750 INDUCTION OF CURRENTS. 



secondary coil is usually the larger of the two and 

 surrounds the primary. 



If the primary coil is connected with a battery and 

 the circuit closed, an instantaneous current will be pro- 

 duced in the secondary coil at the moment of closing 

 the circuit, and the direction of the secondary current 

 will be inverse to that in the primary coil, that is, it 

 will be in a contrary direction. Again, at the moment 

 when the circuit of the primary current is opened there 

 is another instantaneous current developed in the secon- 

 dary coil, but it is direct, that is, it flows in the same 

 direction as the inducing current. 



The inductive power of the primary coil is immensely 

 increased by putting a bar of soft iron in the heart of 

 it. The Amperian currents of the bar of iron, which 

 have previous to the closing of the primary circuit 

 all possible directions, assume parallel positions at the 

 moment of closing the primary current and the same 

 direction as that of the current itself, while at the 

 moment of breaking the primary current they all return 

 to their original irregular positions. It follows that at 

 the instant at which a current begins in the primary 

 coil, a current is also set up in the iron core, because 

 the Amperian currents are now parallel, and again, that 

 at the instant when the current stops in the primary 

 coil a current is made to stop in the core, because 

 the Amperian currents return to irregular positions. 

 Another current which undergoes simultaneously the 

 same changes is thus superadded to the primary current, 

 and the effect is therefore considerably increased. The 

 action becomes still more strengthened if instead of the 

 bar a bundle of soft iron wires is used. 



