ELECTRICITY BY FRICTION 



481 



after it was rubbed with flannel or a cat's skin. Does the glass rod 

 repel or attract the ball? 



Experiment 155. Suspend a pith ball from the ring of a ring- 

 stand by a very fine piece of copper wire no larger than a thread. 

 Wrap the wire around the pith ball in several directions. Bring a 

 rubbed glass rod toward the pith ball. Does it act as it did when 

 suspended by silk? Allow the ball to touch the rod. Does the 

 ball now act as it did when suspended by silk? Try these same 

 experiments, using the vulcanite rod. 



From the previous experiments it has been seen that 

 when glass is rubbed with silk, and vulcanite with flannel 



FIGURE 153 



or a cat's skin, they seem to have two different kinds of 

 electrical charges. The like kinds repel each other and the 

 opposite kinds attract. These two kinds are called posi- 

 tive and negative respectively. 



Whether there are really two kinds of electricity has not 

 yet been fully determined, but electricity acts exactly as it 

 would if there were two kinds, and it has become customary 



