272 THERMOELECTRIC CURRENTS. 



This expression is in conformity with the laws of Gaugain (272). 

 If the straight lines AX and BX were parallel, we should have 



and the couple would have a uniform course. 



281. SPECIFIC HEAT OF ELECTRICITY. Suppose now that the 

 variations of potential, to which electromotive force is due, are of 

 two kinds; sudden variations, resulting from Volta's principle, and 

 continuous variations connected with variations of temperature, and, 

 like the former, capable of producing reversible calorific phenomena. 

 It is clear that, if we designate the variations of the former kind by 

 H, and the sum of the continuous variations which exist between the 



two points A and B of a conductor by I dh t the value of the whole 

 electromotive force will be 



E = 



The variations of the second kind between two points M and M' 

 of the same metal, according to the law of Magnus, only depend 

 on the temperatures / and /', and not at all on the intermediate 

 resistance. We may then put 



If the current is so small that the heating of the circuit on 

 Joule's law may be neglected, the quantity of heat absorbed or 

 developed in unit time in that portion of the circuit in which is 

 produced the heat in question, by the passage of a current I, will 

 clearly be expressed by 



Idh = If(t)dt=I<rdt. 



The quantity a- is the variation of potential, and therefore the 

 thermal work for unit current which corresponds to a variation 

 of temperature equal to unity ; it is a characteristic function of the 

 nature of the conductor, but which varies with the temperature. 

 Sir W. Thomson has given the name specific heat of electricity to this 

 new physical quantity. 



