CH. X.] 



THE INDUCTION COIL. 



117 



In Helmholtz' modification (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). G is raised out of reach, and 

 the lower screw F is brought within reach of the spring. Owing 



Fig. 138. 



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 



Key 



Fig. 139- 



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

 sliding clamp which fixes the reed so that different lengths of it 

 can be made to vibrate. If a long piece of reed vibrates, it 

 does so slowly, and thus successive induction shocks at long 

 intervals can be sent into the nerve. But if one wishes to 



