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D,Bi:ized3,GOOgIC
D,Bi:ized3,GOOgIe
THOMAS A. EDISOR
:lli>t[>r and lovenur of Nu
ighlini, and Other Ekclrica]
3,GoogIe
Cyclopedia
of
Applied Electricity
IKF.CT-CURRENl- nP.NERATORS AND MOTORS, STOKAUE BATTBRIKS,
KLECTROCHEMISTRY, ELECTRIC WIRING, ELECTRICAL MF.AS-
I'KEUENTS, ELECTRIC LIGHTING, ELECTRIC RAILWAYS,
TRANSMISSION, ALTERNATING-CURRENT
WACHINERV, TEI-EGRAPHV, ETC.
F.LErTKICAL EXFEKTR, ENGINEERS, AND DESlIiNEKS Ol* THE II1GKF.ST
UluUy.^h.l -.lUh 0-ir Two Tlwowu/ E>r;raviKSS
SEVKN \'OLl'MES
DignzedoyGOOgle
OOCnilOHT. IME. ISM. 1906. 1909. 1911. 1913
AMERICAN SCHOOL OP CORRESPONDENCE
190G, IMS. 1908. 1909. 1911, ItlS
AUERICAN TECHNICAL SOCIETTT
Capyrishted In Gnat Briton
All RlEhts Roerved
DignzedoyGOOgle
189154./y^
OCT -8 13K ^^^^ CH"^ 4 ^"^
Authors and Collaborators
FRANCIS B. CROCKER, E. M., Ph. D.
Proftuor of Electrtcal EDElDeariiii. Columbia University. Nnr York
Pwt-PnaUent. Anuriuo InatitaU of Elsctricitl Eniinsen
WILLIAM ESTY, S. B., M. A.
Kewl. DiiwrtnunC ef Elsctrhsl Engtitetiriia. Lshlib Univer
Joint Adthot of "Tbs Elements of Elaetrical EnBiBaerlna"
HENRY H. NORRIS, M. E.
PrvtoMor of Electrical EnctnearlOK. Comsll XJniTerBiU
Secretary, Society for tbe Promotion of Eaelaemins Bdncation
Chalnnu. Edueatlonal CommiLtw. Ameiicnn Electric Railway
ROBERT ANDREWS MILLIKAN, Ph. D.. Sc. D.
PrafeKT of PhTila. University of Cbi««a
Co-Aatbwof "PintCoarse'in Phyucs"
Author of "Uechanici, UoImuIk PhyBloand Best." uid Co-Anthor of "Electricity,
Sound and Usht"
n Physical Society
KEHPSTER B. MILLER, M. E.
ConaultlnK Ensinser
or th< firm of HcMem ft MUler. Electrical Ensine
nstitute of Technolosy
CHARLES F. BURGESS, E. E.
ProfeesoT of Chemical Eurineerlne and Applied Electrochemii
PaK-Preiident. Amwlcan Eleetrochemlcat Society
Member. American Ingtitute of Eleclricia GniiiDeerB. America
Society of Cbamlcal InduAry. Weatem Society of Eniineei
Pnaident. Northern Chemical EnglneerinE LaborBlorioi
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Authors and Collaborators -Continued
UORTQN ARENDT, E. E.
Awiataot PrafiHor of EUctrkal EasineerliiK. Columbia Unlvosit;, N«« York
WILLIAM H. FREEDHAN, C. E., E. E., H. S.
Head. DBputniDt of ApuliBd ElMtriclCy, Pntt Inititutc, BrooklrD. Nsw York
Formarly Hevl. Depubnaot of ElKtarlckl EnEiBsariDS. Univenlty of Vrnnont
GEORGE S. HACOMBER, H. E.
AHlitut PiofenoT of BJtbUM Ensinnriiw, Comsl] Uoiversltr
FornHTlr Inatmcter. WuhlnirUm UniTenlty
t, CbtIhii Talaphons UanafuEurilut Cooipuir
CHARLES E. KNOX, B. E.
CoBulttnK Electrical Ensliicer
Amarieu IiuUCatB of Elactrlcal EntHnc
JOHN LORD BACON
EndoMrwicl SuperlDtcndent of CoMlruction with R. P. Shield* A Son. G
inSoclMTOf Mechanical Endnears
EDWARD B, WAITE
Hawl. Initrtiction Department. American School of Cor»Bponden>
Formerly R«(«rcb Aaaiatant. Hirvard Colless ObHrvaCory
American Socwty of Uachanical EaEioeen
W«t«m Social of Ensinaen
GEORGE R. METCALFE, M. E.
Editor. AmaricaD laatltateot Elaetrtcai Bnsineen
rorawlv Head. Technical PiMtcatioa DepartnHnt. WHtlnKhouM Elective A Manafac-
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Authors nnd Collaboraiors— Cojuinucd
HUGO DIEHER, M. E.
Pralcaor of InduBtrlat EnslHarlnB. PeniUTlvanU Stat* Cotka*
A>B«ricaa SocMy of Hechantcal BBi1n»n
THOMAS E. DIAL, B. S.
CouultiiuE En«inMr. DeputmiDt of Civil EnslDe
CdTT«flJIOTld«DGa
JOHN H. JALLINGS
H«hiuilcal EmriiHcr
ForTwenU VmsSuiHrinlendnitmiidChicICanBtruetarfor J. W. Rndr Elevator Co.
DAVID P. MORETON, B. S., E. E.
Aaoociatc PrafcBor of Elactrlul Enilneerii
Amrlcmnlnititateof ElestrhalBDEidHn
GLENN H. HOBBS, Ph. D.
S«crel«TT, Anwrkaa School of Corrapondon
Fomwrly InitructuF In PhyHioa. Univeraity a
Amerlcui Phyiical Society
H. C GUSHING, Jr.
ConaultliiB EUetrical EnsliiMr
Author o( "Standanl WirliiB for Electric Lliht and Pan
J. P. SCHROETER
Consultini Engineer. Department of Electrical GBBin«HBK. American School of
CHAS. THOH
Chief, Quadranln Department. Western Union Main Omce. New York City
JESSIE H. SHEPHERD, A. B.
Bead, Publication Department. American School of CorrcapondeDC*
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Authorities Consulted
THE editors have freely consulted the standard technical literature of
America and Europe in the preparation of these volumes.* Their
desire to express their indebtedness, particularly to the following
eminent authorities, whose well-known works should be in the library of every
electrician and engineer.
Grateful acknowledgment is here made also for the invaluable co-opera-
tion of the foremost engineering firms and manufacturers in making these
volumes thoroughly representative of the very best and latest practice in the
design, construction, and operation of electrical machinery and instruments;
also for the valuable drawings, data, suggestions, criticisms, and other
SCHUYLER S. WHEELER, D. Sc.
FiMident.Ciwlier-Wh«aler Company: Put-Pm
EDElnecn
Joint Author of "Hsnasmnent of Electrlul Unci
ALFRED E. WIENER. E. E., M. E.
Member. Amenan Iiutitots of Electrical Emineei
AuUnr of "Praetlcal Calculation of Dynamo-Eltwt
WILLIAM S. FRANKLIN. M. S., D. Sc.
Fntcnorof Phyaica. Lotaiah Unlvrraity
Joint Author of "The E]«i»Dta of Electrical EnglnwrinE." "The Elemmtl of
WILLIAM ESTY, S. B., M. A.
Head ot Department of Electricul EnsineerinK. LehlRh UniveniCr
Joint Author ot "The ElemenU of ElKtric>l Englneerinc"
HORATIO A. FOSTER
Consoltins EoBinwr: ICtmber of 1
of AnunicHn Society of Mfchan
Author of "Electrical EnsinMi'e P
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Authorities Consulted — Continued
DUGALD C JACKSON, C. E.
Haw] of Daputmant of SlacCriul EnslBearlns. HunehnutU IiutitnM of TKhiiahvT ;
' Hamber, Amaricaa Soclaty of llKhmnlcal EivliHan. AdwHoui liuUtuU ol Glsc-
ADthor of "A THtbODk on Elactronucnatllin and the Cmutmction of Drnamoa"; Joist
Author of "ALCanutloE Carraota and Altanutliv<-Curreat HAChfrwry"
It tiM Oertikon Work*
WILLIAM L. HOOPER, Ph. D
Held of Depvtinant of Electrics]
Joint Aothor of "Eketrlcsl Pmbl
ROBERT ANDREWS MILLIKAN, Ph. D.
Prafcuar of Fhraici. Univenlty of Chhxga
Joint Author of "A Fint Coune In PhraicB." "Electrleity. Sound a]
JOHN PRICE JACKSON. M. E.
;. Pennwlnnia 3UCa Cotlene:
I and Allematlne-Currant Ma
MICHAEL IDVORSKY PUPIN, A. B., Sc. D., Ph. D.
ProfeHor of ElHrtro-Mechani». Columbia Univrralty. New York
Author of "ProDasatlon oF Lode Electric Wivan." and "Wave-Trans
Uniform Cables and Lans-Dlstanca Air Line*"
LAHAR LYNDON, B. E., M. E.
Conauttlni Electrical Engineer; Anociate I
Enslaeara: Hernber. American Electroc
Author of "Storasa Battary EneinaarinK*'
EDWIN J. HOUSTON, Ph. D.
Profowr of Phyala. Franklin Initltute. PenniyWania: Joint Inventor of Thof
Hoaaton Syatem of Arc LiEhllnsc: Electrical Eineri and ConsultlDit Enttlnaer
Joint Author of "Altematlns Currenta." "Arc LlKhtlni." "Electric Heat
'■Electric Motora." "Eleitrkr Rallwayi," "IncandeBxnt Liahting." etc.
ARTHUR E. KENNELY, D. Sc.
Proteaear ot Electrical Enitlacerina:. Har
"Electric Motors." "Electric Railwair
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SILVANUS P. THOMPSON, D. Sc., B. A., F. R. S., F. R. A. S.
Prlneipal and Pnitewor of Phnilci in the City aiid Guflda of Laadou TochDical Coneso
Paat-Prnidmt. ItuUtutloD of EI«tiicaI EnKincsn
Author of "Etoctrielty and HaKMtiim." "Drumo-ElKtric Uachlnerr." "Polyphu*
Elsctrlc CumnU and A Itarnate -Currant Motor*," 'Ths ElKtromisiutt.'' ate.
KEHPSTER B. MILLER, M. E.
Consultins EBKlnnr of tha Firm or HcMeen and HilUr, Eleelrlcal EnKlnecra. Chlcasa
Author af "Ameriean Talaphono Practice", Joint Author of "Telaphony"
MAURICE A. OUDIN, H. S.
Hemlwr. Anwrlcan InitltuU of Elactrleal Enelnaera
Aatbor of "Btandajnl Poiyphaae Apparatus and SyiU
FREDERICK BEDELL, Ph. D.
Profcuorof Applied Electricity. ComaJI UnlTen
Author of "The Principle* of the TnuuCar
H. F. PARSHALL
Hemlwr of American Inatltute of Electrical EnEfneera,
Eniineen. American 8oclely of Uachanicat Gnilneir*. •[
Joint Author of "Armature Wlndlnn of Electric Haehlnea"
, FLEMING, M. A., D. Sc. (London), F. R. S.
FrofeBBor of Electrical EDKlDeeriur Id (Jnlvanily (^lleEe. Londoa: Lata Fellow and
Schobir of St. John'a ColIeK*, Canbridca: Fellow of Univenity Coilese, London;
Member. InatitutloD of Electrical Ensineoia; Member of the Phyaical Society at
London : Member of the RoyMl Inatltution of Grwt Britain, etc.
Author of "The Alternate -Current Transformer. " etc.
LOUIS BELL, Ph. D.
ConsultlnE Btectrical Ensh>«r; Lecturer on Power 1
Inatltata of Technoloiy
Author of "Electric Power Trmnsmisaion," "Power Dtetrtt
-The Art of Ulunilnatlon." "Wireleo Telephony." etc.
CHARLES PROTEUS STEINMETZ
ConsukiDE EnElneer. with Che General Electric Compan
EnifineeHnc, Union Coliese
Author of The Theory and CalculatioD of Alternatins-Cu
retical Elemenu of Bl*ctrieal Eoitneerlns." etc.
CYRILL M. JANSKY. B. S., B. A.
Associate Prof eosoc of Electrical Engine.
Author of "Elacttlc Hetera"
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J. J. THOMSON, D. Sc., LL. D.. Ph. D., F. R. S.
Fellow ot Trislur CollesK Cambridae Ualvanlty: C>Tendiih Profawr of EiiHilineotiil
Phyalo. Cunbrida* Uuivenlty
Author of "Tha Conduction of ElKtrldtythrouBh Guag." "Eloctrlelty and Hattar." eU.
HENRy SMITH CARHART, A. M., LL. D.
PralttMor Emerltui of Phyalca. Univanltr of Uiehlsui
Anthor of "Priimrj Battariaa," "Elamanta of Phyaka." "UDiTar^tjr Phraics." '
trkal Maninnianta." "Hteh School Phrslca," etc.
F. A. C. PERRINE, A. M., D. Sc.
ConaultlDs Englnaar; Formarly Praaident. Stsnlay Elactiie Uunfacinrins Cdit
Pormartj Hbdbsct. Inaulatad Win DapartRtant, John A. Soablioa'a 3on> Coi
Authorof "CoDductocB for Elaetrical Diatiibutlon"
WILLIAM MAVER, Jr.
Ei-ELeetriclaa. Biltlmoie and Ohio Tslasnuli Company
Author of "WInleaa Tetosrachy." "Amarfcan TaleErachy and E
TeLasnrh"
E. B. RAYMOND
TaattnE Departmant. Gflneral Electric Company
Author of "AltntiaClnB-CDmnt BDElnearlDs"
AUGUSTUS TREADWELL, Jr., E. E
AaMciata Uember. American loatitule of i
Author of 'Th« ator»Ba Battery; A Practk
SAMUEL SHELDON, A. M., Ph. D.
Proteaaor ot Phyalea and Electrical Eosineerini, Polytechnic InatitDte
Joint Author of "Dynaoio-Elactric Itachinery." "AltematlDx-CDrrent Ha
V-
HOBART MASON, B. S,, E. E.
AaalstBDt In Eleetrdcal Enslnaerlnc. Polytechnic Inatltute of Brooklyn; At
bee, American Initltute of Electrical Enslneen
Joint Author of "Dynamo- Electric Machinery." "Alternating- Current Hi
H. M. HOBART, B. Sc.
Member. Inatltotlon of Civil Enslneeia. American Inatltute of Electric
Joint Author of "Armature Construction"
ALBERT GUSHING CREHORE, A. B., Ph. D.
Electrical Enslneer: Auiatant Profesaor of Phyalea. Dartnuoth Colleca
Author of "SjnehronouB and Other Uultiple Telesrapha": Joint Aoth
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Fore-word
ONE of the simplest acts in modern life is switchinsr on
the electric current that gives light or power, or that
makes possible communication between distant points.
A child can perform that act as effectively as a man, so
thoroughly has electricity been broken to the harness of the
world's work; but behind that simple act stand a hundred years
of struggle and achievement, and the untiring labors of thousands
of the century's greatest scientists. To compact the results of
these labors into the comi>ass of a practical reference work is
the achievement that has been attempted— and it is believed
accomplished— in this latest edition of the Cyclopedia of Applied
Electricity.
C Books on electrical topics are almost as many as the subjects
of which they treat, and all of them, if gathered into a great
common library, would contain so many duplicate pages that
their use would entail an appalling waste of time upon the man
who is trying to keep up with electrical progress. To overcome
this difficulty the publishers of this Cyclopedia went direct to
the original sources, and secured as writers of the various
sections, men of wide practical experience and thorough tech-
nical training, each an acknowledged authority in his work; and
these contributions have been correlated by our Board of
Editors into a logical and unified whole.
C. The Cyclopedia is, therefore, a complete and practical work-
ing treatise on the generation and application of electric power.
It covers the known principles and laws of Electricity, its
generation by dynamos operated by steam, gas, and water power;
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its transmission and storage; and its commercial application for
purposes of power, light, transportation, and communication.
It includes the construction as well as the operation of all plants
and instruments involved in its use; and it is exhaustive in its
treatment of operating "troubles" and their remedies.
C. It accomplishes these things both by the simplicity of its text
and the graphicness of its supplementary diagrams and illus-
trations. The Cyclopedia is as thoroughly scientific as any
work could be; but its treatment is as free as possible from
abstruse mathematics and unnecessary technical phrasing,
while it gives particular attention to the careful explanation
of involved but necessary formulas. Diagrams, curves, and
practical examples are used without stint, where they may be
helpful in explaining the subject under discussion; and they
are kept simple, practical, and easy to understand.
C. The Cyclopedia is a compilation of many of the most valu-
able Instruction Books of the American School of Corre-
spondence, and the method adopted in its preparation is that
which this School has developed and employed so successfully
for many years. This method is not an experiment, but has
stood the severest of all tests-that of practical use-which has
demonstrated it to be the best devised for the education of the
busy, practical man.
C. Attention is directed to a bibliography of the best literature
in Electrical Engineering, in Volume VII. No attempt has been
made to exhaust the sources but merely to provide the names,
authors, and publishers of books which would appeal to the
widest circle of readers.
C. In conclusion, grateful acknowledgment is due to the staff
of authors and collaborators, without whose hearty co-operation
this work would have been impossible.
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Table of Contents
VOLUME 1
Elements of Electricity . . By R. A. MilHkan t Page *11
yasnaU— Poka— Attncdon ind Rep ulslnn— Induction— Rctentivity ■Dd Perm*-
■bllity—Ltneiof Force— Ths Earth MB M«Bn«—St«tU! Electricity— PtnitWeMlii
uid Theonr— ElMtmn Theory— Electro«copo— Liaht-
ial— Ueasurementof PoUntials— Coadensen— Leyil«n
Jki— Electrical Screcni-Static Uocbloer-GalvBDie Cell— Electrical Uoobuk-
nwnte— E. U. F.— Rnisunce— Ohm'i I«w— Primary Cell* (BJehranal*. DuiiaH.
LeclanchA. Diy)— Cella Connected In Scriea: Id Parallal— Electrolyili— StonEe
Battery —EleclromasDeti—Eleetric Bell— Telerrach —Telephone
The Electric Current . . By L. K. Soger Page 81
E. M. F. -Current— Re»»anc«-UB Its (Volt. Ampere. Ohml— Conductance-
Conductivity— Ohm's Law— Serisa Clrculti— Fall of PoConClal- Divided Circuits
—Battery Circuit*— Quantity. Enarsr, and Power IConkinib, Joale. WattI—
Central SUtiosB— Isolated Plonta
Electrical Measurements By George W. Patterton Page 123
Syitenu of Units— Gal vsnomatert- EleetrodyBa mametera- Elec trometen— Watt-
meters— Rscordlns Voltmeters and Ammeten— IntecntinB Watt-Hour Bleters
— IntCEratiiiE Ampere- Hour Meters— RheosUta and Raalatance Colla— Shanta—
Ohm's Uiw— HstisurameDt i^ BHlatance— Portable TeitinE Set— Ineolatkni
Reitatanca— Re^tanee of Llnee— Loeatine Grounds and Faults— Meaaurement
of Battery RealBtance-Ueasureinent of E. U. F.— Measurement of Current-
Underwriters' Requirehents By Dana Pierce Page 217
Electricity as a Cause of Fbv— Elementary Electrical Terms— Essential Parts
of Electrical InstalUtions- Power Stations and Their Equicment: Geneniton,
Switchboard. Rheostats. LIstatnlns Arresters. Ground Detectors. Motors.
Storace Batlarfee, Transformer— Outside Work: Wlrioa. Electrolysis. High
Tenalon Lines. Moontlns Tranaformers. Grounding of Circuits— Insids Worli-.
General Rules. Constant Current. Constant Potenlisl, Fuses and Gtrcait
Breakers. Fixtures, Tnuufonnera-Ioatallatlan at Wires in BuildinKs: Open
Work, Concealed Work. Conduit Work- Decorative and Commercial Lishthis—
Theater WtrlDg- Hovint Pieture Theaters— Car Wlrine-Llshtins and Power
from Railway Wires- Hish- and ETtra-Hish Potential Syatema— Signallns
Systems— Tastlni by Voltmeter Method- Devices and Materials: Rubber-
Covered Wire. Rigid Conduit and F^ttlnrs. Cut-OuU. Circuit Breakers, Panel
Boards. Sockets. Bell-RlnEing Transformers. Heatini Devices
Review Questions Page 391
Index Page 401
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ELEMENTS OF ELECTRIQTY
AND MAGNETISM'
MAGNETISM
1. Natural and Aitifidal Magnus. It has been known for many
centuries that some specimens of the ore known as magnetite (Fe,Oj)
have the property of attracting small bits of iron and steel. Tliis
ore probably received its name from the fact that it is abundant in the
province of Magnesia in Thessaly, although the Latin writer Pliny
says that the word magnet is derived from the name of the Greek
shepherd Magnes, who, on the top of Mount Ida, observed the
attraction of a large stone for his iron crook. Pieces of ore which
odiibtt this attractive property for iron oi steel are known as natural
magnets.
It was also known to tiie ancients that artificial magnets may be
made by stroking pieces of sted with natural magnets, but it was not
until the twelfth century that the discovery was made that a sus-
pended magnet would assume a north-and-south position. Because
of this property natur^ magnets came to be known as lodestones
Reading stones), and magnets, dther artificial or natural, b^n to
be used for determining directions. The fijst mention of the use of a
compass in Europe is in 1190. It is thought to have been introduced
from China.
Artificial magnets are now made either by repeatedly stroking a
bar of steel, first from the middle to one end on one of the ends, or
poles, of a magnet, and then from the middle to the other end on the
other pole, or else by passing electric currents about the bar in a
manner to be described later. The form shown in Fig. 1 la called a
bar m^net, that shown in Fig. 2 a horseshoe magnet
•TbU paper IB a, modification siul abriagmenc ol the trsatmenti of MoffintUm OM
StictricUf foaod In MllllliMi and Oale's nrtl Covrtt In PhytUi (Qlnn A Oo. , BoaMm). U
wuoli Um ModBut la referred tor a more compLeta preasutatloii ol the subject.
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ELECTRICITY AND MAGNETISM
■"^
^
Fig. 1. HoTMihoe Uagnot.
2. The Poles of a Magn^ If a magnet ia dipped into iron filings,
the filings are observed to cling in tufts near the ends, but scarcely
at all near the middle (Fig. 3). These places near the ends of the
magnet, in which its strength seems
to be concentrated, are called the
poles of the magnet. It has been
Tng.i. BursugiiM. decided to call the end of a freely
suspended magnet which points to the north, the northnseeking or
north pole, and it is commonly
designated by the letter N. The
other end is called the aovih-teek- i
xng or aouih poU, and is desi^ I ^
nated by the letter S. The direc-
lion in which the compass needle
points is called the magnetic meridian,
3. The Laws of Magnetic Attraction and R^mlsion. In the
experiment with the iron filings, no particular difference was ob-
served between the action of the two
poles. That there is a. difference,
however, may be shown by ex-
perimenting with two magnets,
either of which may be suspended
(see Fig. 4). If two N poles are
brought near one another, they
are found to repel each other.
The 8 poles likewise are found to
repel each other. But the N pole
of one magnet is found to be attract-
ed by the S pole of another. The
results of these experiments may be
summarized in a general law:
Magnet poles of like kind repel
each other, while poles of unlike
kind attract.
The force of attraction is found,
like gravitation, to vary inversely
as the square of the distance between the poles; that is, separat-
mg two poles to twice their original distance reduces the foree
DignzedoyGOOgle
ELECTRICITY AND MAGNETISM
acting between them to one-fourth its original value, separating
them to three times their original distance, reduces the force to
one-ninth its original value, etc.
4. Magnetic Substances. Iron and steel are the only conmion
substances which exhibit magnetic properties to a marked degree.
Nickel and cobalt, however, are also attracted
appreciably by strong magnets. Bismuth,
antimony, and a number of other substances
are actually repelled instead of attracted, but
the effect is very small. Until quite recentiy .
iron and steel were the only substances "^
whose magnetic properties were sufficiently
strong to make them of any value as magnets.
Within the last five years, however, it has
been discovered that it is possible to make
certain alloys out of non-magnetic materials
such as copper, magnesium, and aluminum ^^'
which are almost as strongly magnetic as iron.
These are known as the Heusster alloys.
5. Magnetic Induction. If a small unmagnetized nail is sus-
pended from one end of a bar magnet, it is found that a second nail
may be suspended from this first nail, which itself acts like a magnet.
4. SbOirlaB Vftrlk-
Ukgnet PolM.
C
a third ftom the second, etc., as shown in Fig. 5. But if the bar
magnet is carefully pulled away from the first nail, the others will
instantly fall away from each other, thus showing that the nails were
strong magnets only so long as they were in contact with the bar
magnet. Any piece of soft iron may be thus magnetized temporarily
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4 ELECTRICiry AND MAGNETISM
by holding it in contact with a permanent magnet. Indeed, it is not
necessary that there be actual contact, for if a nail is simply brought
near to the permanent magnet it is found to become a magnet. This
may be proved by presenting some iron filings to one end of a nail
held near a magnet in the manner shown in Fig. 6. Even inserting
a plate of glass, or of copper, or of any other material except iron
between S and JV will not change appreciably the number of filings
which cling to the end of S'. But as soon at the permanent magnet
13 removed, most of the filings will fall. Magnetism produced in this
way by the mere presence of adjacerU magnets, wiik or vnthovi contact,
it called induced magn^ism. If the induced magnetism of the nail
in Fig. 6 is tested with a compass needle, it is found that the remote
induced pole S' is of the same kind as the inducing pole S, while the
near pole N ta of unlike kind. This b the general law of magnetic
induction.
Magnetic induction ex^^ins the fact that a magnet attracts an
unmagnetized piece of iron, for it first magnetizes it by induction,
so that the near pole is unlike the inducing pole, and the remote pole
like the inducing pole, and then, since the two unlike poles are closer
together than the like poles, the attraction overbtdances the repulsion
and the iron is drawn toward the magnet. Magnetic induction also
explains the formaUon of the tufts of iron filings shown in fig. 3,
each little filing becoming a temporary magnet such that the end
which points toward the inducing pole is unlike this pole, and the
end which points away from it is like this pole. The bush-like
appearance is due to the repulsive action which the outside free poles
exert upon each other.
6. R^eiitivity and Prameability. A piece of soft iron will very
easily become a strong temporary m^net, but when removed from
the influence of the magnet it loses practically all of its magnetism.
On the other hand, a piece of steel will not be so strongly magnetized
as the soft iron, but it will retain a much larger fraction of its mag-
netism after it is removed from the influence of the permanent magnet.
This power of redsting either magnetization or demagnetizadon is
called retentivUy. Thus, steel has a much greater retentivity than
wrought iron, and, in general, the harder the steel the greater its
retentivity.
A substance which has the property of becoming strongly mog-
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ELECTTRICITY AND MAGNETISM 5
ttedc under the influence of a pemument magnet, whether it has a
hi^ tetentivi^ or not, is stud to possess •permeabUUy in large degree.
Thus, iron is much more permeable than nickel. Fenneability b
me&sured by the amount of magnetization which a substance is able
to recare; while letentivi^ is measured by the tenacity with which
it holds it
7. Magnetic Lines of Force. If
we could separate the N and 8
poles of a small magnet so as to
obtain an independent N pole, and
rig. T. Dtraoutm at uaonMiTLi^ot "^^^ to pUce tMs N pole near the
""""^ N pole of a bar magnet, it would
move over to the S pole of the bar magnet, along some curved
path Bunilar to that shown in Fig. 7. The reason that the motion
is along a curved rather than along a straight path is that the free
pole is at one and the same time repelled by the N pole of the bar
magnet and attracted by its S pole, and the relative strengths of
these two forces are continually changing as the relative distances
of the moving pole from these two poles are changed.
It is not difficult to test this conclu^n experimentally. Hus,
if a bar or horseshoe magnet is pieced just braieath a flat dish con-
taining water (see Fig. 8), and a cork carrying a magnetized needle
jrfaced near the N pole in the manner shown in the figure, the cork
win actually be fotmd to move in a curved path from N around to S.
In this case the cork and the needle actually move as would an
independent pole, since the upper pole of the needle is so much farther
from the magnet than the lower pole that the influence of the fonner
OR the motion is very small.
Any path which an independent N pole would take in going from
.^ to S is called a line of
magrutie force. The simplest
way of finding the direction
of this path at any point
near a magnet is to bold a
compass needle at the point
coondered, for the needle must obviously set itself along the line in
which its poles would move if independent, that is, along the line of
fevce wiiich passes throu^ the given pomt (see C, Fig. 7).
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6 ELECTRICrrY AND MAGNETISM
S. Magnetic Fields of F<wce. The region about a magnet in
which its magnetic forces can be detected is called its field of force.
The simplest way of gaining an idea of the way in which the lines of
force are arranged
in the magnetic
field about any
magnet is to sift
iron filings upon a
piece of paper
placed immediately
over the magnet.
Eadi little fiUng
becomes a tempo-
Pig. 9. Shspa Of Uasnetic Field about ft bm MftgnBt. ^ magnet by in-
duction, and therefore, like the compaas needle, sets itself in the
direction of the line of force at the point where it is. Fig. 9 shows
the shape of the magnetic field about a bar me^et. Fig. 10 shows
the direction of the lines of force about a horseshoe magnet. Fig. 11
is the ideal diagram corresponding
to Fig. 9 and showing the lines of
force emei^ng from the N pole and
passing around in curved lines to the
S pole. This way of imagining
the lines of force to be closed
curves passing on the outside of the
magnet from N around to 5, and
on the inside of the magnet from iS
back to N, was introduced by
Faraday about 1830, and has been
found of great assistance in correla-
ting the facts of magnetism.
9. Molecular Nature of Mag-
n^lsm. If a small test-tube full of
iron filings be stroked from one end p^ ,„. ^ireotion ot Lin«, « i^m»
to the other with a magnet, it will '*'~"' "^ Bo™«>i.oe suBnei,
be found to behave toward a compass needle as if it were itself a
magnet, but it will lose its magnetism as soon as the filings are shaken
up. If a magnetized needle is heated red-hot, it is found to lose its
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ELECTRICITY AND MAGNETISM 7
a completely. Again, if any magnet is jarred or hammered
or twisted, the strength of its poles as measured by thdr ability to pick
up tacks or iron filings, is found to be greatly diminished.
These facts point to the
') I j ,' /,''"""-, \ ! J / ; conclusion that m^netism
\ ■* ' ' ■''—--> ^ ■ ' ' ,' has something to do with the
\\ ,' arrangement of the molecules,
;*V .;; since causes which violently
fjj ;: disturb the molecules of the
'/', "0^ magnet weaken its ma^
' • ^ ^ netism. Again, if a m^net-
/ / \ ized needle is broken, each
' ' ' ' ^ "" ' '' ' I '. 1 part will be found to be a
Fit. II. MertDUpgmof^nj. Of Force about complete magnet. That is.
two new poles will appear at
the point of breaking, a new N pole on the part which has the
original S pole, and a new B pole on the part which has the original
iV pole. The subdivision may be continued indefinitely, but always
with the same result, as indicated in Fig. 12. This points to the
conclusion that the molecules of a magnetized bar are themselves
little magnets arranged in rows with their opposite poles in contact.
If an unmagnetized piece of hard steel is pounded vigorously
white it lies between _
the poles of a mag- fe N B .N S N ''^ S N
net, or if it is heat- g^ TT?* *& ' «>
ed to redness and |nr jrf i
then allowed to s N
cool in this nosi- V^- ^ ^ Manet Broken tulo Smaller Uagnet*^ Sbowluc
«™. w. uua |/vo. Oonnectlon between MaBnetlsm anfl
tion, it will be Moleoolar AmnBement.
found to have become magnetized. This points to the conclusion
that the molecules of the steel are magnets even when the bar as
a whole is not magnetized, and that magnetization consists in
causing these molecular magnets to arrange themselves in rows,
end to end.
In an unmagnetized bar of iron or steel, then, it is probable that
the molecules themselves are tiny magnets which are arranged either
haphazard or in little closed groups or chains, as in Fig. 13, so that,
on the whole, opposite poles neutralize each other throughout the
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ELECTRICITY AND MAGNETISM
bar. But when the bar is brought near a magnet, the molecules are
swung around by the outside magnetic force into some such arrange-
ment as that shown by Fig. 14, in which the opposite poles com-
pletely neutrahze one another only in the middle of the bar. Accotd-
Fig. I& niearMlaKlAmngementot HolecnlM In k Bar ot
OnUnuT Iron or StwL
log to this view, the reason that heating and jarring weaken a magnet
b that disturbances of thb sort tend to shake the molecules out of
alignment. On the other hand heating and jarring facilitate mag-
netization when an unmagnetized bar ia between the poles of a magnet,
because they assist the magnetizing force in breaking up the molecular
groups or chuns and getting the molecules into alignment. Soft iron,
then, has hi^er permeability than hard steel.merely because the mole-
cules of the former substance do not offer so much resistance to a
force tending to swing them into line as do those of the latter sub-
stance. Steel has on the other hand a much greater retenttvity than
soft iron, merely because its molecules are not so easily moved out of
postion when once they have been aligned.
10. Saturated Magnets, Strong evidence for the correctness of
the above view is found in the fact that a piece of iron or steel cannot
be magnetized beyond a certain limit, no matter how strong the
magnetizing force. This limit probably corresponds to the condition
in which the axes of all the molecules are biou^t into parallelism, as
in Fig. 14. The magnet is then said to be aatwaied, since it is as
strong as it is po9«ble to make it.
1 1. The Earth's Magndlsm. The fact that a compass needle
^ways points north and south, or approximately so, indicates that
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ELECTRICITY AND MAGNETISM 9
the earth itself is a great magnet, having an S pole near the geographi-
cal north pole and an N pole near the geographical south pole; for
the magnetic pole of the earth which is near the geographical north
pole must of course be unlike the pole of a suspended magnet which
points toward it, and the pole of the suspended magnet which points
toward the north is the one which by convention it haa been decided
to call the north pole. The magnetic pole of the earth which b near
the north geographical pole was found in 1831 b; Sir James Ross
in Boothia Felix, Canada, latitude 70° 30' N., longitude 95° W. It
was located again in 1905 by Capt^ Amundsen at a point a little
farther west. Its approximate location is 70° 5' N., and 96° 46' W.
It is probable that it slowly shifts its position.
12. Declinatioii. It is, of
course, on account of the fact
that the earth's magnetic and
geographical poles do not alto-
gether coincide, that the magnetic
needle does not point exactly
north, and also that the direction
in which it does point changes
as the needle is moved about ^^ i«p«th,ii.gnedoN«di^
over the earth's surface. This
last fact was first discovered by Columbus on his voyage to America,
and caused great alarm among his sailors. There are other local
causes, however, such as lai^ deposits of iron ore, which cause local
deviations of the needle from the true north. The number of
degrees by which the needle varies from the north and south line at
a given point, is called the declination at that point
13. Inclination or Dip. Let an unmagnetized knitting needle
a (Fig. 15) be thrust through a cork, and let a second needle b be
passed through the cork at ri^t angles to a. Let the system be
adjusted by means of wax or a pin e, until it is in neutral equilibrium
about i as an axis, when a is pointing east and west. Then let a be
strongly magnetized by stroking one end of it from the middle out
with the N pole of a strong magnet, and the other end from the middle
out with the S pole of the same magnet When now the needle is
replaced on its supports and turned into a nortfa-and-south line with
its N pole toward the north, it will be found, in the north temperate
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10 ELECTRICITY AND MAGNBTISM
zone, to dip so as to make an angle of from 60° to 75° with the hori-
zontal, niiis shows that in the latitudes mentioned the earth's mag-
netic lines are not at all parallel to tlie earth's surface. The angle
between these lines and the earth's surface is called the dip, or
inclinatitm, of the needle. At Washington it is 71° 5'; at Chicago,
72* 5C; at the magnetic poles it is of course 90°;and at the so-called
magnetic equator — an irregular curved line passing through the tropics
— the dip b 0°.
14. The Earth's Inductive Action. A very instructive way of
showing that the earth acts like a great magnet is to hold any ordinary
iron or steel rod parallel to the earth's magnetic lines, that is, about
in the geographical meridian, but with the north end slanting down
at an angle of say 70°, and then to strike one end a few blows with
the hammer. The rod will be found to have become a magnet with
its upper end an 8 pole, like the north pole of the earth, and its lower
end an N pole. If the rod is reversed and tapped again with the
■ hammer, its magnetism will be reversed. If held in an east-and-west
position and tapped, it will become demagnetized, as is shown by
the fact that both ends of it will attract either end of a compass needle.
STATIC ELECTRICITY
15. Electrification by Friction. If a piece of hard rubber or a
stick of sealing wax is rubbed with flannel or cat's fur and then brought
near some dry pith balls, bits of paper, or other light bodies, these
bodies are found to jump toward the rod. After coming into contact
with it, however, they become repelled. Tliese experiments may be
very satisfactorily performed in winter with the aid of a pith ball
suspended by a fine silk thread, as shown in Fig. 16.
This sort of attraction was observed by the Greeks as early as
600 B. C, when it was found that amber which had been rubbed with
silk attracted various sorts of light bodies. It was not, however, until
1600 A. D. that Dr. William Gilbert, physician to Queen Eliiabeth,
and sometimes called the father of the modem science of electricity
and magnetism, discovered that the effect could be produced by rub-
bing together a great variety of other substances besides amber and
silk, such, for example, as glass and silk, sealing wax and flannel, hard
rubber and cat's fur, eto.
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ELECTRICITY AND MAGNETISM
11
Fig. W. ElecUlILoBtlan b7 FrlMbm.
Gilbert named the effect which was produced upon these various
substances by friction, electrijkation, after the Greek name for amber,
electron. Thus, a body which, like rubbed amber, has been endowed
-with the property of attracting
light bodies is said to have been
electrified, or to have been g^ven a
charge of elec^ricifyf. In this state-
ment nothing whatever is said
about the nature of electricity. We
amply define an electrically charged
body as one which has been put
into the condition in which it acts
toward li^t bodies Uke the nibbed
amber or the rubbed sealing wax.
To this day we do not know with
certainty what the nature of elec-
tricity is, but we are fairiy familiar
with the laws which govern its
action. It is these laws to which
attention will be mainly devoted in the following sections.
16. Positive and Negative Electricity. If a pith ball has touched
a glass rod which has been rubbed with silk and thus been put into the
condition in which it is strongly repelled by tbb rod, it is found
not to be repelled, but on the contrary to be very strongly attracted by
a stick of sealing wax which has been rubbed with cat's fur or flannel.*
Similarly, if the pith ball has touched the sealing wax so that it is
repelled by it, it is found to be strongly attracted by the glass rod.
Again, two pith balls both of which have been in contact with the
glass rod are found to repel one another, while pith balls one of which
has been in contact with the glass rod and the other with the sealing
wax attract one another.
Evidently, then, the electrifications which are imparted to glass
by rubbing it with silk, and to sealing wax by rubbing it with flannel
are opposite in the sense that an electrified body that is attracted
by one is repelled by the other. We say, therefore, that there are two
kinds of electrification, and we arbitrarily call one positive and the
other negative. Tlius, a positively electrified body is one which acts
with respect to other electrified bodies like a glaaa rod which has been
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12 ELECTRICITY AND MAGNETISM
rubbed vnth nlk, and a negatively det^fied body is one which acts like
a piece 0/ sealing wax which has been rubbed with fiannel. These
facts and definidona ma; then be stated in the following general law:
Ele(^'ical charges of like tign repel each other, while charges of utUike
ngn attract each other. The forces of attraction or repulsion are
found, like those of gravitation and magnetism, to decrease as the
tquore of the distance increases.
17. Conductors and InsuIatMs. If a pith ball is in contact
with a metal body A (Fig. 17), and if this body is connected to another
metal body B by a wire, then, when B is rubbed with an electrified
glass rod, A will be found immediately to repel the pith ball from
itself. That is, a portion of the charge communicated to B evidently
passes instantly over the wire to
A. If the experiment is repeated
when A and B aie connected with
A JL^ ,0\ a thread of silk, or with a rod of
^— ' ^--' wood instead of metal, no effect will
PIr. IT. Bxperlment Sbowlna tbe Con- ■ i j , n .i •,!
diMUnBoriaBaiacing propenrar be Observed at all Upon the pith
^'virloui MawrlalB. u n t* ■ . / ^u j
ball. If a moistened thread con-
nects A and B, the pith ball will be affected, but not so soon as when
A and B are connected with a wire.
lliese experiments make it clear that while electric cha^;e3
pass with perfect readiness thiou^ a wire, they are quite unable to
pass along dry silk or wool, while they pass with considerable difficulty
along moist silk. We are therefore accustomed to divide substances
into two classes, condwiors and nonroondudors or insulators, according
to their ability to transmit electrical charges from point to point. Thus
metab and solutions of salts and acids in water are all conductors of
electridty, while glass, porcelain, rubber, mica, shellac, wood, silk,
vaseline, turpentine, paraffin, and oils generally are insulators. No
hard and fast line, however, can be drawn between conductors and
non-conductors, since all so-called insulators conduct to some extent,
while the so-called conductors differ greatly among themselves in
the fticility with which they transmit charges.
Tbe fact of conduction bnngs out sharply one of the most
essential distinctions between electricity and magnetism. Magnetic
poles exist only in iron and steel, while electrical charges can be
communicated to any body whatsoever, provided they are insulated.
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ELECTRICITY AND MAGNETISM 13
Hese charges pass from point to point over conductors, and can be
transferred by contact from one body to any other, while magnetic
poles remain fixed in position, and are wholly uninfluenced by contact
with other bodies, unless these bodies themselves are magnets.
18. Electrostatic Induction. If a metal ball A, Fig. IS, is
stronf^y charged by rubbing it with a dialled rod, and then brought
near an insulated metal body ^ « a o
which is provided with pith balls or f'^^^f- ^| — • s'
strips of paper, a, b, c, as shown, \^^ ^ '
the divergence of a and c, will show ^^ j^ mecwowatio maoouon.
that the ends of B have received
electrical charges because of the presence of A, while the failure
of i to diveige will show that the middle of 5 is uncharged.
Further, the rod which charged A will be found to repel c but to
attract a. When A is removed alt evidences of electrification in B
will disappear.
From experiments of this sort we conclude that when a conductor
is brought near a charged body the end away from the chained body
becomes electrified with the same kind of electricity as that on the
charged body, while the end near the chaiged body receives a diarge
of opposite sign. This mdhod of prodvcing eledrificatian by the mere
wflvence which an electric charge has upon a conductor placed in ita
neighborhood, is called electrostatic indiiction. The fact that . as soon
asAh removed, a and c collapse, shows that this form of electrifica-
tion is only a temporary phenomenon.
19. The Tw&4^uid Theory of Electridty. We can describe
the facts of induction conveniently by assuming that in every con-
ductor there exists an equal number of positively and n^atively
chaiged corpuscles, which are very much smaller than atoms and
which are able to move about freely among the molecules of the con-
ductor. According to this view, when no electrified body is near the
conductor B, it appears to have no charge at all, because all of the
litde positive diarges within it counteract the effects upon out^de
bodies of all the little negative charges. But as soon as an electrical
charge is brought near B, it drives bs far away as possible the little
corpuscles which carry charges of sign like its own, while it attracts
the corpuscles of unlike sign. S, therefore, becomes electrified like il
at its remote end, and unlike A at its near end. As aoon as the
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14 ELECTRICITY AND MAGNETISM
inducing charge s removed, B immediately becomes neutral again
because the little positive and negative corpuscles come togetbei
under the influence of their mutual attraction. This picture of the
mechanism of electrification by induction is a modern modification
of the so-called tv>o~fluid theory of electricity, which .conceived of all
conductors as containing equal amounts of two wei^tless electrical
fluids, called positive electricity and negative electricity. Although
it is extremely doubtful whether this theory represents the actual con-
ditions within a conductor, yet we are able to say with perfect positive-
ness that the electrical behavior of a conductor is exactly what it would
be if it did eorUain apial amounts of positive and negative electrical
fluids, or equal numbers of minute positive and negative corpuscles
which are free to move about among the molecules of the conductor
under the infiuence of outside electrical forces. Furthermore, since
the real nature of electricity is as yet unknown, it has gradually
become a universally recognized convention to speak of the positive
electricity within a conductor as being repelled to the remote end, and
the negative electricity as being attracted to the near end by an outside
positive charge, and vice versa. This does not imply the acceptance
of the two-fluid theory. It is merely a way of describing the fact
that the remote end does acquire a diarge like that of the inducing
body, and the near end a cbai^ unlike that of the inducing body.
30. The Electron Theory. A slightly different theory has
recently been put forward by [^ysicists of high standing both in
England and in Germany. According to this theory a certain amount
of positive electricity is supposed to constitute the nucleus of the
atom of every substEnce. About this positive charge are grouped
a number of very minute negatively charged corpuscles or electrons,
the mass of each of which is approximately n iW of that of the
hydrogen atom. The sum of the negative charges of these electrons
is supposed to be just equal to the positive charge of the atom, so
that in its normal condition, the whole atom is neutral and uncharged.
But in the jostlings of the molecules of the conductor, electrons are
continually getting loose from the atoms, moving about freely among
the molecules, and re-entering other atoms which have lost their
electrons. Therefore, at a given instant, there are always in every
conductor a lai^ number of free negative electrons and an exactly
equal number of atoms which have lost electrons and which are
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ELECTRICITY AND MAGNETISM 15
therefore positively charged. Suc^ a conductor would, as a whole,
show no charge of either positive or negative electricity. But if a
body charged, for example, n^atively, were brought near such a
body, the negatively (Jiaxged electrons would stream away to the
remote end, leaving behind them the positively chai^ged atoms which
are not supposed to be fiee to move from their positions. On the
other hand, if a positively charged body is brought near the conductor,
the n^ative electrons are attracted and tiie remote end is left with the
immovable positive atoms.
The only advantage of this theoiy over that suggested in the
preceding section, in which the existence of both po^tive and negative
corpuscles is assumed, is that there is much direct experimental
evidence for the existence
of free negatively (jiaif;ed
corpuscles of about j-jVy
the mass of the hydrogen
atom, but no direct evi-
dence as yet for the exist-
ence of positively charged
electrons.
21. TheQoId-Leaf Elec-
troscc^K. One of the most
sensitive and convenient
instruments for detecting the presence of an electrical charge upon
a body and for determining the sign of that charge, is the gold4eaf
electToscope (Fig. 19). It consists of a ^ass jar, throu^ the neck
of which passes a metal rod supported by a rubber stopper or
some other insulated material, and carrying at its lower end two
gold leaves or strips of aluminum foil. To detect with this instru-
ment the presence of an electrical charge, it is only necessary to bring
near the upper end of the electroscope the body which is to be tested.
If it is charged, it will repel electricity of the kind which it possesses
to the leaves and draw the unlike kind to the upper end. The
leaves under the influence of the like chai^;es which they possess
will stand apart or diveige. If the body is not charged the gold
leaves will not be affected at all.
To determine the sign of an unknown diarge with an eleci-o-
scope, we first impart a chai^ of known sign to the electroscope by
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16 ELECTRICITY AND MAGNETISM
touching it with a piece of seating wax, for example, which has been
nibbed with cat's fur. This charges the leaves negatively and causes
them to divei^. The unknown charge is then slowly brought near
the upper end of the electroscope; and if the divergence of the leaves
is increased, the sign of the unknown charge is negative, for the in-
crea^ divergence means that more negative electridty haa been
repelled to the leaves. If the divergence is decreased instead of
increased, the sign of the unknown charge is positive, for, the de-
creased divergence of the leaves means that a part of the negative
electricity already on the leaves has been drawn to the upper end.
32. Charging by Induction. If a positively charged body C
(Kg. 20) is brought near two conductors A and B in contact, we have
seen that a positive charge will
appear upon A and a negative
A / /T-^ C charge upon B. li C were removed
these chaises would recombine and
A and B both become neutral.
But if, before C has been removed,
A and B are separated, and if then
C is removed, there is no opportuni^ for this recombination. Hence
A is left permanently charged positively, and B negatively. These
charges can be easily detected by bringing A and B into the neigh-
borhood of a charged electroscope. One will cause the divergence of
the leaves to increase, the other will cause it to decrease.
Again, if a positively chaiged body C (Fig. 21) is brought near a
conductor B, and if, while C is still in position, the finger is touched
anywhere to the conductor B and then removed, then, when C in
removed, B is found to be negatively chaiged. In this case the body
of the experimenter corresponds to the conductor A of the preceding
experiment, and removing the finger from B corresponds therefore to
separating the two conductors A and B. In the use of this method
of charging a single body by induction, it makes no dilTerence with
the sign of the charge left upon B where the finger touches the body B,
whether at a or at i> or at any other point, for it is always the kind
of electricity which is hke that on the charging body C that is re-
pelled off to earth throu^ the finger; while the chaige which is unlike
that upon C is drawn to the part of B which is next to C, and as soon
as C is removed this spreads over the whole body B. Whenever, Uien,
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"I
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I
D,Bi:ized3,GOOgIe
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ELECTRICITY AND MAGNETISM 17
a, SLigle body is chained by iaduction in this manner, the sign of the
charge left upon it is aiways opposiie to that of the inducing charge.
Thus, if we wished to chai^ an electroscope negatively by induc-
tion from a positively charged glass rod, we should first bring the
rod near the knob of the electroscope, thus causing the leaves to
diverge because of the positive electricity which is repelled to them.
Then while the rod
was still in position
(+ ° ^
near the electro- V"
scope, we should
touch the knob of
the latter with the "*"■ Sln«to Body Charged by inflection,
finger. The leaves would at once collapse. This is because the
positive electricity on the electroscope passes off to earth through
the finger, while the negative is held attracted to the knob of the
electroscope by the positive chai^ on the rod. In this condition
the negative is sometimes said to be bound by the attraction of th^
positive charge on C. We should then remove the finger and finally
the rod. The negative would then be free to pass to the leaver
and cause them to diverge. The electroscope would thus be
charged negatively. This is often one of the most convenient
methods of chaiging an electroscope. It should always be used
where it is desired to obtain a charge of opposite sign to that of the
enlarging body. If it is desired to obtain a charge on the electro-
scope of like sign to that of the cbai^ng body we simply touch the
body directly to the knob of the electroscope, and thus charge it by
conduction rather than by induction.
One advantage of chai^ng by induction lies in the fact that
the dialing body loses none of its own charge, whereas, in charging
by conduction the charging body must of course part with a portion
of its charge.
23. Po^tive and Negative Electricities Always Appear Simul-
taneously and in Equal Amounts. If a strip of flannel is stuck fast
to one side of a rod of sealing wax and rubbed back and forth over a
a second rod of sealing wax, and if then the two bodies are
brought near the knob of a charged electroscope before they are
separated, it b found that they give no evidence at alt of electri-
ficatioD. But if they are separated and brought in successioo
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18 ELECTRICITY AND MAGNETISM
to the knob of the electroscope, they will exhibit positive and-
negative charges of equal strength, the flannel being positive and
the bare sealing wax negative. Similarly, when a glass rod is charged
positively by nibbing it with silk, the silk when tested is always found
to possess a negative charge. These experiments show that in pro-
ducing electrification by friction, po^tive and negative chaises appear
simultaneously and in equal amount. This confirms the view,
alrea>jy brought forward in connection with induction, that the process
of electrification always consists in a
separation of positive and n^ative
charges which already exist in
equal amounts within the bodies in
which the electrification is develop-
ed. Certain it is that it is never
'II"' ' yT* possible to produce in any way what-
Tig-n. Showing ttat Ml Electric chMFo ^^^^ °°® "^^ <>' electridty without
Tj« cu outSTfeurcu* Of canducu«. p^ducing at the same time an equal
amount of the opposite kind.
24. An Electrical Charge Reddes upon the Outside Surface of a
Conductor. If a deep metal cup is placed upon an insulating stand
and charged as strongly as possible, either from a charged rod or from
az tiectrical machine (see Fig. 22), and a metal ball suspended by a
silk thread is touched to the inside of the cup, the ball is found upon
removal to show no evidence of charge when brought near the knob
of the electroscope. If, on the other hand, the ball is touched to the
outside of the cup, it exhibits a strong charge. Or, again, if the metal
ball is first charged and then touched to the inside of the cup, it loses
completely its charge, even thou^ the cup itself may be very strongly
charged. These experiments show that an electnc chai^ resides
entirely on the outside surface of a conductor. This is a result which
might have been inferred from the fact that all the little electrical
charges of which the total charge is made up repel each other and
therefore move through the conductor untii they are on the average
as far apart as possible, that is, until they are all upon the surface.
25. Density of Charge Qreatest where Curvature of Surface is
QreatesL Since alt of the parts of an electrica 1 charge tend, because
of their mutual repulsions, to get as far apart as possible, we mi^t
infer that if a chai^ of either sign is placed upon an oblong cond uctor.
DignzedoyGOOgle
ELECTRICITY AND MAGNETISM 19
like that of Fig. 23 (1), it would distribute it^lf so that the electrifica^
tion at the ends will be stronger than that in the middle. The cor-
rectness of this inference is easy to verify experimentally, for it is only
necessary to attach a penny to the end of a piece of sealing wax and
touch it tirst to the middle of a long chaiged conductor, and then
bring it over the knob of the electroscope, then to repeat the operatioQ
when the penny is touched to the end of the
conductor. The electroscope will be affected ^^^ ~^\
much more strongly in the latter case than *>S- ^ _ _ ^^ _ ... -^J
in the former. If we should test in this
way the distribution on a pear-shaped body,
Fig, 23 (2), we should find the density of
electrification considerably greater on the
small end than on the large end. By density p,^,|, y^
of electrification is meiuit the quantity of ^"'^^^^.-.v.
electric!^ on unit-area of the surface.
26. The Discharging Effect of Points. It mi^t be inferred
from the above that if one end of a pear-shaped body is made more
and more pointed, then, when a charge is imparted to the body, the
electric density on the small end will become greater and greater as
the ciurvature of this end is made sharper and sharper. That this is
the case is indicated by the efifecl
which experiment shows that points
have upon electrical charges; for if
a very sharp needle is attached to
any insulated conductor which is
provided with paper or pith-ball
indicators (as shown in Fig. 18), it
is found impossible to impart to
the body a permanent charge; that
Tig. M. Inflnencfl of Pointed ConilactOT 13 jf one attempts tO chaTCe it
upon Electric CluwgB. ' ^ °
by rubbing over it a charged glass
rod or other chaiged body, the indicators will be found to collapse
as soon as the rod is removed. That this is due to an effect of the
point can be proved either by removing the needle, or by covering up
the point with wax, ■(rfien the charge will be retained, as m the case of
any insulated body. The probable explanation of the phenomenon
is as followa^
>^
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20 ELECTRICITY AND MAGNETISM
The density of the charge becomes ho intense upon the point thkt the
molecules of air immediately adjoining the point are broken apart into poai-
trve and negative parts, and portions which are of unlike sign to the charge
on the point are attracted to it, thus neutralizing the charge upon the body,
while portions of like sign are rapelled away.
The effect of points upon an electrical charge may be shown
veiy strikingly by holding a very sharp needle in the hand and bring-
ing it toward the knob of a charged electroscope. The leaves will
fall together rapidly. Or, if the needle is brought near a tassel of
tissue paper which is attached to an electrified conductor (see Fig.
24), the electrified streamers, which stand out in all directions because
of their mutual repulsions, will at once fall together. In both of these
cases the needle becomes electrified by induction and discharges to the
knob of the electroscope, or to the tassel, electricity of opposite sign
to that which it contains, thus neutralizing its charge.
_, An interesting variation of the last ex-
periment is to mount an electric whirl (see
Fig.. 25} upon one knob of an electrical
machine. As soon as the machine is started^
the whirl will rotate rapidly in the direction
of the arrow. The explanation is as follows:
On account of the great magnitude of the
electric force near the points, the molecules
"^K^biJtEi^tS^""' of tl»e S^ J"s* >" f™"t °f t*^«™ a™ broken
"^'^* into positive and negative parts. The part
of sign unlike that of the charge on the points is drawn to them, while
the other part is repelled. But since this repulsion is mutual, the
point is pushed back with the same force with which the particles
are pushed forward; hence the rotation. The repelled particles
in their turn drag the air with them in their forward motion, and
thus produce the electric imnd, which may be detected easily by the
hand or by a candle held b front of the point.
27. The Ughtning Rod. The discharging effect of a sharp point
is utilized in the lightning rod, invented by Franklin in 1752. The
way in which the rod discharges the cloud and protects the building
is as follows : As an electrically charged cloud approaches a building
provided with a lightning rod, it induces an opposite cbai^ in the
earth and in the rod which is connected to the earth. As soon as the
charge on the point becomes strong enough to break apart the mole-
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ELECTRICITY AND MAGNETISM
cules of the air in &ont of it, a stream of electrified particles, of sign
opposite to that of the chai^ od the cloud, passes from the nei^boiiiood
of the rod to the cloud and thus neutralizes the charge of the cloud. We
are accustomed to say merely that the point dischaifjes the cloud.
28. Electrical Potential. There is a very instructive analogy
between the use of the word potential in electricity and .pressure in
hydrostatics. For example, if water will flow from tank A (Fig. 26)
to tank B throu^ the connecting pipe R, we infer that the hydro-
static pressure at a must be greater than that at b, and we attribute the
flow directly to this difference in pressure. In precisely the same way,
if, when two bodies A and B (Fig. 27) are connected by a conducting
wire r, a charge of positive electricity is found to pass from ^ to B, or
of negative from fi to ^, we are accustomed to say that the electrical
potential is higher at A than at B, and we assign this difference of
poteniicU as the cause of the flow. Thus, just as water tends to flow
from points of higher hydrostatic
pressure to points of lower
hydrostatic pressure, so electricity
is conceived of as tending to
flow from points of higher elec-
trical pressure or potential to
points of lower electrical pres-
sure or potential.
Again, if water is not continu-
ously supplied to one of the tanks
of Fig. 26, we know that the pres-
sures at a and b must soon be-
come the same. Similarly, if no
electricity is supplied to the bodies A and B of Fig, 27, their poten-
tials very quickly become the same. In other words, all points
on a system of connected conductors in which the electricity is in a
staiionaTy, or static, condition are necessarily at the same potential;
for if this were not the case, then the electricity which we imagine
all conductors to contain would move through the conductor until
the potentials of all points were equalized. In other words,
equality in the poteatiab of alt points on a conductor in the static
condition follows at once from the fact of mobility of electrical
charges through or over a conductor.
0-
-<D
Pig.K.
and tT. Illuatratlni; Analotrv Ix
waea Electric Potenlial and
Hrdmstatlc Presaure.
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22 ELECTRICITY AND MAGNETISM
But if water is continually poured into A (Fig. 26) and removed
from B, the pressure at a will remain permanently above the pressure
at b, and a continuous flow of water will take place through R. Simi-
larly if A (Fig. 27) b connected with an electrical machine and B to
earth, a permanent potential difference will exist between A and B
and a continuous current of electricity will flow through r. Difference
in potential is commonly denoted simply by the letters P. D. (potential
difference).
When we speak simply of the potential of a body we mean the
difference of poterUial which exbts between the body and the earth,
for the electrical condition of the earth is always taken as the zero to
which the electrical conditions of all other bodies are referred. Thus
a body which is positively charged is regarded as one which has a
potential higher than that of the earth, while a body which is nega-
tively charged is looked upon as one which has a potential lower than
that of the earth. Fig. 28 represents the hydrostatic analogy of
positively and negatively
charged bodies. Since it
has been decided to r^ard
the flow of electricity as
taking place from a point
of higher to that of lower
potential, it will be seen
that when a discharge takes
pig.18. Hydroaiatic Analogy of Positively and p'ace between E negatively
A^gati^iy Charges Bodi«. charged body and the
earth we must regard the positive electricity as passing from the
earth to the body, rather than the negative as passing from th; body
to the earth. This is, indeed, a mere convention, but it is one which
it is very important to remember in connection with the study of
current electricity. From the point of view of the electron theory
(§ 20), it would be natural to invert this convention exactly, since
this theory regards the n^ative electricity aloii^ as moving through
conductors. But since the opposite convention has become estab-
lished, it will not be wise to attempt to change it until the electron
theory has become more thoroughly established than is at present
the case.
29. S(Hne M^ods of Aleasuring Potentials. One of the
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ELECTEICITY AND MAGNETISM
amplest methods of comparing the potential difference which exists
between any two charged bodies and the earth, is to connect the
diarged bodies successively to the knob of an electroscope, the con-
ducting case of which is in electrical connection with the earth. The
amount of separation of the gold leaves is then a measure of the P. D.
between the earth and the charged body.
Another very conveni^it way of measuring approximately a
very large P. D. is to measure the length of the spark which will pass
between the two bodies whose P. D. is sought. This P. D. is approxi-
mately proportional to the spark length, provided the dimensions of
the bodies are large in comparison with the distance between them,
each centimeter of spark length representing a P. D. of about 30,000
volts.
30. Condensers. If a metal plate A is mounted on an insulating
plate and connected with an electroscope. a« 'n Fig. 2d and *i a
Mmam
Flg.tt. lUaBtratiris the Principle Of the C<
second plate B \s similarly mounted and connected to earth, then,
when a charge is placed on A, it will be found that the gold leaves fall
together as B approaches A and diverge farther as B recedes from A.
This shows that the potential of .4 is diminished by bringing B close
to it, in spite of the fact that the quantity of electricity on A has
remained unchanged. If we convey additional plus charges to A, we
find that many times the original amount of electricity may be placed
on A when B is close to it, before the leaves return to their original
divergence, that is, before the body regwns its original potential.
We say, therefore, that the capaeUy of A for holding electricity
haa been very greatly increased by bringing near it another conductor
which is connected to earth. It is evident from this statement that
uje measure the capacity of a body hy the anwmsd of electriciitf which
must be put upon it m order to raise Us potentiai to a given poirU. The
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24 ELECTRICITY AND MAGNETISM
explanation of the increase of capacity in this case is obrious. As
soon OS B was brought near to ^, it became charged, by induction,
with electricity of sign opposite to that of A, the electricity of sign
like that of A being driven off to earth through the connecting wire.
The attraction between these opposite charges on A and B drew the
electricity on A to the face nearest to B, and removed it from the more
remote parts of ^, so that it became possible to put a very much
laiger charge on A before the tendency of the electricity on ^ to pass
over to the electroscope became as great as at first, that is, before the
potential of A rose to its original value. Under circumstances of
this sort the electricity on ^ is said to be bound by the opposite elec-
tricity on B.
An arrangement of this sort consisting of two conductors sepa-
rated by a nonconductor is called a oondenser. If the conducting
points are very close together and one of them is joined to earth, the
capacity of the system may be thousands of times as great as that of
one of the plates alone.
31. The Leyden Jar. The most common form of condenser is
a glass jar coated part way to the top inside and outside with tinfoil
(Fig. 30). The inside coating is connected by a chain to the knob,
fy.,^^-^ while the outside coating is con-
j ""V nected to earth. Condensers of
I \ this sort first came into use ic
<^t^JS \ — . Ley den, Holland, In 1745.
J T\_^ Y _y Hence they are now called Leyden
BTfll liiliitfM- ^.^ Such a jar is charged by hold-
I iii^r'^ '"^ ^^ knob in contact with one
i i^l terminal of an electrical machine
I'' I Iffll ^""^ connecting the outer coat to
^-L.,,!''*'' earth either by a wire or simply
PiB.«a i^denjar by holding it with the hand. As
fast as electricity passes to the knob, it spreads to the inner coat of
the jar where it attracts electricity of the opposite kind from the earth
to the outer coat, repelling electricity of the same kind. If the inner
and outer coatings are now connected by a dischar^ng rod (as in
Fig. 30), a very powerful spark will be produced. If a charged jar
is [Jaced on a glass plate so as to insulate the outer coat, the knob
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ELECTRicrry and magnetism 25
may be toudied with the finger and no appreciable dischai^ noticed.
Smilarly the outer coat may be touched with the finger with the same
result. But if the inner and outer coats are connected with the'dis-
cbai^r, a powerful spark parses.
The experiment shows that it is impossible to discharge one side
of the jar alone, for practically all of the charge is bound by the opposite
charge on the other coat. Tlierefore, the full discharge can occur
only when the inner and outer coats are connected.
32. Electrical Screens. We have seen that if a poutively
charged body A (Fig. 31) is brought near an uncharged conductor
B, negative electricity is attracted to the near end of the conductor and
positive electridty appears on the
remote end, this separation of the /'a\ ..
positive and negative charges being \^ y ■ ^ ^^ 1)
dependent upon the presence of the ^ ,, ni^^xm PHnoipi. o( ««
charged conductor A. Let us see Kiectncai screen.
how this known fact as to the condition of the charges may be used
to determine the electrical condition at any point p within the conduc-
tor. Since the electricity within the conductor ia free to move under
the influence of any electrical force which is acting upon it, it is clear
that the accumulation of negative electricity at one end and of positive
at the other will cease only when all electrical forces inside the con-
ductor are reduced to zero, that is,when the charges on A and B, acting
jointly, neutralize one another completely at any point p within the
conductor. It appears, therefore, that the distribution of the intjuced
charge on the surface of a conductor in the electrical field of a charged
body must always be such that there is no force whatever inside the
body. This theoretical conclusion was first experimentally verified
by Faraday, who coated a large box with tinfoil and went inside the
box with delicate electroscopes. He found that these electroscopes
showed no effects whatever, even when powerfully charged bodies
were brought near the outside of the box. The experiment is often
repeated in a small way by placing an electroscope under a wire cage
of rather small mesh. A chaiged rod brought near the cage will pro-
duce no effect whatever upon the electroscope. We thus learn that
electrical influences can be completely cut off from a body by sur-
rounding it on all sides with a conductor.
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26
ELECTRICITY AND MAGNETISM
ELECTRICAL GENERATORS.
33. The Electrophorus. The electrophorus is a simple elec-
trical generator which illustrates well the principle uoderlying the
action of all electrostatic machines. All such machines generate
electricity primarily by induction, not by friction. B (Fig, 32) is a
hani rubber plate which is first charged by rubbing it with fur or
flannel. ^ is a metal plate provided with an insulating handle.
When the plate A is placed upon B, touched with the finger, and then
removed, it is found possible to draw a spark from it, which in dry
weather may be a quarter of an
inch or more in length. The proc-
ess may be repeated an indefi-
nite number of times without
producing any diminution in the
aze of the spark which may be
drawn from A.
If the sign of the charge on A
is tested by means of an electro-
scope, it will be found to be posi-
tive. This proves that A has been
charged by induction, not by contact with B, for it is to be remembered
Fig. tt. TbeBlectrophoms.
Fig.S& ToeplBi^HoltBStatlaHBclilll&
that the latter is charged negatively. The reason for this is that even
wben.rl rests upon B it is in reaUty separated from it, at all but a very
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ELECTRICITY AND MAGNETISM
few points, b; an insulating layer of air; and.sincefi is a noo-conductor,
its charge cannot pass off appreciably through these few points of
contact. It amply repels negative electricity to the top side of the
metal plate A, and draws positive to the lower side of this plate. The
negative passes off to earth .when the plate is touched with the finger.
Hence, when the finger is removed and A lifted, is possesses a strong
positive charge.
34. The Toepler-Holtz Static Machine. The ordinary static
machine is nothing but a continuously acting electropborus. Fig. 33
represents one type of such machine. Upon the back of the stationary
glass plate E are pasted paper sectors, beneath which are strips of
tinfoil A B and C D, called indiuiors. In front of £7 is a revolving
glass plate carrying disks I, m, n, o, p, and q, called camera. To the
inductors A B and 2) are fastened metal arms t and u, which bring
these inductors into electrical contact with disks I, m, n, o, p, and q,
when these disks pass beneath the tinsel brushes carried by t and v.
A stationaiy metallic rod r 8 carries at its ends stationaiy brushes as
well as sharp-pomted metallic combs. The two knobs R and S
have their capacity increased by the Iicyden jars L and L.'
The action of the machine is best understood from the diagram
(Fig. 34). Suppose that a small + chai;ge is originally placed on the
inductor C D. Induction takes [dace in the metallic system consisting
of the disks I and
orStiscloUachlo«.
where a part of it passes over to the inductor A B, thus charg-
ing it negatively. When I reaches the position n, the remaindef
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28.
ELBCTEICITY AND MAGNETISM
of its charge, being repelled hj the negative which is now on A B,
passes over into the Leyden jar L. When I reaches the position
o, it again becomes chaif;ed by induction, this time positively,
uid more strongly than at first, since now the negative on A B, aa
well as the postive on C P, is acting inductively upon the rod r t.
When I reaches the position u, a part of its now strong positive charge
passes toC D, thus iacrea:ung the positive charge upon this inductor.
In the position v, the ranainder of the positive charge on / passes over
to L'. This completes the cycle for I, Thus, as the rotation con-
tinues, A B and C D acquire stronger and stronger charges, the induc-
tive action upon r s becomes more and more intense, and positive
and negative charges are continuously imparted to L and L' until a
discharge takes place between the knobs R and S.
Iliere is usually sufficient charge on one of the inductors to
start the machine, but in damp weather it is often found necessary
to apply a charge to one of them by means of a {ttece of sealing wax
or a glass rod before the machine will work.
ELECTRICITV IN MOTION— ELECTRICAL CURRENTS
35. The Magnetic Effect Due to a Charge in Moti<m. An
electrical chai^ at rest produces no magnetic effect whatever. This
can be proved by bringing a charged body near a compass needle or
suspended magnet. It will attract both ends equally well by virtue
of the principle of electro-
static induction. If the
effect were magnetic, one
end should be repelled and
the other attracted. Again,
if a sheet of zinc, aluminum,
or copper is inserted between
the deflected needle and the
charge, all effect which was
produced upon the needle
by the chaige will be cut off,
for the metallic sheet will act as an electric screen (cf. § 32). But if
such a metal screen is inserted between a compass needle and
magnet, its insertion has no effect at all on the magnetic forces
(cf.§5).
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ELECTRICITY AND MAGNETISM 29
If, however, a charged Leyden jar is discharged through a coit
which surrounds an unmagnetized knitting needle in the manner
shown in Fig. 35, the needle will be found after the discharge to have
become distinctly magnetized. If the sign of the charge on the jar b
reversed, the poles will, in general, be reversed also.
This experiment demonstrates the existence of some connection
between electricity end magnetism. Just what this connection is, is
not yet known with certainty ; but it is known that magnetic electa
are always observable near the path of a vwving
electrical charge, while no such effects can ever
be observed near a charge st rest.
An electrical charge in motion is coiled an
electricai current, and the presence of such current
in a conductor is most conmionly detected by the
magnetic effect which it produces.
36. The Oalvanlc Cell. When a Leyden
jar is dischai^ed, but a very small quantiQr of
electricity passes through the connecting wires,
since the current lasts but a small fraction of a
second. If we could keep the current flowing
continuously through the wire, we should expect
the magnetic effect to be more pronounced. This nught be done by
discharging Leyden jars in rapid succession through the wire. In
1786, however, Galvani, an Italian anatomist at the University of
Bologna, accidentally discovered that there is a chemical method for
producing sudi a continuous current. His discovery was DOt under-
stood, however.
Fig. M.
Slmpleal Form ot
Qalyanla Call.
times as the galvanic cell.
Such a cell consists in its simplest form of a strip of copper and a
strip of zinc immersed in dilute sulphuric add (Fig. 36) , If the wires
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30 ELECTRICITY AND MAGNETISM
leading from the copper and the zinc are connected for a few seconds
to the end of the coil of Fig. 35, when an unmagnetized needle lies
within this coil, the needle will be found to be much more strongly
magnetized than it was when the Leyden jar was dischai^ed through
the coil. Or, if the wire connecting the copper and zinc is simply held
above the needle in the manner shown in Fig. 37, the Utter will be
found to be strongly deflected. It is evident from these experiments
that the wire which connects the terminals of a galvanic cell carries a
current of electricity. Historically the second of these experiments,
performed by the Danish physicist Oersted in 1819, preceded the dis-
covery of the magnetizing effect of currents upon needles. It created
a great deal of exdtement at ^he time because it was the first clew
v/bidi had been found to a relationship between electricity and
magnedsm.
It mi^t be inferred from the above experiments that the two
plates of a galvanic cell when not connected by a wite cany static
positive and negative chaiges just as do the two coats of a Leyden
jar before it is dischaiged through the wire. This inference can be
easily verified with aa electroscope.
Thus, if a metal plate A (Fig. 38} covered with shellac on its
lower side and provided with an insulating handle, is placed upon a
similar plate B which is in contact
with the knob on an electroscope;
and if the copper plate, for ex-
ample, of a galvanic cell is con-
nected to A and the zinc to B; then,
when the connecting wires are re-
moved and the plate A lifted away
from B, the leaves of the electro-
scope will diverge and when tested
FiB.SB. sbowinK Existence of scaiio wiU be found to be negatively
^rge.onPU«.oto»iv.n.oceu. ^^^_ If the deflection observed
in the leaves of the electroscope is too small for purposes of demon-
stration, the conditions can be bettered by using a battery of from
five to ten cells instead of the single cell. If, however, the plates A
and B are sufficiently large — say, three or four inches in diameter —
and if their surfaces are veiy flat, a single cell will be found to be
gufficient. If. on the other hand, the copper plate is connected to B
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ELECTRICITY AND MAGNETISM 31
and the zinc to A in the above experiment, the electroscope vill be
found to be positively chafed. Thia shows clearly that the copper
plate possesses a positive electrical charge, while the sdoc plate pos-
sesses a n^ative chai^, these charges originating in the chemical,
action within the galvanic cell.
In this experiment the two metal plates separated by Celiac
constitute an electrical condenser wbidi is charged positively on
one side and negatively on the other by connecting it with the two
plates of the galvanic cell, in precisely the same way in which a
Leyden jar is charged by connecting its two coats one to one terminal
and the other to the other terminal of a static machine. The potential
of plate B is increased bymoving A awayfiom it, just as in the arrange-
ment shown in Fig. 29 the potential of A was increased by moving
B away from it. This device makes it possible to detect very small
potential differences.
37. Coinparison of a Qalvanlc Cell and Static Machins. If one
of the terminab of a galvanic cell is touched directly to the knob of
the gold-leaf electroscope without the use of the condenser plates
A and B of Fig. 38, no diveigence of the leaves can be detected; but
if one knob of a static machine in operation were so touched, the
leaves would be thrown apart very violently. Since we have seen
that the divergence of the leaves is a measure of the potential of the
body to which they are connected, we learn from this experiment that
the chemical actions going on in a galvanic cell are able to produce
between its terminals but veiy small potential differences in com-
parison with that produced by the static machine between its
terminals. As a matter of fact, the potential difference between
the ^enninals of the cell is but one volt (cf. § 40), while that
between the terminals of an electrical machine may be several
Hundred thousand volts.
On the other hand, if the knobs of the static machine are con-
nected to the ends of the wire shown in Fig. 37, and the machine
operated, the current will not be large enough to produce any
appreciable effect upon the needle. Smce, under these same
circumstances the galvanic cell produced a very large effect
upon the needle, we learn that although the cell develops a
much smaller P. D. than does the static machine, it neverthe-
less sends thioufcb the wire a veiy much larger araount- of etectridly
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32 - ELECTRICITY AND MAGNETISM
per second. Thia means merely that the chemical actions which
are going on within the cell are able to recharge the plates when
they become discharged through the electric wire, far more rapidly
than is the static machine able to recharge its terminals after they
have once been discharged.
38. Shape of the Magnet Field about a CurrenL If we place
the wire which coimects the plates of a galTanic cell in a vertical
position (see Fig. 39), and explore with a compass needle the shape
of the magnetic field about the current, we find that the magnetic
lines are concentric drcles lying in a plane perpendicular to the wire
and having the wire as their conmion center. If we reverse the
direction of the current, we find that the direction in which the com-
pass needle points reverses also. If the current is very strong (say
40 amperes), this shape of the field can be shown by scattering iron
filings on a plate through
which the current passes,
in the manner shown in
Pig. 39. The relation be-
tween the direction in which
the current Sows and the
direction in which the posd-
tive end of the needle points
(this is, by definition, the
direction of the magnetic
field) b given in the follow-
ing convenient rule: If the
03 in Fig. 40, to that the
thumb points in the direction in which the positive electricity is moving,
that is, in the direction from the copper toward the zinc, then the mag-
netic lines encircle the -wire in the same direction as do the fingers of
the hand. Another way of stating this rule is as follows: The rela-
tion between the direction of the current in a wire and the direction of
the magnetic lines about it, is the same as the relation between the direc-
tion of the forward •motion of a right-handed screw and the direction
of rotation when it is being driven in. In this form the rule is known
as the right-hand screw rule.
39. The Measureineiit of Electrical Currents. Electrical cu>
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ELECTRICrrY AND MAGNETISM
rents are, in general, measured by the strength of the i
effect which they are able to produce under specific conditions.
Thus, if the wire carrying a current is vound into circular form as in
Fig. 41, the right-hand screw rule shows us that the shape of the mag-
netic field at the center of the coil is similar to that shown in the figure.
