MAGNETISM AND ELECTRICITY 



193 



Experiment 54. Suspend a pith ball with a silk thread as 

 shown in Figure 161. When an electrified glass rod is brought 

 near, the pith ball is attracted to the rod. It clings to the rod for 

 a moment and then springs away from it. 

 The ball has received a positive charge from 

 the positive charged rod. Now as the rod 

 is brought near the ball it moves away as 

 if pushed by some invisible force. Next rub 

 a hard rubber or ebonite rod with flannel 

 or cart's fur and bring it near the pith ball. 

 It will be found that the ball is no longer 

 repelled as with the glass rod, but that it is 

 strongly attracted. To discharge the pith 

 ball, hold it in the finger for a moment. 



Hang two pith balls from the same point 

 of suspension and charge both balls with 

 the same kind of electricity. The balls now 

 repel each other (Figure 162). 



FIG. 161. A Pith Ball 

 Electroscope. 



It is evident from these experiments 



that electrical charges of like kind repel 



each other and charges of unlike kind attract each other. 



Conductors and Insulators. A 

 substance which conducts or trans- 

 mits electricity readily is called a 

 conductor, while . a substance that 

 does not is called an insulator, or 

 non-conductor. All metals and solu- 

 tions of salts and acids in water are 

 conductors of electricity, while glass, 

 rubber, shellac, dry air, wood, silk, 

 and oils are good insulators. When 

 a metal rod is held in the bare hand 

 and rubbed with some substance such 

 as a woolen cloth, a charge does not 

 FIG. 162. appear upon the rod, because the 



