212 



PHYSIOLOGY 



current, and therefore tending to reduce the rate of change of the current from 

 full strength to nothing. In this case, however, the primary circuit being 

 broken, the current of self-induction cannot pass without jumping the great 

 resistance offered by the air, so that its retarding effect on the rate of disappear- 

 ance of the primary current may be practically disregarded. In Fig. 47 the 

 line a, b, c, d, will represent the changes occurring in the primary current at 

 make and break, a b corresponding to the make and c d to the break. The 

 lower line represents the momentary currents induced in the secondary circuit, 

 m 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 



in 



FIG. 47. 



attached to the coil, known as Wagner's hammer (Figs. 48A and 48B). In this 

 case the wires from the battery are connected to the two lower screws (a and &, 

 Fig. 46). Fig. 48A shows the direction of the current when Wagner's hammer 

 is used. The current 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 BJ. 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 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 in the primary will be much stronger than the inter- 

 vening 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 usqd. 



