ELECTRIC GENERATORS. 95 



surface and plate, making them negative. The under 

 surface of the cover also becomes negative and its 

 upper surface positive. Connection being made as 

 before, electricity passes from the cover to the lower 

 plate ; leaving the cover negative, and its charge bound 

 by the positive on the upper surface of the dielectric. 



The cover being removed, and a conductor presented 

 to the knob, a discharge takes place ; electricity now 

 passing from the conductor to the cover, instead of from 

 the cover to the conductor as before. 



The removal of the cover, with its negative charge, 

 having left the upper surface of the dielectric positive, 

 electricity is again repelled from the under surface and 

 plate by induction : and the cover having been restored 

 to zero and replaced, the process may be repeated as 

 before. 



We see, then, that when the charge is negative, 

 electricity is attracted from the earth to the lower 

 plate, then passes to the cover, and then from the cover 

 to the presented conductor; but when the charge is 

 positive, electricity is repelled to the earth from the 

 lower plate ; then an equal amount passes from the 

 cover to the lower plate, and the same amount passes 

 to the cover from the presented conductor. 



Hence, when the instrument receives a positive charge, 

 it gives a negative charge ; and when it receives a neg- 

 ative charge, it gives a positive charge. 



It will also be noticed that the initial charge is given 

 by friction, but all subsequent charges are obtained by 

 induction. 



If the cover be removed, without first making con- 

 nection between it and the lower plate, no charge will 

 be found on it; since it has neither gained nor lost 



