EXCITATION OF MUSCLE 



211 



few turns of comparatively thick wire, is the primary coil, and is put 

 into connection 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 inducing secondary currents. The secondary coil, R 2 , 

 of a large number 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 connected 

 with the nerve or other tissue that we wish to stimulate. Since the electro- 

 motive force of the 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 current 

 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. 



FIG. 46. Diagram of inductorium. EJ, primary ; B. 2 , secondary coil. 

 m, electro-magnet of Wagner's hammer, w, Helniholtz's side wire. 



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-induction. At break of the current, an extra current 

 is also produced in the primary coil in the same direction as the battery 



