CHAPTER II. 

 THE DYNAMO. 



26. The dynamo. The dynamo electric machine, or, simply, 

 the dynamo, is a machine for the production and maintenance of 

 an electric current when the machine is supplied with mechanical 

 power, or conversely for the development of mechanical power 

 when the machine is supplied with an electric current. When 

 used for the former purpose the dynamo is called an electric 

 generator or simply a generator ; when used for the latter purpose 

 the dynamo is called an electric motor or simply a motor. 



The action of the dynamo as a generator is essentially as fol- 

 lows : A wire / centimeters in length, see Fig. 15, is forced by 

 an external source of mechanical power to move sidewise at a 

 velocity of v centimeters per second across a magnetic field of 

 intensity 3f. This motion induces an electromotive force E in the 

 wire in accordance with equation ( I go), namely, 



E = l$v abvolts 



This electromotive force produces a current of / abamperes in 

 the wire, if the wire is a portion of a closed circuit, and because 

 of this current the wire is pushed sidewise by the magnetic field 



with the force : 



F = IldC dynes 



which opposes the motion of the wire. Work is done by the 

 external mechanical agent at the rate of Fv ergs per second in 

 moving the wire against the opposing force F, and the work so 

 spent goes to maintain the current in the wire. 



The action of the dynamo as a motor is essentially as follows : 

 A current of / abamperes is forced by an external source of 

 electric current through a wire / centimeters in length which is 



35 



