PRACTICAL PHYSIOLOGY 15 



gether away from the spring, and the screw S 2 is 

 brought nearly up to the spring. The battery cur- 

 rent passes by the wire, W, from the terminal, T\ 

 directly to T\ thence through the primary coil 

 and through the electro-magnet, M t which draws 

 down the iron bar and brings the spring in contact 

 with the screw, S 2 . A large part of the battery cur- 

 rent now goes directly back to the battery through 

 this contact, and is diverted from the primary coil 

 and electro-magnet. This greatly weakens the 



FIG. 19. HELMHOLTZ'S ARRANGEMENT OF INDUCTION COIL. 



current through the primary coil, and the equiva- 

 lent of a break induced shock is obtained in the 

 secondary circuit ; for any sudden variation in the 

 current of the primary coil is effective in produc- 

 ing an induced current in the secondary coil. But 

 the electro-magnet is also weakened, so that the 

 bar and spring fly up. This breaks the short-cir- 

 cuiting contact which was established between the 

 spring and S 2 , and the whole current again passes 

 through the primary coil, producing the equivalent 

 of a make induced shock in the secondary circuit, 