It, then, the coil is placed in
TT
8 Dorth-and-south plane and
a (»mpass needle is placed
at the center, the passage of
the current through the coil
tends to deflect the needle so
as to make it point east and
west The amount of deflection under these conditions is taken as
the measure of current strength. Ilie unit of current is called the
ampere and is in fact approximately the same as the current which,
flowing through a circular coil of three turns and 10 cm. radius, set
in a Dorth-and-aouth plane, will produce a deflection of 45 degrees
at Washington in a small compass needle placed in its center (as in
Fig. 41). Nearly all current-measuring instruments, commonly
called ammeters, consist essentially ather of a small magnet sus-
pended at the center
of a fixed coil as in
Fig. 41, or of a mov-
able coil suspended
between the poles of
a fixed magnet. The
passage of the cur-
rent through the coil
Fig. 41. ArranEeiDent of Circular Conductor and Oom- produces a deflection,
psss tor Ueasnrlns Current Streugtli. ■ ■ r> ■ i
m the first case,of the
magnetic needle with reference to the fixed coil, and m the second case,
of the coil with reference to the fixed magnet. If the instrument
has been suitably calibrated, the amount of the deflection gives at
once the strength of the current m junperes.
40. Electromotive Force and its Measurements. Hie potential
difference which a galvanic cell or any other generator of electriaty
is able to maintain between its terminab when these terminabare
Tint connected b; a wire, t. «., the total electrical pressure which the
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ELECTRICITY AND MAGNETISM
generator is capable of exerting, is commonly called its electromotive
force, usually abbreviated to E. M. F. Ths B. M. F. of an eleetrieal
generator may then be defined as ^ capacity for prodvcmg eledrical
preaeure, or P. D. Iliis P. D. might be measured, as in § § 29 and 36,
by the deflection produced in an electroscope, or other similaT inatru-
ment, vhea one terminal was connected to the case of the electro-
sMpe and the other terminal to the knob. Potential differences are
in fact measured in this way in all so-called electrostatic voltmeters,
which are now coming more and more into use.
The more common type of potentJal difference measurers,
so-called voUmeiers, comosts, however, of an instrument made like
an ammeter, save that the coU of
wire is made of an enormous num-
ber of turns of extremely fine wire,
so that it carries a very small cur-
rent. Hie amount of current which
it does carry, however, and there-
tore the amount of deflection of its
needle is taken as proportional to
the difference in electrical pressure
existing between its ends when these
are touched to the two points whose
P.D. is sought The principle
underlying this type of voltmeter
will be better understood bom a
consideration of the following water .
analogy. If the stop-cock K (Fig.
42) in the pipe connecting the
water tanks C and D is closed, and
if the water wheel ^ is set in motion by applying a wei^t W,
the wheel will turn until it creates such a difference in the water
levels between C and D that the back pressure against the left face of
the wheel stops it and brings the weight W to rest. In precisely the
same way, the chemical action within the galvanic cell whose termi-
nals are not joined (Fig. 43) develops positive and negative diaiges
upon these terminals, that is, creates a P.D. between them, until the
back electrical pressure throu^ the cell due to this P. D. is sufficJent
to put a stop to further chemical action.
Fig. 42. HfdroBUttlc Analogy of Poten-
tial Dlllerence.
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ELECTRICITY AND MAGNETISM
Now, if the water reservoirs (Fig. 42) are put in oommunicatioD
by opening the stop-cock K, the difference in level between C and D will
b^in to fall, and the wheel will begin to build it up again. But if
the canying capacity of the pipe a & is small in comparison with the
capaci^ of the wheel to remove water from D and to supply it to C,
then the difference of level which permanently exists between C and
D when K is open will not be appreciably smaller than when it is
closed. In this case the current which flows throu^ A B may
obviously be taken as a measure of the difference in pressure -winrii the
pump is able to maintain between C and D when K h closed.
In predsely the same way, if the terminals C and D of the cell
(Ilg. 43) are connected by attaching to them the tenoinab a and b td
any conductor, they at once b^;in to dis-
charge through this conductor, and their P.
D. therefore b^ins to fall. But it the chemi-
cal action in the cell is able to rechai^ C and
D very rapidly in comparison with the abihty
of the wire to discharge them, then the P.D,
between C and D will not be appreciably
lowered by the presence of the connecting
conductor. In this case the current which
flows thiou^ the conducting coil, and there-
fore the deflection of the needle at its center,
may be taken as a measure of the electrical
pressure developed by the cell, that is, of the
F. D. between its unconnected terminals.
lie common voltmeter is, then, exactly like an ammeter, save that
its coil offers so hi^ a resistance to the passage of electricity throu^
it that it does not assist appreciably in discharging, that b, in redudng
the F.D. between the points to which it is connected.
The unit of P. D. may be taken for practical purposes as the
electrical pressure produced by a ^mple galvanic cell consisting of
sane and copper immersed in dilute sulphuric add. It is named a
volt in honor of Volta.
41. The Electromotive Forces of Oalvanlc Cells. When a
Tohmeta of any sort is connected to the terminals of a galvanic cell,
it is found that the deflection produced is altogether independ^it of
the ahi^ or die of the plates or their distance apart But if the
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86 ELECTRICITY AND MAGNETISM
nature of the plates is changed, the deflection changes. Thus, while
copper and zinc in dilute sulphuric acid h&vs an E. M. F. of one volt,
carbon and zinc show an E. M. F. of at least 1.5 volts, while carbon and
copperwillshowanE.M.F. of very much less than a volt. Similarly,
by changing the nature of the liquid in which the plates are immersed,
we can produce changes in the deflection of the voltmeter. We leara
therefore that the E. M. F. of a galvanic cell depejtda simply upon the
materials of which the cell is composed and not at all upon the shape,
nxe, or distance apart of the plates.
42. Electrical Resistance. If the terminals of a galvanic cell
are connected first to, say, ten feet of No. 30 copper wire, and then
to ten feet of No. 30 German-silver wire, it is found that a compass
needle placed at a given distance from the copper wire will show a
much larger deflection than when placed the same distance from the
German-silver wire. A cell, therefore, which is capable of develop-
ing a certain flxed electrical pressure is able to force very mudi more
current through a given wire of copper than through an exactly similar
wire of German-silver. We say, therefore, that German-silver offers
a hi^er resistance to the passage of elec-
E^^^^223~ .© tricity than does copper. Similarly, every
particular substance has its own characteiia-
*^**'(toppSwSL''"'°'^ tic power of transmitting electrical cur-
rents. Silver is the best conductor of any
known substances. The resistances of different substances are
commonly referred to silver as a standard, and the ratio between the
resistance of a given wire of any substance and the resistance of an
exactly similar silver wire is called the specific resistance of that sub-
stance. The specific resistances of some of the conunoner metals
are given below:
Silver 1.00 Soft iron 7.40 Qamsn fdlver 20.4
Copper 1.13 Nickel 7.87 Hard sted 21.0
Aluminum... 2.00 Platinum 0.00 Mercury 62.7
llie unit of resistance is the resistance at 0^ of a column of mer-
cury 106.3 cm. long and I sq. mm. in cross-section. It is called an
ohm, in honor of the great German physicist, Geoi^ Ohm (1789-
1854). A length of 9.35 feet of No. 30 copper wire, or 6.2 inches of
'No. 30 German-silT^ wue, has a resistance of about one ohm.
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ELECTRICITY AND MAGNETISM 37
Copper wire of the size shown in Fig. 44 has a resistance of about 2.62
ohms per mile.
The resistances of all of the metals increase with rise in tempera-
ture. The resistances of liquid conductors on the other hand usually
decrease with- rise in temperature. Carbon and a few other solids
show a similar behavior: the filament in an incandescent lamp has
only about half the resistance when hot that it has when cold. The
resistances of wires of the same material are found to be directly
proportional to their lengths, and inversely proportional to their cross-
sections.
43. Ohm's Law. In 1827 Ohm announced the discovery that
the eurrenis fvmisked by different g(dvanic ceUa, or combinatums of
celU, are always directly proportional to the E. M. F.'t existing in ike
circuits in which the currents flow, and inversely proportional to the
total resistances of these circuits; i.e., if C represents the current in
amp>ere3, E the E. M. F. in volts, and R the resistance of the circuit in
ohms, then Ohm's law as applied to the complete circuit is:
„ E , - Electromotive force jji
R' ' '' Resistance \ )
As applied to any portion of an electrical circuit, Ohm's law is:
„ PD . „ . Potential diflference , >
C— — ; I.e., Current— = — t-: , |2l
where P.D. represents the difference of poteutial in volts between any
two points in the circuit, and r the resbtance in ohms of the conductor
connecting these two points. This is one of the most important laws
in [^ysics.
Both of the above statements of Ohm's law are included in the
equation :
. Volts ,,,
44. Internal Resistance of a Qalvanic Cell. If the zinc and
bopper plates of a simple galvanic cell are connected to an ammeter,
and the distance between the plates then increased, the deflection
of the needle is found to decrease, or if the amount of immersion is
decreased the current also will decrease. But since the E. M. F. of
a cell was shown in § 41 to be wholly independent of the area of the
plates inmiersed or of the distance between them, it will be seen from
Ohm's law that the change in the ciurent in these cases must be due
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ELECTRICITY AND MAGNETISM
to some chaoge in the total resiatance of the citcuit Since the wire
which constitutes the outside portion of the circuit has remuned the
same, we must conclude that the liquid vnthin the cell, as weU as the
external wire, offers resistance to the passage of the curreni. This
internal resistance of the liquid is directly proportional to the distance
between the plates, and inversely proportional to the area of the im-
mersed portion of the plates. If, then, we represent the external
resistance of the circuit of a galvanic cell by Rt aod the internal by
Rt, then Ohm's law as applied to the entire circuit takes the form:
Thus, if a ample cell has an internal resistance of 2 ohms and an
E. M. F. of 1 volt, the current which will fiow throu^ the circuit
when its terminals are connected by 9.3 ft. of No. 30 copper wire
(1 ohm) is ^ = .33 ampere. This is about the current which
is usually obtained from an ordinary Daniell cell (see § 49).
PRIMARY CELLS
45. The Action of a Simfie Cell. If the simple cell already
mentioned — ^namely, zinc and copper strips in dilute sulphuric acid —
is carefully observed, it will be seen that, ao
long as the plates are not connected by a con-
ductor, fine bubbles of gas are slowly formed
at the zinc plate, but none at the copper
cr~\ n ® plate. As soon, however, as the two strips
^ are put into electrical connection, bubbles
instantly appear in great numbers about the
copper plate and at the same time a current
, , .t-ri- manifests itself in the connecting vrire (Fig.
^ r. ^^'1 ;: \ 5 ^)- Th^ bubbles are of hydrogen. Their
' ~° ' oripnal appearance on the zinc plate may be
. . it- .T 1 1 prevented either by using a plate of chemi-
:.S£x-^E£;t!°E3i C cally pure zinc, or by amalgamating impure
■- — I zinc, tfiat is, by coating it over with a thin film
"*' *■ 5to ^°' " ^"^ '^^ mercury. But the bubbles ^on the copper
cannot be thus disposed of. They are an
invariable accompaniment of the current in the circuit. If the cur-
{
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ELECTRICITY AND MAGNETISM 39
rent is allowed to run for a considerable time, it will be found that
the zinc wastes away, even though it has been amalgamated, but the
copper plate does not undergo an; change.
An electrical current in a simple cell is, then, accompanied by
the eating up of the zinc plate by the liquid, and by the evolution of
hydrogen bubbles at the copper plate. In every type of galvanic cell,
actions similar to these two are always found. That is, one of the
plates is always eaten up, and on the other some element is deposited.
The plate which is eaten is always the one which is found to be
negatively charged, while the other is always found to be positively
charged; so that in all galvanic cells, when the terminals are connected
through a wire, the positive electricity flows through this wire from
the uneaten plate to the eaten plate. It will be remembered that the
direction in which the positive electricity flows is taken for conven*
ience as the direction of the current (see § § 19 and 28).
<M. Theory of the Action of a Simple Cell. A simple cell may
be made of any two dissimilar metals immersed in a solution of any
acid or salt. For simplicity, let
us examine the action of a cell
composed of plates of zinc and
copper immersed in a dilute
solution of hydrochloric acid.
The chemical formula for hydro-
chloric acid is HCl. This
means that each molecule of
the acid consists of one atom of
hydrogen combined with one
atom of chlorine. In accordance
with thf» thporV now in VOirue P1«**- lUustratlnK Dlssoclatlonof ions
Wim tue lueorj' uuw in vuguc and Thaory of Action of » Simple CalL
among physicists and chemists,
when hydrochloric acid is mixed with water so as to form a dilute
solution, the HCl molecules split up into two electrically chained
parts, called ions, the hydrogen ion canying a positive charge and
the chlorine ion an equal negative charge (Fig. 46). This phenomenon
is known as dissociation. The solution as a whole is neutral; t. e.,
it is unchained, because it contains just as many positive as negative
ions.
When a zinc plate is placed in such a solution, the acid attacks
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40 ELECTRICITY AND MAGNETISM
it and pulls nnc atoms into soludon. Now, whenever a metal dis-
solves in an acid, its atoms, for some unknown reason, go into solu-
tion bearing little positive chaiges. The correspofuimg negative
charges must be left on the zinc plate In precisely the same way in
which a negative charge is left on silk when positive electrification
is produced on a glass rod by nibbing it with the silk. It is in this
way, then, that we attempt to account for the. negative charge which
we find upon the zinc plate in the experiment described in § 36.
The passage of positively chatged zinc ions into solution gives
a positive charge to the solution about the zinc plate, so that the
hydrogen ions tend to be repelled toward the copper plate. When
these repelled hydrogen ions reach the copper plate some of them
give up their charges to it and then collect as bubbles of hydrogen
gas. It is in this way that we account for the positive charge which
we find on the copper plate in the experiment described in § 36.-
If the zinc and copper plates are not connected by an outside
conductor, this passage of positively charged zinc ions into solution
continues but a veiy short time, for the zinc soon becomes so strongly
chafed negatively that it pulls back on the plus zinc ions with as much
force as the acid is pulling them into solution. In precisely the same
*ay the copper plate soon ceases to take up any more positive elec-
tricity from the hydrogen ions, since it soon acquires a large enou^
plus charge to repel them from itself with a force equal to that with
which they are being driven out of solution by the positively charged
zinc ions. It is in this way that we account for the fact that on open
drcuit no chemical action goes on in the simple galvanic cell, the
zinc and copper plates simply becoming charged to a definite differ-
ence of potential which is called the E. M. F. of the cell
When, however, the copper and zinc plates are connected by a
wire, a current at once flows from the copper to the zinc, and the
plates thus begin to lose their charges. This allows the acid to pull
more zinc into solution at the zinc plate, and allows more hydrogen
to go out of solution at the copper plate. TTiese processes, therefore,
go on continuously so long as the plates are connected. Hence a
continuous current flows through the connecting wire until the anc
is all eaten up or the hydrogen ions have all been driven out of the
solution, i.e., until either the plate or the acid has become exhausted.
47. Polarization. If the simple cell which has been described
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ELECTRICITY AND MAGNETISM
ia connected to an ammeter and the deflection observed for a few
minutes, it is found to produce a current of continually decreasing
strength; but if the hydrogen is removed from the copper plate by
taking out the plate and drying it, the deflection returns to its first
value. This phenomenon is called polarization.
The presence of the hydrogen on the positive plate causes a
diminution in the strength of the current for twc reasons: First, ance
hydrogen is a non-conductor, by collecting on the plate it diminishes
the effective area of the plate and therefore increases the internal
resistance of the cell; second, by collecting upon the copper plate it
low"ers the E. M. F. of the cell, because it virtually substitutes a hydro-
gen plate for the copper plate, and we have already seen (in §41)
that a change in any of the materiab of which a cell is composed
changes its E. M. F.
The different forms of galvanic cells in common use differ
chieSy in different devices employed either for disposing of the hydro-
gen bubbles or for preventing their formation.
The moat common types of such cells are
described in the following sections.
48. The Bichromate Cell. The bichro-
mate cell (Fig. 47) consists of a plate of zinc
immersed in sulphuric acid between two plates
of carbon, carbon being used instead of cop-
per because it pves 'a greater E. M. F. In
the sulphuric acid is dissolved some bichro-
mate of potassium or sodium, the function of
which is to unite chemically with the hydro-
gen as fast as it is formed at the positive
plate, thus preventing its accumulation upon
this plate.* Such a cell has the high E. M. F.
of 2.1 volts. Its internal resistance is low,
from .2 to .5 ohm, since the plates are gener-
ally large and close together. It will be seen,
therefore, that when the external resistance is
veiy small it is capable of furnishing a current of from 5 to 10 amperes.
Since, however, the chromic acid formed by the union of the sulphuric
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42 ELECTRICITY AND MAGNETISM
add vith the bichromate attacks the zinc even when the circuit is open,
it is necessary to Hft the dnc from the liquid by the rod A, when the
cell is not in use. Such cells are useful where lai^ currents are
needed for a short time. The great disadvantages are that the fluid
deteriorates rapidly, and that the zinc cannot be left in the liquid.
49. The Danlell Cell. The Daniell cell consists of a zinc plate
immersed in zinc sulphate, and a copper plate immersed in copper
sulphate, the two liquids being kept apart either by means of a porous
earthen cup, as in
ft the type shown in ,
O Fig. 48, or else by
gravity, as in the
type shown in Fig.
49. This last type,
commonly called
• the gravity, or
crowfoot type, is
used almost ex-
clusively on tele-
graph lines. The
copper sulphate,
being the heavier
remains at the bot-
tom about the copper plate, while the zinc sulphate remans at the
top about the zinc plate.
In this cell polarization is almost entirely avoided, for the reason
that no opportunity is given for the formation of hydrogen bubbles.
For, just as the hydrochloric acid solution described in § 46 consists
cf pontive hydrogm ions and n^ative chlorine ions in water, so the
zinc sulphate (ZnSO^) solution consists of positive zinc ions and
negative SO, ions. Now the zinc of the zinc plate goes into solution
in the zinc sulphate in precisely the same way that it goes into solution
in the hydrochloric add of the simple cell described in § 46. TTiis
^ves a positive charge to the solution about the zinc jdate, and
causes a movement of the positive ions between the two plates from
the Qnc towards the copper, and of negative ions in the opposite
direction, both the Zn and the SO^ ions being able to pass throu^
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ELECTRICITY AND MAGNETISM 43
the porous cup. Since the po^tive ions about the copper plate con-
sist of atoms of copper, it will be
seen that the material which is
driven out of solution at the cop-
per plate, instead of being hydro-
gen, as in the simple cell, is metal-
lic copper. Since, then, the element
which is deposited on the copper
plate is the same as that of which
it already consista, it is clear that
ndther the E. M. F. nor the resist-
ance of the cell can be changed
because of this deposit; i.e., the
cause of the polarization of the
simple cell has been removed.
The great advantage of the
Daniell cell lies in the relatively
high d^ree of constancy in its
E. M. F. (1.08 volts). It has a
comparatively high internal resist-
ance (one to six ohms) and is there-
fore incapable of producing veiy — „ ^-__-_-,, .
large currents, about one ampere
at most. It will furnish a very constant current, however, for a
great length of time; in fact, until all of the
copper is driven out of the copper sulphate
solution. In order to keep a constant supply
of the copper ions in the solution, copper
sulphate crystals are kept in the compart-
ment S of the cell of Fig. 48, or in the bottom
of the gravity cell. These dissolve as fast as
the solution loses its strength through the
deposition of copper on the copper plate.
The Daniell is a so-called cloaal^ireuit
tig-K. LeoiM.ch«c«i. <*•!' ^■''- '*3 <=»""'* s*^-^"'^ be left closed
(through a resistance of thirty or forty ohms)
whenever the cell is not in use. If it is left on open circuit, the copper
sulphate diffuses through the porous cup, and a brownish muddy
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44
ELECTRICITY AND MAGNETISM
deposit of copper or copper oxide is formed upon the zinc. Pure cop-
per is also deposited in the pores of the porous cup. Both of these
actions damage the cell. When the circuit is closed, however, since
flie electrical forces always keep the copper ions moving toward
the copper plate, these damaging effects are to a large extent avoided.
50. The Leclanche Cell. The Leclanchd cell (Fig. 50) con-
sists of a mc rod in a solution of ammonium chloride ( 1 50 g. to a liter
of water), and a carbon plate placed inside of a porous cup which is
packed full of manganese dioxide and powdered graphite or carbon.
As in the simple cell, the zinc dissolves in the liquid, and hydrogen is
liberated at the carbon, or positive, plate. Here it is slowly attacked
by the manganese dioxide. This chemical action is, however, not
quick enough to prevent rapid polarization when large currents
are taken tiom the cell. The cell slowly recovers when allowed
to stand for a
■^ -^ ^ ___-,,, ~ while on open cir-
""^ipppnririnnrirsinnni iiii'in'66 \ cuit. The e. m.
F. of a Leclanche
cell is about 1.5
volts.and its initial
internal resistance
is somewhat less
than an ohm. It
therefore furnishes
a momentary cur-
rent of from one
to three amperes.
The immense advantage of this type of cell lies in the fact that
the zinc is not at all eaten by the ammonium chloride when the cii^
cuit is open, and that, therefore, unlike the Daniell or bichromate
celb, it can be left for an indefinite time on open drcuit without
deterioration. Leclanche cells are used almost exclusively where
momentary currents only are needed, as, for example, on door-
bell circuits. The cell requires no attention for years at a time,
other than the occasional addition of water to replace loss by evapora-
tion, and the occasional addition of ammonium chloride (NH^Cl) to
keep poMtive NH, and negative CI ions in the solution.
51. The Diy Cell. The dty cell is only a modified form of the
Fle.H. Cells Connected
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ELECTRICITY AND MAGNETISM
1 i^rririnnnnniTTO
Leclanch^ celt. It b Dot really dry, since the ^c and carbon plates
are imbedded in moist paste which consists usually of one part of
ciystals of ammonium chloride, three parts of plaster of Paris, one
part of zinc oxide, one part of zinc chloride, and two parts of water.
The plaster of Paris is used to give the paste rigidity. As in the
Leclanch^ cell, it is the acUon of the ammonium chloride upon the
anc which produces the current.
52. ComUnatioRS of Cells. There are two ways in which celb
may be combined: First, in series; and second, in parallel. When
they are connected in series the zinc of one cell is joined to the copper
of the second, the zinc of the second to the cop-
per of the third, etc., the copper of the first and
the zinc of the last being joined to the ends of the
external resistance (see Fig. 51). The E. M. F.
of such a combination is the sum of the E. M.
F.'s of the single cells. The internal resistance
of the combination is also the sum of the internal
resistances of the single cells. Hence, if the ex-
ternal resistances are very small, the current
furnished by the combination will not be taiger
than that furnished by a single cell, since the
total resistance of the circuit has been increased
in the same ratio as the total E. M. F. But if
the external resistance is large, the current pro-
duced by the combination will be very much
greater than that produced by a single cell. Just
how much greater can always be determined by applying Ohm's law,
for if there are n celb in series, and E is the E. M. F. of each cell, the
total E. M. F. of the circuit is n E. Hence if i{« is the external resist-
ance and Ri the internal resistance of a single cell, then Ohm's law ^ves
Plg.EI. OelliOonneeUd
Re-\-nRi
If the n celb are connected in parallel, that b, if all the coppers
are connected together and all the zincs, as in Fig. 52, the E. M. F. of
the combination is only the E. M. F. of a sin^e cell, while tlie internal
resbtance is 1 /n of that of a single cell, since connecting the cells in
thb way b simply equivalent to multiplying the area of the plates n
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40 ELECTRICITY AND MAGNETISM
times. The current iumisbed by such a combinatioD will be given
by the formula:
If, therefore, R, is n^li^bly small, as in the case of a heavy
copper vire, the cunent flowing through it will be n times as great as
that which could be made to flow through Ithy a, single cell. These
considerations show that the rules which should govern the combina-
tion of cells are as follows:
When the external retutanee' is large in comparison wiik the
irUemal resistance of a single cell, the cells should he conneded in series.
When the external resistance is small in comparison with the
inlemal resistance of a single ceU, the ceile should be connected in
fariUlel.
CHEMICAL EFFECTS OF THE ELECTRIC
CURRENT
53. Electrolysis. If two platinum electrodes are dipped into
a solution of dilute sulphuric acid, and the terminab of a battery
producing an £. M. F. of 2 volts or more b ap[^ied to these electrodes,
oxygen gas is found to be given off at the electrode at which the
ciureot enters the solution, called the anode, while hydrogen is ^ven
off at the electrode at which the current leaves the solution, called
the cathode. The modem theory of this phenomenon is as follows:
Sulphuric acid (HjSO^), when it dissolves in water, breaks up into
positively charged hydrogen ions and negatively charged SO^ ions.
As soon as an electrical field is established in the solution by con-
necting the electrodes to the positive and negative terminals of a
battery, the hydrogen ions b^n to migrate toward the cathode, and
there, after giving up their chai^^, unite to form molecules of hydro-
gen gas. On the other hand, the negative SO, ions migrate to the
positive electrode (that is, the anode), where they give up their charges
to it, and then act upon the water, H,0, thus forming H^SO, and
liberating oxygen.
If the volumes of hydrogen and of oxj^en are measured, the
hydrogen is found to occupy in every case just twice the volume
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ELECTRICITY AND MAGNETISM
occupied by the oxygen. This is, indeed, one of the reasons for
believing that water consists of two atoms of hydiogeD and one of
54. Electroplating. If the solution, instead of being sulphuric
acid, had been one of copper sulphate, CuSO^, the results would have
been precisely the same in eveiy respect, except that, since the hydro-
gen ions in the solution are now replaced by copper ions, the substance
deposited on the cathode is pure copper instead of hydrogen. This
is the principle involved in electroplating of all kinds. In commercisl
work, the positive plate, that is, the plate at which the current eaters
the bath, is always made from the same metal as that which is to be
deposited from the solution ; for in this case the SO^ or other n^ative
ions dissolve thb plate as fast as the metal ions are deposited upon
Flg.U. SUT«T-PUtliig Bmth.
the other. The strength of the solution, therefore, remains unchanged.
In effect, the metal is simply taken from one plate and deposited on
the other. Fig. 53 represents a silver-[4ating bath. The bars joined
to the anode A are of pure silver. The spoons to be plated are con-
nected to the cathode K. 'Hte'solution consists of 500 g. of potassium
cyanide and 250 g. of silver cyanide in 10 1. of water.
55. Chemical Method of Measuring Current In 1834, Faraday
found that a given current of electricity flowing for a given time
always deposits the skme amount of a given element from a solution,
whatever be the nature of the solution which contains the element.
For example, one ampere always deposits in an hour 4.025 g. of
alver, whether the solution is of silver nitrate, silver cyanide, or any
other »lver compound. Similariy, an ampere will deposit in an hour
l.lSl g. of copper, 1.203 g. of zinc, etc. Hiis fact is made use of in
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48
ELECTRICITY AND MAGNETISM
calibrating fise ammeters, since it is possible to compute with great
accuracy the strength of a current which will deposit a given wei^t
of metal in a known time. In fact, the Electrical Congress held in
Chicago in 1S93 defined the ampere as the ammmt of current which
tmll defosti ^1118 g. of silver per second.
56. The Storage Battery. If two lead plates are immersed in
sulphuric add and the current sent through the cell, the anode
or plate at which the current enters the solution will be found in
the course of a few minutes to turn dark brown. Tbia brown coat is
a oon^xtund of lead with the ox^en which, in the case of the plat-
inum electrodes, was evolved as a gaa. The
other lead plate is not affected by the
hydrt^n, which is, in this case, as in that
of the platinum,evolved as a gas. Since, then, the passage of the current
through this cell has left one plate unchanged, while it has changed
the surface of the other plate to a new substance, namely, lead per-
oxide, FbOj, it might be expected that if the charging battery were
removed, and these two dissimilar plates connected with a wire, a
current will flow through the wire, for the arrangement is now essen-
tially a »mple .galvanic cell, which in its essentials consists simply of
two dissimilar plates immersed in an electrolyte (a conducting liquid
other than a molten metal). In this case the plate having the lead
peroxide upon it corresponds to the copper of an ordinary cell, and
the unchanged lead plate to the zinc. The arrangement will furnish
a cuitent mitil the lead peroxide is all used up. The only important
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SI
D,Bi:ized3,GOOgIe
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ELECTRICITY AND MAGNETISM
49
difference between a commerical storage cell and the two lead plates
just considered, is that the fonner is provided in the process of
manufacture with a very much thicker coat of the active maierial
(lead peroxide on the positive plate, and a porous, spongy lead on the
negative) than can
be formed by a ^^S
single chai^ng
such as we con- '^
sidered. In one
type of storage
cell this active y/
material is actu- -^
ally formed by the
repeated charging
and discharKin?
° ** Tig. Bft Illustrating the MftBOetlc Properties ol a Helix,
of plates which are
originally ordinary sheets of lead. With each new chai^ng a slightly
thicker layer of the lead peroxide is formed. In the more common
type of commereial cell the active material
,/^'- — ,>x is pressed into interstices of the plate in the
-^iSflB^^l^i^r'' form of a paste. It will be seen from this
''{/^^^T^^i^ discussion that a storage batteiy is not, prop-
-(■■C^^ ""*T-'--i+ ^'^y speaking, a device for storing elee-
piB. B7. Magnetic Fiaia tricity. It is rather a device in which the
SmrounlllngaHelli. i , • i ^ j i_ • i i
electncal current produces chemical cbanges,
and these new chemicals, so long as they last, are capable of genera-
ting a new electrical current.
ELECTROMAQNETISM
57. Magnetic Properties of a Loop. We have seen in § 38 that
an electrical current is surrounded by a magnetic field the direction
of which is given by the right-hand rule. We have seen also that a
loop or coil of wire through which a current flows produces a m^netic
field of the shape shown in Fig. 4L Now, if such a loop is suspended
in the manner shown in Fig, 54 while a current is passed through it,
it is found slowly to set itself in an east-and-west plane, and so that
the face of the loop from which the magnetic lines emerge (see right-
hand rule, § 38 and also Fig. 55) ia toward the north. In other words.
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50 ELECTRICITY AND MAGNETISM
the loop will be found to behave with respect to the earth or to any
other nu^et precisely as thou^ it were a flat magnetic disc whose
boundaiy is the wire, the face which turns toward the north, that is,
that from which the magnetic lines emei^, being an N pole and
the other an S pole.
58. Magnetic Properties of a Helix. If a wire carrying a cu>
rent be wound in the form of a helix and held near a suspended ma^et
as in Fig. 56, the coil will be found to act in eveiy respect like a ma^et,
with an N pole at one end and an S pole at the other.
This result might have been predicted from the fact that a single
loop is equivalent to a flalrdisc magnet For when a series of such
discs is placed side by side, as in the helix, the result must be the
same as placing a series of disc magnets in a row, the N pole of one
being directly in contact with the S pole of the next, etc. These poles
would therefore all neutralize each other except at the two ends. We
therefore get a magnetic Geld of the shape shown in Fig. 57, the
direction of the arrows representing as usual the direction in which
an N pole tends to move.
59. Rules for North and South Poles of a Helix. Hie ri^t-
hand rule, as given in § 38, .is sufficient in every case to determine
which is the N and which the S pole of a helix, i.e., from which end
the lines of magnetic force emerge from the helix and at which end
they enter it. But it is' found convenient, in the consideration of
coils, to restate the ri^t-hand nile in a slightly different way, thus :
Jf the coil is grasped in ihe right hand in svch a way th<U the
fingera point in the diredion in which the current is flowing in the wires,
the thumb wUl point in the direction of the north pole of the helix {see
Fig. 58).
Similarly, if the sign of the poles is known, but the direction of
the current unknown, the latter may be determined as follows:
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ELECTRICrrY AND MAGNETISM 61
// the right hand u placed against the coil imth the thumb pointing
m the direction of the lines of force (i.e., toward the north pole of the
n«.«a ^rsMhoeElecwoniaffiiet, Fig. fl^^Sbowlur UnesotForM in
hdix), the fingers vnU pass around the coU in the direction in which
the current is flowing.
60. The Electromagnet. If a core of soft iron be inserted id
the helix (Fig. 59), the poles will be found to be enormously stronger
than before. This
is because the core
b magnetized by
induction from the
field of the helix in
predsely the same
way in which it ,
would be mag-
netized by induc-
tion if placed in the
field of a permanent
magnet. The new
field strength about
the coil is now the
sum of the fields
due to the core and ^ ^ ^^^ ^.^.^ 3^ ^ connexions.
that due to the coil.
If the current is broken, the core will at once lose the greater part of its
magnetism. If the current is reversed, the polanty of the core will be
reversed. Such a coil with a soft-iron core is called an electromagnet.
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62 ELECTRICITY AND MAGNETISM
The strength of an electromagnet can be very greatly increased
by giving it such form that the magnetic lines can remain in iron
throughout their entire length instead of emerging into air, as they
do in Fig. 59. For this reason electromagnets are usually built in
the horseshoe form and provided with an armature A (Fig. 60)
Chicago Nsw Vtork
Armature
Fig. US. niustmUng tbe Principle ol tlie Electric Telegraph.
thiou^ which a complete iron path for the lines of force is estab-
lished, as shown in Fig. 61. The strength of such a magnet depends
chiefly upon the number of ampere-tumg which encircle it, the expres-
sion ampere-iums denoting the product of the number of turns of wire
about the magnet by the number of amperes flowing in each turn.
Thus a current of ,-Jij- ampere flowing 1,000 times around a core
will make an electromagnet of precisely the same strength as a current
of 1 ampere flowing
10 times about the
core.
<il. The Elec-
tric Bell. The elec-
tric bell (Fig. 62) is
one of the simplest
applications of the
electromagnet.
When the button P
is pressed, the elec-
tric circuit of the
batteiy is closedand
a current flows In at A, through the magnet, over the closed contact C,
and out again at B. But no sooner is this current established than the
electronmgnet E pulls over the armature a, and in so doing breaks the
contact at C. Thb stops the current and demagnetizes the magnet E.
The armature is then thrown back against C by the elasticity of the
PlR. M. Telegrapblo Relay.
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ELECTRICITY AND MAGNETISM 53
spring 8 which supports it. No sooner is the contact made at C
than the current again begins to flow and the former operation is
repeated. Thus the drcuit is automatically made and broken at G
and the hammer H is in consequence set into rapid vibration against
the rim of the bell.
62. The Tdegraph. The electric telegraph is another simple
application of the electromagnet. The principle is illustrated in Fig.
63. As soon as the key K at Chicago, for example, b closed, the
current flows over the line to, we will say, New Yoik. T^ere it
passes through the electromagnet m, and thence back to Chicago
throu^. the earth. The armature h is held down by the electro-
magnet m as long as the key K is kept closed. As soon as the circuit
is broken at K, the armature is pulled up by the spring d. By means
of a clockwoik device the tape e is drawn along at a uniform rate
beneath the pencil or pen carried by the armature 6. A very short
time of closing of K produces a dot upon the tape, a longer time a dash.
As the Moise, or telegraphic, alphabet consists of certain combina-
tbns of dots and dashes, any desired message may be sent ttom
Chicago and recorded in New York.
AMERICAN MORSE CODE
In modem practice the message is not ordinarily recorded on a
tapa, for operators have learned to read messages by ear, a very short
interval between two clicks being interpreted as a dot, a longer interval
Hs a dash.
The first commercial tel^raph line was built by S. F. B. Morse
between Baltimore and Washington. It was opened on May 24,
1844, with the now famous message: "What hath God wrought)'"
63. The Relay and Sounder. On account of the great resistance
of long lines, the current which passes through the electromagnet is
so weak that the armature of this magnet must be made very light in
order to respond to the action of the current The clicks of such
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54
ELECTRICITY AND MAGNETISM
an armature are not sufficiently loud to be read easily by an operator.
Henre at each station there is introduced a local circuit which con-
tains a local battery, and a second and heavier electromagnet which
is called a sounder.
The electromagnet
on the main line is
then called therelay
(see Figs. 64, 65, and
66). The sounder
has a very heavy
armature (A, Fig.
65), which is so ar-
I ranged that it clicks
both when it is
drawn down by ita
electromagnet
against the stop S
and when it is pushed up again by its spring, on breaking the current,
against the stop (. The interval which elapses between those two
clicks indicates to the operator whether a dot or dash is sent. The
FlK- (S- Telegraphic Sounder.
current in the main line simply serves to close and open the circuit
in the local battery which operates the sounder (see Fig. 66). The
electromagnets of the relay and the sounder difFer in that the former
consists of many thousand turns of fine wire, usually having a resist-
ance of about 150 ohms, while the latter ctmsists of a few hundred
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ELECTRICITY AND MAGNETISM 65
turns of coarse wire having ordinarily a resistance ot about 4 otims.
64. Plan of a Tdegraph System. Ilie actual arrangement of
the various parts of a telegraphic system is shown in Fig. 66. When
an operator at Chicago wishes to send a message to New York, he
first opens the switch whidi is connected to his key, and which b
always kept closed except when he is sending a message. He then
begins to operate his key, thus controlling the clicks of both his own
sounder and that at New Yoik. When the Chicago switch is closed
and the one at New York open, the New York operator is able to said
a message back over the same line. In practice a message is not
usually sent as far as from Chicago to New York over a single line,
save in the case of trans-oceanic cables. Instead, it b automatically
transferred at, say, Cleveland, to a second line which carries it on
to Buffalo, where it b again transferred to a third line which car-
ries it on to New York. The transfer b made in precisely the same
way as the transfer from the main circuit to the sounder circuit.
If, for example, the sounder circuit at Cleveland b lengthened so
aatoe:rtendtoBuf-
fak), and if the
sounder itself b re-
placed by a relay
(called in thb case
a repealer), and the
local battery by a
main battery, then
the sounder circuit has been transformed into a repeater circuit,
and all the conditions are met for an automatic transfer of the message
at Cleveland to the Cleveland-Buffalo line. Iliere b, of course, no
time lost in thb automatic transfer.
INDUCED CURRENTS
65. Indoction of Currents by Magnets. If a coil of wire C is
connected to any sensitive current detector, as in Fig. 67, and then
thrust over the pole of a magnet from the position a to the position c,
a momentary durrent b observed to flow through the drcuit. If the
coil is held stationary over the magnet, the needle will come to rest
in its natural position. If the coil b removed suddenly from the pole,
the needle irill move in the direction opposite to that of its first d^eo*
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56 ELECTRICITY AND MAGNETISM
tion, which shows that a reverse current is now being generated in
the coil.
These experiments show that a current of electricity may he in-
duced in a conductor by causing the loiter to move through a magnetic
field. This discovery, one of (he most important in the history of
science, was announced by Faraday in 1831. From it have sprung
directly most of the modem industrial developments of electricity.
When we test the direction in which the current is induced in
the coil C (Fig. 67) by applying the right-hand rule to the direction
of deflection of the needle, we find that while the coil C was mov-
ing from a down to c the induced current flowing through it was
in such a direction as to make its lower face an N pole and its
upper face an S pole. Now if we split up this motion into two
parts, namely, that from a to 6 and that from b to c, we see
that while the coil is being moved from a to 6 the repulsion of
the N pole of the magnet for the N pole of the coil is greater
than the attraction of the N pole of the magnet for the S pole
of the coil, 30 that the motion must be made against an opposing
force. Similarly, while the coil is going from i to c, the S pole
of the coil is neai^r the N pole of the magnet than is the N
.pole of the coil, and consequently the attrac-
tion of the N pole of the magnet for this S
pole of the coil is greater than the repulsion
of the two N poles. Hence the motion from
6 to c also must be made against an opposing
force. When the coil was moving from c to a,
the current was in the reverse direction, hence
the poles of the coil were reversed, so that at
FlK. «S. snowing tUM a .... ,■ t , , , ,
Conductor must cui Lines evcru potnt the motion had to be made aaainst
ot Uurnetlc Force In older . ,
to Induce Ml E.M.F. an oppostng force.
From these experiments and others of a similar kind, it has been
discovered that whenever a current is induced in a conductor by the
relative motion of the, conductor and the magnetic field, the direction
of the induced current is always such as to set up a magnetic field
which opposes the motion. This is known as Lenz's law. It is a law
which might have been predicted beforehand from the principle of
the conservation of enei^, for, since an electrical current possesses
energy, the prnciple of conservation of enei^ tells us that no such
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ELECTRICITY AND MAGNETISM 67
current can possibly be created without the expenditure of vmA. of
some sort. In this case there is no pUce for the eaergy to come from
except from the mechanioU work done in pushing the coil against some
resisting force.
If, instead of moving the coil up and down over the pole, we
had held it in the position shown in Fig. 68, and moved it back and
forth 90 that its motion was parailel to the line N 8, no induced current
would have been observed. By experiments of this sort it is found
that an E. M. F. ia induced in a coH only when the motion takes place
in mtck a way as to change the total nuTnber of magnetic lines of force
which are enclosed in the eoU, Or, to state this rule in a more general
form: An E. M. F. is induced in any element of a amdveior when
and only when that element is moving in svch a way as to cut magnetic
lines of force.
It will be noticed that the first statement of the rule is included
in the second, for whenever the number of lines of force which pass
through a coil changes, some lines of force must cut across the coil
from the inside to the outside, or vice vers&.
In the preceding statement we have used the expression induced
E. if. F, instead of induced current for the reason that whether or not
a continuous current flows in a conductor in which an E. M. F. (t. «.,
a pressure tendmg to produce a current) exists, depends simply od
whether or not the conductor is a portion of a closed electrical drcuit
In our experiment the portion of the wire in which the E. M. F. was
being generated by its passage across the lines of force running from
NtoS was a part of such a closed circuit, and hence a current resulted.
If we bad moved a straight conductor like that shown in Fig. 69,
the E. M. F. would have been induced precisely as before; but since
the circuit would then have been open, the only effect of this E. M. F.
would have been to establish a P. D. between the ends of the wire,
i. e., to cause a positive charge to appear at one of its ends and a
negative chaige at the other, in precisely the same way that the E.M.F.
of a battery causes positive and negative charges to appear on the
terminals of the battery when it is on open circuit
66. Strength of the Induced E.A1.F. The strength of an induced
E. M. F. is found to depend simply upon the number of lines of force
cut per second by the conductor, or, in the case of a coil, upon the
rate of change in the number of lines of force which pass thiou^
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68 ELECTRICITY AND MAGNETTISM
the coil. The str^igth of the current which flows is then givai b;
Ohm's law; i. e., it is equal to the induced E. M. F. divided by the
resistance of the circuit. The iuniber of lines of force which the
conductor cuts per second may always be detennined if we know
the velodty of the conductor and the strength of the magnetic field
ihiou^ which it moves.*
In a conductor which is cutting lines at the rate of 100,000,000
lines per second, there is an induced electromotive force of exactly
one volt.
67. The Dyaamo Rule. Since the experiment illustrated in
Fig. 67 shows that reversiiig the direction in which a conductor ia
*^
v^y
»sws
dDondCamnL
cutting lines of fbree also reverses the direction of the induced electro-
motive force, it is clear that a fixed relation must exist between these
two directions and the direction of. the magnetic lines. What this
relation is may be obtained easily from Lenz'a law. V/ben the con-
ductor was moving upward (Fig. 68), the current fiowed in such a
direction as to oppose the motion, that is, so as to make the lower
face of the coil an S pole. This means that in the portion of the
conductor between N and iS where the E. M. F, was being generated,
• AnugnetloiMla of nolt strengtbls bydaflnlUon » pole which wlien plaoed aX B
dlalance of one centimeter trom an siuitly almU^r pole repels It with a lorce ol one ara*
(elMat one thoneauilth ot a gram, or nin o' an ounce). A msKnetlo fisld at unit
■trenctbla. bydeanitloa, a fleldln which a nnlt-pole la acted apon b; a force o I one dyne.
Benoe, II a nnlt-poLe lafoonil In a given Qeld lo be acl«d upon by a force of one thooaand
d7nea,we uytbat the Deld strength Is one thousand nntu. Now, It la onstomar; to
repreeent a magnetic field by drawing as nuuir Unea per square oentlmeter taken at right
mngles to the direction of the field al the Qeld baa nnlta of strength. Thns, a field ot unit
■trength ta said to contain one line per aqnare oentlmeter, a field ot a thonsand nnlta
■mngth a thousand Unee per egnare centimeter, etc The magnetic fields need In pow.
■tfnl Ornuaos will hare MMnetlmea •■ Ugh as nooo 1 Ine* per square contlmatar.
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ELECTRicrry and magnetism
f(
its direction was kaax back to front, that is, towaid the reader (aee
arrow, Fig. 69). We therefore set up the fbtlowing nite, which ia
found to apply in every case:
// the Jorepnger of ths right hand fomts m the direcHm of the
magnetic linee {see Fig. 70)i and the tkvmi) in the direction in which
the conductor ia cutting these lines, ^len the middle finger, held at right
angles to both thumb and forefinger, mU point in
the diredion of the induct current.
lliis rule is known as the dynamo rtde.
68. ThePrincipleoftheDynaino. Adynamo
is essentially nothing but a coil of wire rotating
continuously between the poles of a magnet Thus, ■
suppose that starting with the coil in the poiution
shown in Fig. 71, it be rotated through 180 d^prees
from left to ri^t as one looks down upon it
During the fiist half of the revolution the wires
on the rii^t side of the loop are cutting the lines
... . , . T L -1 Vis- "- nummtlng
of force while movmK toward the reader, while fta pnnoipiB oi
° ttLeDynuno.
the lines on the left side are cutting the same
lines while moving away from the reader. Hence, by applying the
dynamo rule, we find that a current is being generated which flows
down on the ri^t side of the coil and up on the left side. It will be
seen that both currents flow around the coil In
the same direction. Tte mduced current is
strongest when the coil is in the position shown
in Fig. 72, because there the lines of force are
being cut most rapidly. Just as the coil is bang
moved into or out of the position shown in Fig. 71,
it is moving partdkl to the lines of force and
hence no current is induced, since no lines of force
^ y // are being cut As the coil is now moved through
^^^=-^ X the last 180 degrees of a complete revolution, both
ades are cutting the same lines of force as be-
BSS^^ou*'wtan fore, but they are cuttmg tiiem while moving in
rent wonges ^ oppofflte direction from that in which they were
first moving, hence the current generated during this last half is
oppoate in direction to that of the first half. If the coil is continu-
ously rotated in the field, therefore, an alternating current is set up in
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60 ELECTRICITY AND MAGNETISM
it, which reverses direction erety tinie the coil passes tlirough the
portion shown in Fig. 71. This is the essential principle of the
alternating-current dynamo. The direct-current dynamo differs from
the alternating-current dynamo, only in that a so-called wmmviator
is used for the purpose of changing the direction of the current in
the external circuit eveiy time the coil passes throu^ the position
shown in Fig. 71, so that the current always flows in the same
direction through this external portion of the circuit in spite of the fact
that in the rotating coil it changes direction every half-revolution.
69. The Princi|rie of the Electric Motw. If a vertical wire
a & is made to pass between the poles of a magnet in the manner shown
in Fig. 73, and the current from an outside souree — for example, a
Leclanch^ cell — sent through it from a to b, the wire a b will be found
to move through the mercury, into
which its lower end dips, in the
direction indicated by the anow /,
namely, at right angles to the direc-
tion of the Unes of magnetic force.
If the direction of the cunent in
o 6 IB reversed the direction of the
motion of the wire will be found to
H^ — — *■ be TCveraed also. This experiment
shows that a wire carrying a Cur-
rent in a magnetic field tends to move
in a directum at right angles both to
the direction of the field and to the
direction of the'current. The experi-
ment illjistrates the essential princi-
ple of the electric motor. The
relation between the direction qf the magnetic lines, the direction of
the current, and the direction of the force, is often remembered by
means of the following rule, known as the motor rule. It differs
from the dynamo rule, only in that it is applied to the fingers of the
left hand instead of to those of the right.
Let the forefinger of the left hand point in the direction of ike
magnetic linet of force and the middle finger in the direction of the
current sent through the wire; the thumb will then point in the direo-
Hon of the mechanical force acting to move the vrtre (see Fig. 73).
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EI.ECTRICrrY AND MAGNETISM 61
In pracUce the motor does not differ in construction at all from
the dynamo. Thus, if a current is sent into the right side of the coil
shown in Fig. 72, and out of the left side, the wires on tiie left side
of the coil will be seen, by on application of the motor rule, to be urged
toward the reader, while the wires on the right side are ui^ged sway
from the reader. Hence the coil begins to rotate. After it has rotated
thiou^ the position shown in Fig. 71, if the direction in whicJi the
current flows through it were not changed, it would be urged to
rotate back to the position of Fig. 71; but in the actual motor, at the
instant at which the coil passes throu^ the portion shown in Fig.
71, the commutator reverses the direction of the current as it enters
the coil. Hence the coil is always impelled to n>tate in the same
direction.
70. The Principle of the Induction Coil and Transformer. If a
coil of wire p is wound about an iron core, as in Fig. 74, and connected
to the circuit of a
Fig. 74. Illuitntlng ibe Principle ol tlia Indnotlon
Colt and Transtormer.
the figure, it is found that when the key K is closed, the deflection
of the detector indicates that a temporary current has been induced
in one direction through the coil a, but when it b opened an equal
but opposite deSection will indicate an equal induced current in the
opposite direction.
The experiment illustrates the principle of the induction coil
and the transformer. The coil p, which is connected to the source
of the current, is called the primary coil, and the coil s, in which the
currents are induced, is called the secondary coil. Causing lines of
force to spring into existence inside of » — in other words, magnetizing
the space inside of s (that is, the core about which'the coils are wound)
— has caused an induced current tp flow in a; and demagnetizing
the space inside of a has also induced a current in s in accordance
with the general principle stated in § 65 that any change in the number
of magnetic lines of force which thread through a coil induces a cui>
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62 ELECTRICrrY AND MAGNETISM
rent in the coil. We may think of the lines which suddenly appear
within the iron core upon magnetization as springing from without
across the loops into the core, and as springing back again upon
demagnetization, thus cutting the loops while moving m opposite
directions in the two cases.
71. Direction of the Induced Current. Lenz's law, which, it
will be remembered, followed from the principle of conservation of
energy, enables us to predict at once the direction of the induced cur-
rents in the above experiments; and an observation of the deflections
of the galvanometer enables us to verify the correctness of the pre-
dictions. Consider first the case in which the primary circuit is
made and the core thus magnetized. According to Lenz's law, the
current induced in the secondary circuit must be in such a direction
aa to oppose the change which is being produced by the primary cur>
rent, t. e. in such a direction as to Und to magnetize the core oppositely
to the direction in which it is being magnetized by the primary. This
means, of course, that the induced current in the secondary must
encircle the core in a direction opposite to the direction in which the
primary current encircles it. We learn, therefore, that on making
the current in the primary, the current induced in the secondary is
oppoaiie in direction to thai in the primary.
When the current in the primary is broken, the magnetic field
created by the primaiy tends to die out. Hence, by Lenz's law, the
current induced in the secondary must be in such a direction as to
tend to oppose this process of demagnetization, i.e., in such a direc-
tion as to magnetize the core in the same direction in which it is
magnetized by the decaying current in the primary, llierefore, at
break, the current induced in the secondary is in^ same direction as
that which is dying out in the primary.
72. E. M. F. of the Secondary. If half of the turns of the sec-
ondary s (Fig. 74) are unwrapped, the deflection when K is opened
or closed, will be found to be just half as great as before. Since the
resistance of the circuit has not been changed, we learn from this that
the E. M. F. of the secondary is proportional to the number of titm* of
wire upon it. This result followed also from the statement made in
§ 66 that the electromotive force induced in any circuit is equal to the
rate of cutting of lines of force by that circuit. For all of the lines
friiich pass through the core cut all of the coils on t. If, therefore,
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ELECTRICITY AND MAGNETISM 63
theie are twice as man; coils in one case as in another, twice as many
linea of force cut the circuit, and henee'the E. M. F. is twice as great.
If, then, we wish to develop a very high B. M. F. in the secondary,
we have only to make it of a very large number of turns of fine wire.
The wire must not, howevw, be wrapped so far away from the core as
to include the hnes of force which are returning through the air
(see Fig. 59), for, when this happens, the coils are threaded in both
directions by the same lines, and hence have no current induced in
them.
73. E. AL F. at Make and Break. If the secondary coil a is
replaced by a spool or paper cyUnder upon which are wound 5,000
or 10,000 turns of No. 36 copper wire, and if the ends of the coil are
attached to metal handles and held in the moistened hands, then,
when the key K is closed, no shock whatever will be felt; but a very
marked one will be observable when the key is opened. The experi-
ment can easily be tried with an inexpensive medical coil. It shows
that the E. M. F. developed at the break of the circuit is enormously
greater than that at the make. The explanation is found in the
fact that the E. M. F. developed in a coil depends upon the rate at
which the niunber of lines of force passing through it is made to
change (see § 66). When the circuit of the primary was made, the
current required an appreciable time, perhaps a tenth of a second,
to rise to its full value, just as a current of water, started through
a hose, requires an appreciable time to rise to its full hei^t, on account
of the inertia of the water. An electrical current possesses a prop-
erty ramilar to inertia. Hence the magnetic field about the primary
tdso rises equally gradually to its full strength, and therefore its lines
pass into the coil comparatively slowly. At break, however, by
separating the contact point very quickly, 'we can make the current
in the primary fall to zero in an exceedingly short time, perhaps not
more than .00001 of a second; i.e., we can make all of its lines pass out
of the coil in this time. Hence the rate at which lines thread through,
or cut, the secondary is perhaps 10,000 times as great at break as at
make, and therefore -the E. M.F. is also something like 10,000 times
as great. It should be remembered, however, that in a closed second-
ary the make current lasts as much longer than the break current as
its E. M. F. is smaller; hence the total energy of the two is the sante,
as was indeed indicated by the equal deflections in § 65.
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64 ELECTRICITY AND MAGNETISM
INDUCTION COIL AND TRANSFORMER
74. The Induction Coil. The induction coil, as usually made
(Fig. 75), consists of a soft iron core C, which is composed of a
bundle of soft iron wires; a primary coil p wrapp^ around this core,
and consisting of, say, 200 turns of coarse copper wire {e. g., No. 16),
which is connected into the circuit of a battery through the contact
point at the end of the screw d; a secondary coil s surrounding the
primary in the manner indicated in the diagram, and consisting gen-
erally of between 30,000 and 1,000,000 turns of No. 36 copper wire
the terminals of which are the points t and f; and a hammer b, or
Condenaer
Fie- n. nuloocloti CoU, and DimgrunmAtlo ReprsMOMtlon ol Same.
other automatic arrangement for making and breaking the circuit of
the primary.
When the current is first started in the primary, it magnetizes
the core C Thereupon the iron hammer b is drawn away from its
contact with d and the current is thus suddenly stopped. This
instantly demagnetizes the core and induces in the secondary s an
E. M, F. which is usually sufficient to cause a spark to leap between
t and f. As soon as the core is demagnetized, the spring r which
supports the hammer restores the contact with d and the operation is
repeated. The condenser, shown in the diagram with its two sets
of plates connected to the conductors on either ^de of the spark gap
between r and d, is not an essential part of a coil ; but when it is
introduced, it is found that the length of the spark which can be sent
across between ( and C is considerably mcreased. The reason is as
follows: V/hen the drcuit is broken at b, the inertia of the current
tends to make a spark jump across from dtob; and if this happens,
the current continues to flow through this spark (or are) until the
terminals have become aepar&ited throng a considerable distance.
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ELECTRicrry and magnetism es
This makes the current die down gradually instead of suddenly, as
it ought to do to produce a high £. M. F. But when a condenser is
inserted, as soon as b b^ins to leave d, the current begins to flow into
the condenser, and this gives the hammer time to get so far away from
d that an arc cannot be formed. This means a sudden break and
a hi^ £. M. F. Since a spark passes between t and f only at break,
§ 73, it must alwaj's pass in the same direction. Coils which give
24-in<^ sparks (perhaps 500,000 volts) are not uncommon. Such
coils usually have hundreds of miles of wire upon their secondaries.
75. The Transforms. The commercial transformer is a modi-
fied form of the induction coil. The chief difference is that the
core R (Fig 76), instead of being straight, is bent into the form
of a ring or is pven some other
shape such that the magnetic lines
of force have a continuous iron
path, instead of being obliged to "^
push out into the air, as in the
induction coil. Furthermore, it is
always an alternating instead of an "R^
intermittent current which ia sent Fig. to. Diagnunmstic R^iTesenMUoii
through the primary A. Sending <■( CommercUl Tn«8former.
such a current through A is equivalent to magnetizing the core
first in one direction, then demagnetizing it, then magnetizing it in
the opposite direction, etc. The result of these changes in the mag-
netism of the core is of course an induced alternating current in the
secondary B.
76. Pressure in Primary and Secondary. If there are a few
turns in the primary and a lai^ number in the secondary, the tran»<
former is called a step^p transformer, because the P. D. produced at
die terminals of the secondary is greater than that applied at the
terminals of the primary. Tlius, an induction coil is a step-up
transformer. In electric lighting, however, transformers are mostly
of the step-dovm type; i. e., a high F. D., say 2,200 volts, is applied
at the terminal of the primaiy, and a lower P. D., say 110 volts, is
obtained at the terminals of the secondary. In such a transformer ttu
primary will have twenty times as many turns as the secondary. In
general, Oie ratio between the voliagea at the terminals of the primary
and Becondary is the ratio of the number of turns of wire upon the two.
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Flc-IT. Slmplen Form of Bieetrlo Telephone
06 ELECTRICITY AND MAGNETISM
77. The ^mple Teleptaon& The telephone was mvented in
1876 by Eliaha Gray, of Chicago, and Alexander Graham Bell, of
Washington. In its simplest form it consists, at each aid, of a per-
manent bar magnet A (Fig. 77) surrounded by a coil of fine wire B,
in series with the line, and an iron disc or diaphragm E mounted
close to one end of the magnet. When a sound is made in front of
the diaphragm, the vibiations produced by the sounding body are
transmitted by the air to the diaphragm, thus cau^ng the latter to
vibrate back and forth in front of the magnet. Tliese vibrations of
the diaphragm produce slight backward and tormrd movements of
the lines of force which are continually passing into the disc or
diaphragm from the magnet
Some of these lines of force,
therefore, cut across the coil
B, first in one direction and
then in the other, and in so
doing induce currents in it
These induced currents are
transmitted by the line to the receiving station, where those in one direc-
tion pass around B' in such a way as to mcretue the strength of the
magnet A', and thus increase the pull which it exerts upon E'; while
the opposite currents pass around B' in the opposite direction, and
therefore weaken the magnet A' and Himmi<ili its pull upon £'.
When, therefore, E moves in one direction, £' also moves in one
direction ; and when E reverses its motion, the direction of motion of
E' is also reversed. In other words, the induced currents transnutted
by the line force E' to reproduce the motions of E. E', therefore,
sends out sound waves exactly like those which fell upon E. In
exactly the same way, a sound made in front of E' is reproduced at E.
Telephones of this simple type will work satisfactorily for a distance
of several miles. This simple form of instrument is still used at the
receiving end of the modem telephone, the only innovation which
has been introduced consisting in the substitution of a U-shaped
loagnet for the bar magnet. The instrument used at the tranamit-
. ting end has, however, been changed, as explained in the next
paragraph, and the circuit is now completed through a letum wire
instead of through the earth. A modem telephone receiver is shown
in Fig. 78. G is the earpiece, E the diaphragm, A the U-flhaped
Dignzedoy Google
ELECTRICITY AND MAGNETISM 67
magnet, B the coib, consiatiiig of many turns of fine vire, and D
the terminals of the line.
78. The Modern TransnUtt^. To inerease the distance at
which telephoning may be done, it is necessary to increase the strength
of the induced cui^
lents. Tliis is done
in the modeni trans-
mitter by replacing
the magnet and coil
by an arrangement
which is essentially
an induction coU,
the current in the
primary of which is "«™- 8«=«<««ivtowot«od«nTeKpho™B«»i™r.
caused to vary by the motion of the diaphragm. Iliis is accomplished
as follows: The ciurent from the battery (B, Fig. 79) is led first to
the back of the diaphragm E, whence it passes through a Uttie chamber
C filled with granular carbon to the conducting back d of the trans-
mitter, and tlience through the primary p of the induction coil, and
back to the battery. As the diaphragm vibrates it varies the pressure
upon the many contact points of the granular carbon throng whidi
the primary current fiows. lliis produces considerable variation
in the reastance of the primary circuit, so that as the diaphragm
moves forward, i.e., toward the carbon, a comparatively large current
Racatysr
w
Pig. 79. GoiuieeUoiii of Modam TruumltMir.
flows through p, and as it moves back, a much smaller current. These
changes in the current strength in the primary p produce changes in
the magnetism of the soft-iron core of the induction coil. Currents
are tlierefore induced in the secondary t of the induction coil, and
these currents pass over the line and affect the receiver at the other
end in the manner explained in the preceding paragraph. Fig.
80 shows a section of a complete long^listance transmitter.
DignzedoyGOOgle
68 ELECTRICTTY AND MAGNETISM
79. The Subso^bo^s Tdeptione Connections. In the most
. recent practice of the Bell Telephone Company, the local batter}' at
the subscriber's end is done away with altogether, and the. primary
current is furnished by a battery at the central station. Fig. 81
shows the essential elements of such a system. A battery B, usually
of 25 volts pressure, is always kept
connected at central to all the lines
which enter the exchange. No
current flows throu^ these lines,
however, so long as the subscriber's
receivers R are upon their hooks H;
for the line circuit is then open at
the contact points t. It would be
closed through the bell b were it
not for the introduction of the
condenser C in series with the bell.
"'■'* ^rSa^J^'*"'^'" This makes it impossible for any
direct current to pass from one side
of the line to the other, so long as the receiver is upon the hook.
But if the operator at central wishes to call up the subscriber, she
has only to throw upon the line an aUemaiing current from the mag-
neto M, or from any alternating-current generator whose terminals
she can connect to the subscriber's line by turning a switch. This
alternating current surges back and forth through the bell into the
Subscriber Uno Cgntrai
Fig. SI Essential Elementi ot Subwrlber's Telepbone ConnecUoos.
condenser and out agam, tirst charing the condenser plates in one
direction, then in the other. By making the capacity of this con-
denser sufficiently large, this alternating current is made strong enough
to pull the armature a first toward the electromagnet m, then toward
n. In this way it rings the bell.
On the other hand, if the subscriber wishes to call up central,
he has only to lift the receiver from the hook. This closes the Une
DignzedoyGOOgle
ELECTRicrry and magnetism 69
circuit at t, and the direct current which at once heffns to flow from
the battery B through the electromagnet g closes the drcuit of B
through the glow lamp / and the contact point r. This li^ts up
the lamp I whidi b upon the switchboard in front of the operator.
Upon seeing this signal, the latter moves a switch which connects her
own telephone to the subscriber's line. Then, as the latter tallcs
into the transmitter T, the strength of the direct current from the
battery B, through the primary p, is varied by the vaiying pressure
of the diaphragm E upon the granular carbon c, and these variations
induce in the secondary a the talking currents which pass over the
line to the receiver of the operator. Although with this arrangement
the primary and secondary currents pass simultaneously over the
same line, speech is found to be transmitted quite as di^tinctiy as
when the two drcuits are entirely separate, as is the case with the
arrangement of Fig. 79. When the operator finds what number
the subscriber wishes, she connects the ends d and e of his line with
the ends of the desired line by means of a flexible conducting cord
which terminates in a metallic plug u, suitable for making contact
with d and e. As soon as the subscriber replaces his receiver upon
its hook, the lamp I is extinguished, and the operator thereupon
withdraws w and thus disconnects the two lines.
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ELBCTRIC-UrTni& MAQIIBT HANDUHQ WIKB SCBAP
It Is Swone I
irane over a Pile ot Scnp. &nd When the Current Is Turned OB thti^rr I p
ItersJly Jainp« up to the Hunet. K 1» Than Cmrrita OMT ft '^'*-'X '^^
u- ana the (Srcnlc Broken, «i-'M-'<"g the Wire Serxp. ^'
THE ELECTRIC CURRENT
Blectroniotive Force. When a difFerence of electrical poten*
tial exists between two points, there is aaid to exist aa electro-
motive foreet or tendency to cause a current to flow from one point
to the other. In the voltaic cell ODe plate is at a difFerent potential
from the other, which gives rise to an electromotive force between
them. Also in the induction coil, an eleotromoUve force is created
in the secondary circuit caused by the action of the primary. This .
electromotive force is analogous to the prtttwot caused by a dif-
ference in level of two bodies of water connected by a pipe. The
pressure tends to force the water through the pipe, and the
electromotive force tends to cause an electric current to flow.
The terms potential difference and electromotive force are
oommooly used with the same meaning, but strictly speaking the
potential difference gives rise to the electromotive force. Electro-
motive force is commonly designated by the letters H. M. F, or
■imply E. It is also referred to as pretture or voltage.
Curreat. A current of electricity flows when two points, at
a difference of potential, are connected by a wire, or when the
oircoit is otherwise completed. Similarly water flows from a high
level to a lower one, when a path is provided. In either case the
flow can take place only when the path exists. Hence to produce
a current it is necessary to have an electromotive force and a closed
circuit. The current continues to flow only as long as the electny
motive force and closed circuit exist.
The strength of a current in a conductor is deflned as the
quantity of electricity which passes any point in the circuit in a
unit of time.
Current is sometimes designated by the letter C, but the
letter /will be used for current throughout this and following
sections. Itie latter symbol was recommended by the Interna-
tional Electrical Congress held at Chicago in 1893, and has since
been universdly adopted.
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4 THE ELECTKIO CURRKNT.
Resistance. ReaUtance is that property of matter in Tirtae
of wbioli bodies oppose or resist the free flow of electricity. Water
passes with difficulty tjirough a small pipe of great length or
through a pipe flUed with stones or sand, but very readily through
a large clear pipe of short length. Likewise a small wire of con-
sider&ble length and made of poor conducting material oSeis great
resistance to the passage of electricity, but a good conductor of
short length and large cross section offers very little resistance.
Besistanoe is designated by the letter R.
Volt, Ampere and Ohm. The volt is the practical unit of
electromotiTe force.
The ampere is the practical unit of current
The ohm is the practical unit of resistance. The mtorohm is
ODe millionth of an ohm and the megohm is one million chms.
The standard values of the above units were very accurately
determined by the International Electrical Congress in 1898, and
are as follows :
The International ohm, or true ohm, as nearly as known, ii
the resistance of a uniform column of mercury 106.3 centimeteis
long and 14.4521 grams in mass, at the temperature of melting
. tce.
The ampere is the strength of current which, when passed
through a solution of silver nitrate, under suitable conditions,
deposits silver at the rate of .001118 gram per second. Current
strength may be veiy accurately determined by electrolysis, and
it is used therefore in determining the standard milt.
The volt is equal to the E. M. F. vbioh, when applied to a
conductor having a resistance of one ohm, will produce in it a
current of one ampere. One volt equals ^{^ of tiie K. M. F. of
a Clark standard cell at 15° Centigrade.
RBSISTANCB.
All snbBtanoes resist the passage o£ eieobioity, but the redst-
anoe offered by some is very mucb greater tlian that offered by
others. Metals hare by far the ieast rasistanoe and of these, Bilvei
possesses the least of any. In otber vordb. aalTSr is tlie best eon-
dnctor. If the temperature remains tbo same, the rvsistanoe of a
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THE ELECTTRIO CURRENT. 6
ooncluotor is not affected by the oorrent paaung through it. A
current of ten, twenty or any number of amperes may pass through
a circuit, but its resistance will be unchanged with constant tem-
perature. Resistance is affected by the temperature and also by
the degree of hardness. Annealing decreases the resistance of a
metal.
Conductance is the inveise of resistance ; that is, if a ooodu>
tor has a resistance of R ohms, its conductance ia equal to -^.
Resistance Proportional to Length. The resistance of a
conductor is directly proportional to its length. Hence, if the
length of a conductor ia doubled, the resistance is doubled, or if
the length is divided, say into three equal parts, then the resists
ance of each part is oue third the total resistance.
Example. — The resistance of 128S feet of a certain wire is
6.9 ohms. What is the resistance of 142 feet of the same wire ?
Solution. — Aa the resistance is directly proportional to (ha
length we bare the proportion,
required reBistance :6.9 : : 142 : 1288
required resistance 142
O 1288"
142
Hence, required resistance = 6.9 X ■ -^
^ .76 ohm (approx.)
Ans. .76 ohm.
Example The resistance of a wire having a length of 521
feet is .11 ohm. What length of the same wire will have a
resistance of .18 ohm?
Solution As the resistance is proportional to tei^fth, wa
have the proportion,
required length : 521 : : .18 : .11
required length .18
* 521 Ti
,.18
TeqTtred length :
> 852 feet (approx.)
Ans. 862 £
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6 THE ELECTRIC CURRENT.
Resistance tavtm^y Proportional to Cross^ection. The
resistance of a oondnctor is inversely proportional to its cross-sec-
tional area. HeDce the greater the crosB-section of a wire the less
is its resistance. Therefore, if two wires have the same length,
bnt one has a cross-section three times that of the other, the
resistance of the former is one-third that of the latter.
Example. — The ratio of the cross-sectional area of one wire
257
to that of another of the same length and material is j^ ■ The
resistance of the former is 16.3 ohms. What is the resistance of
the latter ?
SolotioD. — Aa the resistances are ioTerselj proportional to
the cross -sections, the smaller wire has the greater resistance, and
we have the proportion :
required resist. ; 16.3 : : 2B7 : 101
or, reqnired resist. 2B7
16:3 lor
Hence, required resist. = 16.3 X jr^
^ 41.fi ohma (approx.)
Ans. 41.5 ohms-
Example. — If the resistance of a wire of a certain length
and having a oross-eectional area of ,0083 square inch is 1.7 ohms,
what woold be its resistance if the area of its cross-section were
.092 square inch ?
Solation. — Since increasing the croas^ectional area of a wire
decreases its resistance, we have the proportion,
required resist. : 1.7 : : .0083 : .09S
teqnired reaiat. .0088
1.7 ■" .092
Henofl^ zeqoired reeist. » 1.7 X '^^^
= .16 ohm (approx.)
Ans. .15 tAim.
Am tlie area of s circle is proportional to the sqoare of its
diameter, it follows that the resistances of round conductors are
inTersely proportional to the squares of their diameters.
Example.— Tlie resistance of a certain wire Iiavisg a diam.
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THB ELECTRIC CUHRENT. I
flter of .1 inch is 12.6 ohms. What would be its resistance if the
diameter were increased to .82 inch?
Solution. — The reaistanoea being inversely proportional to
the sqaarsB of the diameters, we have,
required resist. : 12.6 ::.!*: .S2<
required reeist.
12.6
.1'
" .82'
teqoired reeiet.
= 12.6 X
.1«
_ 12.6 X
M
.1024
=1.28 ohms (approx.)
Ans. 1.28 ohmB,
5pedfk Resistance. The specific resistance of a substance
is the .resistance of a portion of tliat substance of unit length and
unit erosS'Section at a standard temperature. The units commonly
used are the centimeter or the inc^, and the temperature that of
melting ice. The specific resistance may therefore be said to b^
the resistance (usually stated in microhfns) of a centimeter cube or
of an inch cube at the temperature of melting ice. If the specific
- nsistances of two substftncea are known then their relative resist-
ance is given by the ratio of the specific resistances.
Conductivity is the reciprocal of specific resistance.
Example. — 'A certain copper wire at the temperature of
melting ice has a resistance of 29.7 ohms. Its specific resistance
(resistance of 1 centimeter cube in' microhms^ is 1.594, and that of
platinum is 9.082. What would be the resistance of a platinum
wire of the same size and length of the copper wire, and at the
tame temperature ?
Solution. — The resistance would be in direct ratio qt the
specific reeistancee, and we have the proportion :
required resist. : 29.7 : : 9.032 : 1.694
Heooe, r^red resist. = 29.7 X ^'^^^
.594
s 168. ohms (appros.)
Ans. 168. ohms.
Colculatloa ot Resistance. From the preceding pages it is
evident that resistance varies directl^r as the length, inversely ar
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fe THE ei;kctbio current.
the croas-sectional area, anJ depends upon iho specific resistance
of the materiaL This may be espreesed convenieDtly by tho
formula,
in which A is the resiatance, L the length of the conductor, A the
area of its cross section, and » the specific reeiHtaiu^n of the
material.
Example. — A telegraph relay is wound with l.bOO feet of
wire -OtO inch in diameter, and has a resistance of 150 ohms.
What will be its resistance if wound witii 400 feet of wire .022
inch in diameter?
Solution. — If the wires were of equal length, we sbonld
have the proportion,
Required resistance : 150 : : (.010)* ; (.022)'
or, Required resistance = 150 X \naaii = S0.99-{- ohms.
For a wire 400 feet long, we have, therefore, by direct proportion.
Required resistance = fgOO^ ^**'^^ ~ 6.88+.
Ans. 6.88+ ohms.
If a ciivait is made np of several different materials joined in
series with each other, the resistance of the circuit is equal to the
num of the resistances of its several parts. In calculating the
resistance of such a circuit, the resistance of each part should fiist
be calculated, and the sum of these resistances will be the total
resistance of the circuit.
The table on pf^ 9 ^ves the resistance of chemically pore
substances at 0" Centigrade or 32° Fahrenheit in International
ohms. The first column of numbers gives the relative resistances
when that of annealed silver is taken as unity. For example, mer-
cury has 62.73 times the resistance of annealed silver. The
second and third columns give the resistances of a foot of wire
^01 inch in diameter, and of a meter of wire 1 milhmeter in
diameter, respectively. The fourth and fifth • rolumns give
lespectively the resistance in microhms of a cuHc inch and cubic
oentimeter, that is, the specific resistances.
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THE EI.EOTRIO CURRENT.
lUlc Showing Relative ReslsUiica of Ctacmfcally Pare SubstAoces «
Thirty-two Degrees Fahrenheit in Interaetional Ohnu.
BcaliUnM
BMljUuMln
of » win
ol Birln
Mlorohnu.
.001 luoh 'a
CnblD
Csbla0a»
dtunMcr.
IQ dluiocet.
Inch.
tlmeUr.
Silrer, annealed.
1.000
9.02S
.01911
.6904
1.600
Copper, annealed.
1.06S
9.S85
.02028
.6274
1.594
Silver, hani dravn.
1.086
9,802
.02074
.6416
1.629
Copper, hard diawn.
Gold, annealed.
1.086
9808
.02076
.6415
1.629
1.369
12.86
.02618
.8079
2.062
Gold.hard drawn.
1.S9S
12.66
.02661
.8224
2.088
Alnminnm, annealed
1.986
17.48
.08700
1.144
2.904
Zinc, pressed.
8.741
88.76
.07143
2.209
6.610
PUtinnm, annealed.
6.022
64.84
.1150
3.555
9.082
Iron, annealed.
6.460
58.29
.1284
8.814
9.689
Lead, pressed.
18.05
117.7
.2491
7.706
19.58
Qerman silver.
18.92
126.6
.2659
8.217
20.87
Platinnm-sUver alloy
(J platinnm, J silver.)
16.21
146.8
.8097
9576
24.32
Mercniy.
62.73
670.7
1.208
87.05
94.06
It should be noted that the resistances in the above table are
for chemically pnre subataQces, and also at 82° Fahrenheit. A
very small portion of foreign matter mixed with a metal greatly
increases its resistance. An alloy of two or more metals always
has a higher specific resistance than that of any of ita OODStitnents.
For example, the conductivity of silver mixed wiUi 1.2 per cent
in volume of gold, will be 69 when that of pore silver is taken as
100. Annealing reduces the resistance of metals.
The following examples are given to illustrate the use of the
table above in connection with the formula at die top of page 8,
ind to show the application of preceding laws.
Example. — ^From the specific resistance of annealed alu-
minum OS given in the next to the last column of the table,
calculate the resistance given in the second column of figures for
tliat substance.
BolntiOD. — The resiBtance In microhms of a cubic inch of
annealed aluminum at 32° F. is 1.144, which is equal to
.000001144 ohms. The resistance of a wire 1 foot long and .00'
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THE ELECTRIC CURRENT.
inch in diameter ia required. In the formola on page 8, ve
have • = .000001144, L=l foot = 12 inches and
^ ^ ^ _ 8.1416 X. 001' ^ .0000007854 sq. in.
4 4
Snhstitating these values in the formula.
we have.
R = .000001144 X ^^^^?! „«.
.0000007854
= 17.48 ohms. Ans. 17.48 ohms.
Example. — The reaietance in miorobmB of a cuhic centimeter
of annealed platinum at 32° F. is 9.082. What is the resistance
of a wire of the same substance one meter long and one miUimeter
in diameter at the same temperature ?
Solution. — In the formula for resistance we have the qnan*
lities » = 9.032 microhms = .000009032 ohms ; X = 1 meter s
100 ceutimeteis ; and
^ ^ ^ ^ i.imx.1' ^ .00,854 3,. „^
Hie diameter being equal to 1 millimeter « .1 cm.
Substitating these values we have,
B = .momnx-^
B .1150 ohms. Ans. .115 ohms.
Example. — From the table the resistance of 1 ft. of pure
annealed silver wire .001 inch in diameter at 82° F. is 9.02S
ohms. What ia the resistance of a mile of wire of the same sub-
stance .1 inch in diameter at that temperature?
Solution. — As the resistance of wires is directly proportional
to their length and inversely proportional to the squares of their
diameters, the required resistance is found by multiplying ihe
resistance per foot by 5,280 and the product by the inverse
squares of the diameters.
