INDUCTION ON AX CLOSED CONDUCTOR. 



body, originally in the neutral state, is placed near electrified bodies 

 it becomes itself electrified ; the phenomenon is known as electrifica- 

 tion by influence or induction. If the body under electrical induction 

 is insulated, its total electrical mass, from what we have seen, must 

 remain zero. It will then be charged with two masses, equal, 

 and of opposite signs, distributed according to a certain law. The 

 phenomenon of induction always precedes the attraction of a neutral 

 body by an electrified one, and the action which is exerted is simply 

 that between the electrical masses. We may thus consider it as an 

 experimental fact that there is never any direct action, except that of 

 electrical masses on other electrical masses. 



11. ELECTRICAL EQUILIBRIUM. The essential characteristic of 

 induction is that electricity is produced at every point of a conductor 

 at which electrical force is exerted. Equilibrium can therefore only 

 exist on a conductor, provided that the electrical force is zero at each 

 of its points ; the electricity which it possesses exerts, at each point of 

 its surface, an action equal and of opposite sign to that of the external 

 masses. 



The necessary and sufficient condition for electrical equilibrium 

 in a system of conductors, insulated or not, is then that the electrical 

 force be zero at any point whatever of each of them. 



12. DIELECTRICS. Electrical force can, therefore, only exist in a 

 state of equilibrium, on bad conductors, or insulators. For this 

 reason Faraday gave the name of dielectrics to these bodies, to denote 

 that they are bodies in which electrical forces may exist or be 

 transmitted. 



13. LOCALISATION OF ELECTRICITY ON THE SURFACE OF CON- 

 DUCTORS. The experiments of Cavendish and of Coulomb showed 

 that in any electrical system in equilibrium, conductors have electricity 

 on their external surface only. The surface of any closed cavity 

 hollowed in a conductor, and not containing electrical masses, is 

 destitute of electricity, and the electrical force is null throughout the 

 above extent of the cavity. We shall find that this fundamental 

 property is only compatible with the law of inverse squares. 



14. INDUCTION ON A CLOSED CONDUCTOR. This localisation 

 of electricity on the surface of a conductor leads to several important 

 consequences. 



When a conductor is electrified by induction, each of the positive 

 and negative layers, with which it is charged, forms a mass less than, 

 or at most equal to, that of the influencing body, or inductor. When 

 the influenced, or induced, conductor completely surrounds the in- 

 ductor, the outer electrical layer is of the same kind as that of the 



