146 



MAGNETS AND ELECTRICITY 



ter, and for propelling, lighting, and heating trolley cars 

 within a considerable distance. The control of water 

 power is everywhere giving the energy needed for electric 

 power (Fig. 139). This is true of the great dam at As- 

 souan, in Egypt, the Gatun 

 dam of the Panama Canal, 

 and the dam across the 

 Mississippi, at Keokuk, 

 Iowa (Fig. 140). 



163. Summary. Magnets 

 are natural or artificial. Natural 

 magnets are lodestones. All 

 magnets, when freely suspen- 

 ded, point toward the earth's 

 magnetic poles, unless they are 

 interfered with by large masses 

 of iron, or by local magnetic 

 fields. 



Two magnetic poles of the 

 same kind repel each other; 

 those of opposite kind attract 

 each other. 



Soft iron forms temporary 

 magnets; steel forms permanent 

 magnets. Iron, steel, magnetic 

 iron oxide, cobalt, nickel, etc., are magnetic; that is, attracted by 

 a magnet. 



A magnet attracts a magnetic body by first inducing it to become a 

 magnet. 



The earth is a magnet, with two magnetic poles. These are found 

 by the dipping needle, as well as by the compass. 



The compass has been of the greatest use in the development of 

 navigation and civilization. 



Frictional electricity is commonly developed by rubbing two sub- 



FIG. 139. 



Water from a higher level falls through a 

 penstock against the guides of the turbine. 

 The guides direct the rushing water upon 

 the blades, causing them to revolve rapidly. 

 The whirling turbine turns the shaft and the 

 generator attached to it. 



