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:lli>t[>r and lovenur of Nu 
ighlini, and Other Ekclrica] 




Applied Electricity 






UluUy.^h.l -.lUh 0-ir Two Tlwowu/ E>r;raviKSS 



OOCnilOHT. IME. ISM. 1906. 1909. 1911. 1913 


190G, IMS. 1908. 1909. 1911, ItlS 

Capyrishted In Gnat Briton 
All RlEhts Roerved 


OCT -8 13K ^^^^ CH"^ 4 ^"^ 

Authors and Collaborators 


Proftuor of Electrtcal EDElDeariiii. Columbia University. Nnr York 
Pwt-PnaUent. Anuriuo InatitaU of Elsctricitl Eniinsen 


Kewl. DiiwrtnunC ef Elsctrhsl Engtitetiriia. Lshlib Univer 
Joint Adthot of "Tbs Elements of Elaetrical EnBiBaerlna" 


PrvtoMor of Electrical EnctnearlOK. Comsll XJniTerBiU 
Secretary, Society for tbe Promotion of Eaelaemins Bdncation 
Chalnnu. Edueatlonal CommiLtw. Ameiicnn Electric Railway 


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 


ConaultlnK Ensinser 

or th< firm of HcMem ft MUler. Electrical Ensine 

nstitute of Technolosy 


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 


Authors and Collaborators -Continued 


Awiataot PrafiHor of EUctrkal EasineerliiK. Columbia Unlvosit;, N«« York 


Head. DBputniDt of ApuliBd ElMtriclCy, Pntt Inititutc, BrooklrD. Nsw York 
Formarly Hevl. Depubnaot of ElKtarlckl EnEiBsariDS. Univenlty of Vrnnont 


AHlitut PiofenoT of BJtbUM Ensinnriiw, Comsl] Uoiversltr 
FornHTlr Inatmcter. WuhlnirUm UniTenlty 

t, CbtIhii Talaphons UanafuEurilut Cooipuir 


CoBulttnK Electrical Ensliicer 
Amarieu IiuUCatB of Elactrlcal EntHnc 


EndoMrwicl SuperlDtcndent of CoMlruction with R. P. Shield* A Son. G 
inSoclMTOf Mechanical Endnears 


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 


Editor. AmaricaD laatltateot Elaetrtcai Bnsineen 

rorawlv Head. Technical PiMtcatioa DepartnHnt. WHtlnKhouM Elective A Manafac- 


Authors nnd Collaboraiors— Cojuinucd 


Pralcaor of InduBtrlat EnslHarlnB. PeniUTlvanU Stat* Cotka* 
A>B«ricaa SocMy of Hechantcal BBi1n»n 


CouultiiuE En«inMr. DeputmiDt of Civil EnslDe 



H«hiuilcal EmriiHcr 

ForTwenU VmsSuiHrinlendnitmiidChicICanBtruetarfor J. W. Rndr Elevator Co. 


Aaoociatc PrafcBor of Elactrlul Enilneerii 
Amrlcmnlnititateof ElestrhalBDEidHn 


S«crel«TT, Anwrkaa School of Corrapondon 
Fomwrly InitructuF In PhyHioa. Univeraity a 
Amerlcui Phyiical Society 


ConaultliiB EUetrical EnsliiMr 

Author o( "Standanl WirliiB for Electric Lliht and Pan 


Consultini Engineer. Department of Electrical GBBin«HBK. American School of 


Chief, Quadranln Department. Western Union Main Omce. New York City 


Bead, Publication Department. American School of CorrcapondeDC* 


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 


FiMident.Ciwlier-Wh«aler Company: Put-Pm 

Joint Author of "Hsnasmnent of Electrlul Unci 


Member. Amenan Iiutitots of Electrical Emineei 
AuUnr of "Praetlcal Calculation of Dynamo-Eltwt 


Fntcnorof Phyaica. Lotaiah Unlvrraity 

Joint Author of "The E]«i»Dta of Electrical EnglnwrinE." "The Elemmtl of 


Head ot Department of Electricul EnsineerinK. LehlRh UniveniCr 
Joint Author ot "The ElemenU of ElKtric>l Englneerinc" 


