766 TEXT-BOOK OF PHYSIOLOGY 



current corresponding in direction which assists in the development of the induced current. This 

 is not the case, however, as no break-extra current is developed in the inductoriura as ordinarily 

 used when actuated by a battery current of moderate strength. 



As it is not so much the intensity of the current as it is rapid variations in intensity that produce 

 effects, it is readily apparent why the induced current developed at the break of the primary is 

 more effective as a stimulus than the induced current developed at the make of the primary circuit. 

 The quantity of electricity is, however, the same in both cases. 



If the secondary be pushed further along the slideway until it largely 

 covers the primary coil, a position will be reached when the make-induced 

 current equals in its efficiency as a stimulus the break-induced current; 

 and if the secondary be yet further advanced, a position is reached when the 

 make-induced current becomes more powerful and efficient than the break- 

 induced current, as shown by the greater contraction of the muscle. This 

 result is explained by the fact that the make-extra current is now able of 

 itself to induce a current in the secondary coil, on account of its proximity, 

 which, added to that induced by the battery current, produces a current, 

 greater than that induced on the break of the circuit. 1 



Rapidly Repeated Induced Currents. As the single-induced current is of 

 extremely short duration, it is inefficient as a stimulus in the conduct of 

 many experiments. It is necessary, therefore, to develop it with a frequency 

 that is sufficient to give rise to a summation of effects. The duration of the 

 stimulation may be thus considerably prolonged. This is accomplished 

 by introducing in the primary circuit close to the primary coil an automatic 

 interrupter, usually Neef's modification of Wagner's hammer (Fig. 344). 

 This consists of a vertical post, P', to the top of which is fastened a metallic 

 spring carrying at its opposite end a steel or iron hammer, H, which hangs over, 

 but does not touch, the two vertical bars of soft iron around which the wire 

 of the primary coil is wound. About the middle of the spring on its upper 

 surface there is a small plate of platinum which is in contact with an adjust- 

 able, platinum-tipped screw, S', carried by a plate of brass in connection with 

 binding post S". 



For the purpose of interrupting the primary circuit frequently in a unit 

 of time, and thus developing induced currents in quick succession, the 

 apparatus is arranged in the following way: The positive and negative 

 poles of the electric cell are connected by wires with binding post P' and P", 

 a key being interposed in the circuit. If the screw S' is in contact with 

 the spring, the current on the closure of the circuit will enter P', pass 

 along the spring to S', thence into and through the primary coil R', to 

 the coils surrounding the vertical bars B', then to P", and so back to 

 the cell. 



As the current passes around the vertical bars, they are magnetized. 

 The magnetization draws down the hammer, and, in so doing, breaks the 

 circuit at the tip of the screw, S'. The vertical bars are at once demagnetized, 

 and the hammer is restored to its original position by the elasticity of the 

 spring. The circuit is thus reestablished, the current flows through the 

 coils, the bars are again magnetized, the hammer is drawn down, to be 

 followed by a second break of the circuit. 



The number of times the circuit is thus made and broken per second will 

 vary with the length of the spring. 



1 "On certain peculiarities of the inductorium," Prof. Colin C. Stewart, "Univ. Pa. Medical 

 Bulletin," Feb., 1904. 



