, with seven
{nrtitions alternately meeting either side and stopping 35 short of the
Fig. 176.
NoU. — Theentrancepi[>efoT Ihepentane, shown dotted in Fig. 176, should be mora
nearlf horizontal, aa ebown in Fig. 175.
opposite side to cau.se the air to pass eight times across the box. TheM
partitions must be soldered to tbe top, not to the bottom of the box.
Siphon Tube from Saturator. — Outer diameter, 14 (b&lf-inch full).
India Rubber Tvhe. — Inner diameter, 13 (half-inch).
Steatite Burner. — Outer diameter, 24.
Inner diameter, 14.
30 holes, not less than 1.25 or more than 1.5 in
diameter.
The holes must be evenly spaoed, and in nny one burner they must not
differ from one another in diameter by more than .05 millimetre.
Brass Chimney C. — Outer diameter, 32.
Inner diameter, 30.
Length, 431.
VIOLLE'S ACETYLENE STANDAHD.
353
Braaa Outer Tvhe D, — Outer diameter, 52.
Inner diameter, 50.
Length, 290.
Chimney C projects 68 below and 73 above the tube D,
Brass Ttibe E. — Outer diameter, 25.
Inner diameter, 23.
Length, 5 29 J.
Distance between axL) of tube E and axis of tubes C and i>, 67.
Shade G. — Diameter of base, 102.
Diameter at top, 55.
Height, 57.
Opening 38 within, 34 without.
The structure of tibe actual burner is shown in the sectional drawinir,
Kg. 176.
This proposed standard has been adopted for use in the London Gas
Testing Stations in the place of candles under an agreement between the
Gas Companies concerned and the Metropolitan Gas Referees, the standard
sperm candle, however, still remaining the parliamentary standard.
Fio. 177.
&
I
2
Violle's Acetylene Standard.
VioU^s Acetylene Standard. — In 1895 M. Yiolle proposed to use as a
secondary standard an acetylene flame burning under pressure, in a Man-
chester burner, Fig. 177. The acetylene passing through a small conical
opening like that of a Bunsen burner, draws with it the necessary supply
of air. The mixed gases then pass through a small hole into a cylindrical
tube where they thoroughly mix. The flame is said to be perfectly steady
and remarkably white ; either the whole or a portion of it may be used as
the standard. The flame is enclosed in a box, not shown in the figure, one
of the faces of which carries an iris disc diaphragm by means of which
the amount of light emitted may be regulated, whilst the other face is
fitted with a rotating diaphragm which carries four openings of various
known sizes. The total illuminative value of the flame is xoo candles
(French) when burning under a pressure equal to 30 centimetres of wateri
354 DR. POLE'S LAW.
the flow of the acetylene being equal to 58 litres per hour. The light difiers
slightly from that of the fused platinum used by M. Yiolle as the absolute
unit, but the apparatus is said to be very convenient for practical use.
Methods of Determining the Illuminating Power of Goal Gtaa.
The usual method of testing the illuminative value of coal gas has been
described under the head of photometers and in the notification of the Gsa
Referees in Appendix 0.
The question of the actual illuminating power is however in some doubt,
in consequence of the altered conditions introduced by the changes in the
standard quality of the London gas supply, by means of which the Parlia-
mentary standard has in some case» been reduced from 16 to 14 candles.
When this alteration wns made the question naturally arose as to the
method by which thin lower quality of gas was to be estimated, as the
standard burner was speciHlly constructed to burn i6-caDdle gas. The
Gas Referees prescribed, when the table photometer was employed, that
the gas should be burned at such a rate that the light afforded by the
burner should equal 16 candles and that the illuminating power of the gas
should be ascertained by calculating to a standard rate of consumption of
5 cubic feet per hour. This prescription was made in regard to the slight
variation which prevailed with the ordinary so-called 16 candle gas, but
when Parliament sanctioned the extension of this method to the testing
of 14 candle gas the matter became more involved as the correction for the
rate of consumption gave different results to those which were obtained
when the gas was burnt at the hitherto standard rate of 5 cubic feet per
hour : differences of more than one candle being thus in some cases obtained.
The different illuminating values ascribed to a gas by different methods is
by no means a new point, as the late Dr. Pole, formerly one of the
Metropolitan Gas Refereev«$, in October 1870, published in the Journal of
Gas Lighting a most careful investigation on the " Theory of Gas burners."
Although the results of this research has been much neglected, in con-
sequence of the all but general practice of prescribing that the illuminating
power of the gas shall be such that it will afford a certain degree of
illumination when burnt in a standard burner at the rate of 5 cubic feet
per hour, recent legislation and the practice of the Metropolitan Gas
Referees has placed the question on a different footing, and it therefore
becomes necessary to ascertain how these altered conditions affect the question
as to the quantity of light which is meant to be represented by the term
" illuminating power."
Dr. Pole found that '^ the law which governs the relation between the
quantities (of gas consumed) is that during the normal state of action of a
gas burner the light given varies directly as the consumption, minus a con-
stant quantity. In algebraical language, let q equal quantity of gas con-
sumed per hour in a given burner," and L equal light produced thereby,
then the true law appears to be, L varies as (q - c) where c is constant for
the same burner, ..."
'' The principle involved in this law appears to be somewhat as follows:
When the quantity of gas supplied to the burner is very small, compared
to its normal capacity, it is burnt at a disadvantage having an excess of air ;
it is, in fact, in the position of a Bunsen burner. Hence, as has been
often explained, the deposition of the light-giving particles is impeded, the
flame burns blue, and the light developed is smaller than is fairly due to
the gas employed. But as more and more gas is admitted, this defect
tends gradually to remedy itself, until a point arrives where a normal and
proper condition is reached, and beyond that point every increment of gas
DR. POLE'S LAW. 355
gi^es a corresponding and uniform increment of light. The quantity of
gas necessary to develop this condition in the first instance is represented
in our equation by the constant c ; and, for want of a better name, I may
call it the devdopant for that burner. This quantity, though constant for
the same burner, varies materially for different kinds of burners and also
under other changes of condition. • • ."
Shortly *' the law is that in a given burner, taken through its normal
range, the light given varies as the quantity of gas consumed, minus a con-
stcmi qwMfUity, That is, if L represents the photometric amount of light
produced by the consumption of q cubic feet per hour, the LaA(q-c;,
where A and c are constant for the same gas and the same burner."
In order to apply the law the illuminating value of the gas is observed
at different rates of consumption within the obvious range of the burner.
Each increase of light is then divided by its corresponding increase of con-
sumption : the mean of these several results equal A, i.6., the amount of
candle-power increment per cubic foot throughout the above range. To
find the developant c, divide each individual illuminating value by the con-
stant A and subtract each result from the corresponding gas consumption
from this paiticular illuminating value : the mean of these results equals a
As the standard rate of consumption in ordinary gas testing is 5 cubic
feet per hour, this quantity is taken as representing the value of q.
The table on p. 356 affords an illustration of this method. Compara-
tive results are given in which the illuminating power is estimated by other
methods.
From this it will be seen that Dr. Pole's law gives practically identical
results whether the gas is burnt at a rate which affords 10, 12, 14 or 16
candles of light, i.e., the calculated results from observations at 10 and 12
candles equals 14.0; from those at 12 and 14 candles 13.95; ^^^ from
those at 14 and 16 candles 13.95 candle-power, the average for all results
being 13.96 candle-power for 5 cubic feet per hour, which agrees closely
with the direct observation of 14.0a when the gas was actually burned at
the rate of 5 cubic feet per hour. When the gas was burned at such a
rate as was required to yield a 16 -candle flame and the result calculated
back to 5 cubic feet per hour the illuminating power was 14.76 candles.
356
DR. POLE'S LAW.
-.1
1
1
1
i
s.
1
"Us
■s 1 " S
i
JS
1
e
1 1
O
n H 1 H 3
i i t 5 a
C ^ i s
i i - '
* • A A
1
1
Ipl
5 S II
1
1
%
1
3iSS.°
It
MS'
Is
S"
APPENDIX A.
Ths burner which has been adopted as the standard burner for teeting
commoD gas was designed by Mr. Sugg and was called by him " Sugg's
London Argand, JSo. i."
A section is appended, in which A represents a. supply pipe, B the gallery,
C the cone, D the steatite chamber, E the chimney.
The following are the dimensions of those
parts of the burner upon which its action de- Fra. 178.
Diameter of RDpplr pipes .... 0.08 inch.
Bitanial diamster of aunolai tteadte
chamber 0.S4 „
Intemal ditto 0.48 „
Number of holes 34
DiameMt of each hole 0.043 ••
Internal diameter of aona at the bottom ■ 1.5 ,<
„ „ „ „ top . . 1.08 „
Height of upper sarbu» of cone and of
steatite chamber above floor of gallerj 0.75 ..
Height ot glau chimaey .... 6 „
Interoal diameter ot chimney . - <I •■
The standard burner for testing cannel gas is
a steatite batswing burner, consisting of a cylin-
drical stem the top of which is divided by a slit of
uniform width.
External diameter of top of stem . . 0.31 inch.
Internal diameter of stem . . . . o. 17 „
Width otalit 0.01 „
Depth of lilt a.15 „
APPENDIX B.
Tabular Nnumhers^ being a Table to facilitate tl^e Coirection of the
under different Atmospherie Preeewres
Bar.
Ther.
