616 ELECTRICAL CONDENSERS. 



longer bind the same quantity as before in the lower 

 disc, hence a portion of the electricity in the lower disc 

 becomes free, and causes a divergence of the pen- 

 dulums. There must now clearly be a surplus of 

 negative electricity in the lower disc, for when the 

 upper disc was touched only that quantity of positive 

 electricity was left behind which the negative electricity 

 on the lower disc was capable of binding, that is, a less 

 quantity of positive electricity was left in the upper 

 disc than of negative' electricity in the lower disc. 

 When the surplus of negative electricity in the lower 

 disc is again conducted away through the finger, the 

 lower pendulums drop and the upper pendulum rises ; 

 there is now free electricity in the upper disc, because 

 the quantity in the lower has been diminished. By al- 

 ternately repeating the operation, the apparatus may be 

 completely discharged; each time the surplus of free 

 electricity in one disc is taken away, and thereby a 

 quantity of electricity set free in the other disc. 



The process may be illustrated numerically if, for example, we 

 suppose the distance between the two discs to be such that a given 

 quantity .of positive electricity communicated to the upper disc, 

 which we may represent by the number 1,000, can only bind -Jg- of 

 this quantity of the opposite electricity in the lower disc. Then we 

 should have, at starting, 1,000 of positive electricity in the upper 

 disc, and ^ x 1,000 = 950 of negative electricity in the lower 

 disc. This 950 can only bind ^ x 950 = 902'5 of positive elec- 

 tricity in the upper disc, in which there is therefore 1,000 902 '5 

 ~ 97'5 of positive electricity in the free state. This 97'5 of positive 

 electricity is removed when the upper plate is touched, and the 

 remaining 902 - 5 now binds only ^J x 902*5 = 857*375 of nega- 

 tive electricity in the lower disc, so that 950 857'375 = 92'625 

 is in the free state. This calculation may be continued in the 

 form of the following small table. 



