l8o SOURCES OF ELECTRICITY. 



and the force which acts on one of the surfaces is 



In order to move the plates to a great distance 

 expend the work 



- fra- 



The electrical energy imparted to the system at the moment 

 of contact, is borrowed from the original potential energy of the 

 two plates, and it cannot be supposed to be greater than that which 

 would be available when the two metals are alloyed together. 



Suppose that, with zinc and copper, the ratio of the weights 

 of the plates is that which forms brass. Let p be the total weight, 

 c the sum of the two thicknesses, and 8 the mean density of the 

 system, we shall have 



or S = ^. 

 The electrical energy may then be written 



This equation holds as long as the metals retain their physical 

 properties, and for this it is evident that the total thickness of the 

 two plates must be greater than the normal distance e of the elec- 

 trical layers. 



Suppose now that, without changing the total weight /, we 

 increase simultaneously the surface of the two plates at the expense 

 of their thickness, the energy W increases in proportion to the 

 surface, or inversely as the thickness, so long at any rate as the 

 thickness e is greater than e. The maximum energy corresponds to 

 the case in which e = e, and we have then 



whence 



<= v ''jiS-w- 



