STATIC ELECTRICITY 



137 



the side farther from a will receive a charge of the same 

 kind. The body b is said to be charged by induction, and 

 the charges which it receives are called induced charges. 

 If a ball of pith be suspended by a silk thread, as in 

 Fig. 102, it may be used in several experiments with 

 electrified bodies. Charge a glass rod with silk and 

 bring it near the pith 

 ball ; note how far the 

 rod can be removed 

 from the ball before 

 the force ceases to 

 make it move. Note 

 also how readily the 

 ball moves when the 

 rod is near it. We 

 may get from this an 

 idea of the extent of 

 the field of force about 

 a body bearing even 

 a small charge. The 

 medium between the rod and the ball must be the seat 

 of energy, and the ball moves so as to make this energy 

 less. When the uncharged ball becomes charged by 

 induction, the medium between the rod and the ball 

 must be considered as in a state of strain. 



160. Discharges. Now if the body b is charged by 

 induction from the electrified body a (Fig. 101), it is clear 

 that these two charges bear some relation to each other. 

 In other words, if there is no change in the positions 

 of the two bodies, then a change in the potential of the 



FIG. 102 



