266 THERMOELECTRIC CURRENTS. 



value E', which is little more than H 2 -H 15 the direction of the 

 current would change ; if the absolute value of the strength remains 

 the same, the same quantities of heat would be put in play at 

 each junction, but in opposite directions, and the electrolyte would 

 produce heat instead of absorbing it. In the case of a very 

 feeble current, the thermoelectrical pile would behave as a re- 

 versible calorific engine, and we may apply the principle of Carnot. 

 If T x and T 2 are the absolute temperatures of the two junctions, 

 the quantities of heat Q 1 and Q 2 , absorbed or furnished by the two 

 sources according to the working of the machine, must be proportional 

 to the absolute temperatures T l and T 2 , and we should have 



0,0, 



> 

 or, A being a constant, 



T T 



1 2 1 l 



From this would follow 



1 - = = A, 



T 2 -T X TJ-T! 



and therefore 



Hence the electromotive force of all couples should be pro- 

 portional to the difference in temperature of the two junctions ; all 

 couples would have a" uniform course, and the phenomena of in- 

 version could never be met with. 



276. SIR W. THOMSON'S THEORY. Volta's principle is therefore 

 incapable of giving a complete explanation of thermoelectrical phe- 

 nomena ; we must accordingly assume the existence of electromotive 

 forces other than those of contact, and capable, like them, of pro- 

 ducing reversible thermal phenomena. 



The least changes in the physical condition of metals, such as 

 tempering, torsion, or traction, etc., modify their electrical proper- 

 ties ; it is accordingly natural to assume that the contact of two 

 parts of the same metal at different temperatures also gives rise to 

 a difference of potential. 



