CH. X.] 



THE INDUCTION COIL 



111 



Key 



FIG. 138. 



entirely ceases in the primary coil, but is alternately strengthened 

 and weakened by the rise and fall of the hammer ; the strengthening 

 corresponds to the ordinary make, and is weakened by the make 

 extra current, which occurs in the opposite direction to the battery 

 current ; the break is also incomplete, and so it is weakened by the 

 break extra current, which 

 being in the same direction 

 as the battery current im- 

 pedes its disappearance. 



The two next diagrams 

 show the way the interrupter 

 acts. We are supposed to be 

 looking at the end of the 

 primary coil; the battery 

 wires are attached to the 

 binding screws A and E (fig. 

 138). The current now passes 

 to the primary coil by the 

 pillar on the left and the spring or handle of the hammer as far as 

 the screw (C) ; after going round the primary coil, one turn only of 

 which is seen, it twists round a pillar of soft iron on the right-hand 

 side, and then to the screw E and back to the battery ; the result 

 of a current going around a bar of soft iron is to make it a magnet, 

 so it attracts the hammer, and draws the spring away from the top 

 screw C, and thus breaks the current ; the current ceases, the soft 



iron is no longer a magnet, so 

 it releases the hammer, and 

 contact is restored by the 

 spring; then the same thing 

 starts over again, and so a 

 succession of break and 

 make shocks occurs alter- 

 nately and automatically. 



In Helmholtz's modifica- 

 tion (fig. 139) the battery 

 wires are connected as before. 

 The interrupter is bridged by 

 a wire from B to C (also 

 shown in fig. 137, e). C is 



raised out of reach, and the lower screw F is brought within reach 

 of the spring. Owing to the wire BC, the vibration of the hammer 

 never entirely breaks the current. 



Instead of Wagner's hammer a long vibrating reed constructed 

 on the same principle is often used. This has the advantage that 

 the rate of vibration can be varied at will by means of a sliding 



Key 



FIG. 139. 



