ELECTRICAL MACHINERY 497 



This is the principle of the induction coil, a diagram of 

 the connections being shown in Fig. 374. The circuit 

 leading from the batteries to the inside of the coil is 

 known as the primary and the circuit wound on the out- 



FIG. 372 



FIG. 373 



side of this is known as the secondary. The primary in- 

 duces the current in the secondary, and if the secondary 

 circuit has more turns of wire than the primary it will 

 have a correspondingly greater E.M.F., in other words, 

 the difference in E.M.F. of the two circuits varies directly 

 with the difference in the number of turns in the wire 

 of the two. Since the induced E.M.F. is set up only 

 as the current is made or broken, an automatic device A 



is connected into the pri- 

 mary, whose action is iden- 

 tical with the circuit break- 

 er of an electric bell. In 

 induction coils this, how- 

 ever, is generally known as 

 a buzzer. 



The induction coil is 

 used with jump-spark igni- 

 tion, on gasoline engines. 

 For this work the spark 

 FIG. 374 PRINCIPLE OF THE iNDuc- requires such a high 



E.M.F. that the primary 



consists of only a few turns of coarse wire, while the sec- 

 ondary consists of several thousand turns of fine wire. 



