ELECTRICAL INVENTIONS 241 



of wire. It will be remembered that it was only when the magnet 

 was moving that a current was produced; only, in other words, 

 when the wire was cutting through the lines of force of the 

 magnetic field. The current moved in the wire in one direction 

 when the magnet was being introduced into the coil, and in a 

 reverse direction when the magnet was being withdrawn from 

 the coil. This principle is the basis of the dynamo. 



If the armature of a motor is forced to revolve by sending a 

 current through its wires, it might be expected that a current 

 would be generated in the wires by the rapid mechanical revolu- 

 tion of the armature. This is true. If the magnetic field in 

 which the armature revolves is produced by permanent magnets, 

 the machine is called a magneto; if the magnetic field is produced 

 by electric magnets, the machine is called a dynamo. 



Let us follow in detail the events that would happen if the 

 armature in a motor were revolved mechanically (Fig. 101). 

 Such revolutions may be accomplished by a steam engine, a water 

 wheel, a windmill, or by hand. As the upper of the three coils 

 turns so as to approach the north pole of the magnet, it cuts 

 through the lines of force of the magnetic field, and a current is 

 produced in its wire. Since the current flowing through the 

 wire in a clockwise direction produced a motion of this coil 

 toward the north pole of the electromagnet when the machine 

 was operating as a motor, therefore motion of the coil toward 

 this north pole will now produce a current flowing in the wire in 

 a clockwise direction. Or to state the matter in a slightly differ- 

 ent way: The free end of the core a tends to become a south pole 

 of an induced magnet as it approaches the north pole of the 

 electromagnet, and the current in the wire must similarly tend 

 .to make this end of the coil a south magnetic pole. The current 

 so induced now flows through the coil to the armature strips 

 and thence through the spring clips and binding-posts, coming 

 from the machine at e. But now b is moving away from the 

 north pole, is becoming less strongly a south pole, and so the cur- 

 rent in the coil about the core whose free end is b is flowing in an 



