264 THERMOELECTRIC CURRENTS. 



Let I be the current which traverses the circuit whose total 

 resistance is R. In unit time, the work withdrawn from the heated 

 junction is IH 2 , and the work expended at the cold junction is 

 IHj ; the difference of these two works is transformed into thermal 

 energy, which is disengaged in the circuit in accordance with Joule's 

 law, and we have 



whence 



T _H 2 -H 1 



R 



The system may therefore be looked upon as a heat engine, the 

 boiler of which yields a quantity of heat Q 2 given by the equation 

 JQ 2 = IH 2 , while the condenser absorbs a smaller quantity of heat 

 Qj, defined in like manner by the equation JQ 1 = IH 1 , the difference 

 of these two quantities being employed to heat the circuit, from 

 which it follows that 



The law of Magnus is contained in the hypothesis that there is 

 no electromotive force at the junctions. 



The law of successive temperatures follows from the identity 



B|A 



Lastly, the law of intermediate metals is also evident, for, by 

 definition, we have 



On the other hand, Volta's law of tensions gives, for any given 

 temperature, 



B|C C|A_B|A 



~~ " ~> 



the preceding equation thus becomes 



