ATTRACTION AND REPULSION 



131 



he could have electrified the metal as well as the glass. 

 His own body acted as a conductor, and the charge passed 

 through it to the earth. We now call bodies conductors, 

 or non-conductors. 



Non-conductors are also called insulators, from the Latin, insula, 

 an island. A charge formed on one end of a glass rod remains where it 

 was produced; but if one end of a metal rod is rubbed, the whole rod is 

 charged. The glass is an insulator, the metal a conductor; but both 

 are "electrics." Some of the best conductors are: metals, charcoal, 

 water vapor, and wet substances generally. Some of the best insula- 

 tors are: dry cotton, wool, wood, silk, glass, wax, rubber, and shellac. 

 Since the earth is a good conductor, a charged conductor will lose its 

 charge if it is not insulated from the earth. 



143. Attraction and Repulsion. If an electrified body 

 is brought near bits of paper, they are first attracted, and 

 then repelled. This phenomenon is studied 

 most easily if an "electric pendulum " is 

 used. One is shown in Fig. 119. It is sim- 

 ply a ball of pith, or of cork, suspended by 

 a thread which insulates it. If a glass rod, 

 charged by rubbing it with silk, is held near 

 the ball, the ball is attracted to the rod. 

 After it has obtained a charge from the rod, 

 it is repelled. If a stick of sealing wax, 

 charged by rubbing it with flannel, is now 

 brought near the charged ball, the ball is 

 attracted. Since the charged sealing wax attracts what 

 the charged glass repels, we say that the glass and wax 

 are oppositely charged. 



Fio. 119. 



Like charges repel 

 each other. 



If we call the charge on the glass positive (+), that on the wax is 

 negative ( ). It will be found that the silk has received a negative 



