324 Introduction to the Study of Science 



The direction of the armature may be reversed by reversing the direc- 

 tion of the electric current either in the field coils or in the armature. 



If the armature were connected as a motor is, by a belt or gear to 

 some machine, it would be harnessed to do work or to change electric 

 energy into mechanical energy. This motor is not strong enough to 

 operate any except the lightest machine. 



The electromagnet. The field coils form what is called the 

 electromagnet. The electromagnet differs from the permanent 

 magnet in this respect : its magnetic power is due to the electric 

 current passing along the wire wound around the soft iron cores. 

 The permanent magnet is made of a certain kind of steel 

 which retains indefinitely its magnetic force. The electro- 

 magnet is a temporary magnet, for the soft iron loses its 

 magnetism as soon as the electric circuit is broken. It may be 

 pointed out that there are two complete circuits in the electro- 

 magnet : one the electric circuit by way of the wire wound 

 around the cores, and the other the magnetic circuit produced 

 in the soft iron cores and the bars connecting them. 



The electromagnet is useful because it is a temporary magnet. 

 It is used in such familiar appliances as the electric bell, the 

 electric clock, in powerful magnets employed in metal works for 

 lifting large loads of metal, in the Morse telegraph sounder, the 

 automatic telephone, the galvanometer, and in generators and 

 motors. (Examine the magnets of an electric bell or buzzer, 

 and compare them with the magnets making up the field coils 

 of the motor.) 



The motor used as a generator. The motor when supplied 

 with electrical energy may perform mechanical work; or it 

 may be reversed in function and when supplied with mechanical 

 energy produce electrical energy. This is the case with the 

 motors of the Milwaukee electric locomotive (page 319). If the 

 armature of the motor you have built were made to revolve 

 at a high rate of speed, it would generate an electric current 

 just as a generator does, that is, the motor would become a 

 generator. 



