TABLE 40 (continued) 70 



PROBABLE VALUES OF THE GENERAL PHYSICAL CONSTANTS 



The numerical relation of the int. and abs. ohm rests chiefly on two exten- 

 sive investigations, one ^ = 1.00052 ±0.00004, by Smith, 1 at the National 

 Physical Laboratory (N.P.L.) of England, and the other, /» =1.00051, by 

 Griineisen and Giebe, 2 at the German Reichsanstalt. The latter estimate their 

 probable error, as well as that of Smith, as about 3 parts in io 5 . In 1925 a 

 committee at the N.P.L. began an investigation of the relation of the int. and 

 abs. electrical units. This work is incomplete. It was stated in 1925 3 that a 

 comparison of various manganin with mercury resistances indicates that the 

 former have all increased in resistance by about 2.5 parts per io 5 since 1912, 

 or that the mercury standards (defining the int. ohm) are really smaller by this 

 amount. The latter assumption would give />= 1.000495, in place of Smith's 

 value of 1.00052. In a recent investigation at the Reichsanstalt, Steinwehr and 

 Schulze 4 evidently assume that the N.P.L. 1925 standards are 2 parts in io 5 

 less than the older 1912 standards, giving a mean value of p in exact agreement 

 with the 1920 Reichsanstalt value. Their own experiments in 1928 agree with 

 this same mean value to ± 1 in io 5 . Various intercomparisons at the N.P.L. 5 

 show that the German and American standards lie between the 1912 and 1925 

 N.P.L. values. It seems certain that the best value of p, at the present time, 

 is 1. 00051 (p.e. seems to be not more than 2 parts in io 5 ). 



The most probable value of q is more uncertain. In the older work, the abs. 

 amp., determined with either a current balance or a tangent galvanometer, was 

 compared directly with the int. amp. as measured by a silver voltameter. There 

 was measured by means of a silver voltameter, with certain specifications, the 

 amount of silver, in grams, deposited per sec. by a current of one abs. amp. 

 This mass of silver was then compared with 0.00111800 gram, the defined 

 amount deposited, under the same conditions, by one int. amp. per sec. 



Such a procedure determines q unambiguously, but does not necessarily 

 evaluate the electrochemical equivalent of silver (E Ag ) per abs. coul. The 

 electrochemical equivalent of a substance is the mass actually associated with 

 unit charge, and is independent of experimental imperfections, while the mass 

 deposited in an electrolytic cell per unit charge — the only quantity we can 

 actually measure — is subject to experimental imperfections. This distinction 

 has no bearing on the value of q, so long as one accepts the official definition 

 of the int. ampere. It concerns only the value of electrochemical equivalents 

 and the resulting value of the faraday. The various experimental values of q, 

 determined as explained above, are listed by Henning and Jaeger.* 



1 Philos. Trans., 214, 27, 1914. 2 Ann. Phys., 63, 179, 1920. 3 N.P.L. Reports, p. 94, 

 1925. 'Ann. Phys., 87, 769, 1928. B N.P.L. Reports, p. 8, 1927. 6 H.P., 2, 499. 



Smithsonian Tables 



