10 



STATIC ELECTRICITY 



+ 



-f 



4- 

 4- 



4- 



outside of a conductor may be shown in another way. Placing the 

 can on the paraffin, uncharged, we may introduce within it a 

 charged conductor, such as a proof plane or a metal ball with an 

 insulating handle. On lowering the charged body well within the 

 can and allowing it to touch the inside of the can it becomes part 

 of the inside surface. Its electrification at once leaves it, and on 

 taking it out of the can it is found to be entirely discharged. By 

 aid of the proof plane we can show that there is no electrification on 

 the inside of the can. But before contact the inside of the can was 

 -electrified by induction, the state of affairs being represented by Fig. 

 9 ; the + on the insulated body within the can induces on the 



inner surface and the complementary + 

 is drawn to the outside. But we may 

 still say that the can acts as a complete 

 screen between the inside and the outside. 

 For the + charge on the outside may be 

 shown to remain in the same position 

 wherever the + body is placed within the 

 conductor so long as it is some di>tanee 

 below the top. 



We may, for example, conduct the 

 experiment with the can on the table of 

 a gold-leaf electroscope so that the charge 

 on the outside extends to the leaves. 

 Their divergence remains the same, how- 

 ever the bodv within the can may be 

 moved about. When it is well Mow 



the top, the + on the outside is really related now to a cha 

 which it induces on the surface of the nearest conductors, the 

 table, wall, &c. 



The two electrifications always occur together, in- 

 ducing each other ; or electrification is always in- 

 ductive. The analysis of these simple cases of induction prepares 

 us for the general statement that all electrification is accompanied by 

 induction, i.e. that whenever we have one kind of electrification we 

 have somewhere, facing it, the other kind, and the bodies whose sur- 

 faces are subjected to these related charges are being pulled towards 

 each other. In the next chapter we shall describe experiments 

 which verify this, and shall show also that the two related electri- 

 fications are always equal in quantity, so that if any conducting 

 connection be made between the oppositely electrified surfaces the 

 electrifications coming together exactly neutralise each other. Thus 

 in the experiment represented in Fig. 9, the + n the inside 

 body is equal to the on the inside of the can. The + on the 

 outside of the can induces equal on the inside of the walls of the 

 room. If the body touches the can the first + and come 

 together and only the outside + is left inducing the on the walls. 

 Now, connecting the walls to the can by a wire or by the body, 



FIG. 9. 