Consoltins EoBinwr: ICtmber of 1 

of AnunicHn Society of Mfchan 

Author of "Electrical EnsinMi'e P 


Authorities Consulted — Continued 


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* 


Held of Depvtinant of Electrics] 
Joint Aothor of "Eketrlcsl Pmbl 


Prafcuar of Fhraici. Univenlty of Chhxga 

Joint Author of "A Fint Coune In PhraicB." "Electrleity. Sound a] 


;. Pennwlnnia 3UCa Cotlene: 
I and Allematlne-Currant Ma 


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*" 


Conauttlni Electrical Engineer; Anociate I 

Enslaeara: Hernber. American Electroc 

Author of "Storasa Battary EneinaarinK*' 


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. 


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. 


Consultins EBKlnnr of tha Firm or HcMeen and HilUr, Eleelrlcal EnKlnecra. Chlcasa 
Author af "Ameriean Talaphono Practice", Joint Author of "Telaphony" 


Hemlwr. Anwrlcan InitltuU of Elactrleal Enelnaera 
Aatbor of "Btandajnl Poiyphaae Apparatus and SyiU 


Profcuorof Applied Electricity. ComaJI UnlTen 
Author of "The Principle* of the TnuuCar 


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. 


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. 


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. 


Associate Prof eosoc of Electrical Engine. 
Author of "Elacttlc Hetera" 


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. 


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" 


Ei-ELeetriclaa. Biltlmoie and Ohio Tslasnuli Company 

Author of "WInleaa Tetosrachy." "Amarfcan TaleErachy and E 



TaattnE Departmant. Gflneral Electric Company 
Author of "AltntiaClnB-CDmnt BDElnearlDs" 


AaMciata Uember. American loatitule of i 

Author of 'Th« ator»Ba Battery; A Practk 


Proteaaor ot Phyalea and Electrical Eosineerini, Polytechnic InatitDte 
Joint Author of "Dynaoio-Elactric Itachinery." "AltematlDx-CDrrent Ha 



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" 


Electrical Enslneer: Auiatant Profesaor of Phyalea. Dartnuoth Colleca 
Author of "SjnehronouB and Other Uultiple Telesrapha": Joint Aoth 

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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 

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. 


Table of Contents 

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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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 

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. 





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 



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. 


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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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 

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 

A substance which has the property of becoming strongly mog- 

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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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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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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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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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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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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. 


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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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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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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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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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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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 

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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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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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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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. 



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^ 




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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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. 




and tT. Illuatratlni; Analotrv Ix 
waea Electric Potenlial and 
Hrdmstatlc Presaure. 



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 

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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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 

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 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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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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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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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 


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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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. 

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 



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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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 

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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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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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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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 


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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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. 



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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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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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- 

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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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). 


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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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 

When a zinc plate is placed in such a solution, the acid attacks 

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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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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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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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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.«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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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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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 


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 



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 


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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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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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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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. 


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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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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// 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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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 


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 

<il. The Elec- 
tric Bell. The elec- 
tric bell (Fig. 62) is 
one of the simplest 
applications of the 
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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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. 


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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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 
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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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 


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. 


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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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 




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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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 





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. 


ELECTRicrry and magnetism 


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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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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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 

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> 



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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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 

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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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 

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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3, Google 

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 


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 

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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 



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. 



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 


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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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. ^' 


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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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. 


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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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 

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" 

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 


TeqTtred length : 

> 852 feet (approx.) 

Ans. 862 £ 

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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 

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^ 

^ 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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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. 


" .82' 

teqoired reeiet. 

= 12.6 X 


_ 12.6 X 


=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 ^'^^^ 

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 

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 

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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lUlc Showing Relative ReslsUiica of Ctacmfcally Pare SubstAoces « 
Thirty-two Degrees Fahrenheit in Interaetional Ohnu. 



of » win 

ol Birln 


.001 luoh 'a 




IQ dluiocet. 



Silrer, annealed. 






Copper, annealed. 






Silver, hani dravn. 






Copper, hard diawn. 
Gold, annealed. 











Gold.hard drawn. 






Alnminnm, annealed 






Zinc, pressed. 






PUtinnm, annealed. 






