INTRODUCTION. XXXvii 



This system of units, sufficiently near to the "absolute" system for the pur- 

 pose of electrical measurements and as a basis for legislation, was defined as 

 follows: 



''i. The International Ohm is the resistance offered to an unvarying electric 

 current by a column of mercury at the temperature of melting ice, 14.4521 grams 

 in mass, of a constant cross-sectional area and of a length of 106.300 centimeters. 

 "2. The International Ampere is the unvarying electric current which, when 

 passed through a solution of nitrate of silver in water, in accordance with speci- 

 fication II attached to these Resolutions, deposits silver at the rate of 0.00111800 

 of a gram per second. 



"3. The International Volt is the electrical pressure which, when steadily 

 appUed to a conductor the resistance of which is one international ohm will pro- 

 duce a current of one international ampere. 



"4. The International Watt is the energy expended per second by an unvary- 

 ing electric current of one international ampere under the pressure of one inter- 

 national volt." 



In accordance with these definitions, a value was established for the electro- 

 motive force of the recognized standard of electromotive force, the Weston 

 normal cell, as the result of international cooperative experiments in 19 10. The 

 value was 1.0183 international volts at 20° C. 



The definitions by the 1908 International Conference supersede certain defini- 

 tions adopted by the International Electrical Congress at Chicago in 1893. Cer- 

 tain of the units retain their Chicago definitions, however. They are as follows: 

 ^^ Coulomb. As a unit of quantity, the International Coulomb, which is the 

 quantity of electricity transferred by a current of one international ampere 

 in one second. 

 ^^ Farad. As a unit of capacity, the International Farad, which is the capacity 

 of a condenser, charged to be a potential of one international volt by one 

 international coulomb of electricity. 

 ^^ Joule. As a unit of work, the Joule, which is equal to 10^ units of work in 

 the c.g.s. system, and which is represented sufficiently well for practical use 

 by the energy expended in one second by an international ampere in an 

 international ohm. 

 " Henry. As the unit of induction, the Henry, which is the induction in a 

 circuit when the electromotive force induced in this circuit is one interna- 

 tional volt, while the inducing current varies at the rate of one ampere per 

 second." 



"The choice of the ohm and ampere as fundamental was purely arbitrary. 

 These are the two quantities directly measured in absolute electrical measure- 

 ments. The ohm and volt have been urged as more suitable for definition in 

 terms of arbitrary standards, because the primary standard of electromotive 

 force (standard cell) has greater simplicity than the primary standard of current 

 (silver voltameter). The standard cell is in fact used, together with resistance 

 standards, for the actual maintenance of the units, rather than the silver vol- 

 tameter and resistance standards. Again, the volt and ampere have some claim 