Therefore J2 = 9.028 X 6280 X I ^ j '
*= 4.76 ohms fapprox.)
Ans. 4.76 ohma.
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THE ELECTBIC CDBKENT. 11
Example. — A mile ftod one-half of an annealed wire of pare
iron haB a resistance of 46.1 ohms. What would bo the resist-
aoce of bard drawn wire of pnre copper of the same length and
diameter, as&nming each to he at the temperature of melting ice?
SolatioD. — The only factor involved by this example is the
relative resistance of the two metals. From the table, page 9,
annealed iron has 6.460 and hard-drawn copper 1.086 timea the
reaistance of annealed silver. Hence the resistance of the copper
is to tii&t of the iron as 1.086 ia to 6.460, and the required zesia^
anceis
5= 46.1 X ^^^ = 7.75 ohme (approi.)
Ana. 7.75 ohms.
Example. — If the resistance of a wire 7,428 feet long ia
18.7 ohms, what would be its reaiatance if its length were reduced
to 6,253 feet and its cross-section made one half again as large ?
Solution. — As resistance Is directly proportional to the
length, and inversely proportional to the area of the oroaa^eotioDt
tiu required resistance is
, 6258
^742g"^8"
Ans. 10.5 ohma.
Resistance Affected by. Heatlns. The resistance of metals
depends upon the temperature, and the resistance is increased by
heating. The heating of some substances, amcoig which is carbon,
causes a decrease in tlieir resistance. The resistance of the
filament of an incandescent lamp when lighted is only about half
as great as when cold. All tTietaUy however, have their resistance
increased by a rise in temperature. The percentage increase in
resistance with rise of temperatnre varies with the different
metals, and varies slightly for the same metal at different tern-
petatnres. The increase is practically uniform for most metals
throughout a considerable range of temperatnre. The resistance
of copper increases about .4 per cent, per degree Centigrade, or
about .22 per cent, per degree Fahrenheit. The percentage
increase in resistance for alloys is much less than for the mmple
metals. Standard resistance coils are therefore made of alloys, as
it is desirable that their resistance should be as nearly constant u
possible.
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THE ELECTRIC CURRENT.
The change in resistance of one ohm per degree lise in tem-
perature for a flubstance is called the temperature coeffieient for
that aubetance. The following table g^ves the temperature coeffi-
(dents for a few Bubetances.
TenPERATURE
COEFFICIENTS.
..m,^
loP.
M WBur BBinm
1=0.
Plutiiioid
.00012
.00022
PUtioani'Silver
.00014
.00026
German silver
.00022
.00040
Platmom
.0018
.0086
SilTer
.0021
.0088
Copper, alnminom
.0022
.0040
Iron
.0026
.0046
If the resistance of a conductor at a certain temperature is
known, the resistance the conductor will hare at a higher tem-
perature may be found by multiplying the temperature coefGcient
for the substance, by the nnmber of degrees increase and by the
resistance at the lower temperature, and adding to this result the
resistance at the lower temperature. The product of the temper-
ature coefficient by the niunber of degrees increase gives the in-
crease in reustance of one ohm through that number of degrees,
and multiplying this by the number of ohms gives the increase in
resistance for the conductor. The r^ult obtained is practically
correct for moderate ranges of temperature.
The above method of calculating the resistance of conductors
at increased temperaitnres is conveniently expressed by the follow*
ing formula t
where R^ is the resistance at the higher temperature, Rj that at
the lower temperature, a the tempentore coefficient for the sub-
stance and t the number of degrees change.
From the preceeding formula it follows that if the resistance
•t the higher temperature is known, that at the lower temperature
will be idven by the formula ;
R ^
Dignzedoy Google
-So ,2
Is: I
Pi
i
D,Bi:ized3,GOOgIe
3,GoogIe
THE ELBCTRIC CUKRENT. n
In ooloulatitig resiatanoes at different tempemtnree, the tem-
perature coefficient based on the Fahrenheit scale should be used
if the number of degrees change is given in degrees Fahrenheit,
and that based on the Centigrade scale if given in degrees
Centigrade.
Example. — The resistance of a coil of German silver wire at
12° C. is 1304 ohma. What would be its resistance at a tempeis
ature of 60" C?
Solution. — From the statement of the example Jt^ = 180^
« =s 60 — 12 = 48, and from the table page 12, a = .0004.
Substituting these ^ues in the first of the preceding formulas ws
IwTe*
J2j = 1804 (1 + .0004 X 48)
a. 1304 X 1.0192
H 1829 ohms (approz.)
Ane. 1829 ohms.
Example. — If the resistance of a copper conductor at 95' F.
is 48.2 ohms, what would be the resistance of the same oondactoi
8140' F.?
Soluticm. — In this case £, = 48.2, t = 95 — 40 = 55,
and from the table a = .0022. Substituting these values in tha
tormnla at the foot of page 12, we have,
H - 48.2 _ 48.2
» 1 + .002a X 55 TT2I
= 48. ohms (approx.)
Ans. 48 ohms.
The first table on pt^e 14 gives the resistanoe of the most
common sizes of copper vrire according to the American or Brown
and Sharpe (B. & S.) gauge. The resistance given is for pure
■aopper wire at a temperature of 75° F. or 24" C.
The first column gives the number of the wire, the second
the diameter in thousandths of an inch or mils, and the third the
diameter in millimeters, The fourth column gives the equivalent
nujnber of wires each one mil or one thousandth of an inch in
diameter. This is called the size of the wire in circular mils and
is equal to the square of the diameter in mils. The fifth column
gives the ohms per thousand feet and the resistance per mile is
foond by multiplying these values by 6.28. Ordinar}' commercial
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14
THB ELECTRIC CURRENT.
copper has a coDductivity of aboat 95 to 97 per cent, of that of
pure copper. The resistanca of commercial wire is therefore about
8 to 5 per cent, greater than the values given in the table. The
resistance for auy metal other than copper may be found by mul-
tiplying the resistance given in the table by the ratio of the spec-
ific reaistanc6 of the given metal to the specific resistance of
copper.
American Wire Oause (B. & 5.)
Ohmi
No.
Ho.
Clnmlar
Mill.
HIl*.
Ulnlm.
Mill.
MUllm.
iXV
US«.D
UM.1
M
».U
9.9
109).S»
The followii^ table gives the size of the English or Birming-
ham wire gauge. The B. & S. is however muoh more frequently
used in this country. The Brown and Sharpe gauge is a littla
smaller than the Birmingham for corresponding numbers.
Stubs'
w. a.)
KOb
DIun«t.rlD
DluneMr In
DluietHlji
WU.
Mm...
HUa.
Hn„n..
K,,..
UlUlm.
0000
454
11.53
8
165
4.19
18
49
1.24
00
880
9.65
10
134
8.40
20
35
0.89
I
800
7.62
12
109
2.77
24
22
0.55
4
238
6.04
14
88
2.11
SO
12
0.31
<
208
6.16
16
65
1.65
86
4
0.10
3,GoogIe
THE ELECTRIC CURRENT. 15
eXAilPLES FOR PRACTICE.
1. What is the resistance of an annealed Bilrer wire 90 feet
long and .2 inch in diameter at 32° F.? Ana. .02-{- ohm.
2. What is the resistance of 300 meters of annealed iron
wire 4 millimeters in diameter -when at a temperature of 0° C?
Ans. 2.314- ohms.
3. What is the TOQistancu of 2 miles of No. 27 (B. & S.) pore
copper wire at 75° F.? Ana. 665.+ ohms.
4. The resistance of a piece of copper wire at 32'*F. is 3
ohms. What is its resistance at 4d°F.? Ana. 8.114- ohms.
6. The resistance of a copper wire at 52°F. is 7 ohms.
What is its resistance at 82°F.? Ana. 6.70+ ohnu.
6. What is the resistance of 496 ft. of No. 10 (B. & S.)
pure copper wire at 45*^.? Ans. .483+ ohms.
On pages 16 and 17 is given a table disclosing among other
data the resistance of various primary cells. The resistance of a
circuit of which a hattery forms a part, ia made up of the external
resistance, or the resistance of outside wires and connections, and
the internal reaistance, or the resistance of the batteiy itself.
The table referred to ^ves in the first column the name of the
cell. In the second and third column a^^peara the name of the
anode and kathode respectively. These terms are commonly
Qsed with reference to electrolysis but may also be applied to
primary cells. The current passes from the anode to the kathode
thniugh the cell, and therefore with reference to the cell itself,
the anode m»y be considered the positive element and the hath-
ode the negative element. In regard to tlie outside circuit how-
ever, tlie current passes of course, from the kathode to the anode,
and hence with reference to the outside circuit the kaUiode is
positive and the anode negative; ordinarily the external circuit
is considered. As the anode of almost all primary cells is zinc it
may readily be remembered that the current passes from the other
element to the zinc tlirougli the external circuit. The fourth and
fifth columns of tlie table give the excitant and depolarizer respec
lively. The sixth column gives the E. M. F. of each cell when it
is supplying no current, and tJbe last column gives the internal
resistance in ohms<
oy Google
THE ELECTRIC CUBRENT.
TABLE IN RELATION TO PRiflARY CELLS, ELECTRO-
nOTIVE FORCE, RESISTANCE, ETC.
Pl*tlDlMd
(Oreaet)
two fluid
ChAptroii
Dudell
rilaldln-
EirMJn-
to,«to.)
talk Ana
ZlBC
Qn^Ite
OnphlU
(Cubon)
alphorfo Aoi
{a, BO.)
SolDtlon ot
olphnrlc Aoi
Th.boj
Bolntlon ot
llphartc Add
EH. 60.)
SktnntBd Sola-
slnm Dtahro-
SnlphDiia Acid
Bnlphnrlo JlcU
'"—e (H, BO,;
BalpbiiTio Add
Dtchrom*ts
(E, C[, O,}
Nltrlo Add
0.1 to 0.11
B OJiatoO.U
Oanctio PoU«h
or PoUMiam
Hrdmta (EOH}
Zinc Chlorida
(ZnCl,)
AmmoDlnm
Chloride
(KB. CI)
Sod lam &PoU»
■lam Chlot»t««
[N«C10,+
KOIO.^
Chloride
(NH. CI) In
C^dnm Sal-
phftte (C»80.)
phB^ «
lto»
O.ltoOJ
Snlpbarlo Add
Puts of Solph-
-'m of MercQTT
(Hg. 80.)
pii«ta(0d80U
DignzedoyGOOgle
THB BLBCTRIC CURRENT,
bMSiol
anpblt«
OfaphlU
FlftUnlim
Bnlpharlo knd
OhnnnleAeldi,
dllntB mixed
IftTtMUl]
OBiutlo Potash
or Potawlom
Hydrate S.OH)
Uarauraoa
Obloridi
(B8.0W
Ohiond* at
BilTBT
(AbOI)
* At IS degioM OentlgiadB or 19 degroM Fkbrenhelk
ResfiUacM in last oolamn meaanred in cells standing 6" x 4*
OHH'S LAW^
One of the moat important and moat nsed laws of eleotri<
dly la that first fonnolated hy Dr. O. S. Ohm, and knova u
Ohm's law. Thia law is as follows :
Ihs current fa directly proportional to t/u «iee»'omottw font
and inveriely proportional to the regittance
That is, if the electromotive force applied to h circmt is \n
creased, the ourrent will be inci'easec) in the same pi-opurtioii, and
if the resistance of a circuit is increased then the current wiU 1m
decreased propertjonally. Likewise a decrease 'm the eleotTomotive
force canees a proportional decrease la current and a decrease in
reBiBtance causes a proportional increaee in current. The current
depends only npon the electromotive force and resistance and in
the manner expressed by the above simple law. The law may be
expressed algebraically as follows:
electromotive force
current vanes as •
THB i stance
oy Google
18 THE ELECTRIC CURRENT.
The units of these quantities, the ampere, volt and ohm, have
been so chosen that an electromotive force of 1 volt applied to a
resistance of 1 ohm, causes 1 ampere of current to flow. Ohm's
law may therefore be expressed by the following equation;
where /is the current in amperes, E the electromotive force in
volts and R the resistance in ohms.
It is therefore evident, that if the electromotive force and
resistance are known the current may be found, or if any two of
the three quantities are known the third may be found. If the
current and resistance are known the electromotive force may be
found from the formula:
E = RI
and if the current and electromotive force are known, the resist-
ance may be found from the formula:
Simple Applications. The following examples are given to
illustrate the simplest applications of Ohm's law.
Example. — If the E.M.F. applied to a circuit is 4 volts and
its resistance is 3 ohms, what current will flow ?
Solution. — By the formula for current,
/ ^' * 9
/ = "^ = "y- = * amperes.
Ans. % amperes.
Example. — What voltage is necessary to cause a current of
23 amperes to flow through a resistance of 820 ohms?
Solution. — By the formula for E.M.F.,
^ = 7i;/= 820 X 23 = 18,860 volts.
Ans. 18,860 volts.
Example. — The E.M.F, applied to a circuit is 110 volts, and
it is desired to obtain a current of .6 ampere. What should be
the resistance of the circuit ?
Solution. — By the formula for resistance,
E 110
Ji = -w- = —^ = 183, 4- ohms.
Ans. 183 ohms.
oy Google
THE ELECTrtlC CURRENT. 10
Series Circuits. A circuit made ap of several parte all
joined in series with each other, ia called a series cirouit and the
resistance of the entire circuit is of course the sum of the separate
resistances. In calculating the current in such a circuit the
total resistance must first be obtained, and the current ma,y then
be found by dividing the applied or total E.M.F. by the total
reuBtanoe. This is expressed by the formula,
/= *
^1 T- -^a + 'Jg + etc.
Example. — Three resistance coils are connected in series with
each other and have a resistance of 8, 4 and 17 ohms respectively.
What current will flow if the E.M.F. of the circuit is 54 volts?
Solution. — By the preceding formula,
j__ E 54
Ji, + Jtt-\-Ji, 8 + 4 + 17 ■
Ans. 1.8+ amperes.
Example. — Six arc lamps, each having a resistance of 5
ohms, are connected in series with each other and the resistanoe
of tbe connecting wirtm and other apparatus is 3.7 ohms. What
must be the pressure of the circuit to give a desired current of 9,6
amperes?
Solution. — Tbe total resistance of the circuit is .B = (6 X 5)
+ 8.7 = 83.7 ohms and the current is to be J= 9.6 amperes,
Hence by the formula for E.M.F.,
E:= R 1= 33.7 X 9.6 = 828+ volts.
Ans. 323.+ volts.
Example. — The ourrent passing in a certain circuit was 12
amperes and the E.M.F. was 748 volta. The circuit was made up
trf 4 sections all connected in series, and the resistanoe of three
sections was 16, 9 and 26 ohms respectively. What was the
reustance of the fourth section?
Solution. — Let x =• the resistance of the fourth section, then
JJ = 16 + 9 + 26 +ar=. 51 +a^/= 12. and ^= 742, By
the formula for resistance,
Ji « ^ or, 61 + * = ^= 61'» ohms (appros.)
oy Google
so THB ELECTRIO CURRENT.
I£ 61 -f- ir ^ 61.9 WB have, by laranspoBing 61 to the other
side of the equation,
a> s> 61.9 — 51 £= 10.9 ohms.
Ana. 10.9 ohms.
Example. — A ounent of 54 amperes flowed through a circuit
when tii9 E.M.F. waa 220 volts. What leeistanoe shotild be
added in series with tha circuit to reduce the otinent to 19
amperes?
Solatioo. — The resistance in the first case was.
The resistance in the second mast be,
B = ^ = 11.58 ohms (approx.)
The required resistance to insert in the circuit is the differ*
enoe of these two resistances, or 11.58 — 4.07 = 7.51 ohms.
Ans. 7.51 ohma.
Pall of Poteatlal in a Circuit. Fig. 1 illustrates a series
oircnit in which the resistances A, S, C, D and H are connected
tn series with each other and with the source of electricity.
^ the B. M-. F, is known, the current may be found by divid-
JDg the E, M. F. by the sum of all the reeistanoes. Ohm's law
may, however, be applied to any part of a circuit separately,
as well as to the complete circuit. Suppose the resistances of
A, By O, D and E are 4, S, 6, 8 and 4 ohms respectively, and
assume that the source has no resistance. Suppose the current
flowing to be 12 amperes. The E. M. F. necessary to force a
current of 12 amperes through the resistance >4 of 4 ohms is, by
applying Ohm's law, equal to J?b£7=4X 12 = 48 volts.
Hence between the points a and h outside of the resistance A,
there must be a difference of potential of 48 volts to force the
current through this resistance. Also to force the same current
through .5, the voltage necessary is 8 X 12 ^ 86. Similarly, for
each part (7, D vaA E, there are required 72, 86 and 48 volt«
respectively. *
As 48 volts are necessary for part A and 86 volts for part S,
it is evident that to force the current through both parts a diffeiv
euce of potential of 48 4* ^^ ~ ^^ volts is required ; that is, tbe
oy Google
THE ELECTMC CURRENT. SI
roltage between the points a and e must be 84 volts. For Uie
three parts A, B and 0, 48 -|- 36 -|- 72 = 156 volts are aeceeaaiy,
and for the entire circuit, 240 volts must be applied to give the
current of 12 amperes. From the above it is evident that there
is a gradual fall of potential throughout the circuit, and if the
voltage between any two points of the circuit be measured, tlie
E.M. F, obtained would depend upon the resistance included
between these two points. For example, the voltage between
points b and d wonld be found to be 72 + 36 ^ 108 volts, or
between d and e 36, volts, etc. From the preceding it is apparent
that the fall of potential in a part of a circuit is equal to the
correut multiplied by the rnsistance of that part.
C
Fig. 1.
This gradual fall of potential, or drop as It is commonl}
called, throughout a circuit, enters into the calculations for the
size of oonductoi-s or mains supplying current to distant points.
The resiatancefi of tlie conductors cause a certain drop in trans-
mitting the current, depending upon their size and length, and it
is therefore necessary that the voltage of machines at the supply
station shall be great enough to give the voltage necessary at the
receiving stations as well as the additional voltage lost in the
conducting mains.
For example, in Fig. 1 the voltage necessary between the
points e and b is 144 volts, but to give this voltage the source
must supply in addition the volti^e lost in parts A and J?, which
equals 96 volts.
Example. — The voltage required by 17 arc lamps connected
in series is 782 volts and the current is 6.6 amperes. The leauit'
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22 THE ELECTRIC CURKENT.
anoe of the connecting wires is 7 ohms. What must be the-
E. M. F. applied to the circuit?
Solution. — The drop in the connecting wires ia ^= R I^=
7 X 6.6 = 46.2 volts. The E. M. F. necessary is therefore 782
+- 46.2 = 828.2 volts. Ans. 828.2 volts.
Example. — The source of E. M. F. supplies 114 volts to a
circuit made up of incandescent lamps and conducting wires.
The lamps require a voltage of 110 at their terminals, and take a
CDrrent of 12 amperes. What should be the resistance of the
conducting wires in order that the lamps will receive the necessary
voltage ?
Solution. — The allowable drop in the conducting wires ia
114 — 110 => 4 Tolte. The current to pass through the wires is
12 amperes. Hence the resistance must be
JJ = ^=^ = .88 + ohins.
Ans. .8S -t- ounu.
Divided Circuits. When a circuit divides into two or more
partSt it is called a divided circuit and each part will transmit a
portion of the current.
Sach a circuit is illustrated in Fig. 2, the two branches being
represented by h and e. The current passes fmm the positive
pole of the battery through a and then divides ; part of the
current passing through b and part through e. The current then
unites ar.d passes through d to the negative pole of the battery.
The part e may be considered as the main part of the circuit and
h & as a by-pass about it. A branch
which serves as a by>pas8 to an-
other circuit is called a thuiU
circuit, and the two branches are
said to be connected in parallel.
In considering the \
Fig. 3. of a current through a circuit of
this sort, it may be necessary to determine bow much current
will pass through one branch and how much through the other.
Evidently this will depend upon the relative resistance of the two
branches, and more current will pass through the branch offering
the lesser resistance than through the branch having the higbei
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THE ELECTRIC CURRENT. 28
resistance. If the two parts have equal resistances, tben one half
of the total current will pass through each branch. If one branch
has twice the resistance of the other, then onlj one-half as much
of the total current will pass through that branch as through the
other ; that is, ^ of the total current will pass through the first
branch and the remaining } will pass through the second.
The relative stretch of current in the two branches will he
invereely proportional to their renstancet, or directly proportionat
to their conductances.
Suppose the resistance of one branch of a divided circuit is
r^ (see Fig. 3), and that of the other is r,. Then by the pre-
ceding law,
current in r^ : current in r, : : r^ ! r^
Also,
current in r, j total current '••r^;r^+r^ '
and
current in r^ : total current '•• fi '■ fi+f^
Let /represent the total current, t, the jurrent through the
resistance r, and I'j the current through the resistance r^. Then
the two preceding proportions are expressed by the following
formulas :
/f.
and
.• - ^•■i
Example. — The total current passing in a oircnit is 24
amperes. The circuit divides int» two branches having resist-
ances of 5 and 7 ohms respectively. What is the current in each
branch ?
Solution. — In this case /= 24, r, ^ 5 and r^ = 7. Sub-
Btituting these values in the above formulas we have,
= 14 amperes.
_ Ir, _ 24 X 7 _
' •■i+'-a 5+7
Jr. 24 X 5 irt
. = 1 — = C-^^ = 10 amperes.
i+r, 5 + 7
. ( In 5 ohm branch, 14 amperes.
■ I In 7 ohm branch, 10 amperes.
Joint Resistance ot Divided Circuits- As % divided circuit
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M THB ELECTRIC CURRENT.
offers tTTD paths to tlie current, it follows that Hie joint reaistanoe
of the two branches will be less than the resistance of either branch
alone. The ability of a circuit to conduct electricity is repre-
Bented by its conductance, which is the reciprocal of resistance ;
and the conductance of a divided circuit is equal to the sum of
the conductances of its parts.
For example, in Fig. 8, the conductance of t^e upper branch
equals — and that of the lower branch equals — , If R repro-
r, Tj
Bents the joint resistance of the two ■parts then the joint conduot-
ance equals!
1 _ 1 ■ 1 ^ >-i+>-a
J2 r, r, f,rj
Having thus obtained tiie joint conductance, the joint resists
anoe is found by taking the reciprocal of the conductance, that ia,
This formula may be stated as follows i
The joint renitanefi of a divided circuit la equal to thsproiuet
(f the too teparate reaittaneea divided by their avm.
<^
Big. S.
For example, suppose the resistance of each branch to be
2 ohms. The conductance of the cii-cuit will be,
-— = ~g- -^ -J- E> 1, and hence £ = 1 ohm.
Also by tiie preceding formula,
B=l^l = l ohm.
2 + 2
The resistance of a divided circuit in which each branch has
a leustanoe of 2 ohms is therefore 1 ohm.
Example. — The resistances of two separate conductors are 8
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THE ELECTRIC CURRENT. 86
and 7 ohms reapectiTety. What would be their joint reaistanoe
if oonnected in parallel ?
Solution. — In this ease r, = 8 and r, x 7, hence hy the
formtda,
R =. ^^3 = 2.1 ohms. Ans. 2.t ohmi.
3 + 7
Suppose, as illustrated in Fig^. 4, the conductors haTii^
cesiBtanoeB equal to r,, r, and r^ respecUvelj, are connected in
Fig. 4.
parallel. The joint total oonductance will then be equal to,
2. = -L -L ^ -L ^ ~ '•a fa +'"1^8+ '•!'•>
R ri Tj rj »'i''j»"a
and aa the joint resiatance is the reciprocal of the joint condaofc*
ance, the joint resistance R of the three branches is expressed by
the formula,
'•» ''a + ^ J-s + n »■«
Example. — What ia the joint resistance when eoonected in
parallel, of three wires whose respective resistances are 41, 62 and
29 ohms respectively?
Solution. — Id this case rj a> 41, f, =« 52 and r, s 39.
Hence, by the preceding formula,
R = 41X52x^^ J2.8 + ohms.
fiSx 29 + 41 X 29 + 41x62 T""™.
Ans. '2.8 + ohms.
In general, for any number of conductora oonnected in
parallel, the joint resistance is found by taking the reciprooal ol
the sum of the reciprocals of the separate resistances.
Example. — A circuit is made up of five wires connected in
parallel, and their separate resistances are respectively 12, 21. 2ft,
8 and 42 ohms. What is the joint resistance?
DignzedoyGOOglC
26 THE ELECTRIC CITRRENT.
Solation, — The Bum of the oonduotances Is:
1 + 1 4-_L-t-J_ + l= 58
12^21^28 ^8 ^42 165
Hence the joint reslBtance equals :
iJ = ^ = 3.1 + ohms. Ana. 8.1 + ohmB.
If the resistance of each branch is known and also the poten-
tial difference between tbe points of union, then the current in
each branch may be found by applying Ohm's law to each branch
eeparatelj. For example, if thia potential difference were 96
Tolte, and the separate resistances of the 4 branches were 8, 24, 3
and 48 ohms respectively, then the current in the respective
branches would be 12, 4, 32 and 2 amperes respectively.
If the current in each branch is known and also the poten-
tial difference between the points of union, then the resistance of
each branch may likewise bf I'ound from Ohm's law.
The following examples are given to illustrate the applioa-
tion of the preceding principles.
EXAMPLES FOR PRACTICE.
1. Two conductors liaving resistances of 71 and 19 obmr
respectively are connected in parallel, and the total current pajss
jng in the circuit is 37 amperes. What current passes in the
conductor whose I'esistance is 71 ohms? Ans. 7.S-{- amperes.
2. What is the joint resistance of two wires connected in
parallel if their separate resistances are 2 and 8 ohms respectively?
Ans. 1.6 ohnu.
8. What is the joint resistance of three wires when con-
nected in parallel, whose separate resistances are 5, 7 and 9 ohms
respectively? Ans. 2,2 -j- ohms.
4. Three wires, the respective resistances of which are 8, 10
and 20 ohms, are joined in parallel. What is their joint resist-
ance ? Ans. 3.6 + ohms.
5. Four wires are joined in parallel, and their separate
resistances are 2, 4, 6 and 9 ohms respectively. What is the
joint resistance of the conductor thus formed?
Ans. .97 + ohm.
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THE EliECTEIC CDREENT. 27
Battery Circuits. Fig. 5 illastrateB a simple circuit having
a single cell C connected in series with a resistance. This is the
customary manner of representing a cell, the short, heavy line
representing the ziu£ and the long light line representing the
copper or carbon plate. In determining the amount of current
which will flow in such a circuit, the total resistance of the circuit
must he considered. This is made up of the external resistance
R and the internal resistance r, or the resistance of the cell itself.
If E represents the total E.M.F. of the cell, then the earrent
/ which will flow is expressed by the formula,
-^
It has been shown that whenever a current passes through
any resistance, there is always a certain drop or fall of potential.
The total E.M.F. above referred to, expresses the total poten-
tial difference between the plates of the cell and is the E.M.F.
of thecellon «^«»circnit. When
the current flows, however, there
is a fall of potential or loss of
voltage within the cell itself,
and hence the E.M.F. of the
cell on closed circuit is less than
on open circuit. That is, if the
voltage l>e measured when the
cell is supplying current, it will be found to be less than when
the volt^e is measured on open circuit, or when the cell is sup-
plying no current. The voltage on cloyed circuit is that available
for the external circuit, and is therefore called the external or
availalle voltage or E.M.F.
The external E.M.F. depends of course upon the strength
of current the cell is supplying, and may be calculated as
follows:
If the current passing is I and the resistance of the cell is
7; then from Ohm's law the voltage lost in the cell equals r I. If
E represents the total E.M,F. of the cell and ^, .the external
E.M.F., then,
E^ = E — rI
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THE ELECfTBIC CURRENT.
> the total E. M. F.
Pig. 6.
The E. M. F. of a cell is anderatood to
DDlesB otherwise stated.
When two or more cella are interconnected thej are said to
form a battery.
Fig. 6 illastrates three cells connected in series with each
other and with the external circuit. That is, the positive terminal
of one cell is connected to the negative of the next, and the posi-
tive of that cell to the negative of the adjacent, etc. By thia
method of connecting, the E. M. F. of each cell is added to that
of the others, so that the total E. M. F, of the circuit is three
times that of a single cell. If one of the cells were connected so
that its E. M. F. opposed that of
the other two, it won Id offset
the £. M. F. of one of the cells
and the resoltaot E. H. F. would
be that of a single cell. The con-
necting of cells in series as in
Fig. 6 not only increases the E. M, F. of the circuit bnt also
increases the internal resistance, the resistance of each cell being
added to that of the others. If F eqnals the E. M. F. of each
cell, r the internal resistance of each and H the external resist-
ance, then the current that will flow is expressed by the formala,
r for n. cells connected in series the formula for current is,
R + nr
Fig. 7 illustrates two cells connected in parallel, and sup-
plying current to an ex-
I temal circuit. Here the
two positive terminals are
connected with each other
and also the two negative.
The E. M. F. supplied to
the circuit is equal to that
of a single cell only. In
equivalent to enlarging the
r=-
Fig. 7.
fact connecting cells in parall<.K
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3,GoogIe
or* "
I
D,Bi:ized3,GOOgIe
THE ELECTRIC CURRENT. 29
plates, and the only effect is to decrease the iaternal resistance
It is evident that coupling two cella in p^xallel affords two
paths for the current and bo decreases the resistance of the two
cells to one-half that of a single cell. The formala express-
ing the current that would flow in the external cireait wiUi two
cells in parallel is therefore,
or for n cells connected in parallel, the formala for current is,
Fig. 8 represents a combination of the aeries and parallel
method of connecting and represents four files of cells joined in
parallel and each file having four cells connected in series. The
£. M. F. of each file and consequently of the circnit is 4 B, The
resistance of each file is 4 r and that of all the files -
die formala tor current is,
Hence.
If there were n files connected In parallel and m cella were
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THE BLECTTRIC dTRKENT.
connected in aeries in each file, the fonnola e^treanng tlie ourteut
in the external cirouit would he,
Jt + '!^
where J? is the E. M. F. of each cell, R the eztemal re^tanoe,
and r the internal resistance of each cell.
The most advant^^us metibod of oonuecting cells depends
upon the results desired, the resistance of the cell and the external
teaiBtanoe. Suppose it is desired to pass a current through as
eztemal resistance of 2 ohms, and that Daniell's cells are to be
nsed each having an B. M. F. of 1 volt and an internal resistance
of S ohms.
With one cell only in circuit, the current will be,
and with 6 oells all in series the current will be,
Therefore with 5 cells iu series the current is only .1 ampere
greater than with a single cell, and with 100 cells in series the
onrrent is only,
100^ _ 100 _ Bft «
^Tioo-r=5TW = -*'^""P^
Hence with a oomparatiTely low eztemal resistance, there is bat
little gfun in onrrent strength by the addition of cells in series.
This is due to the fact that, although the E. M. F. is increased
1 Tolt by each cell, the resistance is increased by 8 ohms.
Now suppose 5 Daniell cells to be connected in parallel with
the external oiicuic of 2 ohms. The E. M, F. of the circuit will
then be that of a single cell and the current will be,
= .4 ampere (neariy),
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THE ELEOTRIO OURRENT. 81
and with 100 cells oonnected in parallel tiie oonent will be,
= jj- = .5 ampere (nearly).
A lai^r current is therefore obtained in this case t^ oonneofc-
ing the cells in parallel than by connecting them in series.
With a laige eztemal resistance on the other hand, a lai^r
current is obtained by connecting the cells in series. For example*
suppose the estemal resistance to be 500 ohms. One cell wiU
tlien give a current of .00198 -(- ampere, and 5 cells in series Till
give about .0097 ampere, whereas 100 cells will give .125 ampere.
With 5 cells connected in parallel the current will be .00199 -{-
ampere, and with 100 cells the current will amount to approzi*
mately .002 ampere. With an external resistance of 600 ohms,
there is practically no advaoti^ in connecting the cells in paralleL
The only effect of the latter method is to decrease the internal
resistance which is almost negligible in comparison with the
external reustance.
It may be shown mathematically that for a given external
reBistance and a given number of cells, the largest current is
obtained when the internal resistance is equal to the external
resistance. In order to obtain this result the values of m and n
in the formula on p^e 80, should be ao chosen that —— eqnalB
S. This arrangement, although g^vii^ the largest current
strength, is not the most economical. With the internal resist-
ance equal to the external resistance there is just as much energy
used up in the battery itself as is expended nsefully in the external
cirouit.
In order to obiain the most economical arrangement, the
internal resistance should be made as smalt as possible, that is,
all the cells should be connected in parallel. The loss of power
in t^e battery is then the smallest amount possible.
In order to obbun tbe quickest action of the current the cells
should be connected in series. When the external oiroait possesses
considerable self-indaction, as is the case when electrom^nets are
connected in the circuit, the action of the enrrent is retarded.
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H THE ELECTRIC CITRRENT.
This retardation may be deoioaaed by faaTJn^ a high interna?
resistanoe, which is obtained by connectnng the cells in series.
Example. — Sixteen cells, each having an internal resistance oi
.1 ohm are to be connected with a circuit whose resistance is ,4
ohm. How should the cells be connected to obtain the greatest
current?
Solation. — Here the external resistance R, equals .4 ohm and
the resistance r of each cell equals .1 ohm. For mazimoiD
current,
Vll^R, or l^^A
H n
Therefore, m a 4 n
and as nt n = 16, the only values of m and n which will he tme
for both of these equations are m = 8 and n =:: 2. Hence there
mast be 2 files of cells, with 8 cells in series in each file.
Ans. 2 files, 8 cells in each.
Example.-— The external resistance in a circnit is 4 ohms.
The celts used each have an E. M. F. of 1.2 volts and an internal
resistance of 3.8 ohms. If 20 cells were used, which metiiod of
oonnecfing would supply the larger current, — 5 files with 4 cells
in series, or 4 files with 5 cells in series ?
Ist Solution. — Applying the formula on page 80, we have
A = 4, £ => 1.2, r =a S.8 and with d files and 4 in series, m — 4
and n s 5. Hence, the current is,
mE 4 X 1.2
B + Vtl. 4 +
4X1
.681 + ampere.
With 4 dies and 5 cells in series, m = 5 and n 3
the onneDt is,
^ ^ ^-^ „ = .685 + ampere.
4
The lai^r current is therefore supplied by having 4 files
■ith 5 cells in series. Ana. 4 files, with 5 cells in series.
2nd Solution. — The masrimum current is supplied when the
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THE BLECTRIC CURRENT.
internal resistaDce equals the external resistance or wtien
With 6 files and 4 cells in series,
mr _ 4 X 8.8
= 8.04 ohms,
and with 4 files and 5 cells in aeries,
^ = i2iH = 4.75ohm^
» 4 V
The latter value is nearer to 4 ohms, which is the external resist-
ance, than is 3.04, hence the larger current will be supplied with
4 files and 5 cells in series. Ana. 4 files, with 5 cells in series.
Example. — It is desired to puss a current of .025 ampere
through an external resistAnce of 921 ohms. The cqHb are to be
connected in series and each has an E. M. F. of .8 volt and an
internal resistance of 1.3 ohms. What number of cells must be
iised?
Solution. — From p&ge 28, the general formula for cells in
series is,
,_ tiE
and m this case 1= .025, £= .8, £=921 and r = 1.8. Snbsti*
tuting these values gives,
.025 = "-^
921 +n 1.3
Multiplying hj 921 -1- 1.3 n gives
23.025 + .0825n = .8 n
Transposing .0825 n gives
^n— .0825 n=: 23.025
or .7675 n = 28.026
EXAnPLES FOR PRACTICE.
1. Tea cells in series have an E. M. F. of 1 volt each and
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84 TRK BLECTTRIt; CCTRBENT.
an internal resistance of .2 ohm. The external reststanoe is 8
ohms. What is the oarrent ? Ans. 2 amperes.
2. Sis cells, each of which has an E. M. F. of 1.2 Tolta and
a resistance of 2 ohnLS, are connected in parallel. With an external
resistance of 10 ohms, what is the current? Ans. .116 -f- ampere.
8. What is the current supplied by the same cells if joined
in series and the external resistance is 20 ohms?
Ans. .225 ampera.
4. A single cell whose E. M. F. on open circuit 181.41 volts
and whose internal resistance is .5 ohm is supplying a carreiit of
.8 ampere. What is the available E. M. F. of the cell?
Ans. 1.26 volts.
fi. What would be the available E. M. F. with 8 of the
oells referred to in example 4, when connected in series and sup-
plying the same current? Ans. 10.08 volts.
6. Eight Daniell cells (E. M. F. =: 1.05, resistance = 2.5
ohms each) are joined in series. Three wires A, B and Q of 9*
86 and 72 ohms resistance respectively are arranged to be connected
to the poles of the battery. Find the current when each wire is in*
serted separately, and when all three wires are connected in paralleL
Ans. Through A, .29 ampere nearly ; through By .15 ampere }
throt^h O*) .091 -{- ampere ; and through all three, .81 -{-ampere.
7. A battery of 28 Bunsen cells (E. M. F. = 1.8, resistanoo
^ .1 ohm each) are to supply current to a circuit having an
external resistance of 80 ohms. Kind the current (a) when all
the cells are joined in series, (ft) when all the cells are In parallel,
(«) when there are 2 files each having 14 cells in series, (d) when
there are 7 files each having 4 cells in series.
Ana. (a) 1.58 -(-; (&) .06 nearly; (o) .82-1-; (iQ .28 +
ampere.
QUANTITY, ENERQY AND POWER.
Quantity. The strength of a current is determined by tiu
amount of electricity which passes any cross section of the conduo*
tor in a second; that is, current strength expresses the rate at
vhich electricity is conducted. The quarUit}/ of electricity con-
Teyed evidently depends upon the current strength and the time
tin Minent continues.
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THB ELBCmaO CUimENT. K
The Coulomb. The coulomb is the unit of qttantit/ and is
equal to the amount of electricity which passes anj cross-seotioD
of tho coudaotor in one second when t^e current atrength is one
ampeio. If a current of one ampere flows for two seconds, the
quantity of electricity delivered is two coulombs, and if two
amperes flow for one second the quantity is also two coulombs.
With a current of four amperes flowing for three seconds, the
qnantity delivered is 12 coulombs. The quantity of electricity in
coulombs is therefore eqnal to the current strength in ampeiw
multiplied by tlie time in seconds, or
where Q %. the quantity in coulombs, I the current In amperes and
( tiie time in seconds.
The coulomb is also called the ampere-teeond. The quantity
at electricity delivered in one hour when the current is one
ampere is called one ampere-hour. The ampere-hour is equal to
8,600 coulombs, as it is equal to one ampere for 8,600 seconds.
E^m the formula Q = It, it follows that
Example. — A current of 18 amperes flows throngh a eireidt
lor 2^ hours. What quantity of electricity is delivered?
Solution. — Redudng 2} hours to seconds gives 8,100 secondly
and 8,100 X 18 = 145,800. Ans. 145,800 coulombs.
Example. — What is the strength of ouxrent when 1} ampere
hoars pass in a circuit in 89 seconds?
Solution. — One and one-half ampere-hours equal 5,400
coulombs and as cnrrent strength is expressed by quantity divided
by time, the current is 5,400 -j- 89 = 60. + amperes.
Ans. 60. + an^ienB.
EXAMPLES FOR PRACTIC&
1. How many coulombs are dehvered in 9 minutes, yrbea
die eurreat is 17| amperes? Ans. 9,450 coulombs.
2. What is the ooireut when 480 oonlombs are delivered
^r minate ? Am. 8 ampem.
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86 THE ELEUTlUC UUKKENT.
3. In what time will 72,000 coulombs be delirered when the
oiureDt is 80 amperes ? Ans. 1 5 minates.
4. How many ampere-houra pass in a circuit in 2| houts
when the current is 22 amperes? Ans. 60.5 ampere-hours.
Energy. WheoeTer a current flows, a certain amount of
eaei^y is expended, and this may be transformed into heat, or
mechaDical work, or may produce chemical changes. The uuit of
mechanical energy is ttie amount of work performed in raising a
m^s of one pound through a distance of oue foot, and is called
the foob-pound. The work done in raising any mass through any
height, is found by mfiltiplying the number of pounds in that mass
by the number of feet through which it ia lifted. Electrical work
may be determined in a correapondii^ manner by the amount of
electricity transferred through a difference of potential.
The Joule. The joule is the unit of electrical energy, and is
the work performed in transferring one coulomb through a differ-
ence of potential of one volt. That ia, the unit of electrical energy
is equal to the work performed in transferring a unit quantity of
electricity through a unit difference of potential. It is evident
that if 2 coulombs pass in a circuit and the difference of potential
J one volt, the energy expended ia 2 joules. Likewise if 1 cou-
lomb passes and the potential difference is 2 volts, then the enei^
expended is also 2 joules. Therefore, to find the number of joules
expended in a circuit, multiply the quantity of electricity by the
potential difference through which it is transferred. This is
expressed by the formula,
W= Q E, or W=II!t^
where IF is the work in joules, Q the quantity in coulombs, Eiibe
potential difference in volts, /the current in amperes and t the
time in seconds.
By Ohm*8 law E^ R I and by substituting this value of E
in the equation for energy, we obtain the formula,
W= /» R t,
which may be used when the curreut, resistance, and time are
known, R being the resistance in ohms.
Example. — With a potential difference of 97 volts and a cur*
lent of 14 amperes, what enei^y is expended in 20 minutes ?
Solution. — Work is expressed by the product of the quantity
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THE ELECTRIC CURRENT. 87
ftnd potential difference. The time in seconds equals 20 X 60^
1200, and the w>rk W= 14 X 1200 X 97 = 1,629,600 joules.
Aus. 1,629,600 joules.
Example. — The resistance of a oirauit is .9 ohm, and the
current is 25 amperes. What enei^y ia expended in half an hour?
Solution. — Suhatituting these valuea of renistanoe, curreut
and time in the formula W= /" ij (, we have, W= 25' X -9 X
80 X 60 = 1,012,500 joules. Ans. 1,012,500 joulaa.
Power. Power is the rate of doing work, and expresses the
amount of work done in a certain time. The horse-power is the
unit of mechanical energy, and is equal to S8,000 foot-pounds per
minute or 550 foot-pounds per second. A certain amount of work
may be done in one hour or two hours, and in stating the work
done to be so many foot-pounds or so many joules, the rate at
which the work is done is not expressed. Power on the other
hand, includes the rate of working.
It is evident that if it is known that a certun amount of work
is done in a certain time, the rate at which the work is done, or
the power, may be ohtfuned by dividing the work by the time,
giving the work done per unit of time.
The Watt. The electrical unit of power is the watt, and is
equal to one joule per second, that is, when one joule of work is
.expended in one second, the power is one watt If the number of
joules expended in a certain time is known, then the power in
watts is obtained by dividing the number of joules by the time in
seconds. The formulas for the work done in joules as given
on the preceding pages are,
W= 7 J; e, and W= /« R t.
By dividing each of these by the time t, we obtfun the oor-
lesponding formulas for power as follows :
P=I E, and Pz=I^ R, where P is the power in watts, 1
the current in amperes, E the potential difference in volts, and R
the resistance in ohms.
The power is obtained therefore, by multiplying the current
by the voltage, or by multiplying the square of the current by the
resistance.
The watt is sometimes called the volt-ampere.
For large units the kUowatt is used, and this is equal to 1,000
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S8 THE ELECTRIC CURRENT.
wattB. The common abbreviatioD for kilowatt ia K. W. The
Jeilowatt-houT is a unit of eoergy, and is tihe energy expended in
<me hotu: when the power is one kilowatt.
EXAHPLEd FOR PRACTICE
1. A current of 40 amperes is supplied to a circuit and Hia
Tolt^ is 110. What is the power in watts? Ans. 4400 watts.
2. What is the power in kilowatts supplied to a nomber of
incandescent lamps when the current is 84 amperes and the volt-
age of the oirooit 97? Ans. 8.1 -|- kilowatts.
8. A circuit has a resistance of 60 ohms and the current is
12 amperes. What power is expended in the circuit?
Ana. 7.2 K. W.
4. The voltage of an incandescent lamp circnit is 220 volts,
and the resistance 2 ohms. What power is expended in the cir^
cuit? Ans. 24.2 K. W.
Note. — Flr»t find current by Ohm's law.
Equlvoleacs of Electrical Eaer£y In Heat Units. When-
ever there u any resistance to the flow of a current there is always
a certain amount of electrical enei^ transformed into heat. The
current in passing through such resistance expends a certain
amount of energy in overcoming the resistance, and this energy ia
dissipated as heat. The entire electrical energy of a circuit may
be transformed into heat, as in a lamp circuit, or only part of the
enei^y may appear as heat, the remainder being transformed into
mechanical or chemical work. The enei^ which appears as heat
raises the temperature of the circuit to an amount depending upon
its radiating surface, and the temperature of the surrounding
medium.
When the resistance of a circnit and the current are known,
the electrical energy expended may be calculated by finding the
product of the square of the current, the resistance, and the time,
88 by the formula at the foot of page 36. All this energy is
transformed into heat. Other work may be done by the current,
as would be the case if an electric motor were connected to the
circuit, but this requires additional energy to that which is dissi-
pated as heat. The formula referred to gives only the enei^ lost
M heat, whioll is the total eamgy when no other work is done.
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THE ELECTRIC CURRENT. 89
This formula, which gives the energy in joules, is in accordance
with Jonle'a law, which ia as follows :
The nuTnber of heat units developed in a eondv/stor it proport
ticnal to its resistance, to the square of the currejU, and to the fiW
the current lasts.
As we have seen, the unit of electrical energy is the joule.
The common unit of heat is the calorie, which is the amount ot
heat necessary to raise the temperature of 1 gram of water through
1 degree Ceatignide. By careful InveBtigatioiis it has been found
that the joule is equivalent to .24 of a calorie ; that is, one joule
of electrical energy when transformed into heat is equal to .21
calorie. Electrical energy may therefore he expressed in heat
units by multiplying the number of joules by .24 ; that is,
where lT\a the heat in oaloriea.
As one joule is equivalent to .24 calorie, it follows that one
calorie is equivalent to 4.2 joules approximately.
BXAHPLES FOR PRACTICE.
1. How many calories will be developed by a current of 80
amperes flowing through a resistance of 12 ohms for 10 seconds?
Ans. 25,920 calories.
2. What amount of heat will a current of 20 amperes
develop if it flows through a resistance of 80 ohms for 2 seconds?
Ans. 15,860 calories.
Equlvaleat of Electrical Energy In flechanlcal Units. The
common unit of mechanical energy is the foot-pound, and from
experiment it has been found that one joule is equivalent to
.7878 foot-pound ; that is, the same amount of heat will be
developed by one joule as by .7378 foot-pound of work.
As one horse-power is equal to 550 foot-pounds pei' second, it
follows that this rate of working is equivalent to
550 ,
.7373 "
Hence one horse-power is equivalent to 746 watts. There-
fore ^4 god the equivalent of mechanical power in electrical
^wer multiply the horse-poWer by 746, and to find the equiva-
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40 THE ELECTRIC CURRENT.
lent of electrical power in mechanical pover divide the number of
wattB by 746.
EXAnPLES FOR PRACTICE.
1. A power of 287 watts is equivalent to how many horse*
power? Ana. .38+ H. P.
2. The Toltage applied to a circuit ia 500 and the current is
196 amperes. What is the equivalent horse-power of the circuit?
Ana. 131+ H. P.
3. Wliat is the equivalent of 43 H. P. in kilowatts?
Ana. 82 + K. W.
4. How many horse-power approximately are equivalent to
one kilowatt? Ans. IJ H. P.
THE SUPPLY OP ELECTRICAL ENERQY.
Electrical energy is now made use of on such a large scale
for lighting, power, heating, etc., that it ia generated br pro-
duced by machines of great capacity. The dynamo ia used for
this purpose and inaclunes having a capacity of several thousand
kilowatts are now common.
Central Stations. Large central etations or power houses
are built at convenient places and here are collected the generat-
ing, controlling and meaauiing apparatus. Usually steam engines
or turbines are used to drive the dynamos, and from the latter,
large copper mains conduct the current to the ewitchboard located
within the station. Here are assembled all the regulating devices,
instruments, and switches for the control of the system. From
the switchboard conducting mains run out to various distant
points, where the euei^ is to be used, to the receiving apparatus,
such as electric motors, lamps, heatit^ devices, etc. A complete
sjTstem is therefore made up of three sub-divisions — the generat-
ing plant, the conducting mains, and the receiving apparatus.
Isolated Plants. Besides large central stations which occupy
one or more entire buildings and which are usually built and
designed especially for such purpose, there are the comparatively
small anJ simple plants called isolated power plants. They are
purely local syatems and supply energy to a single building, or to
buildings in the immediate vicinity. The generating apparatus
in this case is usually located in the basement of the building.
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THE ELECTRIC CURRENT. 41
Large hotels and office buildings are frequently provided wiUi
individual generating plants.
Losses In Energy. In operating an electrical machine there
is always some lous in euergy, that is, the machine does not give
oat an amount of energy equivalent to the amount it receives.
Besides ordinary mechanical losses there is iu addition the electri-
cal loss, which always occurs when a current flows through any
resistance. This loss as previously explained, is equal to the
square of the current multiplied by the resistance.
The ratio of the amount of energy which a machine gives
out, to the amount which it receives is called the commercial
effldency of the machine. For example, if the commercial effi-
ciency ot a dynamo is stated to be 80%, then 20% of the energy
given to the dynamo is lost, partly in overcoming frictioD and
partly in electrical losses.
Where electricity is transmitted some distance by means of
conducting mains, there takes place a loss in the line due to lieat-
ing, which is frequently as much as 10 % . Also at the receiving
station, if the electrical energy is converted into mechanical by
means of an electric motor, there will be a further loss.
Illustrative Example. For example, suppose it is desired to
ascertain the losses in a system which comprises a generator,
conducting mains and an electric motor. Suppose the efficient^
of the generator is 92% and that 1000 horse-power are imparted
to it by the driving engine. The output of the dynamo will be
.92 X 1000, or 920 hotse-power, and this is equivalent to
920 X 746, or 686,320 watts. The energy lost in the dynamo
will be 80 X T46, or 59,680 watts. We will assume the voltage
of the dynamo and the circuit to be 1000, and as the power in
watts is equal to the product of the voltage and current, the cur>
rent must be 686,820 -^ 1000, or 686 amperes approximately.
Now suppose the resistance of the conducting mains is equal
to .11 ohms. Knowing the current in the mains and the resis-
tance, the loss therein is obtained by applying the formula I* R
giving 686» X -11, or 51,765 watts. The energy available at the
receiving end of the line will therefore be 686,820 — 61,766, or
d8435d watts.
The remaining loss to be conaidered is that in the electrif
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^ THE ELECTBIC CUBBENT.
motor. ABsaming the effioiencj of the motor to be 90%, the
power lost thereia will be .10 X 634,555, or 63,455 watte. The
oatpat of the motor is therefore equivalent to 634,555 — 63,455,
or 571,100 watts. This in mechanical units is equal to 571,100 -t-
746, OF 765 horse-power approximately.
Hence from an input of 1,000 horse-power at the generating
station, the work the motor is capable of performing at the receiv-
ing station is 765 horse-power. The efficiency of the entire
system under the assumed conditions is therefore 765 h- 1,000, or
76.5%.
AmoDg the great variety of generating machines, systems of
diatributioQ and auxiliary devices, each has its particular advan-
tage for special conditions, and the selection of the type of ma-
chine and system of distribution depends almost entirely apon the
special circamstances. For example, a low voltage system is best
adapted for isolated plants, whereas for the transmission of power
long distances very high voltages are ased. The various types of
marines, systems, etc., with their special advantages and disad-
vantages, will be folly considered in the following Instrnction
Papers.
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II
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D,Bi:ized3,GOOgIe
ELECTRICAL MEASUREMENTS
PART I— ELEMENTARY
SYSTEMS OF UNITS
Physical quantides are measured in terms of quaatities called
units. These units, as a rule, are related to one another and form
systems; as, for example, the British system and the C. G. S. system.
Fuadameatal Units. The arbitrarily chosen units of a system are
called fundamental in distinction to the related units depending on
them, which are called derived units. The C. G. S. system, univer-
sally used in electrical measurements, takes its name from three of its
fundamental units — the cerUimeter, the gram, and the secaiid of mean
solar time. Besides the three units from which it takes its name, the
C. G. S. system includes other fundamental units; for example, the
degree centigrade, the calorie, and the unit magnetic pole. Whenever
the arbitrary choice of a property of a substance enters into the choice
of a unit, the unit itself becomes fundamental. Thus the calorie
depends on the thermal capacity of water; the unit magnetic pole
depends on the miignetic property of air, etc.
Derived Units. Geometrical units, such as area and volume,
are derived from the unit of length. That is, areas are measured in
square centimeters, and volumes in cubic centimeters, involving units
of the second and third degree with reference to the unit of length.
We say that an area has a dimension of 2 and a volume of 3 in terms of
a length. Put algebraically, an area may be expressed as L*, and a
volume as L' in terms of a length L. In mechanics we use derived
units depending on length L, mass M, and time T. Thus velocity,
which may be measured by the ratio of length and time, has as dimen-
sions L T^', and acceleration L T~'. Force is more complicated and
may be defined in terms of the acceleration of a mass. The dimen-
sions of force are then L M T"^. The C. G. S. unit of force is called the
dyne. Work and energy may be measured in terms of force exerted
through space, and the unit, equ^l to one dyne acting through one
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2 ELECTRICAL MEASUREMENTS
centimeter, is called the erg. The dimensions of the erg aveL'MT''.
In the same way power (time rate of doing work) may be expressed
in ergs per second. This unit of power is so small that for practical
purposes we use the watt which is 10,000,000 ergs per second. Even
the watt is small and so we frequently use the kUowaU (one thousand
watts) for measurement of power. As we shall see later, the watt b
used also for the measurement of power for electric circuits. Besides
the C. G. S. units we use many units which are multiples or sub-
multiples and so are related. For example, we use the meter (100
centimeters) and the kilometer (100,000 centimeters) and the milli-
meter (0.1 centimeter). Evidently the meter was intended to be the
fundamental unit, the centimeter and the millimeter submultiples,
and the kilometer a multiple; but in the C. G. S. system the meter
becomes a multiple of the fundamental unit.
In electrical measurements the unit of resistance — the oAm —
is practically taken as 1,000,000,000 C. G. S. units; the unit of elec-
tromotive force (e. m. f.)— the voU—h taken as 100,000,000 C. G. S.
units; and the unit of current — the ampere — is taken as 0.1 C. G. S-
unit. These units were originally recommended by a conmiittee of
the British AssociaUon for the advancement of science in 1873, and
were internationally adopted at Paris in 1881. The watt b the prac-
tical unit of power and is equal to an e. m. f. of one volt multiplied
by a current of one ampere. If the current is constant the product of
current and e. m. t. gives the power. If the current is not constant,
the average product of current and e. m. f. gives the average power.
As we shall see later in the case of alternating currents, the readings
of alternating-current voltm^ers and ammeters cannot be multiplied
together to get the power; but an instrument called a wattmeter must
be used. The wattmeter gives the correct result. The watt is 10,-
000,000, i. e., ICP C. G. S. units.
The unit of charge (or quantity) — the coulomb — is the quantity
of electritnty equal to a flow of one ampere for one second. The
coulomb is 0.1 C. G. S. unit. The farad is the unit of capacity.
A condense has one farad capacity if it can store one coulomb with
a potential difference of one volt at its terminals. Potential difference,
like e. m. f., is practically measured in volts. At higher potential
differences a condenser takes a proportionately higher charge. The
farad is a very large capacity and condensers are practically rated
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ELECTRICAL MEASUREMENTS 3
in microfarads, t. e., in miUionths of a farad. Tlie henry is the unit of
inductance. When a current is started in a coil of wire a magnetic
field is produced. This requires more e. m. t. than to muntain the
current when once started. If the coil requires one volt more to in-
crease the current at the rate of one ampere per second than to main-
tain it, we say the inductance of the coil is one henry. The henry b
1,000,000,000, t. e., 10" 0. G. S. units. These practical units are all
related, as is seen aboye, to the C. G. S. units by factors, of powers
of 10. There are other units in the dectro-magnetic system for which
the reader is referred to more advanced works.
Relatloa of C. Q. S. to British Units. To reduce British to
C. G. S. units and vice vertd, we make use of the relations between
them. One inch equals 2.54 cenUmeters; one pound mass equals .
453.59 grams mass; and a like relation between pounds weight
(force) and grains wei^t. The second of mean solar time is the
same in both systems. It should be kept in mind that for equal
quantities the number of units is inversely proportional to the size
<tf the unit.
ELECTRICAL MEASURING APPARATUS
Qalvanonwtns. In the year 1819 Oersted discovered that a
current flowing through a conductor produced an effect on a magnet.
This effect is now explained by saying that lines of force surround the
conductor, and that the north pole of the
magnet tends to move along the lines of
force in one direction and the south pole
in the opposite direction. In other
words the magnet, if free to move, tends
to take a direction across the conductor.
In the case of a long, straight wire the
lines of force are circumferences of cir-
cles witii the conductor at tiie center. y^.,_ oentea-. Bipwiment
The force on the magnet pole in this
case falls off in proportion to the increase in the dbtance from the
center of the conductor; i. «., the force is inversely proportional to
the distance. If the magnet is already in s magnetic field, such as
that of the earth for instance, a current in a north and south wire
above or bdow the magnet, tends to turn the magnet away from the
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4 ELECTRICAL MEASUREMENTS .
magnedc north and south, the tangeot of the angle through which it
turns being proportional to the current, Fig. 1. The effect of a single
wire is small unless the current is very large.
Tangent Odtvanometer. If the conductor is wound in a coil
whose plane is north and south and vertical, the effect on a magnet at
the center is multiplied many times. Fig. 2. Such an instrument ia
called a tangent galvaTwmeter. If the thumb of the right hand u
placed along the outside of the conductor pointing in the direction
Fig. S. DlKgnm of D'ArsoDval
Pig. S. Tangent QalTanometer. QalTanometer.
of the current, the fingers of the hand may be curled around the con-
ductor and will point in the direction toward which the north pole of
the magnet will be ui^ed by the field produced. A similar arrange-
ment of the left hand wilt indicate the direcdon in which the south
pole will be urged.
D'Araonval Galvanometer. If the magnet is fixed and the coil
free to turn, the latter will turn in the reverse direction. If the
magnet is of the horse-shoe type with the coil of wire between the
poles a similar rule will determine the direction of motion. Gal-
vanometers of the moving coil type were invented by D'Arsonval
and Deprez, and are usually called D'ATsoitval galvanometers.
Fig. 3.
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ELECTRICAL MEASUREMENTS 6
Astatic Galvanometer. An improvement may be made in the
tangent galvanometer, if greater sensitiveness is desired, by mounting
on the same support two magnets of nearly but not quite equal
strength, care bdng taken to turn the poles in exactly opposite direc-
tions. This is very important. One mag- ^
net is placed at the center of the coil through
which the current b sent and the other
magnet is above or below the coil and in-
fluenced relatively litde by the current, Fig.
4. The directive action of the earth's
magnetic fleld is little on such a system —
called astatic— and a small current conse- , pi«- *• a-i»mc S7.t«m.
quently turns the system more easily from the magnetic meridian. A
similar effect is produced if part of the coil is about one magnet attd
the rest, with reversed direction of the current, about the other mag-
net. Another way to produce an equivalent effect on a single, sus-
pended magnet is to mount a powerful control magnet near by
(above, below, or behind) so as to reduce to
a very small amount the magnetic field due
to the earth and the control magnet at the
center oi the coil.
An extremely sensitive galvanometer may
be made by combming the control megnel
with the astatic system of magnets, llie
magnet (or system of magnets) of tangent
and astatic galvanometers is suspended gen-
erally either hy a fine silk or quartz fiber.
The current is led into and out of the coil of
the D'Arsonval galvanometer through two
wires, both above in the bifilar suspension,
_ „ one above and one below in the unifilar sus-
Flg. S. Section at Siwpen-
HloD Of PortabiB Qal- peUSlOU.
Less sensitive galvanometers may have their
moving parts mounted on pivots or .other bearings, and in such gal-
vanometers of the D'Arsonval type the current is brought in and out
through spiral springs which tend to hold the coil in its zero position,
Fig. 5. Galvanometers of this type are used for ammeters — to meas-
ure amperes of current; or for voltmeters — to measure e. m. f. in
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6 ELECTRICAI^ MEASUREMENTS
volts. Such instrumenta are provided with some damping arrange-
ment so that they come to rest quickly. The deflection of such
galvanometers is indicated by a pointer moving on a scale. If the
poles of the magnet are property shaped the deflection may be made
proportional to the current passing.
Mirror Galvarwrneteri. Very seosidve galvanometers must be
made with moving parts of little wei^t It is, however, very desirable
Fls. 8. Thoaiaoa Ulrror OKlvknometer with lAmp and Scale.
that the pointer be very long so that s large number of scale parts
may correspond to small deflections. Hiis may be accomplished by
using a pencil of light rays for a pointer, as shown in Fig. 6, which
illustrates the Ismp-and-scale method, in which a lamp is placed
behind a slit in a screen on which the scale is mounted. A concave
mirror carried on the moving part of the galvanometer focuses an
image of the slit at the reference point of the scale (usually the middle).
When current passes, the mirror is deflected, thus deflecting the rays
of light to another part of the scale. If the mirror turns throu^ 1^,
the image is deflected 2°. In place of a slit an opening of another
form with cross wires may be substituted. Also if desired the lamp
may be mounted at the side, and its light reflected by another mirror
to the mirror on the galvanometer. In this last case it is more conven*
lent to have the scale printed on a strip of translucent ground glass or
paper, and to view the image throu^ the glass or paper. If a tele-
scope is substituted for the lamp, an image of the reference point of the
scale may be made to coincide with the cross wire of the telescope
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ELECTRICAL MEASUREMENTS 7
when no current is passing, and other parts of the scale will take the
place of the reference point when a deflection b produced, Fig. 7.
In this case a plane mirror may take the place of the concave. The
telescope-and-scale method Is more satbfactory for very sensitive
galvanometers than the lamp-and-scale method, though the latter,
usually used in a darkened room, is easier on the eyes unless an ex-
cellent galvanometer mirror and telescope are used.
. Choice of Galvanometers. In choosing a galvanometer for use,
it is desirable that the instrument should not be too sensitive for the
experiment. As a rule the D'Arsonval galvanometer is the most
satisfactory galvanometer for general use, as it is not much affected
Fig. T. BkIUbUc D'ArsoDTal irltb Telescope and Scale.
by changes in the magnetic field, even if of as great amount as pro-
duced by dynamo-electric machinery or moving of masses of iron in
the neighborhood. The astatic galvanometer is, however, as a rule,
far more sensitive and for certain purposes must be used.
Use of the Control Magnet. In using astatic or other galvanom-
eters with moving magnets, the use of the control magnet is some-
times very puzzling to beginners. The galvanometer should be set
up with its coils in a north and south plane. The mirror then faces to
the west (or east sometimes). The control magnet is then placed
in position as far away as its support will allow and turned with its
north pole to the north. The magnets and the mirror of the galvanom-
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8 ELECTRICAL MEASUREMENTS
eter, as a rule, are somewhat deflected because of the presence of die
control magnet. If tbe latter is slightly turned in one direction, the
mirror should turn in the opposite direction. As the control magnet
is brought nearer, the period of swing of the mirror should increase,
and the sensibility should increase in a greater proportion (as the
square). If by chance the control magnet is with its south pole to
the north, the mirror will turn in tbe same direction as the control
magnet is turned, and the period of swing will decrease as the
control magnet is brought nearer. Control magnets as a rule have
the north pole marked in some way, so that there is no need for
any mistake. When the control magnet is brought so close that
the effect of the earth's field is overcome, the magnets and mirror
of the galvanometer will try to turn half way around, thus turning the
back of the mirror to the observer, if the construction of the galva-
nometer will allow. As a rule it does not pay to increase the sensidve-
ness of the galvanometer to the highest possible limit, as the zero
reading will become very easily influenced by slight magnetic changes
due to movement of small masses of iron, or the currents in neighboring
conductors, or even the variation in the magnetic field due to a cloud
cutting the sunlight off from the walls of a red brick laboratory, small
as such an effect must be. If the galvanometer is of the astatic ^pe,
it is presupposed in the above that the sup^rt for the control magnet
is arranged to weaken the field of the stronger magnet of the astatic
pair more than it does the field of the weaker magnet. In some poorly
adjusted galvanometers, the control magnet may produce the con>
trary result, and it may be necessaiy to make appropriate allowance.
If the magnets of the astatic galvanometer take an east and west
position before the control magnet is put on, it b evident that the
magnets of the astatic pair are not exactly in opposite directions and
that the result b a magnetic system having its. effective or resultant
north pole about half way between the north poles, and its resultant
south pole about half way between the south poles of the two magnets.
The line joining these resultant poles lies in the magnetic meridian
and the magnets of the astatic pair lie nearly east and west. To cor-
rect this error in adjustment is a very delicate matt^ and should not
be attempted by the novice.
Ballistic Galvanometer. When a charged condenser is discharged
through a circuit containing a galvanometer, the galvanometer de-
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ELECTRICAL MEASUREMENTS 9
fleets. The period of swing should be long enough for pncticallj
the whole duu^ to pass during the eariy part of the swing. If the
galvanometer has a short period, the return swing may b^in before
the discharge is complete. It may be assumed that the first deflec-
tion b a measure of the quantity discharged; but it is evident that
this is an error if the discharge is slow in comparison with the lime
occupied by the deflection. . To be on the safe side the period of
swing should be large. Galvanconeters which are suitaUe for measur-
ing disdiarges are called baUutic. Depending on circumstances,
their period may be between, say, five and twenty seconds for the
complete swing. Ilie D'Arsonval galvanometer may be made with
high enough period and sensibility to give satisfaction as a ballistic
instrument; but for extreme sensibility an instrument of tbe astatic
type is more generally used. The D'ArsonTal galvanometer is more
nearly free from the drift of the reference point, which b due mostly
to varying magnetic field and somewhat to elastic fatigue or sub-
permanent set in the suspension. Freedom from drift is very impor-
tant, as the deflection is uncertain in iHx>portion as the r^erence point
is in doubt.
Damping of Vibraiioni. The motion of the moving system of a
galvanometer may be impeded by damping. Tliis may be accom-
plished by mounting vanes on the system so fliat the air in an s:;::!r>«ed
chamber impedes die motion, or by electromagnetic damping pro-
duced by eddy currents induced in metal moving in a strong magnetic
field. In D'Arsonval galvanometers if the coil is wound on a metal
frame, currents will be induced in the fnune while the coU is in motion.
Such damping ensures a speedy coming to rest after a deflection and is
vety helpful, especially in ballistic galvEuiometers where certainty of
zero is important. It is evident that any damping reduces the sensi-
bility of a galvanometer. Some galvanometers are provided with ao
much damping that on the return swing the system does not swing
past the zero or refoence point Galvanometers without a period of
complete vibration are said to'be aperiodic (the a denoting without).
As a rule galvanometers have a complete period, that is, they are
damped less than the aperiodic galvanometer. ' Tlie effect of
damping b to shorten the time and amplitude of the outward
part of the swing (though it lengthens the complete period), and tO'
thb extent damping b objectionable. That are, however, counts-
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10
ELECTRICAL MEASXmEMENTS
balancing advantages and 90 for most purposes some damping is
considered wise.
Plunger Tyfe Inatrumenta If in place of the magnet of a gal-
vanometer, some soft iron is substituted in such a position that the
Bcdon of the current b to magnetize the soft iron and to draw it into
a stronger part of the magnetic fidd, we have a curreiU indicator of
the plunger type. The coil frequently takes the form of a solenoid
and the soft iron that of a rod which is drawn by the action of the
current into the solenoid.
Ilie restraining force
may be gravitational or
that of a spring. Such
an instrument is shown
in Fig. S. It is evident
that the direction of the
deflection does not de-
pend on the direction of
the current. In fact,
plunger type instruments
may be used to measure
alternating currents.
There are many possible
variations of this type of
instrument. As the iron
has a certain amount of residual magnetization, tiie deflection with
smaller following large currents b more than would have been pro-
duced by the same current following a smaller pne. For this reason
the plunger type of instrument is less reliable than the usual types
of galvanometers. The scale is usually of unequal divisions as the
pull increases more rapidly than the current.
Etectrodynamometers. If the magnet of a galvanometer is re-
placed by a coil through which the current passes in series with the
other coil, we have what is known as an electrodynamometer. As
in the case of the plunger type instruments, the electrodynamometer
deflects in the same direction for all currents unless disturbed by
being placed in a magnetic field of outside origin. It is desirable to
set up an electrodynamometer with the moving coil (or coib, if more
than one) with its axis (or their axes) along the magnetic meridian.
Pig. S. DUgrftm ol Plunger lutranient.
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ELECTRICAL MEASUREMENTS 11
The disturbing effect of a permanent field b n^ligible when (be
electrodynamometer is used to measure alternating currents. For
direct currents, the action of the oulude field is
eliminated by reversing the connections. The
defiection is approximately proportional to the
square of the current. For the best ^rpes of
electrodynamometers the suspended coil is
brought back to its zero position by twisting
a torsion head which operates through a spiral
spring on the suspended coD. The current
in this type of instrument is proportionid to
the square root of the reading of the torsion
head necessary to restore the moving coil to
its zero position. A direct current producing
the same deflection as an alternating current
is said to be the effective value of the alter-
nating current Fig. 9 illustrates the usual
type of electrodynamometer. Fig. 10 illu»-
trates another form invented by Lord Kelvin
B.ndca.\\ed& Kelvin b<dance. Tie figure shows
the connections viewed from the back of the
balance. The fixed coil is subdivided into
four parts B, and the moving coil into two
parts A, placed symmetrically between the parts of B. The parts of
A are supported on opposite arms of a balance and the btdanoe is
restored to its zero position by displadng a weight along the beam.
Plg.ia DlagrunofCollslnKelTinBaUnce.
The action of the current b thus weighed, and the square of the cur-
rent b proportional to the distance the weight b moved along thb
beam. The beam is divided accurately into equal parts and it b
possible to obtain the reading with a high degree of accuracy. Tike
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12 ELECTRICAL MEASUREMENTS
current is proportional to the square root of the reading. The ef-
fect of dividing the two parts of A is to free the instrument from
the disturbing effect of the earth's magnetic field or any other stray
field of fairly uniform intensity.
Electrometers. Electrometers dep>end on the attraction be-
tween electrostatic charges of opposite signs. The only electrometer
which we shall describe is the eledrogtaiic voltmeter which consists of
fixed and movable me-
tallic parts of relatively
large surface. These
surfaces may be plane or
curved. The terminals
are connected, as a rule,
one to the fixed part and
the other to the movable
part — the vane. These
parts take charges pro-
portional to the potential
difference between them
— e. m, f . applied — and a
certain attraction results
therefrom. If the vane
is allowed to move, the
electrostatic capacity of
the combination in-
creases somewhat, thus
increasing the amount of
the charges and the at-
tractive force. If it is desired, the vane may be brought back to its
zero position by some counter force. As a rule electrostatic instru-
ments are allowed to deflect and are calibrated by comparison with
other fomos of voltmeters. More complicated forms of electrostatic
instruments may have two sets of fixed surfaces and a movable
vane. In some cases a battery of cells of known e. m. f. may be
used to charge the fixed surfaces, and the e. m. f. to be measured may
be applied between one fixed surface and the vane. Electrostatic
voltmeters are generally used to measure high electromotive forces.
Fig. 11 shows an electrostatic voltmeter of an old type, which shows
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ELECTRICAL MEASUREMENTS 13
die genaal scheme more dearly than better and more complicated
electrometers.
Hot Wire Instnimeiits. If current passes through a wire, a
heating effect results and the wire lengthens because of its rise in
temperature. If a pointer is held in a position of equilibrium between
turning moments produced by two wires pulling on opposite anns of
a lever, the headng of one of these wires hy an electric current will
produce a change in the position of equilibrium. It is evident that
change in the temperature of the room affects both wires alike and
produces no change in the zero position. The deflection of a hot wire
instrument is dependent on the square of the current (as the heating
is proportional to the square of the current). For this reason the
hot wire instrument deflects in the same direction for currents in either
direction and for alternating currents as well. As the effective value
of an alternating current is equal to the square root of the mean square,
it is evident that a hot wire instrument calibrated by direct currents,
will ff.ye proper readings for alternating currents also. Hot wire
instruments are made use of as ammeters (low resistance) and volt-
meters (high resistance). As a rule hot wire instruments are used for
alternating currents. They are usually less accurate than electro-
dynamometers of the best types.
Wattmeters. We have seen that an etectrodynamometer has a
turning moment proportional to the square of the current passing
through it. If the current passing through the fixed coil b different
from that passing through the movable coil, the turning moment will
be proportional to the product of these currents. If the power de-
livered to a line is to be measured, the average product of the volts
and amperes gives the result in watts. The current delivered from the
tine to the load may be passed through one coil (usually of low resis-
tance) whose terminals are A and B, Fig. 12, and the e. m. f. may be
applied at the terminab of the other coil (usually of high resbtance)
whose terminab are a and 6, and produce a second current proportional
to thb e. m. f. Id order to avoid measuring the efTect of the pressure
current it b led backward through coil F shown in dotted line, thus
subtracting its effect. The instrument maybe calibrated to read
watts. As a rule it b easier to make the second coil of moderate re-
sbtance and to insert a non-inductively wound high resbtance coil R
io series. If the wattmeter is to be calibrated by the use of current
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14
ELECTRICAL MEASUREMENTS
Fif. II. DUgnm ot Wutmcur.
and e. m. f. in separate circuits, the tenninals i and b are used. The
resistance of £ is equal to that of F. The currents in both coils will
reverse if the e. m. f. is reversed, but the deflection will be unchanged.
The average power of a varying current equals the average product of
current and e. m. f.; consequently a wattmeter calibrated with direct
currents gives correct results for al-
ternating currents. If the current
and e. m. f. are alternating, the mean
product will in general be less than
the product of the effective values
of current and e. m. t. (as measured
by A. C. voltmeters and ammeters);
consequently when dealing with al-
ternating current and e. m. f. the
product must be multiplied by a
factor, called the power factor, which is usually less than unity, if
the correct value is to be computed from anmieler and voltmeter
readings. As a rule the power factor is found by dividing the watts
as measured by a wattmeter by the product of volts and amperes.
Recording Voltmeters and Ammeters. If any of the voltmeters
or ammeters described above are
arranged with a pen which traces
a line on a disk or roll of paper
drawn by clockwork past the pen^
the instrument will record the
variations of e. m. f. or current.
There are several good types of
recording voltmeters and am<
meters on the market. A re-
cording ammeter is shown in
Fig. 13.
lategratlng Watt-Hour M^
ters. Integrating meters show
the total consumption of the thing
to be measured; for example, in- '^' "" '"""^'"^ Ammewr,
te^rating gas meters show the consumption of gas in cubic feet. In
ifae same way an mtegrab'ng watt-hour meter (commonly, though
inexactly, called an int^^ting wattmeter) shows the consumption
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ELECTRICAL MEASUREMENTS 15
of enei^ in watt-hours. Such an instrument is shown in Fig. 14.
The instrument is essentially an electric motor geared to a train of
wheels moving hands over dials. The speed of the motor is propor-
tions! to the power in watts, and the product of the average power and
the time in hours (that is, watt-hours) is indicated by the change in
the position of the hands on
the dial since the last reading.
To give correct readings the
driving motor must be de-
signed for the circuit on which
it is used. The essential fac-
tors of the circuit are the e. m.
f., maximum current, whether
direct or alternating current is
used, etc. In a three-wire sys-
tem a single meter may be
designed to measure the power
of the two or three circuits in-
volved.
lategratlne Ampere>Hour
„ , . . , Pig. 1*. Wttt-Bonr Meier.
Meters. An migrating atn-
pere-hour meter (commonly called integrating ammeter) is similar to
the watt-hour meter. It is used generally in connection with storage
batteries to keep account of the cha^ and discharge. It is of
little general use.
Rheostats and Resistance Coils. The word rheostat means an
apparatus for stopping the current In actual fact it does not wholly
stop the current, but only reduces it to a desired extent. Every
material interposes some resistance to the flow of an electric current.
Substances interposing extremely high resistance are known as insula-
tors, and those interposing relatively tittle resistance, as conductors.
Metals, as a rule, are the best conductors. The metals most used
commercially for electrical transmission are copper, aluminum, and
iron (or steel). Alloys in general have much higher resistance than
the metals of which they are composed. Carbon and solutions of
various salts have much higher resistance than metals. Rheostats
may be made of any of these materials, but those most generally used
are ateel wire or sheets, German silver wire or other alloys, carbon
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16 ELECTRICAL MEASUREME^fTS
rods or plates, and solutions in tanks in which metal plates are im-
mersed, the metal plates being connected to the terminab of the cir-
cuit. Such metal plates are known as electrodes. This last arrange-
ment is usually called a water rheostai. Pure water has a very high
resistance and is never used in water rheostats, but the resistance may
be reduced as desired by dissolving salt in the water. The metal
plates are usually arranged so that
one electrode may be brou^t
nearer the other when it is desired
to increase the current. When the
word rheostat is used, it is generally
understood that the resbtance is
not exactly known. A rheostat la
shown in Fig. 15.