Iron, annealed. 






Lead, pressed. 






Qerman silver. 






Platinnm-sUver alloy 

(J platinnm, J silver.) 












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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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 ^^^^?! „«. 

= 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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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^ 

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 

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 

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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. 





M WBur BBinm 







German silver 









Copper, alnminom 






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 ^ 

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-So ,2 

Is: I 





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 

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 

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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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 

American Wire Oause (B. & 5.) 

















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. 


w. a.) 



DluneMr In 
























































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< 

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two fluid 




talk Ana 




alphorfo Aoi 
{a, BO.) 

SolDtlon ot 
olphnrlc Aoi 


Bolntlon ot 
llphartc Add 
EH. 60.) 

SktnntBd Sola- 

slnm Dtahro- 

SnlphDiia Acid 

Bnlphnrlo JlcU 
'"—e (H, BO,; 

BalpbiiTio Add 

(E, C[, O,} 

Nltrlo Add 

0.1 to 0.11 
B OJiatoO.U 

Oanctio PoU«h 
or PoUMiam 
Hrdmta (EOH} 

Zinc Chlorida 

(KB. CI) 

Sod lam &PoU» 

■lam Chlot»t«« 



(NH. CI) In 
C^dnm Sal- 
phftte (C»80.) 

phB^ « 



Snlpbarlo Add 

Puts of Solph- 

-'m of MercQTT 

(Hg. 80.) 







Bnlpharlo knd 
dllntB mixed 


OBiutlo Potash 
or Potawlom 
Hydrate S.OH) 


Ohiond* at 



* At IS degioM OentlgiadB or 19 degroM Fkbrenhelk 
ResfiUacM in last oolamn meaanred in cells standing 6" x 4* 


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 

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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. 

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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 

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.) 

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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 


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 

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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. 

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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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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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^ 

current in r, j total current '••r^;r^+r^ ' 


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 : 



.• - ^•■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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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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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 

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? 



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. 


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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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 

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 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 


Fig. 7. 
fact connecting cells in parall<.K 

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or* " 




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, 


If there were n files connected In parallel and m cella were 

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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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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 

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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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 

Solation. — Here the external resistance R, equals .4 ohm and 
the resistance r of each cell equals .1 ohm. For mazimoiD 

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 + 


.681 + ampere. 

With 4 dies and 5 cells in series, m = 5 and n 3 
the onneDt is, 

^ ^ ^-^ „ = .685 + ampere. 


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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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 

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 

1. Tea cells in series have an E. M. F. of 1 volt each and 

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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 + 


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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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. 


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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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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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 

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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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. 


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 

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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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. 


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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lent of electrical power in mechanical pover divide the number of 
wattB by 746. 


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. 


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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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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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 

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 

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H E 






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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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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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. 

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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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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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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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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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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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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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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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. 



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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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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die genaal scheme more dearly than better and more complicated 

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 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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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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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- 

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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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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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 
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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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. 


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 

^ 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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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. 

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 




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 
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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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, 
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 



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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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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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- 

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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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? 





Flg.M. Dikgnmof aTestlDBSei. 



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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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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^^^ 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 




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- 


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 

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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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 


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 


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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- — = 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 




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, 

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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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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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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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 

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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- 

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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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 

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 

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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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. 



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. 

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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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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_ 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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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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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 

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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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. 


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 

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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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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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 

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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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. 


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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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- 

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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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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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- 

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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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. 




o ■ 







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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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. 



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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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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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 

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- 

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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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 

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 


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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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„ 

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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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 

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^, 


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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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 

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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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 

° 1,000,000 
From this it follows that 

^_ 1,000,000 7 _ .59,-. I 

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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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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: 


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? 


«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 







remaining the same as in the previous example, what correcdon, if 
any, should be made? 

/. - 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. 


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 


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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1 ft 


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 


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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Usually 6 b greater than a, so we write the formula 

,. , rR J,' 1 1 , 


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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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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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. 

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 



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 

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. 


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 

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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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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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 


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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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,. 

