APPARATUS USED TO PRODUCE MUSCLE CONTRACTION 445 



clamp by which it can be secured to the table. The interval between the pieces of brass 

 can be bridged over by means of a third thinner piece of similar metal fixed by a screw to 

 one of the brass pieces, and capable of movement by a handle at right angles, so as to touch 

 the other piece of brass. If the wires from the battery are brought to the inner binding- 

 screws, and the bridge connects them, the current passes across it and back to the battery. 

 Wires are connected with the outer binding-screws, and the other ends are joined together 

 for about two inches, but, being covered except at their points, are insulated; the un- 

 covered points are about an eighth of an inch apart. These wires are the electrodes, and the 

 electrical stimulus is applied to the muscle through them, if they are placed behind its 

 nerve. When the connection between the two brass plates of the key is broken by depress- 

 ing the handle of the bridge, the key is then said to be opened. 



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. 



FIG. 317. Du Bois Reymond's Induction 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 cir- 

 cuit of wires from the battery to the primary coil (primary circuit) be closed, the current 

 from the battery passes through the primary coil, and 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 completion of the primary circuit, an instantaneous current of 

 electricity is induced 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 open- 

 ing the key, a second current, also momentary, is induced in g. The former induced cur- 

 rent 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 interrupter. 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 cur- 

 rent passes up the pillar from e, and along the springs if the end of d' is close to the spring, 



