730 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



cells, may not cut down the current too much; while the currents 

 induced in the secondary, having a high electromotive force, can readily 

 pass through a high resistance, and are directly proportional in intensity 

 to the number of turns of the wire. 



By means of various binding-screws and the electro -magnetic inter- 

 rupter or Neef 's hammer, shown in the figure and explained below it, 

 the current can be made once in the primary or broken once, or a con- 

 stant alternation of make and break can be kept up. We can thus get 

 a single make or break shock in the secondary, or a series of shocks, 

 sometimes called an interrupted or faradic current. Such a series of 

 stimuli can also be got by making and breaking a voltaic current at any 

 given rate. 



A ' self -induced ' current can also be obtained from a single coil ; for 

 instance, from the primary coil alone of the induction apparatus. The 



Fig. 237. Du Bois-Reymond's Inductorium. B, primary, B', secondary, coil. 

 H, guides in which B' slides, with scale; D, electro-magnet; E, vibrating spring; 

 i, wire connecting wire of D to end of primary; v, screw with platinum point, 

 connected with other end of primary ; A, A', binding-screws, to which are attached 

 the wires from battery. A' is connected with the wire of the electro-magnet D; 

 and through it and t with the primary. 



reason of this is, that when a current begins to flow through any turn 

 of a coil of wire it induces in all the other turns a current in the opposite 

 direction, and, when it ceases to flow, a current in the same direction 

 as itself. The former current, ' the make extra shock,' being in the 

 opposite direction to the inducing current, is retarded in its develop- 

 ment, and reaches its maximum more slowly than ' the break extra 

 shock.' But, as we shall see, the suddenness with which an electrical 

 change is brought about is one of the most important factors in elec- 

 trical stimulation, and therefore the break extra shock is a much more 

 powerful stimulus than the make. Owing to these self-induced cur- 

 rents, the stimulating power of a voltaic stream may be much in- 

 creased by putting into the circuit a coil of wire of not too great 

 reristance. 



The self-induction of the primary also affects the stimulating power 

 of the currents induced in the secondary; the shock induced in the 

 secondary by break of the primary current is a stronger stimulus than 

 that caused at make of the primary. The reason is that with a given 

 distance of primary and secondary, and a given intensity of the voltaic 

 current in the primary, the abruptness with which the induced current 



