CHAP, x.] ELECTRICITY AND MAGNETISM. 



363 



each can induce through the other, when each carries 

 unit current, is 4?rS, which is the maximum value of 

 M. If the circuits are not simple, but have respectively 

 m turns and n turns, then the value of M will be 

 4?rS x ;;/;?, when the circuits, coincide with each other. 



398. The Induction Coil. Induced currents have 

 in general enormously high electromotive-forces, and are 

 able to spark across spaces that ordinary battery cur- 

 rents cannot possibly cross. In order to observe these 

 effects a piece of apparatus invented by Mason, and im- 

 proved by Ruhmkorff, and termed the Induction Coil or 

 Inductorium (Fig. 148), is used. The induction coil con- 

 sists of a cylindrical bobbin having a central iron core 



M 



Fig. 148. 



surrounded by a short inner or "primary" coil of stout 

 wire, and by an outer " secondary" coil consisting of many 

 thousand turns of very fine wire, very carefully insulated 

 between its different parts. The primary circuit is joined 

 to the terminals of a few powerful Grove's or Bunsen's 

 cells, and in it are also included an interruptor, and a 

 commutator or key. The object of the. interruptor is 



