Uses of Electromagnets 223 



listening end electrical energy is transformed back into energy 

 of motion. 



The Induced Current. Owing partly to the great expense 

 involved in using zinc as a fuel in generating large quantities of 

 electricity, we do not depend on batteries to provide electric cur- 

 rents in large industrial plants. There is a cheaper and better 

 way the use of induced currents, caused by moving a coil of 

 insulated wire near a magnet. When the coil of wire is con- 

 nected into a circuit the current flows around the circuit. A 

 current generated in this way is known as an induced current. 



Experiment to Show Induced Current. The following 

 experiment illustrates the induced current by showing the possi- 

 bility of causing a momentary electric current without a battery. 

 The materials needed are a galvanometer, to measure the cur- 

 rent, a coil of many turns of fine insulated copper wire, and a 

 bar magnet. Connect the ends of the coil to the galvanometer and 

 then insert into the coil the N pole of the magnet. Notice that 

 the needle of the galvanometer is deflected as the magnet enters 

 and leaves the coil. Repeat the process, using the S pole of the 

 magnet, and observe what occurs. These observations lead to the 

 conclusion that a magnet moved near a conducting coil induces 

 a current in the conductor. 



The Dynamo. The dynamo is a machine which is used to 

 change mechanical energy into electrical energy. In its simplest 

 form it is merely a coil of insulated wire electrified by being 

 moved within the magnetic field of a magnet. 



The important parts of a practical dynamo consist of the mag- 

 nets, the armature, the commutator and the brushes. The magnets 

 are U-shaped and are used to produce the magnetic field. In con- 

 sequence, they are called field magnets. The armature consists 

 of a coil or coils of wire wound around a soft iron core. The 

 armature rotates in such a way that the coil of wire moves in one 

 direction through the magnetic field for one-half a revolution 

 and then in the opposite direction. Hence the current formed in 

 the coil is an alternating current flowing first in one direction 

 and then in the opposite direction. Most of our houses are lighted 

 by alternating currents. 



