Electricity 



211 



FIG. 1 1 8. The magneto in an automobile is a small dynamo. 



The crank turns a coil of wire between the poles of the 

 magnet and generates the electricity for ringing the 

 bell. These little dynamos, like those in automobiles, 

 are usually called magnetos. 



Alternating current. For the sake of simplicity and 

 convenience we speak of electricity as always flowing 

 in through one wire and out through the other. With 

 batteries this is actually the case. It is also the case 

 where people have what is called direct-current (d.c.) 

 electricity. But it is easier to raise and lower the volt- 

 age (pressure) of the current if instead of being direct 

 it is alternating; that is, if for one instant the electricity 



