MUSCLE 



541 



below it, the current can be made once in the primary or broken 

 once, or a constant 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 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 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 development, and reaches its maximum more slowly than the break 

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



FIG. 155. 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 i with the primary. 



electrical change is brought about is one of the most important factors 

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

 current in the secondary is developed depends upon the rapidity 