40"
4J»
44»
46»
48-
.960
60»
62*
»4'
66»
«8»
•932
28.0
■979
.974
.970
.965
.956
•951
.946
.942
j
' .937
28.1
•983
.978
.973
.969
.964
.959
.955
.95"
.945
.941
.936
28.2
.9S6
.981
.977
.972
.967
.963
.958
•953
■949
1 .944
1
.939
1
28.8
.990
.985
.980
.976
.971
.966
.961
.957
.952
I .947
1
.942
28.4
•993
.988
.9S4
.979
-974
.970
.965
.960
.955
.951
.946
28.6
997
.992
.9S7
.983
.978
•973
.96S
.964
.959
.954
•949
28.6
I.OOI
.995
.991
.986
.981
.977
.972
.967
.962
.958
•953
28.7
T.004
.999
.994
•990
.985
.9S0
.975
.970
.966
.961
.956
28.8
1.007
1.003
.99S
.993
.988
.984
.979
.974
.969
.964
.959
28.9
I.OII
1.006
I.OOI
.997
.992
.987
.982
.977
.973
.968
.963
29.0
I.0I4
I.OIO
1.005
1. 000
.995
.990
.9C6
.981
.976
1 .971
.966
29.1
1. 018
I.0I3
i.ooS
1.004
•999
.994
.989
.984
.979
•975
.969
29.2
1.021
I.0I7
1. 012
1.007
1.002
.99;
.992
.988
.982
.978
.973
29.3
1.025
1.020
1. 015
I on
1.006
I.OOI
.996
.991
.986
.981
.976
29.4
1.028
1.024
I.OI9
1.014
1.009
1.004
.999
.995
.990
.985
.980
29.5
1.032
1.0-7
1.022
1.018
1. 013
I.ooS
1.003
.998
.993
.988
.983
29.6
1.036
1.031
1.026
1.021
i.oi6
I.OII
1.006
I.OOI
.996
.992
.986
29.7
1039
1.034
1.029
1.025
1. 019
1. 015
I.OIO
1.005
I.OOO
•995
.990
29.8
1 043
1.038
1-033
1.028
1.023
1.018
I.0I3
1.008
1.003
.998
.993
29.9
1.046
I.04I
1.036
1.03 1
1.026
1.022
I.OI7
1.012
1.007
1. 00^
.997
80.0
1.050
1.045
1.040
1.035
1.030
1.025
1.020
1.015
I.OIO
1
1.005
I.OOO
80.1
1-053
I.04S
1.043
1.038
1.033
1.029
1.024
1.019
1. 014
1.009
1.003
30.2
1.057
1.052
1.047
1.042
1.037
1.032
1.027
1.022
I.0I7
1.012
1.007
30.3
1.060
1.055
1.050
1.045
1.040
1,036
1.030
1.025
1.020
i.ois
I.OIO
30.4
1.064
1.059
1.054
1.049
1.044
I.C39
1.034
1.029
1.024
1
1. 019
I.0I4
30.5
1.06/
1
1.062
1.057
1.052
1.047
1.042
1.037
1.032
1
1.027
1.022
1. 017
80.6
I.07I
1
1.066 i
i.o6r
1.056
1. 051
1.046
1. 041
1.036
1.03 1
1.026
1.020
80.7
1.074
1
1.069
1.064
I 059
1.054
1.049
1.044
1.039
1.034
1.029
1.024
80.8
1.078
1.073
1.06S
1.063
1.058
1.053
1.048
1.043
I-037
1.032
1.027
80.9
I.08I
1.076
1. 071
1.066
i.c6i
1.056
1. 05 1
1
1.046
1
1. 041
1.036
1. 03 1
81.0
I.0S5
1.080
1.075
1.070
1.065,
1
I 060
1
1.055
1.049
1 .044
1
1.039
1.034
^*^ The numbers in the above table have been calculated from the formula
on the Fahrenheit scale, and a the tension of aqueous vapour at <^ If v is any
pressure.
APPENDIX B.
Volums of Gets Measured over Water at different Temperaturee and
(from the Notification of the Gas Referees).
Bab.
Ther.
68*
64-
66«
68'
.912
70*
72*
74»
.897
78»
.892
78'
80*
82*
84
28.0
.927
.922
•917
.907
'902
.887
.881
.875
.870
28.1
.930
.926
.921
.916
.911
.905
.900
.895
.890
.884
.879
.873
28.2
.934
.929
.924
.919
,914
.909
.904
.898
•893
.887
.882
.876
28.8
.937
.932
.928
.922
.917
.912
•907
.902
.896
.891
.885
.S80
28.4
.941
.936
.931
.926
.921
.915
.910
.905
.900
.894
.8S8
.883
28.6
.944
.939
.934
.929
.924
.919
.914
.908
.903
.897
.892
.886
28.6
•947
.943
.938
.932
.927
.922
.917
.912
.906
.901
•895
.8S9
28.7
.95"
.946
.941
•936
.931
.925
.920
•915
.909
.904
.898
•893
28.8
.954
.949
•944
939
•934
.929
.924
.918
•913
.907
.901
.896
28.9
.958
.953
.948
•942
•937
.932
.927
.921
.916
.910
.905
.899
29.0
.961
.956
.951
.946
.941
.935
•930
.925
.919
.914
.908
•903
29.1
.964
•959
.954
•949
•944
•939
•933
.928
923
.917
.911
.906
29.2
.968
•963
.958
.952
.947
.942
•937
•931
.926
.920
.914
.909
29.8
.971
•966
.961
.956
.950
•945
.940
•935
.929
•923
.918
.912
29.4
.975
.969
.964
•959
.954
•949
.943
•93^
.932
.927
.921
.915
29.5
.978
•973
.968
.962
•957
.952
•947
.941
•936
•930
.924
.919
29.6
.981
.976
.971
.966
.960
•955
.950
•944
•939
•933
•927
.922
29.7
.985
.980
.974
.969
.964
•959
•953
.948
.942
.937
•931
.925
29.8
.988
.983
.978
.972
.967
.962
•957
•951
.946
.940
•934
.928
29.9
.991
.986
.981
.976
.970
.965
.960
.954
■949
•943
.937
•932
80.0
•995
.990
.985
•979
•974
.968
•963
•958
.952
.946
.941
.935
30.1
. .998
•993
.988
.983
.977
.972
.966
.961
•955
.950
•944
.938
30.2
1.002
.996
.991
.986
.980
.975
.970
.964
.959
•953
•947
.941
30.3
1.005
I.OOO
.995
.989
.984
.978
•973
.968
.962
.956
.950
•945
80.4
1.008
1.003
•998
.993
■987
.982
.976
.971
.965
.959
•954
•948
80.5
1. 012
X.006
I.OOZ
.996
.990
.985
.980
•974
.969
•963
•957
.951
80.6
1*015
I.OIO
1.005
.999
.994
.988
•983
•977
.972
.966
.960
•954
80.7
1.018
I.0I3
X.008
1*003
.997
.992
.986
.981
•975
.969
•963
•957
30.8
1.022
I.0I7
I.OII
1.006
I.OOO
•995
.990
•984
.978
.972
.967
.961
80.9
1.025
1.020
1*015
1.009
1.004
•998
.993
.987
.982
.976
.970
.964
31.0
1.029
1.023
1. 018
1.013
1.007
1.002
.996
.991
•985
.979
•973
.967
n ^21lAA — IL^j where h is the height of the barometer in inches, t the temperature
▼olnme at f and K inches pressure and Y the corresponding volume at 60* and 30 inches
APPENDIX G.
METROPOLITAN GAS REFEREES' NOTIFICATION OF
METHOD FOR TESTING THE GAS SUPPLIED TO LONDON.
Afl TO THB StANDABD LaHP TO BB USKD FOB TESTDTa iLLUXIirATZNO
POWEB.
Thb standard to be used in tesUng the illuminating power of gas shall
be a Haroourt pentane ten-candle lamp which has been examined and
certified by the Gas Referees. The residue of pentane in the saturator shall,
at least onoe in each calendar month, be removed, and shall not be used again
in any testings.
All pentane provided by the Gas Companies will be examined and certified
by the Gas Referees, and will be sent to the testing places in one-pint cans
which have been both sealed and labelled by them ; and no pentane shall be
used in the testing places other than that which has been thus certified.
As TO THE Times and Mode of Testing fob iLLUMiNATiNa Poweb.
The testings for illuminating power shall be three in number daily. But
if the average of three testings of illuminating power falls below the pre-
scribed illuminating power, a fourth testing shall be made. It is required
(Gaslight and Coke and other Gas Companies Acts Amendment Act, 1880,
section 7) " That the tests for illuminating power shall be taken at intervals
of not less than one hour/' Also (section S) ** That the average of all the
testings at any testing place on each day of the illuminating power of the gas
supplied by the Company at such testing place shall be deemed to represent
the illuminating power of such gas on that day at such testing place."
The photometer to be used in the testing places shall be the table-photo-
meter as described on pages 284 et seq. The air-gas in the lamp is to be kept
burning so that the flame is near its proper height for at least ten minutes
before any testing is made. At the completion of every testing the air-gas
is to be turned ofif ; but, if the interval between two testings does not much
exceed one hour and the Gas Examiner is present during the interval, he
may, instead of turning it off, turn it down low.
The gas burner attached to each photometer shall be a standard burner
corresponding to that which has been deposited with the Warden of the
Standards in accordance with, among others, section 37 of the Gaslight and
Coke Company Act, 1876. A description of the standard burner to be used
for testing gas is given in Appendix A. No burner shall be used for testing
the illuminating power of gas that does not bear the lead seal of the Gas
Referees.
A clean chimney is to be placed on the burner before each testing.
The gas under examination is to be kept burning, so that the flame is
about the usual height, for at least fifteen minutes before any testing is
made ; and no gas shall pass through the meter attached to the photometer
Gas REFEREES' INSTRUCTIONS. 36 1
except that which is consumed by the standard burner in testing or during
the intervals between the testings made on any day, and that which is used
in proving the meter.
The paper used in the photoped of the photometer shall be white in
colour, unglazed, of fine grain and free from water-marks. It shall be as
translucent as is possible consistently with its being sufficiently opaque to
prevent any change in the apparent relative brightness of the two portions
of the illuminated surface, when the head is moved to either side. This
paper should^ when not in use, be covered to protect it from dust ; and if it
has been in any way mai'ked or soiled a fresh piece is to be substituted.
Each testing shall be made as follows :
The index of the regulating tap shall be so turned that the gas flame
gives rather less light on the photoped than the standard, and shall then be
gradually turned on until equal illumination has been obtained. The position
of the index shall then be noted. Next, the tap shall be so turned that the
gas flame appears to give rather more light than the standard, and shall
then be turned ofi' until equality is again attained, and the position of the
index shall be again noted. The double operation shall be repeated. In
making these adjustments, a hmall alternating movement of the tap may be
employed if the Gas Examiner tinds that he can by this means make more
consistent readings; but, as stated, the tap is to be turned before each
setting, alternately, too high or too low. The mean of the four index
positions shall be taken as that which gives true equality of illumination.
The index shall be set to this mean position, the equality of illumination
verified, and the time that the hand of the meter takes to make two complete
revolutions shall be observed.
In order to make this observation, a stop-clock shall be used by which
the time which has elapsed since the clock was started can be read with
an accuracy of at least half a second. The clock shall be started at the
moment when the meter-hand points either to zero or to some other con-
venient mark, and a note shall be immediately made of the mark chosen.
Exactly at the completion of the second turn of the meter- hand the Gas
Examiner shall stop the clock. The time of two revolutions thus indicated
by the clock is to be read to the nearest half-second. From this and the
reading of the aerorthometer, or a determination of the tabular number
deduced from readings of the thermometer and barometer, the illuminating
power of the gas is to be obtained, either directly or by interpolation. Only
one figure after the decimal point need be entered when the result is
above 16; where a lower result is found, both figures should be noted and
entered. A table giving the tabular numbers for difierent temperatures and
pressures is given in Appendix B.
The method of findir-g the illuminating power from the table by inter-
polation, may be illustrated by the two following examples :
I. Time, i min. 53 sees. Reading of aerorthometer, 1.073. By the
table the iUuminating power corresponding to this time of consumption and
to the reading 1.070 is 16.12, while for the reading 1.080 it is 16.27.
Thus, in this part of the scale, when the reading is 10° higher the illumina-
ting power is greater by 0.15 candle. Hence, when the reading is 3° above
1.070, the corresponding illuminating power is 16.12 +-^ x 0.15 ■■ 16.165
candles, and the number to be returned is 16.2.
