(iKVERAL ACCOUNT OF COMMON PHENOMENA 9 



inanv electroscopes, theiv are no special side plates. In that case 

 the surface of the glass may be dusty or clamp and so become 

 oppositely electrified. If the glass is perfectly insulating the 

 opposite electrification is induced in the nearest conducting 

 surfaces and the leaves are still pulled out by the attractions of 

 those surfaces rather than pushed apart by mutual repulsion. 

 But the nearer the oppositely electrified surfaces the stronger its 

 outward pull. Hence the advantage of having adjustable side 

 plates, which may be brought very near the leaves when the charge 

 is small. If while the electrified rod is over the table of the 

 electroscope we either touch the table or allow the leaves to 

 diverge so far that they touch the side plates, the negative from 

 the leaves either passes away to " the earth," i.e. to the surrounding 

 conductors, or is neutralised by a further supply of positive 

 brought up from the earth, and. ceasing to be electrified, the leaves 

 fall together. On breaking the earth communication the positive 

 on the table above remains, and on removing the vulcanite this 

 positive spreads over the rod and leaves, and induces negative in 

 the side plates. These again attract the leaves, which are once 

 more drawn apart and remain apart as long as the insulation is 

 preserved. 



\Ye can now see how an electroscope, left charged with positive in 

 this way, may be used as a detector of the kind of charge possessed 

 bv another bodv. If a positively electrified body is brought near 

 its table, then the table is negatively electrified by induction and 

 positive is sent down into the already positively electrified leaves, 

 and they are forced still further apart. If a negatively electrified 

 bodv is brought near the table, it is positively electrified and either 

 this positive is drawn up to it from the leaves or negative is sent 

 down to the leaves, and in either case they are less strongly 

 positively electrified and fall together somewhat. If the body is 

 strongly charged with negative and is brought nearer, a point may 

 be reached when the positive in the leaves is just neutralised and 

 they come together. If the body approaches still nearer than this 

 point, then there is a balance of negative in the leaves over and 

 above that neutralising the positive pre\ iouslv there, and the leaves 

 diverge once more. It will easily be seen how the indications arc 

 modified if the initial charge of the electroscope is negative. 



Only the outer surface of a conductor is electrified 

 unless there are insulated charged bodies within it. This 

 may be proved as follows: A narrow, deep, tin can, is placed on a block 

 of paraffin to serve as insulator and is charged with electricity either 

 bv contact or induction. If we place a proof plane in contact with 

 the inside of the can and then remove the plane to an electroscope 

 no charge is detected, however strongly the outside may be charged. 

 In fact, the (in acta as ;i screen protecting the inside space from 

 outside electrification, and the more nearly it is closed the more 

 complete is the protection. The tendency of electrification to the 



