132 MAGNETS AND ELECTRICITY 



charge by being rubbed with glass, while the flannel rubbed with wax 

 or rubber receives a positive charge. 



The laws of charged bodies are like those of magnets (cf. 136) : 



1. Charged bodies attract uncharged bodies. 



2. Bodies having unlike charges attract each other. 



3. Bodies having like charges repel each other. The space sur- 

 rounding a charged body is called an " electric field " (cf. 138). 



144. Induction of Charges. We have learned ( 143) 

 that an uncharged body is charged by contact with a 

 charged body. It may also be charged by induction, 

 without contact. A simple apparatus for showing this 

 is an egg shell covered with tin foil (Fig. 120). If a posi- 

 tively (+) charged glass rod is brought 

 near the shell without touching it, the 

 shell will be electrified by induction. An 

 uncharged cork ball will be attracted 

 by either end of the shell. If the glass 

 rod is removed, the charges disappear. 

 We say they neutralize each other. We 



FIG. 120. can charge the shell permanently if we 



Egg fnducto gedby remove one of the charges induced by 



the glass rod. To do this we hold the 



rod near the shell, as before, and then touch with the 



finger the end of the shell farther away from the rod. 



The repelled positive charge passes through the hand and 



body to the earth. If the glass rod is now removed, 



the shell will have a negative ( ) charge. 



145. Electric Discharge. If the finger is held near a 

 charged glass rod, sparks pass between the two. The 

 sparks are larger if the finger is held near a charged 



