MAGNETISM AND ELECTRICITY 



199 



moving the finger from the disk and test its charge. 

 What kind of charge should be on the disk? What kind 

 of charge passes through the body to the earth when the 

 finger is placed on the disk? 



When the charged disk is 

 brought near an object, a spark 

 will pass with a slight cracking 

 noise. When the spark passes, 

 the air is heated and its sudden 

 expansion and contraction causes 

 the noise. 



Atmospheric Electricity. It 

 was Benjamin Franklin who first 

 demonstrated that lightning is 

 simply a great spark of frictional 

 electricity. On the approach of 

 a thunderstorm he sent up a kite 

 having at its top a pointed metal 

 rod. The kite string was insu- 

 lated from the earth. As soon 

 as this string became moistened 

 by the rain electric sparks were 

 drawn from a key attached to 

 the string. These sparks proved 

 to be the same as those obtained 

 by rubbing a glass rod with silk 

 (Figure 169). 



The air between two charged clouds or between a cloud 

 and the earth acts like the insulating substance in a con- 

 denser. The rapid formation of raindrops generates large 

 quantities of electricity and finally the charge on the 

 cloud becomes so great that it bursts through the inter- 

 vening air to the neighboring cloud or to the earth. 



FIG. 169. Franklin's Experi- 

 ment to Test the Identity of 

 Lightning. 



