1 82 SOURCES OF ELECTRICITY. 



number of points touching. We may further add that the pene- 

 tration of electricity in the dielectric still further complicates the 

 experiments. 



193. ELECTRIFICATION BY FRICTION. In electrification by fric- 

 tion, the electricity seems to have no other cause than the contact 

 of two bodies ; the friction simply multiplies the number of points 

 of contact. 



When the bodies rubbed are the same, they cannot in general be 

 electrified ; very feeble traces of electricity are sometimes seen, but 

 in that case the development of the electricity may always be attri- 

 buted to a more or less visible dissymmetry between the two bodies 

 rubbed. 



194. ELECTRICAL MACHINES. We are thus led to the con- 

 clusion that there are two modes of producing electricity contact 

 and induction. All electrical machines bring into play one or other 

 of these modes, and their only object is to accumulate on a con- 

 ductor the charges produced. 



The general theory of these machines is very simple. Let us 

 consider a hollow insulated conductor that of Faraday, for instance. 

 On the other hand, let us take the cake of an electrophorus of resin 

 or ebonite charged with negative electricity. A metal disc held by 

 an insulating handle, placed on the cake and connected with the 

 ground for a moment, will become charged with positive electricity, 

 and, if it is carried to the cylinder and made to touch the inside, an 

 equal charge will be produced on the outer surface of the cylinder ; 

 the disc itself will be neutral on being taken out, and the same pro- 

 cess may be repeated indefinitely. It is clear that in theory, there 

 is no limit to the charge of the cylinder, for, whatever be the charge 

 which it has acquired, a conductor, placed in the inside and put in 

 communication with it, can retain no electricity. This, reduced to 

 its simplest expression, is the mode of action of machines based on 

 induction. 



Let us now suppose that instead of the cake of the electrophorus, 

 we have a disc of copper in connection with the earth, and that we 

 touch it with a zinc disc held by an insulating handle ; from Volta's 

 law, the zinc disc would be charged with positive electricity; this 

 charge, as in the preceding case, could be transferred to the cylinder, 

 and the experiment could be repeated for an indefinite number of 

 times. 



This would also be the case if we took a piece of cloth, or of 

 caoutchouc, instead of a copper disc, and glass disc instead of one 

 of zinc. The insulating nature of the bodies employed must be 



