EXCITATION OF MUSCLE 



213 



In this arrangement the side wire w, shown in Fig. 46, and diagramrnatically 

 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 surface of the spring. If we consider the direction of the current now, 

 we see that it enters as before at the terminal, travels up the Helmholtz's wire 

 w to the screw c, thence through the primary coil %, then through the 

 coil m of the Wagner's hammer, and so back to the battery. The coil TO, 

 thus becoming an electro -magnet, draws down the hammer h. In this act 

 the under surface of the spring comes in contact with the screw y. The 

 current then has a choice of two ways. It may either go through the coil as 

 before, or take a short cut from the terminal a, up the pillar, along the spring, 

 through the screw y, and down to the terminal b back to the battery. As 

 the resistance of this latter route is very small compared with the resistance 

 of the primary coil, &c., the greater part of the current takes this way. The 

 infinitesimal current which now passes through the coil of Wagner's arrange- 

 ment is insufficient to magnetise this, and the hammer springs up again ; thus 



R, 



FIG. 48A. Diagram showing course of 

 current in inductoriuru when Wagner's 

 hammer is used. 



I a 



FIG. 48s. Diagram showing course of 

 current when the Helmholtz side wire 

 is used. 



the process is restarted, and the spring vibrates rhythmically. With this 

 arrangement the primary current is never broken, but only short-circuited, 

 and so diminished very largely. Hence the retarding influence of self-induction 

 is as potent with break as with make of the current, and the effects on the 

 secondary coil in the two cases are approximately equal. In Fig. 47 ce 

 represents the change in the primary current when the current is short-circuited 

 instead of being broken, and b represents the effect produced in the secondary 

 coil. It will be seen that the currents m and b are practically identical 

 in intensity and duration. 



When the induction-coil is used for stimulating, it is usual to graduate the 

 strength of the shock administered to the excitable tissue by moving the 

 secondary coil nearer to or further away from the primary coil. It must be 

 remembered that the strength of the induced current does not vary in numerical 

 proportion with the distance of the two coils from one another. If one coil 

 is some distance, say, 20 cms. from the primary coil, the induced current pro- 

 duced by make or break of the primary current is very small, and on moving 

 the secondary from 20 up to 10 cms. the increase in strength of the current 

 will not be very rapid. The increase will however become more and more rapid 

 as the two coils are brought closer together. Using the same strength of current 

 in the primary coil and the same resistance in the secondary coil, we can say 