II. Time, 2 mins. i^ sec. Beading of aerorthometer, .984. The
numbers in the table under .980 are 15.81 for 2 mins. i sec, and 15.94 for
2 mins. 2 sees.; therefore the number corresponding to i^ sec. is the half-
way number 15.875 ; the number found similarly under .990 is 16.035.
The increase for 10 is here 0.16 ; the number corresponding to the reading
36j gas referees* instructions.
984 is accordingly 15.875 + yi^ x 0.16 » 15-939 » and the number to be
leturned is 15 94.
If, in very exceptional circumstances, the aerorthometer scale or the
tables do not include the conditions that are met with, the Gas Examiner
shall determine the illuminating power by means of one or other of the
formulas printed below the tables.
Each testing place must be provided with a chemical thermometer, divided
into degrees on the Fahrenheit scale, and with a standard clock that will go
for a week without re-winding.
The Gas Examiner shall, at least once a week, compare the stop-clock in
the testing place with the standard clock or with his watch.
The Gtas Examiner shall enter in his book the particulars of every testing
of illuminating power made by him at the testing places, during or immediately
after such testing ; and in the case of any testing which he rejects he shall
also state the cause of rejection. No testing is to be rejected on the ground
that the result seems improbable.
As TO THE Times and Mode of Testing for Puritt.
The testings for purity shall extend over not less than fifteen hours of
each day, and shall be made upon 10 cubic feet of gas. The gas shall be
tested successively for sulphuretted hydrogen, ammonia, and sulphur com-
pounds other than sulphuretted hydrogen, in the manner hereinafter pre-
scribed. These testings must be stai-ted between 9 a.m. and 5.30 P.M., and
must be concluded before 9 a.m. on the following morning. They are con-
cluded by the action of an automatic lever-tap attached to the meter, which
stops the passage of the gas when 10 cubic feet have passed. A clock con-
nected with the lever-tap is stopped at the same moment, leaving a record
of the time ; and the tap of an aerorthometer is turned, leaving a record of
the final conditions under which the gas was measured by the meter.
The liquids in the sulphur and ammonia tests, and the slips of paper in
the tests for sulphuretted hydrogen then contain the sulphur and ammonia
which were present in the gas supplied to the testing place during the day
which ended at 9 a.m. The chemical examination of these liquids may be
made on the following day — that is to say, after 9 A.M.*
All connections between the following pieces of apparatus, in which the
purity of the gas is tested, are to be on or above the surface of the table on
which the apparatus stands.
I. Svlphv/retted Hydrogen, — The gas, as it leaves the service pipe, shall
be passed through a small dry governor and thence through an apparatus
in which are suspended slips of bibulous paper, impregnated with basic
acetate of lead.
The test-paper from which these slips are cut is to be prepared, from
time to time, by moistening sheets of bibulous paper with a solution of oue
part of sugar of lead in eight or nine parts of water, and holding each sheet
while still damp over the surface of a strong solution of ammonia for a few
moments. As the paper dries all free ammonia escapes.
If distinct discolouration of the surface of the test-paper is found to
have taken place, this is to be held as conclusive evidence that sulphuretted
hydrogen is present in the gas. Fresh test-slips are to be placed in the
apparatus every day.
In the event of any impurity being discovered, one of the test-slipe shall
* The gas- testing day begins at 9 A.M. of one civil day and terminateffat 9
of the next. The date is that of the civil daj on which it begins (The City of London
Qas Act, 1 868, section 2).
GAS BEFEREES* INSTRUCTIONS. 363
be placed in a stoppered bottle and kept in the dark at the testing place ;
the remaining slips shall be forwarded with the daily report.
II. Ammonia, — The gas which has been tested for sulphuretted hydrogen
shall pass next through an apparatus consisting of a glass cylinder filled
with gkuss beads, which have been moistened with a meaaured quantity
of stimdard sulphuric add. A set of burettes^ properly graduated, is
provided*
The maximum amount of ammonia allowed is 4 grains per 100 cubic feet
of gas ; and the examination of the liquid shall be made so as to show the
exact amount of ammonia in the gas.
Two test-solutions are to be used — one consisting of dilute sulphuric acid
of such strength that 25 measures (septems) will neutralise i grain of am-
monia ; the other of a weak solution of ammonia, 100 measures of which
contain i grain of ammonia.
The correctness of the result to be obtained depends upon the fulfilment
of two conditions :
I. The preparation of test-solutions having the proper strength.
3. The accurate performance of the operation of testing.
To prepare the test-solutions the following processes may be used by the
Gas Examiner :
Measure a gallon of distilled water into a clean earthenware jar, or other
suitable vessel. Add to this 94 septems uf pure concentrated sulphuric acid,
and mix thoroughly. Take exactJy 50 septems of the liquid and precipitate
it with barium chloride in the manner prescribed for the sulphur test. The
weight of barium sulphate which 50 septems of the test-acid should yield is
13.8 grains. The weight obtalnea with the dilute acid prepared as above
will be somewhat greater, unless the sulphuric acid used had a specific gravity
below 1.84.
Add now to the diluted acid a measured quantity of water, which is to
be found by subtracting 13 8 from the weight of barium sulphate obtained
in the experiment, and multiplying the difference by 726. The resulting
number is the number of septems of water to be added.
If these operations have been accurately performed, a second precipita-
tion and weighing of the barium sulphate obtainable from 50 septems of the
test-acid will give nearly the correct number of 13.8 grains. If the weight
exceeds 13.9 grains, or falls below 13.7 grains, more water or sulphuric acid
must be added, and fresh trials made until the weight falls within these
limits. The test-acid thus prepared should be transferred at once to stop-
pered bottles which have been well drained and are duly labelled.
To prepare the standard solution of ammonia, measure out as before a
gallon of distilled water, and mix with it 50 septems of strong solution of
ammonia (specific gravity 0.88). Try whether 100 septems of the test-alkali
thus prepared will neutralise 25 of the test-acid, proceeding according to
the directions given subsequently as to the mode of testing. If the acid is
just neutralised by the last few drops, the test-alkali is of the required strength.
But if not, small additional quantities of water, or of strong ammonia solu-
tion, must be added, and fresh trials made, until the proper strength has
been attained. The bottles in which the solution is stored should be filled
nearly full and well stoppered.
The mode of proceeding is as follows : — Take 50 septems of the test-acid
(which is more than enough to neutralise any quantity of ammonia likely to
be found in the gas), and pour it into tho glass cylinder, so as to well wet
the whole interior surface, and also the glass beads. Connect one terminal
tube of the cylinder with the gas supply, and the other with the meter, and
make the gas pass at a rate nf not more than two-thirds of a cubic foot per
hour. Any ammonia that is in the gas will be arrested by the sulphuric
364 GAS REFEREES' INSTRUCTIONS.
acid, and a portion of the acid (varying with the quantity of ammonia in
the gas) will be neutralised thereby. At the end of each period of testing,
wash out the glass cylinder and its contents with distilled water, and collect
the washings in a glass vessel. Transfer one-half of this liquid to a separate
glass vessel, and add a quantity of a neutral solution of litmus, or other
indicator in ordinary use, just sufficient to colour the liquid. Then pour
into the burette 100 sept^ms of the test-alkali, and gradually drop this
solution into the measured quantity of the washings, stirring constantly.
As soon as the colour changes (indicating that the whole of the sulphuric
acid has been neutralised), read off the quantity of liquid remaining in the
burette. To find the number of grains of ammonia in 100 cubic feet of the
gas, multiply by 2 the number of septems of test-alkali remaining in the
burette, and move the decimal point one place to the left.
The remaining half of the liquid is to be set aside, in case it should be
desirable to repeat the volumetric analysis. This portion of the liquid is to
be used in either of the two following oases :
(i) If the analysis of the first portion of the liquid show an excess of
impurity, the Gas Examiner shall forthwith give the notice provided for in
Acts of Parliament (the Gaslight and Coke Company Act, 1876, sect. 40,
and others) ; and if the Company think fit to be represented by some officer,
the second portion of the liquid shall be examined in his presence.
(2) If the analysis of the first portion of the liquid should miscarry, or
the Gas Examiner have any reason to distrust the result, he shall be at
liberty to make an analysis of the second portion, provided that before doing
so he give notice to the Company In order that they may, if they think fit,
be represented by some officer.
Unless thus used it is to be preserved, in a bottle properly labelled, for
a week.
III. — Measurement of Gas and of ike Rate of Flow. — ^The gas which has
been tested for sulphuretted hydrogen and ammonia shall pass next through
a m^ter by reference to which the rate of flow can be adjusted, and which
is provided with a self-acting movement for shutting off the gas when ten
cubic feet have passed, for stopping a clock so as to indicate the time at
which the testings terminated, and for turning the tap of the recording
aerorthometer. The Gas Examiner shall enter in his book the time thus
indicated, as also the time at which the testings began.
The clock required ^is a good pendulum clock with a wire passing trans-
versely through the case behind the pendulum. Outside the case a lever
arm is clamped to the wire, so that when liberated the arm will drop and
turn the wire. Inside the case an arm is clamped to the wire, and at the
end of the arm a flexible wire is fastened; when the lever drops, this flexible
wire is brought into gentle frictional contact with the pendulum so as to
stop it without shock.
The clock should be wound from the front, and both hands should be
mounted so that they can be set independently also from the front. It is
desirable that the clock should be able to go for a week with one winding,
and the Gas Examiner must satisfy himself from time to time that the
rating is nearly correct.
IV. — Sulphur Compounds other than Sulphuretted Ilydrogeji, — ^This test-
ing shall be made in a room or closet where no gas is burninj? other than
that which is being tested for sulphur and ammonia. A description of the
apparatus to be employed is given in Fig. 180, on p. 371.
Pieces of Besqui-carbonnte of ammonia, from the surface of which any
efflorescence has been removed, are to be placed round the stem of the
burner. The index of the meter is to be then turned forward to the point
at which the catch falls aud will again support the lever-tap in the
GAS REFEREES* INSTRUCTIONS. 365
horizontal poBition. The lever is then made to rest against the catch so a^ to
turn on the gas. The index is then turned back to a little short of zero,
and the burner lighted. When the index is close to zero the trumpet-tube
is placed in position on the stand and its narrow end connected with the
tubulure of the condenser. At the same time the long chimney-tube is
attached to the top of the condenser.
As soon as tbe testing has been started, a reading of the aerorthometer
is to be made and recorded. The mechanism for stopping the clock is then
to be connected with the lever-tap of the meter, so that both may be stopped
at the same moment when ten cubic feet of gas have passed through the
meter. The clock is to be started and set right, and the time is to be
recorded.
After each testing the flask or beaker, which has received the liquid
products of the combustion of the ten cubic feet of gas, is to be emptied
into a measuring cylinder and then replaced to receive the washings of the
condenser. Next the trumpet-tube is to be removed and well washed out
into the measuring cylinder. The condenser is then to be flushed twice or
thrice by pouring quickly into the mouth of it 40 or 50 cubic centimetres
of distilled water. These washings are brought into the measuring
cylinder, whose contents are to be well mixed and divided into two equal
parts.
