INTRODUCTORY. 



inductor, and the charge at each point is independent of the position 

 of this latter. Nothing is altered, even if the inductor comes in 

 contact with the internal surface ; but then if it also is a conductor 

 it only forms with the induced body a single conducting mass, and 

 the internal surface retains no charge of electricity. There was, 

 accordingly, in the inside of the induced body, an electrical layer 

 equal and of opposite sign to that of the inductor, and hence on the 

 external surface a layer equal and of the same sign. 



The quantity of electricity induced by an electrified body on a 

 conductor which completely surrounds it, is thus equal to the quantity 

 of inducing electricity. This property also holds if the inducing body 

 is a bad conductor, and more generally if the electrical masses are 

 distributed in any manner whatever in the cavity of the conductor. 



15. ADDITION OF CHARGES. We have seen above that the 

 electrical charge of a conductor may be divided into two. In like 

 manner any electrical masses whatever may be added to a conductor. 

 It is sufficient for this if the conductor has a cavity almost entirely 

 closed through which electrified conductors may be introduced, and 

 which, by contact, transmit the electricities with which they are 

 charged to the outer surface. 



16. We may then increase or diminish at pleasure, the algebraic 

 sum of the electrical masses contained in the interior of a closed 

 surface, provided we introduce, or give exit to, positive or negative 

 masses. It is, however, important to remark that, if no mass tra- 

 verses the surface in one direction or the other, whatever be the 

 actions to which the enclosed body is submitted friction, induction, 

 contact, physical or chemical actions, it is impossible to modify the 

 total quantity of electricity of the system. We can neither create 

 nor destroy, on any body, a determinate quantity of electricity, without 

 at the same time creating or destroying, on the same body, or on 

 another, an equal quantity of electricity of the opposite sign. 



17. HYPOTHESES RESPECTING THE NATURE OF ELECTRICITY. 

 Electricity, defined and measured as we have explained above, is a 

 magnitude of a particular kind, perfectly definite from the mechanical 

 point of view, affected with a sign like a quantity of motion, and the 

 theory of electrical phenomena may be established from experimental 

 laws without having recourse to any hypothesis. From the facility 

 with which electricity is transmitted in conductors, it has often been 

 compared to a fluid, just as formerly the effects of thermal con- 

 ductivity were explained by the propagation of a special fluid. The 

 character of duality, which electrical phenomena present, has been 

 accounted for in two ways. 



