268 



THERMOELECTRIC CURRENTS. 



and the maximum will take place at the temperature / m , which is 

 evidently independent of f lt and is denned by the condition 



H 2 dh A 

 - + =0, 

 dt dt 



We shall see that at this instant the value of H 2 is zero, and 

 that it then becomes negative. The difference of potential near 

 the junction is then simply due to the variations of temperature 

 on the two metals (Fig. 65). 



Cu 



Fig. 65. 



As the temperature continues to rise, H 2 changes its sign ; 

 the iron which was positive to the copper becomes negative; the 



Fig. 66. 



distribution of potential is represented by Fig. 66, and heat is 

 disengaged at the two junctions. 



Inversion takes place at the moment at which 



The electromotive force changes its sign at a higher temperature 

 at the heated junction, and we have 



