758 NUMERICAL CONSTANTS. 



according to M. La Roux (284 and 1034), the specific heat of 

 electricity of lead is null, then, referring all metals to lead, we 

 may write 



dE 

 *(/)- 



The coefficient k= - B, on Professor Tait's hypothesis (284), 



is the constant ratio of the specific heat of electricity to the absolute 



^ 



temperature ; the constant t n = - - represents the neutral point of 



B 



the couple formed by the metal compared with lead. 



For two different metals M and M' the thermolectric power is 



at 



the electromotive force E'J, between two temperatures / x and / 2 , the 

 mean of which is = , is expressed by 



r 



[ 







B'-B 



This electromotive force is equal to the product of the difference 

 of temperatures / 2 - / x of the two functions by the difference of the 

 thermoelectric powers of the metals in respect of the mean tem- 

 perature 0-, we see thus that the neutral point of the couples is 



A -A' 



It is not surprising that the numbers found by different experi- 

 menters are not in close agreement, for traces of foreign substances 

 and the least change of physical state are sufficient to modify the 

 thermoelectric power. It is difficult, for instance, to obtain bismuth- 

 copper couples, in which the bismuth bar has been prepared by 



fusion, which are identical to within th ; the addition of a tenth 



10 



of antimony to the bismuth almost doubles the electromotive force 

 of the couple. 



