THOMSON EFFECT. 



269 



In this case, which is represented by Fig. 67, the current absorbs 

 thermal energy at the two junctions, IH 2 at the hot one, IH 1 at the 

 cold one, and a quantity \h is liberated at those points where the 

 temperature varies. 



Such is a general idea of Sir W. Thomson's theory, the 

 mathematical consequences of which we shall proceed to develop. 

 We shall apply the term Thomson effect to the difference of 

 potential due to the differences of temperature which form the 

 basis of this theory. 



277. THERMOELECTRICAL POWERS. We have seen that, by 

 the law of successive temperatures, the electromotive force of a 

 couple is the difference of the values of one and the same function 

 for the temperatures of the two junctions. If these temperatures 

 / and t + dt are infinitely near, the electromotive force is infinitely 

 small, and is expressed by 



= dt; 



dt 



we may therefore write 



dt 



Sir W. Thomson calls the function <J>(t) the thermoelectrical 

 power of the two metals at the temperature t. This function is 

 nothing but the angular coefficient of the tangent to Gaugain's 

 curves. We can deduce from it the electromotive force of the 

 couple for the temperatures ^ and / 2 of the two junctions by the 

 formula 



278. This function possesses a remarkable property, in virtue 

 of which thermoelectrical phenomena may be very simply expressed. 



