jS Table 40 (continued) 



PROBABLE VALUES OF THE GENERAL PHYSICAL CONSTANTS 



The int. ohm can be constructed as a definite laboratory standard. This is 

 not true of the int. amp. (a). Hence the 1908 London conference appointed a 

 committee to determine the e.m.f . of the Weston normal cell, in terms of the 

 int. ohm and int. amp. The final value adopted by the committee (Jan. 1, 191 1) 

 was 1.0183 int. volts, at 20°C, which, to avoid ambiguity, is written 1.01830. 

 This is effectively a new definition of the int. volt and to distinguish it, if neces- 

 sary, from the primary definition, Doctor Birge again follows the I.C.T. in 

 writing int. volt (v). Similarly all units involving the Weston normal cell will 

 be designated by "(v)." Let 



1 int. volt (v) =r abs. volt (8) 



as contrasted with eq. (4). It is now possible to use the int. volt (v) and the 

 int. ohm to obtain a new (subsidiary) definition of the int. amp. Thus 



1 int. amp. (v)=r/p abs. amp. (9) 



as compared to eq. (2). Finally, in many investigations, a so-called " semi- 

 absolute " volt has been used. This is defined as the e.m.f. required to force 

 one abs. amp. of current through one int. ohm resistance. Hence from eq. ( 1 ) 



1 semiabs. vo\t = p abs. volt. (10) 



From eqs. (8) and (10) one obtains 



1 int. volt (v)=r/p semiabs. volt. (11) 



We have now to consider the most probable value of p and of q, and the 

 difference, if any, between rand pq (or between r/p and q). These questions 

 are discussed by Henning and Jaeger in the H.P., and they conclude, 



q — 1 , p — 1 .00050, r = pq=i .00050. 



On the other hand, the I.C.T. gives 



q — 0.99993, />= 1.00052, r= 1.00042, while £5=1.00045. 



Hence r/p = o.ggggo^=q. The correct determination of the best values of 

 p and q is a very technical and extremely involved matter. Unfortunately, 

 as just seen, there is no exact agreement on the subject. Part of the present 

 disagreement in the values of p and q is due to the fact that there is no standard 

 international unit of resistance or of voltage. Each national laboratory has its 

 own standards which differ more or less among themselves, and also may 

 change with time. The values of p and q finally adopted here represent, as well 

 as possible, mean values both in respect to place and to time. Fortunately the 

 accuracy of these quantities is so great that any possible error in the finally 

 adopted values is entirely immaterial in its effect on the many constants 

 derived later in this paper. 



Smithsonian Tables 





