28 



STATIC ELECTRICITY 



outside of the can and the gold leaves become negatively electrified. 

 We may at once prove this by again bringing up the ebonite rod, 

 when the leaves will diverge still further. Now, discharging the 

 electroscope and again inserting in the can 

 the positively charged body, we know from 

 our experiments on induction that the quan- 

 tities on the inside and the outside surfares 

 of the can are equal, though opposite. 

 Touching the charged body to the inside of 

 the can, it is entirely discharged, and at the 

 same time the inside charge of the can dis- 

 appears, while the outside charge is un- 

 affected. Hence the inside charge must 

 liuvr been exactly equal in amount, though 

 opposite in kind, to the charge on the 

 inserted Ixxlv. Faraday used the cube 

 already described to show that a charge 

 within a conductor induces an equal and 

 opposite charge on the inner surface of the 

 conductor. For this purpose he passed a 

 long glass tube through the wall of the cube 

 so far that it reached well within it. A wire 

 from a large electric machine passed through 

 the lube, and bv discharge from the end of the 

 wire the air within the cube could be highly 

 electrified. If while the machine was being 

 worked the cube remained insulated, the 

 outside became strongly charged. If it was 

 put to earth this charge was conducted away. If at the moment 

 that the machine was stopped the cube was insulated again, no 

 charge gathered on the outside, showing that the electrification 

 of the air within \\a> exactly neutralised by the charge which it 

 induced on the inner surface of the cube. 



Imagined construction of multiples and sub- 

 multiples of an arbitrary unit of charge. 1 fa fore pro- 

 ceeding further with the account of electric induction it may 

 assist us if we consider a method of making quant itathc measure- 

 ments of charge which depends upon the foregoing experiments. 

 The method is impracticable owing to the impossibility of perfect 

 insulation, but. as a conception, it is a legitimate deduction from 

 the principles to which we have been led, and it KTrefl to show 

 that we can attach a definite meaning to the term ' quantity of 

 electricity." 



Let us suppose that perfect insulation is possible and that we 

 have a conductor on a perfectly insulating stand. Let us impart 

 to A, Fig. 22, some arbitrary positive charge. With a perfect 

 insulation this charge will remain on A for ever, and so being 

 definite and consistent we may chooie it as the unit of charge. 



FIG. 21. 



