CHAP, I.] ELECTRICITY AND MAGNETISM. 25 



surface because it is repelled there by the negative 

 charge on the cake) will be repelled away through the 

 hand and body of the experimenter into the earth. The 

 attracted positive electricity will, however, remain, being 

 bound as it were by its attraction towards the negative 

 charge on the cake. Fig. 15 shows the condition of 

 things after the cover has been touched. If, finally, the 

 cover be lifted by its handle, the remaining positive 

 electricity will be no longer " bound " on the lower surface 

 by attraction, but will distribute itself on both sides of 



Fig. 15. Fig. 16. 



the cover, and may be used to give a spark, as already 

 said. It is clear that no part of the original charge has 

 been consumed in the process, which may be repeated 

 as often as desired. As a matter of fact, the charge on 

 the cake slowly dissipates especially if the air be damp. 

 Hence it is needful sometimes to renew the original 

 charge by afresh beating the cake with the cat's skin. 

 The labour of touching the cover with the finger at each 

 operation may be saved by having a pin of brass or a 

 strip of tinfoil projecting from the metallic " sole " on to 

 the top of the cake, so that it touches the plate each 

 time, and thus allows the repelled negative electricity to 

 flow to the earth. 



Since the electricity, thus yielded by the electrophorus 



