ELECTROMOTIVE FORCE OF CONTACT. 175 



CHAPTER VIII. 

 SOURCES OF ELECTRICITY. 



186. We have already mentioned Volta's most important dis- 

 covery that two conductors, and, more generally, any two bodies 

 placed in contact, assume different electrical conditions on each side 

 of the surfaces in contact. 



In the case of two different conductors in contact and in equi- 

 librium, the potential is constant on either of them, -but experiences 

 a sudden change, on passing from one surface to the other. 



We need not mention here all the experiments by which this law, 

 of such fundamental importance, has been established; we shall 

 merely adduce the following experiment, which will serve to define 

 the conditions of the phenomenon. 



If two plates, one of zinc and the other of copper, in the 

 neutral state and at the same temperature, while held by insulating 

 handles, are placed in contact with their parallel faces, and are then 

 separated from each other, each of them will be found to be elec- 

 trified, the zinc positively and the copper negatively. The electrical 

 charge on each of the plates is proportional, other things being equal, 

 to the extent of the surfaces in contact. 



The phenomenon is as if the plates were the coatings of a con- 

 denser, between which there existed a given difference of potential ; 

 the corresponding electrical layers being on each side of the surface 

 of separation in the two metals respectively, and at a very small 

 distance apart, so that the capacity of the system is simply pro- 

 portional to the extent of the surfaces facing each other. 



187. ELECTROMOTIVE FORCE OF CONTACT. If V 1 and V 2 are 

 the potentials of the zinc and of the copper, 3V their difference 

 Vj - V 2 , or the electromotive force of contact, and C the capacity 

 of the condenser formed when they are in contact, the charge of the 

 plates will be 



