GENERAL ACCOUNT OF COMMON PHENOMENA 15 



a few points. The charged ebonite therefore acts inductively 

 across the intervening layer of air on the cover, and some positive is 

 induced on its under surface and the complementary negative 

 on its upper surface. The cover is now touched with the finger 

 and so earth-connected. The positive charge on the sole can 

 now get very much nearer to the upper surface of the ebonite 

 and we may regard it as passing round through the earth and the 

 body of the operator to the cover, which receives nearly all the 

 charge previously in the sole. The negative charge in the cover we 

 may regard either as neutralised by the addition of positive or as 

 }>;i>sing away through the operator. Now removing the finger, the 

 cover is insulated, and on lifting it by the insulating handle 

 it contains a positive charge which is quite large enough to be 

 useful for many purposes. The charge on the ebonite still remains 

 practically undiminished and at once induces another charge on 

 the sole. By discharging the cover and replacing it on the ebonite 

 the process may be repeated, the original charge of the ebonite 

 sufficing to give a great number of charges to the cover. Though 

 the original charge is thus the exciter of all the succeeding charges, 

 it must not be supposed that it is also the source of their energy. 

 The cover when placed on the ebonite and charged oppositely is 

 attracted by it, and in lifting the cover up again more work 

 inu*t be done than that required merely to raise its weight, and 

 it is this excess of work which gives to the charge on the cover 

 the energy it possesses. The operator therefore supplies the 

 energy of each charge in the act of raising the cover. 



As illustrating the fact that the charge on the ebonite acts induc- 

 tively through a thin layer of air on the ordinary cover, it is 

 interesting to note that if a layer of mercury be poured on to the 

 cake and touched to earth the cake is discharged. Evidently the 

 mercury fits so closely on to the electrified surface that the two 

 electrifications are able to unite. 



It is important to notice that the charge taken away on raising 

 the cover is nearly all derived from the sole. When the cover is 

 fir>t placed on the cake a small electrification of the cover, 

 -f below, above, no doubt occurs. But the greater part of the 

 charge on the cake is still occupied in inducing the charge on the 

 sole. When the cover is touched the sole charge rushes up into 

 the cover, and really forms the charge carried away when the 

 cover is raised. This may easily be proved by placing the electro- 

 phonis when excited on an insulating pillar. If the cover is put 

 on the cake and touched, the sole charge cannot now pass into the 

 cover and a very small result is obtained. 



The electrophone is a convenient instrument only when a 

 Mnall quantity of electricity is required. For though any number 

 of iH.-.-irly ( (jual charges can be obtained on the cover in succession 

 from the same electrification of the resinous cake, and may be 

 communicated to a suitably arranged conductor, it would be both 



