EXCITATION OF MUSCLE 



191 



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 wire w to the screw c, 

 thence through the primary coil B 1? then through the coil m of the Wagner's hammer, 

 and so back to the battery. The coil m, 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 arrangement is insufficient to 

 magnetise this, and the hammer springs up again ; thup 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 hi 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 



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 produced by make or break of the primary current is very 

 smallj and on moving the secondary from 20 up to 10 cms. the increase in strength of 

 the current will not bo 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 that the 

 make or break current will be uniform so long as the distance of the coils remains 

 constant. We are not able however to say by how much the current will increase 

 as the secondary coil is moved, say, from 11 to 10 cms. distant from the primary coil. 

 If it is required to know the exact increment in the exciting current which is used, it 

 is necessary to graduate the induction-coil by sending the induction shocks, obtained 

 at different distances of secondary from primary coil, through a ballistic galvanometer. 



Another method which may be adopted for the excitation of muscle or nerve is the 

 discharge of a condenser. The advantage of this method is that we can determine 

 not only the amount of electricity discharged through 

 the preparation, but the actual energy employed. If 

 two plates of metal separated from one another by a thin 

 insulating layer of dielectric such as air, glass, mica, or j 

 paraffined paper, be connected with the two poles of a 

 battery, each plate acquires the potential of the pole of 

 the battery with which it is connected, and receives 

 therefrom a charge of electricity (positive or negative). 

 If the connections be broken the two plates retain their 

 charge. If now they be connected by a wire they 

 discharge through the wire, and if a nerve be inserted in 

 the course of the wire, it may be excited by the discharge. 



The amount of electricity that may be stored up in 

 this way will depend on the extent of the plates and 

 their proximity to one another, as well as on the E.M.F. FIG. 

 of the charging battery. In order to get great extent 

 of surface, a condenser is built up, as in the diagram 

 (Fig. 49), of a very large number of plates of tinfoil, 

 separated by discs of mica or paraffined paper. Alternate discs are connected together : 



49. Diagram to show 

 the mode of construction of 

 a condenser. 



