EXCITATION OF MUSCLE 



189 



another coil through which a current may be led from a battery, it is found that on 

 make and break of the current of the second coil a momentary current is induced in the 

 first. The induced current on make is in the reverse direction, that on break in the 

 same direction as the primary current. The electromotive force of the induced current 

 is proportional to the number of turns of wire in the coils. The induction-coil consists 

 of two coils, each containing many 

 turns of wire. The smaller coil (R 15 

 Fig. 46), consisting of a few turns of 

 comparatively thick wire, is the pri- 

 mary coil, and is put into connec- 

 tion with a battery. It has within 

 it a core of soft iron wires, which 

 has the effect of attracting the 

 lines of force, concentrating them, 

 and so increasing its power of in- 

 ducing secondary currents. The 

 secondary coil, R 2 * a large num- 

 ber of turns of very thin wire, 

 is arranged so as to slide over the 

 primary coil. It is provided with 

 two terminals, which may be con- 

 nected with the nerve or other FlG< 46 Diagram of inductorium. R t , primary: 

 tissue that we wish to stimulate. R 2 ? secondary coil, m, electro-magnet of Wagner's 

 Since the electromotive force of the hammer, w, Helmholtz's side wire, 

 induced current is proportional to 



the number of turns of wire, it is evident that the electromotive force of the current 

 delivered by the induction coil may be many thousand times that of the battery cur- 

 rent flowing through the primary coil. The induced currents increase rapidly in 

 strength as the coils are approached to one another ; the strength of these therefore 

 may be regulated by shoving the secondary up to or away from the primary coil. 



A short-circuiting key is always placed between the secondary coil and the nerve 

 to be stimulated. If only single induction shocks are to be used, a make-and-break 

 key is put in the primary battery circuit, and the two wires from the battery and key 

 are attached to the two top screws of the primary coil (c and d, Fig. 46). It is then 

 found that the shock given by the induced current on break of the primary current 

 is much stronger than that on make. 



In endeavouring to explain this difference in the intensity of the make-and-break 

 induction shocks, it must be remembered that the intensity of the momentary current 

 induced in the secondary coil at make or break of the primary current is proportional 

 (1) to the number of turns of wire in each coil ; (2) inversely to the mean distance between 

 the coils (i.e. the nearer the coils, the stronger the induced current) ; (3) to the rate of 

 change in strength of the primary current. Now, when a current is made through the 

 primary coil, induction takes place, not only between primary and secondary coils, 

 but also between the individual turns of the primary coil itself. This current of self- 

 induction, being opposed in direction to the battery current, hinders and delays the 

 attainment by the latter of its full strength, and so slows the rate of change of current 

 in the primary coil. Hence the intensity of the momentary current induced in the 

 secondary coil is less than it would have been without the retarding effect of self-induc- 

 tion. At break of the current, an extra current is also produced in the primary coil 

 in the same direction as the battery 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 dis- 

 appearance 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 & corresponding to the make and c d to the break. The lower line represents 