One-half of the liquid so obtained is to be set aside, in case it should be
desirable to repeat the determination of the amount of sulphur which the
liquid contains. This portion is to be examined under the same conditions
as have been prescribed for the examination of the second portion of the
liquid obtained from the apparatus used in testing for ammonia; unless
thus previously used, it is to be preserved, in a bottle properly labelled, for
one week.
The remaining half of the liquid is to be brought into a flask, or beaker
covered with a large watch-glass, treated with hydrochloric add sufficient
in quantity to leave an excess of acid in the solution, and then raised to
the boiling point. An excess of a solution of barium chloride is now to be
added, and the boiling continued for five minutes. The vessel and its con-
tents are to be allowed to stand till the barium sulphate has settled at the
bottom of the vessel, after which the clear liquid is to be as far as possible
poured off through a paper filter. The remaining liquid and barium sulphate
are then to be poured on to the filter, and the latter is to be well washed
with hot distilled water. (In order to ascertain whether every trace of
barium chloride and ammonium chloride has been removed, a small quantity
of the washings from the filter should be placed in a test-tube, and a drop
of a solution of silver nitrate added ; should the liquid, instead of remaining
perfectly clear, become cloudy, the washing must be continued until on
repeating the test no cloudiness is produced.) Dry the filter with its con-
tents, and transfer it into a weighed platinum crucible. Heat the crucible
over a lamp, increaaing the temperature gradually, from the point at which
the paper begins to char, up to bright redness.* When no black particles
remain, allow the crucible to cool ; place it when nearly cold in a desiccator
over strong sulphuric acid, and again weigh it. The difference between the
first and second weighings of the crucible will give the number of grains of
barium sulphate. Multiply this number by 1 1 and divide by 4 ; the result
is the number of grains of sulphur in 100 cijbic feet of the gas.
This number is to be corrected for the variations of temperature and
atmospheric pressure in the manner indicated under the head of Illuminating
* An eqaally good and more expeditious method is to drop the filter with its
contents, drained but not dried, into the red-hot crucible.
366 GAS REFEREES' INSTRUCTIONS.
Power, with this difference, that the mean of the aerorthometer readings
found at the beginning and at the end of any testing shall be taken as the
reading for that testing. The reading at the beginning of the testing is to
be made by the Gas Examiner, who before leaving the testing place will
set the columns of mercury level in the two tubes of the instrument and
will connect the lever- tap of the aerorthometer with that of the meter.
The fall of the lever of the meter will release a similar lever turning a tap
which closes the tube of the aerorthometer. The reading of the aeror-
thometer as it stood at the end of the testing will require a small correction
for the difference in level of the mercury in the two tubes, which is to be
made in the following manner :
Let R be the corrected reading, r, the actual reading of the aeror-
thometer, r, the reading of the companion tube, h the mean height of the
barometer in units of the aerorthometer 8ca.e, a number which will be
printed on each instrument and is commonly 0.76. Then
a = r, X *±iiJ:JX
h
The correction by means of the aerorthometer readi^ig may be made
most simply- and with sufficient accuracy in the following manner :
When the aerorthometer read-
ing is between . . . .9SS-.965, .966-.975, .976-.985, .986-.995,
diminish the number of
grains of sulphur by • . 4, 3, 2, z per
cent.
When the aerothometer reading is between .996-1.005, no correcticHi
need be made.
When the aerorthometer read-
ing is between . . . 1.006-1.015, 1.016-1.025, 1.026-1.035,
increase the number of
grains of sulphur by • • x, 2, 3 per
eent.
Example :
Grains of barium salphate from 5 cubic feet of gas 4.3
Multiply by 1 1, and divide by 4. 11
4)47-3
Aerorthometer
reading 1.018
Grains of sulphur in 100 cub. ft of gas (uncorrected) 1 1.82
Add 1 1.8 X Y^v = 24 Besult:
Grains of sulphur in 100 cub. ft. of gas (corrected) 12.06
12.1 grs.
The aerorthometer reading is the reciprocal of the tabular number. The
Gas Examiner shall, not less often than once a month, compare the aeror-
thometer readings with the reciprocal of the tabular number deduced from
observations of the barometer and thermometer, aud if there is a difference
of more than one-half per cent, the instruments are to be readjusted*
As TO THE Mods of Testing the Pressttbb at whioh Gas is Suppubo.
Testings of pressure shall be made at such times and in such places as
the controlliDg authority may from time to time appoint. In order to
make this testing the Gas Examiner shall unscrew the governor and burner
of one of the ordinary public lamps, and shall attach in their stead a port-
able pressure-gauge. In places where incandescent burners are used for
GAS REFEREES' INSTRUCTIONS. 367
street lighting, one street lamp in each street or group of streets may be
provided under the lantern with a branch closed by a screw stopper. The
Gas Examiner shall in such cases connect the pressure gauge by screwing
to it an L-shaped pipe fitted with a union, by means of which it may be
connected to the service pipe in the place of the screw stopper. The
L-shaped pipe is to be of such dimensions as to enable the pressure gauge
to be fixed outside the lantern but at about the same level as the incan-
descent burner. It should be provided with a tap.
The gauge, see Fig. 181, p. 372, to be used for this purpose consists of
an ordinary pressure-gauge enclosed in a lantern, which also holds a candle
for throwing light upon the tubes and pcale. The difference of level of the
water in the two limbs of the gauge is read by means of a sliding scale, the
zero of which is made to coincide with the top of the lower column of
liquid.
The Gas Examiner having fixed the gauge gas-tight, and as nearly as
possible vertical on the pipe of the lamp, and having opened the cocks of
the lamp and gauge, shall read and at once record the pressure shown.
From the observed pressure one-tenth of an inch is to be deducted to correct
for the difference between the pressure of gas at the top of the lamp
column and that at which it is suppUed to the basement of neighbouring
houses.
The pressure prescribed in the Acts of the three Metropolitan Gas
Companies is to be such as to balance from midnight to sunset a column
of. water not less than six-tenths of an inch in height, and to balance from
sunset to midnight a column of water not less than one inch in height.
Meters.
Each of the meters used for measuring the gas consumed in making the
various testings is constructed with a measuring drum which allows one-
twelfth of a cubic foot of gas to pass for every revolution. A hand is
fastened directly to the axle of this drum and passes over a dial divided into
one hundred equal divisions. The dial and hand are protected by a glass.
In the meter employed in testing the purity of gas the pattern of dial for
showing the number of revolutions and the automatic cut-off hitherto in use
shall be retained, but in the meter employed fur testing illuminating power,
only the dial above described is needed. The stop-clock may be either
attached to the meter or separate.
The meters used for measuring the gas consumed in making the various
testings having been certified by the Referees, shall, at least once in seven
days, be proved by the Gas Examiners by means of the Referees' one-twelfth
of a cubic foot measure. The following is the Gas Referees description of
this instrument, which is represented in Fig. 179 ; it consists of a vessel of
blown glass of a cylindriacal form with rounded ends terminating in short
tubes about 40 millimetres in diameter outside, which are reduced at their
outer ends to about 20 millimetres in diameter outside. Lines are etched
round each tubular neck in such positions that the capacity of that portion
of the vessel included between these marks is exactly one-twelfth of a cubic
foot when the glass is at the ordinary temperature. No correction is
needed for the cubical expansion of the glass. The two tubular necks of
the instrument pass through two boards placed below and parallel to the
top of a small four-legged table. For convenience the upper one of these
two boards is made in two parts and hinged to the legs.
Into each end of the instrument a glass tube about 8 millimetres in
diameter outside is fitted gas- and water-tight by means of india-rubber
corks, in such positions that the inner end of the upper tube lies exactly in
368
GAS EEFEREES' INSTRUCTIONS.
the plane of the mark as its end of the instnunent, while that of the lower
is about I mm. below the mark.
The upper tube terminates in a T, each branch of which is provided
with a stop-cock.
A separate stand carries two shelves, the upper one about 40 milli-
metres below the level of the upper mark and the lower one below the level
of the lower mark. The lower shelf is adjustable, and must be so placed
that the action about to be described shall take place.
Fio. 179.
A water vessel is provided having a capacity of about one-tenth of a
cubic foot. It should be made of brass or copper, tinned on the inside.
It has a tubulure near the bottom, to which is fitted a metal tap. The end
of the tap is to be turned slightly downwards, and is to have a diameter
outside of about 8 millimetres. The size of the way through the tap and
of the connections is such that when a meter is being proved in the manner
to be described, the water fills the instrument from one mark to the other
in about one minute. The water vessel has a tubulure in the cover, to
which a narrow glass tube is fitted by means of a cork, so that air may
enter or escape. The end of the tube is bent round upon itself in the form
QAS REFEREES' INSTRUCTIONS. 369
of a crook, so as to exclude dust and dirt. An india-rubber tube connects
the tube at the base of the measure with the stop-cock of the water vessel.
An ordinary chemical thermometer is provided for taking the temperature
of the water.
The pipe supplying gas to each meter is provided near the meter with
a three-way stop-cock carrying a short branch pipe, so formed that it either
connects the gas supply only with the branch pipe, the meter only with
the branch |Hpe, or the gas supply with the meter, in which latter case the
branch pipe is cut off from both. The index of the tap shows which
communication is open. In order to avoid sending the gas used in proving
the sulphur meter through the sulphuretted hydrogen and ammonia
apparatus, a separate gas supply is provided. The branch pipe is so shaped
as to be convenient (at the attachment of an india-rubber tube.
In order to put the instrument in adjustment the water vessel is placed
upon the upper shelf^ and water is poured into it until it rises about one-
quarter of an inch in the upper narrow tube. One branch of the glass T
is then connected by an india-rubber pipe with the branch of the three-
way stop-cock. This is now turned so as to connect the branch pipe with
the gs%a supply. The stop-cock in the •branch of the glaBs T to which the
rubber-tube is attached is turned on, and the water vessel is placed on the
lower shelf. The water will run back into the vessel. The flow should cease
when the water has just begun to descend in the lower tube ; if not, the
height of the lower shelf must be adjusted until this is the case.
The space above the upper mark is always filled with gas, and that
below the lower mark with water, so that the capacity of these portions of
the instrument has no effect upon the measurements. The narrow tubes
are so small that a variation of even an inch of the level at which the
water stands in them has no appreciable effect upon the meter reading.
The apparatus «hall only be used in proving a meter when the tempera-
ture of* the meter and of the water in the water vessel have been found not
to differ by more than two degrees Fahrenheit.
