THE MUSCULAR SYSTEM. 409 



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 grooved and graduated scale. 

 To the binding screws to the left of ff, the wires of 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 cur- 

 rent from the battery passes through the primary coil, and across the 

 key to the battery, and continues to pass as long as thekey 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 elec- 

 tricity is induced in the secondary coil g, if it be sufficiently near; 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 whilst the primary circuit is complete. When, however, 



ff 



FIG. 285. Du Bois Raymond's induction coil. 



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

 tary, current 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 as, the primary current. 



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

 by means of another and accessory part of the apparatus at the right of 

 the fig., 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, a, the course of the current is indicated in Fig. 286, the direc- 

 tion being indicated by the arrows. The current passes up the pillar from 

 c, and along the springs if the end of d' is close to the spring, the cur- 

 rent passes to the primary coil c, and to wires covering two upright pillars 

 of soft iron, from them to the pillar a, and out by the wires to the 

 battery; in passing along the wire, #, the soft iron is converted into a 

 magnet, and so attracts the hammer,/, of the spring, breaks the connec- 

 tion of the spring with d' , and so cuts off the current from the primary 

 coil, and also from the electro-magnet. As the pillars, #, are no longer 