When it b desired that the re-
sistance have a certain exact value,
metals are the only practical ma-
terials to use. Coils of wire ex-
"*■ "■ ^SS'toASJcimaE," ""* °°'" «":*'/ adjusted «« called resistance
coils. They are adjusted to certain
values in ohms. For very low resistances, e. g., small fractions of an
ohm, metal strips may be used. As most pure metals increase their
resbtance with increase of temperature by about 0.4% per degree
centigrade, resistance coib are almost always made of certain alloys
which change little in resbtance with change in temperature. One
alloy in particular, mangantn, changes so little in resistance with
change in temperature that it b usually chosen for standard resiatance
coils. Figs. 16 and 17 show standard resistances in the form of a coil
and a strip. As mentioned above alloys have relatively high resbtance
and for thb reason also the alloy manganin is preferable to any pure
metal for resistances.
Lamp Rheostats. A very convenient form of carbon rheostat
is a bank of incandescent lamps. The usual 16 c. p. lamp for a 110-'
volt circuit has a resistance when hot of about 220 ohms. Its resbt-
ance when cold b about twice as much. Carbon and solutions,
unlike metals, are better conductors when hot than cold. It is evident
that incandescent lamps, because of their change in resbtance from
cold to hot, are not suitable for standard resbtances. . If two lamps
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ELECTRICAL MEASTJREMENTS 17
are arranged in series, i. e., if the current is made to pass through one
after the otlier, the resistance of the combination is twice that of a
single lamp. On the other hand if the lamps are connected in parallel,
i. e., if the cur-
rent divides be-
tween them, the
resbtance of the
combination is
only half of that
ofasinglelamp.
This result is
evident as the
same e. m. t. pro-
duces twice as
much current in
two lamps as in
a single one. In
the same way
ten lamps in
„ , , Fig. 16. A Standard BeBlRlance Coll.
parallel have a
combined resistance only one-tenth as much as a single lainp.
Multiplying Power <rf Shunts. The word shunt b the Britbh
name for a side track (or as we would call it, switch) on a railway.
Any electrical side path
is called a shunt. If the
current has two or more
paths in parallel offered
to it, the current divides
in inverse proportion to
the resbtance or, as more
~.. ... .^^. „_ simply expressed, in di-
Plg, IT. A Standftrd RealatoD'W Strip. t^ ^ >^ _ >
rect proportion to the
conductivity of the various paths. Thus if a galvanometer has
a shunt across its terminals whose resistance b one-ninth of
that of the galvanometer, nine times as much current will go
through the shunt as through the galvanometer and consequently
only one-tenth of the current will go through the latter. The total
current is then ten times the current through the galvanometer and we
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18 ELECTRICAL MEASUREMENTS
say that the multiplying power of the shunt is ten. If the galvanom-
eter has a shunt of one ninety-ninth of the former's resistance, one
one-hundredth of the current will pass through the galvanometer and
the multiplying power of the shunt will be one hundred. In general,
if the resistance of the shunt is — — - of that of the galvanometer,
the multiplying power of the shunt will be m. The evident effect of
the shunt is to reduce the resistance of the galvanometer circuit to
— of its former value, i. e., to — of the resistance of the salva-
m m °
nometer itself; the resulting fall of potential over the galvanometer
and shunt b, therefore, only — as much as if the shunt were not
there. Galvanometers are provided by their makers, if desired, with
shunts having a multiplying power of 10, 100, and 1,000, marked to
go with the particular galvanometer. It is evident that the usual
shunt cannot be used with other galvanometers without recalculation
of its multiplying power, which under such circumstances would
probably he some inconvenient number.
Professor Ayrton ha^ devised a form of shunt box with extra
resbtance which is automatically connected in series in proper amount
to keep the total resistance constant but allowing only Vif. tiiti or
tifVv of the current to pass through the galvanometer.
OHM'S LAW
In 1827, Dr. G. S. Ohm of Berlin published a treatise, now
famous, entitled Ths Galvanic Circuit Investigated MatkematicaUy,
in which he announced the fundamental law of electric circuits now
known as Ohm's law. This is usually stated in the algebraic
formula:
^ R
In words, the current (in amperes) equals the e. m. f. (tn volts)
divided by the resistance (in ohms). It is truly a surprising fact that
the resistance of an electric circuit is a constant not dependent on the
current passing. Many experimenters have tried in vain to find any
inaccuracy in Ohm's law. If any two of the three quantities involved
are known, the third may be found by solving the equation. Thus,
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ELECTRICAL MEASUREMENTS 19
As will be seen later, one of the most convenient methods of
measuring low resistances, as of a dynamo armature, is a simple
application of Ohm's law.
MEASUREMENT OP RESISTANCE
Resistance Boxes. Measurement of resistance is made by com-
parison with certain standards of known resistance, the different
methods of measurement varying to a great degree. The standard
resistance coils are made of such alloys as manganin — an alloy of
manganese copper and nickel— which has a high specific resistance
and changes its resistance with rise in temperature to a much less
extent than other metals. It is of course desirable that this change
should be as small as possible. The size and length of the coils are
such that they have resistances of a definite number of ohms at a
certain temperature. The coils are insulated with silk or paraffined
cotton and are very carefully wound. Each wire is doubled on itself
before being coiled up, and then wound as
shown at ^ and £ in Fig. 18; or,asi3some-
times preferred, the wire may be wounid
single in layers, the direction of winding
being reversed for alternate layers. In-
ductance and capacity effects are by these ^^^ B^bwnce box coin
means reduced to a minimum. The ends of showto^Non^iEauctiv.
the coils are soldered to brass pieces as C, D,
E. Removable conical plugs F and G of brass are made to fit accurately
between the brass pieces. When these are inserted as shown, the coils
will be short circuited and a current will pass directly through C, F,
D, 0, and E without going through the coils. If F is withdrawn the
coil A will then be inserted in the circuit; if G is also withdrawn then
coils A and B will both be inserted, as the current cannot pass from
C to E without going through the coils.
Resistance boxes are constructed consisting of a large number
of resistance coils, and of such resistances that by withdrawing plugs
varying resistances may be built up. A common form of resistance
box has coils of the following ohms resbtance: 1, 2, 2, 5, 10, 20, 20,
50, 100, 200, 200, 500, 1,000, 2,000. A resistance of 497 ohms
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20
ELECTRICAL MEASUREMENTS
could be miule up by withdrawing plugs corresponding to the coils
200 + 200 + 50 + 20 + 20 + 5 + 2 = 497, or 768 by cmIs 500 +
200 + 50 + 10 + 5 + 2 + l« 768.
Resistance by Substitution. By Ohm's law the greater the
resistance inserted in a circuit the less becomes the current, provided
the e. m. f. remains constant. This gives us a simple although not
very accurate method of measuring electrical resistance. If a battery
of constant e.m. f., the unknown resistance, and a simple galvanom-
eter are connected in series, the strength of the current passing will
be indicated by the latter. Suppose the unknown resistance to be
replaced by known resistances, enough resistance coils being inserted
so that the deflection of the galvanometer needle is the same as when
the unknown resistance was in circuit. The current will then be
the same, and as the e. m. f. remains unchanged, the resistances
must be equal in each case. The sum of the known resistance coils in-
serted will then be equal to the unknown resistance.
The advantages of thb method are that it is rapid, and that only
crude apparatus is required, as the galvanometer and resistance
box can be very simple in form. The resbtance of the battery
and galvanom-
B eter should be
but a few ohms,
otherwise small
. resistances can-
not be measured
closely. Only
small currents
should be used so
that the error
fromheatingmay
be negligible.
Wheatstone's Bridge. All ordinary measurements of resistance
are usually made by use of a Wheatstone's bridge.
The principles of this instrument will be understood from Fig.
19. There are four arms to the bridge with the resistances Af, ^,
X, and P. From the points of junction A and C, wires connect with
a battery E. A galvanometer G is connected between the junction
points B and D. The current from the battery divides at A and
Fig. IS. Theonttol Dlasr»m oj
1 WhratsloDe'i BrMgs.
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ELECTRICAL MEASUREMENTS
passes through the resistances M and X, and N and P, uniting again
at C. The fall of potential between A and C must of course be the
same in amount through the resistances M and X as through N and
P. If no current passes through the galvanometer then the points
B and D will be at the same potendal, and there will be the same fall
of potential in the resbtances M and N, and in the resistances X
and P. Under these circumstances the ratio of the resistances of
if to JV will be the same as X to P, or
K A
N^ P
If M, N, and P are known resistances, the resistance of X is readily
found by the formula.
The method of using the bridge will be better underatood from
Fig. 20. The bridge arm M has coils of 1, 10, 100 ohms p
and arm N, coils
10, 100, 1,000.
The series of
coils P for ob-
taining a balance
usually has re-
sistances of 1,2,
2, 5, 10, 20, 20,
50,100,200,200,
500, 1,000, 2,000
ohms, but coils
up to 100 only
are shown. There is a key K in the galvanometer circuit and a key
H in the battery circuit The battery key H should always be
closed before the galvanometer key K, and should be kept closed
until after K is opened. This not only insures steadiness in all
currents when the galvanometer circuit b closed, but also protects
the galvanometer from aelf-induction currents which would occur if
the battery circuit were closed after that of the galvanometer. A
double successive contact key, Fig. 21, may with advantage be sub-
stituted for the two single keys, thus insuring that the battery and
galvanometer branches will be closed and opened in the proper ae>
FIk- n. DlAgrom Sbowlng Method ot HaktuK BrUgs
Ueaiurements.
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22 KLECTRICAL MEASUREMENTS
quence. A reflecting galvanometer is uaed for accurate measuremeDt
In making a measm«ment of an unknown resistance it is first
necessary to gain a knowledge of its approximate resistance. For
this purpose the 100-ohm plug b withdrawn from both arms M
and N, the unknown resistance being connected at X, The ratios
of Jlf to ^ will then be unity, and hence for a balance the number
of ohms required in the resistance coils P will be the same as the
resistance X. The 1,000-ohm plug in P should first be drawn and
the keys depressed In their proper order for an instant only. The
galvanometer needle or mirror, as seen by the light reflected on the
Fig. H. l-polnt Contact Kej.
scale, b deflected — say to the right, and the resistance is probably too
large. The plug is replaced and the I-ohm coil withdrawn. On
depressing the keys suppose the spot of li^t is deflected to the left.
Then the 1 ohm is too small and the 1,000 ohms too large; also in
this case deflections to the right mean that the resistance inserted is
too large, and to the left, that the resbtance inserted b too small.
The 1-ohm plug b now replaced, and 500, 200, etc., are successively
tried until it b found that 12 ohms b too lai^ and 11 ohms too
small, that b, the unknown resbtance b between 11 and 12 ohms.
Suppose that it is desired to find the correct value of the unknown
resistance to the second place of decimals. The rado of the arms
Jf to ^ must then be changed so that the resbtance coib P will have
a value of between 1,100 and 1,200 ohms when a balance is obtained.
The ratio of X to P will then be 11 to 1,100 approximately, or about
1 to 100. To obtain a balance the ratio of the arms M toN must also
be 1 to 100. Hence the 100-ohm plugs first withdrawTi are replaced
and the 10-ohm plug withdrawn from M and the 1,000-ohm plug
from N giving the required ratio. The same ratio could be obtained
by withdrawing the I-ohm plug in M and the 100-ohm plug in N.
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ELECTRICAL MEASUREMKNTS 23
The bridge is now arranged for the final ineasuTement. As the
resistance inPwill now be over 1,100 ohms, the 1,000- and 100-ohm
plugs are first removed. Suppose the 50-ohm plug to be also removed,
and a deflection to the right shows that this is too great. The plug
is replaced and 20 withdrawn, which proves to be too small. The
next twenty plug is also withdrawn and a deflection to the left shows
the resistance to be still too small. The 5-, 2-, and 2-ohm plugs are
Fig. 21. Portable TflttluB Set.
successively withdrawn, the last two ohms proving to be too great
This is replaced and ihe 1-ohm plug withdrawn, and suppose no
deflection is then obtained. The total number of ohms in P is now
1,000 +100 + 20 + 20+5 + 2 + 1 = 1,148. The value of X is
therefore yJH X 1,148 = 11.48 ohms.
The above example illustrates the general method of using the
bridge. Usually the resistance to be measured is known approxi-
mately and the required ratio between M and N can be determined
without making a preliminary measurement. The possible changes
in the ratio between M and N ffves the bridge a great range of
meadurementt 'When M isl and N is 1,000 ohms, measuremepts of
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24 ELECTRICAL MEASUREMENTS
resistance as small as .001 ohm may be made. Bridges are usually
arranged with a reversing key so that M and N may be interchanged,
hence Af could be 1,000 and N 1, and measurements of resistance as
high as 4,110,000 ohms could be made with the bridge we have con-
sidered.
Portable Testing Set. There are many different varieties of
bridges and their form always differs from that of the diagrams in
Figs. 19 and 21. A p>ortable testing set including Wheatstone's bridge,
galvanometer, battery, and keys,is illustrated in Fig. 22. The rheostat
of the bridge is made up of coils, 16 in number, of denominations
1,2,3,4,10,20,30,40,100, 200,300,400, 1,000,2,000,3,000,4,000
ohms— 11,110 ohms in all. Bridge coils are I. 10, and 100 on one
side and 10, 100, and 1,000 on the other. A reversing key admits of
any ratio being obtained in either direction so that the range of the
set is from .001 to 11,110,000 ohms. It
is, however, impossible to construct a
portable galvanometer of su£Bcient sensi-
tiveness for these measurements, and the
Fig. n jteTeniDB'jce7s. actual limits are from .001 ohm to 300,-
000 or 400,000 ohms.
The reveling key, shown in Fig. 23, consists of the blocks M, N,
P, and X .and two plugs which must both lie on one diagonal or the
other. The blocks are connected with the resistances indicated
by their letters. In the left-hand figure M b connected with X and JV
with P, and the bridge arms have the relation
In the right-hand figure M is connected with P and N with X, the
bridge arms then having the relation
The advantages of having a reversing key in the bridge arms are:
the increase in range obtained, six coils being made to do the work of
eight, and also that any error in the initial adjustment of the
bridge arms can be detected by having the two arms equal, bal-
ancing and reversing. Unless the resistance of the coils inserted in
M and JV are exactly equal, the system will be unbalanced after re-
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ELECnUCAL MEASUREMENTS
The galvanometer, the needle and scale of which are shown at
the left in Fig. 22, is of the D'Arsonval type, and the coil is mounted
in jewels. As this galvanometer is not affected by external magnetic
fields or electric currents, it b suitable for dynamo or shop testing.
The key for the galvanometer circuit b shown in front at the right.
The battery b made up of chloride of silver cdls mounted in
the bottom of the box. The cells will last a number of months even
with daily use. Flexible connectiDg cords, running from the cells,
have their terminal sockets combined with smalt binding posts so
that connection maj be made to an extra battery or other source of
e. m. f. if desired. Ilie left-hand key controls the battery circuit.
A plan of the connections of thb testing set is shown in Fig. 24.
The two lower
rows of coils
(marked 1 to
4,000) are con-
nected beneath
the top at the
right by a heavy-
popper rod and
constitute the
rheostat arm, or
what corre-
sponds to P in
the formula.
By withdrawing the proper plugs in these rows any number of ohms
h^m 1 to 11,110 may be obtained. The upper row of coils
con»3ts of the two bridge arms, M at the left and N at the right,
with the reversing key between them. The two extremes of the
upper row are joined by a heavy copper connection and correspond
to the point A in Fig. 19. The upper block X of the reversing key
b connected with the binding post B, the block P b joined to the left
of the middle row of coils while the other end of the rheostat combi-
nation is connected with the binding post C. The resbtance to be
measured X b connected between the terminals B and C.
Exavtfle. Suppose a balance b obtained with an unknown r^
sbtance connected between B and C, when the plugs are withdrawn
as shown is Fig. 24* What is the value of the unknown resistance?
.-^rv-sa
W\
f^^iMiMillr'
^^^?
Flg.M. Dikgnmof aTestlDBSei.
DignzedoyGOOgle
26 ELECTRICAL MEASUREMENTS
j Solution. The reversing key is arranged so that M is coDoected
with P and N with X, hence
In the figure JV = 1010, M = 101, and P = 4,000 + 3,000 + 400 +
40+20+10+4+1 =7,475.
Therefore X = ~-X 7,475 - 74,750 ohms
Ans. 74,750 ohms.
Use and Care of Bridge. Before beginning a measurement it is
essential that each plug be examined to see that it is firmly twisted
into place, also in replacing a plug the same care should be used. A
slight looseness will considerably increase the contact resbtance and
so introduce errors in the result. Moderate force only is needed in
placing plugs. A strong person may damage the apparatus. For
the same reason the plug tapers should be kept clean and the top of the
bridge should be free from dust and moisture. Special care should
be taken with the surfaces between adjacent blocks. The plugs
should be handled only by their vulcanite tops, and care should be
taken not to touch the blocks.
Pis- IC Slide wire Wbeatitone'a Bridge.
The plug tapers may be cleaned with a cloth moistened with
alcohol and then rubbed with powdered chalk or whiting. The pow-
der should be entirely removed with a clean cloth before the plugs
are replaced. Sand paper or emery cloth should never be used to
clean the plugs or bridge blocks. If there are no idle sockets for the
reception of the plugs when they are withdrawn, they should be stood
on end or placed on a clean surface.
Slide Wire Bridge. The simplest form of Wheatstone's bridge
is the slide wire Inidge. Fig. 25 illustrates the apparatus. The
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ELECTRICAL MEASUREMENTS 27
foundation of the bridge is a board well braced to avoid war[nng,
which, after being well dried, is saturated with hot paraffin to make
it a good insulator. The bridge wire is usually one meter long and
stretched between substantial anchorages at the ends. Heavy straps
of copper or brass serve as connections of negligible resistance to the
other parts of the bridge. The known resistance is inserted at R
and the unknown at X. Openings at A and A' are closed by heavy
metal straps for the usual method of use, or in more complete methods
by resistances which sie, in effect, extensions of the bridge wire.
The battery and its key are connected between B and B, and the gal-
vanometer between and G. The heavy rod back of and above the
bridge wire is a support for the galvanometer key K and the index
which is adjacent to the meter scale shown. The key and the index
may be moved along the rod to find the balancing point. A com-
mutator shown at the center of the apparatus serves to exchange the
relative position of X and R in the arrangement. The commutator
makes connection in four mercury cups. If the portion of the bridge
wire to the left of the galvanometer key is a cm. long, the rest of the
wire is 100-a long. If the commutator is arranged so that R is con-
nected to the left end and X to the right end of the wire, when a balance
is reached we have
R a „ o 100 - o
"F = ??iS ,otX = R
X 100 — o a
If the commutator b reversed and the new reading is a' we get
If the balancing point is near the end of the wire, it b evident that
any small error in the reading and the assumption that the connec-
tions are of negligible resistance, will result in greater error in the
final formula. For thb reason it b well to treat the first balance as
only approximate and after calculating X to take as known resistance
a new value of A as nearly as possible equal to X. In thb way the
balancing point will be brought near the center of the wire.
For very exact comparison of two nearly equal resbtances, we
insert auxiliary resbtances at A and A', These are in effect extensions
of the bridge wire. Call these resbtances equal to A and A' cm. of the
wire. When a balance is obtained at the points a and a' for the two
posidons of the commutator, we have ,
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28 ELECTRICAL MEASUREMENTS
^^^ X^ + lOO-g A + a'
A + a A'+lOO-a'
While it is still of advantage under these conditions to have a
and a' somewhere near 50 cm. it is no longer necessary, for with the
bridge wire extended by A and A' any point of the actual bridge wire
is now near the center.
If A is materially larger or smaller than X, the balondng point
may be beyond the end of the actual wire, i. e., in one of the exten-
sions, and no balance can be obtained. It 's necessary then to ad-
jurt R until a balancing point is found on the wire. We may then
proceed with the experiment.
A variation of this method, known as the Carey-Foster method,
is used for the comparison of two standard resistances to discover
small differences in adjustment.
Example. If with the openings A and A' closed with straps of
negligible resistance and a resistance of 150 ohms for R, the mean
balance point comes so that a = 68.4 cm. and b = 3L6 cm., what
is the value of ^? Ans. 69.3 ohms.
Examjde. If A and A' are equivalent to 500 cm. each, and R =
ISO ohms, and the mean balance point makes a = 68.4 cm., and b—
3L6 cm., what is the value of X^ Ans. 140.29 ohms.
Low Resistance Measurement. The bridge methods described
above are not suitable to use in
the measurement of small re-
sistances, for the lead wires (lead-
ing in wires) used in connecting
the unknown resbtance to the
— jy^^^^/yy^ 1 ITSSt^ — ' bridge may have more resistance
I I ' than the unknown. The am^
I |^^*'[ nwter-Doftm^ter rnHhod is that
l^JSP-' jQQg( generally used. The ap-
Flg. M. AmmeMT-Vollmeter Metbod of . . . _i i_ •
LiOwRealBtanceMeasuremem. paratUS IS connected as SllOWn m
Fig. 26. The current from the
battery b led through the ammeter to the unknown low resbtance R.
An adjustable resbtance r of a rheostat may be introduced into the
circuit to control the current. The actual resistance r need not be
known. The fall of potential V through R is measured by the volt-
meter, and the current / by the ammeter. Ohm's law then gives
■Q-i
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ELECTRICAL MEASUREMENTS 29
Ammeter-VoUmeter Method. It b evident that although the
voltmeter is of very high resistance, a small current included in that
measured hy the ammeter passes through the voltmeter. In strict-
V
ness this current, equal to — ; r-i ; , should be
resistance of the voltmeter
subtracted from the ammeter reading to get the value of 7 to be used
in the formula. This correction is easily made, as aU makers of volt-
meters give the value of the resistance, usually marked on the volt-
meter case; but the error resulting from neglecting the correction is
generally immaterial. Instruments of suitable range should be
used.
Examjile, The reading of the ammeter is 50 amperes, that of
the voltmeter 1.5 volts; what is R^ Ana. 0.03 ohm.
In the particular case chosen the ammeter had a range to 75
amperes, and the voltmeter to 3 volts. The resistance of the volt-
meter was 300 ohms. The current through the voltmeter was 1.5 -;-
300 » 0.005 amperes. It is evident that the correction is far smaller
than the probable error of reading the ammeter, and any attempt at
correction would be absurd.
Tliis method may also be used te measure the resistance of a
burning incandescent lamp. In such a case the bridge method is
useless as the resistance of a cold lamp is probably double its resistance
when hot.
Example, The voltmeter, to 160 volts range and resistance
15,000 ohms, reads 110 volts; the ammeter, to 1 amperes range,
reads 0.5 ampere. What is the resistance Rt .\aa. 220 ohms.
High Resistance MeaauFement. Direct Defiecticm Method. An
excellent method of measuring resistances of one megohm (one million
ohms) or more, is the direct deflection method. The main instru-
ments needed are a sensitive galvanometer, usually of high resbtance
and fitted with appropriate shunts; some standard resistances of
100,000 ohms (0.1 megohm) or more; and a battery of relatively low
resbtance and constant e. m. f. (a storage battery of many cells, if
available). The resistance of the galvanometer both alone and com*
bined with its shunts must he known. That of the battery and the-
connections b usually n^lected. The connections are shown id
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30 ELECTRICAI, MEASUREMENTS
Pig. 27. The known resistance R is first connected in series with the
galvanometer G and the testing battery B, through a key K, Care
should be taken that the insulation of the apparatus be very high. The
shunt iS is adjusted to give a suit-
able deflection of the galvanometer
and from this deflection what is
known as the constant is calculated.
The value of this constant is the
resistance that must be inserted in
the circuit to reduce the deflection
Tfl
I— wwwvwv — °
P1B.W. oK*ot.Dea*<«ioiiM«bodof ^ o"* scalc divisioH. The value
tiBbE«i.i«««M6«u«mei... ^f tj,g constant is therefore equal to
deflection d, and the multiplying power of the shunt m. We thus get
Constant = (R -{ ) d m
m
As an illustration, suppose R = 0.1 megohm, G = 20,000 ohms
= 0.02 m^ohm, m = 1,000, and d = 200 divisions; then
Constant = (0.1 + 0.00002) X 200 X 1,000
= 20,004 megohms
This means that if the galvailometer were unshunted and the total
resistance in the circuit were 20,004 megohms, a deflection of one
division would result.
After the constant has been determined the known resistance
R is replaced by the unknown resbtance X. The galvanometer shunt
is readjusted if necessary and the deflection obtained is again noted.
Hie value of the total resistance is then found by dividing the value
of the constant by the product of the deflection d^ and the multiplying
power m, of the shunt used. To continue our illustration suppose d,
= 50 divisions, and tk, = 10. The deflection, if the full current vrent
through the galvanometer, would be 50 X 10 = 500 divisions. A
deflection of one division is produced with a resistance of 20,004
megohms; hence a deflection of 500 divisions must correspond to
1^ of thb, or 40.008 m^ohms. Subtracting the resistance of the
shunted galvanometer 20,000 -f 10 = 2,000 ohms, or 0.002 megohm,
leaves the unknown resistance 40.006 megohms. The algebraic
equation expresung this b
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ELECTRICAL MEASUREMENTS
(iiH
- — = 40.006 megohms
Neglecdng the resistance of the galvanometer in both cases, a simpler
formula would give
„ Rdv
d, m.
' 40 megohms
It may be noted that the difference between these results is an amount
corresponding to a difference in deflection of 0.0075 of a sinf^e scale
divbion, which is far smaller than the probable error which an;
observer would make. It is then clearly permissible to use the simpler
formula, ,, ,
(/, m,
Example. In a high resistance measurement by the above
method the known resistance was .2 me^hms, and gave a deflection
of 237 divisions, the multiplying power of the shunt being 100. With
the unknown resistance inserted, the deflection was 178 divisions
with the full current passing through the galvanometer. What was
the value of this resistance? Ans. 26.6 mc^hms.
Vcitmder Method. Another method of measuring high resistance
is that in which a sensitive high
resistance voltmeter such as the
Weston is used. This method,
however, is not as accurate as the
preceding and is not adapted to
measurements of resistance greater
than a few mcfiohms. The volt-
meter b connected in series with
the unknown resbtance and a
source of constant e. m. f., as shown in Fig. 28. With such an ai^
rangement the resbtance X will be to the resistance of the voltmeter
R, as the volts drop in ^ b to that in the voltmeter. The drop v in
the voltmeter b given by its reading, and if the applied electromotive
force V is known, the drop in X will be F — t>. We therefore have
the proportion,
X : R : : V — V : V, and
Mraaaremiiit-
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32 ELECTRICAL MEASUREMENTS
The voltage V, which should be at least 100, may be first determined
by measurement with the voltmeter.
Example, A voltmeter having a resistance of 15,000 ohms,
was connected in series with an unknown resistance. The e. m. f.
applied to the circuit was 110 volts and the voltmeter indicated 6 volts.
What was the value of the unknown resistance?
Solution. Applying the preceding formula
V - no, « = 6, and /i = 15,000,
therefore
X = "^"'^ X 15,000 = 260,000 ohms, or .26 me^hms
Ans. .26 megohms.
Insulation Resistance. The measurement of insulation resist-
ance is performed by either of the two preceding methods of measuring
high resistance. The voltmeter method is the simpler, but since it
cannot be used to measure resistances greater than a few megohms,
the direct deflection method proves to be the more valuable. The
insulation of low potential circuits, however, need not exceed five
megohms, and in testing such circuits the voltmeter method may be
used. If little or no defi^ction is obtained it is then evident that the
insulation is at least several megohms, which is all that is desired. As
the insulation of high potential circuits must be greater than five or
ten megohms, the direct deflection method should then be used.
The connections in testing the insulation of a circuit by these
two methods are similar to those shown in Figs. 27 and 28, the resbt-
ance X being replaced by the insulation of the circuit This is ac-
complished by connecting one wire to the line and the other to the
ground such as, to a gas or water pipe. The insulation of the line
from the earth is then included in the testing circuit; the current
passing from the battery, or other source, through the voltmeter or
galvanometer to the line, from the line through the insulation to the
ground, and then to the battery.
The insulation of a dynamo, that is, the resistance between its
conductors and its frame, is tested in a similar manner. This re-
sistance should be at least one megohm for a 110-volt machine, but
two megohms is to be preferred and is customary. This insulation
is measured by connecting one wire to the frame and the other to the
binding post, brushes, or commutator. The insulation is then in-
cluded in the circuit. Insulation lesistance decreases with increase of
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ELECTRICAL MEASUREMENTS 33
cemperatuTe so that this test of a machine should be nmde after a full
load run of several hours.
The e. m. f. used should be constant and of one to two hundred
Tolts value. Secondary batteries are the best" for this purpose,
but silver chloride testing cells are much used. The resistance of
dielectrics increases by continued action of the current and this pro-
perty is known as electrificaiion. For this reason the deflection should
not be read until after a certain period of electrification — usually one
minute. This action is quicker in some materiab than in others, and
f is also greater at low than at high temperatures.
Insulatimi Resistance ot Cables. In the preceding cases of
insulation resistance only a part of the insulation is under electric
strain. In the case of submarine cables and lead covered cables used
on land, the whole of the insulation is subjected to the electric straJn.
To test the resbtance of a waterproof insulation, the insulated wire or
cable may be immersed in a tank of water. Care should be taken to
leave enough of the cable out of the water so that surface leakage
near the ends may not interfere with the test. For short lengths of
cable the resistance of the wire inside the insulating material may be
ignored. The resistance between the wire and a metal plate im-
mersed in the tank is practically the resistance of the insulation. Fig.
27 shows the arrangement of the apparatus. As a cable takes a cer-
tain charge as a condenser when subjected to an e. m. f., it is necessary
to protect the galvanometer, by a short-circuiting switch between the
galvanometer terminals, during the rush of current on first closing the
circuit. The switch box S, Fig. 27, has such a short-circuiting switch.
. This is important as otherwise the galvanometer may be injured. If
the insulation resistance is not too high the direct deflection method
above described may be used. If the insulation is excellent the de-
flection produced by the leakage should be very small and some other
method must be used.
Charge and Recharge Method. An excellent method in such
cases is the charge and recharge method. In this method. Fig. 27, the
cable is first charged for several minutes, care being taken to short-
circuit the galvanometer. The circuit is then opened for, say, one
minute and the circuit closed again, the short-circuiting switch of the
galvanometer meanwhile having been opened. While the circuit was
open, a certain part of the charge leaked out and this is now replaced
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34 ■ ELECTRICAL MEASUREMENTS
by an equal added charge. The galvanometer makes a sudden throw
due to this added charge and after many oscillations comes to rest.
If the relation between the added charge and the galvanometer throw
is known, the quantity added may be computed, and the leakage cur-
rent is equal to the added charge (equal to that lost) divided by the
time in seconds for which the circuit was open. If the steady leakage
produces a measurable deflection, account should be taken of this in
estimating that part of the sudden throw produced on closing the
circuit again. This correction we here suppose to be negligible. To
find the relation between charge and throw, a condenser of knowr.
capacity (farads) is charged by a known e. m. f. and then discharged
through the galvanometer. The charge equals the product of capac-
ity and e. m. f. used. The charge divided by the throw produced,
gives the constant of the instrument as a ballistic galvanometer. As
condensers are rated in microfarads (millionths of farads) care must
be taken to use the value in farads if the value in ohms insulation
resistance is required. If the value in microfarads is used, the final
result will come out in megohms. If an e. m. f. £, volts and capacity
C microfarads produces a ballistic throw d^, and if an e. m. f. of £,
volts produces a throw oid^ on closing the circuit through the insulation
under test after the circuit has been open for ( seconds, ignoring the
resistance of other parts of the circuit, the insulation resistance is
X =• ,' — ^megohms
£, X rfj X C ^
Example. If E, is 1.44 volts, C is 0.5 microfarad, d, is 28.8 cm.,
£, is 100 volts, dj is 20 cm., and i, is 60 seconds, what is XI
Ans. 12,000 megohms.
As a rule the insulation resistance per mile is required. The
longer the cable the more surface is exposed to leakage; consequently
it is evident that the insulation resistance per mile is found by multi-
plying the resistance of the sample by its length in fractions of a mile.
That is, a mile of cable would have, say, one-quarter as much insula-
tion resistance as a quarter of a mile of cable.
In the case of lead covered cables, no tank is required, as con-
nection may be made with the lead covering instead of the immersed
metal plate before mentioned.
Resistance of Lines. Telegraph, telephone, and power trans-
mission lines may be measured in place to best advantage if one or
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ELECTRICAL MEASUREMENTS
35
more additional lines are available between the terminals. If only
one wire is available both ends may be connected to ground and the
resistance, which involves that of the connections to ground and that
of the earth return, may, be measured by one of the methods described
above. Such a method though unsatisfactory may be the best avail-
able. The resistance of the earth return is generally low, but there is
always much uncertainty as to the resistance to earth at the ends.
Earth currents of electricity, due
to many causes, may much com-
plicate the problem. When a
second line of resistance X^ is
available, that and the unknown
resistance X^ may be connected
together at the distant end and
the combined resistance R, which
equals^, 4- .Yj, measured. Next,
the distant junction may be
grounded and the two wires con-
nected as the proportional arms
of a Wheats tone's bridge, as
illustrated in Fig. 29. The re-
sistances in the other propor-
tional arms are R^ and ii,. One
terminal of the battery is ground-
ed. When a balance is obtained the proportion of the whole resist-
ance R in X is
In a similar way, X^ = R g- ' - „-
It is well to connect the battery in that branch of the bridge
which includes the earth return as there may be a difference of potential
due to earth currents, which does not disturb the bridge as it simply
adds to or subtracts from the battery e. m. f. If a third line is avail-
able it may be used in the battery branch in place of the earth return.
It will be noted that the resistance to earth at both ends is not in any
of the four proportional arms and consequently does not affect the
result.
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36 ELECTRICAL MEASUREMENTS
Example. The two wires lo>^>e<l together have a resistance of
248 ohms. When a balance is obtained with them aa arms of the
bridge, R^ = 1,000 ohms and K, = 1,127 ohms, the proportion being
«,:/(,: ^.X^■.X,. What are J, and Z,f
Ans. X^ = 116.6 ohms; X^ = 131.4 ohms.
Locating Grounds. In case a line wire is grounded at some
UDknown point, the following method may be used in locating
the ground. The grounded wire and a second wire free from
grounds may be looped together and their combined resistance
measured. The loop, as before, is connected as two arms of a
bridge, but the junction is left insulated. X^ is now the resist-
ance from the testing station to the point where the wire is
grounded. X^ is the combined resistance of the rest of that wire
and the whole of the other wire. The resistance to the grounded
point is then
As a rule the resistance of every line is part of the office data,
and therefore fl = X, + X, is known in advance and need not be
remeasured. As the resistance per mile is also usually part of the
office data, the actual distance corresponding to T, may be computed
and a lineman sent to the point to make the repair. If in a severe
storm several grounds occur on the same wire, this method, of course,
cannot be used to locate the trouble. In the case of ocean cables
this method is used with excellent results. The cable repair steamer
can be sent to the point of trouble where the cable is raised and re-
paired.
Locating Faults. This method may be used in the care of a
broken submarine cable if both ends are exposed to the water,
but it cannot be used for broken land-lines because the ends,
even if both on the ground, are too imperfectly grounded. If
the conductor of a submarine cable is broken but the insulation
left intact, thb method cannot be used. A method, however, in
which the distributed capacity of the cable is measured in mi-
crofarads (see Capacity Measurements later) can be used to de-
termine the location of the break. This latter method may also be
used for a broken land-line where the end of the wire hangs free of
the ground.
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ELECTRICAL MEASUREMENTS
MEASUREMENT OF BATTERY RESISTANCE
Vohmeter Method. The following voltmeter method may be
used to measure battery resistance. The battery of one or more cells
is connected in circuit through a key K, with a known resistance R.
The voltmeter of appropriate range
is connected, as shown in Fig. 30,
to the terminals of the battery.
Vttth the key K open, T,— the e.
m. f. of the battery — is measured.
Tlie key K is then closed and K, -
—the reading of the voltmeter —
is observed. By Ohm's law the
V,
current is -^f . A part of the bat-
R '^ FigX). VoltmeMrHetbodoeifeuarlns
tery's e. m. f., equal to F, — F,, i
DOW lost inside Uie battery because of the resbtance X of the bat-
tery. We then have the relation,
V,- F, = Z^, and
If the resistance R is not known, an ammeter may be introduced
into the circuit in series with R, and the current / me€tsured directly
Tien
a:/ = F, - F„ and
It will be noticed that it is tacidy assumed that when the key K
is open, not enough current will pass through the voltmeter to intro-
duce any error. If the battery resistance b large this error is not
n^ligible and a sensitive high resistance galvanometer with con-
siderable additional resistance, perhaps 100,000 ohms besides, may
be substituted for the voltmeter. If the deflections of the galvanom-
eter in the two cases (open and closed) are rf, and d,, we then have
As the battery when furnishing a current begins at once to fall
off in e. m. f., that is, polarize, a small error due to polarization makes
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ELECTRICAL MEASUREMENTS
the battery resistance appear too high. Such a value of the resistance
R should be chosen as to make the deflections materially different.
Otherwise a slight error in V^ and K, or d^ and d^ will make their
difference V^ — K, or d, — (^ many per cent in error.
Example. A cell has an e. m. f., V^ = L47 volts whenXis open
and L12 when 5 is closed, it is 5 ohms. What is X?
Ans. X = 1.56 ohms.
Mance's Method. Another method is Mance's method, in
which the battery, whose resistance X is to be determined, forms one
arm of a Wheatstone's bridge, as indicated in Fig. 3L No key is
placed in the galvanom-
eter branch and no ad-
ditional resistance, in
the branch which in-
cludes the key S. The
resistance in B, is ad-
justed until the galva-
nometerdoes not change
its deflection on closing
the key iS. It is usually
necessary to put consid-
erable additional resist-
ance R in the galvanometer arm to keep the deflection small. If
the deflection does not change on closing the key S, it is evident that
the decrease in the potential difference at the terminals of the cell due
to its increased current when S is closed, must exactly equal the de-
crease in the potential difference between the terminals of fl, due to
this path being robbed of a part of its current because of the new
path. Otherwise the potential difference at the galvanometer termi-
nals, which is the difference of the potential differences over the two
arms, would change and the deflection change. Similar reasoning
applies to R^ and R,, only here the difference over 71, increases by
just the amount that that over R^ falls, thus keeping the total amount
constant for the combination of R^ and Ry The arrows show the
direction of the currents in the various arms. If no change in the
galvanometer current occurs, the changes in R, and R, must be equal
and so also the changes in fi, and A'. It follows then if the galvanom-
eter deflection remains constant whether S is open or closed, that
pig. SI. Bridge Diagram for Mance's Method.
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ELECTRICAL MEASUREMENTS 39
R,:n,::R,:X, or
y II,XR,
— «r
Should the battery polarization change on closing the key S, the
galvanometer deflection will change. For this reason the key S
should be closed for an instant only.
Besides these methods there are excellent methods in which
alternating currents are used, but they are too advanced to be de-
scribed in this course. Such alternating-current methods should be
used in measurement of the resistance of solutions (so called elec-
trolytes) which are decomposed by a direct current.
MEASUREMENT OF ELECTROMOTIVE FORCE
Voltmeter Method. The simplest method of measuring an elec-
tromotive force is by the use of a voltmeter which indicates directly
Fig. SS. Commercial Portable VoltmetW.
the number of volts. A voltmeter of the proper range should be
chosen. For very small e. m. f 's the miUivoltmeter, usual range 1 to
300 millivolts, i.e., 1 to 0.3 volt, may be used. For higher e. m. f.'s,
voltmeters reading to 1.5, 15, 150,and 300 volts respectively, are made
by the Weston Electrical Instrument Co. and others. A voltmeter
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40 ELECTRICAL MEASUREMENTS
is simpl}' a galvanometer calibrated to be read in volts. It is evident
that if an additional resistance equal to that of the voltmeter is placed
in series with the latter, twice the voltage will be required to produce
the same deflection as before. In general, any resistance in series
which makes the total resistance n times that of the voltmeter alone,
may be used as a multiplier; and the reading of the voltmeter must
be multiplied by n to get the value of the e. m. f. Such a multiplier
may be bought with a voltmeter in order to make its effective range
greater. For example, if the resistance of a voltmeter of range to
150 volts, is 15,000 ohms, a multiplier having a resistance of 60,000
ohms will bring the total up to 75,000 ohms, and the constant n of the
multiplier is 5. With this multiplier in the circuit the upper limit of the
voltmeter is extended to 750 volts. If the multiplier is mounted
inside the voltmeter, and if on using the binding posts marked and
15 the range b to 15 volts, and using the binding posts marked
and 150 the range is to 150 volts, the multiplier evidently must
have nine times the resistance of the main part Such a voltmeter,
shown in Fig. 32, is said to be a two-scale voltmeter, and it may be
used equally well on either range. As the extra expense of providing
the multiplier and extra binding post is slight, a two-scale voltmeter
is a very inexpensive substitute for two voltmeters. It b also evident
that a low range voltmeter may be used in connection with any resist-
ance box as a multiplier. The Weston voltmeters have approximately
100 ohms resistance per volt of range and, therefore, take a maximum
of about 0.01 amperes when used on an e. m. f. which is the maximum
of the range. As a rule the current taken by a voltmeter b negligible
in comparison with the current in the rest of the circuit.
The voltmeter method may be used equally well with both direct
and alternating electromotive forces.
Potenti<Mneter Method. For the comparison of e. m. f.'s, the
potentiometer is the most accurate apparatus. When a balance b
reached the e. m. f.'s to be compared are not allowed to furnish any
current, and consequently no polarization results in their source. The
effect of internal resbtance b absolutely nH also. The arrangement of
apparatus b shown in Fig. 33.
Two resbtance boxes M and N, each of 10,000 ohms capacity,
are arranged to have plugs withdrawn to a total of 10,000 ohms, and
are connected in series with a battery B. To avoid injuting B, plugs
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ELECTRICAL MEASUREMENTS
41
_ o|ofo| _
M O- © AC
correspoDcling to 10,000 ohms should be withdrawn before connecting
it in circuit. The circuit has a high resistance and the effect of polari-
zation of the battery S quickly reaches its limit and a steady current /
flows through the circuit. If the resistance in box Af is iZ ohms, that
in box N is (10,000 — R) ohms; and the fall of potential over M is
R I volts, and over the box JV is (10,000 - R) / volts. One of the
celta to be compared, a standard cell of e. m. f. S, is connected in
series with some high resbtance A, a sensitive galvanometer G, and
a key K. In general, on
closing the key K the gal- lljl
vanometer will deflect; but
if the resistances in M and
JV are adjusted until the
potential difference over M
is exactly equal to the e. m.
r. of the cell S, the latter is
in perfect balance and can
neither supply current to
the general circuit supplied
by B nor can current be
forced backward through
the cell S. In that case the ^<i- » PotwiUo-^^tw Method
e. m. f. of S equals the fall
of potential through M, and, calling R, the resistance in M,
S = I R,
If now a second cell of unknown e. m. f. ^ is substitued for S, and
the resistances in M and N readjusted— but their sum kept 10,000
— until on closing the key K no deflection results, calling the new
value of the resistance in M, R,, we have the relation
X = IR,
It follows that
V ^ R,
If S is known, X may be computed. It b well to repeat the
balance with S to be quite sure that no change has meanwhile oc-
curred in the m«n battery B. Precaution should be taken in setting
■ up the apparatus that B is greater than either 5 or ^ and that they
ore connected into the circuit so that they are in oppo^tion to B. If
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42 ELECTRICAL MEASUREMENTS
these conditions are not obeyed, it is evident that no balance can be
obtained. If no exact balance can be obtained, but a change of one
ohm changes the galvanometer deflection from up to down the scale,
the fraction of an ohm needed for an exact balance can be obtained by
interpolation. In the next article on the calibration of a voltmeter
such an interpoladon is made.
Example. With a total resistance in Af and N of 10,000 ohms
of which the resistance in M was B, = 5,267 ohms, when the standard
Ctark cell of 1.433 volts was in the galvanometer circuit, and the
resistance was R, = 5,470 ohms when the Leclanch^ cell was in the
galvanometer circuit, what is the e. m. f. X of the Leclanch^ cell?
Ans. X = 1.4882 v(Ats.
Calibration of a Voltmeter, If during the previous experiment,
a voltmeter had been connected for the whole time between the ex-
treme terminals of M and N, the potential difference V between the
terminals of the voltmeter would have been
V = 10,000 / = 10,000 -|-
Tbis gives a convenient method of calibrating a voltmeter by means
of a standard cell of known e. m. f. jS. The calibration may be ex-
tended to various points of the voltmeter by chan^ng the e. m. t.
of the main battery B. In such cases the resistance in M, to obtain
a balance, will change in the inverse ratio. It is evident that the
calibration cannot by thb method be extended to points below the
e. m. f. of the standard cell. In the case of high e. m. f.'s, it is desir-
able to increase the total resistance in M and N beyond 10,000 ohms.
For example, if the e. ro. f. produced by B at the terminals of the volt-
meter is 150 volts, a total resbtance of 100,000 ohms would be about
right Id this case over 99,000 ohms would be in ^ and less than
1,000 in M. If several standard cells are available, they may be con-
nected in series in the galvanometer branch, thus increasing the
resistance for a balance in the box M, As most boxes have one ohm
for their smallest resistance, a greater per cent of accuracy is ob-
tained if the resistance in i/ is large. If an exact balance cannot be
obtained and the nearest smaller resistance produces a deflection d^
one way, and the nearest larger resistance produces a deflectioD tf,
in the opposite direction, the fraction of an ohm which would have
produced a balance b evidently
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ELECTRICAL MEASUREMENTS 43
d, + d^
Example. With a total resistance of 100,000 ohms in M and N,
and 987 ohms in M producing a deflection of 5 divisions down the
scale, and 388 ohms producing a deflection of 15 divisions up the scale,
and a standard Clark cell of L433 volts e. m. f. in the galvanometer
drcuit, what is the correction to be added to the voltmeter reading
which was 144.9 volts? Ans. Correction = + 0.25 volt.
Suggestion of Solution. The change in deflection by change of
one ohm is 5 + 15 scale divisions; therefore, 987 is 0.25 ohm too
small and 988 is 0.75 ohm too large. The e. m. f. Ggures out 145.15
Totts; therefore, 0.25 volt will be added to the voltmeter reading to
give the correct result.
This method, as will be seen later under the head of "Measure-
ment of Current," can be used with a standard cell to measure a
current.
Condenser Method. If a con-
denser of capacity C is connected
by means of a charge and discharge
key K, which baa an upper and a
lower contact, as shown in Fig. 34,
alternately to a standard cell of e.
m. f. B, and a ballistic galvanometer
G, the throw d of the galvan<mieteT
will be a measure of the charge of
the condenser equal to B X C If,
now,aceII of unknown e. m. f. Z is '"^b " ^eTt'^^M^ST'""'™^
substituted for B, the deflection
d^ will be a measure of the charge of the condenser now equal to
X XC. It follows that
X = B^
This method is free from difficulties due to polarization and
internal resbtance of the cells; because the very small charge taken
by the condenser produces no measurable polarization, and the effect
- of internal resistance is only to lengthen the time of charging of the
condenser, but not to change the total quantity.
The accuracy of the method, however, is limited to that of the
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44 ELECTRICAL MEASUREMENTS
reading of the deflections d, and d^ and it is difficult to get reaults
much closer than, say, j- of 1%. The potentiometer method will
give results easily to tbVit °f 1% if t^^ resistances used are accurate;
and in general the accuracy of the potentiometer method is limited
only by the accuracy of the resistances. For accurate comparisons
the potentiometer method is always used.
VOLTAIC CELLS AND BATTERIES
A voltaic cell is usually composed of a pair of electrodes
immersed in a liquid or in two liquids separated from one another
by a porous partition. The Uquid, or liquids, must be what is
known as an dectrolyte; and must undergo a chemical breaking
up, called eUdrolysia, with chemical action on one or both elec-
trodes when the circuit is closed and a current flows through the
cell.
Some cells fall rapidly in e. m. f. when the circuit is kept closed.
TUs phenomenon b known as ■polarization, and it frequently is due
in large part to the deposit of a film of hydrogen gas on the surface
of one of the electrodes (the cathode). This gas is one of the products
of the electrolysb of the liquid. If the cathode, for example a copper
plate, as in the case of the gravity cell, b surrounded by a soludon
of copper sulphate, the hydrogen does not reach the copper plate but
is intercepted by the copper sulphate solution, and copper instead of
hydrt^n is deposited. Naturally, the deposit of copper on a copper
plate produces no polarization. The copper sulphate solution is
called a depolarixcr. The other electrode of the gravity cell is zinc
and is immersed in a dilute solution of either zinc sulphate or sul-
phuric acid.
In the Grove and the Bunaen cells, the depolarizer is nitric acid
in a porous cup in which the cathode is immersed. The nitric acid
is rich in oxygen which it ^ves up to oxidize the hydrogen gas, thus
forming water (H,0) which makes the solution more dilute but
causes do polarization. Celb with liquid depolarizers cannot be left
on open circuit as the liquids diffuse into one another and the cell b
spoiled. Cells for open circuit use must have either a solid or a
paste for a depolarizer. The Leclanch^ cell has manganese dioxide
(a solid) packed in a porous cup about its cathode of carbon. The
data of various cdls can be found in books on cells.
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ELECTRICAL MEASUREMENTS 45
Standard Cells. There have been various cells used as standards
of e. m. L of which the Clark and the Weston have received most at-
tention. A standard cell must be composed of materials which,
white the cell is in use, do not change; that is, no new substance may
be formed by the action of the cell; the cell should have little or no
polarization when used by zero methods like the potentiometer
method described above; the cell also must not deteriorate when left
on open circuit.
Both the Clark and the Weston cells fulfill these requirements.
At the International Electrical Congress, held in Chicago, 1893, the
normal Clark cell was recommended for international legalization and
its e. m. f. at 15" C. was voted to be considered for practical purposes
as 1.434 volts; and a coounittee, consbting of Professors von Hdm-
holtz, Ayrton, and Carhart, was charged with the duty of dramng up
specifications for the precise form of the cell. Von Helmholtz died
soon afterward and the other members of the committee could not
agree on the specifications, with the result that the principal countries
(in electrical matters) have never agreed on a form for the cell.
It has now been displaced by the Weston cell, which, in 1908, was
recommended by an International Conference in London as an
international standard. It is now known that the Clark cell bas an
e. m. f. slightly below 1.433 volts, instead of 1.434 volts as thought in
1893. The normal Clark cell uses as materiab zinc amalgam in a
saturated aqueous solution of zinc sulphate, with an excess of ^nc
sulphate crystals present, and pure mercury in the presence of mercu-
rous sulphate in the form of a paste which acts as the depolarizer.
The action of the cdl is to form more zinc sulphate and reduce some
of the mercurous sulphate to mercury, or viee versd, when the current
flows in the direction of the e. m. f. or is driven in the opposite direc-
tion by a greater outside e. m. f. The principal objection to the
normal Clark cell is that its e. m. f. changes by a considerable amount
with change in temperature, falling about 0.08% for every Centigrade
degree rise in temperature.
The Weston normal cell is similar to the Clark cell except that
cadmium replaces the zinc, and cadmium sulphate the zinc sulphate
The Weston normal cell has a much lower temperature coefficient
than the Clark cell, its e. m. f. falling about 0.00406% for each Centi-
grade degree rise above 20^ C. and vice versA, The e. m. f. of the
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46 ELECTRICAL MEASUREMENTS
Weston normal cell at 20° C. was recommended by the London
conference of 1908 to be taken provisionally as 1.0184 volts.
borage Cells. Many voltaic cells when exhausted, may be re-
charged by forcing current through the cell in the reverse direction
by the application of an outride e. m. f. greater than the e. m. f, of
the cell. Such cells are called storage cells. In general only such
cells as fonn no tuw kind of material when discharging are reversible,
and evidently a cell to be charged and dischaif;ed repeatedly must be
reversible.
All standard cells must be reversible. ReversibUi^, however,
is not the only requirement of storage cells to be used commercially.
Other qualities required are low internal resistance, large capacity
for chai^ measured in ampere hours in comparison with size and
wdght, long life under service, ability to stand without harm in open
circuit, moderate cost, etc.
The storage cell most used commercially has both plates of lead
with dilute sulphuric acid as electrolyte and lead peroxide as the
depolarizer. The lead peroxide is a solid or paste which adheres to
the positive pole of the battery. The e. m. f. of a lead cell is about
2.2 volts when fully charged and may safely be discharged until its
e. ID. f. is reduced to 1.8 volts. When the cell b chaif;ed one plate
has a deport of lead peroxide, and the other has a spongy texture,
due to its reduction from an oxide or a su'phate of lead in its previous
history. When the battery is discharged, the sulphuric acid is elec-
trolyzed; the hydrogen formed reduces some of the lead perox-
ide of the positive, and the sulphion forms some insoluble lead
sulphate from the ne^tive. The sulphuric acid becomes more
dilute. On rechar^ng the cell the lead sulphate is reduced to
spongy lead at the negadve, some additional lead peroxide is
formed on the positive, and the density of the sulphuric acid in-
creases. For detaib as to the manufacture of the various variedes
of lead cells and other ston^ cells, the reader is referred to works
on storage cells.
To increase the capacity of a cell the negative consbts generally
of a number of plates connected together both electrically and mechan-
ically, and the positive consists of one plate less in number and
connected together in the same manner. The posidve plates are
interlarded between the negative plates.
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ELECTRICAL MEASUREMENTS 47
Storage celb are also called secondary cdls or aecumvlaion by
some writers.
Batteries. The word battery is technically used to mean a group
of cells. In common parlance the word is used sometimes to mean a
sin^e cell; but this use is not to be recommended.
MEASUREMENT OF CURRENT
ElectrodynamtHiieters. In the choice of a unit of current, it was
decided that a unit of current in a straight conductor at right angles
to a unit magnetic field, should exert a force (at right angles to both
the directions of current and field) of one dyne per unit length of the
conductor. As mentioned earlier, this unit of current was thought to
be inconveniently targe by the committee of the Britbh Association
for the Advancement of Science, which had the matter of electrical
units in charge, and as a consequence for practical purposes they
recommended that one-tenth of this theoretical unit should be taken
as the practical unit. The latter unit is called the ampere. As the
magnetic field due to the flow of an electric current in coils, may be
computed from the data of the coils and the current, it is evident that
absolute electrodynamometers may be made to measure current with-
out the intervention of other electrical measuring apparatus. These
absolute electrodynamometers may take various forms, including
that of current balance. By means of an absolute electrodyna-
mometer and a standard resistance, the e. m. f. of standard cells may
be determined with a high degree of accuracy according to the
principle of Ohm's law.
By comparison either directly with the standard electrodyna-
mometer or indirectly by means of standard resistances and standard
cells, other forms of electrodynamometers and all forms of ammeters
may be adjusted so as to read amperes. It is evident from the above
that as more improved absolute electrodynamometers are constructed,
we may expect greater exactness in the determination of the e. m. f.
of the Weston normal cell, which for the present is taken as 1.0184
volts at 20° C.
Ammeteis. An ammeter Is a galvanometer graduated so that
it reads current directly in amperes. Thb graduation is obtained
directly by comparison with either an absolute electrodynamometer
or indirectly by means of a standard eel! and standard resistances.
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48 ELECTRICAL MEASUREMENTS
Ammetera for any desired range of current are on the market, and
the accuracy of their readings is in proportion to the care with which
they have been constructed and calibrated. Even the best are
moderate in price and the poorest should not be one per cent in error.
Ammeters to measure large currents carry the main portion of
the current through shunts which differ theoretically in no respect
from the shunts used with other forms of galvanometer.
To measure current by means of an ammeter involves intro-
ducing the instrument into the circuit, care being taken to connect the
terminal marked + to the positive terminal of the source of e. m. f.
The exact position of the ammeter in an undivided circuit is not im-
portant, as the current is the same throughout the circuit. Care
should be taken that the ammeter has a range which the current does
not exceed; otherwise the»pointer may be bent or even the ammeter
may be burned out by the action of excessive current. An ammeter
has an exceedingly small resistance, and to connect an ammeter with-
out additional resistance between the terminals of a dynamo or a
battery is to produce a short circuit practically. Excessive current
will flow through the ammeter and it probably will be destroyed.
Ammeters designed for direct-current circuits cannot be used
on alternating-current circuits. Some forms of A. C. ammeters may
be used on D. C. circuits; but as a rule ammeters should be used on
the type of circuit for which they are designed.
CallbratiMi of Ammeters. An ammeter may be compared
directly with another in the same circuit by putting them in series and
observing their readings with various values of the current. The
current may be varied by changing the resbtance in the circuit or the
e. m. f. The potentiometer method may be used to calibrate an am-
meter.
Potentiometer Method. The most exact method of measuring a
current, assuming that the e. m. f. of a standard cell and the resistance
of standard coils are known, is the potentiometer method. This is
not an absotvie method. The arrangement of the apparatus is some-
what complicated; but if it is compared with the potentiometer
method as used for the comparison of e. m. f.'s (page 41) it
will be seen that the commutator C, Fig. 35, is used to insert in
the galvanometer branch either the standard cell of known e. m. f.
S or a potential difference over a known resistance B due to the
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ELECTRICAL MEASUREMENTS
current to be measured 7. The current / may be passed also
through an ammeter A in circuit with R, a rtieoatat and an
amdliaiy battery £, which causes the current / to flow through the
circuit In the lower part of the diagram the galvanometer Q
and, if desired, a high resbtance to protect the galvanometer, are
connected between the left end of the commutator and one tei^
minal of the resistance
box L; the right end of i 1 | 1 1-
the commutator is con-
nected to the other tet^
minal of L. The auxil-
iary battery B, serves
to send a constant cur-
pent through L and M,
whose combined resist-
ance is kept Constant at,
say, 10,000 ohms. In
setting up the appara-
tus, 10,000 ohms should
be inserted in L and M
before connecting the
battery £,; otherwise the
battery will become badly polarized due to an excessive cutrenL
This precaution is very important. When a balance is obt^ned no
current passes through the galvanometer on closing the key K.
llie order of procedure is as follows: first, connect S in circuit
by means of the conmiutator C and shift resistance from L to M, at
vice versd undl, on closing the key K a balance is reached, as indicated
by a zero deflection of the galvanometer. The fall of potential
through L, which under these conditions is exactly equal to the e. m. f.
of the standard cell jS, is then equal to its resbtance it, muldplied by
the current / from the battery B„ or S = ii, /,; second, close the
upper circuit by K^, reverse the commutator to the position shown in
the figure by dotted lines, which throws the p. d. over R into the
galvanometer circuit. Adjust L and M until no current flows through
the galvanometer on closing the key K,. The total resbtance in L
and M b sdll kept 10,000 ohms. The resistance in L now haa a value
R^ and the potential difference over L is now it, 7, and is also in the
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50 ELECTRICAL MEASUREMENTS
galvanometer circuit. If no current flows through the galvanometer.
Rj I^ must exactly equal and oppose the fall of potential R I over the
resistance R. We then have
S = R,I„ and RI = R,I„
or, combining these equations,
I A X S
^R XR,
By adjusting the rheostat in the upper, or the battery £, circuit,
the current to be measured /, and consequently the point at which the
calibration of the ammeter is desired, may be changed at will. The
only limitation is that the battery B, must have a higher e. m. f. than
the standard cell's e. m. f., and also higher than the largest p. d. over
the resistance R.
As the e. m. f. of B^ may be increased, if necessary, by intro-
ducing additional cells into B^, there is no limit to the current / (in the
upper circuit) which may be measured. In the case of very large
currents the standard resistance R should be of relatively low re-
sistance. If too much heat is developed in R its temperature may
rise with consequent change of resistance.
Standard resbtances are made of manganin which will carry
any reasonable current without undue healing or change of re-
sistance. These standards are arranged so that the wire may be
immersed in an oil bath (pure petroleum) which may be kept
stirred so that the heat may be carried away. A thermometer in
the oil bath may be used to measure the temperature. Allowance
thus may be made for any change in resistance due to change in tem-
perature.
Such standards are made by the best manufacturers for 100, 10, 1,
0.1, 0.01, and 0.001 ohms resistance. Standard resistances calibrated
by the Bureau of Standards at Washington are moderately expensive,
but the cost is not higher than the cost of manufacture and testing at
the Bureau would warrant. The certificate which accompanies each
coil states the resistance at 20° C. and the change of resistance per
degree change of temperature.
Silver VoUamder Method. If a current b passed through a solu-
tion of silver nitrate, each ampere deposits 0.001118 gram of silver
from the solution each second. This is closely equal to 4 ^\ grams
per hour. The silv,er voltameter is very difficult to handle with the
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ELECTRICAL MEASUREMENTS 51
d^^ree of accuracy that is necessary for good results^ so it is f.ot
recommended for general use.
To set up a voltameter, a platinum bowl is used as a cathode and
a plate of pure silver as an anode. The electrolyte, 15 to 20 parts by
weight of pure silver nitrate added to 100 parts of distilled water, la
placed in the bowl, and the anode immersed in the solution. A cur-
rent density of -J- ampere or less per sq. cm. is allowed at the anode,
and of ^ ampere or less per sq. cm. at the cathode. Care is to be
taken that no particles of silver mechanically detached from the
anode shall reach the cathode. This may be accomplished by
wrapping the anode in clean filter paper.
B^ore weighing the cathode to determine its increase in wei^t,
any trace of the solution must be removed by careful working with
dbtilled water and the cathode dried. This seems easy, but it is dif-
ficult, in fact.
The solution should be made anew for each experiment.
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ELECTRICAL MEASUREMENTS
PART II— ADVANCED
MEASUREMENT OF CAPACITY
Ballistic Qalvuometer. In the measurement of the capacity
of a condenser by the methods given in the subsequent pages, the
charge of electricity from the condenser b allowed to flow as a mo-
mentary current throu^ a galvanometer, giving, the suspension a
sudden kick. In order to calculate from this deflection the quantity
of electriciiy in the condenser, it is necessary to assume that the
galvanometer suspension is so heavy that it will not have moved
very far before the diarge has completely passed. This requisite,
viz, a heavy suspension, b the distingubhing feature of the ballistic
type of galvanometer. (See Fig. 7, Part I.)
As a rule the methods of measurement involve only a comparison
of the deflections produced in the ballistic galvanometer by chained
condensers of known and unknown capacity, so that, as long as the
capacity of a standard condenser is known, the unknown factors,
die galvanometer constant, etc., are unimportant. Nevertheless it
may be instructive to know how these unknbwns can he determined
and the deflections can be made to give the actual quantity of elec-
tricity in the given condensers.
Because of the fact that the deflection of the galvanometer b not
proportional to the current which produces the deflecUon, it is neces-
sary to know the factor called the constant of the galvanometer before
measurements can be tak^i. Thb constant b used in various forms
but can be briefly stated as the constant ratio between the eurrerU and
the defiection produced by it. Vilien put in more definite form it can
be pven as follows :
in which / b the current flowing in the galvanometer, D is the dbtance
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ELECTRICAL MEASUREMENTS
from the galvanometer mirror to the scale, and cj is the deflecdon pro-
duced on the scale.
With this in mind let us consider how to find the quantUy of elec-
tricity Q from the throw 9 of the galvanometer, ths gtUvanometer con-
tant K, and the half period of the susfension.
As has been stated above, while
the quantity Q is passing through
the galvanometer, short though
it xa&y be in duration, it consti-
tutes a current and the magnetic
effect of thb current exols a
turning moment on the coil.
If / represents the mean value
of this current, then the mean
moment of force Fh acting on the
coil while the current is flowing is
Fh = IHA
in which H is the strength of the
field and ^ is the area of the
galvanometer coil. If t is the
duration of the discharge, then
the moment of force times the time can be given by
Fhr = ItHA = qWA
in which Q is the quantity of electricity, equal to It.
If the moment of inertia M of the suspension and the angular
velocity, which is pven to it by this kick, are taken into consideration,
the quantity of electricity Q may be obtained from the above equation
as follows:
e - ■^"-TT
in which u b the angular velocity and T„ is the torsion constant of the
suspension. By taking the half period of the suspension, which b
easily obtained by counting the time of a given number of swings,
and expressing &> in terms of the angle of throw 6, the expression
for the quantity of electricity is given by the following equation:
in which K is the galvanometer constant, 6 the angle of throw (ob-
Flg. 34. Damtilug DiKnni.
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ELECTRICAL MEASUREMENTS 55
tained by dividing the deflection d hy twice the dbtance from mirror
to acale D), t the half period of the suspension, and ir 3.1416.
For accurate work 6 must be muItipHed by a damping factor
|//», derived as follows: With the suspension swin^ng freely. Fig.
36, take a deflection 6^, then after a given number of swings (n-m)
take another deflection, d^ ^ b the (n-m) root of the ratio — -^ .
P n
Condensers. A condenser consists in its simplest form of two
metal sheets separated by a nonconducting material, Fig. 37. If an
e. m .f . is applied to the two metal sheets, they will take a static charge,
one positive and the other n^;ative. The nonconducting material
is called a dielectric, as the electric force acts through it (dia meaning
through). The capacity of such a condenser is propordonal to the
Fig. 37. CoDdeDKr Sheets.
area of the sheets and inversely proportional to' the thickness of the
dielectric. If the condenser is in the form of a glass jar coated out-
side and inside with tin foil, the arrangement is called a Leyden jar.
A considerable portion of the surface near the edges of the jar should
be free from the tin foil coating in order that the charge may not leak
over the surface of the glass. The best condensers are made of many
sheets of mica with sheets of tin foil interlarded, every alternate one
being connected to one, and the others to the other terminal of the
condenser. Fig. 38. By using many sheets of tin foil the capacity is
increased in proportion to the total area. IVKca is an excellent ma-
terial for the dielectric as its resistance is extremely high, and very
thin sheets have enough strength to withstand the mechanical stress
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ELECTRICAL MEASUREMENTS
due to the electric charges without breaking down. The mica and tin
fml are clamped in place and the whole immersed in melted paraffin
and then withdrawn, carrying out a coating of paraffin which pro-
tects the condenser from the effects of moisture. Several condensers
of assorted capacides are frequently mounted in one box. Fig, 39.
Aim. f. box will frequently have condensets of 0.5,0.2,0.1,0.1,0.05
FfK. 3a Slmfile CondeDBer.
and 0.05 microfarad, Fig. 40. Cheaper condensers have paraffined
paper or other materials in place of mica; but are usually poor since
the dielectric, though it does not break down, is apt to yield gradu-
ally to the strain of the charge, producing an effect which is known as
ubsorption of tha charge. It seems as though some of the chai^ had
been lost for when the condenser is discharged, less charge comes out
than was put in. It
is true that real
leakage causes a
loss of part of the
charge, but we find
also that a poor con-
denser, if set aside
after being dis-
charged, will, on a
later test, show a
small charge which
has come from the
gradual return of the dielectric to its original state. The Leyden
jar (glass dielectric) absorbs a considerable portion of its charge.
Standard condensers are sometimes made with massive plates of
metal and with air, which has no absorption, as the dielectric. They
are very expensive and have small capacity. For practical purposes
Fig. 39. Vfu4able CDDdennr.
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ELECTRICAL MEASUREMENTS
57
the beat condensers have mica for the dielectric, for mica shows
almost QO absorption of the charge.
Single conductor submarine and land cables have the properties
of condenses, the water acting as the second sheet in the caae of the
Hh Hh
Hh
Hh Hh
Fig. 40. PIui Of Tarl&ble Oondeiiwr.
former, and the usual lead covering in the caae of the latter. In
telephone cables, each pair of wires and the insulation between make
up a condenser. These cabfes almost always show considerable
absorption of the charge.
Direct Deflection Method. Two
condensers may be compared as to
their capacity, if first one and then
the other is charged by a cell B of
known e. m. f., and then discharged
through a ballistic galvanometer G,
Fig. 41. If the deflection with the
standard of capacity C is d,, and
that with the unknown of capacity JV
is dj then
A charge and discharge key K, Fig.
42, connects the condenser first to the battery and then to the galva-
nometer.
This method is convenient, but the accuracy of the results de-
pends on the accuracy with which the throw of the galvanometer can
be read. The accuracy is not much better than 1% even if neither
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58 -ELECTRICAL MEASUREMENTS
condenser shows absorption of charge. If either condenser absorbs
charge, the ratio of the deflections Tyill depend on the time of charge
and the slowness of clearing out of the ckarge. There are better
methods.
Example. With a. condenser of 0.5 microfarads in circuit, the
defleclJon is 46 divisions; with the unknown capacity X in circuit,
the deflection is 69 divisions. What is Xi
Ans. X = 0.75 m. f.
Bridge, Methods. The W^eatstone's bridge method of comparing
resistances may be adapted to the comparison of two capacities.
The apparatus may be arranged as shown in either Fig. 43 or Fig. 44.
In the former a charge and discharge key is used to charge and dis-
charge the condensers.
If all the chaige taken
by Cj passes through
A, and all taken by C^
passes through R^,
both in charging and
discharging, then the
galvanometer will not
deflect ; otherwise it
will deflect in opposite
directions for the
Fig. 42. Ksmpe Discharge Key. charge and discharge.
As the current divides
in parallel circuits in inverse proportion to the resistances; and as
the charge, and therefore the current, taken by condensers in parallel
is directly proportional to their capacities, it is evident that if no
current passes through the galvanometer
therefore
In Fig. 44 the battery circuit produces a current through flj and
Rj, and they take potential differences between their temunab pro-
portional to their resistances when the current becomes steady. The
condensers will take a certain charge; and if the galvanometer b
open, both condensers must take equal charges r^ardless of their
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ELECTRICAL MEASUREMENTS
capacides. If one has a smaller capacity than the other, the former
'Will acquire a smaller potential difference than the latter, as the charge
is equal to the product of the capacity and the potential difference.
If Cj and R, acquire equal potential differences, and C, and R^ also
equal' potential differences, then on closing the galvanometer key no
deflection will result. If on closing the galvanometer key a deffec-
don results, it is evident that the above relation is not satisfied. If no
deflecdon results
C,:C^: :R,:R„
and
Fig. M. FlB. «.
DlxKTuns tor Bridge Hethods lor Heamiriiig C>padtle> of CoDdeimn.
The order of closing and opening keys is important. It should be
as follows ; First close K^, then K, and note deflection if any; second,
open Kf, then K^, then close K^ and note deflection (or discnarge)
which should be in opposite direcdon. Then open Ky It is neces-
sary to discharge the condensers before recharging them, otherwise
they will take no new charge beyond what is necessary to make up
for leakage or absorpdon of charge. If the condensers absorb charges
it is impossible to get a perfect balance.
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60
ELECTRICAL MEASUREMENTS
L— ©
Method of Mixtures. In the method of mixtures the positive
charge taken by one condenser is mixed with the negative charge of
the other, and vice verad, and the remaining difference. of charges ts
discharged through the galvanometer. This method allows the time
of chai^ and the time of mixing the charges to be varied at will,
thus allowing the absorbed charge more or less time to make itself
felt.
In Fig. 45, the Pohl's commutator P, by bringing the points e and c
and/and d into contact as indicated, allows the condenserC, to charge
undl its potential difference b equal to that over /I, (due to the current
from the battery B), and also allows C, to charge until its potential
difference equals that over Rj. The commutator is then reversed,
brining e into contact with a, and / into contact with b. The points
a and b are permanently
connected together. The
+ charge on C, can mix
with the — charge on C,,
and the other charges on
C, and C, — which were
previously so-called
bound charges — now be-
come free and can mix
also. The remaining
charges are divided between the two condensers in proportion to
their capacities. If now the key K is closed, these remaining chaises
are discharged through the galvanometer. If no deflection occurs
the chaise remaining must be nil. As the charge taken by a con-
'ienser is equal to the product of its capacity multiplied by its
potential difference, and aa the potential differences to which C, and
C, were charged were proportional to R^ and ftj, then, by Ohm's
taw, when the charges on C^ and C, are equal we must have the
relation that
C, R, ^ C^R^,
1
Fig. *B. DlagTun tor Msthod ot Mlxturea.
If condenser C, absorbs part of its charge, its total chai^ will increase
an charging for a longer time. If the chaises are allowed to mix
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ELECTRICAL MEASUREMENTS 61
for a longer time there is more opportunity for the absorbed chaige to
be given up. It follows that if the time of charging is short, the capa-
city of the C, will appear to be smaller than if a longer time of charging
were allowed. With good mica condensers litde effect of absorption
will be found. With most other dielectrics the absorption is quite
marked.
ExatafU. If in the bridge method or in the method of mixtures,
C, - 0.5 microfarads, fl, = 2,340 ohms, and R^ = 1,000 ohms, what
is C,? Ana. C, = 1.17 microfarads.
Absolute Method. If a condenser is rapidly charged through
a galvanometer and then discharged by short circuiting the condenser,
the defection of the galvanometer will be the same as though an equal
charge had passed through the galvanometer in the form of a steady
current during the same
time. The difficulty with
the method b that the
galvanometer obstructs
the complete charge of
the condenser when the
charges become very fre-
quent. To get around
this difficulty a second
circuit to the galvanom-
eter is arranged to carry
a steady current equal in
value but opposite in direction to the pulsating current due to the
charge of the condenser. The result is that the galvanometer carries
only the difference of these two currents, and when a balance is
obtained, the resultant current is a small alternating current, altera
nately helped and hindered by the resistance and inductance of the
galvanometer. The method then becomes somewhat Uke the Wheat-
stone's bridge method.
To r^ulate the number of charges to a uniform number per
second, a smaU motor running at a constant known speed or an elec-
trically driven tuning fork of known frequency of vibration, may be
used. The apparatus is arranged as in Fig. 46. If the condenser
of capacity C is charged when the movable piece P makes contact
with S, part of the charge will pass through the galvanometer of re*
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62 ELECTRICAL MEASUREMENTS
sistance G. The condenser is discharged when P makes contact
with R. The battery B tends to send a steady current through the
divided bridge circuit, part passing through F and G (both of fairly
high resistance) and part through A (of low resistance). The re-,
sistance in the battery arm must be made very low in comparison with
F, D, and G. Let us suppose D a fixed resistance, say, 1,000 ohms,
and A, 1 ohm. In this case two proportional arms of a poatoffice box
may be used. Adjust F until a balance is obtained, with no deflection
of the galvanometer. Let n be the number of charges of the condenser
per second. We' then have the closely approximate relation
„ A (F + G)X Kf
nF{DG + DF + AG)
The capacity C is in microfarads. If the factor 10* is omitted, the
formula will give C in farads.
If the resistances in the battery branch and in A are not small,
it is necessary to use a more complicated fonnula.
Example. When a balance is obtained ^ = 1 ohm, F = 2,340
ohms, G = 10,000 ohms, D = 1,000 ohms, and n = 32 periods per
second. What is the capacity of C? Ans. C = 0.01334 microfarad.
Altmiating^urrent Method. If a circuit through which an
alternating current is flowing includes a condenser, the chai^ and
dischaif;e of the condenser is repeated with every alternation of the
current. The quantity of each charge is equal to the capacity multi-
plied by the p. d. to which the condenser is charged. The rate of
charge or discharge is the value of the current at any particular instant.
It is proved in treatises on alternating currents that the effective
value of a current or an e. m. f. is the square root of the average square
of its instantaneous values. Alternating-current ammeters and volt-
meters such as those shown in Figs. 47, 52, and 53, calibrated with
direct currents, show the eifective values of A. C. currents and electro-
motive forces. The theory of alternating currents shows that the
current passing in and out of a condenser, if the e. m. f. follows a sine
law, is
27rnCE
° 1,000,000
From this it follows that
^_ 1,000,000 7 _ .59,-. I
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ELECTRICAL MEASUREMENTS 63
when I is the effective value of the current in amperes, E the effective
value of the e. m. f. in volts, n the number of cycles per second of the
e. m. f. (and consequently of the current too), and C the capacity
of the condenser in microfarads. If the capacity is rated in farads,
omit the factor 1 ,000,000. In Fig. 48 the current 1 flows through the
ammeter A, and (he condenser of capacity C in series. A hi^ re-
sistance voltmeter V measures the potential difference E at the termi-
nals of the condenser.
Fig. 47. Commercial Form of Portable A. C. Volimeter.
Example. The ammeter reads 1 ampere, the voltmeter 220
volts, the frequency n is 60 cycles per second. What is the capacity
of the condenser?
Ans. C — 12.06 microfarads.
If the voltmeter takes much current in proportion to the whole, a
correction must be made for it. From the theory of alternating
currents a condenser takes a current one-quarter of a period in phase
ahead of its potential difference. A circuit in which there is both
resistance and inductance takes a current lagging behind the potential
difference. As voltmeters have some inductance, which should be
small in comparison with the resistance, it will be seen that the current
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64 ELECTRICAL MEASUREMENTS
in the voltmeter tags a little over a quarter period in phase behind the
condenser current. The relation of the currents with apparatus set
up as in the previous figure is shown in Fig. 49, in which /„ 7„ and
/„ are the currents in the
I I ammeter, condenser, and
voltmeter respectively.
7, and 7, are practically
of equal length. The
directions of the arrows
show, the phase relations.