In order to prove the meters used in the various testings, the position
of the index is taken when the instrument has been put in adjustment and
filled with gas as described. The tap of the Water vessel is turned off; the
three-way tap is turned half-way towards the position which will connect
the instrument with the gas-meter, and the pressure of the gas in the
instrument is veduoed to atmospheric pressure by momentarily opening the
tap in the free branch of the glass T. The water vessel is placed upon the
upper shelf, the regulating tap (Fig. 179) is turned on, the three-way tap
is turned into such a position as will connect the instrument with the
meter, and the tap of the water vessel is turned on. One-twelfth of a cubic
foot of gas will then be discharged through the meter. Fig. 179 represents
this operation in progress. The three-way stop-cock is then turned so as
to fill the instrument with gas, the water vessel is placed upon the lower
shelf, the gas is reduced to atmospheric pressure as bef<Mre, and a second,
and again a third quantity is discharged through the metw. Should the
hand attached to the axle of the measuring drum have travelled in the
three revolutions as much as one division beyond the point from which it
started, some water must be removed from the meter ; if the travel of the
meter hand is as much as one division short of this point, ^'ome water must
be poured in. The operation is then to be repeated until the error is found
to fnll within the specified limits.
The following is an example of the form in which Returns are to be
made:
9 A
370 GAS REFEREES' INSTRUCTIONS.
COUNTY OF LONDON GAS-TKSTINO STATION,
I, Oabltlb Squabb, Chblsba, S.W.
BBPOHT ON GAS SUPPLIED BT THE GAS LIGHT AND COKE COMPANY.
Dates of Bopplt of Gab.
■ •
Meaa Lighting
Power, 1q Cuidlei,
oorvected.
r
Bnlphnr in zoo
oablo feet of Gaa,
Ingnine.
Salphnretted
Hydrogen.
Ammonia in zoo
cubic feet of Gu,
in graine.
Teniogof
PnasttJre.
StreeL
Time.
Pretsure.
OoM Examiner,
No meter other than a w^t meter shall be used in testing the gas nnd^
these instructions.
EBsui;rs of Tksts.
The results of the daily testings for illuminating power and purity shall
be recorded^ and delivered as provided in the Acts of Parliament.
Afl TO Illuminating Powsb.
By the Acts of Parliament the illuminating power of the gas supplied
by the Gas Light and Coke Company shall be 16 candles, and of the
gas supplied by the Oommercial Gas Company and by the South Metro-
politan Gas Company shalkbe 14 oandles.
As to thb Maximum Amounts of Impubitt
in each form with which the gas shall be allowed to be charged.
Sulphuretted Hydrogen. — By the Acts of Parliament all gas supplied must
be whoUy free from this impurity.
Ammonia. — ^The maximum amount of this impurity shall be 4 grains
per 100 cubic feet.
Sulphwr Compounds other than Stdphuretted Hydrogen, — ^The maximum
amount of sulphur with which gas shall be allowed to be charged shall be
in the six months from April i to September 30, 1 7 grains of sulphur in
every 100 cubic feet of gas, and in the other months, 22 grains of sulphur in
every 100 cubic feet of gas.
SuLPHUE Test,
The apparatus to be employed is represented by Fig. 180, and is of the
following description : — The gas is burnt in a small Bun sen burner with a
steatite top, which is mounted on a short cylindrical stand, perforated with
holes for the admission of air, and having on its upper surface, which is also
perforated, a deep circular channel to receive the wide end of a glass
trumpet-tube. There are both in the side and in the top of this stand
fourteen holes of five millimetres in diameter, or an equivalent air-way.
GAS EEFEEEES' INSTEDCTI0N3. 37 1
On the top of the stand, between the narrow Btem of the bamer and the
DUiTounding gl&se trumpet-tube, are to be placed pieces of commercial
setiqui-ciirboDate of ammonia neighing in all about two ounces.
The products both of the combustion of the gae and of the gradual
volatilieation of the ammonia salt go upwards through the trumpet-tube
into a vertical gloAS cylinder with a
tubulure near the bottom, and dt«wn ^lo- iSo.
in at a point above this to about half
itd diameter. From the contracted
part to the top the cylinder is packed
with balls of glass about lifteen milli-
metres in diameter, to break up the .
current and promote condensation.
From the top of this condenser there
prooeeds a long glass pipe or chimney
slightly bent over at the upper end,
serving to effect some further con-
densation, as well as to regulate the
draught and afford an exit for the
uncondensable gases. In the bottom
of the condenser is fixed a small glass
tube, through which the liquid formed
during the testing drops into a flask
placed beneath.
The following cautions are to be
observed in selecting and setting up
the apparatus :
See that the inlet-pipe fits gas-tight
into the burner, and that the holes in
the circular stand are clear. If the
burner gives a luminous flame> remove
the top piece, and having hammered
down gently the nozzle of soft metal,
perforate it afresh, making as small a hole as will give passage to two-thirds
of a cubic foot of gas per hour at a convenient pressure.
See that the tubulure of the condenser has an internal diameter of
not leKs than iS millimetres, and that its outside is smooth and of the same
size as the small end of the trumpet-tube ; also that the inlernal diameter of
the contracted part is not less than 30 millimetres.
See that the short piece of iudia-ioibber pipe fits tightly both to the
trumpet-tube and to the tubulure of the condenser.
The email tube at the bottom of the condenser should have its lower end
contracted, so that when in use it may be closed by a drop of water.
The india-rubber pipe at the lower end of the chimney-tube should fit
into or over, and not simply rest upon, the mouth of the condenser.
A central hole, about 50 millimetres in diameter, may with advantage be
made in the shelf of the stand. If a beaker is kept on the table below, the
liquid will still be preserved if by any accident the flask is not in its place.
Ths Gas Beferebs' Stbeet Lamp Pressure Gadgb.
This instrument has been designed in compliance with sect. 6 of the
Gas Light and Coke and other Gas Companies Acts Amendment Act,
1880, for the purpose of testing in any street at any hour the pressure at
which gas is supplied. ItM construction and mode of use are as follows :
Within a lantern provided with a handle for carrying and feet for
372 GAS RfiFEREES' INSTBUCTIOKS.
reaticig on the ground, ia placed a oandle-lamp, to give Bght for reading the
gauge. In front of the candle-lamp is a sheet of opal glass, and in front of
this a glass U-tube, partly filled nich cwloured water, and oonunanicating at
one etid with the air, at the other with a metal pipe, which passes through
the bottom of the lantern. In wder to read easily and accurately the
difference of level of the liquid in the two limbs, a scale divided into tenths
of an inch is made to slide between them with sufficient friction to retain it
in any position. The zero of the scale having been brought level with the
surface of the liquid which is pressed upon by the gas, the height above this
of the surface which is pressed upon by the air can bo read directly. The
lantern is closed in front by a glass door, at each side <d which is a reflector
tor throwing l^ht upon the sctde of the gauge.
FiQ. iSi. Above each limb of ^e U-tube is a tap which can
be closed when the instrument is not in use, to
prevent the liquid being accidentally spilt
To make a testing of pressure the governor and
burner of a street lainp are to be removed, and the
pressure-gauge is to be screwed on to the gas-pipe,
by which it is supported. In places where iucan-
desoent burners are used, the L-ahaped pipe de-
scribed on p. 367 is to be used for the attachment
of the preesur»^uge. The cock is then turned
on, and a reading made. If on turning off the cock
the level of the liquid is unohajiged, or cbangea
slowly, tbe reading is correct ; but if the level
changes quickly, the junction between the lamp and
the gauge must be made more perfect, and tbe
testing repeated. A small leakage is immaterial,
provided the cock is turned fully on.
The pressure at the top of a lamp ccdnmn is
greater by about o.i inch than that at the main,
which is the pressure required. Aooordingly a
deduction of o.i inch from the observed pressure is
to be made.
Tbe following ia the procedure which the Gas
Befereea have arranged with the Gas Companies
for the provision and testing of pentane ;
Each of the Qas Companies shall keep upon
their premises one or more properly closed vessels
capable of containing from fifty to one hundred
gaJlons of pentane.
When a supply of pentane is needed for use in the testing-plaoes a
number of one-pint metal cans with screw stoppers, of a pattern approved
by the Gas Beferees (of which a specimen can be seen at their offic^ shall
be provided sufficient to contain the whole quantity required.*
The Gas Referees shall then be informed by letter that this quantity of
pentane awaits their examination; and they will arrange to attend at the
premises where the pentane is stored. They will see the cans filled, and
will affix a numbered lead seal to each can ; or where it ia convenient to
* The size of can to be Dsed ia limited to one pint bj the regnlatioDS of tbe Home
QtAB BKFF1BF1F18' INSTfiUOTIONS. 373
send the cans to the teBting-plaoes in groups or in boxes they will place one
seal on each group or box.
They will then take away one or more of the cans for examination ; the
remaining cans must be kept unto the Qas Referees have reported on the
quality of the pentane.
If the results of their testings are satisfactory, they will prepare aa
many labels as there are cans, or groups or boxes of cans, of pentane. Each
label will bear the embossed stamp of the Gas Referees, and will be numbered
with the numbers impressed upon the lead seals on the cans or groups or
boxes. These labels will then be sent to the Company for attachment.
No cans of pentane which the Gsa Referees have certified by the attach-
ment of their lead seals and labels, are to be supplied to or used by any
person or persons other than the Gkis Examiners at the several testing-
places without the written permission of the Gas Referees, and a record
must be kept by the Gas Company of all cans to which tbe lead seal has
been attached. If, however, application should be made to the Gas Referees
by the London County OouncU, the Corporation of London, or any of the
Metropolitan Gas Companies, to examine and certify pentane in reasonable
quantities for non-o£Gic£al testings, they will be wDling to do so.
If the Gas Referees, after examination, find that the sample of pentane
taken from any vessel does not satisfy the requirements of their notification,
they will inform the Gas Company of the fact ; and in such case the lead
seals axe to be out off from the other cans filled from the same vessel, and
returned to the Gas Referees.
The Gas Companies will send the certified cans of pentane to the testing-
places in their several districts. The Gas Examiner at any testing-place
will take the presence of the Gas Referees' lead seal and label, bearing
identical numbers, upon any can, group or box of cans, as evidence that the
pentane therein has been certified, and no pentane shall be used in any
testing that has not been so certified.
INDEX.
