ELECTRICITY BY FRICTION 



483 



now jump across between the knobs? Why are houses provided 

 with lightning rods ? 



About the middle of the eighteenth century, Benjamin 

 Franklin proved by his notable kite experiment that light- 

 ning was simply an electrical discharge between the clouds 

 and the earth, or be- 

 tween different clouds. 

 This discharge is simi- 

 lar to that which takes 

 place on an electrical 

 machine. The elec- 

 tricity in the clouds 

 attracts as close as 

 possible the opposite 

 kind of electricity on 

 the earth's surface and 

 tends to hold it ac- 

 cumulated on high 

 objects. If the attrac- 

 tion is sufficient, the 

 electricity discharges 

 between the cloud and 

 the object, and we say 

 the object was struck 

 by lightning. 



If a sharp point, 

 such as a lightning 



rod, is present on the object where the electricity tends 

 to accumulate, it allows the electricity to pass off gradually 

 before enough accumulates to cause damage. Lightning 

 rods, however, must be continuous conductors and properly 

 terminated in the ground. 



A TREE COMPLETELY SHATTERED BY A 

 STROKE OF LIGHTNING 



