INTRODUCTION xlix 



national committee since 1910, but no agreement upon final specifications has 

 yet been reached. 



Resistance Standards Used in Current Measurements. — Precise measure- 

 ments of currents require a potentiometer, a standard cell and a resistance 

 standard. The resistance must be so designed as to carry the maximum current 

 without undue heating and consequent change of resistance. Accordingly the 

 resistance metal must have a small temperature resistance coefficient and a 

 sufficient area in contact with the air, oil, or other cooling fluid. It must have 

 a small thermal electromotive force against copper. Manganin satisfies these 

 conditions and is usually used. The terminals of the standard must have suffi- 

 cient contact area so that there shall be no undue heating at contacts. 1 It must 

 be so designed that the current distribution does not depend upon the mode of 

 connection to the circuit. 



Absolute Ampere. — The absolute ampere (io _1 c.g.s. electromagnetic units) 

 differs by a negligible amount from the international ampere. Since the dimen- 

 sional formula of the current in the electromagnetic system is [L*M */ Tyft~\ which 

 is equivalent to \_F*/yfi~\, the absolute measurement of current involves funda- 

 mentally the measurement of a force in a medium of unit permeability. In most 

 measurements of high precision an electrodynamometer has been used of the 

 form known as a current balance. 



The best value may be taken as (Birge, 1930) 



1 international ampere = 0.99995 ±0.00005 absolute ampere. 



The result may also be expressed in terms of the electrochemical equivalent of 

 silver, and thus equals 0.00111805 g per absolute coulomb. The value is 

 0.00111800 g per international coulomb. 



ELECTROMOTIVE FORCE 



International Volt. — "The international volt is derived from the interna- 

 tional ohm and ampere by Ohm's law. Its value is maintained by the aid of the 

 Weston normal cell. The national standardizing laboratories have groups of 

 such cells, to which values in terms of the international ohm and ampere have 

 been assigned by international experiments, and thus form a basis of reference 

 for the standardization of the standard cells used in practical measurements." 



Weston Normal Cell. — The Weston normal cell is the standard used to 

 maintain the international volt and, in conjunction with resistance standards, 

 to maintain the international ampere. The cell is a simple voltaic combination 



1 See "Report to the International Committee on Electrical Units and Standards," 191 2, p 

 199. For the Bureau of Standards investigations see Bull. Bureau of Standards, 9, pp. 209, 493; 

 10, p. 475, 1912-14; 13, p. 147, 1915; 9, p. 151, 1912; 13, pp. 447. 479. 1916. 