■»» I
AccnmiiJLTOSs, 95-1x7
AlMmattng omrenu, 141-
Altenuton» zcfi^ 167
in pualult 903
AmmonlA In ooal-gi^ 36a
AmortlMenr, 66
Amyl-aoetate* 333
Amyl-Metate flAinSi 335
oandlA-power, 338
eondttotlon o^ 232
eLeotrodn, 334
eaoIoMd, 358
ndiatton, 337
regnlatioii, 343
reBistanoe, 343
temperatare of, 333
Arg»nd burner, itandnrd, 3^
Armntnre,
cloied-ooll, 54
drum, 59
Onmme, 58
FiuinotU, 57
T M O t lon, 6z
ring»S4
■tren on^oa
winding, &, 70
Ayrton, Mr&, are lamps, 341
AjTton, FraL W. B^ 7, 338
B
Babcook ft WUooz boilen, 366
Bare-strip aystema, 25
Barometrical ooireotlona, table for, 358
Becquerel'i photometer, 310
Bellla engine, 373
Board of Trade Begulationa, 8, 15, 187, 193
Boilen, 364
» Boiling** of oellB,iz6
Booatem, 1x7
Bongner*! photometer, 387
Boxei, ■treet,24
Bristol aceumnlator, 108
British Aasooiatlon'B report on candles, 331
Brockle-Pell are lamp, 350
Brooks' oil-lnsolatlon, 38
Bnuh, arc lamp, 347
dynamoa, 70, 71, X38
Geipel regulator, 133
transformer, X77
Bnnaenls dlae, 3x9
photometer, 387
d
CAI.LBNDBB ^JStemS, 31-33
Candle-power of incandescent lamps, 3x8-338
of are lamp^ 337, 34X
Candles, standard, 330
British AssoolationlB report on, 331
gas zefereDoes on, 331
Helsch and Hartley on, 33X
proposed snbetltntes for, 336
table for oorreetlon, 333
Oapaeity of batteries, xoi
central station plant, 376
dynamos, 87
tranafonners, X85
Carbon are lamp, 334
filaments, 31 x
Carbonisation, 314
Caroel lamp, 334^
Cardew earthing 'doTice, 194
Casing of conductors, 17
Characteristics, 74, 30i
external, fj
■eriee,78
shnnt, 79-8X
Ohaiglng, rate ot, X07
▼ariable scale of, 376
Chelsea system, 133
Chord- winding, 68
Closed-eoU armature, 54
Coal-gas, illnminattng power ot 354
method of testing, 360
CoerdTC force, 46
Commutator, 60^-63, 130
coils, 66
Compounding, 3X, 84, 90, sos
ConductlYity of add solution, 99, 100
of gases, 331
of metala,7
testing, 87, 38
Conductors, ooncentrle, z8
siie ot 8, XX, X36, 187
Con?enion. 5ee Transf orm.ition
CouTerter, rotary, 198
Connterbolanoe eoils, 65
Crater, 333
Critical points, 80, 83
Crompton alternator, 158
system of mains, 25
Crooke's photometer, 3 10
Cross-magnetisation, ox
n^
INDEX.
Cnrrent-density, 9
Cat-onta, i8-3z
D
Demagnetibivo-tubns, 63
Density, carrent, 9
of battery acid, 99, loa
of anpply, 189
Deptford, 189, 193
Discbargo, rate o^ 108
DiBtribntion, 3
parallel, 29
■erlee, 124
DiHCM, photometer, 3x9
Doulton conduits, 22
Drake-Plant^ cells, 1x2
Drum-winding, 59
l>nst-deBtraetor8, 266
Datch standard of light, 335
Dynamo, definition, 37
alternating cnrrout, 156
are lighting, 127
eloaed-coil, 52
design and regiihitor, 74
efficiency of, 93
DynamotoxB, 118
E
Eartbiivg deyioes, 194
*' Economic life," 220
E.C.C. cont. cnrr. transf., 121, 123
arc lamp, 252
Sdison, armature- winding, 68
incandescent lamp, 2x1
Efficiency,
of arc lamps, 239
of batteries, xoz
of boilers, 264
of cont. cnrr. transf., 178, z8i, 184
of distribution, 274
of dynamos, 93, 94, 274
of enginesi 267
of incandescent lamps, 219
Eickmeyer, armature- winding, 69
Electric strength, 14
Electrolyte^ 99, 102
Electromagnetism, 37-52
Electromotive force,
limits of, 30
of altemators, 159
of batteries, 102
of dynamos, 60
Elwell-Parker alternator, 165
EmisedTity, 208
Enclosed arcs, 258
K.P.S. cells, 104, 105
Epstein cells, 109
Evan's photometer, 283
Kwiiij,^ Prol J. A, 41, 51, 169
F
Fall of potential, 32, 64
Feeders, 34
Feldmuuu, 226
Ferrantl, S. Z. de, alternator, 160, 167
mains, 193
rectifier, 136
transformer, 175
Filaments, dimensions of, 221
maniifacture of, 213
Fleming, Dr. J. A., 172, 184, 194, 225
Formation of lead plates, 96
FoucaulfS photometer, 281
French standard of light, 324
o
Gas-ooal, illuminating-power tests, 360
Gas-enginea, 273
Gausa,39
referees on candles, 321
instmctioos, 360
Gaulard & Gibbs transformer, 173
German standard of lig^t, 325
Gladstone, J. H., 99
Goldston lamp, 141
Gramme armature, 56
GnQy*s photometer, 3x1
GK»e*8, 3x3
HAJtcouRT*B holo-photometer, 30Z
z-oandle pentane fljune, 337
pentane lamp No. 2, 348
xcHsandle pentane lamp, 351
table photometer, 284
Hartley's universal photometer, 29Z
Hedgehog transformer, 175, 184
Hef ner-Alteneck amyl-acetate lamp, 396
Hefner Ton Alteneck, 69
Henry, 145
Heisch photometer, 310
Helsch & Hartley on candles, 321
Holmes, pole-pieces, 66
train-lighting, XX4
Holo-photometer, 301
Hopkinson, Dr. John, 42, 47, 50, 74, 94, 903
Horiiontal rays, 294
Hydraulic analogy, 5, 29
Hysteresis, 47, 51, 148, 171
iLLUMiNATiirG power, 327, 360
Impedance, 146
Insulated tubes, vj
Insulation,
testing of, 15, 27, 28
thickness of, X3
Inverse squares, law of, 279
Jablockhoff arc lamp, 209
Jandns arc lamp, 259
I^
Kapp, Gisbert, 60, 68, 163
alternator, X63
Kapp & Snell transformer, 178
Kelvin's economic law, 33
Kennedy system of mains, 96
Kennelly, A. E., xo
King's photometer, 282
KruBB flame measure, 334
Lag, Z46
Lamination, 51, 169-172
Lamp, carcel, 324
Lane-Fox, 210
INDEX.
377
Law of inyene Bqnsrai, 079
Lead of bnishea, 6a. 66, 131
Leny lair, 53
Lethebj*! photometor, 98a
Life of Incandeioent lampa, aao-aaS
Ligbt, cnrrent, 337
power, 327
itandards of, 319
supply, 337
Lines of force, 38, 39. 5S» 57
Lithanode, 109
Lirerpool, Ag^ Insnraiioe Oo^ zi
Lowrie-Hall transformer, 175
Lommer Brodhun's photometer, 3x6
Lana aro lamp, 257
Lux, the, 327
M
Maonetio Force, 40
clT0nlt,44
ey^e, 48, 148
Induction, 40
screening, 169
Magnetisation, 43
MagnetlBing current, 153
Magnetism resldnal, 46, 85
Magneto-motlTe force, 45
Magnets, field, 70-77
Mains, street, ax
Ferranti, 193
Marks'aro lamp, 959
Mather A Piatt, 63, 69
Meters for gas testing, 367
MethTcn's screen standards, 34Z
Mordey, W. M^ alternator, i6x
on dynamo losses. 93, 94
on frequency, x86
on ylrtual resistance, 193
transformer, 177
Multiple winding, 68
wire systems, 35
N
KitRNST lamp, 399
Ketwork, 34, 189
Obscuratioh photometers, 308
Oil insulation, 28, 189
PAcnroTTi, 56, 57, 89
Paraffin candle, 325
Parallel distribution, 99
dynamos In, 200
transformers In, 190
Parchmentising, 212
parfltt system, 140
Parson's turbine, 156, 269
Pasted plates, 97, 108
Pentane standards, 337, 344, 348, 35X
▲rgand, Dibdin's, 343
storage of, 373
Periodicity, 143
Perry, Prot J., 226
Phoenix aro lamp, 954
Fire Office, 11, 15
Photometer-head, Lummer Brodhun's, 316
Photometers, Beoquerers, 5x0
Bouguer's, 281
Bunsen's, 281
Crooke's, 310
Dibdin's radial, 29X
Btmis*, 283
Foncault's, 28X
Qrosse's, 313
Guoy's, 3x1
Haroourt's table, 284
Hartley's uniTersal, 99X
HelMh, 3x0
holophotometer, 30X
King's, 282
Letlieby's,989
polarisation, 309
Preeoe's illumination photomebar, 304
radial, 290
Bumf ord's, 28X
Tooley's, 283
Trotter's, 305
Weber's, 311
Photometry, law of, 279
Pllsen aro lami), 247
Plants 96
Platinum, VioUe's standard of light) 34X
Polarisation photometers, 309
Pole, Dr., law of, 354
Polyphase currents, 195
Potential equaliser, 37
fall of, 32, 64
Power, 3, 53
curyes, 79, xsx
factor, 181
Preece's photometer, 304
Preeoe A Trotter's photometer, 30$
Pressure of gas supply, 366^ 37X
Pulsadng current, 53, 138
Purity of coal-gas, testing the^ 369
B
BadiaIi photometer, Dibdin's, 99X
photometry, 290
Badiatlon, 208, 237
Bam, G. S., 218, 225
Bayleigh, Lord, 193
Beactance, 153
Bectifler, X36
Beferees, gas, on candles, 39X
notification, 360
Beflectors, 295
Belnctance, 45
Besidual magnetism, 46, 8^
Besistance, specific, 6, 12, 99, 100
of arc,24x
Beynler, 209
Bing-winding, 54
Bobertson, loi
Bumford's photometer, 28X
Byan, Prof. H. J., 64, 65
s
Salomons, Sir David, 107
Sawyer, 211
Sayers, W. B., 66
Screening, magnetic, 169
Search-lights, 259
Secohm, 145
Self-inductance, 145
Semi-incandescent Ijimps, 209
Series distribution, 124
Shades, 295
Siemens armature, 66
378
INDEX.
Stomam^ alternator, Z58
mn lamm 955
Sina law, 143, 149
SparkUng dlntanoe, 14, 155, ^z
Spermaoetl oaodle, 390» 395
Splnl oomwwtloiii 09
Btaadardi of lights 3x9
Stone, traln-Ughtlng^ tx$
Btoiage, 95, 965
BabdiTlslon of power, 4, za5
flwlntattoni, zzob Z74, z88
Bugs*! ten-eandle teati 34Z
BnlphaUng, zoz
Bnlphor In eoal-gaa, taatlng for, 36a, 37Z
Balphuretted hydrogen In coal-gai, 31M, 370
Bun lamp, 909, 099
Bortioe iUamlnatlon, 397
8wan,9Z
Bwinbame, Jamei^ 69
Bjrnoluonldng, 906
TABza for eoneoltng eandle oonfnmption, 393
earoel lamp, 396
tfiermometrieal, fte^ oboerratloni, 358
Temperature^ ooeffldent,
of arc lampa, 933
of oondnoton, 9^ 13, 907
of dynamoa, 91
of incandeaoent lamps, 9zz, 994
of trsnaformeiB, z8a
Testing, 97, Z96
Thomson, Prof. Elihn, 66, Z34
Thomson, Prot J. J., 169, 931
Thomson-Hooston dynamo^ Z34
transformer, 139
Thiee-idiaaeh Z97
Three voltmeter teat, zSt
Three-wire qrstem, 35
Tool^y Street photometer, 983
Traln-Ughttngt ZZ3
Tranaformatlon, zzS
Traaafonnflr, alternating enrrent, Z53, i6f
eoostaat eorrent, 139
eontlnnoDa eorrent, lao
rotary, Z98
Tranimlftoa of power,3
Trotter^ photometer, 305
Tnrhlne^ Z56, 969
Uswiv, FML W. On 873
YioziiE, 933
platinnm standard of lights 341
aoetylenob 353
Ylrtnal Tolts, 144
zeaistanoe, Z46^ 19a
w
Watbk-powbb, 96z
Weber^s photometer, 3ZZ
Werdermann, 2x0
Weston transformer, zyS
Wlllan*s engine, 97Z
z
ZiraamumatxtumtaauUftn
Catalogue of the Medical, Dental, Phar-
maceutical, Chemical, and Scientific Books
Published by P. Blakiston^s Son & Co.,
IOI2 Walnut Street, Philadelphia.