If the condenser cur-
rent is small and the voltmeter current large relatively, it will be seen
from Fig. 50 that 7, may be materially larger than I,, and that the
ammeter reading must be corrected. It
may be shown by trigonometry that if the
voltmeter cunent lags by an angle a behind
its potential difference, we wilt have
Fl8. 4B. Dlngmn tat A. O. UMbod of C&pkdty
7, = 7, sin o + l^ 7,» - (7, cos a)'
By the theory of alternating currents tan a = — ^ — , where n i:
the
frequency of the system, L the self-inductance, and R the resistance of
the part of the circuit considered. If n, L, and 71 are known, a may
be found.
„ . Example. What correction, if any, should be
made in the previous cTcample, 7^ = 1 amp., E, =
220 volts, n - 60 cycles per second, if the volt-
meter takes a current 0.06 amp. lagging 1" behind
n«- w its potential difference?
S(dvtion.
«n 1° = 0.01745, cos 1° = 0.99985
7. = 0.00105 + ^"0.996401 2 =- 0.00105 + 0.99820
= 0.99925 amp.
As the correction of the current is far below the accuracy with which
the ammeter may be read, no correction in the computed capacity of
the condenser should be made.
Example. If the current observed was 0.1 amp., the other data
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3,GoogIc
St
6&
4
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ELECTRICAL MEASUREMENTS 65
remaining the same as in the previous example, what correcdon, if
any, should be made?
Solution.
/. - 0.00105 + 1^0.0064012
= 0.00105 + 0.08001 = 0.08106 amp.
The uncorrected formula gives C = 1.2Q6 microfarads
The corrected formula gives C = 0.978
The difference is 0.228
which is 19%, a difference much too large to be ignored.
MEASUREMENT OF SELF-IN DUCTANCE
Most methods of measuring inductance are too di£5cult for the
readers of this book. The difficulty is due to the fact that a coil of
wire which has inductance, has resistance also. During the increase of
a current the inductance acts as a false resistance which makes the
resistance appear too high. Diuing the current's decrease, however,
the inductance acts as a n^ative resistance, which makes the resist-
ance appear too low. In the case of an alternating current the effect
is to make the apparent resistance, called the impedance, bigb^ than
the real resistance. Algebraically expressed we have
E
Impedance = .JiP + \ j? v? V ^ -=-,
in which R is the real resistance in ohms, x = 3.1416, n the frequency
in cycles per second, L the inductance in henrys, E the e. m. f. in
volts, and I the current in amperes. The impedance of a coil is there-
fore not a constant quantity if R and L are constant, but depends on
the frequency n. For commercial lighting n is usually 60, and for
power circuits 25 cycles per second. Coils which have an iron core
do not have a constant self-inductance, for the latter, with increase
of current, rises slighdy to a maximum and then falls off greatly for
large values of the current.
Alternating-Current Method. If in a circuit. Fig. 51, the current
through a, coil of known resistance R and unknown inductance L, is
measured by means of an ammeter A (two views of a well-known
commercial instrument are shown in Figs. 52 and 53), and the
potential difference E over the coil is measured by a voltmeter V,
we have the following relation
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ELECTRICAL MEASUREMENTS
1 ft
R*
Example. In a circuit for which the frequency n -• 60 cycles
per aecond, R = 0.1 ohm, E •= 110 volts, and / = 10 amperes, what
is the value of the inductance L7
Ans. L = 0.00S2 henry.
If the resistance or inductance is very high, so that the total cui^
rent is small, a correction
must be made for the por-
tion of the current pass-
ing through the volt-
meter. The corrected
~^^~]-rmmy~
Fig. 61. Diagram tor A. C. Method of Selt-Inductuice formula 13 very COmpli-
cated when expressed
directly in terms of L, I, R, r, the inductances and resbtances of the
coil and the vdtmeter respectively, n the frequency, £.the potential
difference, and 7^ the current through the ammeter. If the tangents of
the lag of the current in the two
branches of the circuit are for
the coil
and for the voltmeter circuit
Fig. 52. Tbomaoo lodined (Mi A. 0.
we get the equation for the
cosine of the difference be-
tween a and b
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ELECTRICAL MEASUREMENTS 67
co.(±(«-M)-f(¥-^-i)
Usually 6 b greater than a, so we write the formula
,. , rR J,' 1 1 ,
cos(6-a)--(-----^)
As the constants of the voltmeter are supposed to be known, the angle
a is known; therefore b maybe determined and we finally get
. R tan b
If b is so small that the difference between a and b is not greater than
Fig. £
a, there is always the possibility that the previous formula is the cosine
of (o — 6). To determine which result to take, it is necessary to
repeat the experiment with additional resistance in one of the two
branches, and to take the value of L that equals one of the previous
solutions. As a rule, however, the difference between a and b will be
greater than o, and the incorrect result will lead to a negative value for
6. As £ must be positive, the negative result is rejected as impossible.
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68 ELECTRICAL MEASUREMENTS
Bridge Method. Two self-inductances may be compared by a
modification of the Wheatstone's bridge method. In the simplest of
many bridge methods the coil of unknown inductance X is one arm of
the bridge; a double coil. Fig. 54, whose inductance L may be varied
by rotating one part to various positions inside the other part, and
whose inductance is known for each position and marked by a pointer
on a circular scale for each position, is inserted in an adjacent arm.
The other arms are non-inductive resbtances A, and R^. The ar-
rangement is shown in Fig. 55. The inductive branches have cer-
tain resistances it, and R^. A regular Wheatstone's bridge balance is
obtained by adjusting the resistances B, and ft,, care being taken to
close the key K^ in the galvanometer branch several seconds after
closing the key K^ in the
battery branch. This pre-
caution b necessary to inr
sure that the currents are
steady when the galvanom-
' eter key is closed, as
inductive effects are pro-
duced only when the current
is changing in value. AVhen
a balance is obtained
R^:R^■.■.R^•.R^. If now
the galvanometer key K^ is
closed first, the fabe resbt-
ance due to inductance, will
' cause the galvanometer to
deflect when the battery cir-
cuit is closed. After oscil-
» .. «_... a.. . ^ — _ latine a number of times the
galvanometer finally comes
to rest at its zero position. On opening the battery circuit the first
deflection b in the opposite direction. If, now, the variable induct-
ance is adjusted until there is no deflection on closing the battery
circuit, the galvanometer circuit having been closed in advance, the
false resistances due to inductance must increase the apparent resist-
ances in both inductive branches in proportion to their real resist-
ances, from which it follows that
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ELECTRICAL MEASUREMENTS
Fig. 5G. DIasrwn or Bridge Method ot &elt-IttduMsnce
X '. L '. '. ftj : it, : : it, : itj,
and we get the relation
If no variable standard of inductance is available, there are various
modifications of the bridge method which may be used. These
methods are as
a rule very com-
plicated and
consequently be-
yond the scope
of this course.
Condens,er
Method. The
self -inductance
of a coil may be
compared with
the capacity of a
condenser. The
bridge is set up
as indicated in Fig. 56. The condenser of capacity C b in parallel
with M which is one part of a constant resistance R^. R^ = M + N.
iJj and R^ are
resistances, one
or both of which
may be varied at
will. The coil
of resistance R^,
whose induct-
ance L is to be
measured, is in
the fourth arm of
the bridge. The
galvanometer G,
battery B, and
keys Ki and if,
are as usual. The resistances are adjusted in it, and A, tindt a
balance is reached, when K, is closed several seconds after closing Ky
DignzedoyGOOglC
70 ELECTRICAL MEASUREMENTS
Then R, : fi, : : R, : R^, the usual Wheatstone's bridge relation. The
galvanometer key K^ is next kept closed and resistance shifted between
M and N, care being taken to keep M + N = R^ constant, until
no deflection is produced at the instant of closing K^. The explana-
tion of (he method is quite complicated, but leads to the simple
result,
R,' 10*
C is in microfarads, R^, R,, R,, and M in ohms, and the result for L
in henries.
Example. The resbtance R, was kept constant at 1,000 ohms,
part of which M, when a balance was reached, was 516 ohms. R,
= 1,000 ohms,R, = 1,260 ohms, and C = 1 m. f., ibakes up the
balance of the data. What was L?
Ans. L = 0.3355 henry.
MEASUREMENT OF MUTUAL INDUCTANCE
If two electric circuits are in the neighborhood of. one another,
the increase or decrease of the current of one will produce a change
in the magnetic field which will act to produce an e. m. f., and conse-
quently a current in the second circuit if it is closed. If the current
in one circuit varies at the rate of one ampere increase or decrease per
second and an e. m. f. of one volt is produced in the second circuit,
we say the mutual inductance is one henry. If the mutual inductance
is constant, the e. m. f. produced in the second circuit is proportional
to the rate of change of current in the first; also larger inductances
produce proportionately larger e. m. f.'s in the second circuit with
equal rates of change of current in the first. Algebraically expressed,
if /, and /, represent any two values of the current, supposed to be
increasing at a uniform rate, and ( is the interval of time between these
values of the current, and if E is the e. m. f. produced in the second
coil whose mutuid inductance with respect to the first is M, then
t
The inductance between the coils is called midual because a
certain increase of current in either will produce the same e. m. t.
in the other regardless of which circuit has the original current. The
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ELECTRICAL MEASUREMENTS
71
relation is therefore mutual. If the circuit has an iron core the mutual
inductance is not strictly constant, but with increasing current in-
creases to a maximum and then falls again. A^ it is very difficult,
if not impossible, to control the precise rate of chai^ge of the current
so that it will increase at a uniform rate, either of two methods may be
used to obtain the value of M without keeping the rate of change of
I constant.
Ballistic Qalvanraneter Method. In this method the second
circuit, called the tecondary circuit, is of known resistance it,, and
includes a ballistic galvanometer G and sufficient extra non-inductive
resistance T, to control
the deflection within
reasonable bounds. The
other circuit, called the
primary circuit, includes
a key K, an adjustable
resistance r,, a battery B,
and an ammeter A. The
arrangement is shown in Fig. 57.
secondary S.
On closing the key K the current in P b^ns to increase and the
galvanometer G b^DS to deflect. If we make no allowance for the
false resistance due to the self-inductance of the secondary circuit, the
Fig. 57.
The primary coil is P and the
current 7, in the secondary is 7,
As the current 7, in the
primary requires a considerable part of a second nearly to reach its
max^um or steady value, it b. evident that the secondary current 7,
will require at least an equal time to rise from zero to its maximum
and to fall nearly to zero again. TheoreticaDy it takes an infinite
time for this to happen, but if the total resistance 71, in the primary and
Rj in the secondary are reasonably large in comparison with their self-
inductance, the time practically necessary is a second or so. The
current I^ in the secondary will, therefore, during the times that it
flows, cause a certain quantity Q of electricity to pass through the
galvanometer. If the ballistic galvanometer has a long period of
swing — a condition required of ballistic galvanometers — practically
the whole of the quantity Q will pass before the galvanometer gets far
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72 ELECTRICAL MEASUREMENTS
from its zero position. As the e. m. f. of the secondary is proportional
to the rate of increase of the primary current /,, it is evident that the
total quantity flowing in the secondary will depend on the total rise of
current in the primary in precisely the same way as the current at any
inatarU in the secondary depends on the rate of increase of the current
in the primary. As before mentioned, we have ignored the false
resistance due to the sdf-inductance of the secondary. We have
learned, however, in the chapter on self-inductance, that during the
rise of a current the self-inductance increases the apparent resistance,
but during its fall the opposite effect is produced. Therefore, during
the rise and fall of the current in the 3cx;on<jary, no appreciable error
is caused by ignoring the self-inductance of the secondary. The
final result is J/ 7, =■ Q i{,, or
To get the value of Q we must know the relation between the
throw ij, of the ballistic galvanometer to the quantity of electricity
producing the throw. This may be found by charging a standard
condenser of known capacity C by a standard cell of known e. m. f. E^,
and noting the defiection d on dischar^ng the condenser through the
ballistic galvanometer. As the charge of the condenser is E, C, we
obtain the result
Q:d^::E,C:d,
or Q ^ d^E.C
Substituting in the earlier equation the value of Q and expressing the
capacity of the condenser in microfarads,
„^ d^E.CR,
d A X 10-
Exam/j^. The current /j in the primary coi! rises on closing
the circuit to 1 ampere. The total resistance in the secondary circuit
is 10,000 ohms. The defiection produced is 21 .3 cm. TOth a Weston
normal cell of L0184 volts and a condenser of 0.2 microfarads, a
deflection of 23,0 cm, was produced on discharging the condenser
through the galvanometer. What b the mutual inductance Mt
Ans. M = 0.001886 henry.
Altemating^urrent Method. The previous method may be
varied by putting in the secondary an alternating-current voltmeter
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ELECTRICAL MEASUREMENTS 73
of hi^ resistance, and in the primary an A. C. ammeter, an e. m. f.
source of sine form, and whatever resistance is needed to control the
current. From the theory of alternating currents, the e. m. f. pro-
duced in the secondary is
E, = 27tnML,
when n is the frequency in cycles per second, M the mutual inductance,
and /, the primary current. This rdation assumes that the current
in the secondary is too small to affect the flux of magnedc lines crossing
over from primary to secondary. If the resistance of the secondary
coil is small in comparison with that of the voltmeter, the volt-
meter reading E, is taken as Ey If the coil is of too high resistance
to make the last assumption allowable, the reading must be multiplied
resistance of the voltmeter. We then get
E =^^E, = 2 7tnM I,.
or
M ^^'
If the secondary current /, is too lai^ for its magnetic effect to
be ignored, or if the current in the primary does not follow a simple
sine law, the problem becomes too complex for easy solution.
Example. The current /, in the primary is 1.1 amperes, the fre-
quency n is 60 cycles per second, the resistance of the secondary coil
is negligible. The reading of the voltmeter £, is 110 volts. What
is the mutual inductance Ml Ans. M = 0.2653.
Carey-Foster Method. Let a battery of constant e.m.f. be
connected in series with one of the two coils P whose mutual inductahce
is to be determined, a known resistance /£,, and a key K, Fig. 58. Let
the ballbtic galvanometer G and another resistance R, be connected
in series with the other coil S. Then if 7 be the steady current pro-
duced by the battery B through P, and M be the mutual inductance,
and r be the resistance of the circuit through S, R^, and the galva^
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ELECTRICAL MEASUREMENTS
nometer, then the quantity of elcctricitvQ, passing through the galva-
nometer on closing or opening the circuit will be
«. = ^-^
Next, if the galvanometer be removed from this circuit and put
in series with a condenser whose capacity is C, which is connected
as a shunt to the resistance R,, on opening or closing the battery cir-
cuit the quantity of electricity
Q, = IRf
By combining these two equations it is possible to find the relative
values of C and M. In practice it is much more desirable to combine
■GJ
t.
Fig. sa Dlagnun ot Cany-FosMr HeUiod ot Meaauiing Mutual loductuioe.
these two circuits, as shown above, so that the charge and discharge of
the condenser and the currents produced at l^e same time in S by
mutual induction are in the same direction through C, 7?,, and S.
Then if the resistance R^ and fl, and the capacity C are adjusted until
no deflection of the galvanometer is produqed, the following may be
written :
Q,r = MI, butQ, = IRfi; hence M = Cft,r
Attention b called to the fact that in order that the galvanometer
current may be at every instant during the establishment of the
steady current, it is essential that the coefficient of self induction of the
coil S should be equal to the coefficient of mutual induction. Under
this condition it is possible to replace the galvanometer by a telephone.
Examjde. Small Induction Coil, no iron core. Resistance of
secondary, 194 ohms. Capacity of condenser, 4.926 microfarads
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ELECTRICAL MEASUREMENTS
75
The secondary coil could slide endways remaining coaxial with the
primary. The following are the results with the centers of the two
coils as nearly coincident as possible:
"■
'
(CG.S.Cmitb)
A.
'.
B,r
IC. O. 8. IJbit«)
15
I'M + 217
616fi X 10»
10
194 + 423
6170 X 10«
14
+ 247
6174
9
+ 490
6156
!3
4- 282
6188
H
+ 576
6180
12
+ 322
6192
7
+ 688
6174
11
+ 367
6171
6
+ 835
6174
Hence
M = 4.926 X 10-" X 6172 X 10" =-3.04 X 10' or .0304 henrys.
MAGNETIC MEASUREMENTS
Certain materials, notably iron (or steel), nickel, and cobalt, have
a property known as magnetiam. These materials when magnetized
have the property of attracting soft iron. The modern view of mag-
netism is that it is a property of the individual molecules of a body.
A body which seems to be unmagnetized probably has its molecules
arranged in more or less irr^ularly formed closed chains. Fig. 59,
which produce no outside effect. To magnetize a body, it is, accord-
ing to this theory, necessary to break the chains and to rearrange the
connections of the molecules so that the ends of the chains of mole-
cules come out at points on the surface where so-called magnetic
charges appear, Fig. 60.
The centers of action of
these chain ends are
called poles. In the sim-
plest magnets there are ^- ^
two poles, and if the
magnet is free to turn in a horizontal plane, one of the poles is
turned toward the north and the other toward the south approxi-
mately. In general, the magnetic meridian, determined by the line
joining the poles when at equilibrium in the horizontal plane, does
not agree exactly with the geographical meridian. The line of no
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76
ELECTRICAL MEASUREMENTS
variation at present b located near the eastern shore of Lake Mich- .
igan and is moving westward. A century ago it was near the eastern
end of Lake Erie. For points to the east of the line of no variation,
the magnetic compass points west of north', and for points west of
this line of no variation, it points east of north. The north seeking
pole of a magnet is commonly called the north pole and the other the
south pole of the magnet. If the magnet is free to turn in all direc-
tions, the north pole will dip downward and the south pole rise up-
ward for points in the northern hemisjJiere and vice versd for points
in the southern hemisphere. The dip in Chicago is in the neighbor-
hood of 70*.
Suppaam Moleculur CondlUon of » MtgiwtlEed Piece of 8ted.
From the above it appears that the earth has a magnetic fidd,
meaning by magnetic field an extent of space where magnetic forces
are to be found. If a magnet with poles 1 cm. apart and of unit
strength is in a unit magnetic field, it will act on each pole with a
force of one dyne; and if the poles are turned so that the directions
of the forces are at right angles to the line connecting the poles, a unit
turning moment, or torque, is exerted on the magnet. In the first
chapter of this book, unit magnetic pole was defined as a pole which,
at a distance of one centimeter in air from a like pole, produces a
repulsion of one dyne of force. It appears then that a magnetic field
has both direction and magnitude, and is what b called a vedor
quantity. To define a vector quantity both magnitude and direction
must be known.
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ELECTRICAL MEASUREMENTS 77
Methods til Magnetizing. Besides natural magnets, composed of
the magnetic oxide of iron and known as loadstones, artificial mag-
nets may be made by subjecting hard steel to a magnetic field.
The Btagnettc field used may be due to a loadstone, or an
artificial magnet previously made, or a magnetic field due to an
electric current in a coil of wire. Before 1819, when Oersted dis-
covered the magnetic field produced by a current, the source of all
artificial magnets was directly or indirectly the loadstone. Fer^
manent magnets nowadays are practically all magnetized by means
of electric currents.
Lines ol Force and Penneability. To Michael Faraday we owe
the notion of lines of force to express the vector quantity defining the
magnetization. The direction of the lines is used to indicate the
direction of magnetization, and the number of lines per square centi-
meter indicates the magnitude of the magnetization. The numbers
of lines per square centimeter is commonly called the fiux density or
^Ma;o//(Wce per square centimeter. Theunitvalueoccurs when there
is one line per square centimeter, and is called the gauss. The sym-
bol B is used to express this quantity algebraically. As different
materials when put in equal fields take different degrees of magneti-
zation, the relation of field strength to flux, known as the perjtisabUitt/,
must be known. If H indicates the field strength, B the flux per sq.
cm., and ft the permeability, we have the relation
B c^ /t H
When the section considered has an area A sq. cms. and the average
flux intensity is B, the total flux designated by 4» k algebraically
expressed as
* = B^ = /iH^
The unit in which * is measured is the Maxwell and is equal to
one line of force.
If the lines of foree pass from one material into another of dif-
ferent permeability f*. the lines representing B and 4> will usually
suddenly change direction at the surface of separation between the
materials, commonly called media, except in the case that the lines are
normal to the surface. If the permeabilities of two media are /*^ and
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78 ELECTBICAL MEASUREMENTS
/»„ and the angles between the normal and the lines in the two media
Are a^ and a,, we have the relation
tan a, _ /*,
tan 0, fi^
Lines of force follow by preference paths of hi^ per-
meability, thou^, other things being equal, they tend to
follow the shorter paths. Consequently it rarely happens that the
lines are straight, as they converge toward spaces filled with bodies
of high permeability and there diverge again in spaces of low per-
meability. In general the total Qax 4> distributes itself so that,
length of path and permeability consiHered, it takes the easiest course.
A line of force never ends, but always returns on itself. Many writers
carelessly confuse and use the same unit (the gauss) for measuring
B and H, for both are vector quantities and may be represented by
lines. To avoid confusion we shall not represent H by lines. For
highly magnetic materials, B b much greater than H. In the case of
iron the ratio may be as high as ^ = 3,000 for moderate values of B.
The value of n is not constant for the same material for different values
of B. In the case of soft iron the permeability for low values of B may
be about >t = 120, rising to m = 2,000, or in good samples /t = 3,000,
when B reaches a value between B = 5,000 to B = 8,500. For
nickel and cobalt the highest value of the permeability is about i* ->
200. As a standard of comparison the permeability of air is taken
as /< = 1 and b believed to remain sensibly constant for ali values
of B. It follows that in air H = B (numerically). Materials more
magnetic than air, for which M > 1, are called paramagnetic. Ma-
terials less magnetic than air, for which /* < I, are called diamagnetic.
No magnetic material is without permeability, that is /* » 0; in
fact, even the most diamagnetic material, bismuth, is within a fraction
of one per cent as permeable as air. Magnetic insulation is therefore
impossible and to avoid magnetic leakage of lines of force from a pre-
arranged path, it b necessary to dbtribute the magnetoTtiotive force
over the wh<Je path.
Magnetomotive Force. By analogy with electric circuits, where
the potential difference over each unit length of the circuit is found by
multiplying the current by the resistance of that unit length, or dividing
the current by the conductivity (the reciprocal of resbtance) for the
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ELECTRICAL MEASUREMENTS , 79
unit length, we see that in a part of a magnetic circuit, one centimeter
long and one square centimeter in section, t* is analogous to the con-
ductivity per cm. cube of an electric circuit. B is analogous to the
current per sq, cm, of section of the conductor and H is analogous to
the potential difference per centimeter length. H is called the mag-
netomotive force per unit length, and the total magnetomotive force
is the average value of H multiplied by the length of the circuit I.
It follows that the magnetomotive fdrce (m. m. f.) is
m. m. f . - H /
If the magnetic field is produced by a current in a wire, the intensity
of the field is greatest at the surface of the wire and falls in value to
zero at the middle of the wire, and outside the wire falls in value ac-
cording to the law of the reciprocal of the dbtance from the center.
If, however, the wire is coiled into a long, straight coil of uniform
section, called a solenoid, the magnetic field for the portion far from
the ends b practically zero outside the solenoid and of uniform value
inside the solenoid. If there are n turns of wire per centimeter and a
current of/ amperes, the inside magnetic field is
H = 0.4 IT « / = L2566 n I
It the whole number of turns of wire is N, the magnetomotive force
is.
m. m. f. = l.25mN I
The unit of m, m. f. is the gilbert, or that value of magnetic force which
will establish one line or one maxwell per centimeter cube of air.
Many practical authorities prefer to express the m. m. f. in ampere
(unu N I omitting the factor 1 .2566. This leads to some confusion if
the fact is not made clear by stating that the m. m. f. is in ampere
turns. If a long solenoid is bent into the form of a ring, it is called
a ring solenoid. If the width of the ring is small in comparison with
its diameter, it is assumed that the average value of H is equal to its
value along the central line of the ring.
Reluctance. If a circuit is I centimeters long and averages A
sq. cm. in section, the reluctance R of the whole circuit is
A (•
The unit in which reluctance is measured is the oersted.
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ELECTRICAL MEASUREMENTS
By analogy with Ohm's law the magnetic flux 4' is
m. m. r. m. m. t. A II
4> - —r— — i B A - f*n A
reluctance ■ I
Hystnssis. When iron or steel has been magnetized and the
magnetizing force .removed, a portion of the magnetization will still
remain as more or less permanent magnetization. If next the mag-
netizing force is applied in the reverse direction, the magnetization will
not be reversed until the m. m. f. has reached a certain value, t. «.,
until H reaches a certain value. The residual value of B when the
field is reduced to zero is called the remanence or Teteniivenesa by some
writers. The reverse field, m, m. f., necessary to reduce B to zero
is called by the barbarous term (as Professor Mascart puts it) of
coercive force. Further increase of the reverse m. m. f. will cause
a rapid rise of B. With repeated cycles of change between positive
and negative m. m. f.'s the
values of H and B go through
cycles. The tendency of B
to lag behind H is called
magnetic lag or magnetic
hysteresis, hysteresis being
the Greek word for lagging
behind. A certain amount
of energy is expended in the
cycle and appears in the
form of heat generated in
the iron. This loss of energy
per cycle is repeated n times
per second, and the power
used is known as the hys-
teresis loss, and is measured
in watts per cubic centi-
meter. Sometimes the
energy lost per cycle is meas-
ured in joules. One joule
Fig. 61. Hy«ere»i» Di»gr»nj. equals lO.OOO.OOOcrgs. The
relation of H and B during
the hysteresis cycle may be expressed in the form of a curve plotted in
terms of H (horizontally) and B (vertically). The curve shown in Fig.
a
~« ^5^
Z. //
ft
Z ;""'
L ■it
«<« H H
Z. Tl
9000 .J. X
z ^>
Z>i::e:: ,:
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ELECTRICAL MEASUREMENTS 81
61 is known as a hysteresis curve. If H and B are plotted to scale, the
area of the curve divided by 40,000,000 tt, or 125,660,000, gives the
energy lost per cycle in joules. For measurements of hysteretjc losses
the sample may be made into the form of a ring with smalt difference
between its la]^;est and smallest radius. The ring may then be wound
with a coil of wire in the form of a ring solenoid and the current and
turns in the coil will de-
termine the m. m. f. and
consequently H in the
core.'
Alagnetic Dip. By
Dij> Needle. If a long
and slender magnetic
needle, Fig. 62, with
pointed ends is mounted
on an axis passing pre-
cisely through its center of
gravity, at the middle of
an accurately graduated
circle standing vertically
in thema^etic meridian,
the north pole of the
needle will point down-
ward from the horizontal
by an angle equal to the
magnetic dip. The angle
of dip may then be read
directly from the circle.
As, however, it b difficult ^b ^^- mwo'ic dip NmcUo.
to magnetize the needle
with exact uniformity, the bearing should be made reversible so
that the needle may be turned over. Any irregularity of magneti-
zation may be eliminated by taking the mean of the two readings of
the magnet in the two positions. In case the bearing fails to pass
exactly through the center of gravity, the error may be compensated
by reversing the magnetization of the needle, care being taken to use
the same magnetic field as before, and then repeat the observations
of the dip with the magnet in both positions. The mean of the four
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82 ELECTRICAL MEASUREMENTS
readings will give the value of the dip. If the divided circle is not io
the magnetic meridian, an error will be caused which is slight for
slight deviations from the meridian. A compass needle mounted
over the divided circle will locate the magnetic meridian well enough
for practical purposes.
Earth Indvdor Method. Another method is by use of the earth
inductor which is a coil of wire mounted in a frame and which may
be turned about an axis in the plane of the coil. No iron or other
magnetic material should be used in the apparatus. The frame is
carried on a support
by which the axis of
rotation may be made
vertical or horizontal.
The apparatus is
shown in Fig. 63. All
heavy parts are made
of brass which is prac-
tically non-magnetic.
If the coil is turned
about a vertical axis,
as shown in the figure,
from an east and west
plane through 180° to
the reverse position in
the same plane, the
number of lines = B
Big. 63. Evth iBductor. -4 COS S of the earth's
magnetic field passing
through the coil, will be cut by the coil, and if a ballistic galvanometer
is in the circuit, its deflection d, will be proportional to the flux cut. If
now the axis of rotation be made horizontal and the coil horizontal, a
reversal in position will cut the number of lines = B ^ sin £ of the
earth's magnetic field passing through the coil. The resulting de-
flection dj of the ballistic galvanometer is proportional to the flux cut.
We then have the relation
tan o =-r
a,
If the deflections are small they may be increased by reversing
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ELECTRICAL MEASUREMENTS
the position of the coil on the return swing of the galvanometer, and
continuing until the amplitude of swing becomes constant. The
deflections in two positions of the axis of the coil are proportional to
the horizontal and the verticai components respectively of the mag-
netic field, and the ratio of the second to the first gives the tangent of
the dip of the earth's field.
The angle of dip
varies from + 90" at
the earth's north mag-
netic pole to — 90° at
the south magnetic
pole. It is zero at the
magnetic equator.
The Earth's Ma^
oetlc Field. If in the
previous method, the
data of the earth in-
ductor and of the bal-
listic galvanometer are
known, the value of B i
may be determined. *^
As the experiment is mk
performed with air as -^Wo
the medium, the value ' ^^^
of /* is 1, and H is f^^
numerically equal to
B. The horizontal
component of the
earth's magnetic field
is frequently spoken of
as H, meaning thereby the horizontal component. In the same way
V is the vertical component. The tangent of the angle of dip is the ratio
H
Magnetometer Method. The horizontal component may be
measured by means of two magneto, one of which b of light weight
and the other relatively large and heavy, both carrying mirrors. The
little magnet and mirror may be suspended at the center of an instru-
Flg. 84. Hagnetometer.
D,Bi:ized3,GOOgIe
84 ELECTRICAL MEASUREMENTS
ment called a magnetometer, Fig. 64. The second magnet, of con-
siderable size, is mounted on a support at its center at a fixed distance
to the east of the little magnet and in an east and west position (point
g of the figure). The small magnet takes up a position parallel to
the resultant of the horizontal component of the earth's field and that
of the large magnet. By means of a telescope and scale at a known
distance, the angle of the deflection is measured. The other pole of
the l&tge magnet k now turned toward the small magnet by turning
the large magnet end for end. The deflection measured by the tele-
scope and scale should be as before, only in the opposite direction.
The large magnet is now transferred to a support 6 at an equal dis-
tance to the west of the small magnet, and the observations repeated,
obtaining two more deflections. All four deflections should be equal.
If they differ slightly the mean is taken; if they differ much, some-
thing b wrong with the arrangement and the apparatus should be
examined and the trouble corrected. The observations are now
repeated at a larger distance (points a and k), and four more observa-
tions taken. From the data, assuming that the magnetic field falls
off according to the inverse square of the distance from the poles of
the magnet, a pair of equations may be formed from which the
strength of the magnet's poles in terms of the horizontal component
(tf the earth's field and their distance apart may be calculated. Thb
distance apart of the poles will be found to be somewhat less than the
length of the large magnet. The product of the pole strength and
length between poles is called the magnetic moment of the magnet,
represented by M, If r, = the lai^;er distance, r, the smaller distance,
and •!>, and *, the deflections, we get
„ H rS tan«I>, - rf tan*.
1,1 __ _^_.,
The large magnet is now hung up by a fine wire and set to vibrating.
The period of vibration 3*, is determined by measuring with a stop
watch the time of a considerable number of vibrations and finding the
time of a single swing. If K, is the moment of inertia and the cor-
rection for the rigidity of the wire, we have
T,~^J ^. I
\ »iH(i + e)
As there may be some difficulty in computing the moment of inertia
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ELECTRICAL MEASUREMENTS 85
K^, it is usual to add a brasa ring of rectangular section andeasil^r
computed moment of iner^a K,. This ring must be placed on top of
the magnet so that its center lies on the prolongation of the suspending
wire. The moments of inertia are then added to get the total mo-
ment. There are reference marks on opposite ends of a diameter
of the ring and corresponding marks on portions of a circle of the same
radius of the ring marked on the top surface of the magnet to ensure
precise centering. With the ring in position, the new time T, of vibra-
tion is determined.
' \MH(i + tf
is determined by turning the torsion head, from which the magnet
is hung by the wire, through a considerable angle and determining by
the telescope and scale the angle through which the magnet follows.
This enables one to compute how much effect the rigidity of the wire
has on the restoring force which is due principally to the earth's field.
If on turning the torsion head an angle oc, the magnet follows an
a
Combining the earlier
■n'K,
Substituting the value of M previously obtainetf we get on solving
for H
H -^ J
2 K, ( r
(l+5)(r,»- r,*) (r,*tan<l>, -r,'tan4»J
The value of H in the southern part of Michigan is about 0.18
and the vertical component is about three times as strong or about
0.54. As the presence of masses of iron in the neighborhood has a
considerable effect on the dip of the magnetic field and abo on the
value of the field, a measurement made in any place with iron masses
near by, cannot be assumed as valid for even other parts of the same
building. Laboratories for the study of the earth's magnetic field
should have all iron excluded from the building materials and from
the apparatus except the magnets.
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86 ELECTRICAL MEASUREMENTS
If the angle of dip S has been found by one of the previous
methods, the total value of the earth's magnetic fidd F is
cos o
The bars N and L shown in Fig.51 are not used in this experiment.
Magnetic Flux and PenneaU'ity. There are a number of ex-
cellent methods of determining flux and permeability, of which the
following will suffice for the purposes of this work.
Divided Bar Method. The divided bar method assumes that the
material under test is in the form of two long iron bars or rods with
the ends ground and polished into accurately plane surfaces. One
bar, with the polished end upward, is mounted in a long solenoid,
the polished end being at the middle of the 3<Jenoid. The other bar
is placed on top of the first with the polished ends resting one on the
other and accurately centered. The upper piece is attached to a
spring balance which is used to measure the tension necessary to
separate the bars. If the weight of the upper piece is subtracted,
the remainder gives the pull. The bars and the solenoid must be
long enough to have the magnetic field H at the surfaces in contact
practically equal to what it would be in an infinitely long solenoid, for
which / is the current in amperes and n the number of turns of wire per
centimeter length, otherwise a correction must be made for the ends.
H = 0.4 ;rn 7 = L2566 n /
If the area of the ends of the bar is iS sq. cm., and the force in grams
(weight) F, the value of gravity g (= 980 about), we get for the flux
density B,
If the pull F is measured in pounds and the area S in square inches,
we must allow for the ratio of the units. We then obtain
^ S (sq. m.)
In using the method, the spring balance should be supported in guides
and drawn upward gradually by means of a turn-buckle or analogous
means. The last reading before the bars separate is the one to be
taken. If the bars are rounded at the corners an error will be made
because the value of B will be increased at the smaller section to a
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ELECTRICAL MEASUREMENTS 87
greater value than back in the rod, as the total flux ^ is spread over a
smaller area. The pull will be increased because from the above
formula it appears that the pull is proportional to the product SB*.
Therefore avoid rounding the edges. If the surfaces do not fit one
another, the air where they do not touch will have a lower permea-
bility and there will be a tendency for some of the flux to escape at the
side, thus reducing B and consequently the pull.
To obtain the permeability t* divide B by H. As mentioned eariier
the permeability increases as B increases, reaching a maximum for
moderate valuesj>f B and then falb off rapidly for further increase of B.
A magnetization curve (B, H curve) or a permeability curve
(B, fi curve) may be plotted from values obtained for different values
of B, H, and ft. A bar of iron which has never previously been
magnetized, will behave for small values of H differently from what
it will again. For this reason the values of H used should increase
gradually from lower to higher values. A bar once magnetized
cannot be brought back to its ori^na! condition by any process except
heating it to the temperature at which it becomes practically unmag-
netic and then cooling it again, retempering it if Dccessary. If it is
demagnetized by reversing the direction of the field and reducing the
latter to lower values gradually on each reversal, most of the magnet-
ism may be removed. This is supposed to reduce the magnetization
of the bar to a set of concentric magnetizations in opposite direc-
tions in successive concentric layers. This is not quite equivalent
to the irregular chains of molecules in a bar which has never been
magnetized. A bar which has been demagnetized by a simple re-
versal of the field to a value apparently reducing B to zero, results in
reversing the outer layer only, making the total flux zero algdtratxally
as the sum of two ajutU and opposite fluxes in concentric layers.
Divided Ring Method. The divided ring method has the ma-
terial in the form of a ring which has bcsn cut in two and the opposite
surfaces polished. The surfaces should be exactly in the same plane
to insure a close fit when the ring b put together again. The pull
necessary to separate the ring b twice as much as for one surface; so
the total pull, after allowing for the weight of the upper part, should
be divided by two before applying the previous formula. The ring
is magnetized by a ring solenoid surrounding it. The solenoid is in
two parts which separate with the parts of the ring.
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88 ELECTRICAL MEASUREMENTS
BoUiatic Method. If the material to be tested is in the form of a
very long rod, say, 50 diameters in length, or better in the form of a
ring. Fig. 65, with little difference between the outer and the inner
radius, surrounded by a solenoid of n turns per cm. length through
which the current / amp. passes, the field is H = 1,2566 n I. A
secondary coil of a
small number of
turns (of which all
or a part only may
be used) is wound
about the ring and
is connected to a
ballistic galvanom-
eter whose throw for
a known quantity
passing is known.
The ratio K of
Vlg. 65. Iron In Ring Form (or Ballistic Ueamirement. ouantitv to thrOW
may be found by charging a condenser of known capacity C micro-
farads by a standard cell of e. m. f . E volts, and dischar^ng it through
a ballistic galvanometer, producing a deflection (2. The constant K
is then K = -j rx;- Or the galvanometer constant may be deter-
mined by winding a solenoid of n^ turns per cm. on a core of wood
or other material of the same permeability as air, for which f- equals
L H then equals B, and the flux passing through the core whose
area is A^, the current being /,, b
*, = L2566 Til A^ Z,
The total quantity of electricity Q, passing through a ballistic galva-
nometer in series with a secondary circuit of m, turns and in a circuit
of total resistance R,, on making or breaking the primary circuit /,, is
^ m, *. L2566 n,m, A^ 7,
^' - ~Rr ^ K
If the deflection is d^, the quantity per unit deflection is
R. _ 1-2 566 », m. A, 7, _ „
< Rd,
K is called the amitant of the ballistic galvanometer.
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ELECTRICAL MEASUREMENTS 89
If the primary circuit, the solenoid, has n turns per centimeter
length, and the secondary circuit a total number m of turns, and the
area of the section of the ring b A, and the resistance of the secondary
circuit a R, and the deflection is D on reversing the primary circuit of
/ amp>eres, we have the flux
RD K
2m
and
A 2 Am
The current must be reversed suddenly, some form of commutator
being used, otherwise the galvanometer may not feel the full efiFect.
We had previously
H = 1.2566 n 7
The value of the permeability for any value of H is
B
In using the method the current in the primary is reversed because
otherwise the residual magnetism will produce a disturbance. If in
the experiment the current starts at small values, the disturbing effects
of previous magnetization is a minimum. We then insert a high
resistance in the circuit at first and note the deflection of the galva -
nometer on reversing the current. Then increasing the current, the
process is repeated. Several reversals should be made at each value
of the current until enough observations have been made to ensure
an accurate result. The deflection on the Grst reversal is apt to be
different from those following. The B, H curve obtained by thb
method starts from the origin.
Hyttereait Curvet. If instead of reversing the current it a
simply changed by a sudden change in the resistance, the deflection
will measure the change in B. Starting with no current in the primary
and the ring unmagnetized, the circuit is suddenly closed with auxiliary
resistance in circuit and the throw of the galvanometer noted as well
as the current in the primary. The values of B and H are determined.
Next the current is suddenly increased by cutting out part of the re-
sistance and the deflection noted and the ammeter read. The deflec-
tion of the galvanometer measures the increase of B. The value of B
is found by adding the increase to the previous value. The current is
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90 ELECTRICAL MEASUREMENTS
again increased and the deflection of the galvanometer will measure
the increase of B. This is added to the previous total, and so on
undl the value of fi is found for the highest value of H which is to be
used. The current, and consequently H, is now reduced by steps,
and the deflections of the galvanometer in the opposite direction will
measure the decrease of B. To obtain the value of B, these decreases
in B are subtracted from the previous value. When the zero value of
the current, and consequently of H, is reached, the value of B will stilt
be a considerable amount The current is now reversed and built up
by steps and the deflections will measure the continued decrease of
B to zero and its reversal and building up in the reverse direction.
This is condnued until the maximum reverse value H is reached,
equal, we will say, to its previous positive maximum. The current is
reduced by steps to zero, reversed, and built up again in the first
direction. The deflections measuring changes in B are noted and the
total computed by the algebraic sum of all that precede. The sum
of all the deflections corresponding to ail the steps from the positive
maximum to the negative maximum, should equal the sum in the
reverse direction. The B, H curve will then make a closed curve
for each cycle after the first quarter cycle. The curve for the first
quarter cycle starts from the origin and never returns there again.
As explained before, the magnetization lags behind the field H, pro-
ducing the magnetization.
If the curve of B and H, as obtained by this method, is plotted,
it is called a kystereais curve. Fig. 6L The curve plotted to scale has
an area to 4 n- times the energy in eigs expended per cubic centimeter
per cycle. Dividing the area by 4 X t X 10' gives the enei^y in
joules per cycle. If tfie cycle were run through n times per second,
40,000,000 7
in each cubic centimeter. This is what happens when an alternating
current is used.
There are two sources of error which may cause trouble in this
experiment and which have not been mentioned above. The first is
the effect of the current in the primary producing eddy currents in
the material of the ring, just as currents are produced in the secondary
circuit. In fact, the material of the ring is in itself a secondary cir-
cuit of a single turn, and currents in the material of the ring will be
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ELECTRICAL MEASUREMENTS 0\
parallel to the current in the secondary outside. These eddy currents
produce a magnetic field in the ring which opposes the rise of H and
B in the ring. Therefore, H in the ring rises slower than the current
in the primary coil ; and unless the period of the galvanometer is high,
some of the effect of the increase of B, which may be slow in increasing,
will come too late to be measured by the galvanometer deflection.
The other trouble is that if there is any vibration the magnetization
may change by small steps one for each shock. Tbb causes the
magnetization to creep up or down, depending on whether the field
has last been increased or decreased. To avoid the first error it is well
to have the material in the form of thin sheets which give little chance
for eddy currents, and to avoid the second the ring should rest on a
pad of felt or other material which will absorb the vibrations.
Hysteresis Tester. A method of comparing the hysteresis loss
of different samples of iron is to compare their effect in drag^ng the
magnetic field when samples are rotated in a constant magnetic field.
Suppose the sample takes the form of a disk between a pair of field
magnet poles. If the disk is at rest the field will produce a fiux
density B in the disk parallel to H. If the dbk is now rotated the
residual magnetization will cause the flux to rotate with the disk until
the tangential component arrests further rotation of the flux. H and
B then make a small angle with one another for very soft iron in the
disk, and a proportionately larger angle for harder iron which shows
more hysteresb. The turning moment required to rotate the dbk
will be proportional to the enei^ expended per cycle. If the poles
are free to turn, they will follow the disk. If the poles are kept from
rotating by some counter moment due to springs or gravitational
action, the displacement of the poles in the direction of rotation of the
disk will measure the relative hysteresis loss. If with one sample
dbk the dbplacement b twice that produced by another sample, the
hysteresis loss b about double. If the hysteresb loss is known for
one disk that of the other may be computed.
As the relative twbt of B with respect to H is the same for all
moderate speeds of rotation, it b not necessary to be careful about
the exact speed of rotation. Also it b not necessary that the sample
be in the form of a dbk. The same relation for samples in the form
of bundles of equal size strips will be found to hold true. The absolute
an^es of twist will be different, but they will have a ratio of equal value.
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92 ELECTRICAL MEASUREMENTS
Professor Ewing has devised a hysteresis tester in which the
sample, in the form of a bundle of strips, is rotated by means of a
crank and gear train between the poles of a pennanent magnet which
is mounted on knife edges at a point above its center of gravity. The
magnet follows the rotating bundle until the gravitational force gives
an equal torque in the opposite direction.
If the sample is in the form of a solid bar, eddy currents of con-
siderable magnitude may be produced which will complicate the re-
sults and introduce more or less uncertainty. Moreover, the eddy
currents will be greater at higher than at lower rates of rotation, thus
introducing different corrections at different speeds. For these rea-
sons the bundle must be welt laminated to obtain reliable results.
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UNDERWRITERS' REQUIRE-
MENTS
INTRODUCTION
The iitsuiance interests are concerned with electric work only
tar the reason that such work if done improperly constitutes a fire
hazard, and underwriters' rules on electrical installations are, there-
toie, confined to such questions as concern proper methods and
materiab to be employed to minimize the chance of fire arising from
the use of electricity for light, heat, and power. Electricity as
employed for signaling work, such as telegraphs, telephones, call
bells, burglar alarms, and similar purposes, is not covered by in-
surance rules except in so far as such installations may become dan-
gerous because of the liability of wires in such systems becoming
crossed with electric Ught, heat, or power circuits.
The many applications of electricity for municipal fire-alarm
systems, factory or isolated plant fire alarms, watchman's time-
recording appliances, and automatic alarms employing electric
thermostats, are not covered by the general rules for electric work.
They do not in general tend to cause a fire hazard of themselves
but come rather under the head of protective devices and the dis-
cussion of them does not, therefore, come within the scope of this
paper.
The earlier installations of electrical lighting and power ap-
pliances were very crude and were often made with no considera-
tion of what are now generally admitted to be questions of great
importance from the viewpoint of fire protection. It was natural
that experience should have been necessary to demonstrate the
need of protection against unreliable or hazardous methods of apply-
ing electricity and to develop improved materials and devices.
The comparatively mysterious nature of electricity as viewed by
the general public led at first to a habit, not yet wholly outgrown.
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2 UNDERWRITEBS' REQUIREMENTS
of attributing to electrical causes all fires for which no other cause
was readily apparent. Today, however, the producdoD of light
and the transmission of power are without question accomplished
more safely as weU as more conveniently and economically by
electricity than by any other means.
In judging electrical work as affecting the fire hazard, the fire-
protection engineer and underwriters cannot undertake to do for
the assured, the work whidi lies within the province, of the electrical
engineer on whom properly falls the responsibility of designing
installations and choosing the methods and materials to be used
to accomplish the results desired. In electrical, as in other engineer-
ing, consideration of first cost, economy of operation and main-
tenance, efficient^ of machinery and appliances, depreciation, and
reliability are of prime importance. It is the proper function of the
fire-protection engineer to act as critic of the plans and their execu-
tion in order that other considerations involved shall not be allowed
to dictate methods which do not afford suitable and reasonably
safe fire conditions. Electrical engineering is far too complicated
a profession to be fully mastered by the fire-protection engineer
or by an inspector, but the general principles of safe electrical work
can be mastered and applied without presuming to encroach on the
work of either the consulting or the installing engineers.
Architects are now very generally prepared to recognize the
necessity of providing in their plans for the electrical instaltarions
which in more elaborate buildings must be given very careful atten-
tion if adequate provision is to be made for electric wires and ap-
pliances. It is in the smaller and less carefully planned structures
that the electric work is most likely to be left to the installers to be
put in as best it may. A well considered plan adapted to the type
of building and the uses to be made of current b an essential to
successful and safe work. Makeshifts are usually dangerous. For
thb reason, also, it is often much more difficult to secure proper
electrical work in wiring old buildings than new ones.
Electrical contractors are usually willing to do good work for
a fair price and most poor work is due to an attempt to secure busi-
ness at too low rates in order to meet competition. Since a faulty
or dangerous electrical installation may perform its appointed wcffk,
and since electrical defects are not easily discovered, often develop-
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UKDERWRITERS' REQUIREMENTS 8
ing after some time, only thoroughly good work from the start
affords rdal protection against disastrous failure from causes which,
after a fire, may be very difEcult to assign but which might have
been easily foreseen and removed.
Electrical inspectors either of the underwriters or of the munic-
ipality should be fully ' acquainted with approved methods, rules,
reasons for rules, and must have a great range of practical knowl-
edge which only actual field experience can ^ve; and such inspectors
must be s{>ecialist3 of a high order. The fire-protection en^neer
must refer to such specialists the technical details while undertaking
himself to be a competent judge of general methods and principles-
Electricity as a Caii$e of Fires. Electricity may in general be
the cause of a fire in either one or both of the following ways :
First: By causing wires, cables or other conductors to be
overheated by the passage of the "electric current," thereby setting
fire to the insulations on such conductors or to nearby combustible
materials. All conductors are heated by any current however small,
but if the conductor is badly overloaded it may become red- or
even white-hot. Hence it becomes necessary to prescribe the safe
carrying-capacity of wires, and all conducting parts of electric
appliances must be properly proportioned. Such parts as are pecu-
liarly liable to become heated or which must be heated to perform
their proper function must be suitably protected and separated
und» all circumstances from materials which might be ignited.
Fuses, circuit breakers, and other automatic appliances have
to be installed to afford protection in case of accidents which may
result in conductors or other devices being overheated by abnormally
large currents. Such protective devices may themselves becoiue
dangerous and the rules, therefore, prescribe carefully how they
must be constructed and installed. Overheating of conductors is
thus one of the two general ways in which electricity may cause a
fire and provisions against such accidents, therefore, form an im-
portant part of the rules.
Second: Electricity may form "arcs." An arc is the visible
evidence that the current is passing from one conductor to another
or across a break or gap through the air or over the surface of an
insulating material. An arc always causes heat and if any appreciable
current passes, the arc will be very hot and if continued is capable
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4 UNDERWRITERS' REQUIREMENTS
of melting the adjacent metal at the gap or setting fire to any com-
bustible matter near by. It, thefelore, becomes necessary to pre-
scribe rules, the observance of which tends to lessen the chance of
accidental arcs being established, and to so locate all appliances, the
operation of which produces arcs, that no harm will result. Switches,
circuit breakers, and fuses are all "arcing" devices. The problem
of insulation, spacing between conductors and excellence in materials,
methods of installation and workmanship, all have to do chiefly
with the prevention of accidental or dangerous arcs.
From the viewpoint of fire hazard, it is well to treat all con-
ductors, however well insulated, as bare, and to proceed to furnish
adequate protection against grounds or short-circuits on that basis.
In juc^ing of instalhitions it must constantly be kept in mind
that conditions are liable to become worse rather than bett«' after
the wiring and appliances have been in use for some time. Require-
ments are, therefore, made to antiinpate in part such deterioration
as is inevitable in even the best equipments, and which may be sui^
prisingly rapid when inferior materials are put in by careless work-
men and used and abused by those having little or no understaniUng
of electrical affairs and no appreciation of the hazards involved.
The following are the chief general requisites for a safe electrical
installation: Excellence of material; simplicity in design so far as
compatible with the results to be secured; ease of inspection and
repurs of all wiring and appliances; thoroughly good mechanical
execution of the work; the choice wherever possible of the more
protected and safer forms of wiring; and the use of '.'approved
fittings." No rules can take the place of good designing nor will
perfunctory obedience of rules make a poorly executed job, safe.
Tie architect, the owner, the manufacturer of devices and materials,
the electrical contractor, the electricians who install and those who
operate the plant — each and all must share the responsibility with
the insurance and the municipal inspector.
Elementary Electrical Ideas and Terms. Electric power may
be transmitted by either direct or alternating current.
Direct Current. Direct current is current of such character
that what is usually called the "Erection" of the current is always
the same, or, more exactly, the magnetic effects of the cmrent are
not being reversed from instant to instant. If a small compass be
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UNDERWRITERS' REQUIREMENTS 5
held near a wire carrj'ing direct cuireat, the needle may be caused
to turn away from its natural north and south line. Thus in Fig.
1, if i> is a direct-current dynamo connected to a wire from south to
north, a compass needle over the wire, before the switch T is closed,
will point along the wire; when the switch is closed, if current flows
as shown by the small arrows on the wire, the needle will turn as
shown. The amount it will turn is an indication of the amount of
current, but the needle will remain stationary in its new position
if the current is direct current. A battery gives direct current and
so does a direct-current dynamo.
Alternating Curreni. If, however, the current came from an
alternating-current dynamo or from a transformer supplied by such
a dynamo, the needle would fend to swing very rapidly first to one
side of the wire, and then to the other. This would abo be the case
if the connections on the
direct -current machine, Fig.
1, were rapidly exchanged
bick and forth. Such re-
versals of current direction
are made automatically by
an alternating-current dy-
namo. The number of
changes per second is called
the frequency and 25, 60,
and 135 are the commonest
commercial frequencies of
alternators. Evidently no
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compass needle could actually vibrate so fast, but it tends to do ao,
and the result is that the needle does not appear to be affected by
tbe alternating current.
Both direct- and alterqating-current systems are in common use
for light and power. Street railways use at present chiefly ditect-
current systems. Where power must be conveyed to considerable
distances, alternating current is used because it b more economical
under these conditions. Different motors and somewhat different arc
lamps are required for direct and for alternating circuits. Incandes-
cent lamps are the same for either system as are also most heating de-
vices. Transformers can be used only on alternating-current systems.
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6 UNDERWRITERS' REQUIREMENTS
The distinction "between direct and alternating current U not
one which has many important consequences in the safeguarding of
electrical work. There are, however, a few important coses where
different rules are established. It should be remembered that direct
and alternating currents of the same strength produce the same
heating effect in a given conductor. In some cases, however, with
alternating currents an additional heat effect is produced in certain
appliances by the magnetic action of iron cores, of coils, or other parts
of the apparatus. In general, alternating current produces less
severe and persistent arcs than direct current of the same strength
and voltage. Furthermore, alternating-current motors in particular
are somewhat less liable to emit sparks and, therefore, have a certun
advantage. As a whole, however, no distinction may be made as
regards fire hazard between direct- and alternating-current installa-
tions which should be made with the same care in workmanship
and with the same precautions as to insulations, fuses, and all pro-
tective devices.
Current. Current b measured in amperes. It may be com-
pared to the number of "gallons per minute" carried by a water pipe
through which e stream is flowing. More "current" will, other
conditions being equal, do more work, and will always cause more
h^t in the conducting wires and cables and in the appliances, lamps,
heaters, resistuices, motors and the like which use the current.
Furthermore, the heating effect in conductors such as metals varies
with "the square of the current," i. e., if one unit of current pro-
duces a certain amount of heat in a wire, tvxice as much current will
cause /our times as much heat in the same wire, three times the current
will cause nine times the heat and so on. An instrument for measur-
ing current is called an ammeter.
Note. The heat liberated ie a measure of energy or power coDsumed
in the wire. The lemperalure of the wire will not neceBaarily foUow the rule
given above, this being dependent not only upon the heat developed but
also upon the BUrroundings of the wire as affecting the readiness with which
the heat may be radiated or otherwise gotten rid of.
Voltage. Voltage or potential is measured in volts. VoUi
measure the propulsive force which causes "current" to flow through
a conductor. It may be thought of as an electric pressure produced
by the dynamo, battery, or other generator of electricity. Through
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rNDERWRITERS' REQUIREMENTS 7
a given circuit a higher voltage will in general cause a proportionally
greater current. Since it is the voltage which may cause electricity
to pass from its proper path and seek other and perhaps dangerous
paths, higher voltages require better insulation on wires and in all
electrical appliances. The voltage used thus becomes an important
factor in determining the protection necessary for safety.
Difference of PoientiaJ. Two points,' as on conductors or be-
tween a conductor and the earth, are said to be at different potentials.
When such an electrical condition exists on them a current tends to
pass between them either along the conductor or across a gap be-
tween the points. Along the conductor such a current produces
heat, while if the current "jumps the gap" or arcs, heat is produced
in the spark or arc formed.
Resistance. Resistance is measured in ohms. All substances
offer resistance to the passage of current. This b true of metals,
liquids, and gases. A good conductor, such as copper, has com-
paratively little resistance. Other materials, such as slate, porce-
lain, and rubber, are very poor conductors and may generally be
considered as insulators. Since heat is always produced, the con-
ductors roust be of suitable size and material to keep the tempera-
tures below the dangerous values, or, in cases where the heat is the
result desired, suitable protection must be provided. The rcsbtance
of conductors b thus a necessary but undesirable property in some
ways and a usable and valuable property in others. In all cases
the fact that current produces heat in conductors must be reckoned
with in electrical problems.
The loss of power from the heat expended in a supply wire is an
illustration of the undesired property of resbtance. The electric
flatiron is an appliance where the resistance produces a useful result.
Ohm's Law. For our purpose here the relation between cur-
rent, voltage, and resbtance in a direct-current circuit may be stated
as follows: The current (amperes) in a circuit equa's the voltage
E
(volts) divided by the resbtance (ohms), or 1=-^' Thb b true both
H
of entire circuits and parts of circuits, provided E and R are the
voltage and resistance of the whole circuit or the part under con-
sideration, respectively. For a. c. circuits a somewhat more elaborate
formula must be used.
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8 UNDERWRITERS' REQUIREMENTS
Power. Fon-er b measured in w<UU. (A kilowatt b 1,000 watts.)
In direct-current circuits the power expended in any portion of the
circuit is obtained by multiplying the current in amperes by the
Fie. 2. Pmllel Lamp Ci
voltage across the portion of the circuit. Hius: If the current in
an incandescent lamp is ^ ampere and if a voltmeter shows that the
voltage across the lamp terminab b 110 volts, the power b 55 watts.
The corresponding mechanical term is horse-power; 1 horse-power
is equivalent to 746 watts and 1 kilowatt equab about IJ horse-
power. With altemating current the power in watts is not always
to be obtained by multiplying amperes by volts. With alternating
currents, a third factor must be used, called the "power factor" of
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UNDERWRITERS' REQUIREMENTS 9
the circuit. It is sufficient for our purpose here to remember that
the power actually delivered and used with alternating current
may be less than the simple product of amperes and volts. Thus,
leb lad Double-Pole Fiue*
if a current of 50 amperes as read by an ammeter, passes through
a coil of wire, and the voltage across the tenninab of the coil is 100
volts, the power consumed, if the current is direct, is SOX 100= 5,000
watts or 5 kilowatts. If, however,* the coil surrounds an iron core
(as in an electromagnet) and alternating current b used, the power
consumed will not be 5 kilowatts. The power factor may be 60
per cent for this coil, and the power consumed will then be 50X lOOX
.60=3,000 watts or 3 kilowatts, instead of 5 kilowatts. The follow-
ing definitions will be useful to those unfamiliar with electrical terms.
Mulii'ple Conneclioti. When a number of devices such as lamps,
motors, etc., are so connected that the current has a path through
AboT* CfauuleHat
each device separately from one supply wire to another, they are
said to be connected in multiple. See Fig. 2. Incandescent lamps
are almost always connected in multiple. In Fig. 2 if each of the
7 lamps shown takes 1 ampere of current the total current at A
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10 UNDERWRITERS' REQUIREMENTS
and at B will be 7 amperes, while at C and at D it will be 2 amperes.
Series Connection. When a number of devices are so connected
that they come one after another, they are said to be connected
"in series," In Fig. 3, the arc lamps are shown so connected. In
this case the same current traverses all parts of the circuit and the
Fi|. T. EDecU at > Short-Circult in Wirag Uadai Floor
total current is no more and no less than the current in each con-
nected series device.
Shunt. A shunt is a by-path between two points so connected
that part of the current will traverse it. The division of the total
current between the main path and the shunt will depend on the
comparative resistances, the larger current going by the path of
lower resistance. In Fig. 4 are shown the connections of a "shunt
motor" and resistance box "shunted" by a wire.
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UNDERWRITERS' REQUIREMENTS 11
Cvi-Out. A cut-out is a device for automatically breaking a
circuit, usually when the current reaches a predetermined value.
Thus, a 60-ampere fuse is a cut-out designed to burn out when
currents in excess of 60 amperes pass through it. A circuit breaker
is an electro-mechanical switch which is also used as a cut-out.
Switches, fuses and other appliances are said to be aiwj/e-yiofe if
they are for but one wire of a circuit; double-pole if for two, triple-pole
if for three. Fig, 5 shows a single-pole switch and a double-pole fuse.
He. S. Am Bstaesa Wire* of 2BO-VoU Cirooit
Ground. A ground is a connection either intentional or accidental
between a part of an electric circuit and the earth, or any metal or
other conducting substances which are in electrical connection with
the earth, such as water and gas pipes, iron beams, etc.
Skori-Circuii. A short-circuit is a connection which permits
current to flow from one part of a circuit to another by any path
which it is not intended it should take. Since such a connection is
the result of accident or the failure of some insulation, and since it
usually allows excessive currents to flow, a short-circuit may be
very dangerous and liable to cause a fire, especially as the accidental
connection may afford very poor contact and cause arcing and
burning at the junction of the conductors. Fig. 6 illustrates a short-
circuit in a floor between the two supply wires to a chandelier in-
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UNDERWRITERS' REQUIREMENTS
stalled on the ceiling below. The effect of this short-circuit is shown
in Fig. 7. Fig. 8 is from a photograph of an arc produced by a 250-
. m SOfOPATCV?
volt ampere current between two wires which were touched together
at a point where the insulation had been destroyed.
Constant-Potential System. A constant-potential system is one
in which the voltage between the main supply wires is approximately
the same at all points. Such a system is supplied by a constaot-
» Arc-Lamp G«DBrHtor
potential generator, which k of a character to furnish greater cmrent
as more devices, lamps, motors, etc., are put into use. Most in-
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UNDERWRITERS' REQUIREMENTS
13
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S
k^-
lOi-
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at k Three- Win Circuit
candescent-lamp, motor- and street-railway circuita ate of this type
and also many arc-lamp circuits. V\g. 9 shows such a llO-voh
system and if the distributing
wires are properly chosen the
voltmeters at the points Ab
and C will all read approxi-
mately 110 volts. Devices on
such currents are connected
in multiple.
Constani-Current System.
A constant-current system is
one in which the current is
automatically maintained approximately the same regardless of the
number and character of the power-cotuuming devices in use on the
circuit. Devices, usual}y arc lamps, are connected in series and the gen-
erator b of a type which automatically increases the vottage or electric
pressure as more lamps are turned on. Fig. 3 shows such a circuit and
Fig. 10 is a picture of a generator used for series arc-tamp work.
Two-Wire System. A two-wire system is one having a single
pair of distributing wires.
Three-Wire System, A three-wire system is a special form of a
multiple system usuallj' employing two generators. Fig. 11 shows
such a system with two-wire branches to incandescent lamps.
Trantformers. The chief advantage of alternating over direct
current lies in the fact that electric power can by alternating
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current be transmitted at a high voltage from the generating station
to the point where the power is to be used, and there tranaformed
with small lo3s to a voltage better suited to motors and lamps.
It is far more economical to transmit power at highvolt^es.
It can be shown by electrical theory that by using twice as high
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14 UNDERWRITERS' REQUIREMENTS
voltage, a wire only one-fourth as large is needed to transmit a given
amount of power with the same percentage of loss on the trans-
mission line. It should be remembered that if the voltage is doubled
the current need be only half
as great in order to have the
power delivered the same.
But half as much current
heats the wire which carries
it only one-quarter as much
andj since the heat dissipated
on the wire is lost power, it
is evident that economy re-
quires that transmission ot
electric power be accomplish-
, ed by high voltage and rela-
tively small currents rather
than the reverse. Thus it
may be commercially possi-
ble to utilize a water power
at a distance from a city to
make high -voltage current
which can economically be
transmitted to the city over
a line of rather small copper
wires. It would never do,
however, to carry this hlgh-
voltage current into buildings
int. on account of the great fire
risk involved, for the insula-
tions in wiring cannot be made safe for high voltages. With direct
current there is no very economical way to change a small current
at high voltage into a larger current at a lower and safer voltage.
With alternating current, however, this can be done very readily
by means of a "transformer," a de\'ice which consists in its simplest
form of two entirely separate coils of wire wound on the same iron
core; in fact, a sort of double-coil magnet.
Fig. 12 shows two shapes which a transformer may take. Id
each diagram suppose an alternating current at 2,200 volts and 1
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UNDERWRITERS' REQUIREMENTS 15
ampere is supplied to one coil of 100 turns. Then a current of 220
volts and 10 amperes will be induced in the other coil which has but
10 turns, that is, the voltage will be "stepped down" in the same
ratio as the number of turns on the primary (power^upply) coil and
secondary (power-using) coil, and if there were no power loss in the
transformer itself, the current would be multiplied by the same ratio
inverted. In the case illustrated there are one-tenth as many|turns
in the secondary coil as in the primary and, therefore, the secondary
2200X1
voltage will be — rz — =220 volts, and the secondary current will be
approximately ten amperes for every ampere supplied to the primary.
If the power were supplied to
the coil of the transformer
having few turns, and used
by the current induced in the
coil having more turns, we
should have a "step-up" in-
stead of "step-down" trans-
former.
It should be especially
noted that the action of a
transformer depends wholly
on the principle that the
rapid changes and reversals
of current in one coil will
induce currents in the other
coil. Alternating currents are
rapidly and systematically
changing and reversing cur- Fig. U. Eiterlor view ot IncundMcem
rents, and thus transformers " "°* "'"''•™'"'
can be used to change the voltage and currents in a. c. drcuits.
Direct currents do not change in amount and direction and, there-
fore, transformers cannot be used at all on direct current.
Figs. 13 and 14 show the interior and exterior views of a small
transformer of a type used for incandescent lighting. The case Is
designed to be filled with oil. Transformers are constructed of all
sizes up to very large units, and are cooled either by air, air blEist,
oil, or by forced circulation of oil through the windings and core.
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16
UNDERWRITERS' REQUIREMENTS
Alternating-current dynamos and transformers are constructed
and connected in several ways which differ in regard to the number
of partially independent opcuits which they supply. An a. c. circuit
in which there are hut two wires from the generator to the trans-
Ft|. IS. Diagrtm of BJnglo-PtiMe Light and Power Ci
former or to lamps or motors is called a single-pkrae «rcuit. Fig.
15 shows the path of the current in a single-phase circuit which in-
cludes a generator 6, switchboard with ammeter A and voltmeter
V, transformers TT supplying power to lamps L and single-phase
motor M. Single-phase circuits are used for lighting, both incan-
descent and arc, and for power.
The other most common type of a. c. circuit is called three-phase,
and is also used both for light and power. The essential parts of
such a system are shown in Fig. 16 where it will be noted that the
dynamo has three slip rings, the drcuit has three wires and the
transformer lias three coils in the primary Ti, and three in the sec-
ondary Tt. In a three-phase circuit each pair of wires carries what
may be considered a separate alternating current which varies in
each pair in regular cycles, so that the maximum current in each
rtg. IS. Dugr*m of Tbne-Fbi
phase or pair of wires is readied at different instants of time. Such
circuits are very widely used, especially for power, with what are
known as three-phase induction motors.
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UNDERWRITERS' REQUIREMENTS
17
Essential Parts of Electric Installatioiis. For the general pur-
poses of this book the essential parts of electric installations are as
follows :
Generators. Generators include all dynamos and storage
batteries. These present the hazards inherent in all machines of
any sort which produce or tra^^orm large amounts of power.
Cables. Cables and wires used for transmitting electric power
are of concern horn a fire viewpoint because of the possibility of
their becoming overheated by the current, and because failure of
the means i»t>vided to confine the electric current to them, namely,
insulation, may j^ve rise to arcs, t. e., filamcs from powerful electric
sparks or discharges. The rules for safe wiring, therefore, are de-
signed to limit currents on wires
to safe values and to insure in-
tegrity and reliabifity in the in-
sulations employed.
Closely associated with the
conducting wires and cables are
those devices which serve to reg-
ulate the amounts of' current, to
open and close electric circuits,
or to change the characteristic-s
of currents at or near tbe'appa*
ratus utilizing the power. Of
this sort are rheostats, swiichea.
a
3
Fi,. 17. For
voltage-regvlators, Tnotor-geTisTotor sets, recliJUrs, and the very common
aUemating-curre t tran^ormeT. All these must have suitable insula-
tion and mechanical protection, and there must be due protection
ftvm the heat or arcs which may be produced in them.
PovxT-Conaummg Devices. The utUiziation of electric poWCT,
as of all power, is always accompanied by the production and dis-
sipation of heat. Power-consuming electric appliances such as
motors; all lamps, both arc and incandescent; heaters; and every
other appliance using electric current, are, therefore, heat producers
to a greater or less extent and must be so treated by the fire-preven-
tion engineer.
Protective Devices. Protective devices are the safety valves of
electric circuits. A fuse. Fig. 17, b a portion of an electric drcuit
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18 UNDERWRITERS' REQUIREMENTS
purposely made so that it will melt and open the circuit when the
current rises to a value which creates dangerous conditions in other
parts of the line. The proper design and use of fuses thus involves
■ the two questions of suitable protection to the wires and appliances,
and the liabiUty of the fuse itself to cause a fire when it operates.
Circuit Breakers. Circuit breakers. Fig. 18, are automatic
switches arranged to open the circuit if currents reach too great
values. They thus accomplbh mechanically the same results as
fuses. As a general principle it may be remembered that high-
voltage circuits are more hazardous than low-voltage circuits, be-
cause tliey are more able to injure insulations and to produce arcs
across gaps or from one wire to another. Large currents are not
more dangerous than small currents so long as they have propor-
tionally larger wires to carry them. In case of failure, however,
large currents (many amperes) will cause more trouble. The per-
sistency of an arc and the diiBcuIty of suppressing when once started,
are dependent upon both the voltage and the current (amperes), but
it is the voltage which causes arcs to form, as across air gaps or by
^ puncturing insulation on a cable.
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D,Bi:ized3,GOOgIe
UNDERWRITERS' REQUIREMENTS 19
The. production, transmission, and use of energy are necessarily
attended with certtun hazards which are in a sense proportionate
to the amount of energy employed. The peculiar property of elec-
trical energy is that a failure or acddent to electrical appliances
often permits a lai^ amount of energy to be suddenly expended in a
very short space of time, as a^ an arc or short-drcuit, and great heat
b, therefore, developed.
NATIONAL ELECTRICAL CODE
As the result of the united efforts of insurance, electrical, archi-
tectural, manufacturing, and allied int««3ts, there has been de-
veloped a comprehensive body of rules governing electrical work.
The development -and eicperience leading to these rules covered
many years, but in 1897 what is known as the National Electrical
Code*, was first issued by the National Board of Fire Underwriters
with the approval of a large number of engineering associations of
national scope, whose endorsement has given to these rules a stand-
ing and reputation unequaled by that of any other body of rules on
an en^neering topic in this or other countries.
The Code b generally admitted to represent the best opinion
on electricity with relation to the fire hazard under the conditions
existing in the United States. It is in general use by insurance
companies, rating and inspection bureaus and departments, and
engineering organizations. It is either adopted unchanged as the
official body of rules for cities, or is, in some cases, used as the baas
for the ndes prescribed by the city government with a few changes
or additions which are demanded by peculiar local conditions or
which give greater protection against injury to persons than can
be prescribed by insurance companies, which are, of course, not
primarily conc^ned with the life hazard. These changes are, in
nearly every case, of minor importance and consist usually of cer-
tain more stringent requirements than have found acceptance as
a part of the Code. Few, if any, city rules are less exacting than
the Code and nearly all are identical with its provisions.
It should be noted, however, that the Code does not attempt
to prescribe the most efficient or economical means of applying
electric currents, nor does it formulate en^neering rules or practice^
•The NaUoiul Elrctrioal Code will hecektMc be ninred to u tbe "Code."
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20 UNDERWRITERS' REQUIREMENTS
except in relation to fire hazards. It b published every two years
following a public meeting in New York City at which all proposed
changes or additions are thoroughly discussed. These meetings are
held under the direction of the Electrical Committee of the National
Fire Protection Association, a society the active members of which
are insurance boards and bureaus, electric and architectural associa-
tions and institutes and other societies of national character. The
Code has, therefore, the authority not only of underwriters, but also
the indorsement of all the chief associations in the electrical and
building industries.
Twice every year (April and October) is issued the Lift of EUe-
trical Fittings which gives, under the names of manufacturers, the
devices which have been approved for use after samples have been
examined and tested in accordance with the Construction Rules by
Underwriters' Laboratories, an institution located in Chicago,
Illinois, and maintained by insurance interests for the purpose of
making investigations having a bearing on the fire hazard.
In the pages which follow, no attempt will be made to restate
all of the rules of the Code, but rather to indicate the cluef under-
lying principles of the rules and explain some of the reasons for
them.
Copies of the Code can be obtained without cost from any
insurance bureau and reference to the Code will be made freely in
this book. It is, therefore, assumed that the reader has the Code
before him and will refer to it for the official statement of the several
rules, and for the relatively less important detaib not repeated in
this text.
POWER STATIONS AND THEIR EQUIPMENT
There are some peculiar hazards present in stations or rooms
used for generating electricity aside from those always involved
where larger amounts of power are produced and controlled.
In the case of companies furnishing electricity for light and
power, the generating and transforming stations are often of large
size, and contain apparatus which is very expensive and also of
enormous importance for the uninterrupted maintenance of the
service they render. Modern practice, therefore, tends toward the
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UNDERWRITERS' REQUIREMENTS 21
housing of such central plants in separate buildings of strictly fire-
proof construction, and the employment of every reaouree of en-
gineering skill and experience to prevent fire from causing loss,
either by the destruction of the equipment, or by interruption of
service resulting therefrom.
The protection against fire in such stations thus becomes one
of the chief considerations in the design of the whole plant and
passes, therefore, in great measure into the field of the designing
engineer. In any modern, well-designed central station the original
equipment is usually fairly good from the viewpoint of fire hazard.
The chief troubles often result from crowding equipment when it is
found necessary to enlat^ the capacity of the plant. Ample room
for operating and for inspecting is absolutely essential. Accessibility
of all portions of electrical equipment, the use of fireproof msteriab
throughout, the installation of protective devices of approved de-
sign, and of ample capacity and arrangements, by which any trouble
may be readily confined to a limited portion of the equipment, are
chief requisites of a central-station equipment from the viewpoint
of the electrical fire hazard.
Standard methods of installing generators, switchboards, and
all transforming devices, should be followed, but the engineering
requirements of special cases and the complexity and variety of such
equipments in present-day stations render it manifestly impossible
to describe in definite terms the details of construction and installa-
tion. In general, generators and motors of themselves present less
hazard than the switchboards with the mass of wiring often found
on them, the transformers with their charges of inflammable oil, or
the large conductors carrying heavy currents and presenting large
surfaces of combustible insulations.
Strict compliance with alt accepted rules for wiring is of as great
importance in central stations as elsewhere, both because of the
large values involved, and because of the large amounts of energy
to be controlled on the circuits. Central stations, therefore, should
never be considered as outside of "Code rules" so far as they apply.
Qenerators. Generators for either central stations or isolated
plants should always be located in a dry, light place as the presence
of moisture is apt to injure the machines or may in extreme cases
cause short-circuits in leads to the machine. No combustible ma-
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22 UNDERWRITERS' REQUIREMENTS
terial should be permitted near a dynamo, and it is desirable that
there should be good ventilation to maint^ reasonably low tem-
perature, thus increasing the capacity of the machine.
The presence of generators in rooms where any hazardous
process is carried on is not to be allowed and it should be remem-
bered that sparks from generator brushes or rings may readily set
fire to inflammable gases or to flyings of lint. Generators should
be raised above the floor and should preferably be mounted on
wood bases (F'ig. 19) which insulate the machine frame. Such
insulation will prevent a failure of insulation in the machine windings
Fie- 19' Qenentor on Woodsn Bue
from grounding the system. Such a ground might cause a short-
circuit if the system became grounded at another point. If the frames
are not insulated a reliable ground connection is required in order
to give a good path for any leak currents which may occur, instead
of an accidental path which may be dangerous.
In generating plants supplying current to railway power lines,
such as trolley and third-rail equipments, the current usually is
carried to the trolley or third rail and returns after passing through
the car motors by means of the track raib and the ground. Ali feed
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UNDERWRITERS' REQUIREMENTS
23
wires from such generating stations must be protected at the station
by an approved circuit breaker or similar device which will auto-
matically cut oS the current from the feed wires in case of an acci-
dental groimd which might allow abnormally large currents to pass
over the wire.
Dynamos in manufacturing plants are usually installed in the
same rooms with the engines which drive them, and such rooms,
therefore, present possible hazards from the exposure of wiring to
steam piping and to mechanical injuries from whatever work may
be done in repairs or alterations of machinery. The most common
faults in such locations arise from crowding the electrical equipment
into too small and inaccessible locations from a desire to use space
Z-WKPif. D.C GENERATOR
J
I
2- WIRE 6EN£f?AT0/f
ON S-WIFE SY6TEM M0T0/?-6ENEffATQR BALANCEft
Fi|. 20. DikcruD Shovini FuM ProtHtion in Vuiou* Typ«a of Circuits
either not available for other uses or not especially planned for
electrical apparatus. Convenience of operation, cleanliness of sur-
roundings, and a workman-like layout of equipment, all contribute
to the maintenance of good fire conditions. The requirements for
overload protection at constant-potential generators are shown in
the diagram, Fig. 20, where f in each figure represents the fuses or
drcuit breakers required by the Code. These fuses and breakers
wiU usually be located on the main switchboard. This leaves
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24 UNDERWRITERS' REQUIREMENTS
the large main cables from generators to switchboard unprotected
and for this reason special pains should be taken with their instal-
lation since an accident to them might be very serious,
Dynamo-Room Wiring. All wiring in dynamo rooms should be
done in the most substantial manner, and special attention should
be given to simplicity and orderliness of arrangement. All supports
for conductors must be very substantial. Fig. 21 shows an excellent
example of a large mass of wiring well installed.
