Chap. IV.] 



INDUCTION. 



37 



For Ampere showed tliat two currents that are parallel 

 but in opposite directions repel one another, and that 

 two in the same direction attract one another. 



In order to increase the effect of induction a 

 greater amount of wire than a single turn is neces- 

 sary. The primary circuit is made of a considerable 

 length of wire wound on a bobbin (Fig. 20). The 

 wire is comparatively thick, to diminish resistance. 



Fig. 20. Induction. 



and is covered with silk or cotton so as to insulate 

 one turn from the other, and, to increase the in- 

 sulation, the turns are varnished with shellac. The 

 two ends of the wire are connected with brass 

 screws, to which the wires from the element can 

 be attached, when the current will pass round the 

 coil. The secondary coil is of similar construction, 

 but for it thin wire must be used of very much greater 

 length than that of the primary coil, since it is found 

 that the intensity of the induced currents depends on 

 the number of turns in the coil and the fineness of the 



