7 o 4 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



paratively small number of turns of fairly thick copper wire, the latter 

 a large number of turns of thin wire. The object of this is that the 

 resistance of the primary, which is connected with one or more voltaic 

 cells, may not cut down the current top 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. 229. Du Bois-Reymond's Inductorium. B, primary, B', secondary, coil, 

 H, guides in which B 7 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 i 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 currents, 

 the stimulating power of a voltaic stream may be much increased by 

 putting into the circuit a coil of wire of not too great resistance. 



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 