All wiring sliould be readily renewable and accessible. Thb
usually requires either open wiring or some form of conduit. A
Fie. 21. Exunple of lAtet M*ta ot Wirinc W«l] Iiwtdled
fiber conduit b largely used for this work and gives good results.
The large amount of wire and cable often found in dynamo rooms,
together with the large currents carried on the conductors, makes
it necessary especially about switchboards to cover the highly in-
flammable rubber and braided coverings with a tight jacket of some
r.on-combustible material, such as asbestos, to reduce the probability
of fire spreading rapidly o\'er all the exposed wiring.
Bus bars are usuall,\' left bare but there should be as little other
bare current-carrying metal as possible.
All conductors should have ample current capacity, and in their
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UNDERWRITERS' REQUIREMENTS 25
installation special care must be given to securing excellent insula-
tion and reliable supports. Due regard should be given to the
possibility of injury to conductors by tools, belts, ladders, etc., or
by any vibration which may cause a gradual wear on wire coverings.
In general, the requirements of Class C of the Code apply to
dynamo-room wiring with such special precautions as are dictated
FLg. 22. 01d-8iyle Type or Wood-Frame Switolibo»rd
by the large currents to be cared for and the presence of a great
number of conductors which may have to be placed rather closely
together.
Switchboards. The danger of fire at switchboards lies in the
large number of wires usually concentrated on them, and the use of
rheostats, fuses, and other appliances which are possible sources of
fire. The switchboard itself should always be of slate or porcelain.
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26 UNDERWRITERS' REQUIREMENTS
The old wood racks (see Fig. 22) once used are not desirable
thou^ still permitted if nf hard wood well filled. Fig. 23 shows a
small, well-arranged switchboard. While the front of the switch-
board b the operating side, the rear is the part where trouble is more
likely to occur. The back of the board should, therefore, be easily
accessible and neatness of arrangement and reliable supports for all
Fl(. 23,* Approved Type ol SmiJI SiHtehbaud
conductors be imperative. No makeshifts are tolerable on switch-
boards under any conditions. Fig. 24 shows the back of a board.
Every precaution must be taken to keep water or even moisture
off switchboards since a short-circuit would be easily started and
the result would often be highly disastrous. The Code, Rule 3,
gives in detail some requirements for location and care of switch-
boards, the reasons for which are apparent without further explana-
tion. Fig. 25 shows a large modern switchboard. In general the
exact arrangement of parts on switchboards is determined by en-
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Fit. 24, Bocli Viev of Well
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28 UNDERWRITERS' REQUIREMENTS
gineering and operating requirements, and the interest, from the
viewpoint of fire, centers in the protection of conductors and the
location of board and instruments to reduce the prohability of fire
spreading from the board to the other parts of the building. Order-
liness and cleanliness should be insisted upon.
Resistance Boxes or Rheostats. These are used in a great
variety of sizes and designs with dynamos and motors aud at switch-
boards. They may be mounted
either on tlie machines or switch-
boards or separately, but in every
cose they must be considered as
' 'stoves" and liable to become
"red-hot." The fact that they do
not become hot when used under
normal conditions is not to be
considered as in any way lessening
the requirements for protection.
A burn-out of a rheostat may
result in a mass of fiames from
^''' ^"eisr"^" RhcMut' ^°"" *^^ numerous wires, usually con-
tained in or leading to the device,
and there may be drops or spurts of melted solder or other metal.
Fig. 26 shows the interior of a small starting rheostat.
All resistance devices used for starting, regulating, and controlling
machines are to be classed as "resistances" in the intent of the Rules,
and so in general are all devices or parts of devices where coils of
wire, iron grids, or other similar objects are made parts of the circuit.
Lightning Arresters. The purpose of these devices is to prevent
lightning, or external high-voltage currents from foreign cu^niits,
entering the station and there causing fire or damaging machinery
and instruments. Lightning cannot be "stopped" but may be
dioeried to the earth. A large variety of lightning arresters are on
the market but they are all designed to afford a piath by which dis-
charges may pass to "ground" instead of into the station apparatus
and to prevent so far as possible the normal currents on the lines
from following this path. The operation of many lightning arresters
is accompanied by sparks which may in severe cases assume the
size of powerful arcs. The location of these devices away from all
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UNDERWRITERS' REQUIREMENTS 29
combustible material is thus imperative. They are often installed
in se[>arate buildings with other emergency appliances.
The full discussion of lightning-arrester construction, opera-
Uon, and installation is impracticable in this book. Suitable loca-
tion, the running of connections in straight lines with the fewest
possible bends, the use for ground wires of copper wire not smaller
than No. 6, and especially the securing of permanently good ground
connections are the chief considerations. Two separate ground con-
nections are often desirable. Fig. 27 shows a type of electrolytic
lightning arrester for 7,000 volts maximum potential to be used on
three-phase circuits, grounded or neutral service.
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UNDERWRITERS' REQUIREMENTS
Ground Etetectors and Tests. Except where circuits are inten-
tionally and permanently grounded, a leak to "ground" may be
the cause of arcs at any point in tlie system which may be dangerous.
By "ground" is to be understood the earth, walls or floora of masonry,
pipes of any kind, iron beams or floors and the like. It is, there-
fore, required that suitable instruments and circuits be provided to
indicate either continuously, or by frequent tests, whether there
are such ground connections which would indicate a failure of some
insulation. If such a failure is shown by the detector the trouble
can usually be located and repaired before injury is done.
Ground detectors may take the form of special instruments on
the switchboard or of arrangements of switdies with lamps or volt-
^^i> t
'^
aWirCHBOARD BUS BAK OR 7P/MSfOK*t£ff
•secof/OAJty maihs.
meters which can be used to test the insulation of each side of each
circuit in turn. The connections should be such that the detector
cannot be left out of circuit. For the various arrangements of ground
detectors reference must be made to the standard electrical treatises.
Attention may be called to the following points which are fre-
quently neglected:
(1) Lamp receptacles should be keyless and any switches in the
detector circuits should be so connected that the detector will con-
tinuously give an indication on at least one side of the circuit.
(2) The wires should always be protected by small fuses where
they connect to the bus bars.
(3) If the detector is of a type depencUng on relative bright-
ness of two lamps, the lamps should be very close together.
(4) Special care should be taken to secure excellent ground
connection for the ground wire.
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UNDERWRITERS' REQUIREMENTS 31
The accompanying diagram, Fig. 28, shows a very good and
simple detector for any two-wiie low-voltage system, and is typical
of many in use. (The diagram and description are taken from a
publication of the Associated Mutual Fire Insurance Companies.)
The lamp for the detector Bhould be of the some CEindle power and voltage,
the voltage bwig about the aame as that of the regular lampa ax the plant,
and two lamps should be selected which, when connected in seriee, bum with
equal brilliancy. Although somewhat greater sensitivenees can be obttuned
with low-candle-power lamps, such aa 8 c. p. for example, it is believed in
genu^ to be preferable to use lamps of the same candle power aa those through*
out the plant, as then a bumed-out or broken-detector lamp can be imme-
diately replaced by a good lamp from the regular stock, thus avoiding the
neceeaity of keeping on hand a few spare apecisl lamps.
The detector lamps, being two in series acroaa the proper voltage for
one lamp, bum only dimly. If, however, a ground occurs on any circuit,
as at a, the current from the positive bus bar through lamp No. 1 divides on
reaching b, instead of all going through lamp No. 2, as it did when there was
no ground. Part now goes down the ground wire and through the ground to
a, as indicated by the broken line, and thence through the wires to the n^ative
bus bar. This reduces the resistance from b to the negative bus bar, and,
therefore, more current flows through lamp No. 1 than before, whUe less cur-
rent flows through lamp No, 2. Lamp No. 1 consequently brightens and
lamp No. 2 dims. If the ground had occurred at e instead of a, lamp No. 2
would have brightened and lamp No. I dimmed. Thia detector, whilenot able
to indicate extremely email leaks, will show any leak that ia likely to be dan-
geroua from a fire standpoint.
Motors. Electric motors are very generally used in manu-
facturing plants of all sorts and their use b still rapidly increasing.
They are also used very extensively for elevators, cranes, and for
an infinite variety of uses in all classes of buildings. Recently port-
able motors of small size have come into very genial use ior drills,
and other portable tools, vacuum cleaners, washing machines, and
scores of other purposes where their cleanliness, economy, and
adaptability to all service recommends them. These portable
motors, however, do not in general come under the rules prescribed
for stationary machines, though their use involves similar hazards
and some others peculiar to themselves. These will be discussed
later.
In general the same precautions should be taken with motors
as with generators. Dry, clean, well-lighted locations should be
chosen for them. It is a very conmion fault to instaU a motor in a
space too small for it, or in a place where rubbish and dirt are allowed
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32 UNDERWRITERS' REQUIREMENTS
to accumulate. The fact that modern motors are very sturdy ma-
chines, and will continue to work even when neglected and abused,
encourages carelessness in maintenance which often results in hazard-
ous conditions. Except where unavoidable, motors must never be
located in the vicinity of combustibles, in wet places, or in very dusty
or dirty places. ^Mien sucli locations are unavoidable, an "enclosed"
type of motor should be used, or a special room, Fig. 29, should be
provided. Many modern tj^pes of alternating-current motors have
no exposed live parts and do not have commutators or brushes and
are, therefore, well suited to dusty or Unty places. Fig. 30 shows a
rough commutator of a motor. It is evident that as the commutator
, revolves, the brush which carries the main motor current will not
fit well at all points and, therefore, sparking will result. In extreme
cases this sparking may be dangerous, especially in dusty places or
where inflammable gases are present. Where motors are used, not
of enclosed tjpes with brushes, commutators or slip rings, where
sparking may occur, special enclosures should be provided. The
installation of motors on ceilings. Fig. 31, or built in as parts of the
special machines which they drive, often affords both the neatest
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UNDERWRITERS' REQUIREMENTS 33
aad the safest aTrangement. Motors operating at voltages over 550
volts require special precautions in re-
spect to wiring leading to them. Motors
for less than 550 volts are wired accord-
ing to .the general rules for low-poten-
tial systems.
Wiring to Motors. The supply leads
to motors must be of a size to carry at
least 25 per cent more current than that
for which the motor is rated. This ap-
plies to all tjpes of motors both d. c. and
a. c. and is required even though the motor
is not actually being used for its full load. This is called for to
provide for the currents required
to start the motor which are
almost always greater than the
current needed for continuous
full load running, and also to
provide a margin of safety at all
times.
Wiring for Direct-Current
Motors. The Code prescribes
the safe carrying capacity of
wires of different sizes. Page 65.
Thus if a direct-current motor
requires 40 amperes when work-
ing under full load, the lead wire
must be of such size as to carry
50 amperes. A reference to Rule
16 shows that this will call for a
No. 5 wire. As No. 5 wire is not
usually made for electrical pur-
poses it would be necessary to
use a No. 4 wire.
The factors which are in-
volved in the determination of
the proper size of leads to direct-
current motors are the following : '^^ ^'*' ^''Vot Mow"'""' "'""'"''■
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UNDERWRITERS' REQUIREMENTS
TABLE I
Sizes ol Conductors In Direct Current*
f^z-
no voLw
2ao Volts
fiOO
Volu
Open
Coiice«l*d Open
Open
1
14
14
14
14
14
2
10
14
14
14
14
3
8
10
12
14
14
4
6
3
10
14
14
fi
6
R
10
12
14
7i
3
4
6
8
14
10
1
2
6
6
12
12J
1
4
5 •
10
15
00
3
4
10
m
000
000
2
3
8
20
0000
c, m.
000
c. m.
1
2
8
25
250,000
250,000
1
6
30
350,000
250,000
00
5
35
400,000
300,000
000
000
4
40
500,000
350,000
0000
c. m.
000
3
50
700,000
500,000
260,000
250,000
1
*The queatloa of drop ii not Uken inlo coMidcrmtioD in the above table.
E= volts required by motor
t= efficiency of motor in per cent
h.p. = rated horse-power of motor
Then if /= current in amperes at full load and at the pven
voltage
h.p .X746
EXk
When I is found, a reference to the Wire Capacity Table, Page 65,
will show what size of wire is required for current 25 per cent in ex-
cess of /.
In Table I arepven the sizes of conductors in Bronnand Sharpe
gauge required for direct-current motors of respective sizes in h.p,
and voltages, on the basis of 90 per cent efficiency and 25 per cent
overload on motor leads. The two columns refer to wiring run
"concealed" as in conduit or "open" as on cleata.
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UNDERWRITERS' REQUIREMENTS
TABLE II
Sizes of Conductors la AiteraatlaE Carreat*
SINGLE PHASE
H.P.
110 VoLn
220 VoLn 1
*SS»""
Siioo
Wir.
AroM. on
Si» o( Win 1
Op.n
Op™
1
12
12
14
6
14
14
2
23
8
10
11
12
14
3
33
6
6
16
10
12
4
44
4
5
22
8
10
S
53
2
3
26
6
8
7*
85
42
4
5
10
110
000
000
66
2
3
A
B
A
1
THREE PHASE, 220 VOLTS
Sua o
HanB-Pomr
ot'-JSr.
Conculed
OP.Q
1
3
14
14
2
6
14
14
3
9
12
14
4
11
12
14
fi
14
10
12
7*
20
8
10
10
27
6
8
15
40
4
e
20
SO
3
4
30
75
1
SO
125
0000
000
75
185
300,000
250,000
100
250
600,000
350,000
ISO
370
800,000
600,000
A
B
Note. Column "A" gives currents to be used in calculating me of
isins Bupplying more than one motor.
Column "B" gives aice of wire for branches and mains supplying one
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36 UNDERWRITERS' REQUIREMENTS
Wiring for AUemating-CuTrent Motors. The calculations for
proper sizes of wires for alternating-current motors, are somewhat
more complicated than for direct-current motors.
Table II gives the wire sizes prescribed by the rules of the
Department of Electricity of the City of Chicago. Somewhat dif-
ferent values are, however, in use in other places and no universally
accepted rule has yet been developed.
Switchea and Pratedm Devices with Motors. Every motor of
over 1 horse-power must be protected by fuses or circuit breakers,
and controlled by a switch; and all of these must be located within
sight of the motor, and be arranged so as to open and to protect all
the wires. An automatic circuit breaker which will dbconnect all
the wires of the circuit is considered the equivalent of both the fuses
and the switch.
m o! Mnlor CLr.:i
The chief purpose of tht fuses is to protect the motor and espe-
cially the wiring to it from overloads resulting from accidents tothe
motor or from the excessive current which will flow if an attempt is
made to start the motor when it is for any reason unable to start
and attain its proper speed. The fuses nmst, therefore, always be
of such size as to blow with currents not in excess of the specified
carrying capacity of the supply wires. Circuit breakers must not
be set more than 30 per cent above the carrying capacity of the wire
unless fuses are also used.
For a. c. motors, each phase (t. e., each main motor circuit
whether there are one or more) must have a fuse whether circuit
breakers are used or not. This b because of the necessity of setting
drcuit breakers on such circuits so that they will not open with the
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UNDERWRITERS' REQUIREMENTS 37
large currents which flow on starting the motor. The breakers,
therefore, may not protect the wires properlj' but the fuses act less
promptly and even if of lower rating will not flow before the motor
has come up to speed and the currents have been reduced to the
normal amounts for actual running power. An exception to this rule
is made in cases where the protective devices are on main switch-
boards or under constant expert super\'ision. Also for single-phase
a. c. motors, one fuse and one circuit breaker are allowed, one in each
of the two motor supply
wires.
The switch at a motor
is required so that the line
can be readilydisconnected
from the motor when the
latter b not in use or in
case of accident. On two-
wire systems, a double -
pole switch is required, on
three-wire, a triple-pole.
If a circuit breaker is
used without fuses it must
be of a tjpe which will
protect the motor under Fig. 33. SmsJl VrntiJatinn OutSt lo be Coonected
,, . . ~, lu Lamp Socket
all circumstances. 1 bus
in Kg. 32 the single-coil circuit breaker does not comply with
the rule, since, if a ground should occur on the main at G and
another on the motor at C, the coil C of the circuit breaker would
be cut out of the circuit and the breaker would fail to operate, for
no provision is made to open the other line. In this case a fuse of
proper size should be installed at f or a circuit breaker having a trip
coil in each side should be substituted for the fuse and breaker shown
in Fig. 32. If the circuit breaker takes the place of the switch at the
motor it must be such that one line cannot be opened without open-
ing also all the others.
Motors of J h.p. or less may be connected to circuits less than
300 volts in the same way as incandescent lamps, provided the
proper fuses are used in the branches supplying the power to them.
Such motors, illustrated by desk-fan motors and all portable motors
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Fit- 34. Common Typt ot Motor
38 UNDERWRITERS' REQUIREMENTS
of small size, do not usually require starting rheostats. Fig. 33 shows
a small ventilating outfit to be connected to a lamp socket.
Rheoftaia with Matora. Rheostats are used with motors for start-
ing them and for controlling their
speed. They should always be re-
garded as somx^ of heat and installed
accordingly. Fig. 34 shows a common
type of motor-starting rheostat. Such
rheostats (called motor starters) need
to be in the circuit for only the very
short time usually required to bring
the motor up to speed and, therefore,
they are usually so designed to carry
the necessary current for only this
brief period. Such motor starters,
if left with current passing through their coils or grids for longer
periods, will become very hot and may bum out. The manner in
which a simple motor starter is connected b shown in Fig. 35 when A
represents the armature of the motor, 3f
the field coil or electromagnet of the motor
and R the resbtance coils in the starter
rheostat box. The required fuses and
switch are shown at FF and S. In thb
design the starter, fuses, and switch are
all mounted on a single slate base. To
start the motor, the switch S b first
closed and the handle 7/ is slowly moved
from to the position shown in the dia*
gram. It will be seen that the resbtance
R during the process of starting serves to
limit the current supplied to the motor ar-
mature, which would otherwise, until the
motor came up to speed, be excessively
Fia. 35. StartiDc Rheoaut wirinc large. When the handle has reached the
position shown, the resistance is all cut
out of the circuit. A rheostat of this sort cannot be used safely
except to start a motor of the proper size for it. The resistances will
get excessively hot if // b allowed to remain at any intermediate point.
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UNDERWRITERS' REQUIREMENTS 39
Motor starters for d. c. motors must, therefore, be made ao that
a spriQg or other device will return the lever to the "off" position in
case the operator attempts to leave the starter with current passing
through its resistance coils. This b because these coils are not
designed for this continuous duty. When, however, the lever has
been moved clear across to its final position, it is held there by a small
electromagnet (V in Fig. 35) but in this position the connections
have been shifted so that the resistance coils are no longer in the
circuit. If now it should happen that the supply of current should
fail, the electromagnet (called the no-voUage releaae) will release the
lever which will fly back to the original "off"
position. Then when the supply is re-estab-
lished the motor will not be injured by start*
ing too suddenly, or by the severe arcing at
the motor commutator.
Since a. c. motors usually have no com-
mutators and are less liable to injury from
sudden stuping, a no-voltage release is not
required in a. c. starters.
A somewhat different device called an
autostarter, potential atartsr, or compemator, is
often used for starting a. c. motors. It is a
form of transformer which allows only a por-
tion of the line voltages to be applied to the
motor until it b well started. Autostarters
are to be regarded as sources of heat, and
much the. same precautions are necessary as
with ordinary starters containing wire coils or Fie. m. iDMnar vim of
iron grids. If they contain oil-immersed
switches or coib, the oil adds a further hazard on becoming overheat-
ed or spattered about. Fig. 36 shows the interior of a compensator.
In general, motor starters, either d. c. or a. c, involve both the
hazards of switches and those of possibly overheated coils of wire ot
other parts, and must be treated accordingly.
Another class of rheostats includes those which are intended tor
regulating the speed of motors and generators. They are necessarily
used continuously whenever the machine b running and, there-
fore, must be proportioned so that their coils or other resistances
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40 UNDERWRITERS' REQUIREMENTS
will not become overheated, even when the necessary current traverses
them for hours at a time. Fig. 37 shows a large rheostat of thb type.
Among these rheostats are classed field rheostats for dynamos, con-
trollers for motors (as distinguished from motor starters), theater
lamp dimmers, and, in fact, all rheostats or resistances which are
required to be continually in the circuit. Some motor starters are
designed to serve also as controllers and may have two sets of re-
sistances, one for starting only and the other for continuous duty
in varying the speed of the motor. These are especially useful in
connection with machine tools and similar apparatus.
Special forms of rheostats having their resistances so enclosed
that a burn-out will not cause sparks, are required in dusty or linty
Fi«. 37. Closed snd Open Views ol Large RhcoeUt (or CoDtinuoin Etrvice
places, such as flour mills, various textile mills, etc. If such spe-
cially protected starters are not supplied, the starting apparatus
should be in special rooms where the dust or lint can be kept out.
The dangers to be guarded against in all rheostats of every type
arise from the fact that these a])pliances must from their very nature
be sources of heat. By liberal design and good ventilation the
temperatures can be kept low; but all rheostats should be installed
with the idea that they may become overheated by failure or mis-
use, Fig. 38. The large amount of insulated wire often used, and
the oil in which parts are sometimes immersed, may furnish a con-
siderable amount of fuel, and in severe bum-outs, flames may result
and molten metal be ejected. A large amount of smoke usually
accompanies a burn-out of a rheostat.
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UNDERWRITERS' REQUIREMENTS 41
Storage Batteries. Batteries of large size are now in common
use. These are practically alwajs storage batteries, that is, bat-
teries which are recharged by having current from a generator
if Badly lUBlalled RheoaMt Bi
supplied to them from time to time. What are- called primary
batteries, that is cells in which there is a chemical action which sets
up a current without any machine being used to charge and recharge
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42 UNDERWRITERS' REQUIREMENTS
them, are sometimea of large size, and develop fairly high voltages.
They are, however, used chiefly for telegraphs and other signaling
work and need not often come under the supervision of underwriters
or fire-protection engineers.
Storage cells, however, where they are capable of developing
the same voltage (100 to 600 volts not uncommon) require the same
general precautions as dynamos or motors since they produce like
amoimts of energy. Furthermore, it is a property of such batteries
that in case of a short-
drcuit they can for a
short time supply very
large currents, far larger
than they are normally
called upon to supply.
Thus they may be term-
ed reservoirs of energy,
capable of produdng
trouble if their output is
not properly controlled.
Storage battery rooms
should be thoroughly
ventilated. The action
of the current in charging
the battery, liberates at
times large quantities of
hydrogen and oxj'gen,
and if these should ac-
cumulate in the right
n«^30 Geomi EiKtriF An LuhtiDc proportions, they would
Ttuutarmor with S«nea ReoUfiec *^ *^ t j
form an explosive mix-
ture which might be exploded by any accidental spark.
The water and acid used about storage batteries make it neces-
sary to provide especially good insulation. The battery jars are
best mounted on glass strips set on special porcelain insulators. The
usual precautions for rooms where acid fumes exist should be taken
in battery room wiring, and metal parts must not be such as to be
affected by corrosion, since a decrease of the cross-section area of
any current-carrying part may ultimately reduce the metal to a
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UNDERWRITERS' REQUIREMENTS 43
degree that will cause overheating or, in case of an actual break, a
dangerous arc.
Transfomm?. In central or substations or in large power
rooms of isolated plants or factories, the chief point in the installa-
tion of transformers is the provision for preventing injury, the smoke
resulting from burning out of the coils or, in the case of oil-filled
cases, from the boiling over of the oil. These effects are, of course,
produced onlj jfrom some accident to the apparatus or circuits,
but transformer fires are peculiarly difficult to fight, and the oil and
the insulations on the windings produce, at high temperature, large
volumes of smoke which may damage goods in other parts of the
building than the transformer room itself, or which might be mis-
taken for a fire and result- in water being thrown into the building
entailing a heavy "water loss." Transformers should always be
located in clean, dry places with ample space about them. Fig. 39
shows a General Electric constant-current arc-lighting transformer
with series rectifier outfit for 4-aiDpere 50-light system.
OUTSIDE WORK
Defects or failure of electric light or power circuits outside of
buildings, as on poles or over the roofs or walls of buildings, may
become the cause of fire by setting up abnormal conditions in ap-
pliances connected to them, by setting fire, either by overheating or
arcing, to buildings on which they are supported, or especially by
crosses. By a crass is meant an accidental touching by wires of one
drcuit or line with those of another line. By such accidental con-
tacts lines may become charged with a higher voltage than they or
their appliances are suited for. Thus a cross between a 2,200-volt
arc-light wire and a 110-voIt house-lighting circuit may cause current
from the 2,200-volt circuit to pass into one or more houses where
it will perhaps blow fuses with an explosive action, or cause very
dangerous arcs as it passes to the ground through some defective
insulation. Fig. 40. Similarly it is very dangerous for an ordinary
lighting circuit to become crossed with a telephone wire, since the
insulation and carrying capacity of telephone wiring is not such as to
resist the effects which might be produced. For reasons such as these,
outside wiring has a definite relation to fire hazard even in buildings.
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44 UNDERWRITERS' REQUIREMENTS
Wiring. In very many cities and towns insufficient attention
was paid to outside wiring until the streets and alleys became crowded
with circuits of all sorts, each put up as was cheapest or most
convenient at the time it was constructed. Where some of these cir-
cuits are of high power and high voltage exceedingly dangerous
FLj. 41). RnuU of Crusg Detwecn 2,200-VoU Lifhting Circuit Bod 1 10- Volt House Citcuil
conditions exist, and the hazard may be extended over large areas
remote from the place where actual failure occurs, since the resulting
accidental currents may pass over any of the lines involved to build-
ings at considerable distances. The efforts of fire-protection engineers
and insurance interests should be directed toward the correction
of faulty outside wiring conditions, which if neglected are bound to
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UNDERWRITERS' REQUIREMENTS 45
become progressively worse and constitute a serious menace to lives
and property.
Where an insured property is clearly jeopardized by outdoor
wiring, excellent electrical conditions indoors can at best afford only
a partial safeguard, and fire-protection engineering should apply
itself to remove the defects out-of-doors as well as in.
Underground System. The placing of electric wires underground,
especially where low voltage and signal circuits are carried in separate
conduit sj'stems, affords the most thorough solution of the hazards
of town and city wiring. Underground wires are, of course, less liable
to accidents of weather and storm, and are, therefore, preferred by
operating companies although the expense of putting wires under-
ground often prevents or postpones a change.
Legislation prescribing a gradual change, a
certain amount of wiring being put under-
ground each j"ear, has often produced results
without undue hardship to electric com-
panies. However, there are certain well de-
6ned precautions and standard methods for
outside work which can reduce hazards to a
great extent.
Standard Practice. Line wires should
have either rubber insulating or weatherproof covering. The
latter consist? of three tight cotton braids, each thoroughly im-
pregnated with" a waterproof compound. All tie wires, as at
insulators, must have an insulation equal to that of the wires
they confine. The insulation of outside wires on poles is re-
quired as an additional protection, but is not in any way de-
pended on for insulation at the supports. The use of bare wires
would greatly increase the probability of crosses in cases of break-
age of wires.
Standard practice is to use glass or porcelain insulators at least
1 foot apart on pole cross-arms and to ground all metallic sheaths
of cables. Fig. 41 shows a cross-section of one form of porcelain
insulator. The "petticoats" will nearly always have a dry space
underneath their lower edges, and even if not dry, the length of the
path offered to the current escaping over the wet surface is so great
that the leakage is small.
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46 UNDERWRITERS' REQUIREMENTS
All joint wires should be soldered and carefully insulated. "Hie
joint should be made mechanically and electrically secure before
the solder is applied. Good joints are requisite to lessen the chance
of wires falling as well as to prevent arcs on the wires themselves.
Proximity of Wires to Electric Light and Power Lines. Probably
the most conmion and also the most dangerous fault in outside work
FLg. 42. PrHMn™ or Overbed Wire. Hinder. Flramep
is the running of telephone and other signal wires too close to electric
light and power lines. Wires of this sort should never be on the
same cross-arms and it is much the best practice wherever possible
to keep telephone wires on poles on one side of a street, and light
and power wires on the other, giving special attention to the neces-
sary crossings, if any. In all outside pole work in towns due regard
should be gi^-en to the fact that lines consisting of many wires are
a serious hindrance to firemen in attacking a fire in the adj^nt
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UNDERWRITERS' REQUIREMENTS 47
buildings. Fig. 42. The disposition of the lines should be such as
to reduce this as far ss practicable; but pladng wires underground is
the only complete solution of this problem.
Ks. 43.* Well-IoBMUed Root Wiring
OuUide Wires on Buildings. Where outside lines are supported
on buildings, they should be at least 7 feet above the highest point
of flat roofs and at least 1 foot above the ridge of pitch roofs over
Fig, if Approved Form of Roof StruMurc for Sale InsMillation
which they pass and the roof structures should be of the most sub-
stantial description. Figs, 43 and 44 show two forms of roof struc-
ture which hold the wires high enough so that they cannot sag and
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48 UNDERWRITERS' REQUIREMENTS
touch the roof and so that they are not liable to be touched or dis-
turbed by persons walking on the roofs. Where outside wires are
brought from pole lines into buildings special precaution must be
taken. Such wires are spoken of as service wires or services.
The portion of such wires from the service switch in the building
and the first outside support must be rubber insulated. Every pre-
caution must be taken to keep service wires free from contact with
cornices, awning-frames, shutters and the like, under all conditions.
Usually, for low-voltage cir-
cuits, it is best to put the
wires in metal conduit, as
shown in Fig. 45. This draw-
ing also shows the drip loops
which should always be pro-
vided, the insulators on brack-
ets secured to the wall and the
special pipe-cap on the top of
the conduit to keep rain out
of the pipe. Where conduit
is not used, porcelain bushings
may be used, slanting upward
through the wall toward the
inside.
In general it is better to
have service wires enter
through the basement rather
than through an upper story
or attic. Where wires are
carried along side walls out-
*"'''*lho^"'pmvisfun'"A"flilsrMoi»iu^'''"'' •'^'*'^ "^ buildings, the;- should
be supported on glass or por-
celain insulators, not more than 15 feet apart. Where not exposed to
weather, porcelain knobs may be use<l if not more than 4^ feet apart.
Trolley Wires. Trollej' wires must be of ample size for mechRnical
strength (No. B. & S. gauge copper or Xo. 4 B. & S. gauge silicon
bronze). Protection against crosses must be ample and street rail-
way trolleys and feeder cables must be capable of being disconnected
at the power station or of being divided into sections so that in case
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UNDERWRITERS' REQUIREMENTS 49
of fire on the railway route, the cuircDt may be cut off from the
particular section and not interfere with the work of the firemen.
Electrolysis. Whenever an electric current passes between a
pipe or wire underground into damp earth an electro-chemical effect
is produced which may, under cert^n conditions, produce a disin-
tegrating effect on the metal pipe or wire and eventually destroy it.
Whenever an electric current passes through a conducting liquid
which is not a chemical element, the liquid is decomposed. The
salts and acids in the liqujd are thus decomposed and metal plates
by which the current is led into and out of the liquid are also affected.
This process is called electrolysis, that is, breaking up of chemical
composites by electric currents.
It is at once evident that water and gas pipes of iron buried in
moist earth provide the necessary combination for electrolytic effects
and that pipes in areas traversed by currents through the earth may
be injured by such chemical disintegration. In cities and towns
where there are large electric currents which use the earth as a
"return path," these effects, if improper conditions are allowed to
exist, may become very serious and have a bearing on the fire hazard
chiefly as they may jeopardize the water mains and supply pipes to
an extent sufficient to render the fire-fighting facilities unreliable in
emergencies. Since the direction of alternating currents reverses
many times each second, the chemical effects which they can produce
are also reversed and, therefore, in general alternating currents in the
earth are not liable to produce dangerous electrolysis. There are,
however, direct currents which usually employ ground returns and
by far the commonest are the currents of the usual overhead trolley
street railways. It is to these, therefore, that the following explana-
tion applies. Fig. 46 illustrates the way in which the current in a
trolley circuit returns to the generator. Such circuits in this country
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50 UNDERWRITERS' REQUIREMENTS
are almost invariably direct current at 500 to 650 volts and the
positive side of the dynamo b regularly -connected to the overhead
wire. The current thus passes out over the trolley, through such
cars as are in operation and back to the negative side of the dynamo,
by the rails, or through pipes, moist earth or other conducting sub-
stances underground. The negative side of the generator is, of
course, connected to the rails and ground. The retiun current divides
according to the conductivity of the different paths afforded it and
the actual distribution of currents in the earth may be very com-
plicated and is subject to great variations from point to point.
Careful "electrolytic surveys" are necessary in order to discover
fully the actual current arrangements exbting underground in any
town or city.
In general, however, it is important to notice that wherever
the current leaves a pipe for the earth, as at £ in Fig. 46, the iron of
the pipe is carried away into the earth, just as in a silver plating
battery, silver is carried away from the silver plate toward the
article which b being plated. It is this action which is meant by
electrolysis as the term is usually employed with reference to under-
ground pipes and earth currents and the result, as has been stated,
may be a destruction of the pipe at one or at many points.
For the diminution of such electrolytic corrosion every means
should be taken to afford a low-resistance path for the return circuit.
This is also of advantage from the viewpoint of economical opera*
tion since it takes power to drive the current through a high-resistance
return path. Large copper conductors are often laid between the
raib and parallel to them, frequent heavy cross wires being used to
connect this copper cable to the rails. Good bonds between raib
either of low-resistance tie plates, or still better of electric welds
between rail ends, are of great value.
The general arrangement and size of return wires and rail
should he such that the difference of potential in volts between the
grounded terminal of the generator and any point on the return cir-
cuit will not be more than 25 volts.
WTiere pipes or other underground metal work are found to be
electrically positive to the raib or surrounding earth, that is, so
that current tends to flow from the pipes to the rails or earth, special
conductors should be provided to connect the pipes to the rwla to
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ill
in
m
I
i
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UNDERWRITERS' REQUIREMENTS 51
carry such currents and thus prevent, as far as possible, the effects of
electrolysis. Very elaborate investigations of electrolysis as affecting
water mains, gas pipes and other underground metal have been made
in many cities and the co-operation of city authorities, water com-
panies, and electric power and railway companies, has in many places
resulted in greatly lessening, if not in entirely removing, the dangers
from this cause. Surveys should be repeated occasionally since
new electric drcuits or new pipe lines may create an altered condi-
tion in underground distribution of currents.
H^h Tension Lines. In recent years there has, in most parts
of the country, been a very great increase in the number of high-
tension power-transmission lines and the voltages have also become
much higher. Overhead hues of this class present certain problems
Hg, 47.* Croswver ArMmgement lot Hieh-Teiuion Cireuiw
requiring very careful attention. The Code states the following
causes of fire which may come from high-voltage (over 5,000) lines :
Accidental crosses between auch lines and low-potential lines may allow
the high-voltage current to enter buildings ovet a large section of adjoining
country. Moreover, such high-voltage lines, if carried close to buildings,
hamper the work of firemen in case of fire in the building. The object of the
rules is eo to direct this class of construction that no increase in fire haEord
will result, while at the same time care has been taken to avoid restrictions
which would umrcasonably impede progreea in electrical development.
It is fully understood that it is impossible to frame rules which will
cover all conceivable cases that may arise in construction work of such an
extended and varied nature, and it is advised that the Inspection Depart-
ment having jurisdiction be freely consulted as to any modification of the
rules in particular cases.
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52 UNDERWRITERS' REQUIREMENTS
The very best way to guard against accidental crosses between
high-tension lines and other circuits is to have them follow different
, routes. This can often be ac-
complished by mutual agree-
ment of the parties interested
even when a change in one
of the routes will be neces-
sitated.
High-tension lines should
not approach other pole lines
nearer than a distance equal
to the height of the taller
pole and such lines should
not be on the same poles
with any other lines except
such signal lines as may be
K,. 48.- Joiat-Poi. c,™ag with M,.h.m«i "^^ V the company opera-
^"^'^ ting the high-tension system.
Where such lines must necessarily be carried nearer to other pole
lines than is specified above, or where they must necessarily be carried
on the same poles with other wires, extra precautions to reduce the
liability of a breakdown to a minimum must be taken, such as the
use of wires of ample mechani-
cal strength, widely spaced
cross-arms, short spans, double
or extra heavy cross-arms, extra
heavy pins, insulators, and poles
thoroughly supported. If car-
ried on the same poles with other
wires, the high-pressure wires
should be carried at least 3
feet above the other wires, but
this arrangement should never
be adopted unless it is impos- p^^ „. „„^, a,r«i, on Hi«h.T-=.-_
sible to do otherwise. Where CtoMover
such lines cross other lines, the poles of both lines must be of heavy
and substantial construction.
The Code contains quite detailed specifications for the safe
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UNDERWRITERS' REQUIREMENTS 53
oonatruction of crossovers and other details of high voltage lines
which should be carefully studied and followed.
Fig. 47 shows one arrangement for a crossover. A joint-poU
crossing may sometimes be used and Fig. 48 shows such a crossing
with mechanical guards and wires on the upper or high-tension line
cross-arms. Sometimes a screen either supported on high-tension
insulators or grounded may be suspended between the lines at the
crossover as illustrated in Fig. 49. When necessary to carry high-
tension lines near buildings they should be at such height and dis-
tance from the building as not to interfere with firemen in event of a
fire. Such interference might arise either from the difficulty of plac-
ing tadd^s or ht)m the danger
of shocks to firemen holding
hose nozzles, streams from which
might strike the high-tension
lines.
Mounting of Transformers.
Oil-cooled transformers should
not, in general, be installed in
buildings, and an outside location
b always preferable; first, be-
cause it keeps the high-voltage
primary wires entirely out of the
building; and second, because of
the possible injury from smoke j.,^ „ Appro™i p„i, i™tdi«™ of
and oil in case the transformer Tr»ii»roni»r
burns out or is overloaded. Figs. 50, 51, and 52 show transformers
installed on a pole, on the outside wall of a mill and in a special fire-
proof vault, respectively,
Qrounding of Circuits. One of two courses should always be
followed with regard to any electrical connection between circuits
or apparatus and the earth ; either arrange to have no such connection
at all, and secure good insulation between live parts of circuits and
the earth, or provide a good earth connection of sufficient capacity
well installed to care for any current liable to pass to earth, either
regularly or in case of accident.
In general it should be remembered, that if an entire electric
drcuit including the generator, lines, and all connected apparatus
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64 UNDERWRITERS' REQUIREMENTS
and devices are thoroughly well insulated from the ground, one
accidental ground will produce no effect since there is no return
path from the ground to another point on the system. However,
we cannot be sure that an installation will be kept free from grounds
even if originally so installed, since wear, deterioration of insulating
materials, breaking of parts of devices, or the effects of dirt and
damp may bring on a "ground"
which will not be discovered
until a second ground connection
is established which permits cur-
rent to flow with consequent
heating and arcing. Under such
conditions, the fault is liable to
become rapidly worse. Not the
least important thing about such
failures of electrical drcuits, is
the danger of injury to persons
who may become a part of the
ground circuit. Thus a mechanic
working on a line shaft in a mill,
by touching simultaneously a live
part of the circuit or a poorly
insulated live wire and the shaft-
ing, may make his body part of
a path for the current to ground-
While 110 volts or 220 volts
either a. c. or d. c. are rarely
p ,, . . , _ , , . „ ,■ enough to kill a person, painful
tJt' S^ Approved TranaforinQr InstallAlioQ ^ r ' r
on ouuide of Building accidcHts may bc caused directly
or indirectly by shock, and voltages of 440, 500, or higher may
readily be the cause of death if the conditions are such as to permit
any but very small currents to pass through the body. While, of
course, liability of injury to persons is not a "fire hazard ," still
good construction should provide (botli in original installation
and in upkeep) all reasonable protection, to persons and property.
In direct-current three-wire systems of electrical distribution,
the middle or neutral wire is regularly grounded at the central station.
In systems where the cables are underground, the neutral must abo
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UNDERWRITERS' REQUIREMENTS 55
be grounded at each distributing box, through the box; and in over-
head systems the neutral should be grounded every 500 feet. The
neutral in such systems is supposed, normally, to carry either no
current, or such small amount of current as may result from a dif-
ferent amount of power being temporarily taken from the two outside
wires. If in any system the neutral is grounded at all, it should be
done thoroughly so as to prevent the current escaping to ground
where the connections may be so poor as to cause unsafe heating.
Two-wire direct-current systems are not to be grounded at all.
Suppose in Fig. 53 some current-carrying part of the dynamo D is
grounded, that is, in electrical
connection with the earth, as
through the bolt and the damp
wood base of the machine. If
this be all, no current will fiow
so long as there is no other
ground connection to any other
part of the circuit. But suppose
that somewhere in the building a
wire touches a gas pipe as at £
and the insulation on the wire at
B is worn by vibration or is de-
fective for any reason. The gas
pipe is, of course, connected to the
earth and the current then has a
path through the pipe, earth, ng. G3.* TranslormerlnatBllBUan in
bolt, base, and frame of the ma-
chine. The whole voltage of the dynamo may, therefore, drive current
through this path, causing an arc to form at B between the wire
and the pipe which will burn a hole in the pipe and set fire to the
escaping gas. Evidently the greatest pains should be taken to prevent
wires from coming in contact with grounded piping or other metal.
Alternating-current systems almost invariably include trans-
formers in which the higher line voltage is "stepped down" to the
voltage required for tamps and motors. Common line voltages are
1,100, 2,200, and 3,300 volts, while the seeondarj- voltages may be
100, 110, 220, 440 or 550. With such systems as are commonly used
for distributing light and power, it b preferable to ground the sec-
DignzedoyGOOglC
56 UNDERWRITERS' REQUIREMENTS
ondary or low-voltage side. The ground connection may be made
either at the transformers or at individual service entrances. At
transformers the connection is made at the neutral or middle point
of the secondary winding or coil. With three-wire a. c. distributing
systems the neutral wire itself b grounded. Sometimes, when a
neutral point is not accessible, one side of the secondary circuit may
be grounded, provided it will not establish, between the ungrounded
side and the earth, a difference of potential of over 250 volts. A
greater potential difference than this would cause all insulators on
the system to be subjected to undue strain and perhaps cause trouble.
It should be remembered
that a poorly made earth con-
nection may be worse than none.
The rules, therefore, prescribe in
detail how to secure good con-
nections. The wire should be of
large size and should be run in
as nearly a straight line as po3>
sible, avoiding kinks, coils, and
sharp bends which are objection-
able since they impede the flow
of alternating-current or light-
ning discharge.
Individual transformers and
building services may be ground-
ed to water pipes by carrying
Kig, 53. Fsuiiy loauiutionof T*o-wire the gTound wipc into the base-
DiKct-curnat Dyumo mcut and counccting it to the
alreel side of meters, main cocks, etc., so that any resistance which
these might offer in the ground path might be avoided.
The underwriters' rules give the following directions for making
ground connections:
la connecting a ground wire to a piping eystem, the wire ahould be
sweat into a lug attaclied to an approved clamp, and the clamp firmly bolted
to the water pipe after all rust and scale have been removed; or be soldered
into a brass plug and the plug forcibly screwed into a pipe-fitting, or where
the pipes are cast iron, into a hole tapped into the pipe itaelf . For large stations,
where connecting to underground pipes with bell and spigot joints, it is well
to connect to several lengths, as the gipe joints may be of rather high resistance.
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UNDERWRITERS' REQUIREMENTS 57
Wbere ground plates are used a No. 16 Stubbs' gauge copper plate, about
3X6 feet in site, with about 2 feet of crushed coke or charcoal, of pea size,
both under and over it, would make a ground of sufGcieat capacity for a moder-
ate-aiied station, and would probably answer for the ordinary substation or
bank of transformers. For a large central station, a plate with considerably
more area might be necessary, depending upon the other underground con-
nections available. The ground wire should be riveted to the plate in a num-
ber of places, and soldered for its whole length. Perhaps even better than a
copper plate is a cast-iron plate with projecting forks, the idea of the fork
being to distribute the connection to the ground over a fairly broad area, and
to give a large surface contact. The ground wire can probably best be con-
nected to such a cast-iron plate by soldering it into brass plugs screwed into
holes tapped in the plate. In all cases, the joint between the plate and the
ground wire should be thoroughly protected against corrosion by painting it
with waterproof paint or some equivalent.
In the past few years, there has been much discussion of this
question of grounding, but the general tendency is no doubt more
and more in favor of grounding all circuits where so doing will
protect life and will not introduce extremely hazardous conditions as
regards fire. The following statement of arguments for and against
grounding is taken from the explanatory notes of the Associated
Factory Mutual Fire Insurance Companies:
If the primary and secondary coils of a transformer come into contact
electrically, the high-voltage primary current may flow to the secondary
system. If this should happen, the life of any one handling any part of the
secondary system would be endangered, and fires would probably be started
by arcs caused by breaking down of the insulation of the wires or fittings on
the secondary system. If, however, the secondary coil is grounded, a break-
dawn in the transformer cannot^ cause a dangerous difference of potential be-
tween the secondary system and the ground, and only with certun unusual
combiDations of contacts between the primary and secondary wires outside
of the transformers will this protection fful to prevent the voltage of the sec-
ondary system from being raised above its normal limit. In order to secure
the full benefit of the ground connection, reliable primary fuses of proper
carrying capacity must be provided.
The middU of the secondary coil is the proper point to ground, as there
is then only half the normal secondary voltage between either side and the
ground, thus reducing the liability of a breakdown of insulation and also
materially lessening the danger of fire if a breakdown does occur.
There is an objection to grounding the secondary on the other hand,
for when this is done, the first breakdown of insulation may mean a short-
circuit and a possible fire. With a system free from grounds, a breakdown
must exist on each side of the system to cause a short-circuit, and with proper
ground detectors the first can generally be discovered and remedied before
the second occurs.
Grounding is, therefore, a choice of evils, but in many cases it is beUeved
to be a lesser one than to risk getting the primary current on the secondary
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58 UNDERWRITERS' REQUIREMENTS'
Byetem. This is eapecially true where the primary voltage ia high, aay 3,500
or over. For this reason it ia advised that all traosformers be bo designed and
connected that the middle point ot the aecondary coil can be reached if, at
any future time, it should be desired to ground it.
After the traneformer secondary has been properly grounded a test
should be made, especially if the transformer is some distance from the build-
ing supplied, in order to determine if the protection expected from the ground
connection at the transformer is really efTective inside the building in ques-
tion, and if not the connection should be extended to accomplish the desired
Fit. M. EII«t ol Short-Cirsuit from Lighting
Wins to Bell Circuit
reeult. Cases have been known where the effectivenesB of a ground connection
has been limited to a comparatively small area, due to the exact conditions
of the earth in the neighborhood of the ground plate and between it and the
point where the protection due to the grounding was deaired, The entire
ground connection should be carefully examined at least once a year.
INSIDE WORK
Under this heading are included the rules for wiring and ap-
pliances for light, heat, and power distribution and use. These cover
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UNDERWRITERS' REQUIREMENTS 59
the most general cases of electrical installations as they relate to fire
prevention and protection in buildings of all classes. The installa-
tion of wires and apparatus for signaling systems, such as electric
bells (battery current bells), telegraphs, telephones, fire alarms, and
the like, is not covered in the general rules for "Inside Worit" since
they usually present no hazard in themselves but only as they may
become dangerous because of their liability to become crossed with
light, heat, or power wires either outside or inside of buildings,
Fig. 54.
WIRING SYSTEMS
The present approved methods of electrical work inside build-
ings in thb country have been developed through many yews of
experience, beginning with the first applications of electricity for
lighting buildings and gradually changing as the possible dangers
became more generally recognized, and as improved means of guard-
ing against them were devised. In this development the efforts of
insurance and municipal authorities have been supplemented by
an immense activity on the part of inventors and manufacturers in
supplying new devices and materials. The net result has been on
the one hand an elaboration of rules and an approach to a few stand-
ard systems of construction and on the other hand the production of
an almost endless variety of materials available for electrical pur-
poses. Methods and materials which at first seemed adequate have
become obsolete after a few years' use. At present, however, few
important changes appear to be in progress but in many minor
detaib the development is still going on.
At first, electric wires were laid as seemed most convenient in
floors, partitions, and over walls and ceilings, either in channels cut
for them, in wood casings, or supported on wood cleats with
almost no regard to protecting the wires from injury, or the
adjacent combustible materials from being ignited by overheated
wires or by arcs. Today, however, these earlier crude methods
are wholly abandoned and it is generally conceded that the best
protection against electrical fires lies in the adoption of the most
approved methods even when the first cost of an installation is
increased to some extent. Pig. 55 shows some defective wire joints
as found in actual use.
In buildings of the better class, the electric installation is care-
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60 UNDERWRITERS' REQUIREMENTS
fully coimdered in the plans, and provision is made for its safety
as well as for its efficiency and economy. However, in cheaper
buildings, and very often in small stores, apartments, and residences,
the electrical work is left to be arranged as best it may and to be
installed by careless workmen without expert supervision. In
Fie. EG' TyiHcal Def«ti*e Joints in £lectric Initallation
electrical matters as in most other affairs, cheap work is generally
poor work, and deviation from the methods shown by experience to
be reliable are usually prompted by a desire to save money at the
expense of safety and permanence.
The method of bringing supply or "service" wires into builcUngs
naturally demands first attention. Where street mains are under-
ground the Unea should, wherever possible, enter through the base-
ly GoOglc
UNDERWRITERS' REQUIREMENTS 61
ment or cellar walls, the lead-covered cables being carried through
the foundations through tubes tightly sealed.
Where nuuns are overhead, the supply wires may enter either
through the basement, being carried down the outside of the wall
in iron conduit, or through some upper portion of the wall, though
the former entrance is almost always preferable. Where transformers
are used they may be on poles near the building or mounted on the
outside wall in a substantial manner. An entrance through a roof,
near a cornice or into little used or inaccessible attics or lofts, should
be avoided. Where wires or cables pass through the outside walls
there must be either iron conduit or insulating bushings sloping
upward toward the inside, and the wires outside must have drip
loops which will prevent moisture following along them into the wall.
The fastenings of the wires to the
building must be most substantial
and good insulators must be provided
few the supply wires. Fig. 56 shows
the method where bushings are em-
ployed. If the entrance is made
through conduit, the inner end of the
conduit should always be extended to
the service fuses. At the nearest _
., , , . , , .. 1. ^C- "B- Mathod of UiiD( Buahinsi
accessible place m the building must
be placed what are called the service fuses and the service switch.
llie switch is usually of the knife-blade pattern and must be such
as to cut off all the wires. Single-pole switches must never be used
as service switches. The purpose of service switches b to provide
means for cutting off current for repairs or in case of fire or other
acddent. The service fuses should be placed between the service
switch and the mains to the outside of the building. Ilieir purpose
b to protect all the wires inside the building from overloads, and
they should be such as to melt or "blow" with current not much
in excess of the normal full current likely to be taken by the entire
installation which the service supplies.
Service switches and fuses take many forms, from a pair of
small "plug fuses" and knife switch on a porcelain base to quite
elaborate switchboards which carry both these and other sub-fuses
for the main circuits within the building. The same principle, how-
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62 UNDERWRITERS' REQUIREMENTS
ever, applies to all ; all wires must be protected, the service entrance
must be accessible and fuses must be of proper capacity. From the
service switch the lines usually extend through meters to distribution
centers which are panels from which the several sul)-circuits branch
ofF to lamps or motors through the building. Fig. 57 shows such a
center. There may be only one such center in a small installation,
or in a larger one there may be a main distributing panel and numer-
ous smaller ones at various places in the risk.
General Rules on Wires. Size. No wire smaller than No. 14
B. & S. gauge is allowed (except in fixtures and for pendant or flexi-
ble cord) since no smaller size has both the conductivity and abo suf-
fident mechanical strength to stand the strains of installation and use.
JoinU and Spliees.
All joints- and splices
must be made both
mechanically and elec-
trically secure and then,
be soldered except when
one of the very few ap-
proved splicing devices
is used. For general
wiring, the underwriters
have never found any
equivalent for pood sold-
ered joints when all the
possible effects of cor-
rosion, alternate heat-
Fig. 57. DialnbutiDB Pansl lor Lightini Circuit ■ j >• -L
mg and cooling, vibra-
tion, and mechanical strains are considered. The neatness and
thoroughness of the soldered joints are two of the best general in-
dications of the excellence of the workmanship on any job. After
being soldered wire joints must be covered with an insulation equal
to that at other places on the conductors. This is usually done by
winding the joints with a good pure rubber tape over which is wound
a "friction tape" of fabric impregnated with a compound.
Wires in WalU, Floors, etc. Wires must always be separate^
from walls, floors, timbers, and partitions by non-combustible, non.
absorptive, insulating tubes such as glass or porcelain and must be
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UNDERWRITERS' REQUIREMENTS, 63
kept free from all contacts with pipes or any conducting material.
This general rule b establbhed without any reference to the insula-
tion which is on the wires themselves, the idea being that the insula-
tion of the conductors from each other and from other conducting
materiab must be sufficient to furnish the necessary protection in
case the wire coverings are defective or become injured in any way.
This principle does not, however, prevent the wires being drawn into
metal conduits which are specially designed as wire raceways, nor
can it apply to fixtures in which the wires must be in the metal stems
FU- 68* Approved Ovarheui Wirint
and arms. 'For such cases special rules are established. Fig. 58
shows a good example of overhead wiring in which an iron pipe may
be seen protecting wires up the post, while the wires on ceiling and
around beams are very well arranged and supported.
In damp or wet places, the relative arrangement of pipes and
wires should be such that the wires cannot touch the pipes and so
that watCT cannot drop from the pipes on the wires. The subject of
electrical work in damp places will be considered in another place.
Carrying Capacity. The Code prescribes the maximum cur-
rent which shall be carried on copper wires of different sizes.
This table of carrying capacities has been unchanged for many years
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64 UNDERWRITERS' REQUIREMENTS
and was originally based upon an elaborate series of «»eful experi-
ments. Table III gives the capacity for rubber-covered -mres,
and for all types of wire insulation such as slow-burning and
weatherproof braids.
The table for rubber-covered wires is lower than the otha- be-
cause high temperatures such as might result from a wire carrying
too much current have a harmful effect on the insulating properties of
rubber. I^e table is for indoor work only. It is stated that for any
given size of wire, a current about three times as great as that ^ven
in Table III will cause all ordinary insulations to smoke. The table
does not consider the question of drop as it is called. Thus in Fig.
59, suppose O is a dynamo supplying current to ^, a motor 250 feet
away from it, and suppose the motor requires 90 amperes at 220 volts.
From Table III it is seen that No. 2 wire could be used. But
some power is lost in driving the current through the 500 feet of
line wire and, if the wire b small, its resistance will be targe and so
more power will be "lost on the line wires." The part of the
dynamo voltage required to
' drive the working current over
the supply wires is called the
"drop." Suppose it is pre-
scribed that the drop shall not
be over 1 per cent of the total
voltage. One per cent of 220
volts b 2.2 volts. The current in the line equals the voltage to force
the current over the line divided by the ohms resbtance of the line.
2.2
In thb case 90 = — ; , or resistance of the 500 feet of wire
resistance
2.2
must not be more than ~ ohms or .024 ohms. From a suitable wire
table it will be found that a No. 0000 wire will be required. Thus
the necessity of keeping the loss of power low on the line may neces-
sitate the use of a larger wire than would be needed for safety nadtx
the underwriters' rules.
Constant-Current Systems. Ilie nature of these has already
been explained on page 13. Such systems are used nowadays almost
exclusively for street lighting with arc lamps and the voltage runs
Fi(, 59. Simple Electric Power <^rcuit
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UNDERWRITERS' REQUIREMENTS
Carrying Capacity of Wires*
1
B. 4 S. G.
Ampere*
Ampera
Cinulu MiU
18
3
5
1,624
16
6
8
2,683
14
12
16
4,107
12
17
23
6,530
10
24
32
10,380
8
33
46
16,510
46
69
26,250
54
77
33,100
65
92
41,740
76
110
52.630
90
131
66,370
107
166
83,690
127
185
105,500
00
150
220
133,100
000
177
262
167,800
0000
210
312
211,600
Circuit Mil*
200,000
200
300
300.000
270
400
400,000
330
600
500,000
390
590
800,000
450
680
700,000
500
760
800,000
560
840
900,000
600
920
1,000,000
650
1.000
1,100,000
ego
1,080
1,200,000
730
1.160
1,300,000
770
1.220
1,400,000
SIO
1,290
1,500,000
850
1.360
1,600,000
800
1,430
1,700.000
030
1,490
1.800,000
970
1,660
1.900,000
1,010
1,610
2.000,000
1,060
1,670
a md IS B. A 8. t
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66 UNDERWRITERS' REQUIREMENTS
from 2,000 to 3,300 volts. The arc lamps on such circuits are arranged
so that each line has enough lamps in series to use the available
dynamo or transformer voltage iillowing the necessary margin for
regulation.
The high voltages generally employed, call for the very best
insulation and only rubber-covered wire should be used ; all wires in
buildings must be in plain sight and never encased. The bringing
of such circuits into buildings is not very general and arc lamps
designed to be connected in multiple on ordinary low-voltage circuits
are much to be preferred.
There are special rules for series arc-lamp wiring in buildings
which cover the method of bringing supply wires through the walb,
provision for a special form of switch at points where the lines enter
and leave the building, requirement for 1 inch separation between
wires and the surfaces over which they pass and 8 inches from each
other and extra protection of all wires by running boards or guard
strips. The service switch required on eonstant-ciurent systems
must be a denible-contad switch, that is, it must be so arranged as to
first place a cross connection or short-circuit on the lines into the
building and then disconnect these lines from the supply altogether.
This leaves the drcuit unbroken, but cut out of the building. An
attempt to actually break the circuit would be sure to cause a very
destructive arc. Series arc lamps must be carefully isolated from all
inflammable stuff and unless they are of the "enclosed arc" type
must be provided with screens and nets to prevent the escape of
sparks from the carbon or melted copper. All connections must be
made in a most reliable manner and in all series arc work it must be
remembered that the voltage across any break in the circuit is very
high and will cause verj' severe arcing.
Incandescent lamps are not generally connected to series
circuits since their use involves an automatic cut-out at each lamp
which will shunt the current around the lamp in case the lamp be-
comes loose or its filament breaks. These devices are expensive,
difficult to keep in order and generally undesirable. Formerly, com-
binations of incandescent lamps on series circuits were used,
consisting of groups of lamps in series or in multiple, but these
arrangements are not now in use and are forbidden in the rules. It
b evident that incandescent lamps on series circuits should never be
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I
D,Bi:ized3,GOOgIe
UNDERWRITERS' REQUIREMENTS 67
allowed on gas fixtures ^ee an arc to the grounded gas pipe would
be very severe and would be liable to bum through the pipe and
ignite the escaping gas.
Constant-Potential Systems. The character of these systems
has been explained on page 12. Almost all systems for light and
power in buildings of all sorts are of this type. The most common
are llO-volt two-wire direct-current systems; three-wire systems
with 220 volts between outside wires and 110 volts between the
neutral and either outer wire ; 500- to 600-volt d. c, street and elevated
systems with "ground return"; 440- to 600-voh a. e. systems for
motors. In addition to these there are 1,100-, 2,200-, and 3,300-volt
a. c. power circuits and the so-called a. c. transmission lines at all
voltages from 1 ,000 up to 80,000 or 100,000 volts. While occasionally
a. c. motors are made for direct operation at 1,100 or 2,200 volts,
in general for voltages above 600 volts, alternating-current trans-
mission lines are employed which are connected to transformers at
the factories or mills where the power is used and which "st^i-down"
the voltage to 440 volts or some other voltage which can conveniently
be used in the motors and for lighting purposes. Both the primary
and secondary circuits in this case are constant potential systems.
In street rulway work large machines called rotary coQV^^rs
are employed which are driven by the high-voltage alternating
current from transformers connected to the transmission lines and
which deliver direct current at about 600 volts to the trolley system.
Such rotarj' converters are usually placed in substations so located
as to conveniently and economically supply the different sections of
a city. A similar practice is followed in cities where direct current
is to be furnished for general hghting and power, and the original
generating station is more or less remote. Such substations come
under the same rules as generating or dynamo stations.
All so-called isolated plants, that b, plants in individual factories
or large buildings, are constant-potential systems abo. In the
underwriters' rules constant-potential systems are subdivided into
hwpoteniiai systems 10 to 550 volts; higk-poteTiHal systems 550 to
3,500 volts; and extra-kigk-potential systems over 3,500 volts. Of
these the low-potential systems are of most importance since they
include the very great majority of equipments for using electricity
in buildings for light, heat, and power.
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68 UNDERWRITERS' REQUIREMENTS
QENBRAL INSTALLATION RULES FOR CONTROLUNQ AND
PROTECTINQ DEVICES
Switches, fuses, and circuit breskers may all be described as
arcing devices, that b, their operation always produces an arc.
This arc may be small or large but it is impossible to break a circuit-
carrying current without some arc even if it is so small that it is a
mere spark. The duration and intensity of an arc depends upon the
strength and voltage of the current, the rapidity with which the gap
in the drcuit is widened, and the design and condition of the arcing
device, switch or fuse. Dust or inflammable gases may be ignited
by an arc of sufficient intensity. No arcing device, therefore, should
be placed near easily ignitible stuff, or exposed to inflammable gases,
or dust, or flyings of any combustible material. When so exposed, as
in flour mills, textile mills, etc., all switches and fuses should be en-
closed in dust-tight boxes or cabinets. Open-link fuses are espe-
daUy liable to flash violently and throw out molten metal and they
must, therefore, be given special attention, and should never be
installed outside of proper cabinets except on switchboards in fire-
proof rooms, such as engine rooms, generating stations, or where
they will be under constant and expert supervision. Even in ordi-
nary rooms, houses, stores, or factories, where there is no dust or
combustible flyings in the air, it is much better to have all kmfe
switches and all fuses placed in cabinets to prevent accidental short-
circuits, caused by laying a metal object across the exposed parts.
Switches immersed in oil are in common use for large currents
and are quite safe as regards arcing, though the oil involves a certain
hazard since it is combustible.
It should be remembered that any switch or circuit breaker
which is automatic, that is, which is not operated by hand by a
person at the actual device itself, requires better protection since in
case of failure the arcing may be severe and no one may be at hand
to take the needful steps to prevent its starting a fire. Such auto-
matic current-breaking devices including time-switches worked by
clocks, sign flashers, and the like, should always be enclosed in very
substantial non-combustible cases or cabinets of ample size and so
arranged that they are not liable to be left open.
Switches. The general requirements for service switches have
already been discussed. While it is true that the service switch
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UNDERWRITERS' REQUIREMENTS 69
and a switch for every motor are the only ones that are absolutely
required by the rules, still couvenience and economy of operation
naturally call for a number of switches in practically everj- installa-
tion, and the correct placing of them becomes, therefore, a matter
of importance. The description of some of the very numerous types
of switches will be given later in this book, but we consider here the
general rules for installing all types.
Aa a general principle, switches must always be placed in dry,
accessible places and it is well to group them together 90 far as pos-
sible for the reason that this will often reduce the amoimt of wiring
and also render it easier to use them in case of oeed.
Knife Switches. Knife switches consist of copper blades, one
for each pole, hinged at one end to copper clips or posts and closing
at the other into other clips. Where such a switch is made to close
ng. 60. Simla. Double, >Dd Triple Pole Euile Switcheg
into clips at only one side of the hinge end it is called a smgle-tkrow
switch and where the blades can be thrown into clips at either side
of the hinge it is called a double4hT<nv switch. Single-throw switches
must always be installed so that gravity will tend to open rather
than close them since otherwise they might fall and, by only partly
closing, cause arcs and burning. Double-throw switches may be
instaUed so that the throw is either vertical or horizontal as preferred.
Fig. 60 shows a double-pole single-throw switch and a triple-pole
double-throw switch correctly placed and a single-pole single-throw
switch wrongly placed.
WhenevCT practicable, knife switches should be so wired that
the blades will be dsad when the switch is open as this leaves less
exposed live metal and also makes it easier and safer to make any
repairs or adjustments of the switch blades and hinges. In Fig. 61
if the supply wires (from the service or dynamo) enter at the top
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70 UNDERWRITERS' REQUIREMENTS
and the lamps are connected from the bottom of the switch, the
blades will be dead when the switch is open. If the arrangement ia
reversed, the switch blades will be
connected to live wires all the
time, whether the switch b open
or closed. The illustration also
shows an excellent t>'pe of cast-iron
cabinet for such a switch combina-
tioiL
Surface Snap SwUchea. These
are the common porcelain base
switches, usually round in shape
and having metal covers with the
operating handle at the center.
They are commonly mounted on
side walls and the wires are
fT«. fli. Approved M.tal Swiwh Bo. j^^^^^j^^ .^^^^ ^^^^ j^^ ^^^ ^^^
It is not possible to fasten them very securely to a lath-and-plaster
wall unless some block is provided for the screws to be driven into.
For this reason, wherever possible, at all switch or fixture outlets,
a J-inch block must be fastened between studs or floor timbers flush
with the back of lathing to hold tubes, and to support switches or
fixtures. When thb cannot be done, wood biise blocks, not less
than 3 inch in thickness, securely screwed to lathing, must be pro-
vided for switches, and also for fixtures which are not attached to
gas pipes or conduit. Figs. 62 and 63 show these blocks with the
wires brought through them and through the lath and plaster in short
lengths of flexible tubing. The switches can thus be firmly screwed
to the blocks and the wires connected to them.
If snap switches are used with exposed wiring on cleats, there
must be a porcelain sub-base under eacli switch so made that the
wires will be kept J inch from the surface wired over, A a
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UNDERWRITERS' REQUIREMENTS
71
4/ ' 'mis im.
Vy n?OM WAU,
I
w
sub-base must be used where sucli a switch is used with wood
molding, but in this case it may be of liard wood instead of porcelain.
Figs. 64 and 65 show how this is done.
flush. Swilckes. These are made to be inserted
into walls so that only the operating push buttons
or handle will extend out beyond the surface, and
are now in very general use. Inasmuch as their
operating parts are conceale^l in the wall they
should invariably be set into small steel boxes
through the back of which the wires may ent^
either throu^ lengths of flexible tubing or through
iron conduit. The same requirement applies to all
small fittings such as receptacles from which flexi-
ble cords are run to heaters and other portable de-
vices. Fig. 66 is a sketch of such a switch box set
into a lath-and-plaster wall. The sketch shows the
box as it would appear from the back of the wall.
Where it is desired to control the same electric
lamps from either of two switches at different places, t^_. oi
what are called three-^way switches are installed.
These are chiefly used in residences, as for the control of hall lights
_from either upstairs or downstairs. Under
"the rules these are classed as single-pole
switches and are preferably wired so that only
main of the circuit is carried to either
switch. Three-way switches are usually of the
common round-surface porcelain-base type or
push-button wall variety. Fig. 67 gives a dia-
gram of the way to connect them.
Fuses and Circuit Breakers. These may
be compared with "safety valves" on steam
boilers, that b to say, they are primarily de-
signed to act in case of an improper condition
of affairs and prevent by their automatic action
any serious trouble resulting. Although this
is the purpose of fuses and overload circuit
breakers, it is altogether too common for users
i them, and so de-
t. EipoHd
iganCleaM
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72 UNDERWRITERS' REQUIREMENTS
3troy the protection intended. It is evident tliat a fuse which is so
large that it will not melt until a current passes through it which is
far too large to be safely carried by the
wires or other parts of the areuit, is
worthless. The fusible part of a fuse is
usually a strip or wire of soft lead or
zinc of such size that if any considerable
current over that for which it is designed
passes through it, it will melt ofF and so
open the circuit. Whenever a fuse of tlie
proper Mze for its circuit blows or meka,
the first thing to be done is to seek out
the cause, for the operation of the fuse b
proof thaX there is, or has been, something
wrong. Thus if in a house a lighdng-
Fig. 6fl, Fiiuh Switch B« in L»ih- circuit Is propcrly protected by 6 ampere
iM utitioQ £yg^ ^j^j these fuses blow, one may be
sure that more than 6 amperes, and, therefore, more than a safe current
has, for some reason, traversed the wires for a time long enough to melt
the small fuse strip. Now, unless this excess of current was due to
some momentary accident, known and recognized as such, the same
condition that once allowed the unduly large current to fiow probably
still exists, and until this trouble is sought out and remedied, the
same unsafe condition exists. It is, therefore, very unwise to replace
the blown fuses with some of larger current-carrying capacity, for
n«. 67. Wirins DiaccuD foe Three- Way Switch Ciieuit
this is merely redudng the protection without removing the source
of danger.
Still worse is it to replace blown fuses by fuses which have been
filled up with metal, or across, or through which, extra metal strips
have been fastened in a misguided attempt to keep the fuses from
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UNDERWRITERS' REQUIREMENTS 73
blowing again. One might as reasonably tie down a steam boiler's
safety valve. These principles when thus stated appear very ele-
mentary and self-evident and yet the misuse and abuse of fuses is,
perhaps, the commonest fault observed in the maintenance of electric
installations. The only reason that disaster does not always follow
ignorant or culpable misuse of fuses, is to be found in the fact that
wires and other parts of the system are installed with a fairly large
margin of safety. This does not, of course, in any degree justify
over-fusing circuits or tampering with fuses or other safeguards,
and insurance inspectors should not tolerate any deviation from
standard rules for the protection of circuits or fail to demand the
use of only approved protective devices of proper rating for every
orcuit. Fig. 68 shows some fuses which have been "doctored" in
ways unfortunately all too coomion.
l^e following excellent statement is taken from the book of
rules of the Associated Factory Mutual Companies:
Fii. SS. Typical Eumplaa ot "Doitond" Piun
Specifications for fusee require that they shall be rated at a certain per
orat of the maximum current which they will carry indefinitely, as follows:
link fuses 80 per cent and enclosed fuses 90 per cent. The margin thus pro-
vided between the rating of the fuse and its actual melting point will permit
the ordinary fluctuations in current without opening the circuit. If fuses
selected to conform-to the above rule are not large enough to carry the load,
it is evident that the wires also are overloaded, and either the load should be
diminished or the size of the wire increased.
Circuit breakers are so sensitive that it is often necessary to set them
much above the ordinary current to keep them from being constantly opened
by momentary rises in the current, such as might be caused by starting a motor
or by a rise in the voltage of the dynamo due to a sudden decrease of load.
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74 UNDERWRITERS' REQUIREMENTS
When this is the case, a fuse may be neeeBsary to protect the wire from a
steady current above the safe carrying capacity of the wire but below the point
at which the circuit breaker is set to open. The fuse requires a little time to
heat, and bo does not melt with the momentary rises of current which would
open the circuit breaker if it were set as low as it would have to be if the fuses
were not provided.
It has already been pointed out that "service entrances" must
be fused, that is, there must be a fuse in each wire where the current
supply is brought into a building. An exception b made in the case
of three-wire (not three-phase a. c., however), systems. In these the
fuse may be omitted in the neutral wire, provided this wire is of
equal carrying capacity with the outside wires and is reliably grounded,
since in such a three-wire system the neutral wire cannot under any
condition carry more current than either of the outside wires; the
fact that it is grounded adds a certain safety, because it is espe-
cially desirable that the neutral should not be opened unless the
outside wbes are also opened, as might occur if a neutral fuse alone
blew, or was removed without the others also opening. Fig. 69
illustrates this point. When the fuse in the neutral at iV is in place,
lamps A can have only 1 10 volts across them and lamps B the same,
but if the fuse N is removed, the group A will be in series with the
group B across the 220 volts of the outside wires, and as there are
. 4 lamps at A and only 2 at B, the resistance and volts drop across A
will be only one-half the resbtance and drop across B. Thus A
will get only one-third of 220 volts or 73.3 volts, and B will get 146.7
volts. The lamps B will thus burn oner bright or even burn out, and
those In A will be dim. Of course, if there were the same number of
lamps at ^ as at B, it would make no difference whether the neutral
wire were open or not, but when the system b unbalanced as in I^lg.
69 it is better tp avoid having the neutral opened tuid, therefor^
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UNDERWRITERS' REQUIREMENTS
75
the fuse In it is often omitted, and the wire carried through unbroken.
Beyond the service entrance fuses and switch, the circuits are
usually divided; thus in a house several circuits of smaller wire will
lead to the lights on the different floors, or in a larger building or a
factory, sub-mwns will be carried up to the distributing centers
through the building from which in turn smaller wires will branch
out. The proper proportioning of these cables and wires for the
economical distribution of current is a problem for the electrical
en^eer. The point to be noted here is that fuses or breakers must
be placed at every point where a change is made in the size of wire
unless the fuse next back will also protect the smaller wire. The
rated capacity of fuses must not exceed the allowable carrying capacity
of the wire as given on page 65, and circuit breakers must not be set
more than 30 per cent above the capacity of the wire unless a fuse
is also used when it may be set 100 per cent higher. F^g. 70 shows
the sizes of wire and the fuse arrangement for a typical case. The
arc lamps require 30 amperes, the 20 incandescent lamps 10 amperes,
and the motor has a full load current of 20 amperes. The motor,
however, must have leads for 25 per cent above full load current or
25 amperes or No. 10 wire. Allowing 60 amperes for the total normal
current the service fuses at A might be 60 amperes and the wires
next beyond them No. 4, Tlie other wire sizes are shown and the
fuses would have to be placed as shown and have the following
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76 UNDERWRITERS' REQUIREMENTS
ratings: B, 30 amperes; C, 6 amperes; Z>, 30 amperes; £,20 amperes;
and F, 6 amperes. The 6-ampere fuses are required at C and F,
because by a special rule mentioned below this is the limit for fuses
for branch circuits at 110 volts supplying incandescent lamps. A
pair of fuses is also required for each arc lamp and these would in the
case shown usually be of a 10-ampere rating. If circuit breakers
were substituted for the motor fuses at E they should be set to open
at not over 30 per cent above the capacity of No. 10 wire or about 31
amperes.
It wili frequently be found necessary to provide cut-outs where
taps are taken from large mfuns. In such cases, if the clamps on the
cut-outs are not sufficiently large and strong to pve a firm and secure
connection, a short length of smaller wire may be soldered to the
main wire and then carried direct to the cut-out, which should be
located as near as possible to the point of connection with the mains.
Special care should be taken to guard these leads from accident as
they may not be properly protected by the fuses in the main circuit.
Fuses are always installed in pairs (or sets of 3 on three-wire
circuits), so that each side of the circuit is fused for the reason that a
"ground" or "cross" might occur so as to cause a large current to flow
over a path not including any fuse. Furthermore, fusing both sides
gives a much greater factor of safety and insures protection under
ah conditions.
la installing incandescent lamps, fuses must be so placed that
no set of himps requiring more than 660-watt5 power will be depend-
ent on a single pair of fuses. Some city rules state this in terms regu-
lating lamp sockets or receptacles, and limit the number of sockets
to 10 or 12 since this amounts to about the same thing. The purpose
of thb rule is to secure such a subdivision of the fuses that no very
large currents can flow for any long time over any part of the small
wiring without opening a fuse and thus the effects of a short-circuit
or other accident will be very much minimized. Suppose we had a
long line of No. 10 wire protected by 25 ampere fuses on a 110-volt
circuit, and 50 incandescent lamps in multiple on this line with no
other fuses. Suppose also that only one of the lamps were burning.
Now if this lamp were hung on a flexible cord and a "short-circuit"
occurred on the cord or in the lamp socket, we should have to wait
until the short-circuit became severe enoi^ to allow about 30
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UNDERWRITERS' REQUIREMENTS 77
amperes of current to flow before the 25-an)pere fuses would open.
(All fuses will carry, for a short time, more than their rated currents.)
But 30 amperes at 110 volts may cause an arc quite Intense enough
to set fire to the cord or cause molten metal to drop bom the over-
heated socket. If the lamps were on a branch circuit of No. 14 wire
protected by 6-ampere fuses, these fuses would probably melt before
serious harm were done. Thus it may be seen that it b wise to sub-
divide the lunp circuits and protect each circuit with smaU fuses.
In exposed wiring in large mills an exception is made permitting
incandescent-lamp circuits with 25-ampere fuses to be used provided
each lamp is protected by a very small fuse placed in a ceiling rosette.
This is allowed to prevent running an excessive amount of wiring
through lai^ rooms where the crowded wires might be themselves
a source of danger. It was formerly the custom to place small fuses
in the canopies of fixtures or ceilings and side walls but they were
always troublesome and dangerous to an extent that did not offset
the slight extra protection they afforded and all such "bug" fuses
have long been forbidden.
Enclosed fuses, plug and cartridge, are allowed by the Code rules
to be installed without cabinets except in dusty or linty places but
some municipal ordinances require aU fuses to be in cabinets. While
this is safer, there would be a tendency to use the open-link lead fuse
everywhere if cabinets were universally required since they are much
cheaper than enclosed fuses, and this is a tendency not to be en-
couraged. If proper 'locations are chosen for enclosed fuses and if
only approved fuses are used, there is little hazard under ordinary
conditions. Enclosed fuses either of the cartridge or the plug type
and of makes having the underwriters' approval will open the circuits
for which they are rated with practically no explosive action and
without emitting flame or molten metal even on heavy short-circuits.
Even so, however, they should never be placed near combustible
material.
