GENERATION OF ELECTRICITY 



395 



Thunderstorms. Thunderstorms usually occur late in the after- 

 noon or early evening of a hot sultry day. They are classed as travel- 

 ing storms. Before the thunderstorm breaks, heavy masses of clouds 

 are seen slowly rising and collecting near the horizon. The air which 

 is very warm begins to be slightly cooler. Small detached clouds, form- 

 ing in front of the large clouds, rapidly increase in size and unite with 

 the advancing storm cloud. Ragged squall clouds travel underneath 

 the dark heavy clouds. Some of the storms travel from twenty-five 

 to fifty miles, carrying clouds of dust. At first large raindrops form 



FIG. 275. Voltmeter measures the pressure. Ammeter measures the amount 

 of current. Why is it necessary to have all the current go through A? 



which soon become smaller in size and greater in number until there 

 is a heavy downpour. Occasionally hail attends thunderstorms. 



The most dangerous place in a thunderstorm is under a tree, as 

 lightning usually strikes the highest points. Cattle are frequently 

 killed in the pasture by the lightning striking the wire fence. Farmers 

 should ground wire fences every few feet so as to allow the lightning 

 to travel into the ground. 



The Dynamo. Attach a coil of wire to a galvanometer. Move 

 a bar magnet slowly up and down in the coil. The end of the needle 



