VOLTAIC ELECTRICITY. 257 



magnet near the end of the soft iron. In this case the 

 induced currents are due to the magnetism of the soft 

 iron, this magnetism being due to the inductive influence 

 of the magnet ( 311). Thus we see that when the in- 

 tensity of the magnetism of a bar of iron is in- 

 creased or diminished, currents are induced in the 

 coil. 



408. The Telephonic Current. An electric 

 current may be induced in a coil of insulated wire sur- 

 rounding a bar magnet by the approach and withdrawal 

 of a disc of soft iron. The disc a (Fig. 203) is magnetized 

 by the inductive in- 



fluence of the mag- 



net m ( 311). The 



disc, thus magnet- 



ized., reacts upon 



the magnet m and 



changes the distri- 



bution of magnetism therein. By varying the distance 



between a and m, the successive changes in the distribution 



of the magnetism of m induce to-and-fro currents in the 



surrounding coil. 



409. The Telephonic Circuit. If the wire sur- 



rounding the magnet mentioned in the last paragraph be 

 continued to a distance and then wound around a second 

 bar magnet, as shown in Fig. 204, the currents induced at 

 A would affect the magnetism of the bar at B ( 392) or 

 the intensity of its attraction for the neighboring disc ~b. 

 A vibratory motion in the disc a would induce electric 

 currents at A ; these currents, when transmitted to B, 

 would affect the magnetism of the bar there, and thus tend 