Electric Heaters. Under this heading are included all devices
in which use is made of the heat developed by the current (usually
by causing it to pass through coils of wire). These are electric
pressing irons, air and water heaters, toilet articles, such as curling
irons, cooking devices of all sorts, and a vast and constantly increas-
ing variety of domestic and industrial appliances. All these present
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78 UNDERWRITERS' REQUIREMENTS
the same hazards as other heaters of equal capacity, except that
the dangers of open gas flames and of matches are eliminated, and
they all require the same precautions in use and in their installation.
Each should be protected by fuses either on the device or preferably
in the branch circuits supplying them and must be controlled by
separate switches so made as to indicate whether the current b "on"
or "off." These must be double-pole switches if more than 660 watts
of energy is required. In general, such heaters should never be "built
in" but should be in plain sight. However, for many industrial pur-
poses electric heaters are constructed as parts of toob or machines
and when well made and used with due care are not more hazardous
than other methods of heating.
Portable heaters are in general more dangerous than stationary
ones since the latter may be safeguarded by suitable heat-resisting
material placed between the device and its surroundings, such as
sheets of tin or steel with an air space between them or by alternate
layers of sheet steel and asbestos with a similar air space.
The electric flatiron is perhaps the cause of more trouble and
danger from fire than any other form of heater. The temperature
of the iron required for ironing damp fabrics is necessarily high (at
least 500° F. b common) and if the iron is left with current on and
is not in use it will become red-hot in from ten to twenty minutes.
If it has been left on a table or on clothing a fire is almost inevitable.
Fig. 71 shows a cloth-covered board burned by electric irons. Few
irons have any automatic cut-off to guard against such an accident
and the fact that many such irons are used by persons not familiar
with the possible danger, makes these devices rather hazardous.
It is often desirable to connect in multiple with the heaters and
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UNDERWRITERS' REQUIREMENTS
79
between the heater and the switch controlling same, an incandescent
lamp of low candle power, as it shows at a glance whether or not the
switch is open, and tends to prevent it being left closed through
oversight. An approved stand, of a pattern such that the iron may
be safely left on it even with the current on, should be used with every
electric pressing-iron. The ordinary plain iron stand for cast flatirons
is not adequate as it will become hot enough to set fire to a table.
It should be remembered that stove-heated irons get cooler when
taken from the stove, while an electric iron will get hotter and hotter
if left connected to the circuit and
not used. Fig. 72 shows an iron
properly installed with indicating
switch, pilot lamp, and stand.
Portable heaters, if they re-
quire over 250 watts, should be
furnished with approved "heater
cord" which consists of stranded
copper conductors with a thin
rubber and a thick asbestos yam
covering over each with a good
braid over all. In factories and
shops where a large number of
flatirons or rather portable electric
heaters are used, the circuits lead-
ing to them should be so arranged
and provided with switches that -t
any department or tier of benches Rg. 72. p„jwrjy [lutiiikd EiMtrio
can be cut off when not in use and "° '"""'
pilot tamps in conspicuous places should be provided to call attention
to the fact that the circuit is closed to the heaters. Domestic cooking
devices electrically heated, are, in general, fairly safe as made by well-
known manufacturers, and are preferable from the viewpoint of fire
hazard to similar appliances heated by gas or by alcohol lamps.
The use of flatirons or other heaters on ordinary lighting drcuits
is undesirable since they often require more current tlian can be taken
through the proper fuses for such lamp drcuits. It is very desirable
that special circuits be run of large wire properly fused and with large
capacity fittings and that all heating and small power devices such
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80 UNDERWRITERS' REQUIREMENTS
03 washing and vacuum cleaning machines be suppHed from such
special circuits. The ordinary wall or ceiling fixtures and the lamp
sockets attached are very ill suited for
connecting portable heaters because of
their lack of current-carrying capacity;
and, too, because they are not me-
chanically strong enough to with-
stand the comparatively rough usage
to which they are inevitably subjected.
HXTURBS AND PIXTURB WIRING
Fixture Details. Electric fixtures
an of two types known as straight
electric and combination, the former
carrying only electric lamps and the
other having both electric and gas
lights. The chief parts of a fixture are
the canopy at ceiling or wall, the stem,
the body, the arms, and the sockets.
Most fixtures consbt of plain iron pipe,
often of small size, threaded into
special castings in the body or central
ball with arms branching from the
body. Over this pipe body b a casing
of brass pipe in pressed or spun forms.
In straight electric fixtures the wires
are usually drawn through the iron
pipes, but in combination fixtures the
gas runs in the pipes and the wires lie
along the outside of the pipes between
them and the casing or are drawn
through cored holes in cast-brass arms.
The necessarily small spaces and chan-
nels for wires in fixtures and the slight
amount of insulation which the wires
can carry, require very careful work
in order that the best conditions pos-
sible may be maintained. The pipes in which wires are drawn should
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UNDERWRITERS' REQUIREMENTS 81
be carefully reamed at ends and all sharp angles, burrs, and comers
on which wires may be injured should be carefully rounded ofT. These
details should be attended to by the fixture maker but are often neg-
lected, resulting in such accidents as are shown in Fig. 73, where the
ciurent has burned holes through the gas pipe and ignited the gas.
Nothing but rubber-covered wire having a rubber wall, and no wire
smaller than No. IS B. & S. gauge, should be used. Stranded wire
is preferable to solid wire. A rubber insulation A-inch thick is
permitted on No. IS wire for fixtures, but all No. 16 wire and all
flexible cord if used for fixture work should have at least A-inch
wall. Fixtures are not allowed on circuits of over 300 volts.
Inaulaiing Joint. On lath-and-plaster ceilings and walls wh««
steel outlet boxes are not used, fixtures are usually fastened to some
form of "crowfoot," a small tripod casting into which the stem of
the fixture is screwed. Combination fixtures are screwed on the pro-
jecting nipple of the gas pipe. When fixtures are thus supported
on gas pipes or when they are attached to any grounded metal work
of a building or are on walb or ceilings of plaster on metal lathing,
an approved "insulating joint" must be inserted between the fixture
and its support. An insulating joint is a coupling, the two ends of
which are reliably insulated from each other by some substance,
usually mica. For combination fixtiu-es there b a hole for the gas
through the center of the joint. Such joints are required to be made
of materials which will not be affected by the gas; no soft rubber is
allowable and they must be capable of withstanding a voltage test
of 4,000 volts a. c. between the two ends. Such insulation is required
because the fixture wire is necessarily poorly insulated and liable
to permit the conductors to become "grounded" on the fixture at one
or more points. The insulating joint prevents such a failure in the
fixture from causing current to flow to the earth in case the circuit is
either purposely or accidentally grounded at other places. It also
prevents voltage sufficient to puncture the fixture wiring from arcing
across broken insulation to the fixture and so to the ground.
Canopy InsuhioTS. Since the canopy or bell covering the base
of the fixture at the ceiling or wall b in electrical connection with
the fixture stem, it also must be insulated at its upper edge from the
wall wherever an insulating joint b required. Canopy insulators
take the form of molded rings of composition, fiber rings riveted to
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82 UNDERWRITERS' REQUIREMENTS
the canopy edge or insulating linings and flanges of micanite or other
suitable material. Fig. 74 shows a correctly mounted combination
fixture with insulating joint and canopy insulator. The cut also
shows the pieces of flexible tubing in which the wires are separately
encased from the last porcelain support, through the ceiling and to
a point below the insulating joint. B in the figure b a piece of
insulating tubing which should be placed about the gas pipe above
the insulating joint. In case the wires were in steel conduit a steel
outlet box would be placed in the ceiling, the gas pipe would enter
through a central bole in the back and the conduit through another
hole at one side. In such cases the flexible tubes would not be used
but the canopy insulator and
the insulating joint would still
be required. Most trouble from
fixtures occurs in the canopy
from poorly made wire joints and
crowded or jammed wires which
gradually give way until finally
an arc is formed and the wire
coverings are ignited, Hg. 75.
Canopies are often made too
shallow and too small and work-
men are often careless in connect-
Fig. 74. Correctb; Moiintsd CombiDslion >1g fixtUrCS.
"" Sockets and Receptacles.
These are the devices into which incandescent lamps are put and
are today made in a great variety of forms. The most common is
the familiar brass shell socket either with a key switch or keyless,
screwed on the ends of fixture arms or hung on flexible cords. Where
a lamp holder is desired to be fastened rigidly to walls or ceilings, re-
ceptacles are used and this in general constitutes the distinction
between receptacles and sockets. For outdoor use or in damp
places weatherproof sockets or receptacles should be used. These
usually have porcelain or composition outer shells, with connecting
wires sealed in or with some form of encased terminal so placed
and covered as to keep moisting from exterior and interior.
It is good practice to use only porcelain sockets in locations
where a person might touch them while at the same time in contact
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UNDERWRITERS' REQUIREMENTS 83
in any way with any grounded metal work. Thus in bath rooms if a
person attempted to turn on a. lamp by means of the key of a brass
covered socket while at the same time he was touching a water pipe
or faucet, he might receive a painful or dangerous shock if any portion
of the electric circuit were grounded. While such a shock would
not be given, if everything were in proper condition, still there are
numerous instances where persons have been killed and it is cer-
tainly wise to prevent even the chance of such an accident by either
using all porcelain sockets or by putting the sockets out of reach,
and controlling the lamps by means of a switch on a side wall. This
practice is especiallj' to be commended where the lamps are supplied
with a. c. transformers the primaries of which are supplied by circuits
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84 UNDERWRITERS' REQUIREMENTS
of voltages of 1,100, 2,200, or higher values. The voltages of 110 ot
even 220 either d. c. or a. c, whicti are almost universally used for
incandescent lighting indoors, are not of themselves at all liable to
injure persons, but if there is a
fault in the transformer the high
primary voltage may get into the
house over the secondary lines
and such a circuit will then be
distinctly dangerous to life.
In rooms where inflammable
gases may be present, as the re-
sult of some manufacturing proc-
ess or otherwise, the incandes-
cent lamp and its socket must
be enclosed in a vapor-tight globe
and supported on a pipe hanger,
wired with approved rubber-
covered No. 14 wire soldered directly to the circuit wires. Even the
minute spark caused by breaking a 16-candle-power lamp has been
known to set fire to vapors such as gasoline and air in the proper
mixture and the reason for taking every precaution against sparks
where such vapors exist, becomes
very apparent. Fig. 76 shows
a lamp so enclosed and sup-
ported.
In damp or wet places weath-
erproof sockets are required
and these must either be made
upon fixtures or hung as shown
in Fig. 77. Here stranded No.
14 rubber-covered wires are
shown passing from the socket to
the circuit wires to which they
iDcandi-Bcent.Lamp ^^ soldcrcd. The sockct and
lamp must not hang direct on the soldered joints but some other
support must be supplied as by holding the socket wires under one
of the porcelain cleats. The ordinary brass shell socket is a fairly
standard device as now furnished by the best makers, but it is of
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UNDERWRITERS' REQUIREMENTS
85
necessity small, and not very strong. It is decidedly better
never to use such a socket except for an incandescent lamp and
where an outlet is desired from which to take current foi' portable
heaters, fans, and the like, to provide special wall or floor receptacles.
This insures adequate current-carrying capacity and avoids mechan-
ical injury to the comparatively frail sockets.
Flexible Cords. There are two chief classes of flexible cords,
the plain twisted pairs and the various types of reinforced cord for
portable use and where extra protection and strength is needed. It
will be observed that the use of any type of flexible cord constitutes
an exception to the general rule that conductors must be well sepa-
rated. From one point of view it seems inconsistent to reqube wires
to be well separated in walls, floors, and partitions and then permit
the two conductors of a cord to
be twisted closely together, with
only a thin rubber insulation and
a cotton br^d on each. It is, in
fact, a concession made to the
necessities of the case and it
cannot be denied that flexible
cords may be and often are the
weakest part of an ordinary wir-
ing installation. It thus be-
comes at once evident why very
definite limitations must be made
in the use of cords. Fig. 78 shows
types of flexible cords.
The common "twisted pair" cord c(»isists of stranded copper
conductors having a total carrying capacity equal to that of No. 18,
16 or 14 B. & S. gauge solid wire. No conductors smaller than No. 18
are allowed even for very small currents in order that the mechanical
strength may not be too little. (It should be noticed also that No.
18 and No. 16 wires are permitted only for flexible cord and for fixture
wiring. No. 14 or larger being required everywhere else.)
The copper strands are twisted or cabled together and wound
with a wrapping of cotton thread both to keep the rubber from direct
contact with the copper which it tends to corrode and to prevent a
broken strand of wire puncturing the insulation.
Fig. 78. Types of Fleii)
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86 UNDERWRITERS' REQUIREMENTS
The rubber insulation should be W^nch thick on No. 18 and
No, 16, and A-inch on No, 14 cords and over each should be a fairly
close braid of cotton thread. It is apparent that the finished cord so
made with its two conductors twisted closely together does not afford
any very great protection against a short-circuit resulting from broken
or worn insulations and braid, and that the rubber and cotton braids
supply a very fairly good fuel for any flame which is started.
The foregoing is not intended to lead to the conclusion that
flexible cords should not be used, but rather to point to the reason why
their use should be restricted and why they are open to objections not
applying to ordinary separate and fixed wires.
Flexible cord should never be used as a substitute for regular
fixed wiring. If the wiring provided does not give outlets at the
proper places it should be changed in a proper
and reliable manner. The common practice of
festooning flexible cord along wails and ceilings
and even through doorways and walh b to
be strongly condemned as an unsafe and wholly
inexcusable misuse of material. The use of
flexible cord is limited to 300 volts. It should
not be used to support lamp clusters as they
are not capable of being secured well enough
^ under binding screws to hold any considerable
weight. The ordinary cord should be used only
to hold lamps which under all usual conditions
hang freely in air and which are not likely to
Fig 79 Wire Guud (or ^ movcd Sufficiently to come into contact with
PoTiabie L«np suTTOunding objccts. lu brief, this cord is for
"pendant" use only as its common name implies. The use of pen-
dant cord for reinforced cord b a very common fault. Ragged half-
broken lamp cord lying about on floors and looped over all kinds of
supports constitutes a very common defect in installations, otherwise
perhaps in fair condition. "Flexible cord fires" are as common as
might be expected from the frequent misuse of this form of wire.
For all portable work including those pendants which are liable
to be moved about, some form of reinforced cord should always be
employed. In these the ordinary pendant cord is covered either
with an outer rubber jacket and a stout "over-all" braid or where
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UNDERWRITERS' REQUIREMENTS 87
the exposure to injury is less (as in offices and residences) with an
outer woven braid only. These reinforced cords are far from in-
destructible and should be frequently inspected and renewed before
an accident makes it absolutely necessary. The use of all kinds of
flexible cord in show windows is expressly forbidden by the Code,
because it has been found that it is subject to exceptionally hard
Elf. SO. RmuIO at C»nle« Haodling of PorUble Ump*
usage there, and more especially because of the practical certainty
that it will be used as a support for window decorations or goods of
inflammable nature which have often been found pinned to it or
supported by wires strung across it. A special kind of cord having
a metal armor is, however, allowed in show windows.
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88 UNDERWRITERS' REQUIREMENTS
In connecting flexible cords to sockets, rosettes or other devices,
special attention should be paid to seeing that all the small copper
strands are well tucked in under the heads of binding screws and
that stray ends are not left sticking out to cause short-circuits. At
all places where cords pass out of sockets or other fittings, smooth,
well-rounded insulating bushings should be provided and a knot
should be tied in the cord inside the socket cap or rosette, so that
this knot will take the strain from the binding screws. Several
special little fittings for thb purpose are also on the market as sub-
stitutes for the knot. Pendant lamps wherever exposed to injury
or at all liable to be brought into contact with inflammable material,
and all portable lamps should be provided with substantial wire
Fii[. 81. Hole Burned by Incuideicent Lamp Bulb
guards such as are shown in Fig. 79. Figs. 80 and 81 show the results
of carelessness with hanging or portable incandescent lamps.
Arc Lamps on Constant-Potential Circuits. Each lamp, or each
series of lamps, should be separately fused and the branch conductors
should have a carrying capacity about 50 per cent in excess of the
current required to provide for the heavy current required by the
lamp in starting or when the lamp carbons accidentally become stuck.
If this were not done it would be necessary, generally, to over-fuse
the wires, which is objectionable.
Arc lamps are of many patterns but aU of them contain resist-
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UNDERWEITERS' REQUIREMENTS -89
ances or regulators which become very hot and, therefore, all the parts
of the lamp and its case must be of BODK»mbustible materia] and
must be treated as sources of heat, that is, they must be installed
well away from all inflam-
mable stuff. The globes and
netting about the carbons
must be used in all cases. In
general the resistances and all
Fig. 81. EtMrior uid iDlsrior Vi«ir of Enclosed Arc Lunp
Other accessories of the lamp except the controlling switch should
be contained in the lamp case itself. Fig. 82 shows a modern type
of enclosed arc lamp and its mechanism.
The "flaming arc" lamps now in common use call for the same
precautions as the older patterns. In dusty or linty places special
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90 UNDERWRITERS' REQUIREMENTS
precautions must be taken to prevent the accumulation of lint, etc.,
either on hot resistances, inside the lamp or on the switch usually
furnished on the lamp. In general the same rules apply to mercury
are lamps as to the carbon arc lamps, except that the former do not
present the hazards due to the hot carbon points.
TRANSFORMERS IN BUILDINGS
Except in central stations and substations, an outside location
for transformers is always preferable and the underwriters do not
allow any oil-cooled trans-
formers in buildings except
by special permission. This
is because of the danger
from the oil which may be
boiled over or set on fire in
case the transformer becomes
overheated.
Air-cooled transformers
having the highest voltage of
both primary and secondarj'
under ;)50 volts may be in-
stalled inside buildings if the
case is kept at least- one foot
from combustible material or
separated from such material
by being mounted on a suit-
able slab of slate or marble.
Fig. 83. Comman Type of Oil-Cooled Translormer _ , . ,
Iransformers sometimes be-
come very hot in case of a partial or complete burn-out of the coils
or from overloading and, therefore, should never be mounted di-
rectly against wood posts, beams, or walls. Fig. 83 shows a com-
mon type of oil-cooled transformer.
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STANDARD BYnBOLS FOR WIRIHQ FLAMS
TK lUTimu tucniui cniucnis ushuihh *r m cna sc«ns mi tk utucu nmrni «r AKsncn.
s:^ti jss? N »• »i»«^ »«*fi»d o
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UNDERWRITERS' REQUIRE-
MENTS
INSTALLATION OF WIRES IN BUILDINGS
Hie proper choice of wires and their safe installation con-
stitutes the most important part of all electric equipment from the
viewpoint of the fire hazard. It luis already been stated that in all
electric work, conductors, however well insulated, should always be
treated as bare, and from one point of view, it may be said that the
value of conductors as regards safety lies in their insulation rather
than in the copper, for if we assume that a wire of adequate carrying
capacity is chosen for a given purpose, there remains only the choice
of a suitable covering or insulation on the wire and a reliable and work-
manlike method of placing it.
No one material has yet been produced which has every desir-
able property as a covering and insulation of electric wires and
cables. Among the desirable properties of a wire covering are
elasticity, flexibility, waterproofness, good insulating quality and
resistance to voltage strains, resistance to effects of changing tem-
perature, acids, vapors, etc., and permanence. All of these prop-
erties are possessed by rubber in greater or less degree and, all t«ld,
to a greater degree than any other material. The properties of
rubber-covered wires will be treated at greater length in another
place, the foregoing statement being made here to emphasize the
reason why rubber-covered wires are used almost exclusively in all
inside wiring.
"Slow burning" wire b a copper conductor covered with three
closely woven cotton braids saturated with a fire-resisting compound.
Its use is limited to places where rubber is liable to be rapidly injured
by high temperatures. Its insulating value is slight and it b not
capabl" of resisting mobture.
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92 UNDERWRITERS' REQUIREMENTS
"Weatherproof" wire consbts of a copper conductor covered
with three braids saturated with a mobture-proof compound. Its
insulating value when new is low (much less than rubber) and it is
ver>' inflammable. Its use b practically confined to outdoors.
In the following sections it may be assumed that all references
to wire mean "rubber-covered" wire.
ClassificaticHi and General Principles. There are two classes of
wiring which may be named for convenience: enclosed wiring and
ntm-encloeed wiring.
Wires run on insulators such as cleats and knobs exposed on
walb and ceilings or on knobs and through tubes concealed in floors
and walls are the chief types of non-enclosed wiring. It will be
observed that the distinction consists in the presence or absence of
\ special wire-ways or channek for the wires. Wires must not be laid
in plaster, cement, or similar finish, because such materiab may
contain either alkalies or acids which will injure the insulation and
corrode the copper. Wires must never under any circumstances be
fastened with staples because of the probable injury to the wire
coverings, the insecure fastening obtained, and the possibility of
such staples affording a path between wires in case twin conductors
are used.
Twin wires must never be used except in conduit or where flexible
conductors are necessary. The nearness of the two wires, on the
opposite sides of the circuit, renders twin wire of any description
somewhat more liable to failure and an injury to one wire generally
involves an injury to both with resultant certainty of a short-circuit.
The added safety of keeping the wires of a circuit separated is lost
in twin wires.
In any scheme of wiring it is essential that all electric wires be
installed so that they cannot come into contact at any point with
any materials other than those expressly intended for their enclosure
or support. Thb means that they should be kept absolutely free
from contact with gas, water, or other piping, and from all metal
work of any description unless it be that of piping, boxing, or molding
provided as wire enclosures. Wires should also be installed so as
not to touch woodwork or other combustible material, even if such
materia) b not a good conductor. This general principle finds an
exception in the case of wood molding.
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.UNDERWRITERS' REQUIREMENTS 93
Contact of wires with metallic substances may, in case of a ffuiure
of the insulation, permit dangerous arcs, short-circuits, or grounds,
while contact with wood or combustible material is objectionable on
account of setting fire to it in case of overheated wires or from leakage
due to the presence of moisture on materials which when dry would
be good insulators. These are, therefore, the general principles of
wiring under the established rules and will be illustrated in the dis-
cussion of the various classes of wiring which follow.
Open Work in Dry Places. Wires in open work may be either
rubber-covered, slow-burning, or — special and now little used wire
having a weatherproof braid covered by a slow-burning braid — but
as a matter of fact only rubber-covered wire is used to any extent
Fig. S4. Lttf Ftedst WIra Eipoaed on lonilston
and it is much to be preferred except in exceptionally hot places aa
over steam boilers, where rubber insulation will deteriorate very
rapidly. The rubber-covered wire used for open work has a single
braid over the rubber. The chief advantages of open work are its
cheapness and its accessibility. The latter may be of great advantage
in eases where frequent changes and additions are likely to be re-
quired or where renewals of wire are frequent because of peculiarly
unfavorable conditions such as exist in packing houses.
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94
UNDERWRITERS' REQUIREMENTS
Open work finds its chief use in mills and factories and for large
ooaductors which it is especially difficult and expensive to enclose in
^__^ conduit. Fig. 84 shows
an example of a set of
largefeeders run exposed
on insulators. It should
be noted that such a
large group of heavy
cables covered with the
inflammable braid and
rubber insulation fur-
Tig. 8S. PoroaUin Tvo-^
oishes a very considerable amount of fuel for fire and the necessity
for excellent spacing and reliable fastening is obvious. The heavy
porcelain blocks carried in metal frames as shown in the illustra-
tion are of an approved type. In all open work, wires or cables
must be ri^dly supported on non-combustible, non-absorptive
insulators. Formerly wood cleats were used but these are now
obsolete and have been replaced by porcelain. Where the voltage
is less than 300 volts, wires must be separated from each other at
least 2) inches and from the surface wired over at least § inch in dry
places — in damp places at least 1 inch. For voltages from 301 to
550 voltj, the limit for 'low-potential systems," the wires must' be
kept 4 inches apart and 1 inch from the sur^ce wired over.
The neutral wire of an Edison direct-current three-wire system
(110-220 volts) may be placed in the center of a three-wire cleat
Elf. Sa, PornUini
Fi8- 37. Poroelun Oas-Win Clwt
which will keep the two outside wires 2J inches apart. Fig. 85 shows
the form of a porcelain two-wire cleat and Figs. 86 and 87 show good
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UNDERWRITERS' REQUIREMENTS 95
forms of one-wire cleats for heavier conductors. No. 6 wire is about
the largest which should be installed in cleats like those in Fig. 85,
and for cables larger than No. 0000 B. & S. gauge, some form of iron
rack for the insulators is desirable in order to secure the needed
mechanical strength and rigidity.
The rigid supporting necessary for open wiring requires under
ordinary conditions along flat surfaces, supports at least every 4J
feet. This distance should be decreased wherever wires are liable
to be disturbed especially if the wires are small. The following
comment is from the Rules of the Associated Factory Mutual Fire
Insurance Companies:
The proper distance between insulators depends l&rgely on the 8ur>
roundingB. In placee where ceilings are low, or where belts, shafting, or other
Fig. BS. Wiling on CtillDg Bbowiog (Jn of Btnin InauUton
machinery may require frequent attention, insulators should be plaeed every
few feet, ia order to prevent the wires from being displaced by careless or
unavoidable blows from workmen. On the other hand, with a high ceiling
and no chance of deranf;ement, a greater distance would be allowable.
The whole idea is to so rigidly secure the wires that they cannot come
in contact with each other or any other conductors, if loosened by shrink-
age of timbers and floors or by careless knocking.
Special methods must be followed at corners and in wiring over
broken surfaces as on ceilings of mill-constructed buildings. Fig.
88 shows a use of "strain insulators" in making a turn on a ceiling.
These are insulating balls having rings set in each side, the wire
being looped through one ring and the hook on the beam through
the other, Turnbuckles may be used to keep the wires taut. Ordi-
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96
UNDERWRITERS' REQUIREMENTS
nary cleats will not hold heavy wires at comers. With conductors
of No. 8 B. & S. gauge or over, it is not necessarj' to "break around"
beams but smaller wires should
■ be carried around the beams
as shown in Fig. 89. The
cleats on the ceiling should be
set off from the timbers 3 or 4
inches. If they are closer, the
Kg. 89, Approved Method of Carrying Wires shrinkage of the timber or
Aroun ctmt rough usage is liable to bring
the wires into contact with the timber. On the other hand if the
distance is greater, the wires are too much exposed to injury from
brooms, ladders, and the like. With this arrangement any slack
wire can be taken up by
moving the cleats a lit-
tle nearer the corner.
\Miere beams are widely
spaced some such method
of support as that shown
in Fig. 90 should be fol-
lowed. Fig. 91 shows a
less desirable method of
support. In low ceiling rooms where wires are exposed to mechani-
cal injury, wood guard strips (see Fig. 92} may be used to ad-
vantage, HTiere wires pass through partitions or walls they must
be protected by tubes of porcelain or iron pipes lined with flexible
tubing.
Wires on side walls must be protected from injury to a height of
at least 5 feet from the floor either by wood boxing or by iron pipe as
C
FiK. 01. M<^thad<
shown in Figs. 93 and 94. When iron pipe or conduit is used the
insulation of each wire must be reinforced by flexible tubing extend-
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UNDERWRITERS' REQUIREMENTS
97
ing from the insulator next below the pipe to the one next above it
(see Fig. 93), For alternating current both wires of the circuit must
be in the same pipe.
Open Wiring in Damp Places.
The installation of electric wires
in places exposed to dampness
presents some peculiar difficul-
ties requiring special methods.
A film of water such as may be
formed by steam or otherwise
by condensation is a very fair
conductor of electricity, and may reduce or even entirely destroy
insulation which would be quite adequate in dry places. Acid or
alkaline fumes or vapors are also good
conductors in some eases and in addition
they are liable to injure both the insula-
tors and the copper of electric wires.
Fig, 93.' Wood Boiina OS Protpptioo Fig. B4 " Protecting
for Wiroi, on Si e Walls '7ran"pip'ng *'
In paper mills, breweries, soap factories, packing houses, dye
works, and cold storage rooms special pains must be taken to insure
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98 UNDERWRITERS' REQUIREMENTS
permanence and reliability of all electric fittings and appliances.
The Code does not prescribe at length precautions to be taken
in damp or otherwise exceptionally troublesome places, but merely
specifies- for open wiring that only rubber-covered wire be used and
that the separation between wires shall be at least 2^ inches for
voltages up to 300 volts (4 inches for higher voltages) and that all
wires be kept 1 inch from surface wired over instead of only J inch
as in dry places.
There are two objections to the use of steel conduits in damp
places, first, that all metal work is especially liable to corrosion even
when well enameled or galvanized; and second and more important,
Pl|. 9S, Corrodod RoeetM Impnipsrly Mounted on Damp Ceillua
that water is apt to collect in the pipes and gradually deteriorate the
insulation. This water results from moisture condensed ^m the
air during changes in the temperature and amount of water vapor
present in the atmosphere. This condensation is often sufficient to
be very troublesome and often leads to the adoption of open wiring of
special forms rather than a complete conduit installation. Usually
conduit is used in the more exposed or crowded places only and in
such parts of the plant as are less liable to dampness.
In many breweries, packing houses, and other plants, walls and
ceilings are continually dripping with water and in some factories
fumes and vapors are present in lai^ quantities at all times. A
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UNDERWRITERS' REQUIREMENTS
fairly good solution of the problem
is possible where only water b to
be guarded against, but where cor-
rosive vapors exist, no thoroughly
satisfactory method has been de-
vised to resist indefinitely the cor-
rosive actions. A good asphaltum
paint will protect cabinets and con-
duits for a time and frequent re-
painting will extend the life of
these parts of the equipment for a
considerable period. It b evident
that current-carrying metal parts
should be enclosed in tight boxes
wherever feasible and very frequent
and thorough reinspections of the
entire equipment should be made
followed by renewals as faults de-
velop. Fig, 95 shows the corrosion
on a rosette improperly mounted.
In rooms where dampness is
excessive the wires are sometimes
run open in inverted wood troughs,
one form of which is shown in Fig.
96 and in detail in Fig. 97. This
trough serves to separate the wire-
way entirely from a wet ceiling and
the sloping surfaces serve to carry
the moisture away from the knobs
on which the wires are held. The
V-shaped blocks are spaced about
41 feet apart and care is taken to
make a close joint where the run-
ning boards come together at the
top. The whole is thoroughly
painted with an insulating paint. Fk. w. iDverud Trou«hi
Fig. 98 shows oj)en wiring on flat
running boards for moderately damp places like basements c
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100 UNDERWRITERS' REQUIREMENTS
Fii. 67. Seclion ol Wiring Trough
Fig. as, Eumple of Wirim on RuDUiai BosrdB in D»mp PUoo
D,Bi:ized3,GOOgIe
UNDERWRITERS' REQUIREMENTS 101
All drop cords ahould be of extra heavy reinforced type or of
standard rubber-covered wire and only weatherproof keyless sockets
should be used with all wire joints soldered, taped and painted in
the best manner. Motors and their resistance boxes or starters
should be kept out of damp rooms if possible but if in such rooms,
they should be installed with special reference to accessibility,
cleanliness, and separation from wet floors or walls. Posts in middle
spaces of rooms will generally afford better locations than side walls,
which are always wet. Wood cabinets lined with slate or with stiff
asbestos board are preferable in many wet places to metal enclosures
if they are kept well painted inside and out with an asphalt or in-
sulating paint. A single incandescent lamp in such cabinets if kept
constantly burning will tend to keep the interior of the box dry, if
the box is tight and if the door is kept closed. Such cabinets may
well have a glass panel in the door to show the lamp and incidentally
to mark the location when the room is dark.
Wires la Molding. Wood Molding. Wood molding b one of
the commonest forms of protection for wiring and when properly
used affords a cheap and fairly satisfactory installation. From one
point of view it seems inconsistent to take every precaution in other
fcmns of wiring to keep wires away from direct contact with wood
surfaces, such as ceilings and walls and hidden spaces in frame par-
titions and floors, and on the other hand to permit them to be run
in small grooves in strips of wood, as in molding work. The solution
to this somewhat theoretical objection is found in the fact that in
molding, the wires are completely enclosed in a «-ire-way especially
designed for them, rather than allowing them to hang or be drawn
over wood objects in a more or less accidental manner with no re^
protection, and a still better solution is to be found in the unques-
tionable fact that experience has proven the use of molding under
jB-oper conditions to be satisfactory.
Wood molding should never be used in concealed spaces or in
damp places or for voltages over 300. The rule as to damp places
precludes its use in cellars and basements, anj-where out-of-doors
and generally on outside brick walls which are often more or less
damp and likely to "sweat" and thus introduce moisture back of the
molding. Ji the wood molding becomes soaked with water there
is a liability of leakage of current from one conductor to the other or
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102
UNDERWRITERS' REQUIREMENTS
to "ground." If conductors in molding become overheated by
excess current the wood may become charred, and charred wood b a
fiur conductor. The possibility of fire from such a cause is evident.
Molding should be made of hard
wood and should be thoroughly
impregnated inside and out with
a paint- or moisture-repellant. It
must be made in two pieces, a
backing contuning the wire
grooves, and a capping. The
tongue between wires must be at least J inch thick and the wood
under the grooves must be at least ^ inch thick. Figs. 99 and 100
give the fonns of two-
Two- wire Molding
[ > Acj-AbJ-Aa-j-Ab J-Aa-j-AbJAo ■ J,
F1(. 100. SectioD of Tbree-Wjre Molding
and three-wire moldings.
Larger moldings are
sometimes used for
heavier wires but gener-
ally only smull conduc-
tors are placed in
wood moldings. Only good rubber-covered wire should be used
and no joints or splices made in the wires in the molding, but where
branch taps are necessary some form of fitting approved for the
purpose should be employed. Such
fittings as the one shown in Fig.
101 provide porcelain bases with
suitable binding screws for the main
and the branch wires and a cover
over the joints.
A large variety of receptacles
(both for lamps and plug connec-
tors), rosettes, etc., are available for
use with wood molding and should
be used instead of the ordinary pat-
terns. Where it is desired to insert
Fit. 101. t^"^^' ['°'^ w»'''°» 'T'P '" a snap switch, either a special switch
approved for mounting directly on
the molding, or else a sub-base of porcelain or hard wood on which
the switcli can be securely fastened should be employed.
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UNDERWRITERS' REQUIREMENTS 103
Wood molding is often used in connection with other types of
ndring and in such cases special attention should be given to making
a good mechanical job where the conductors enter or leave the mold*
ing. Fig. 102 shows one form of protector for use in protecting
wood molding at floor levels. The capping should be carefully
and tightly nailed in place and under no circumstances should fix-
tures be attached to molding or any hooks or nails be driven into it
for the support of lamp cords or other objects. The use of wood
molding in show windows is undesirable: ^rsf, because of the damp-
ness apt to exist there; and second, because in the process of decorat-
ing windows and arran^^ng displays of merchandise nails will surely
be driven into the molding with resultant injury to the insulation of
the wires.
In conclusion, it may
be said that wood-molding
work is cheap and may be
used properly, but is in-
ferior to most types of wir-
ing.
Metal Molding. Re-
cently several types of metal
molding have been intro-
duced which are free hoxa
some of the objections ad-
■ . . J IJ- Pis- 103' Moldioc Protection at Floor Lovet
nenng to wood molding,
and make possible a neat, inexpensive, and convenient installation.
In these moldings, as m those of wood, the wire is laid in, not drawn
in as in conduits, and is covered by a metal capping. At present the
underwriters' rules limit its use to circuits requiring not more than
660 watts of energy.
Special fittings are provided for angles, bends, taps, and crosses,
and for switch, rosette, and receptacle bases. The molding can be
bent for slight curves or offsets and when carefully installed gives
good results. Tig. 103 shows such a molding and fittings.
Metal molding must be continuous from outlet to outlet and
where it passes through floors it must be enclosed in an iron pipe for
added protection. The backing must be secured by screws or bolts
with heads countersunk so as not to obstruct the wire-way. Between
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104 UNDERWRITERS' REQUIREMENTS
lengths of molding and at all fittings and outlet boxes the joints must
be mechanically and electrically secured. The fundamental idea
is to secure an absolutely continuous metaihe conductor throughout
the entire run of molding and in addition the molding must be well
grounded in a permanent manner. In this respect the molding is
regarded in the same light as rigid metallic conduit. The reasons
for requiring good electrical continuity and grounding will be ex-
plained later when the subject of conduit is considered.
Concealed Work, This kind of wiring is also often called "knob'
and tube" work since the wires are held on knobs and passed through
tubes of porcelain. The great advantage of this method of wiring
consists in the cheapness and ease with which a building, especiall\'
a frame building, can be wired. The greatest objection to the knob
and tube work lies in the fact that the wires are wholly unprotected
from mechanical injury and the building is not protected from the
results of arcing between wires in case of a cross or short-circuit.
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UNDERWRITERS' REQUIREMENTS
105
It is no doubt true that knoband tube wiring is the least reliable
form of electric work and inferior to good conduit or armored cable.
Where wires can be run open and are not exposed too much to
mechanical injury, they are probably somewhat safer than when
concealed on porcelain supports in walls, floors, and partitions. Open
wiring is not possible in residences or where good appearance is a
requisite, and the somewhat greater cost sometimes prevents the
use of conduit. Therefore, recourse is made to concealed wiring and
where an installation is carefully
made a reasonably good result
may be obtained. In many cities
concealed work is entirely for-
bidden within "fire limits", that
is, in the closely-built sections,
but it is still very extensively
employed in places where first
cost and quickness of installation
are the prime factors. Only
good rubber-covered wire is al-
lowable in concealed work.
Approved Installation in a
Residence. The wiring of a resi-
dence may be taken as an
illustration of how the work
should be done. The service
wires are brought in through
the wall near the ground pre-
ferably in iron conduit but allowably through bushings with
drip loops outside. Fig. 104. As near as practicable to the point of
entrance is placed the main service fuse and switch. These should
be in a suitable cabinet though this is not obligatory. It should be
understood that the wiring is placed during the. erection of the
building. In an ordinary frame building the wiring will be done
just after the rough flooring and the partition studding has been
placed, but before the lathing or any plastering is done. It should
all be completed except the final connection of service wires, fixtures
and fittings Ijefore any of it is enclosed or hidden, so that inspection
may be made while all parts are accessible and visible.
g Building
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106 UNDERWRITERS' REQUIREMENTS
From the service cabinet and meter as many circuits are run as
may be required to feed the lamps and other devices to be connected.
each being fused at the service center unless there is no change at that
point in the size of wires. These circuits will pass up through the
floor within porcelain tubes and will thence be carried on porcelain
'knobs fastened to the timbers and studding not more than 4} feet
apart. Where the wires pass through floor timbers, porcelain tubeS)
straight and smooth, must be used in holes bored in the wood and
just large enough for the tubes. In general we may say that knobs
are used where the wires
run parallel with floor
beams, and tubes where
the wires are run at right
angles to the beams.
Fig. 105 shows the gen-
eral method and Fig. 106
Fi,.iOfl. Two.PL«.Koob(orHoidi,.,wi„ illusttates a two-piece
knob of common type.
These are preferable to one-piece knobs with which a tie-wire b
necessary. The wires should preferably be run singly on separate
timbers or studding, and must, except as noted below, be kept
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UNDERWRITERS' REQUIREMENTS 107
everywhere 5 inches apart. Hg. 107 shows clearly the use of the
knobs and tubes, but the tubing shown on two of the upper wires
Fig. 107. Knob md Tube Wiring Showing ObJMtio
is objectionable and knobs should have been used instead. It is
very desirable to use metal outlet boxes at all outlets and the added
expense of so doing b not large. Where flush switches or receptacles
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108 UNDERWRITERS' REQUIREMENTS
Fig. 108. Proper Uw or Outlet Boiee Fig. 109. Wirine in Partitions Shawios Vat
PuiceluD Tubca
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UNDERWRITERS' REQUIREMENTS
109
are used, metal boxes are absolutely necessary and in Fig. 108 several
sucb boxes are shown as well as the method of mounting them on
cross strips between the uprights. This illustration shows also an
allowable use of flexible non-metallic tubing on wires in concealed
work where it is impracticable to maintain the 5-inch separation.
Fig. 109 shows at the bottom the extra porcelain tube which
should be put (HI each wire passing through timber at the bottom of
a plastered wall to protect it from the droppings of wet plaster which
will fall on it during the process of closing in the wall surface. This
picture also shows very clearly the flexible tubing which must sepa-
rately enclose each wire at every outlet, reaching from the last porce-
lain support into a switch box, or on ceiling or wall outlets, where
no box is used, at least 1 inch beyond the surface. In the case of
combination gas and electric fixtures the tube must extend at least
fiush with the outer end of the gas pipe as in Fig. 1 10.
When in a concealed knob and tube sj stem, it is impracticable
to place the whole of a arcuit on non-combustible support of glass
P
wing Use o( Tubine or Loom
or porcelain, that portion of the circuit which cannot be so supported
must be installed with approved metal conduit, or approved armored
cable, except that if the difference of potential between the wires is
not over 300 volts, and if the wires are not exposed to mobture, they
may be fished if separately encased in approved flexible tubing, extend-
ing in continuous lengths from porcelain support to porcelain sup-
port, from porcelain support to outlet, or from outlet to outlet.
Thre can, of course, be no assurance that such fished wires do not
lie in close contact with gas or water pipes, or other wires, and so there
is need of the protecting tubing.
In judging an installation of wires in concealed work, special
attention should be paid to the wire joints. The rules allow these
to be made in concealed spaces by means of soldered joints well
covered witli both rubber and friction tape. The wiring b usually
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110 UNDERWRITERS' REQUIREMENTS
on the so-called "tap plan," that is, taps or branches taken off wher-
ever convenient. The necessity for good workmanship is evident
as bad joints may easily set fire in dry floor and wall spaces. Fig.
Fia. 111. Detective Joint F<
111 shows such a defective joint which was found behind a latb-and-
plaster partition. The joint was not taped and was hot when dis-
covered. What is called the "loop-plan" is occasionally employed,
all joints or taps being made at outlets where suitable boi^s are
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UNDERWRITERS' REQUIREMENTS 111
provided. This involves the use of more wire, makes the conditions
at outlets more crowded, and is perhaps not as good for concealed
work under the conditions usually obtainable. Either system is
pennitted by the rules.
Armored Cable. Armored cable for interior wiring consists of
double-braided, rubber-insulated, twin wires, covered with a spiral
steel strip armor which protects the conductors from injury and is
at the same time flexible. It is largely used in wiring old buildings
since it can be drawn into concealed spaces without fear of injuring
the conductors. It is also used for new work and is in some ways
easier and cheaper than rigid conduit. Fig. 112 shows a piece of
twin-conductor house cable and some of the fittings for use with it.
Fi«. 112. Armored Cable sod Pittiutg
With armored cable all joints must be made at outlets or in cabinets
or junction boxes which are always accessible. No taps or joints
are permissible except at such outlets or boxes. Great care should
be taken to secure the armor very firnjly to all outlet boxes in a way
to give a good connection both electrically and mechanically, and
the armor system must be permanently and reliably grouTided. Where
dampness may be expected there must be a lead sheath between the
braided wires and the outer armor since the cable is not thoroughly
moisture-proof.
Armored cable installations are superior to molding, open work,
or concealed work, in that the wires are better protected. The fact
that twin wire is used instead of separate conductors is at least a
theoretical disadvantage, and the cable unless leaded is not absolutely
waterproof.
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112 UNDERWRITERS' REQUIREMENTS
The chief disadvantages of armored cable as compared with
rigid conduit, consist in the somewhat greater difficulty of making
good connections to the armor at outlets and still more in the im-
possibility of drawing out wires which have proved defective. How-
ever, for many places a thoroughly good job can be done with armored
cable. Fig. 113 shows a characteristic piece of work of this sort.
Conduit Work. The earlier forms of conduit for interior wiring
were made of paper or fiber and later of paper with a thin brass
casing but these forms are now obsolete and entirely displaced by
steel conduits made of either rigid pipe or flexible steel spirals. A
limited use is still made of a form of conduit having a steel pipe
lined with a heavy paper impregnated with some material to exclude
moisture. This is, however, going out of use and almost all conduit
work is now done with unlined pipe. Conduit, however, differs from
ordinary commercial pipe such as is used for gas, water, or steam,
in that it is carefully cleaned in the process of manufacture and then
protected from rust both inside and outside by a good baked enamel
or by some form of zinc coating. Rigid conduit, Fig. 114, gives a
rather more workmanHke job than flexible conduit. Fig, 115, but the
latter can* be used in some places, especially in old buildings where
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UNDERWRITERS' REQUIREMENTS 113
the use of rigid pipe would be impossible. If well fastened at ail
outlets and at all bends it affords a protection to wires second only
to the rigid conduit. The complete protection afforded by conduit
pis, 114. Rigid Conduit
both of the wires from mechanical injur>', and of the surrounding
parts of the building from fire resulting from a burn-out of the wires,
makes conduit work undoubtedly the safest form of electrical in-
stallation and one which is becoming more and more used, not only in
the more expensive type of buildings but also in cheaper work as well.
The essential principle of conduit work is to furnish a complete,
strong and unbroken metal enclosure for conductors between outlets
and to give to this metal-pipe system an electrical continuity and
carrying capacity sufficient to serve as a safe path for any current
which the failure of conductors within may impose upon it, for a
time long enough to operate the fuses or circuit breakers protecting
such conductors. It is also an essential characterbtic of a correct
conduit installation that all conductors can be drawn in after the
entire conduit system is put into position and can at any subsequent
time be dravm, ovi in case any wire fails and must be replaced.
These fundamental ideas will explain the reasons for most of the
details prescribed in underwriters' rules for conduit work.
The construction details of
steel conduit will be briefly
discussed later and we consider
here only methods of installing it.
There must be no breaks in
the conduit system, that is, the i
pipe must be continuous from
outlet to outlet or junction
boxes. At every outlet metal ^.^ ^^^ F..ribie cood«i
boxes must be provided which
the conduits must properly enter and to which they must be me-
chanically secured. This is usually accomplished by means of either
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114 UNDERWRITERS' REQUIREMENTS
threaded lock nuts on the pipe outside the box and threaded bushings
on the pipe inside the box or by threading the conduit into tapped
hohs in the box. With flexible conduit a special approved clamp must
be used for this purpose. Fig. 116 shows a box with a rigid conduit
on one side and a flexible conduit on the other side together with a
few of the fittings used in connection »ith it. No conduit having
an internal diameter less than | inch is allowable since this is as
small as will permit nires of required minimum size to be drawn in
without injury.
All elbows and bends in the piping must be so made that the
conduit will not be injured and there should not be more than the
equivalent of four quarter turns from outlet to outlet, not counting
bends at the outlets themselves. If more turns are required, or
wherever it would be difficult
to draw in the wires, addi-
tional outlets called junc-
tion- or pull-boxes should be
put in to facilitate the inser-
tion of the wires. There
should be no sharp edges,
burrs or other obstructions
O either in conduits, at coup-
lings between lengths, or at
^v outlets, as they are apt to
^^^ ^^ injure the wire coverings.
\^ Therefore, all ends of pipe
should be reamed out before
— they are put into the pipe
couplings or into boxes, and
all bushings should ha^'e smoothly rounded edges.
The entire conduit system must be installed complete and, in
fact, all the mechanical work on the building must be completed as
far as possible, before anj' conductors are drawn into the conduits.
Pains must be taken to make all joints tight, so that there will
be no bad electrical connections between parts of the metal system.
It is not, of course, expected that under normal conditions the con-
duits will carry any current, but under some circumstances if the
insulation of a wire fails, current may pass over the conduit. If the
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UNDERWRITERS' REQUIREMENTS 115
conduit is well grounded and all joints are well made a safe path to
"ground" is afforded for a current large enough to blow the fuses
protecting the circuit, thus cutting off the current before the pipe is
overheated or burned through at the point where the bared wire has
come into contact with it.
Conduits and gas pipes must be securely fastened in metal
outlet boxes so as to secure good electrical connection. Where boxes
used for centers of distribution do not afford good electrical connec-
tion, the conduits must be joined around them by suitable bond wires.
Where sections of metal conduit are installed without being fas- '
tened to the metal structure of buildings or grounded metal piping,
they must be bonded to a permanent and efficient ground con-
nection.
It is rarely possible to perfectly insulate a conduit system
throughout, and a positive ground is, therefore, required, so as to pro-
vide a definite path for leaking currents and thus prevent them from
escaping through parts of a building, etc., where they might do harm.
The size of conduit which should be used for different sizes of
wire or for different numbers of wires of specified size, depends upon
the length of run between outlets where wires can be pulled or fed in,
upon the number and the radius of bends and the thickness of the
msulation on the wires. The rules, however, state that the same
conduit must not contain more than four two-wire, or three three-wire
circuits of the same system, except by special permission of the
Inspection Department having jurisdiction, and must never contain
cireuits of different systems, that is, from different generators
whether of the same voltage or not or whether both d. c. or a. c. or
one of each.
In tall buildings special provision must be made to support the
conductors in the vertical conduits to remove their weight from the
connections, and the spacing of supports in such cases is prescribed as
follows: No. 14 to every 100 feet; No. 00 to 0000 every 80 feet;
0000 to 350,000 c. m. every 60 feet; 350,000 c. m. to 500,000 c. m.
every 50 feet ; 500,000 c. m. to 750,000 c. m. every 40 feet ; 750,000 e.
m. every 35 feet.
The following methods of supporting cables are recommended:
(1) A turn of 90 degrees in the conduit system will constitute a satis-
factory support.
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116 UNDERWRITERS' REQUIREMENTS
(2) ' Junction boxes may be iiieerted in the conduit Byetem at the re-
quired intervals, in which insulating supports of approved type must be in-
stalled and secured in a satisfactory manner so as to withstand the weight o[
the conductors attached thereto, the boxes to be provided with proper covere.
(3) Cables may be supported in approved junction boxes on two or
more insulating supports so placed that the conductors will be deflected at
FiK. 117. Method of Supportinf Condvit RiMn
an angle of not 1p»s than 00 degrees, and carried a distance of not less than
twiue the diameter of the cable from its vertical position. Cables so suspended
may be additionallv secured to these insulators by tie wires.
The second method is illustrated in Fig. 117 where a space has
been left in conduit risers. The conductors will be held to the back
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UNDERWRITERS' REQUIREMENTS U7
plates by clamps and the whole will finally be enclosed in a heavy
steel box which will form the connecting bond in the riser system.
Fig, 1 18 shows one form of cable clamp used for this purpose.
Wires for conduit must be rubber-
covered and have a double braid over
the rubber. Twin wires are universally
used for the smaller sizes. No. 14 to No,
10, and these have a braid over the rub-
ber of each conductor and a second braid
over both conductors together. For
larger sizes single conductors, double
braided, are used and for the largest
sizes only one condnctor is usually run in ^g. ng. c»bie cump u»d in
a pipe It should be noted, however, Coodmi SyBiem
that for alternating-current systems the two or more wires of any
one circuit must be drawn in the same pipe since otherwise there
will be excessive heating of the metal pipes due to a magnetic action
peculiar to alternating currents and known as "induction." Fig.
119 shows three lafge feeder ducts passing out of a steel service box.
In this case the circuit was three-wire direct current.
The design of a conduit system of wiring for a large building
may be a problem of some magnitude involving no little engineering
skill and experience if the most economical, efficient, and sightly
results are to be secured. A full exposition of the methods followed
and the reasons for them does not fall within the scope of this book
but the following general considerations are of some value in judg-
ing both the advantages of conduit work and the excellence of any
given installation. In all conduit work it should be remembered
that no taps or joints are permitted in conductors except at outlets,
junction boxes, and cabinets, and that all these must be in accessible
places. By accessible is meant accessible not only during the process
of installation of the system but also after the building is completed
and in use. A junction box is not "accessible" if it is necessary in
order to get at it, to take up portions of floors or make openings in
ceilings or side walls. This prime requisite of conduit work makes
it necessary to run all conductors on the "loop" plan which means
that all lines, branches, and control circuits (as to switches) will con-
sist of a pair of wires starting at the same point at some outlet box
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118 UNDERWRITERS' REQUIREMENTS
or cabinet and running generally in the same pipe to some other out-
let where the socket, switch, or other device la placed. Conduit
systems may be made with the piping concealed or exposed as may
be dictated by the character of the building and the finish desired.
In frame buildings where the electric installation is put in during
the original construction the conduits .are placed after the main
framing and partitions are In position and will be arranged to pass
through walls and floors with the outlet boxes and cabinets set and
fastened securely at or very near the places where they will be when
Pig. 119. SweI Scrvire Boi and Feeder Pipe*
the building is done. In laying out such work little regard need be
paid to having pijies run straight or parallel to each other so long as
they are so shaped at bends and otherwise disposed as to make it
easy to draw in the wires. The chief considerations are the position
of outlets, the proper pipe sizes for the wires, and the proper subdi-
vision to secure correct fusing of incandescent lamp circuits accord-
ing to the rule which permits not over 660 watts on any such circuit.
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UNDERWRITERS' REQUIREMENTS 119
Characteristic Conduit hiatallatlon. Fig. 120 shows a char-
acteristic layout for conduits and outlets for a small apartment.
At the point marked "riser" a main conduit rises from the "service"
in the basement and on each floor has a cabinet where the supply
w ires for that floor are taken
oir and fused and where the
meter may be installed. In
lajing out such a system of
conduits or in inspecting it
one should start from the
service entrance cabinets
from which the conduits
should enter. If this cabi-
net contains not only the
service fuses but also the
sub-fuses of the branches,
the several circuits should be
examined to see that they are
properly fused, and that they
are distributed in pipes of
adequate size. The several
runs of conduit should then
be followed throughout their
entire length to see that all
couplings between pipes and
Fil, 121. Approved Ceiling Support for Conduila ,, . . , , , . ,
all attachments to outlet
boxes are thoroughly tight and that both conduits and boxes are
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120 UNDERWRITERS' REQUIREMENTS
fastened by straps or screws so that no strain will be put upon the
screwed joints. Care should be taken to see that there are no sharp
bends anywhere and that no more outlets are set in one circuit than
b proper and that all are accessible. Observe whether due provision
has been made for installing fuses wherever there will be a change in
the size of wire. It is often cheaper and more satisfactory to continue
a somewhat larger wire than is required, than to change to a smaller
size and insert fuses. In no case should fuses be installed in small
outlet boxes, but only in junction boxes or cabinets of ample size.
After the wires have been drawn in, observe whether it is pos-
sible to trace a given pair from outlet to outlet and if there is doubt
\ /
Fig. 122. nttinfs Used [or EipoKd CoDduit Work
on any point it may be necessary to demand that certain wires be
drawn out for inspection. There should, of course, be absolutely
no comers, Y-shaped fittings, or other devices in any place which
will prevent any or all wires from being taken out and replaced at
any time even after the building is completed. It is weU to observe
whether there are bends or traps in the piping in which water result-
ing from condensation or otherwise can collect. All conduit should
be practically uninjured by the work done on it, all pipe ends should
be threaded and reamed and the entire metal system should be firmly
supported and secured so as to be free from vibration or rattle. The
system must be electrically continuous throughout and must be
permanently and effectively grounded.
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UNDERWRITERS' REQUIREMENTS 121
Where conduits are run exposed, more attention should be paid
to appearances. All runs should be in straight lines parallel to walls
of the rooms and arranged in a symmetrical and orderly manner.
Smaller sizes of conduit can usually be fastened to ceilings or walls
. by ordinary pipe straps, but where large conduit b used some form
of pipe hanger b required and there are special patterns of a variety
of designs. Where a number of conduits are run parallel the form
of supports shown in Rg. 121 is excellent. With exposed conduit a
variety of fittings can be used for outlets, Junction boxes, and the
like, which add much to the neat appearance and also provide for
joints and traps as well as for mounting sockets, switches, and other
small devices. Fig. 122 shows a few such fittings for exposed conduit
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UNDERWRITERS' REQUIREMENTS
i made with threaded nipples into which the conduit may
be screwed and when so used these fit-
tings need not be fastened to the wall
or ceiling in anyway, but are considered
to be firmly enough held by the pipe
itself. However, if boxes requiring lock
nuts and screw bushings on pipes are
employed they must be separately se-
cured to the surface of wall or ceiling
by screws and all cabinets of any size
whatever should be so fastened. Ex-
posed conduit work is usually employed
in factories, warehouses and elsewhere
if the finish of room does not require all
piping to be out of sight.
Some special problems arise in con-
duit work in fireproof buildings. In
these tj-pes of structure the conduit
work is done after the rough floor work
is completed, the conduits being usually
laid on the tile floors so as to be covered
by the top finish of cement usually laid
over the tiling. Fig. 123 shows such
an installation and it will be noted
that the baseboard and all side-wall
outlets have been placed, before the
tile partitions between the rooms, have
been set. In a fireproof building it will
generally be necessary to run the main
risers before the floors are made and to
arrange for the distributing cabinets on
each floor. Fig. 124 shows a cabinet
in place in the unfinished partition
with two conduit risers entering it from
below and the branch circuits going
out from the top to various parts of the
building. The cabinet in this case has
« Dutributina Cabinet been fastened by rods above and below.
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UNDERWRITERS' REQUIREMENTS 123
In concrete buildings it is often somewhat difficult to place
the conduits and the boxes so as to clear the reinforcing materia
and at the same time keep the metal parts weQ covered at aU points.
Special methods have to be adopted varying with the type of con-
crete construction employed but in all cases an effort should be made
to secure proper draining of conduits. The water from the concrete
Fie- 12&. Method of Placioc Outlet Born in THe PBrtiliooB
is very liable to get into the conduit system at the outlets or else-
where, and when once this occurs it is difficult to get it out. Its
presence b, however, most objectionable as it tends to injure the
piping by rusting and also to deteriorate seriously the insulation
coverings on the wires. This is a difficulty which should be considered
in locating outlets in such a building and which reqiures very great
care to entirely overcome. It is a matter of some difficulty at times
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124 UNDERWRITERS' REQUIREMENTS
to secure outlet boxes on the concrete forms or in correct position
where tile is to be used so as to be sure that they will be ia the right
position and firmly held when the building is completed. Ceiling
boxes should be held by special hangers from each box up into the
tile or concrete and reliance should not be placed on the conduits
alone to hold them. Boxes set where tile partitions are to be sub-
sequently erected should preferably be held by iron straps to adjacent
steel work wherever this is practicable. Fig. 125 shows how this
may be done.
SPECIAL INSTALLATIONS
DECORATIVE AND COMMERCIAL LIOHTINa
Decorative Lighting. Decorative lighting by means of incan-
descent lamps is often desired either inside or outside buildings. The
chief hazards of such work he in the use of inferior materiab hastily
put together and poorly located and fused. The voltage should never
exceed 150 volts in such work and not more than 1,320 watts of
energy should ever be allowed to be dependent for protection upon
a single cut-out. It is highly improper to take current for such sys-
tems from ordinary outlets which are wired for only small currents,
since by so doing the wires to the outlets will be overloaded and the
proper fuses for the wires will have to be replaced by others of too
great capacity to furnish safe protection. The supply should be
taken only from points on the circuit (as at distribution boards or
paneb) where the correct fusing and wiring can be provided for.
There is obviously no real distinction between decoroHve lighting
and ordinary lighting arrangements except that the former is usually
put up for temporary use only. If these temporary circuits are in-
stalled in a workmanlike manner and if good wire and good sockets
are used there need be no special hazard provided the wires are not
tacked up in a manner liable to injure the insulations and are not
made to serve as supports for inflammable decorations of cloth ot
paper. Festoons of keyless weatherproof sockets well connected to
standard rubber-covered wire (not flexible cord), and kept separated
from all combustible stuff, do not present any great hazard but no such
installation should be made without great care and should never be
allowed to remain as a permanent installation unless it is fully pro-
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UNDERWRITERS' REQUIREMENTS 125
tected and made the equivalent of standard work in every respect.
The use of extra "carrier" wires for such festoons is very desirable
as they remove all strain from the current-carrying wires and coa-
nections. Show windows are very frequently decorated by tem-
porary electric displays of all sorts especially during the holiday sea-
son and such window displays are very often wired in utter disregard
of all safe rules. The wires are often made to serve as supports for
merchandise of very inflammable material, and where tinsel, cotton
batting, and similar materials are employed, the conditions are ex-
cellent for a rapidly-spreading fire if any electric failure should
occur. Incandescent lamps are often allowed to lie against inflam-
mable materials and small motor-operated advertising or other nov-
elties may contribute their share to the danger. The fact that an
installation of this sort is "temporary" does not lessen the hazard
so long as it b in operation and should not in any sense be considered
as an excuse for allowing practices which are known to he danger-
ous. Property owners are often ignorant of the extent to which
ill-advised temporary decorations jeopardize their biuldings.
In ^ow-window lighting, the lamps, whether temporary or
permanent, should be placed either in the front or the top of the
window, proper reflectors being used to throw the light upon the
goods displayed. This plan almost invariably gives better results
in lighting than lamps placed in sight, and scattered through the
window and among the goods, and is also very much safer.
Oviline Lighting. A very common form of decorative work con-
sbts of what is known as ovtliTie lighting. In this class of work rows
of incandescent lamps are used on the exterior of buildings to outline
the chief architectural features, to mark entrances, etc. Lamps
and wiring used for such purposes inside buildings come under the
regular rules for inside work but outside outline lighting may have
some special characteristics. Only low potential circuits (under 550
volts) should be used and the wiring either open work or in conduit.
Molding, either wood or metal, is not allowable since it does not
afford protection against moisture. In open work the same spacing
of wires from the surface (1 inch) and from each other (2| inches)
must be maintained as in all exposed wiring in damp places. If the
use of flexible tubing is necessary, as at crosses or where regular
spacing cannot be maintained, the ends of the tubing must be sealed
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126 UNDERWRITERS' REQUIREMENTS
and painted so as to exclude moisture, and for a similar reason
armored cable, if used, must be of the type having a lead sheath over
the conductors and under the armor.
In order to assure proper control and fusing of outline-wiring
circuits they should have their own separate switch and fuses enclosed
in a suitable metal cabinet which must be watertight if placed out-
of-doors. Small candle-power lamps (2 to 4} are almost always used
for outline wiring and may be so grouped on the circuits that not
more than 1,320 watts will be dependent on a single pair of fuses
but in no case should more than 66 sockets or receptacles be con-
nected to a single circuit. This limitation of the number of lamps
is to prevent any trouble in a single socket causing too much arcing
or burning before the fuse protecting its circuit will open. All the
sockets and receptacles in outline wiring must be of the keyless
porcelain type and the wires connecting them must be soldered to
the lugs. With conduit work in outline wiring special attention
should be given to making the entire system as water tight as pos-
sible. The receptacles set in the covers of steel outlet boxes or
conduits should be provided with rubber gaskets, and similar rubber
rings may also be used to advantage about the bases of the lamps to
prevent the entrance of water into the receptacles themselves.
Electric Signs, An endless variety of electric signs are shown
on the streets of cities and towns today, from the simplest illuminated
panels to huge structures containing many hundreds of lamps and
operated by very complicated "flashing" machines. An elaborate
treatment of all types of signs cannot be given here. The large
signs erected on roofs should come under the supervision of the
building department of a city or under both electric and building
departments. Many of these signs are hazardous because of the
obstruction they offer to firemen, the large amount of power they
use, and the inadequate provision made for cutting off current from
them ^ther automatically or by hand switches in accessible places.
No combustible material should be permitted either in or near such
signs. It is preferable that all wiring be in conduit but where this
is impracticable, the supports and spacing of wires should be excellent
and the sides of the circuits should be kept apart preferably by
bunching all wires of one polarity into cables covered by waterproof
and slow-burning jackets. The mass of inflammable rubber-covered
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UNDERWRITERS' REQUIREMENTS 127
and braided wire necessary for a large sign requires skilful arrange-
ment to avoid a serious blaze in case of failure.
Signs of every size should be "all metal," sheet steel being not
less than No. 28 U. S. gauge and frames being in all cases of ample
stiffness and strength. Metal parts should be well protected by
paint or enamel and the sign structure should be reasonably water
tight but have drainage holes in the bottom to let out any water
that may collect inside. The wires should be soldered to all recep-
tacle terminals, should be only rubber-covered and should be brought
out through the sign structure either in conduit or in well spaced
porcelun bushings with drip loops. In supporting the sign attention
should be paid to preventing wear or abrasion of the leading-in wires
by swinging of the sign. Many cities have especially elaborate rules
for the placing and supporting of signs designed to reduce the pos-
sibility of their falling or of their interfering too much with the use
of fire-escapes or the wta-k of firemen in case of fire in the building.
Sign Flaahera. The flashing, and other very elaborate effects of
electric ugns, are accomplished by the use of what b known as sign-
AAA iU ^if^^l?l^?f^^!w ^^^^^^^^^mm\_
Fi(. 12e. Sign Fluher for Diiplayina Electric Signi
machines or flashers, Fig. 126, whidi consbt essentially of motor-
driven drum switches often of great size and complexity. These
machines are hazardous chiefly from their use of many necessarily
rather frail single-pole brush switches with large numbers of connect-
ing wires leading to them and from the fact that being essentially
automatic, such machines are liable to be neglected and poorly
maintained. The location of a sign machine should be such as to
remove it from the chance of accidental injury or tampering, to
permit the direct and well arranged running of circuits to it, and
to render it accessible for cleaning, adjusting, and inspecting at all
times. A flasher should never be installed in a closet or other con-
cealed space. If it b impracticable to mount it outside the build-
ing near the sign, as b preferable, a location should be chosen that
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128 UNDERWRITERS' REQUIREMENTS
will reduce as tsr as possible the amount of wiring required. A plat-
form or shelf over a doorway and just inside the building b often
the best place available. A heavy metal cover must be provided
but thb should be removable so as to permit adjustment of all operat-
ing parts within.
THEATER WIRINQ
Qeneral Specifications. The electric equipment of theaters ia
of great importance both because of the large value of these properties
and also because of the life hazard involved in any public building
in which large numbers of people assemble. The electric equipment
of a modem theater of the first class is generally voy elaborate,
involving as it does not only the brilliant illuminatioQ of lai^ rooms,
the stage, and many smaller apartments, but also the power equip-
ment for handling curtains, scenery, and other paraphernalia and
the numerous devices of special character for producing stage effects.
In the meaning of the Code a theater is defined as a building or
part of a building in which it is designed to make a presentation
of dramatic, operatic or other performances or shows for the ent^-
tainment of spectators, which is capable of seating at least 400
persons, and which has a stage for such performances that can be
used for scenery and other appliances. All theater wiring not
specifically covered by special rules should conform to standard
rules and requirements for work of its class. The special rules
naturally divide into those which concern the protection of life
chiefly and those which concern primarily the fire hazard, though,
of course, the latter have a direct bearing on the life hazard also,
since even a relatively small fire in a theater may produce a panic
resulting in injury to many persons.
For the purpose of insuring the most reliable maintenance of
proper lighting and consequent safety and freedom from panic in the
audience, the underwriters' rules suggest that where the source of
electric supply b outside the building there must be at least two
separate and dbtinct services where practicable, fed from separate
street mains, one service to be of sufficient capacity to supply current
for the entire equipment of the theater, while the other service
must be at least of sufficient capacity to supply current for all
emergency lights. By "emergency lights" are meant exit tights
and all lights in lobbies, stairways, corridors, and otha* portions o£
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UNDERWRITERS' REQUIREMENTS 129
the theater to which the public have access which are kept normally
lighted during the performance. Where source of supply is an
isolated plant within the same building, an auxiliary service of at
least sufficient capacity to supply all emergency lights must be in-
stalled from some outside source, or a suitable storage battery within
the premises may be considered the equivalent of such service.
The principal hazards in a theater result from the stage equip-
ment since it ia there rather than in the auditorium that large cur-
rents are used; many conductors are required, and all sorts of special
devices are employed often in close proximity to combustible material
such as scen^y. Furthermore, the constant changes, additions and
special requirements for different productions result in subjecting
the electric equipment to exceptionally hard wear and tear on theater
stages so that proper upkeep can be secured only by the use of the
most approved materials and methods and constant re-inspections.
All permanent electric work on the stage side of the proscenium
arch must be in conduit or armored cable except in border lights
and on the stage switchboards.
All of the stage circuits, footUghts, borders, arc lamps, and
usually all house lifting are controlled from a switchboard located
on the stage at one side of the proscenium arch. This results in a
very large number of cables and wires being concentrated at this
board and calls for very careful planning if anything like safe and ■
orderly arrangement is to be secured. The space available is often
small and circuits are, therefore, liable to be unduly crowded and
confused. The design of a large stage switchboard calls for no tittle
en^neering skill, which is too often noticeable chiefiy from its entire
absence especially in older houses. The space back of the board
should be of ample size, readily accessible but entirely enclosed by
absolutely fireproof walls and doors and preferably directly ven-
tilated to the outside by a special brick chimney or large flue. All
circuits should be controlled by substantial knife switches and fused
by standard enclosed cut-outs in accessible places where their operar
tion will not in any way be liable to set fire to the wire insulations.
It is very desirable that all wires on the back of the board be covered
with asbestos jackets and be well spaced.
Dimmers. Tlie dimmers ot special rheostats for gradually
reducing the intensity of incandescent lamps either in the auditoriiun
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130 UNDERWRITERS' REQUIREMENTS
or on the stage constitute often a very large installation of heat-
developing apparatus which should be carefully located on iron
brackets or on open galleries absolutely separated from any possible
contact with burnable stuff. Fig. 127 shows a bank of dimmers.
Footlights and Borders. Among the more important fixed-
appliances are the footlights, border and strip lights, and the siage-
flooT poclds. The footlights should be installed either in conduit or
on special steel boxes which entirely enclose the lamp receptacles.
Border lights must be substantially made of hea^'y sheet metal (at
least No. 20 metal gauge) and must be wired with slow-burning wire
because they necessarily get very hot from the lamps they contain.
Footlights, on the other hand, do not get so hot and their location
makes them susceptible to moisture during the cleaning or washing of
the floor of the stage. The;' should, therefore, be wired with rub-
ber-covered wire.
Border lights must be raised and lowered and, therefore, current
must be carried to them by means of cables composed of stranded
rubber-covered wires. Such cables should be in rigid conduit to the.
point where the cable leaves for the border and at this point a suit-
able junction box should be providc-d. The method of bringing the
cable from this box and also of leading it into the border structure
should receive special attention to prevent undue strain being brought
to bear upon it at these connections. The cable should be of the
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UNDERWRITERS' REQUIREMENTS 131
special type designed for this service. I'igs. 128 and 129 show sec-
tions of footlights and borders ready for installation on a stage.
Stage Pockets. Stage pockets (see Fig. 130) are iron boxes,
having trap doors flush with the stage 0oor and containing receptacles
for special plugs and cables by means of which arc lamps, flood lamps,
and other temporary and portable devices maj- be connected. The
circuits should have a capacity of
' at least 35 amperes for arc lamps
or 15 amperes for incandescent
lamps, wired to the full capacity
and controlled from the stage
board. They should contain no
fuses and should be so constructed
that accumulations of dust or rub-
bish in them cannot interfere with
ng, 130. sttsc Pocket their safe use or be liable to cause
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132 UNDERWRITERS' REQUIREMENTS
a fire from any arc resulting from withdrawing the plug. These
pockets may be for one or several plugs.
Portable equipments for use on theater stages include, in addition
to special devices of all sorts — arc lamps, bunch lamps, strip lights,
pin plug connectors, and portable
plugging boxes. Fig. 131 shows a
poor form of stage arc lamp of
the type supposed to have caused
the Iroquois theater fire in Chi-
cago, while Fig, 132 shows a
Pl(. 131. NoD-ApproTpd Form of Stsge Fig. 132. Approved Form ot BUce
Arc I-Kiap Aio Lunp
modern approved form. These should be of approved pattern,
made of non-combustible materials only and fitted with standard
stage cable the specifications for which are as follows: To consist
of not more than three flexible copper conductors each of capacity
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UNDERWRITERS' REQUIREMENTS 133
not exceediDg No. 4 B. & S. g&uge, built up of wires not smaller
than No. 26 B. & S. gauge; each conductor to be covered with a
cotton wind and with rubber insulation of thickness equivalent to
standard rubber-covered wire and a saturated braid; the conductors
to be twisted together with a filler (jute or a similar material) to
make the cable round and to act as a cushion; the whole to be
covered with two weatherproof braids. Such a cable will withstand
considerable hard usage and has sufficient mechanical strength to
endure the severe conditions of this service.
Special Lighting Circuits and Stage Effects. The proper
installation of all of the stage devices just mentioned is such an
imimrtant feature of the safety of the theater that special rules are
established for portable conductors, for lights on scenery, for fes-
toons, and for the many special effects, such as lightning, etc.
Requirements for Stage Auditoriums. The special requirements
for the lighting of stage auditoriums are as follows : All wiring must
be in either conduit, metal moldings, or armored cable; all fuses
must be in enclosed cabinets; exit lights must not have more than
one set of fuses between them and the mwn service fuses, land
together with all hall and corridor lights, must be supplied inde-
pendently of the stage lighting and be controlled only from the lobby
or front of the house. All of these requirements are made to insure
adequate lighting for the audience in case of a fire, to render the
exit and corridor lights independent of any accident to circuits re-
sulting from a stage fire, and to insure a certain amount of illumi-
nation at all times in order to permit firemen to work to advantage
in case of a fire in the auditorium or corridors leading thereto.
MOVINa PICTURE THEATERS AND MACHINES
Interior Equipment. The very large number of moving-picture
theaters which have come into use in the past few years has intro-
duced a special hazard of considerable extent and a large number of
disastrous fires in such installations have bfeen recorded. See Fig.
133. These theaters have often been established in rooms originally
intended for stores and their equipment has been of the poorest
character. This condition is now somewhat improved as the danger
has become more generally recognized and has been made the sub-
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134 UNDERWRITERS' REQUIREMENTS
ject of special legislation both by insurance interests and by municipal
and town ordinances.
Causes of Danger. The lighting of such theaters comes under
the ordinary rules for electrical work but the peculiar danger lies
in the use of the arc lamp projection machines with the highly in-
flammable "films" on which the pictures are printed. These films
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UNDERWRITERS' REQUIREMENTS 135
are of cellulose coated with a chemical which burns with great
intensity and rapidity and with an almost explosive force. A burn-
ing film cannot be extinguished easily and evolves an enormous
amount of heat and a dense and suffocating smolie. In displaying
the moving pictures the film (usually 500 to 1,500 feet in length) is
caused to pass rapidly in front of a special arc lamp taking from 25
to 50 amperes at 1 10 volts, the light and also the heat of the arc being
concentrated on a small area of the film about as large as a postage
stamp by means of lenses set in the head of the nuichine. \VIule
the machine is in normal operation, the ribbon of film is moved so
rapidly that no portion of it has time to become heated. If, however,
for any cause, the advance of the film b checked so that a portion of
it remains for even a few seconds exposed to the concentrated heat
of the arc, it is at once ignited and the Same spreads to the adjacent
parts of the film and the whole reel b liable to bum up with a very
intense fire which may endanger the life of the operator and create
a serious fire hazard to the building. The film may also be ignited
by coming into contact with the hot lamp housing or with the resbt-
ance box which it b usually necessarj' to use in the arc lamp circuit
to regulate the current. \Mien alternating current b used special
forms of transformers or coils may be used instead of the resbtance
box or rheostat and these are to be preferred since they do not get
so hot.
The arc lamp and rheostats should be constructed similarly to
arc lamps and rheostats used on theater stages. Tight metal boxes
made without solder must be used both to hold the reel from which
the film is unwound in order to expose to view, and also to receive
the film after it has passed through the machine head, and the open-
ings in these boxes must be as sqiall as possible, being regulated by
rollers between which the film may pass or by shutters which can
be instantly closed. The handle or crank used in operating the
machine must be secured to the spindle or shaft, so that there wlQ be
no liability of its coming off and allowing the film to stop in front
of the lamp. A shutter must be placed in front of the condenser,
arranged so as to be readily closed. Extra films must be kept in a
metal box with tight-fitting cover. Machines must be operated by
hand and never by motor, since the failure of the motor, the slipping
or breaking of a belt, or other like accident might stop the film. It
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136 UNDERWRITERS' REQUIREMENTS
is essential that the film and the operation of the entire equipment
receive the constant attention of the operator. The picture machine
must be placed in an enclosure or house maue of suitable fireproof
materia), must be thoroughly ventilated, and should be made large
enough for the operator to walk freely on either side of or back of it.
All openings into this booth must be arranged so as to be entirely
closed by doors or shutters constructed of the same or equally good
fire-resbting material as the booth itself. Doors or covers must be
arranged so as to be held normally closed by spring hinges or equiv-
alent devices.
The construction, location, and wiring of this booth are the
chief considerations in the equipment of a moving-picture theater
and no pains should be spared to obtain the highest possible degree
of security in every detail, in its construction and equipment, and
the greatest care in its maintenance.
CAR WIRINO
The electric wiring of cars requires some special rules which
make certain exceptions to the general rules on the one hand and
which make some special extra requirements on the other hand.
These special rules have to do chiefly with the protection of car
bodies and woodwork over all the electrical apparatus such as motors,
resistances, contactors, and the like, and over such of the conductors
as are not run in conduit. Other somewhat special requirements
apply to wires, cables, and methods of making joints and connections
in them, to the location and type of fuses and circuit breakers to be
used, to special forms of conduit and wood molding, and to details
of the lighting, heating, and air-pump circuits. Very carefully detailed
requirements have been worked out for the main motor circuits
and for the devices used in connection with them. As all of these
requirements are of a very special character, the underwriters' rules
in this connection should be studied in detail by those directly
interested in car wiring.