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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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 



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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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: 







IC. O. 8. IJbit«) 


I'M + 217 

616fi X 10» 


194 + 423 

6170 X 10« 


+ 247 



+ 490 



4- 282 



+ 576 



+ 322 



+ 688 



+ 367 



+ 835 



M = 4.926 X 10-" X 6172 X 10" =-3.04 X 10' or .0304 henrys. 


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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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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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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/»„ 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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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 

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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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. 


~« ^5^ 

Z. // 


Z ;""' 

L ■it 

«<« H H 

Z. Tl 

9000 .J. X 

z ^> 

Z>i::e:: ,: 

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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 

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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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 

If the deflections are small they may be increased by reversing 

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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 


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. 



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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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 


Combining the earlier 


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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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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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. 



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. 



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 


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 

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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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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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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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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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 

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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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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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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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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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 


Tig. I. Dia^iiin of Simpls Dvdbh 
BbowiiiaESsct on M^nede Ni 

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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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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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 

(volts) divided by the resbtance (ohms), or 1=-^' Thb b true both 

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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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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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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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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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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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- 


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- 










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 




V 1 


ooTvms L 

VUl- 12. DUcnm of Two TypM of Tniufornier 

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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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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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 

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- 

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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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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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. 



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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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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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. 


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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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 

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. 


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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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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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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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 




2- WIRE 6EN£f?AT0/f 


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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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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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 

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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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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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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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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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 


•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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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 

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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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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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 


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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Sizes ol Conductors In Direct Current* 


no voLw 

2ao Volts 



Coiice«l*d Open 















































5 • 














c, m. 

c. m. 























c. m. 









*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 

h.p .X746 

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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Sizes of Conductors la AiteraatlaE Carreat* 



110 VoLn 

220 VoLn 1 




AroM. on 

Si» o( Win 1 























































Sua o 






























































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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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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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 

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 


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. 



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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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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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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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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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. 


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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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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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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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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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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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 

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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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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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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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. 



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- 

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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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 

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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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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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- 



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. 



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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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. 


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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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. 


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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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- 

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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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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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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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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. 


In thb case 90 = — ; , or resistance of the 500 feet of wire 


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 



Carrying Capacity of Wires* 


B. 4 S. G. 



Cinulu MiU 


























































Circuit Mil* 


























































a md IS B. A 8. t 



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 

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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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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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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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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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- 

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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4/ ' 'mis im. 
Vy n?OM WAU, 



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 

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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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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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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^ 




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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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 

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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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 

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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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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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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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. 

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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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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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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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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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- 

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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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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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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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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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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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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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. 


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 

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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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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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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"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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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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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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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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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 

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 


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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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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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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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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Fii. 67. Seclion ol Wiring Trough 

Fig. as, Eumple of Wirim on RuDUiai BosrdB in D»mp PUoo 



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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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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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- 

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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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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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 



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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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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Fig. 108. Proper Uw or Outlet Boiee Fig. 109. Wirine in Partitions Shawios Vat 

PuiceluD Tubca 




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 


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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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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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 

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 

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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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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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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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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- 

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. 



(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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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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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. 

Dignzedoy Google 


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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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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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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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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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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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. 


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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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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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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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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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. 


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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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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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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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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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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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. 


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- 



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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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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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. 


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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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. 

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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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- 

\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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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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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. 

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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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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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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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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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. 


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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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 

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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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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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 


E'R+E' R,-ER 


E' R.~E R-E' R-R (£-£") 


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 ' 

"■ y^ 

and assuming that the readii^ of the voltmeter with the insulation 
resistance connected is 5, we have 

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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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. 


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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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. 


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- 



Thlckneu of Rubber losulatloa 

B. A 8. a»nas 


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 

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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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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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. 

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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MIflimniii W«IChts of Conduit for Reqnlred Wall ThlckacM 



Poundi per 

100 FMt 














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 



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. 


