ELECTRICAL CONDENSERS. 615 



disc are alternately touched with the finger, a small 

 spark will be drawn each time when one of the discs 

 is touched, and when, after repeating this several 

 times, thumb and finger are used as in the first experi- 

 ment for discharging the apparatus, a smart shock will 

 still be felt. This proves that when each plate was 

 touched separately by a conductor, only a very small 

 quantity of electricity could really have been removed. 

 If the two electricities of the discs were to bind each 

 other completely, it would not be possible to withdraw 

 any electricity from them when they are touched by the 

 finger one at a time. But the two electricities cannot bind 

 each other completely, inasmuch as they are separated 

 by a sensible distance, which in this case is equal to the 

 thickness of the intervening insulator. A given quantity 

 of electricity can, at any distance, only bind a quantity 

 of the opposite electricity less than itself, and if of two 

 electricities at some distance from one another, one is 

 completely bound, there must be a certain surplus of 

 the other. When the upper disc is charged while the 

 lower communicates with the ground, the induced 

 electricity of the lower disc is completely bound by the 

 original charge, while a free surplus of the latter exists 

 in the upper disc. This free portion manifests itself by 

 the divergence of the upper pendulum ; the lower pen- 

 dulums hang down because there is no free electricity 

 in the lower disc which could cause them to diverge. 

 When the conducting wire is removed, and the upper 

 disc touched by the finger, the upper pendulum drops. 

 But now the lower pendulums diverge ; for the quan- 

 tity of electricity in the upper disc being diminished, 

 because the free portion has been removed, it can no 



