MAGNETISM AND ELECTRICITY 



209 



true. When the magnet is placed in or removed from a 

 coil of wire, a current is produced in the wire. 



Experiment 58. Connect a coil of several hundred turns of 

 insulated wire with a galvanometer (Figure 184). Now bring the 

 pole of a bar or horseshoe magnet suddenly into the coil and notice 

 the deflection of the 

 pointer of the galvanom- 

 eter. When the pointer 

 comes to rest, remove 

 the magnet quickly ; the 

 pointer will again be de- 

 flected, but in the op- 

 posite direction. 



Experiments will 

 show that it makes 



FIG. 184. When the permanent magnet is 

 placed in the coil a current of electricity is in- 

 duced in the coil as is shown by the deflection 

 of the galvanometer needle. 



no difference whether 

 the coil be held sta- 

 tionary and the magnet placed in it, or the magnet held 

 stationary and the coil moved over it. The current in 

 the coil of wire is made when it moves through a mag- 

 netic field. This is the principle of the dynamo (Figure 



185). A dynamo consists of a 

 coil of wire revolving in a mag- 

 netic field. The magnet may 

 be a permanent magnet or an 

 electromagnet. In the com- 

 mercial instrument the arma- 

 ture is usually made up of a 

 large number of separate coils 

 ynamo ' of insulated wire. The dynamo 

 is used to produce electricity wherever it is used in very 

 large quantities. Through its agency almost any kind of 

 power may be changed to electric power (Figure 186). 

 The Falls of Niagara furnish power to run many dynamos 



