246 ELECTROSTATICS. [PT. II. CH. VI. 



that A is now negatively electrified, and B positively. By intro- 

 ducing A and the glass together into a larger metal vessel C, 

 its outside will be found to have no charge, consequently the 

 induced charge on A is equal and opposite to that of the glass. 

 In like manner the charge of B may be shown to be equal and 

 opposite to that of the resin. The charge of A, which is not 

 apparent as long as the glass is within, is said to be bound by the 

 inducing charge of the glass, and resides on the inside of A. 

 By the withdrawal of the glass it becomes free, and appears on the 

 outside of A. We have thus a method of charging a vessel with 

 an electrification equal in amount and opposite in kind to that of a 

 given electrified body without changing its electrification. 



EXPERIMENT VII. Let the vessel B, charged with a quantity 

 of positive electricity, which we shall take for a provisional unit, be 

 introduced into the vessel C without touching it. C will be found 

 charged on the outside with a unit of positive electricity. Now let 

 B touch the inside of C. The external electrification is unchanged. 

 If B be now removed from C without touching it, and taken to a 

 distance, the field external to C is still unchanged, that is, C is 

 charged with a unit of electricity, but B is completely discharged. 

 If B be now recharged with a unit of positive electricity, and 

 again introduced and made to touch (7, on removal it will again be 

 found to be completely discharged, and the charge of G will be 

 increased by one unit. This may be repeated indefinitely, and no 

 matter how highly C may become charged, it will be found that B is 

 always completely discharged. This is a cardinal point in the 

 theory of electricity. Since when in contact B virtually forms a 

 part of the conductor (7, we may state that there is no electrifica- 

 tion on the inside of a charged conductor left to itself. We now 

 have a means of charging a body with any number of units of 

 electricity. A machine for the purpose of generating electricity 

 on this principle is Kelvin's Replenisher, whose theory will be 

 considered later. 



The last experiment may be modified by examining the field of 

 force within a hollow charged conductor. This cannot be done by 

 introducing anything through a hole, but was accomplished by 

 Faraday by building a closed conductor large enough for a person 

 to remain inside. Even when the outside was so highly electrified 

 that large sparks were flying off from it, the strength of the field 

 at points within was absolutely zero. 



