14 PRACTICAL PHYSIOLOGY 



magnet being thus set in action, draws down the 

 iron bar and with it the spring, which leaves the 

 screw and breaks contact so that a break induced 

 current is set up in the secondary coil. But, the 

 current being broken, the electro-magnet is no 

 longer active, the bar springs up again, and con- 

 tact is re-established between the spring and screw; 

 this produces a make induced current in the sec- 

 ondary coil. Thus the spring vibrates to and fro, 

 and break and make induced currents are set up in 



Short-cir- 

 cuit key 



Electrodes +\ v Mercury 



key. 



FIG. 18. INDUCTION COIL ARRANGED FOR AUTOMATIC INTERRUPTION OF PRIMARY 



CIRCUIT. 



the secondary coil many times a second, according 

 to the rate of vibration of the spring. These make 

 and break shocks are unequal owing to the extra 

 current (see below) which is self-induced within 

 the primary coil, and which diminishes the make 

 effect. This inequality is, however, got over by 

 a modification introduced by Helmholtz. In this 

 arrangement (Fig. 19) a wire, W , connects the 

 terminals, 7\ T 3 ; the screw S* is raised alto- 



