CHAP, iv.] ELECTRICITY AND MAGNETISM. 235 



total charge will be n ~ n CV, or CV, or equals the charge 

 of one jar of capacity C raised to the same potential V. 



LESSON XXIII. Phenomena of Discharge. 



280. An electrified conductor may be discharged in 

 at least three different ways, depending on the medium 

 through which the discharge is effected, and varying 

 with the circumstances of the discharge. 



281. Disruptive Discharge. In the preceding 

 Lesson it has been shown that induction across a non- 

 conducting medium is always accompanied by a mechani- 

 cal stress upon the medium. If this stress is very great 

 the non-conducting medium will suddenly give way and 

 a spark will burst across it. Such a discharge is called 

 a " disruptive " discharge. 



A very simple experiment, carefully considered, will 

 set the matter in a clear light. Suppose a brass ball 

 charged with + electricity to be hung by a silk string 

 above a metal plate lying on the ground. If we lower 

 down the suspended ball a spark will pass between it 

 and the plate when they come very near together, and 

 the ball will then be found to have lost all its previous 

 charge. It was charged with a certain quantity of 

 electricity, and as it had, when suspended out of the 

 range of other conductors, a certain capacity (numeri- 

 cally equal to its radius in centimetres), the electricity 

 on it would be at a certain potential (namely = ^), and 

 the charge would be distributed with a certain surface 

 density all over it. The plate lying on the earth would 

 be all the while at zero potential. But when the sus- 

 pended ball was lowered down towards the plate the 

 previous state of things was altered. In the presence 

 of the + charge of the ball the potential 1 of the plate 



1 The student must remember that, by the definition of potential in 

 Art. 237, the potential at a point is the sum of all the separate quantities of 

 electricity near it, divided each by its distance from the point. 



