ELECTRICITY AND MAGNETISM 



129 



treated like the first and brought near it, is repelled by the 

 first. But if one ball is charged from the glass and one 

 from wax, they attract instead of repel each other. The 

 silk thread prevents the electricity from passing off, so that 

 the balls remain charged for a long time if the air is dry. 



140. Conductors and Insulators. If, after an object is 

 charged with electricity, the hand is drawn over its surface, 

 the electricity disappears. It has been conducted away 

 through the hand and the body 



to the earth. We call the 

 human body a conductor of 

 electricity. Many of the metals 

 are much better conductors. 

 Moist air and damp wood are 

 also conductors, but not such 

 good ones as metals. Hard 

 rubber, glass, dry wood, dry air, 

 porcelain, and sealing wax are 

 non-conductors or insulators. 



Frictional electricity is also 

 called statical electricity, from 

 a Latin word which means "to 

 stay." This electricity remains 

 for a long time upon the surface 

 of insulated bodies. 



Machines have been con- 

 structed that will produce very 

 strong charges of statical elec- 

 tricity. Some electrical machines will cause a spark to pass 

 through several feet of air. Statical electricity is used by 

 physicians in electrical treatment of diseases and in X-ray 

 work. It was useful in the study which led to the develop- 

 ments of wireless telegraphy. 



141. Lightning and Thunder. If a sufficiently large 

 charge of electricity accumulates upon an insulated conductor 



FIG. 63. An ELECTRIFIED 

 PITH BALL 



1. The pith ball is attached 

 to a metal frame by a silk thread; 

 why not by a wire? 2. The glass 

 rod has been rubbed with silk; 

 how does it affect the ball? 3. 

 How does the position of the 

 thread show that electricity acts 

 like a force? 



