CHAPTER X . 

 ELECTRICITY AND MAGNETISM 



138. Frictional Electricity. Twenty -five hundred years 

 ago a Greek discovered by experiment that if a piece of 

 amber is rubbed with wool or fur, it will attract very light 

 bodies, such as bits of dry paper, lint, or pith. These light 

 bodies cling to the surface of the amber for a time and then 

 fly off as if repelled by it. Similar effects can be produced 

 by rubbing a piece of sealing wax or hard rubber with fur, 

 or by rubbing glass with silk. The rubber, glass, or similar 

 body is said to be charged with electricity by the friction. 

 The bits of paper or pith by contact become charged with 

 the same form of electricity as the rubber or the glass, and 

 they are then repelled. 



139. Positive and Negative Charges. The electricity 

 upon the surface of glass is called positive electricity; that 

 upon rubber is called negative. The body causing the fric- 

 tion receives an opposite charge at the same time. The 

 silk, causing friction on the glass, received a negative charge; 

 and the fur, rubbed on wax or rubber, received a positive 

 charge. A body which is positively charged is attracted 

 by one negatively charged and repelled by one positively 

 charged. 



A ball made of pith suspended at the end of a silk thread 

 shows very well the behavior of a charged body. If an 

 electrified glass rod is brought to the ball, the ball flies to 

 the glass and clings to it for a time. But the ball receives 

 from the glass a positive charge of electricity. Then it 

 jumps away, and instead of hanging vertically, seems pushed 

 ,away from the glass by an invisible agent. A second ball, 



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