160 



ELECTRICITY 



alternating current is sent through the coil of coarse 

 wire ; the current induced in the fine wire coil will 

 have less strength but higher potential than the other. 



185. The Induction Coil. By means of this last- 

 named principle, currents from batteries are often 

 changed into currents of high potential. For this pur- 

 pose an induction coil is used. The induction coil has 

 two coils of wire, a fine and a coarse 

 one as has the transformer, the 

 battery current generally passing 

 through the coarse wire a (Fig. 117). 

 This coil a, through which the 

 battery current passes, is called the 

 primary, and the other (in which 

 a current is induced) is called the 

 secondary coil. Of course the wire 

 of the secondary coil cuts many 

 of the lines in the magnetic field 

 around the primary, and a current is 

 induced in the secondary whenever 

 there is any change in the number of 

 lines of force that it cuts ( 179). In 

 the transformer this change is secured by using an alter- 

 nating current ; but since a battery gives a direct cur- 

 rent, the induction coil must be so arranged as to 

 produce the necessary change in some way. This is 

 usually done by a make-and-break piece c (Fig. 117), 

 which opens and closes the circuit very rapidly. The 

 current thus rapidly flowing and stopping causes a 

 change in the number of lines in the field surrounding 



FIG. 117 



