STATIC ELECTRICITY 139 



of vapor that causes sudden showers. These heavy rain- 

 falls occur more often in the summer, so that lightning 

 is more common at that season. 



Lightning may be explained as follows. Suppose a 

 positively charged cloud passes near the earth: build- 

 ings or the earth itself may become charged by induc- 

 tion, receiving a charge of the opposite kind from that 

 of the cloud. If now the positive charge in the cloud 

 increases in potential, the induced charge in the earth 

 becomes more negative. This may go on until the dif- 

 ference in potential between the two charges is great 

 enough to overcome the resistance of the air between 

 them, and then a discharge takes place from one to the 

 other. The passage of this discharge through the air pro- 

 duces electrical changes which cause the spark of flash. 



Discharges may pass from clouds to the earth, or from 

 the earth to a cloud ; more often they go from one cloud 

 to another. Objects through which the discharge passes 

 are commonly said to be " struck." Tall buildings, 

 towers, spires, trees, and the like are more in danger of 

 being struck, because a charge induced on them is 

 nearer the cloud. " Heat lightning " is the reflection 

 of distant lightning. Thunder is the sound caused by 

 waves of air set in vibration by the discharge. 



QUESTIONS 



1 . How may a body become charged with electricity ? Name 

 some substances that can be charged in this way. Do they, in 

 general, belong to the class of conductors or insulators ? 



2. What name is commonly given to the electricity in a 

 charge of this sort? Why are not conductors easily charged in 

 this way? 



