APPARATUS USED TO PRODUCE MUSCLE CONTRACTION 475 



An induced current is developed by means of an apparatus called an 

 induction coil, and the one most employed for physiological purposes is Du 

 Bois Reymond's, the one seen in figure 317. 



Wires from a battery are brought to the two binding-screws, d' and d, a 

 key intervening. These binding-screws are the ends of a coil of coarse covered 

 wire, c, called the primary coil. The ends of a coil of finer covered wire, g, are 

 attached to two binding-screws to the left of the figure, one only of which is 

 visible. This is the secondary coil, and is capable of being moved nearer to 

 c along a groove and graduated scale. To the binding-screws to the left of g, 

 the wires or electrodes used to stimulate the muscle are attached. If the key 

 in the circuit of wires from the battery to the primary coil (primary circuit) 

 be closed, the current from the battery passes through the primary coil, and 



FIG. 317. Du Bois Reymond's Induction Coil. 



across the key to the battery, and continues to pass as long as the key continues 

 closed. At the moment of closure of the key, at the exact instant of the com- 

 pletion of the primary circuit, an instantaneous current of electricity is in- 

 duced in the secondary coil, g, if it be sufficiently near and in line with the 

 primary coil ; and the nearer it is to c, the stronger is the current induced. The 

 current is only momentary in duration and does not continue during the whole 

 of the period while the primary circuit is complete. When, however, the 

 primary current is broken by opening the key, a second current, also momen- 

 tary, is induced in g. The former induced current is called the making, and 

 the latter the breaking shock; the former is in the opposite direction to, and 

 the latter in the same direction as, the primary current. 



The induction coil may be used to produce a rapid series of shocks by means 

 of the accessory apparatus at the right of the figure, called the magnetic inter- 

 rupter. If the wires from a battery are connected with the two pillars by the 

 binding-screws, one below c and the other at a, the course of the current is 

 indicated by the arrows in figure 318. The current passes up the pillar from 

 e, and along the springs if the end of d' is close to the spring, then to the 

 primary coil c, and to wires covering two upright pillars of soft iron, 6, to the 

 pillar a, and out by the wires to the battery. In passing along the wire b the 

 soft iron is converted into a magnet, and so attracts the hammer, /, of the 



