SIR w. THOMSON'S THEORY. 267 



The electromotive force resulting from variations of temperature 

 is null in a homogeneous wire (law of Magnus), for the total fall 

 of potential on either side of the maximum is of the same value ; 

 but this compensation no longer holds on each side of the junction 

 of two different metals, and we must take into account the continual 

 change of potential which variations of temperature determine along 

 conductors. 



To give greater definiteness to these conceptions, let us consider 

 a copper-iron pair, for example, working between the temperatures 

 /! and / 2 , and let Hj and H 2 (Fig. 64) be the electromotive forces 

 of contact at these two temperatures; suppose, further, that the 

 potential has increased along the copper C M , in consequence of a 

 rise of temperature from ^ to / 2 , by a quantity c independent of the 

 strength of the current ; and that conversely there is a fall of potential 



!H2 



1 



Fig. 64. 



fj on the iron Y e for the same excess of temperature ; the potential 

 near the hot junction will be higher by a quantity f+c=h, and the 

 electromotive force of the couple will now be 



We have implicitly assumed that the temperature / 2 is lower than the 

 temperature of inversion. The current goes from copper to iron 

 through the hot junction; the thermal energy absorbed at the hot 

 junction, as well as on the adjacent points, is equal to (H 2 + ^)I, and 

 that which is expended at the cold junction H 1 I. 



The lower temperature / x being fixed, the electromotive force 

 of the couple will increase as long as H 2 + h increases that is, so 

 long as 



dh 



dt dt 