Car houses or barns present some hazards which can be reduced
by the observance of a few special rules which are determined chiefly
by the fact that the railway circuit is normally grounded and thus
requires somewhat different treatment from the ordinary under-
ground light or power circuits,
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UNDERWRITERS' REQUIREMENTS 137
LIQHTINO AND POWER FROM RAILWAY WIRES
It should be especially not«d that under no circumstances is it
propCT to take lighting or power circuits from trolley or third-rail
nulway circuits with a ground return. This, of course, does not and
cannot apply to the electric railway cars, car houses, power, passenger
and freight houses connected with the operation of electric railwaj-a
in which the use of such circuits is obviously necessary and which
can be specially guarded and supervised. For all other factories or
buildmgs of aU sorts, power from grounded railway circuits is for-
bidden becaxise of the danger of introducing into a building a circuit
which has so much capacity back of it and which is thoroughly con^
nected with the earth on one side. The inevitable fluctuation in
voltage would also frequently require overfusing of the lighting
circuits to prevent blowing fuses under normal conditions.
HIQH- AND EXTRA HIQH- POTENTIAL SYSTEMS
Classification. Under low, constant-potential systems are in-
cluded all such as have voltages not over 550 volts but it should
be noted that this distinction b an arbitrary one adopted for the
classification of the rules on electrical work in relation to the fire
hazard and is not made in just this way in classifying power-trans-
mission lines and conmiercial systems from other viewpoints. In
the underwriters' rules any circuit attached to any machine, or
combination of machines, which develops a difference of potential,
between any two wires, of over 550 volts and less than 3,500 volts,
shall be considered as a high-potential circuit and as coming under
that class, unless an approved transforming device is used, which
cuts the difference of potential down to 550 volts or less. Similarly
if the difference of potential between any two wires is over 3,500
volts the circuit is classed as an extra high-potential circuit unless
an approved transforming device is used. This means that where
the power is brought to a transformer on primary lines of not over
3,500 volts and the secondary lines are not over 550 volts, the sec-
ondary system of light or power is considered a low-potential system,
but if the primary wires of 550 to 3,500 volts are direct to the power-
consuming devices, as motors, without the outside transformer,
the system is considered a high-potential system. If the primary
lines are over 3,500 volts, the secondary lines beyond the transformers
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138 UNDERWRITERS' REQUIREMENTS
are classed as high-potential and wh^e no transformer b employed
the circuit is extra high-potential.
High- and extra high-potential systems are, of com^e, usually
alternating-current circuits since only alternating current is ordi-
narily used for over 600 volts. There are, however, a number of
railway installations in the United States operating at 1,200 volts
direct current and this type of power distribution is becoming some-
what conamon. A 1,200-volt circuit b, of course, classed as high-
potential. Use is made of high-potential circuits for both light and
power purposes and its advantages over low voltages lie in the smaller
sized copper that can be used to transmit a given amount of power
over a certain distance with a specified percentage of loss due to the
line wires.
Requirements for Safety. For lighting, 2,200- to 2,500-volt
circuits are used to transmit the power from the generating station
to standard transformers at or near buildings where the li^t is used
and these transformers step down the voltage either to 110 or 220
volts or to a three-wire 110- to 220-volt system on the secondary
side for the lamps. A very few cities and towns employ a 220- to
440-volt three-wire secondary system but this is not usual or de-
sirable in general, as most fittings, fuses, etc., are designed for not
over 250 volts.
Motors designed for alternating current may, of course, be
operated on these secondaries and such secondary circuits whether
for light or power are simply low-potential circuits provided the
higher voltage lines end at transformers suitably installed outside
the buildings or as near as possible to the point where the primary
wires enter the buildings. The outside location is much to be pre-
ferred,
\Vhen circuits having high-potential transformers are located
inside of buildings they should be placed in an enclosure made of
fire-resisting material such as brick, tile, or concrete. The enclosure
should be used for nothing but the transformers and should be kept
locked. Fig. 134 shows such an enclosure or vault for transformers.
It is a good plan to arrange the transformer room or enclosure so that
it can be entered only from outdoors, since then, even if the door
should happen to be open at the time of a fire in this room, it is
probable that no especial harm would be done. Moreover, the fire
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UNDERWRITERS' REQUIREMENTS 139
could doubtless be better handled from the outside. The trans-
formers must be thoroughly insulated from the ground, or per-
manently and effectually grounded, and the enclosure in which they
are placed must be practically air-tight, except that it must be
Fig. 134 • Approved Melhod of Protecting Tr»nsfornieni in s Vault
thoroughly ventilated to the outdoor air, if possible through a. chim-
ney or flue. There should be at least six inches of air space on all
sides of the transformer. This rule will permit of either the insulating
or grounding of transformer cases as seems most advisable under the
conditions, but will require that with either arrangement the work
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140 UNDERWRITERS' REQUIREMENTS
be well done, and that unless good insulation be provided the cases
be definitely grounded. The object of an air-tight enclosure is to
prevent smoke ftvm escaping or fire from spreading. In case the
transformer coils should become overheated from an overload or
should be ignited by a break-down in the insulation between the
primary and secondary coils. This is especially important with oil-
cooled transformers in which the danger from an oil-fire is added to
the usual electrical hazards involved.
Wherever high-potential circuits are brought into a builditag
only rubber-covered wire should be used and special care should be
taken with the wire supports to protect the wires from mechanical
injury since a failure resulting in an arc is very dangerous at this
high voltage. Substantial boxing about wires on side walb and
running boards where circuits cross floor timbers are very necessaiy,
as, in fact, are all the precautions and rules for good wiring and good
workmanship throughout. A very large number of motors are now
used taking 2^00 to 2,500 volts at the motor terminals. Such power
installations can be made reasonably safe by careful planning of the
drcuits and by excellence of installation and upkeep of all wirijig
and connected apparatus. No multiple-series or series-multiple
system of lighting is allowed on high potential circuits.
Extra high-potential circuits are not allowed either in or oyer
buildings except power stations and substations. Where extra
high-potential primary circuits (over 3,500 volts) supply transformers
the secondary wiring must be installed as high-potential circuits
(550 to 3,500 volts) unless the primaries are installed in complete
compliance with the rules governing outside work on constant-
potential pole lines over 5,000 volts or are wholly underground
within city, town, and village limits.
In concluding the consideration of these higher voltage cir-
cuits, it is proper to refer again to the fact that the higher the volt-
age employed the greater the need of care in installation, upkeep, and
operation since the results of accidental arcs are more serious.
SIONALINQ SYSTEMS
Wiring Requirements. The wiring and other devices in the
great variety of signal systems now employed in buildings, present
as a general rule no hazard except their liability to become crossed
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UNDERWRITERS' REQUIREMENTS 141
either outside or inside buildings with electric light, heat, or power
circuits. Such signal systems include telephone, telegraph, district
messenger and call bell, fire and burglar alarm, and all simitar ap-
pliances and circuits. It is seldom that the wires of any of these
systems are installed in buildings with the same care as are those
ioT lighting or power, and the insulations employed on signal wires
are very generally far inferior to those specified for light and power
circuits. Furthermore, since signal systems are usually operated
from either primary or secondary (storage) batteries of low voltage
and limited current capacity, they may be and commonly are in-
stalled with little attention to separation of the wires, either from
each other, or from the surfaces, walls, and floors to which they are
attached.' It follows, therefore, that all care should be taken to
prevent light and power wires carrying currents of large capacity and
?elatively high voltages from coming in contact with signal wires
aince in such event dangerous fires might very readily be caused.
The same advantages in having wires underground instead of being
.phiced on polea apply to signal as vdl as to light and power circuits,
but the two classes should never occupy the same imderground
duct, manhole, or handhole even when cables are used, since in the
mechanical work or repairs on the lines an injury resulting in a cross
between the systems might cause a dangerous current from the
higher voltage lines, to outer buildingSj over the weakly insulated
and poorly protected signal wires. The liabiUty of accidental cross-
ing of overhead signaling circuits with electric light and power cir-
cuits may be guarded against to a considerable extent by endeavor-
ing to keep the two classes of circuits on different sides of the same
street. He Code prescribes that signal wires on pole lines also
carrying electric U^t or power wires shall generally be placed on
the lower cross-arms. This arrangement is not, however, favored
by many engineers or by all municipal authorities, who prefer that
signal wires be put on the top arms or on extensions above the
tops of the poles. The arguments in favor of putting the signal
wires lowest on the poles are that they are by many judged to be
more liable to break and fall, especially when loaded with sleet,
because of their lesser size and strength; also in case of city fire alarm
circuits which are, of course, very important, the lower position
enables linemen making repurs to get at the fire-alarm wires without
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142 UNDERWRITERS' REQUIREMENTS
passing up through the light and power wirea which may be chwged
with dangerous voltages.
The arguments in favor of putting signal wires at the top are —
they are small and, therefore, collect less ice or sleet and so are less
liable to break; the light and power wires are often better Insulated
and a signal wire breaking is less liable to make a real live contact
with them; the fall of a heavy power cable may wreck all of the
^gnal wires if the latt«r are below; in the case of city fire-alarm
drcuits the upper location removes them from misuse and injury
when wiremen are working on the other lines. However, it may be
said that there is no general agreemeni either in theory or practice
on this subject.
Single wires of signal circuits on the outside of buildings should '
have rubber insulation and where attached to frame buildings should
be secured toglass or porcelain insulators or knobs. Only copper
wire should be used for the span from the last pole to the build-
ing and the wires should pass through outside walb through in-
sulating bushings and have drip loops the same as electric light «
wires. Inside of buildings neat arrangement and secure fastening
of all wires is essential to keep them properly placed and no signal
wire should come nearer than three inches to any light or power wire,
unless separated therefrom by some continuous and firmly fixed
non-conductor creating a permanent separation, this non-conductor
to be in addition to the regular insulation on the wire as the wires
would ordinarily be insulated, but the kind of insulation is not
specified, as the protector described below is relied upon to stop all
dangerous currents. Porcelain tubing, approved flexible tubing, or
rigid conduit may be used for encasing wires where required as above.
Wires where bunched together in a vertical run within any building
should have a fire-resisting covering sufficient to prevent them from
carrying fire from floor to floor unless they are run either in non-
combustible tubing or in a fireproof shaft, which shaft should be
provided with fire stops at each floor.
Signaling wires and electric light or power wires may be run in
the same shaft, provided that one of these clasiies of wires Is run in
non-combustible tubing, otherwise the two classes of wires should be
separated from each other by at least two inches. In no case should
signaling wires be run In the same tube with light or power wires.
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UNDERWRITERS' REQUIREMENTS 143
Protecting Devices. In signal installations vhere the current-
carrying parts of the apparatus installed are capable of carrying
indefinitely without overheating a current of ten amperes (as in
some telegraph or special systems) the inside wires should be of
copper at least as large as No. 16 B. & S. gauge and must have the
same insulation and be supported the same as electric light or power
wires for 600 volts. At the entrance to the building each wire should
be protected by a 10-ampere 600-voIt fuse. Such signal circuits as
the above are much less common than those not suited for a 10-
ampere current. Telephone, district messenger, private watchmen's
time recorders, burglar alarms, and fire-alarm circuits, are never
capable of carrying 10 amperes continuously and for these a special
"protector" is required located as close as possible to the entrance of
■ the building. The purpose of this protector is to prevent any foreign
current or any lightning discharge from entering the building over
the signal wires. For telegraph circuits this protector takes the form
of a 2,000-volt fuse in each wire. The commoner "protector" such
as is used on telephone lines should have the following parts mounted
on a porcelain or slate base on which all parts are well insulated: a
lightning arrester which will operate at 500 volts or more from a
ground wire not less than No. 18 B, & S, gauge; a fuse in each side of
the circuit which will blow with small currents (J to 8 amperes) and
will operate well on the voltages likely to reach the protector in case
of accident. ^Tiere very sensitive instruments are in the circuit,
such as contain magnet windings which are easily overheated, there
must be a heat coil on each side of the line. The heat coil is designed
to warm up and melt out with a current large enough to endanger
the instruments if continued for a long time, but so small that it
would not blow the fuses ordinarily found necessary for such instru-
ments. The smaller currents are often called aneak currents. On
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144 UNDERWRITERS' REQUIREMENTS
those telephone circuits which are supplied with current entirely
from the central telephone headquarters sneak or heat coils are not
Fig. ISe. Common Form of TelepbOH Pnit«(or
necessary. The fuses must be so placed as to protect the arrester
and heat coils, and the protector terminab must be plainly marked
litte, instrumejit, or ground.
The relative arrangement of parts is shown in diagram in Fig.
135 and a common form of protector in Fig. 136.
TESTING
Where possible, two tests of the electric wiring equipment should
be made, one after the wiring itself is entirely completed, and switches,
cut-out panels, etc., are connected; and another one after the
fixtures have all been installed. The reason for this is that if a ground
or short-circuit is discovered before the fixtures are installed, it is
more easily remedied; and also, because there is no division of
the responsibility, as there might be if the first test were made only
after the fixtures were installed. If the test shows no grounds or
short-circuits before the fixtures are installed, and one does develop
after they are installed, the trouble, of course, b that the short-circuit
or ground is one or more of the fixtures. As a matter of fact, it is a
wise plan always to make a separate test of each fixture after it is .
delivered at the building and before it is installed.
While a magneto is largely used for the purpose of tesUng, it is
at best a crude and unreliable method. In the first place, it does
not give an indication, even approximately, of the total insulation
resistance, but merely indicates whether or not there is a ground or
short-circuit. In some instances, moreover, a magneto test has
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UNDERWRITERS' REQUIREMENTS 145
led to .serious ertorSj for reasons that will be explained. If,- as is
nearly always the case, the magneto b an alternating-current instrur
ment, it may sometimes happen — particularly in long cables, and'
especially where thoe is a lead sheathing on the cable — that the
magneto will ring, indicating to the uninitiated that there is a ground
or short-circuit on the cable. This may be, and usually is, far from
being the case; and the cause of the ringing of the magneto is not a
ground or short-drcuit, but is due to the capadty of the cable, which
acts as a condenso' under certain conditions, since the magneto pro-
ducing an alternating current repeatedly charges and discharges the
cable in opposite directions, this changing of the current causing the
magneto to ring. Of course, this defect in a magneto could be
remedied by using a commutator and changing it to a direct-current
machine; but as the method is faulty in itself, it is hardly worth while
to do this.
A portable gtdBatumeter with a resbtance box and Wheatstone
bridge, b sometimes employed; but thb method b objectionable
because it requires a special instrument which cannot be used for
many other purposes. Furthermore, it requires more skill and time
to use than the votirtieier method, which will now be described.
Voltmeter Method. The advantage of the voltmeter method
b that it requhes merely a direct-current voltmeter, which can be
used for many other purposes, and which all engineers or contractors
should possess, together with a box of celb having a potential of
preferably over 30 volts. The voltmeter should have a scale of not
over 150 volts, for the reason that if the scale on which the battery
b used covers too wide a range (say 1,000 volts) the readings might
be so small as to make the test inaccurate. A good arrangement would
be to have a voltmeter having two scales — say, one of 60 and one of
600 — which would make the voltmetw available for all practical
potentiab that are likely to be used inside of a building. If desired,
a voltmeter could be obtained with three connections having three
scales, the lowest scale of which would be used for testing insulation
resistances.
Before starting a test, all of the fuses should be inserted and
switches turned on, so that the complete test of the entire installation
can be made. When thb has been done, the voltmeter and batteiy
should be connected, so as to obtain on the lowest scale of the volt-
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146
UNDERWRITERS' REQUIREMENTS
meter the electromotive force of the entire group of cells. This
comiection is shown in Fig. 137. Immediately after this has been
done, the insulation resistance
to be tested b placed in dr-
cuit, whether the insulation to
be tested is a switchboard, slate
panel-board, or the entire wiring
installation; and the connections
are made as shown in Fig. 138.
A reading is again taken of the
voltmeter and the leakage is
thus obtained, as it is in pro-
portion to the difference be-
tween the first and second voltmeter readings. The explanation
given below will show how thb resistance may be calculated. It is
evident that the resistance in the first case was merely the resbtance
of the voltmeter and the internal resistance of the battery. As a
rule, the internal resistance of the battery is so small in comparison
with the resistance of the voltmeter and the external resistance, that
it may be entirely neglected, and this will be done in the following
calculation. In the second case, however, the total resistance in cir-
cuit b the resbtance of the voltmeter and the battery, plus the
entire iosuhttion resbtance on all the wires, etc., connected in circuit.
To put thb in mathematical form, the voltage of the cells may
be indicated by the letter E; and the reading of the voltmeter when
Bafore Circuit ii Cotmecl
the insulation resistance b connected by the circuit, by the letter £1'.
Let R represent the resistance of the voltmeter and R^ represent the
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UNDERWRITERS' REQUIREMENTS 147
insulation resistance of the installation which we wish to measure.
It is a fact which the reader undoubtedly knows, that the e. m. f . as
indicated by the voltmetw in Fig. 138 is inversely proportional to the
resistance; that is, the greater the resistance, the lower will be the
reading on the voltmeter, as this reading indicates the leakage or cur-
rent passing through the resistance. Putting this in the shape of a
fomula, we have from the theory of proportion
E :E' :: R+R, : B
or
E'R+E' R,-ER
TVansposing
E' R.~E R-E' R-R (£-£")
and
Or, expressed in words, the insulation resistance is equal to the resist-
ance of the voltmeter muliiplied by the difference between the first
reading (or the voltage in the cells) and the second reading (or the
leading of the voltmeter with the insulation resistance in series with
the voltmeter), divided by this last reading of the voltmeter.
Exampk. Assume a resistance of a voltmeter R of 20,0(X) ohms,
and a voltage of the cells £ of 30 volts; and suppose that the insula-
tion resistance test of a wiring installation, including switchboard,
feeders, branch circuits, panel-boards, etc., is to be made, the insula-
tion resistance being represented by the letter fl,. By substituting
in the formula '
R{E-E')
"■ y^
and assuming that the readii^ of the voltmeter with the insulation
resistance connected is 5, we have
20,(XI0X(30-5)
fi,-— = ^ =100,000 ohms
If the test shows an excessive amount of leakage, or a ground or
short-circuit, the location of the trouble may be determined by the
process of elimination — ^that is, by cutting out the various feeders
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148 UNDERWRITERS' REQUIREMENTS
until the ground or leakage disappears, and, when the feeder on which
the trouble exists has been located, by following liie same process
with the branch circuits.
Of course, the larger the installation and the longer and more
numerous the circuits, the greater the leakage will be; and the lower
will be the insulation resistance, as there is a greater surface exposed
for leakage. The rules of the Code give a sliding scale for the require-
ments as to insulation resistance, depending upon the amount of
current carried by the various feeders, branch circuits, etc. The rule
of the Code covering this point, is as follows:
The wiring in any building must test free from groundB; i. e., the com-
plete inatallation muet hftve an ineulation between conductors and between
all conductors and the ground (not including attachments, aockete, recepto-
olee, etc.} not leas than that given below:
Up to 5 ftmperee 4,000,000 ohms
Up to 10 amperes 2,000,000 ohms
tip to 25 amperes 800,000 ohms
Up to SO amperes 400,000 ohms
Up to 100 amperes 200,000 ohms
Up to 200 amperes 100,000 ohms
Up to 400 amperes 50,000 ohms
Up to 800 amperes 25,000 ohms
Up to 1,600 amperes 12,500 ohms
The teat must be made with all cut-outs and safety devices in place. If
the lamp sockets, receptacles, electroliers, etc., are also connected, only one-
half of the resistances specified in the table will be required.
DEVICES AND MATERIALS
No care in installing electrical equipments will entirely com^
pensate for the use of inferior or defective devices or materials.
The National Board of Fire Underwriters has for many years main-
tained a system of tests and examinations of electrical appliances,
and issues twice a year a "List of Electrical Fittings" which con-
tains in a classified form under the names of their manufacturers all
of the standard and special fittings and materials which have been
approved. These tests and examinations are made and the approvab
are issued by Underwriters' Laboratories, Inc., a thorou^y equipped
institution maintuned in Chicago by the fire insurance interests, for
the express purpose of examining and testing all kinds of devices and
materials, electrical and otherwise, which have any bearing what-
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UNDERWRITERS' REQUIREMENTS 149
soever on the fire h&zard. Thia "List of fittings" issued semi-
annually, is universally recognized as the only complete and reliable
guide to properly safeguarded electrical appliances and it should be
consulted and followed in the choice and purchase of supplies. The
Underwriters' Laboratories also maintain an elaborate system
whereby manufacturers may obtain and place on their wares spedid
labels issued by the Laboratories, which thus become a guarantee on
the goods themselves that the articles bearing such labeb are in con-
formity with underwriters' rules and have been examined and tested
by special underwriters' inspectors at the factories where they are
made. This system of label service and inspection is not yet extended
to all classes of approved electrical devices, but for those classes of
appliances which are now included, the label may be regarded as
affording to prospective purchasers and users reliable evidence not
only of the general approval of the design but also that the particular
sample bearing the label is made in accordance with requirements
and is suitable for use.
I The constructional detuls of electrical fittings and materials
together with the chief tests to which they are subjected, prior to
approval, are contained in what is known as "Class D" of the Code,
This pamphlet should be consulted for full details or inquiry should
be addressed to Underwriters' Laboratories, 207 East Ohio Street,
Chicago, Illinois, for information not given in the Code or in the
semi-annual "List of Fittings."
In the following pages is given a brief discussion of the chief
characteristics and requirements of some of the more common
materials and of the more important classes of devices used in elec-
trical construction worit.
RUBBBR-COVBRED WIRE
Qenera] ^>ecificatIoiu. A considerable variety of grades of
rubber-covered wire is manufactured, some naakers offering several
grades and oth»s only one or two at most. The chief flistinction
lies in the quality and quantity of real new, pure, fine rubber gum
used in the compound. It is not possible to determine or grade the
excellence of a rubber compound by any direct or readily applied
tests, but somewhat elaborate tests, phy^cat, chemical, and elec-
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UNDERWRITERS' REQUIREMENTS
TABLB IV
Thlckneu of Rubber losulatloa
B. A 8. a»nas
Tbicenbu
18 to 16
15 to 8
7 to 2
1 to 0000
Cinniltr Mib
250,000 to 500,000
600,000 to 1,000,000
Over 1,000,000
1-32 inch
3-64 inch
1-16 inch
Scinch
3-32 inch
7-64 inch
1-8 inch
trical, are necessary to arrive at any correct estimate. The following
properties of good wire may, however, be noted. The rubber should
be neither hard and dry, nor soft and spongy. When examined
minutely it should appear of a close uniform texture free from small
bits of unmixed matter or pinholes. It should adhere closely to the
timied copper. The thickness of rubber wall should correspond to
the data given in Table IV.
Measurements of insulating wall are to be made at the thinnest
pcotion of the dielectric and it should be very carefully noted whether
the copper is exactly centered in the rubber covering so that the full
prescribed insulation is maintained on all sides.
The rubber insulation should exhibit a fair degree of elasticity
when pieces cut from the wire are stretched and released. If the
rubber breaks with a very slight pull and shows no ability to stretch
and recover its first length it is probable that a. very poor grade of
gum has been used, or that the manufacturing process is defective,
or both.
After the braid has been carefully removed it should be possible
to wind the smaller sizes of wire about a cylinder of the same diameter
as the rubber-covered wire without the rubber showing any breaks
or cracks either at once or after several days.
The foregoing should be considered as only rough tests and not
susceptible of exact apphcation except under conditions which can
be maintained in a regular testing laboratory. No directions can be
given which will permit any but an expert chemist to make cbemiad
examinations of rubber compounds.
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UNDERWRITERS' REQUIREMENTS 151
The bmds should be of close weaye and should be very thor-
ou^y saturated with the compound. All wires should have through
their entire length a marker indicating by whom they were made
and each coil should bear a tag giving, beside the maker's name, the
maximum voltage for which it is designed, tiie words "National
Electrical Code Standard," and the month and year when it was
manufactured. Every coil of approved wire is separately tested by
the maker at the factory by at least two electrical tests, one^esigned
to show that the insulation is free from mechanical defects and the
other to show at least a minimum Insulation value.
Special Insulation. Most makers are prepared to furnish at a
special price a grade of wire conmionly known as "thirty per cent
Para," This wire is understood to have an insulation containing at
least 30 per cent of "fine, pure, up^iver Para" gum which is much
more than common commercial rubber-covered wire contains. This
30 per cent wire is also made according to certain rather exacting
specifications designed to insure a high grade of insulation with good
lasting properties. Wire of this description b often spedfied where
an extra good quality is desired for first class work.
A good compound should contain a large percentage of pure,
fine, new rubber of excellent quality. Para rubber is universally
admitted to be the best for imparting life, strength, and durability
to the insulation. The use of reclaimed rubber or any of the so-called
rubber substitutes reduces the excellence of the compound approxi-
mately in the proportion in which it is used. The other ingredients
of a good compound are solid, waxy, hydrocarbons, suitable mineral
matter and sulphur. The sulphur ptays an important part in the vul-
canizing of the compound, the process whereby the rubber is trans-
formed from its original and almost crude state into the substance
familiar tons as manufactured rubber in anyoneof itsnumerous forms.
Fixture Wire. Fixtures may be wired with flexible cord or
standard rubber-covered wire, and for other wires for use in fixtures,
the following rules apply : The wire may be either solid dr stranded
and not less than No. 18 B. & S. gauge. Solid conductors must be
tinned and stranded conductors must be of strands not less than
No. 30 B. & S. gauge and must have a cotton wind between copper
and rubb^. The No. 18 wire may have a rubber insulation A-inch
thick, but No. 16 and also flexible cord used in fixtures must have
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152 UNDERWRITERS' REQUIREMENTS
at least A-inch rubber. All sizes must be covca^ with a good
braid. The concession of so thin a rubba' wall on No. 18 wire has
been made because a very small wire must be uaed to pass through
the arms and other parts of many fixttues.
In wiring certain designs of show-case fixtures, criling bull's-
eyea and similar appUances, in which the wiring ia exposed to tem-
peratures in excess of 120°F. (49°C.)i from the heat of the lamps,
slow-burning wire may be used.
Insulation for Conduit and Armored Circuits. For all conduit
work and in all armored cable the wire is regular standard rubber-
covered with an extra braid. For twin or duplex wires, this outer
br^d, which should be at least A-inch thick, b made as a covering
over the two regular rubber-covered and braided conductors. These
twin wires are generally used in conduit work but where single con-
ductors are used they must also have double braid. The purpose of
this extra outer braid is primarily to withstand the abrasion and
strain resulting from hauling the conductors through the condmts
from outlet to outlet, and the braids on the individual conduct<ffs
are to hold the rubber insulation in place and prevent jamming and
flattening which might reduce the thickness of rubber between the
two wires and thus weaken the insulation at many points.
RldlD CONDUIT AND CONDUIT FITTINOS
Unlined Steel Conduit. The following description applies
only to standard unlined steel conduit. This is made of mild steel
with a butt weld joint lengthwise of the pipe. Sizes run from normal
)-inch to 4-inch pipie. The raw pipe is thoroughly cleaned inside
and outside and then ^ven a protective coating either of an enamel
baked on or of zinc applied either by electroplating or by a special proc-
ess known as sherardizing. With either form of zincing the interior
b given a coat of enamel also. The finbhed pipe should be smoothly
coated and able to stand bending without injury to the enamel or the
zine. The conduit should be of sufficiency true circular section to
admit of cutting true, clean threads. The enamel applied to con-
duit b not considered as an insulation but either enamel or zinc b
rfequired to protect the steel from rusting away and also to give a
smooth surface for the conductors to be drawn over in inserting the
wires. Ordinary commercial pipe should never be used as electric
conduit since it b not free from rou^ edges, is not maintained at
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UNDERWRITERS' REQUIREMENTS
TABLE V
MIflimniii W«IChts of Conduit for Reqnlred Wall ThlckacM
FouDdipw
Siw
Poundi per
100 FMt
i
75
U
260
104
2
350
1
152-
3
710
U
209
tiniform size or wall thickness, is not protected against rust and in
general is not made with the care and rigid inspection which have
been found by experience to be necessary for electric conduits.
Table V gives the minimum weights per 100 feet of finished conduit
which are required to give the specified thickness of wall.
Conduit Fittings. A very great variety of boxes and small fittings
fra use with conduit is available. All boxes including flush switch
boxes should be either of cast iron with walb at least ^-inch thick or
of sheet steel at least .078 inch thick. They must be well enameled
or galvanized to protect them from rusting and must have no open-
ings not closed by the entering pipes, by a metal cover of the same
thickness of the box or by the switch, receptacle, or canopy of the
device attached to them. Under no circumstances is it allowable to
place any such box so that it will not be accessible. There should be
no rough edges or comers which are liable to injure the coverings of
wires as they are drawn in. Fig. 139 shows the form of a common
type of box. Boxes for use with combina-
tion gas and electric fixtures must be pro-
vided with an arrangement for making a
tight electrical connection between the
box and the gas pipe at each outlet so
that tliere may be no arcing between box
and pipe in case any ftulure of wire insu-
lation causes a current to flow over the
box. Otherwise such an arc may bum a
hole in the gas pipe and ignite the gas.
All threaded parts of boxes and all threads
on locknuts and metal bushings must be clean cut and well fitted in
order to insure that permanent and reliable electrical continuity of the
conduit system which is one of the chief requirements for conduit work.
Tit. 130, Common Typa ol
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154 UNDERWRITERS' REQUIREMENTS
Set screw connections have been found unsatisfactory as they
loosen with the vibration of buildings and with changes of tempera-
ture, and only regular screwed thread joints or substantial clamps
should be used at all conduit and armored-cable connections.
E^(. 140. CommoD Form oC Floor Oatlet Box
Where a floor outlet in a conduit system is desired, a special
type of box should be used known as a floor outlet box. Such boxes
(see Fig, 140) provide ample room for making splices in wires, for
mounting receptacles or other fittings and especially, provide a sub-
stantial, watertight top or cover which can be set flush with the floor
surface. The practice sometimes followed of setting flush wall
receptacles in floors is to be strongly condemned since such fittings
are not strong enough for such service and are not watertight, thus
permitting water to enter the conduit box and system.
FUSES OR CUT-OUTS
Classification. Three forms of fuses are at present employed
in thb country for general wiring work, oyenAiiik fuses, cartridge fuses
and plug fuses, the last two being further described as enclosed fuses
to distinguish them from the open links. The bases to which or in
which the fuses are secured are called cut-outs.
Link fuses are extensively used on large switchboards and their
use on such boards is open to less objection than for general wiring
since such boards are usually under expert supervision and located
in well-protected or fireproof rooms. With link fuses there is always
the possibility of a larger fuse being put into the cut-out than it was
designed for, which is not true of enclosed fuse cut-outs classified as
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UNDERWRITERS' REQUIREMENTS
TABLE VI
Opea-LInk Fuse Spacing
ot Opiwrito Polmrity
Break
125 Volts or lesa
10 amperes or less
11-100 amperes
301-1000 amperes
i
1
1
u
i
1
126 to 250 Volts
10 amperes or lesa
11-100 amperes
101-300 amperes
301-1000 amperes
li
u
2
2i
u
1)
2
required below. Again, the voltage in most plants can, under some
conditions, rise considerably above the normal. The need of some
margin, as a factor of safety to prevent the cut-outs from being
ruined in ordinary service, is therefore evident. When tablet-boards
or single fuse-blocks with such open-link fuses on them are used in
general wiring, they must be enclosed in cabinet boxes. Thb is
necessary, because a severe flash may occur when such fuses melt,
so that they would be dangerous if exposed in the neighborhood of
any combustible material. Link fuses should never be mounted on
porcelain cut-outs because a severe short-circuit is liable to break
this rather fragile material and the molten metal is apt to fuse into
the porcelain, partly reducing its insulating properties.
There is no filler surrounding the fusible metal of open links
and, therefore, the ability of the fuse to open the circuit depends
on having enough of the metal burned away, when the fuse blows, to
break the arc. For thb reason the terminals for link fuses, as far
as practicable, should be made of compact form instead of being
rolled out in thin strips; and sharp edges or thin projecting pieces,
as on wing thumb nuts and the hke, should be avoided. Thin metal,
sharp edges, and projecting pieces are much more likely to cause
an arc to start than a more solid mass of metal. It is a good plan
to round all corners of the terminals and to chamfer the edges. Plain
fuse wire or fuse strip should never be used for links but only fuses
made up with solid metal terminab as shown in Fig. 17.
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156 UNDERWRITERS' REQUIREMENTS
In general work open-link fuses should not be used on circuits
of voltages above 250 volts and where large currents are involved
the use of approved circuit breakers is very much to be preferred to
extra large link fuses. The spacings for open-link fuses are given in
Table VI.
Plug Fuses, The form of fuse which is used in larger num-
bers than those of any other type, b shown in Kg. 141. Fig. 142
shows one of the many forms of cut-out bases for plug fuses and
Fig. 57 shows similar blocks arranged in an asbestos-lined cabinet
to make a tablet or panel-hoard for distributing current to branch-
lighting circuits. The cabinet should have a tightly fitted asbestos-
lined or metal-lined door. Plug fuses are approved for use only on
circuits of not over 125 volts, including 3-wire circuits with grounded
neutral, and not over 250 volts between outside wires. Fig. 143
Fi«. 141. Stsndard Fiue Flui Fig, li2. Cut-Out Bue for Fiue Pluss
nhows the effect if the fuses are blown on a 220-volt circuit. These
plugs are limited to ratings of 30 amperes and less because their
form and strength is not such as to make them safe for use with larger
currents. They are, therefore, chiefly adapted for small lighting and
motor branch circuits. They are decidedly safer than open-link
fuses of equal capacity and are cheaper than enclosed cartridge fuses.
There are a number of patterns of unapproved plug fuses on
the market which should be avoided as they usually lack some of the
essential properties of safe fuses, although they may appear from
casual inspection to he almost identical with them. It b unfor-
tunately true that plug fuses of the present form can be "doctored"
in several ways so as to carry larger currents than they should. Tin-
foil, solder, and bits of copper wire are often found put around or into
plug fuses so as to completely destroy their usefulness as protective
devices. Inspectors and property owners should be on the lookout
for thb highly dangerous practice and also observe carefully whether
plugs of too large current capacity have been substituted for those of
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UNDERWRITERS' REQUIREMENTS 157
ratings which afford real protection to the wiring of the drcuits of
which they are a part.
Fia- 143. ES«t of Plug FuMB Blown on SSO-Volt Circuit
Cartridge Fuses. A cartridge fuse consists of a cylindrical tube
of fiber or strong paper to which are fitted metal caps by means of
which connection is made to the cut-out terminals. Within the tube
is the fusible metal wire or strip extending between and firmly secured
to the inside of the caps. The tube is filled with a powdered or
granulated material packed closely about the fusible strip. The
purpose of this filler b to conduct the heat from the strip to the outer
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158
UNDERWRITERS' REQUIREMENTS
casing, to smother the arc when the fuse blows, to make it possible
to adjust more exactly the carrying capacity of the fusible strip, and
to absorb some of the gases evolved from the molten metal. The chief
ingredients of fillers now used are magnesia and plaster of Paris.
Fig. 144 shows in section the internal construction of two typical
cartridge fuses of 30 and 200 amperes capacity. The illustration
is full size. The small wire extending from one terminal to a point
on the tube and thence to the other terminal is designed to bum
off at the tube when the fuse blows and fuse & bit of powder under
a slip of thin paper on the outside of the casing, thus indicating that
the fuse has operated. As will be seen from the description a cart-
ridge fuse will confine the are, flame, or molten metal within the filler
5^
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Kg. 14S. Dimendon* snd Clainfications of Cutridge FuMi
and tube when its fusible element is melted by an overload or by a
short-cireuit and is, therefore, a safer device than an open-Hnk fuse.
In 1905 a standardization of cartridge fuses and their cut-out
bases was agreed upon in order to bring them all to uniform dimen-
sions, to arrange the different ratings under a classification which
would make it impossible to put a large fuse into a cut-out base of
a smaller class, and to make it possible to use any approved fuse in
any approved base whether fuse and base were of the same or different
make. Under this classification two types of terminals were stand-
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UNDERWRITERS' REQUIREMENTS 159
ardized known as femde and knife-biade contacts, and the current
ratings were grouped into two sizes for the ferrule and four sizes
for the knife-blade contacts. The dimensions and classification
are fully shown on the diagrams and in the table, Fig. 145.
Knife Switches. Knife switches must always be mounted on
separate bases of slate, marble, or porcelain, or on slate or marble
switchboards or panels. The parts carrying the contact clips and
the blade hinges must be secured to the base by two screws, a screw
and a dowel-pin, or otherwise, so that the parts will always be in
correct alignment. If the contact jaws or hinge clips get turned so
as to be out of line, it may be impossible to close the switch, espe-
cially at the first attempt, and severe arcing may result from the
efforts to do so. Even if the blade enters the jaws, the contact may
be imperfect, causing undesirable heating. The cluef points to note
in jud^ng a knife switch are the following: Excellence of fit of
blades both at the hinge and in the contact clips; stiffness and size
of all metal parts to secure good contact surfaces and ample carrying
capacity. No part should become heated over 50° F, when the
switch is carrying its full rated current, and at all sliding contacts
there should be at least 1 square inch of surface contact for every
75 amperes of current. The cross bars should be very securely fas-
tened to the blades and the workmanship throughout should be
excellent. If each biade is secured to the cross bar by only one screw,
without dowel-pins or a square shoulder fitting closely in a recess in
the bar, a slight loosening of the screws will allow one blade to close
and open the circuit before the other, resulting in arcing and ultimate
injury to the switch. Such construction is also liable to result in a
weak switch. Too little attention is frequently given the question
of mechanical strength, with the result that after a comparatively
short time of service the switches rattle to pieces or break unless
very carefully handled, and even then repairs are often necessary
to keep them in working order. A cheap switch is seldom a rugged,
durable device. All switches should be marked with the name of
their maker and the rating in both volts and amperes.
The Spacings of switches must be at least as great as those given
in the Code, a copy of which is given in the following table. This
table specifies the limits necessary for both direct-current and
alternating-current systems.
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UNDERWRITERS' REQUIREMENTS
TABLE VII
Approved Spacing for Knife Switdies
of NevM M«td
Bi«.k
^"*'l^chS' ^°''
Inolwr
Not over 125 Volta D. C. and A. C.
tor Switchboards and Panel Boarde*
10 amperes
i
i
30 amperes
1
{
60 amperes
u
1
Not over 126 Volta D. C. and A. C.
for Individual Switchest
30 amperee
u
1
60 and 100 amperes
u
U
2i
2
400 and 600 amperee
21
21
800 and 1,000 amperes
3
21
250 Volts only D. C. and A. C. for
All Switches
H
u
Not over 250 Volts D. C. nor over
500 Volts A. C. for All Switcheet
30. 60 and 100 amperes
2i
2
200 and 300 amperes
2)
21
400 and 600 amperes
2»
2*
800 and 1,000 amperes
3
21
Not over 600 Volte D. C. and A. C.
for All Switches**
30 and 60 amperes
4
3i
100 amperes
4*
4
at HUdKCA Bdviiifld for
JThe aOO-ampera bw
I spsc«d tor SOO vait tiues, and
•Tho 30-»nipere awitch oi
^^m.
ughh. ,dod b..
•iffr
"T„,
«foT«"bt1^
dWwh
eh caw
theawi
chM mu
■J5'«?:*«'.y
-?!! S
STI
tly I.
.mperea, the
led for D. C. ivitolia denned for oret
,. -- , -- _. _. deugnod lor u«6 in breaiing ouminta
peaur Ihui 100 *inper«a at a Toltage of ovei 2S0.
Note. For three-wire direct-current and three-wire single-phase systems
the separations and break diatances for plain three-pole knife switches must
Dot be less than those required in the above table for switches deigned for the
voltage between the neutral and outside wires.
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UNDERWRITERS' REQUIREMENTS 161
Sn^ Switches. Under this term ate included the eontmon
round base surface switches, the rotary and puah-bvUon switches,
which are set in boxes in side walk flush with the surface, ■pendant
switches, and all such as are operated by the motion of doors, by a
cord and all switches mounted on fixtures. The distinguishing
feature of them all consists in the fact that the motion of the parts
which open and close the circuit b produced by a spring contained
in the mechanism. As the handle or button is turned or pushed
this spring is wound up and at the proper tension b released, thus
throwing the switch blades into or out of the contacts. Thus the
quickness with which the drcuit is opened or closed is not directly
detennined by the motion of the operator's hand but by the spring,
and if the switch is of proper design and in good condition, the
action is prompt and reliable even though the person using the
switch is not careful to do just the right thing. Such snap switches,
therefore, differ from knife-blade switches in that their proper use
does not depend upon the user and they are correspondingly better
. suited for general purposes for unskilled persons.
The bases should be of non-combustible material, usually
porcelain, and all covers should be Uned with a non-conducting
material such as fiber unless they ,are of porcelun. Wthout this
lining there is danger of the cover forming a short-circuit in the
switch, especially if the cover is removed or replaced while the
switch is "alive," The side lining should extend beyond the lower
edge of the cover.
The binding posts should be of a type in which the end of the
connected wire is held under a screw head or equivalent device and
not by a set-screw the end of which drives against the side of the
wire, as a set-screw is likely to become loosened and b almost sure
to cut into the wire. Indicating switches are much preferred for all
work, as by showing at once whether the current is "on" or "off"
they tend to save mistakes and possible accidents. The fact that
lights do not bum or that a motor does not run is not necessarily a
sure sign that the current is off, but the indicating switch makes it
possible to tell at a glance whether the circuit is open or closed.
Fig. 146 shows a variety of approved snap switches of common
type. Snap switches to be approved are required to operate success-
fully at 50 per cent excess current above that for which they are
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162 UNDERWRITERS' REQUIREMENTS
rated at the voltages for which they are designed. This is to provide
a margin of safety but should not be made an excuse for using them
for larger currents than those marked on them. They are made in
a great variety of sizes and patterns, single- and double-pole, three-
and four-way, and in special designs for turning on the lamps of a
chandelier one after the other or for controlling small motors or heat-
ing devices. The standardized ratings include 125, 250, and 600
volts for currents of 3, 5, and 10 amperes in the more commonly
used patterns. Certain large-sized double-pole surface snap switches
aierated at 20 or 30 amperes while a few of the more special and less
substantial forms are limited to 1 ampere only. In judging snap
ii?-'
■fA
Fif, 140. Group of Approved Snap Svilcbes
switches of ali kinds samples are tested by Underwriters' Labora
tories in the following way; they are connected to control groups of
tamps taking full rated current of the switch at full rated voltage
and are then put on a special machine which operates them slowly
and continuously for 6,000 cycles, that is 6,000 full "on and off"
operations. It is required that the samples stand this endurance
test without failing either mechanically or electrically. This test is
re-appUed to new and recent samples from time to time, and similar
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UNDERWRITERS' REQUIREMENTS 163
tests are constantly in progress at the factories where the switches
are made, so that any important defect in material or construction
is soon detected and a fairly uniform grade of snap switches is sure
to be produced for the user. Such persbtent tests have done much
to improve the quality of these and other electrical products.
CIRCUIT BRBAKSRS
Circuit breakers are automadc switches so designed that an
excess current will cause the switch to open. They thus share the
properties of both switches and protective devices such as fuses, and
in their choice and installation both functions must be considered.
Breakers are made for currents of all capacities and for circuits of
every voltage both d. c. and a. c. Those most commonly used on
lighting and power switchboards and in genera! commercial work
are for voltages of 600 volts or less (occasionally 2,000 volts). For
higher voltages the breakers are of massive form and are often set
in cells or compartments of brick, shite, or concrete. Suck breakers
are employed only where expert supervision is always available and
their form and operation is, therefore, not prescribed by under-
writers' rules but rather by the engineering necessities of the system
of which they are a part.
The ordinary commercial breaker as used on low-voltage cir-
cuits may be one, two, or three poles and these may be independent
of each other or may be interlocked so that an overload on any one
wire will cause all the lines to be opened. The latter is preferable.
Breakers are usually made with an adjustment regulating the point
at which they will open. Thus a 100-ampere breaker may be set
to open at any current with a certain range above and below 100
amperes.
In installing breakers the same care should be taken as with
fuses of like capacity. They should never be placed near any in-
flammable material, as their operation under severe overloads
results in a severe though brief arc and often in the spattering of bits
of molten metai quite capable of igniting waste, shavings, etc. The
use of orcuit breakers instead of fuses is to be recommended for
very large currents and for all circuits such as many motor drcuits
where operating conditions are liable to produce frequent overloads.
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UNDERWRITERS' REQUIREMENTS
ClassHication. Circuit breakers may be divided into two chief
sses, carbon or air-break patterns and oH-immeraed patterns. The
f^. 147, Modern Oil Circuit Breaker
former may be used on either d. c. or a. c. circuits. They have
copper blades and fixed contacts with carbon secondary contacts
arranged to open just after the heavy copper contacts. Thus the
current is carried by copper parts of ample size when the breaker is
closed, and the arcing on opening is largely confined to the carbon
contacts which are better for this purpose than metal. Such breakers
may become dangerous either from overheating of the coils, from
_n r,
»*
S
V-U-
III
ria. 148. TypiwJ Pan
'BoATd Bu»-Bu ArranccmenU
arcing upon opening heavy currents, or from failure to act in emer-
gency as they are intended to do. However, the better types of
modern breaker are very well made and form reliable protective
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UNDERWRITERS' REQUIREMENTS 165
CKl-immersed breakers are coming more and more into use.
They are made with their contacts immersed in a heavy oil contuned
in a can or case of suitable form. When the contaqts open, the oil
aids very greatly in quenching the arc, and such breakers can,
therefore, be made quite compact since a long break distance is not
so essential. These breakers are made both for switchboards and
wall mounting. They are not so well suited to direct-current circuits
since the action of the d. c. arc carbonizes the oil too rapidly. Fig.
147 shows a modern oil breaker. They present a hazard due to the
oil which may become overheated or even ignited. This is unlikely
to occw with a well designed oil switch or breaker, but the usual
precautions should be taken to keep the switch and its immediate
neighborhood clean and free from accumulations of rubbish or any
inflammable material which may become oil soaked.
MISCBLLANBOUS DEVICES
Panel Boards and Cabinets. Panel bou^ls are distributing
boards, or switchboards from which the branch circuits are led off
from the mains. They must have slate bases on which are mounted
the necessary bus bars, switches and fuses. A very great variety
of panels is made, the arrangement of parts in a few patterns
being shown in Fig. 148, while a complete panel set in a steel
cabinet is shown in P^g. 149. Fig. 150 shows the wiring channel
often provided ^id a cabinet with wood door and trim. The
panel base and the two partitions shown in the section drawing
are of slate ^nd all other interior surfaces of the cabinet including
the door should be lined with sheet steel.
Wood cabinets should not be used on conduit, armored
cables, or metal molding systems of wiring as they do not form a
metallic connection between parts of the system and any attempt
to overcome this by bonding around the box by wire is not liable
to result in a good job. Metal cabinets are preferable in all cases
except possibly in very damp locations where they are liable to
rust rapidly. All cabinets whether for panels or for individual
switches or cut-outs should be thoroughly dust tight and fitted with
tightly-closing doors. No metal thinner than No, 16 U. S. Metal
Gauge should be used and heavier metal is necessary for all but the
smaUer sizes of box to secure the requisite stiffness and durability.
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166 UNDERWRITERS' REQUIREMENTS
Sockets and Receptacles. The almost endless variety of these
fittings may be classified in various ways, as for dry or for wet places;
key and keyles^ypes; brass shell or porcelain types; conduit boxes;
molding signs; miniature and candelabra sockets and receptacles,
etc. AH standard sockets and receptacles are now made with what
Fie. US. ComplBM Panel Board Id St«e1 Cabioet
are called Edison screw-shells into which the base of the incandescent
lamp is screwed, the shell being connected in the socket to one of the
lead wires and the center contact to the other lead wire. In all types
the design of the socket or receptacle should be such that when a
lamp is inserted there will be no cmrent-cairying part of the lamp
base exposed. This calls for a minimum depth of the socket of tt
inch and sockets and receptacles which do not have such depth should
not be used. Sockets and receptacles were formerly rated in candle-
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UNDERWRITERS' REQUIREMENTS
167
power of the lamps designed foF them or in amperes and volts but the
introduction of the newer high-efficiency lamps such as the tungsten
and tantalum has rendered this method of rating inapplicable. All
key or pull sockets and receptacles of standard types are now rated
250 watts, 250 volts, with a provision that this shall not be inter-
preted to permit the use at any voltage of current above 2J amperes.
Keyless sockets and receptacles of standard types are rated
660 watts, 250 volts, but not over 6 amperes at any voltage.
Miniature and candelabra sockets are rated 75 watts, 125 volts.
Fig. 1£0- P*hI Boud and Wood Cabinet Si
: Melbod ol Canitruotioi
Weatherproof sockets having no exposed current-carrying parta
may be rated 660 watts, 600 volts, and thus may be used in series
on GOO-volt circuits.
The most common abuse of sockets is to employ them as out-
lets for currents far in excess of what they or the wiring immediately
connected to them should carry and the above limits should be
rigidly adhered to. The assigned ratings do not imply that the full
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168 UNDERWRITERS' REQUIREMENTS
power can be taken from each and every socket on a circuit at once.
Thus twelve carbon 16 c, p. lamps in sockets may take 6 amperes
on a 110-volt circuit and be fused with 6-ampere fuses. At 250 watts
per socket, twelve sockets will take 3,000 watts which, at 110 volts,
is over 27 amperes, an amount of power forbidden by the rules
limiting lighting branch circuits to 660 watts and a current mani-
festly too large for the No. 14 wire which would usually be used on
such a cb-cuit. The ratings 250 watts, 250 volts, for key sockets, and
660 watts, 250 volts, for keyless sockets, are intended to express the
maximum safe carrying capacity of each socket or receptacle alone
and do not warrant employing them in the manner indicated above,
which would seriously overload ordinary drcuits.
fig. 161. Common Typei ol Fused uid Unfuoed RoHtiM
Rosettes. These devices are usually of porcelain and provide
a means of connecting flexible cords to the main or branch circuits.
Types are made either with or without small link fuses in the base
but the unfused type is much to be preferred and should be used
exclusively in general work since the use of link fuses in porcelain
fittings is undesirable because of the possible results following a short-
circuit blowing the fuses violently. It is much better to place all
fuses at distribution centers, such as panel boards, and by keeping
the fuses of proper capacity, depend on them for protection rather
than on fuses scattered about in fittings, rosettes, etc. Fig, 151
shows common types of fused and unfused rosettes.
Bell-Ringlt^ Transformers. Within the last few years small
transformers have been brought out designed for the purpose of
ringing door bells or for other light signaling work, deriving their
power for such service directly from alternating-current lighting
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UNDERWRITERS' REQUIREMENTS 169
circuits in houses. Th^ take the form of small, totally enclosed
transformers with two very thoroughly insulated coils, the primary
coil to be connected directly across an alternating circuit and the
secondary to be connected to the bell circuit, as shown in Fig. 152.
The push button is, of course, usually open so that no current
flows over the bel! circuit. When the button is pushed a current of
low voltage (10 to 20 volts) and small ampere capacity (not over 2
amperes) flows over the secondary or bell circuit, the power being
derived from the primary winding connected to tbe alternating-
current hne as in the case of any transformer. Absolutely no de-
pendence can be put upon the insidation of the bell circuit which is
often of cotton -covered paraffined wire (annunciator wire) in-
stalled in the most unreliable manner and the bells and push buttons
are not designed for anything but very low voltage currents. It is,
ft
BUTTOM
/O
SO VOLTS
therefore, imperative that the design and construction and insula-
tion of the transformer be such that under no conditions, dther of
service or from an accident, can the 110-volt current act directly on
the bell circuit. Furthermore, the design of the transformer must
be such that even if the push button be left closed or the bell wires
become short-circuited only a very small current will flow over the
bel! wiring. In approved bell-rin^ng transformCTs both these results
are secured with reasonable safety.
Heating Devices. The rapid introduction of all sorts of electric
heating devices for domestic and industrial use has brought with it
a special hazard which it is peculiarly difficult to control. These
devices are useful only as they are capable of developing a consider-
able heat in a short time. The normal use, of course, tends to draw
.away the heat and thus prevent a dangerously high temperature
being reached, but if these devices are left connected to the circuit
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170 UNDERWRITERS' REQUIREMENTS
and unused, many of them will reach a dull red heat and may thus
become a serious fire hazard. His is true of electric fiatlrons espe-
dally and also of many cooking utensils and other apparatus for
domestic or light factory use. It is evident, therefore, that all such
heating appliances should be made wholly of non-combustible ma-
terial and, so far as their use will permit, be fitted with legs, guards,
or other parts which will -keep the heated parts well separated from
wails, floors, tables, etc.
Stationary heating devices should be installed only when every
precaution has been taken and the fact that perhaps only a low
temperature will be produced by the proper and normal operation of
the heater should not be made an excuse for omitting any of the
precautions that would be taken for the higher temperatures which
may easily result from accident or misuse. There should be ample
air spaces and proper protection of adjacent surfaces by asbestos
board and metal sheathing. In general all electric heating devices
must be installed and used as possible sources of great heat.
Portable heating devices are not easily protected from misuse
or accident. The chief protection against fires from such appliances
appears to depend upon the original excellence of design and con-
struction of the devices themselves, the fact that many of them
employ but a small amount of energy, and, finally, upon a slowly
growing appreciation by users and the public generally, that elec-
trically-heated appliances, while usually fairly safe, if properly used,
may very readily become dangerous if abused or improperly used.
Electric Oas Lighters. A battery, spark coil, and similar
appliances are often used for the purpose of lighting the gas on gas
fixtures without the use of matches. In such installations the wires
from the battery and coil are led to the fixtures in any convenient
manner and on the fixtures themselves small wires are carried down
the outside of fixture stems and arms to the burners. The line wires
are not insulated or installed in a manner comparable as to safety
with electric light wires, and on the fixtures the insulation is espe-
cially weak and exposed to injury. It is, therefore, evident that such
gas-lighting systems should never be installed on the same fixture
with electric lights, since a breakdown is very liable to permit the
electric-light current to pass over the gas-lighting wires and cause
a fire at some point perhaps concealed in a partition, either from ov^-
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UNDERWRITERS' REQUIREMENTS 171
heating the small wires or from arcing to other wires or to grounded
metal piping.
Marine Work. The Code contains special rules governii^ the
installation of electric light and power wires and apparatus on ship-
board. These differ from the standard rules in only a few partic-
ulars, as indicated by the need of spedal care to provide against
the effects of constant and severe vibration, dampness and extreme
hard usage to which marine installations are always subjected.
The provisions of the Code should be referred to in detail by those
who are called upon to install or inspect work of this special type.
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3,GoogIc
REVIEW QUESTIONS
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3,GoogIe
REVIEW QTTKSTIONS
ELKMENTS OF EliBOTRIOITr
AKX> MAON^ETISM
1. Explain, from the standpoint of induced nugnetizadon,
the process by which a magnet attracts a piece of soft iron.
2. What are the differences in the magnetic behavior of soft
iron and hard steel?
3. What is meant by a magnetic line of force?
4. What reasons have we for thinlting that magnetizatioD
is a molecular phenomenon?
5. State how you would test the sign of an unknown charge
of electricity by means of the gold-leaf electroscope.
6. Describe the process of charging an electroscope by induc-
tion.
7. In charging an electroscope by induction, why must die
finger be removed before the removal of the diatged body?
8. If you hold a brass rod in your hand and rub it with silk,
the rod will show no sign of electrification; but if you hold the brass
rod with a piece of sheet rubber and then rub it with silk, you will
find it electrified. Explain.
9. Why is a pith hall attracted to an electrified rod and then
lepelled from it?
10. What differences can you mention between magnetism
and electricity?
11. Explain the principle of the condenser.
12. Explain the principle of the lightning rod.
13. Why is the capacity of a conductor greater when ano^er
sonductor connected to the earth is near it than when it stands alonef
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ELEMENTS OF ELECTRICITY AND MAGNETISM
14. Why cannot a Leyden jar be appreciably diaiged if the
outer coat is insulated?
15. If the potential difference between the terminals of a cell
on open circuit is to be measured by means of an instrument con-
luting of a coil and magnet, why must the coil have a vety hig^
resistance?
16. How much current will flow between two points whose
P. D. is two Tolts, if they are connected by a wire having a resistance
of ten ohms?
17. If a voltmeter placed across the terminals of an incan-
descent lamp shows a P. D. of 110 volts, while an ammeter con-
nected in seiies with the lamp indicates a current of .5 ampere,
what is the resistance of the incandescent filament?
IS. If a certain Daniell cell has an internal resistance of 2 ohms
and an E. M. F. of 1.08 volts, what current will it send through an
anmieter whose resistance is negligible? What current will it send
through a copper wire of 2 ohms resistance? Through a German
silver wire of 100 ohms resistance?
19. A Daniell cell indicates a certain current when connected
to a galvanometer of negligible resistance. When a piece of No. 20
German-silver wire is inserted in the circuit, it is found to require a
length of 5 ft. to reduce the current to one-h(ilf its former value.
Find the resistance of the cell In ohms, No. 20 German-silver wire
having a resistance of 190 . 2 ohms per 1,000 ft.
20. Why is a Daniell cell better than a bichromate cell for
telegraphic purposes?
21. Why is a I«clanch^ cell better than a Daniell cell for
ringing door-bells?
22. If the internal resistance of a Daniell cell of the gravity
type b 4 ohms, and its E. M. F. 1.08 volts, how much current will
40 celb in series send through a telegraph line having a resistance
of 500 ohms? What current will one such cell send throu^ the
same circuit? 'VVhat current will 40 cells joined in parallel send
through the same circuit?
23. What current will the 40 cells in parallel send through an
ammeter which has a resistance of . 1 ohm? What current would
the 40 cells in series send through the same ammeter? What current
would a single cell send through the same ammeter ?
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REVIEW QUESTIONS
THE ELECTRIC CURRENT.
1. (a) Explain what is meant by electromotive force, (Jj
n''hat is its unit of measurement, and by what value is it repre-
wnted?
2. (a) What is necessaiy to cause an electric current V>
flow? (J) Whit is meant by *he strength of a current? (e)
What ia its unit of measurement, and by what value is it repre-
sented?
S. What is tlie unit of resistance and by what value is it
represented?
4. Upon what three general factors does resistance depend ?
5. What length of copper wire 2 millimetera in diameter
will have the same resistance as 12 yards, 1 millimeter in diameter?
6. State Ohm*s law.
7. Two wires, whose lesistances are reapectiTely 28 and 24
ohms, ate placed in paiallel in a circuit so that the current divides,
part passing through each. What resistance is offered by them to
the current?
8. Fif^ Grove's cells (E. M. F, = 1.8 volts') are in series,
and united by a wire of 15 ohms resistance. If the internal resist
ance of each cell is .3 ohm, what is the current?
9. (a) What is the unit of quantity of electric!^? (6)
Define the ampere-hour.
10. What is the power in watts when 4000 jonles of work
are done in 50 minutes?
11. How many hors&power are equivalent to 88 kilowatts T
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THB ELECTRIC OUBRENT.
13. What k s shunt circuit P
18. A cturent of 18 amperes flows In a etnmit vhose remsfe
moe ia 116 olinu. What is the voltage 7
14. The resistance of 812 feet of a certain wire is 2.08
ohms. What would bfi the resistance of 240 feet of the same
wireT
16. A total current of 56 amperes passes through a divided
oircoit having the resistance of its branches equal to 28 and 4
ohms respecdvely. What is the current la each branch f
16. Wbnt Is the value of the current when 4 ampere-hours
■redeliver J In a oirouit in 20 minutes?
17. jt) Define the joule, (i) Define the watt.
1?.. A 220-voIt circuit supplies a current of 18 amperes.
What is tie power in kilowatts ?
19. If the resistance of a certain wire is 2.8 ohms per 1000
feet, how many feet of the wire will be required to make up a
reustance of 17.8 ohma?
20. Wliat is the resistance of a wire having a diameter of
.3 inch if the resistance of the same length of similar wire having
a diameter of .04 inch is 64.2 ohms ?
21. Define specific resistance.
22. The resistance of a circuit is 1.8'ohm8 and the voltage
ia 110. What is the current?
28. A circuit contains a voltuo cell generating an electro-
motive force of 1 volt. Its electrodes are connected by three
wires in parallel of 2, 8, and 4 ohms resistance respectively. The
teeiBtance of the cell is ^ ohm. What Is the current?
24. Eight cells each having an E. M. F. of .9 volt and aa
internal resistance of .6 ohm are connected in parallel, and the
external resistance is 8.4 ohms. Find the current,
25. What quantity of electricity will be conveyed by a cui>
rent of 40 amperes in half an hour?
26. The resistance of a circuit is 10 ohms, and the current
it 83 ampeiea. What is the power in wa^ ?
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REVIEW QUESTIONS
ELECTRICAL MEASUREMENTS
PART I.
1. What b the distinction to be made between fundamental
and derived units? Give examples of each.
2. Describe briefly the different types of galvanometers and
explain wherein they differ and the advantages of each.
3. Voltmeters and ammeters are really galvanometers. Why
do they fall into this class and to which type do they belong?
4. Explain the lamp and scale and the ieleacope and gccde
methods of reading galvanometer deflections.
5. Describe the control magnet as used with needle galvanom-
eters and explain its function.
6. Describe and explain the electrodynamometer and the
wattmeter. How do they differ?
7. Describe the rheostat. What materials may be used for
the lesbtance?
8. How do resistance coils differ from the riieostat mentioned
in Question 6? What material is generally used for accurate resbt-
ance units and why?
9. Describe and explain the use of shunts for galvanometers.
10. Explain the Wheatstone's bridge. Describe the two usual
forms of the bridge.
'■ 1. Make the usual '*diamond" diagram of the connections of
a bridge and And the value of X when M = 1,000, N = 10, and P
-3,247. ■
12. Describe a good method for measurement of a low resist-
ance.
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RKNTIEW QUESTIONS
KliKCTRICAL MEASUREMENTS
PART II.
1. Describe the ballistic galvanometer and explain why its
period of swing should be long.
2. Describe a condenser, mentioning materials used, method
of connection, etc. What b the unii of capacity in which condensers
are rated?
3. Make a diagram of connections for the direct-deflection
method of comparison of capacities of condensers.
4. Describe and explain either the bridge method or the
method of mixtures for comparison of capacities of two condensers.
5. If in question 4, R^ = 2,100, fi, = 1,000, C, = 0.2, what
iaC,?
6. Describe the alternating-current method of determinatioa
of the capacity of a condenser.
7. What is sdf indvctionT What is impedance and how is it
expressed in terms of resbtance and inductance?
8. Describe the alternating-current method of measuring an
inductance.
9. What is a variable standard of self inductance and how is
its inductance varied?
10. Describe the bridge method of comparison of two self
inductances, one being variable.
11. Show how a capacity is compared with a self inductance
by the condenser method.
12. What is mutual induction and why is it so called?
13. Describe the ballistic galvanometer method of measuring
mutual inductance.
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REVIEW QUESTIONS
UNDERWRITERS* REQUIREMENTS
PART 1
1. Explain briefly the electrical causes of fires.
2. State in general terms the means to be taken in every
tDstaliation to guard against such fires being started.
3. As affecting the fire hazard, describe the essential differ-
encea between
(a) constant-current systems and constant-potential sys-
tems;
(b) low-voltage and high-voltage circuits.
4. What is the National Electrical Code?
5. Into what main subdivisions are its rules divided?
6. Desoibe the chief desirable characteristics of a generating
station with regard to the fire hazard.
7. In judging the safety of an instaUation of electric motors,
what are the chief items to be considered?
8. Describe a motor-starting rheostat.
9. What is an autostarter?
10. What is the object of a continuous duty resistance?
11. What are transformers?
12. State the advantages of the use of transformers with regard
to the fire hazard.
13. What precautions must be observed in installing trans-
fcmnws?
14. Outiine the standard practice for outside wiring.
15. What is electrolysis?
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RKVIKW QUESTIONS
UNDEIRWKll.'ElRS' RBQUIRKMKNTS
PART II
1. Wliat 13 open-work wiring?
2. What is the easential prindple of conduit work?
3. What are the advantages and disadvantages of concealed
work?
4. Outline the chief things to be considered in electric in-
stallations when subject to a continual dampness.
5. When should wood molding never be used?
6. What are the chief points to be observed in wiring a theater?
7. How should the footlights be installed?
8. Vfith a voltmeter having a resbtance of 24,000 ohms and
a battery of 26 volts, it was found that the voltmeter read 6 volts
when connected in series with the battery and the wiring of a house.
What was the insulation resistance of the wiring?
9. Why are rubber-<»vered wires used almost exclusively in
all inside wiring?
10. What b a "slow-bmming" wire?
11. Describe a "weatherproof" wire.
12. Give the dimenmons of a cartridge fuse for a circuit of 110
volts, 20 amperes.
13. In what way do telephone wires in a home affect the fire
hazard and what fire precautions should be taken?
14. What b a snap switch and what is its distinguishing feature?
15. Describe a characteristic conduit installation for on apart-
ment'
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INDEX
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INDEX
The page numbers □/ ikit ooIutm totU be found at the bottom qf the paga;
the numbers at the brp refer oiUy la the section.
Action of simple cell
AmericaD Morse code
Ammeters
Ampere
ArcUmpa
Armored cable
Artificial magnets
Astatic galvanometer
Ballistic galvanomet«r
Bar m^net
Batteries
Battery circuits
Bell-ringing transformers
Bichromate cell
Borders
Buneen cell
Calculation of resistoace
Calibration of ammeters
potentiometer method
silver voltmeter method
Calibration of voltmeter
Capacity. meosuremeDt of
absolute method
alternating ciureBt
ballistic gBlvanometer
bridge methods
condensers
direct deflection method
method of mixtures
Car wiring
Cartridge fuses
Cells
bichromate
Page
Cells
combination of 55
Daniell 62
dry 64
Leclanch6 54
simple 48
Central stations 118
Charging by induction 26
Chemical effects of electric current 66
Chemical method of measuring current 57
Circuit breakers 289, 381
Circuits
battery lOS
divided 100
fall of potential in 98
grounding of 289
Commercial efficiency
Condensers 33
Conductance
Conductivity
Conductors
Conduit fittings
Conduit work
Control magnet, use of
Conlomb
Current
chemical method of meaaurii^
measurement of
Current systems (constant)
Cut^-outs *
knife switches
snap switches
D
Damping of vibrations
D'Arsonval galvanometer
Declination
Density of charge
NaU.—Ferpc
ri ue foal o/ pa
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Derived uuilB 1!
Devices (miBcellaneouB) 3)
bell-ringing transformers 3i
electric gaa lighters 31
heating devices 3i
marine work 3i
panel boards and cabinets %
rosettes 3i
sockets and receptacles 3!
Devices and material 3i
circuit breakers Si
fusee or cut-outs 3'
miscellaneous 31
ri^ conduit and conduit fittings 3'
rubber-covetfid wire 3i
Dimmers 3'
Direction of induced current '
Discharging efFect of points :
Dissociation '
Divided circuits II
Diycell I
Dynamo, principle of I
I^namorule I
Earth's inductive action
Eart,h'B magnetic field
Electric bell
Electric current
circuits
currtet
eIectrc»notive force
Ohm's law
power
quantity
rewstance
supply of electrical energy
Electric gas Ughters
Electric heaters
Electric motor, principle of
Electric signs
Electrical chat^
Electrical cunents
measurement of
Electrical cne^y
equivalent of, in heat units
KtU.—Fer pop* nambm •*• fool af IB0M
Electrical energy
equivalent of, in mechanical ui
supply of
isolated plants
central stations
118
119
Electrical generators 36
electrophorus 36
static machine 37
Electrical measurements 123, 214
f4>paratus 125
battery resistance 159
capacity 175
current 169
electromotive force 161
magnetic measurements 197
mutual inductance 102
Ohm's law 140
resistanoe 141
self-inductance 187
systems of imits 123
voltaic cells and batteries 166
Electrical measuring apparatus 125
electrodynamometera 132
electrometers 134
galvanometers 125
hot-wire instruments 135
int^rating ampere-hour meter 137
int^irating watl^hour meter 136
recording ammeter 130
recording voltmeter 136
resistance coils 137
rheostats 137
wattmeters 135
Electrical potential 31
Electrical resistance 46
Electrical screens 35
Electricity
static 20
two-fluid, theory of 23
Electricity in motion 38
galvanic cell 39
measurements of currents 42
shape of field about a current 42
Electricity and magnetism 11, 79
Electrification by friction W
Electrodynamometers 132, 169
Electrolysis 56, 166,266
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rage
Galvanometers "
Electromagnet
61
69
D'Arsonval
electric beU
62
■ mirror
61
plunger type
60
tangent
magnetic properties of a loop
59
use of control magnet
relay and sounder
63
Generators 3
telegraph
63
Gold-leaf electroscope
134
Ground detectors and testa S
Electromotive force
81
Grove cell
galvanic cells
45
at make and break
73
H
measurement of
43,161
165
Helix
potentiometer method
162
magnetic properties of
voltmeter method
161
rules for north and south poles of
of secondary
72
Electron theory
24
of electricity
Electroplating
Electroscope, gold-leaf
24
67
terns a
26
23
Horseshoe magnet
134
Hot-wire inatrumento
Energy
114
Hysteresis S
testOT S
7
Fall of potential in circuit
98
I
Fixture wire
369
Inclination or dip
Fixture wiring
296
Induced currents
Fixtures
296
direction of
Flexible oorda
301
dynamo rule
Footlighta
348
eloctromotive force at make and
Friction, electrif action by
20
break
123
Fuses 287,372
ary
cartridge
375
plug
374
strength of induced e. m. f.
Induction, charging by
Galvanic ceU
39
Induction coil and transformer, prin-
electrical resistance of
46
ciple of
45
Induction of currents by magnets
internal reaiatance of
47
Inductive action, earth's
Galvanometer
125
Inside work S
astatic
127
InHtallfltion rules 3
ballistic
130
choice of
129
constantr-potential systems S
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InstEtllation rules ^^
fixtures and fixture wiring 296
arc lampa on constant-poten-
tial circuits 304
fixture detul 296
flexible coiAa 301
sockets and receptacles 298
general installatioQ rules for con*
trolling and protecting
devices 2S4
electric heaters 263
fuses and circuit breakers 287
switches 284
transformers in building 306
wiring systems 275
Installation of wires in buildings 309
armored cable 329
clasBification and general prin-
ciples 310
concealed work 322
conduit work 330.
open wiring in damp places 315
open wiring in dr; places 311
wires in molding 319
Insulation resistance 154
of cables 155
Insuktors 22
Integrating ampere-hour mel«ra 137
Integrating watt^hour meters 136
Internal resistance of galvanic cell 47
Ions 49
Isolated plants 118
Lsmp rheostats
Leclanch^ cell
Ley den jar
Lighting (decorative)
Lighting and power from railway w
Lightning arresters
Lightning rod
Lines (high tension) 267
Locating faults 15S
Locating grounds 158
Losses in energy 119
Low resistance measurements 150
M
Machines (moving picture) 351
Magnet
bar 11
horseshoe i 11 '
poles of 11
saturated . 18
Magnetic attraction and repulsion,
laws of 12
Magnetic effect due to charge in
motion 38
Magnetic field about a current, shape
of 42
Magnetic flux and permeability meas-
urement 208
ballistic method 210
divided bar method 20S
divided ring method 209
hysteresis curves 211
Magnetic induction 13
Magnetic lines of force 15
Magnetic measurements 197
hysUresis 202
lines of force and permeability 199
magnetic dip 203
magnetic flux and permeability 208
magnetometer method 206
magnetomotive force 200
methods of magnetizing 199
reluctance 201
Magnetic meridian 12
Magnetic properties
helix 60
of loop 59
Magnetic substances 13
Magnetism 11, 18
earth's 18
magnetic attraction 12
magnetic fields of force 16
magnetic induction 13
magnetic lines of foroe 15
NiiU.-For
N numbtri ut loot oj todh.
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Magnetism ^^
magnetic repulaioD 12
magnetic Bubatancea 13
magnets 11
molecular nature of 16
penneability 14
retentivity 14
Magnetometer method 205
Magnetomotive force 200
Measuremeat of battery resistance 159
Measurement of
capacity 175
current 169
electrical currents 42
electromotive force 161
mutual inductance 192
resistance 141
selT-inductance 187
Methods of measuring potentials 32
Mirror galvanometer 128
Modem transmitter 77
Molecular nature of magnetism 16
Motors 247
Moving picture theaters and machines 3S1
causes of danger 352
interior equipment 351
Multiplying power of shunts 139
Mutual-inductance measurement 192
altemating-current method 194
ballistic galvanometer method 193
Carey-Poster method 195
N
National electrical code 235
Natural magnets 11
Negative electricity 21, 27
p Pf^e
Panel boards 383
Permeability 14
Plan of telegraph system 65
Plug fuses 374
Plunger type instruments 132
Points, discharging effect of 29
Polarisation SO, 166
Poles of a magnet 12
Portable testing set 148
Positive electricity 21, 27
Potential systems (constant] 283
Potentials, methodg of measuring 32
Power 115
Power factor 136
Power stations and their equipment 236
generators 237
ground detectors and tests 246
lightning arreeters 244
motors 247
resistance boxes or rheostats 244
storage batteries 257
switchboards 241
transformers 259
Pressure in primary and secondary 76
Primary cells 48
bichromate cell 61
Daniell ceU 52
dry cell £4
Leclanchfi cell 54
polarisation 60
simple cell 48
Principle of *
dynamo 69
electric motor 70
induction coil and transformer 71
O
Ohm's law
Outside work
electrolyus
grounding of circuits
h^ tension lines .
mounting of transformers
wiring
47, 82, 95, 140 Receptacles 298, :
269 Recording voltmeters and ammeters
265 Relay and sounder
269 Reluctance
267 Resistance
39 affected by heating
iO calculation of
D,Bi:ized3,GOOgIe
ReaiBtaiice
inveraely proportional to croaB'
section 8
of lines Ifi
proportional to length 8:
specifio &
by subfltitutioa 14
lUaiBUuice boxes 14
Resistance boxes or rheoetata 24
}{«t«ntivity 1
RheoBtata 13
lamp 13
wat«r 13
Rheoetata and resiBtance coils 13
Rigid conduit and conduit fittings 37
fittings 37
unlined steel 37
Roeettes 38
Rubber-covered wire 36
fixture wire 36
iunilation for conduit and armored
Saturated n
Self'inductance meaeuremeDt
alternating-current m'ethod
bridge method
condenser method
Series circuits
^ protecting devicM
wiring requirements
Snap switches
Simple cell
action of
theory of action
Single telephone
Slide wire bridge
Sockets 26
Sounder and relay
Special installations
car wiring
decorative lighting
electric signs
high- and extra high-potential hi
Pa^
Special mstaUations
lighting and power from railway
wiree 355
moving picture theatera and ma-
chinee 351
signaling Byst«ms 358
theater wiring 346
Specific reustance 85
Stage pockets 349
Standard cdb 1S7
Static electricity 20
charging by induction 28
condeneeiB 33
conductora 22
electrical potential 31
electrical screens 35
electrification by friction 20
electron theory of 24
electrostatic induction 23
insulators 22
Leyden jar 34
lightning rod 30
negative electricity 21
positive electricity 21
two-fluid theory of 23
Static machine, Toepler-HolU 37
Steel conduit (unlined) 370
Storage batteries 58, 257
Storage cells 168
Strength of induced e. m. f. 67
Subscriber's telephone comiections 78
Switchboards 241
Switches 284
Table
American wire gauge 92
conductors in alternating current,
Biiea of 251
conductors in direct current, sises
of 260
conduit, minimum weights of, (or
required wall thickness 371
fuse spacing, open-link 373
knife switches, approved spacing
for
378
Vdc— Ar yoft mmbtn «m /Mt a/ foatt.
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relative refastonce o( chemically
pure Bubfltancee S7
rubber insulation, thickness of 368
Stub's or Birmingham wire gauge 92
t«mperatuTe coefficients 90
nires, carrying capacity of 281
Tangent galvanometer 126
Telegraph 63
Telegraph system, plan of 65
Temperature coeffioienta 90
Testing of electric wiring equipment 362
voltmeter method 363
Tbeat«r wiring 346
dimmera 347
footlights and bordera 348
general specificatbns 346
requirements for stage audit-
oriums 351
special lighting cireuita and stage
effects 351
stage pockets 349
Theaters (moving picture) 351
Toepler-Holtz static machine 37
Transformer 75, 259, 306
mounting of 3$9
Two-fluid theory of electricity 23
V
UnderwriteTs' requirements 217-389
devices and materials 366
electric installations, essential
parts of 233
ll«U.—Fcr tag* immbtt tt /m* a/ paoH,
Underwritcn' requirements
electricity as cause of fires 21
elnnentary electrical ideas and
terms 2S
installation of wires in
national electrical code 23
outside work 25
power stations and their equip-
ment 23
special installations 34
testing electric wiring 36
Units 12
derived IS
funduDental 12
relation ot C. G. S. to British IS
Voh.
VoHato cells
Voltmeter method for testing
Voltmeters
Watt
Wattmeters
Wheatatone bridge
Wires
general rules on
installation of in buildings
in molding
Wiring
Wiring systems
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