350 HOW ELECTRICITY IS MADE ON A LARGE SCALE 



322. Powerful Dynamos. The power and efficiency of a 

 dynamo are increased by employing the devices previously 

 mentioned in connection with the motor. Electromagnets 



FIG. 240. Thomas Edison, one of the foremost electrical inventors of the present day. 



are used in place of simple magnets, and the armature, 

 instead of being a simple coil, may be made up of many coils 

 wound on soft iron. The speed with which the armature is 

 rotated influences the strength of the induced current, and 

 hence the armature is run at high speed. 



The average dynamo, such as is used for lighting fifty 

 incandescent lamps, has a horse power of about 33.5, and 

 large dynamos are frequently as powerful as 7500 horse 

 power. 



323. The Telephone. When a magnet is at rest within a 

 closed coil of wire, as in Section 319, current does not flow 

 through the wire. But if a piece of iron is brought near the 