Established 1843.
SPECIAL NOTE,
The prices as given in this catalogue are absolutely net — no discount will be
allowed retail purchasers under any consideration. This rule has been established
in order that every one will be treated alike, a general reduction in former prices
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MEDICAL AND SCIENTIFIC PUBLICATIONS. 11
Cohen. Physiologic Therapeutics. — Continued.
Serotherapy — Organotherapy — Blood-Letting, etc. — Principles of
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%* Compute descriptive circular upon application,
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use of mere drugs, forgetful and careless of the importance of 'the therapeutic value of the methods
of which this series of books will speak. "—7i>^»j Hopkins Hospital Bulletin.
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Synopsis of Contents.
Volume I. — ^Upper Extremity — Back of Neck, Shoulder, and Trunk — Cranium
— Scalp — ^Face.
Volume II. — Neck — Mouth, Pharynx, Larynx, Nose — Orbit — Eyeball — Organ
of Hearing — ^Brain — Female Perineum — Male Perineum.
Volume III. — ^Abdominal Wall — ^Abdominal Cavity — Pelvic Cavity — Chest —
Lower Extremity.
See next page for Reviews.
MEDICAL AND SCIENTIFIC PUBLICATIONS. 13
Deaver's Surgical Anatomy
The illtistrationSy which at the first glance appear as the prominent feature of
tlie book — ^but which in reality do not overshadow the text — consist of a series of
pictures absolutely unique and fresh. They will bear comparison from an artistic point
of view with any other work, while from a practical point of view there is no other
volume or series of volumes to which they can be compared. When originally an-
ftounced, the book was to contain two hundred illustrations. As the work of prepara-
tion progressed, this number gradually increased to nearly five hundred full-page
plates, many of which contain more than one figure. With the exception of
SL few minor pictures made from preparations in the possession of the author, they have
all been drawn by special artists from dissections made for the purpose in the dissecting-
roor..s of the University of Pennsylvania. Their accuracy cannot be questioned, as
each drawing has been submitted to the most careful scrutiny.
From The Medical Record^ New York.
** The ttaidet is not only taken by easy and natufal stages from the more sciperfictal to the
deeper regions^ but the varioas important regional landmarks are also indicated by schematic
tracdng tspon the limbs* Thus the courses of arteries, Teins, and nerves are indicated in a way that
makes the lesson strikingly impressive and easily learned. No expense, evidently, has been
spared in the preparation of the work, judging from the number of full-page plates it contains, not
counting the smaller drawings. Most of these have been * drawn by special artists from dissections
made for the purpose in the dissecting-rooms of the University of Pennsylvania.' In summing up
the general excellences of this remarkable work, we can accord our unqualified praise for the
accurate, exhaustive, and systematic manner in which the author has carried out his plan, and we
can commend it as a model of its kindt which most be possessed to be appreciated*'^
From The Philadelphia Medical Jotimal*
" Many members of the profession to whom Dr. Deaver is well known either personally or by
reputation as a suigeon, writer, teacher, and practical anatomist, have awaited the appearance of
his Surgical Anatomy with the expectation of finding in it a guide in this difificult branch of medi-
cine of much more than ordinary practical value, and their expectations will not be disappointed.^
From The Journal of the American Medical Association.
** In order to show its thoroughness, it is only necessary to mention that no less than twelre
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MEDICAL AND SCIENTIFIC PUBLICATIONS, 15
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are excellent, and contain a large amount of information in a limited space. The anatomical tables
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Medical Science.
16 P. BLAKISTON'S SON 6- CO:S
Gould. The Pocket Pronouncing Medical Lexicon. Fourth Edition.
(30,000 Medical Words Pronounced and Defined.)
A Student' s Pronouncing Medical Lexicon. Containing all the Words, their Defini-
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elaborate Tables of the Arteries, Muscles, Nerves, Bacilli, etc., etc.; a Dose List in
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simple. Full Limp Leather, Gilt Edges, $1.00 ; >Vith Thumb Index, $1.25
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a medical defining vocabulary — many of the words not yet being found in any other dictioDary,
large or small, while all of the words are those of the living medical literature of the day.*' — Tkt
Virginia Medical Monthly.
*»* 145.000 copies of Gould's Dictionaries have been sold.
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The Origin of the Ill-Health of DeQuincy, Carlyle, Darwin, Huxley, and Brown-
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Gould and Pyle. Cyclopedia of Practical Medicine and Surgery.
72 Special Contributors. Illustrated. One Volume.
A Concise Reference Handbook, Alphabetically Arranged, of Medicine, Surgery,
Obstetrics, Materia Medica, Therapeutics, and the various specialties, with Particular
Reference to Diagnosis and Treatment. Compiled under the Editorial Super\'ision
of Drs. George M. Gould and W. L. Pyle. Illustrated, Large Square Octavo.
Uniform with Gould's "Illustrated Dictionary.*' Full Sheep or Half Dark-Green
Leather, ;^io.oo; With Thumb Index, |ii.oo; Half Russia, Thumb Index, $12.00
%*The great success of Dr. Gould's •* Illustrated Dictionary of Medicine" sug-
gested the preparation of this companion volume, which should be to the physician the
same trustworthy handbook in the broad field of general information that the Dictionary
is in the more special one of the explanation of words and the statement of facts. The
aim has been to provide in a one-volume book all the material usually contained in the
large systems and much which they do not contain. Instead of long, discursive papers
on special' subjects there are short, concise, pithy articles alphabetically arranged, giv-
ing die latest methods of diagnosis, treatment, and operating — a working book in w^ch
the editors and their collaborators have condensed all that is essential from a vast
amount of literature and personal experience.
The seventy-two special contributors have been selected from all parts of the
country in accordance with their fitness for treating special subjects about which they
may be considered expert authorities. They are all men of prominence, teachers,
investigators, and writers of experience, who give to the book a character unequaled by
any other work of the kind.
"The book is a companion volume to Gould's 'Illustrated Dictionary of Medicine/ which
every physician should possess. With these two books in his library, every busy physician will save
a vast amount of time in having at hand an instant reference cyclopedia covering eveiy subject in
surgery and medicine.'* — Chicago Medical Recorder,
Pocket Cyclopedia of Medicine and Surgery.
Based upon Gould and Pyle's Cyclopedia of Practical Medicine and Surgery.
Uniform with Gould's Pocket Dictionary.
Full Limp Leather, Gilt Edges, $1.00 ; With Thumb Index, 51.2$
See next page for List of Contributors,
MEDICAL AND SCIENTIFIC PUBLICATIONS.
17
Gould and Pylc^s Cyclopedia of Medicine
LIST OF CONTRIBUTORS
Samuel W. Abbott, A.M., M.D., Boston.
James M. Anders, M.D., LL.D., Pbila.
Joseph D. Bryant, M.D., New York.
James B. Bullitt, M.D., Louisville.
Charles H. Burnett, A.M., M.D., Phila.
J. Abbott Cantrell, M.D., Philadelphia.
Archibald Church, M.D., Chicago.
7v. Pierce Clark, M.D., Sonyea, N. Y.
Solomon Solis-Cohen, M.D., Philadelphia.
Nathan S. Davis, Jr., M.D., Chicago.
Theodore Diller, M.D., Pittsburg.
Augustus A. Eshner, M.D., Philadelphia.
J. T. Bskridge, M.D., Denver, Col.
J. McFadden Gaston, A.B., M.D., Atlanta,
Ga.
J. McFadden Gaston, Jr., A.M., M.D., At-
lanta, Ga.
Virgil P. Gibney, M.D., New York.
George M. Gould, A.M., M.D., Phila.
W. A. Hardaway, A.M., M.D., St. Louis.
John C. Hemmeter, M.B., M.D., Baltimore.
Barton Cooke Hirst, M.D., Philadelphia.
Bayard Holmes, M.D., Chicago.
Orville Horwitz, B.S., M.D., Philadelphia.
Daniel £. Hughes, M.D., Philadelphia.
James Nevins Hyde, A.M., M.D., Chicago.
£. Fletcher Ingals, A.M., M.D., Chicago.
Abraham Jacobi, M.D., New York.
William W. Johnston, M.D., Washington,
D. C.
Wyatt Johnston, M.D., Montreal.
Allen A. Jones, M.D., Buffalo.
William W. Keen, M.D., LL.D., Phila.
Howard S. Kinne, M.D., Philadelphia.
Ernest Laplace, M.D., Philadelphia.
Benjamin Lee, M.D., Philadelphia.
Charles L. Leonard, M.D., Philadelphia.
James Hendrie Lloyd, A.M., M.D., Phila.
J. W. MacDonald, M.D. (Edin.), F.R.C.S.
Ed., Minneapolis.
L. S. McMurtry, M.D., Louisville.
G. Hudson Makuen, Philadelphia.
Matthew D. Mann, M.D., Buffalo.
Henry O. Marcy, A.M., M.D., LL.D.,
Boston.
Rudolph Matas, M.D., New Orleans.
Joseph M. Mathews, M.D., Louisville.
John K. Mitchell, M.D., Philadelphia.
Harold N. Moyer, M.D., Chicago.
John H. Musser, M.D., Philadelphia.
A. G. Nicholls, M.D., Montreal.
A. H. Ohmann-Dusmesnil, M.D., St.
Louis.
William Osier, M.D., Baltimore.
Samuel O. L. Potter, A.M., M.D., M.R.
C.P. (London), San Francisco.
Walter L. Pyle, A.M., M.D., Philadelphia.
B. Alexander Randall, A.M., M.D., Phila.
Joseph Ransohoff, M.D., F.R.C.S. (Eng.),
Cincinnati.
Jay F. Schamberg, A.M., M.D., Phila.
Nicholas Senn, M.D., LL.D., Chicago.
Richard Slee, M.D., Swift water. Pa.
S. E. Solly, M.D., M.R.C.S., Colorado
Springs, Col.
Edmond Souchon, M.D., New Orleans.
Ward F. Sprenkel, M.D., PhUadelphU.
Charles G. Stockton, M.D., Buffalo.
John Madison Taylor, A.M., M.D., Phila.
William S. Thayer, M.D., Baltimore.
James Thorington, A.M., M.D., Phila.
Martin B. Tinker, M.D., Philadelphia.
James Tyson, M.D., Philadelphia.
