Electromotive Forces of Contact, 45 



And my first point is to show that this action is confined to 

 molecules within the layer of variable potential. 



For consider a particle P (fig. 1) within the conductor A, 

 so that all around P the potential is constant. Then if P 

 takes part in any way in maintaining a difference of potentials 

 between A and B, it must in turn be reacted on by the 

 electric forces which tend to equalize the potential through- 

 out the system. Now, the only forces tending to equalize 

 the potential are the attractions and repulsions inter se of the 

 electric masses distributed on the conductors ; and the par- 

 ticle P being situated in a region where the potential is 

 constant, the electric force at P is zero. 



Therefore P suffers no reaction from the electric masses of 

 the system and cannot, therefore, play any part in maintain- 

 ing the difference of potential. 



Hence the molecular action which gives rise to a contact 

 E.M.F. between two conductors is confined to the immediate 

 neighbourhood of the junction. 



Now, suppose that two conductors A and B (fig. 2), whose 



Biff. 2. 



contact-difference of potentials is P, are originally at the 

 same potential, and are then brought into contact at C. As 

 soon as this is done an E.M.F. E (= — P) will act between A 

 and B, and will drive electricity across the junction at C, until 

 the difference of potentials of A and B becomes equal to P. 



At the beginning of the operation, when contact first 

 occurs, there is no E.M.F. opposed to E, so that when E 

 drives a quantity dM. of electricity across the junction, the 

 work done is EdM. As there is no opposing E.M.F. to be 

 overcome, the whole of this work is spent in producing heat, 

 according to the Joule effect. 



At a later stage, when the difference of potentials has be- 

 come (say) p, the total E.M.F. between the conductors 

 = E +p ; p being of course of the opposite sign to E. Hence, 

 when the quantity c/M crosses the junction, the work 

 (E+/))c/M is spent in producing the Joule effect ; while the 

 electric potential energy of the system increases by — pdM ; 

 so that the total work done on dm by the E.M.F. E = EcZM. 



