136 ELECTRICITY 



These experiments show that when glass is charged 

 by contact with silk, both the glass and the silk are 

 electrified, but their charges are opposite in effect. It 

 has been agreed to call the charge upon the glass posi- 

 tive, and the charge upon the silk negative. 



The effect of these opposite charges upon each other 

 is such that when the bodies bearing them are light and 

 easily moved, these bodies will move toward each other ; 

 for example, the glass and the silk. But when two bodies 

 bearing like charges are brought near each other, these 

 charges act so as to push the bodies apart ; for example, 

 the two rods of glass. Many experiments may be made 

 with small electrified bodies to prove the general law 

 that Like charges repel, and unlike charges attract, 

 each other. 



The sort of electrification that any body receives 

 varies according to its own nature and that of the sub- 

 stance with which it is rubbed. Thus, while glass may 

 be positively electrified by rubbing with silk, it is nega- 

 tively charged by flannel. Other conditions may also 

 affect the sort of charge received. 



159. Electrostatic Induction. If a body that is not 

 charged is brought near a charged body, and both are 

 separated from the earth by insula- 

 tors, the first shows signs of being 

 electrified. In Fig. 101, let a be an 

 electrified body and b an uncharged 

 FIG. 101 k 0( jy . SUS p enc i them by silk strings 



and bring b near to a. The side of b that is nearer a will 

 receive a charge of the opposite kind to that of a, while 