J. Hilton Waterman, M.D., New York.
H. A. West, M.D., Galveston, Texas.
J. William White, M.D., PH.D., PhUa.
Reynold W. Wilcox, M.A., M.D., LL.D.,
New York.
George Wilkins, M.D., Montreal.
DeForest Willard, M.D., PhUadelphia.
Alfred C. Wood, M.D., Philadelphia.
Horatio C. Wood, M.D., LL.D., Phila.
Albert Woldert, PH.G., M.D., Phila.
James K. Young, M.D., Philadelphia.
<* It is difficult to describe the volume before us, and one must imagine all that is clinical
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New York Medical Journal,
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MEDICAL AND SCIENTIFIC PUBLICATIONS. 19
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Vi 'CIRCULAR WITH SAMPLE PAGES AND ILLUSTRATIONS FREE,
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43
PRACTICAL GYNECOLOGY
A Modem Comprehensive Text-Book
By E. E. MONTGOMERY, MJ).
Profenor of GyncooIog7» Jcffcnon Medical College i Gynecologiit to the Jefienoo Medical
College and St» Joseph's Hoapttale^ Consulting Gynecologist to
the PhiladelphiA Lying-in Charity
WITH FIVE HUNDRED AND TWENTY-SEVEN
ILLUSTRATIONS
Nearly all of which have been Drawn and Eo^javed Spedafly for thb
"Workf for the most part from Origkial Sources
OCTAVO* 819 PAGES. CLOTH, $5*00? LEATHER^$6jOO
From THE JOURNAL OF THE AMERICAN MEDICAL ASSOCIATION
" Fashion in medical book-making seems to be running to the composite, which
may be advantageous and the means of producing a better book than one written by
an individual. It may be the old-fashioned notions of the reviewer, but he belives in
the old idea of one book, one author, and he should have all the responsibility, all the
criticism, and all the glory that attach to it. The composite is likely to be written
under a * rush ' order — so much space, in so much time, for so much money. The work
before ui Is the work of one IndMdtsal, and the personattty of that indivtdtial b evident
throfsgh the whole book. • • • The result shows painstaking effort in every detail,
in conciseness of statements, in arrangement of subjects, and in the systematic order
and completeness in which each is considered. . . . The author is neither too
radical nor too conservative in his consideration of the conditions that may need radical
operations. In the introduction he tells us that the true gynecologist must be * so con
servative that he will sacrifice no organ whose physiologic integrity is capable of being
restored ; so bold and courageous that his patient shall not forfeit her opportunity for
life or restored health through his failure to assume the responsibility of any operative
procedure necessary to secure the object.' This is the basal idea that permeates the
book : the tsltra-fadical operator will find no endorsement, and the * tfnkmng * ^fynecologist—
he who treats all diseases of women by means of a pledget of cotton and a speculum-
no encouragement in its pages*
"The book is one that can be recommended to the student, to the general practi-
tiooer — who must sometimes be a gynecologist to a certain extent whether he will or not
— and to the specialistt as an ideal and in every way complete work on the gynecology of
to-day — a practical work for practical workers.''
DESCRIPTIVE CIRCULAR UPON APPUCATION.
44
JACOBSON'S
OPERATIONS OF SURGERY
The Operations of Surgery. By W. H. A. Jacobson,
F.R.C.S., Surgeon to Guy's Hospital, Consulting Surgeon
Royal Hospital for Children and Women, Member Court
of Examiners Royal College of Surgeons, Joint Editor
Annals of Surgery; and F. J. Steward, F.R.C.S., Assistant
Surgeon Guy's Hospital and to the Hospital for Sick
Children. Fourth Edition, Revised, Enlarged and Im-
proved. 550 Illustrations; Two Volumes ; Octavo; 1524
Pages. Cloth, $ 1 0.00 ; Sheep or Half Morocco, $ 1 2.00
This printing has been increased in size over the former edition
by 200 pages and contains 1 50 additional illustrations.
jPj^ess notices of former editions
** Far more than a mere guide to operating, it is essentially a clinical work and in
that lies one of its conspicuous merits. ' ' — TA^ London Lancet
"The author proves himself a judicious operator as shown by his choice of
methods, and by the emphasis with which he refers to the different dangers and com-
plications which may arise to mar success or jeopardize life." — New York Medical
Record.
"Many of the difficulties met with at the time of an operation, and the troubles
ensuing after an operation, which are only known to the practical surgeon, are brought
out prominently in this book." — Boston Medical and Surgical Journal.
'* The important anatomical points are clearly set forth, the conditions indicating or
contraindicating operative interference are given, the details of the operations them-
selves are brought forward prominently, and frequently the after-treatment is considered.
Herein is one of the strong points of the book." — Actjo York Medical Journal.
*jj.* Jacobson's Operations of Surgery is not intended d^/^/y for
those of great surgical experience or skill, but is intended largely as
an authoritative guide for the General Physician and Hospital
Resident who, in emergencies where immediate surgical intervention
is demanded, must act quickly, and often rely solely upon his own
judgment.
Comprehensive in scope, exhaustive in detail, rich in its exposi-
tion of the latest and most uniformly successful methods in operating,
and modern throughout in its treatment of each branch or surgical
work, particularly that of abdominal surgery, this book easily ranks
among the very foremost works in its particular field.
45
Carpenter on ThE MICROSCOPE
AND ITS REVELATIONS
EIGHTH B'DITIOM
Edited by W, H. Dallin^cr, D^Sc, D.CL, F.R.S.
With 23 Plates and nearly 900 Engravings
OCTAVO. 1181 PAGES. CLOTH, $8.00 1 HALF MOROCCO* $9-00
*:ic* Eight of the chapters have been entirely rewritten and the text
througfhout reconstructed^ enlargfed, and revised with the aid and advice
of EL M* Nelson^ ez-President of The Royal Microscopical Society;
Arthur Bolks Lee^ author of ^The Microtomist^s Vade Mecum''; Dr* E
Crookshank^ the well-known Bacteriologfist ; Prof* T* Bonney^ F«R^4
W. J* Pope^ FXC*, YXIS^ etc^ Chemist to the Goldsmith's Technical
Institute ; Prof « A« W. Bennett, Lecturer on Botany at St* Thomas' Hos-
pital; and F* Jeffrey Bellf Professor of G>mparatiYe Anatomy and 2joology,
King's College, London*
''':ic'^A thorough and complete revision of the entire text has
been made ; eight chapters have been entirely reconstructed, and
everything of importance to Microscopy which has transpired in
the interval has been noted* This applies to the theory of the
Microscope as well as to its use* Many new illustrations have
been included and it has been very materially increased in size*
M
CARPENTER" is the only complete and exhaustive modem work on
the Science of Microscopy
46
Diseases of tke Digestive Tract
Thcif Special Pathology^ Dtagaosis» and Treatments With
Sections on Anatomy and Physiology^ Analysis of Stomach
and Intestinal Gmtents^ Secretions^ Feces» Ufine» Bacteria^
ParasiteSy etc^y Suf gery » Dietetics^ Diseases of the Recttf m» etc*
AN EXHAUSTIVE SYSTEMATIC TREATISE
By JOHN C. HEMNETER, N.D.
IVof «aaor In the McdScal DcpactmcnV of fhc University of Maryland t ConKihant to the Univcnity Hoapltal toad
Director of the Clinical Laboratory^ etc.f formcrfy Clinical Pirofcaior of Mcdidne
at the Baltimore Medical College, etc.
DISEASES OF THE STOMACH. Third Edition,
With 15 Plates and 41 other Illustrations, some of which
are printed in Colors. Octavo. 894 pages.
Cloth, $6.00 ; Sheep, $j.oo
DISEASES OF THE INTESTINES. Two Volumes.
With 19 Plates and no other Illustrations, some of which
are printed in Colors. Octavo. 1421 pages.
Vol. I. Anatomy, Physiology, Pathology, Diagnosis, Thera-
peutics, Intestinal Clinic, etc. Cloth, 15.00; Sheep, $6.00
Vol. II. Appendicitis, Occlusions, Intestinal Surgery, En-
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eases of the Rectum, etc. Cloth, I5.00 ; Sheep, $6.00
*5jc* These books form a complete treatise on Diseases of the Digestive Tract.
The subject is covered thoroughly and systematically by an author of well-known
reputation and ability. The results of recent investigation, by which so much
progress has been made in the Pathology, Diagnosis, and Medical and Surgical
Treatment of disorders of the intestinal tract, make their issue at this time of
special importance. They are handsomely illustrated, exhaustive, and written
for the general practitioner, taking into special consideration American habits of
living, diet, and climate.
'< We wish to express unqualified approval of the tendency which is shown to emphasize the
simple and more practical methods of dii^nosis.'* — JVrw York Medical Journal ^ Review of "Dis-
eases of the Stomach.*'
DESCRIPnyE CIRCULAR UPON APPUCATION
47
IN PRESS
EDGAR'S Obstetrics
A NEW TEXT-BOOK
By J. CLIFTON EDGAR, M.a
Pk«{«aMe of Obstetrics, Medical Depwtmcnt ol ComtU Unhrtnhy, New York Oty; PhyiiclaA to Motbcn' and
Babies^ Ho^ltai and to th« Emccgencr Hosphal, etc
Octavo, about 1000 Pages; 900 Illustrations
The Illustrations in Edgar's Obstetrics surpass in number, in artistic
beauty and in practical worth those in any book of similar character. They are
largely from original sources. Those which follow other works have been
redrawn with modifications so that the entire series is new. All have been drawn
by artists of long experience in this department of medical illustration, and
whenever of advantage to do so are reproduced at a stated scale.
No attempt has been made at display. When a small cut serves every pur-
pose drawings are not reproduced to occupy a large space; when black and white
are equally expressive an elaborate colored plate has not been used. So fat
as possible, cuts have been inserted in the text where they are wanted and where
the eye catches them at the place the text explains them. Relative importance
has determined the selection, the size, and the character of each figure. Then
are many explanatory diagrams which add greatly to the teaching values of th<
pictures. The aim of author, artist, and publisher has been to make a series o
pictures useful to the student and reader, and no time, labor, or money ha
been spared to gain this end. The lack of uniformity in quality and failure t<
observe scale — the great faults in books on this subject — have been kept constantl]
in mind, and every endeavor has been made to avoid similar defects.
The Text has been prepared with great care. The author's extensiv
experience in hospital and private practice and as a teacher, his cosmopolitai
knowledge of literature and methods, and an excellent judgment based upon al
these fit him specially to prepare what must be a standard work for both student
and physicians.
In the text as in the illustrating, uniformity and consistency have been kep
constantly in view. The subjects of monstrosities and malformations, for example
do not take up space which could be better used for more practical and usefi]
matters, though these topics like others of their class receive due consideratioi
and are illustrated by a very complete series of small figures. Nothing o
importance remains unsaid, and the relative value of each subject has been care
fully planned out and fixed by deliberate thought. The author's reputation i
sufficient guarantee of the merit of this book ; the publishers, however, ask i
comparison with other works, with confidence that this will be found the mos
useful.
48
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