134 



MAGNETS AND ELECTRICITY 



spark of electricity. One day in June, 1752, when a 

 thunderstorm was coming on, he sent up a kite having 

 at its top a pointed wire. On the lower part of the string 

 he had a key, and the kite-string was insulated from the 

 earth (Fig. 123). 



When the kite-string became wet, so that it could con- 

 duct electricity, Franklin obtained sparks from it. The 



sparks were ex- 

 actly like those 

 obtained by fric- 

 tion. 



When two 

 charged clouds 

 approach each 

 other, the strain 

 in the ' ' electric 

 field" between 

 them becomes so 

 great that, finally, 

 the charge bursts 

 through. This is 

 lightning (Fig. 

 124). If the 

 charge passes be- 

 tween a cloud and 



the earth, we say that the lightning strikes. The two 

 clouds, or a cloud and the earth, with the non-conduct- 

 ing air between them, thus form a huge Ley den jar. 



Thunder is due to the rapid expansion and contraction 

 of the air heated by the Hghtning. 



148. Lightning Rods. The electric charge "induced" on the earth 



Insulator 



FIG. 123. 



Franklin's kite was "charged by induction" from a 

 charged thundercloud. Note the spark. 



