PHYSIOLOGIC APPARATUS. 681 



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 contrac- 

 tion 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.* 



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 fre- 

 quency that is sufficient to give rise to a summation of effects. The dura- 

 tion 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. 342). 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 adjustable, 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 posts 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 re-established, 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. 



As each interruption of the primary circuit develops an induced current, 

 it follows that the latter must succeed each other with a frequency corre- 

 sponding with the frequency of the former. If while the primary circuit 

 is thus being interrupted the wires of the secondary coil be placed in con- 



* "On certain peculiarities of the inductorium," Prof. Colin C. Stewart, "Univ. 

 Pa. Medical Bulletin," Feb., 1904. 



