Middle of the Nineteenth Century. 267 



Since the metals A and B are quite independent, this gives 



This equation connects Thomson's " specific heat of electricity" 

 S A (T) with the Peltier effect. 



In 1870 P. G. Tait* found experimentally that the specific 

 heat of electricity in pure metals is proportional to the absolute 

 temperature. We may therefore write S A (T) = a A T, where 

 a A denotes a constant characteristic of the metal A. The 

 thermodynamical equation then becomes 



_d \U A (T)) 

 dT ( T ~ 



or 



where TT A denotes another constant characteristic of the metal. 

 The chief part of the Peltier effect arises from the term ir A T. 



By the investigations which have been described in the 

 present chapter, the theory of electric currents was considerably 

 advanced in several directions. In all these researches, how- 

 ever, attention was fixed on the conductor carrying the current 

 as the seat of the phenomenon. In the following period, interest 

 was centred not so much on the conductors which carry charges 

 and currents, as on the processes which take place in the 

 dielectric media .around them. 



* Proc. R. S. Edinb. vii (1870), p. 308. Cf. also Batelli, Atti delia R. Ace. di 

 Torino, xxii (1886), p. 48, translated Phil. Mug. xxiv (1887), p. 295. 



