TAIT'S HYPOTHESIS. 275 



283. The discussion of this formula leads to the different cases 

 the examination of which we have anticipated (276). Let T n be 

 the temperature corresponding to the neutral point ; for this point 

 the thermoelectric power is zero, we have then 



<(/) -0 and H n = 0. 



So long as the temperature T 2 of the hot junction is lower than 

 T n , the function H 2 is positive, and the current cools the hot 

 junction. 



When T 2 = T n , the heating effect is null at the hot junction. If 

 the temperature T 2 is between the maximum temperature and that 

 of inversion T f , H 2 is negative ; the current heats the hot and the 

 cold junction at the same time. 



Finally, if the hot junction is at a higher temperature than that 

 of inversion, the current cools the two junctions. 



According to Sir W. Thomson, and in conformity with Gaugain's 

 experiments, the electromotive force of a couple may be empirically 

 represented by the formula 



It follows from this that, for an infinitely small difference 

 between the temperatures of the two junctions, 



and, therefore, 



The electromotive force of the couple and the electromotive 

 force of contact between the two metals, expressed as a function 

 of the temperature, will therefore both be represented by parabolas. 



284. TAIT'S HYPOTHESIS. Professor Tait arrived at the same 

 result by assuming that the specific heat of electricity o-, characteristic 

 of each metal, is proportional to the absolute temperature. Hence, 

 denoting by k and k' the constants for each metal, 



T 2 



