190 



PHYSIOLOGY 



the momentary currents induced in the secondary circuit, ra being the current of low 

 intensity and long duration produced by the make, and B the shock of high intensity 

 and short duration caused by the sharp break of the primary current. 



When we desire to use faradic stimulation that is, secondary induced shocks 

 rapidly repeated 50 to 100 times a second we make use of the apparatus attached 



to the coil, known as Wagner's 

 hammer (Figs. 48A and 48s). 

 In this case the wires from the 

 battery are connected to the 

 two lower screws (a and 6, Fig. 

 46). Fig. 48A shows the direc- 

 tion of the current when Wag- 

 ner's hammer is used. The cur- 

 rent enters at a, runs up the 

 pillar and along the spring to the 

 screw x. Here it passes up 

 through the screw, and through 

 the primary coil R r From the 

 primary coil it passes up the 

 small coil m, and from this to 

 the terminal b and back to the 

 battery. But in this 



FIG. 47. 



in tnis course 



the coil m is converted into an 

 electro-magnet. The hammer h 



attached to the spring is attracted down, and so the spring is* drawn away from the 

 screw x, and the current is therefore broken. The break of the current destroys the 

 magnetic power of the coil, the spring jumps up again and once more makes circuit 

 with the screw x, only to be drawn down again directly this occurs. In this way the 

 spring is kept vibrating, and the primary circuit is continually made and broken, with 

 the production at each make-and-break of an induced current in the secondary coil. 



It is evident that, when the primary current is made and broken fifty times in the 

 second, there will be a hundred momentary currents produced during the same period 

 in the secondary coil. Every alternate one of these produced by the break of current 



I 



FIG. 48A. Diagram showing course of 

 current ininductorium when Wagner's 

 hammer is used. 



FIG. 48s. Diagram showing course of 

 current when the Helmholtz side wire 

 is used. 



in the primary will be much stronger than the intervening currents produced by the 

 make. In order to equalise make and break induction-shocks, so that a regular series 

 of momentary currents of nearly equal intensity may be produced, the arrangement 

 known as Helmholtz's is used. In this arrangement the side wire w, shown in Fig. 

 46, and diagrammatically in Fig. 48s, is used to connect the binding screw o with the 

 binding screw c at the top of the coil. The screw x is raised, so as not to touch the 

 spring, and the lower screw y is moved up till it comes nearly in contact with the under 



