THE INDUCTION COIL. 751 



A permanent magnet acts quite similarly to a 

 primary coil, and apparatus are constructed in which 

 an induced current is developed solely by the in- 

 ductive action of Amperian currents. Apparatus of 

 this kind are called ' magneto- electrical.' A bar of 

 soft iron is surrounded by a spiral in which a current 

 is to be induced; a permanent magnet is made to 

 approach and to recede alternately, and the position of 

 the Amperian currents of the soft iron bar is thus alter- 

 nately rendered parallel and again irregular. The 

 effect is perfectly analogous to the closing and opening 



FIG. 378 (an. prqj. ; real size). 



of the circuit in a primary coil, and a secondary current 

 is induced in accordance with the laws enunciated 

 previously. 



Fig. 378 shows a small apparatus, or so-called induc- 

 tion coil, for conveniently developing powerful induced 

 currents. A rectangular block of wood supports a 

 bobbin, the ends of which are formed by round discs of 

 black polished wood, or ebonite. In the centre of the 

 bobbin is a bundle of soft iron wires. The primary or 



