STATIC ELECTRICITY 201 



either an ebonite rod or a glass rod, repel each other, a ball 

 charged from a glass rod will attract one charged from an 

 ebonite rod. This shows clearly that there are two kinds of 

 electricity, and that, as in magnetism, like charges repel and 

 unlike charges attract. The electricity produced on glass 

 is known as positive (+) electricity and that produced on 

 rubber is negative ( ) electricity. These two kinds of elec- 

 tricity appear to be so evenly balanced on all objects-, that one 

 cannot be produced without the other. When an ebonite 

 rod is rubbed with a woolen cloth, for instance, negative elec- 

 tricity is developed on the rod, but positive electricity is 

 developed on the cloth. It should be noted that, unlike mag- 

 netism, electrification is not more effective at one point on an 

 object than another. It seems to be evenly distributed over 

 the surface of an electrified body and may be removed at any 

 point. Smooth surfaces prevent the escape of electricity 

 much as they prevent the escape of heat. A series of points 

 is always most effective in discharging an electrified body. 



165. The Electroscope. Taking advantage of the fact that 

 like charges repel, it is easy to construct a device that will 

 show when a body is electrified. Such a device is called an 

 electroscope. Two pith balls attached to a common- support 

 by silk thread will serve the purpose, but the more usual form 

 consists of two strips of gold or aluminum foil attached to a 

 metal rod and enclosed in a glass flask to protect the delicate 

 strips of foil. The rod is supported by a cork in the neck of 

 the flask. When an electrified body is brought toward the 

 electroscope, it becomes charged by induction, as it is called, 

 and the strips of foil or the pith balls spread apart in con- 

 sequence. When the electrified body is withdrawn, they fall 

 together again. If, however, one touches the electroscope 

 while the leaves are still separated, withdrawing the electri- 

 fied body does not cause the leaves to fall together again, 

 because one kind of electricity (the opposite of the one on the 