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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Opea-LInk Fuse Spacing 

ot Opiwrito Polmrity 


125 Volts or lesa 

10 amperes or less 
11-100 amperes 

301-1000 amperes 




126 to 250 Volts 

10 amperes or lesa 
11-100 amperes 
101-300 amperes 
301-1000 amperes 






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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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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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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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 


mtMH-fosKHZ cornier 















tmm tr 























- % 













il i 


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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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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Approved Spacing for Knife Switdies 

of NevM M«td 


^"*'l^chS' ^°'' 


Not over 125 Volta D. C. and A. C. 

tor Switchboards and Panel Boarde* 

10 amperes 



30 amperes 



60 amperes 



Not over 126 Volta D. C. and A. C. 

for Individual Switchest 

30 amperee 



60 and 100 amperes 





400 and 600 amperee 



800 and 1,000 amperes 



250 Volts only D. C. and A. C. for 

All Switches 



Not over 250 Volts D. C. nor over 

500 Volts A. C. for All Switcheet 

30. 60 and 100 amperes 



200 and 300 amperes 



400 and 600 amperes 



800 and 1,000 amperes 



Not over 600 Volte D. C. and A. C. 

for All Switches** 

30 and 60 amperes 



100 amperes 



at HUdKCA Bdviiifld for 
JThe aOO-ampera bw 

I spsc«d tor SOO vait tiues, and 
•Tho 30-»nipere awitch oi 


ughh. ,dod b.. 





eh caw 


chM mu 


-?!! S 


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. 



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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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 



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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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 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 

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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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, 





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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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. 


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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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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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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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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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, 





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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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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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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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- 

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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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^ 

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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1. (a) Explain what is meant by electromotive force, (Jj 
n''hat is its unit of measurement, and by what value is it repre- 

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- 

S. What is tlie unit of resistance and by what value is it 

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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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 

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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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- 

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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 

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 

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. 





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- 

(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- 

14. Outiine the standard practice for outside wiring. 

15. What is electrolysis? 

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1. Wliat 13 open-work wiring? 

2. What is the easential prindple of conduit work? 

3. What are the advantages and disadvantages of concealed 

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- 

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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 
Armored cable 
Artificial magnets 
Astatic galvanometer 

Ballistic galvanomet«r 
Bar m^net 
Battery circuits 
Bell-ringing transformers 
Bichromate cell 
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 

direct deflection method 
method of mixtures 

Car wiring 

Cartridge fuses 




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 


battery lOS 

divided 100 

fall of potential in 98 

grounding of 289 

Commercial efficiency 

Condensers 33 




Conduit fittings 

Conduit work 

Control magnet, use of 



chemical method of meaaurii^ 

measurement of 
Current systems (constant) 

Cut^-outs * 

knife switches 
snap switches 

Damping of vibrations 
D'Arsonval galvanometer 
Density of charge 


ri ue foal o/ pa 


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 



eIectrc»notive force 

Ohm's law 




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 


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 

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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Galvanometers " 





electric beU 


■ mirror 


plunger type 



magnetic properties of a loop 


use of control magnet 

relay and sounder 


Generators 3 



Gold-leaf electroscope 


Ground detectors and testa S 

Electromotive force 


Grove cell 

galvanic cells 


at make and break 



measurement of 




potentiometer method 


magnetic properties of 

voltmeter method 


rules for north and south poles of 

of secondary 


Electron theory 


of electricity 

Electroscope, gold-leaf 


terns a 



Horseshoe magnet 


Hot-wire inatrumento 



Hysteresis S 
testOT S 


Fall of potential in circuit 



Fixture wire 


Inclination or dip 

Fixture wiring 


Induced currents 



direction of 

Flexible oorda 


dynamo rule 



eloctromotive force at make and 

Friction, electrif action by 




Fuses 287,372 






strength of induced e. m. f. 
Induction, charging by 

Galvanic ceU 


Induction coil and transformer, prin- 

electrical resistance of 


ciple of 


Induction of currents by magnets 

internal reaiatance of 


Inductive action, earth's 



Inside work S 



InHtallfltion rules 3 



choice of 


constantr-potential systems S 


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 


Machines (moving picture) 351 


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 


N numbtri ut loot oj todh. 


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 


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 


Ohm's law 
Outside work 


grounding of circuits 

h^ tension lines . 

mounting of transformers 


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 



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 

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 


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 



Vdc— Ar yoft mmbtn «m /Mt a/ foatt. 


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 

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 


VoHato cells 

Voltmeter method for testing 



Wheatatone bridge 

general rules on 

installation of in buildings 

in molding 
Wiring systems 

oy Google