536 A TEXTBOOK OF PHYSIOLOGY 



The constant current is not often employed, since it is of low 

 electromotive force (E.M.F.), and, owing to its comparatively long 

 duration, it tends to cause polarization of the tissues, due to the 

 dissociation of electrolytes from the colloid of the muscle substance. 

 The induced current is therefore more convenient, since it has, as 

 compared with the constant current, a comparatively high E.M.F., 

 and, be^ng of very short duration, does not induce so much polariza- 

 tion of the tissues for ordinary experiments it is practically nil. 



The induction coil comprises two coils the primary and the 

 secondary. The primary coil is made up of a few turns of thick copper 

 wire wound around an iron core. The secondary coil consists of a 

 large number of turns of insulated fine copper wire. Each turn of 

 wire in the primary coil induces an effect in every turn of the wire 

 of the secondary coil. By this means, therefore, the low E.M.F. of 

 the current in the primary circuit is transformed into a current 

 of high E.M.F. "n the secondary circuit, the intensity of the current 

 being proportional to the number of turns of wire in each coil. It 

 has been found that the E.M.F. of this current varies 



1. Directly with the intensity of the change of current in the 

 primary circvit. The greater the change, the greater the induction. 



2. Eirectly as the rate of change. The mere rapid the change, the 

 greater the induction. 



3. With the angle between the coils. When the secondary coil 

 is at right angles, there is no induction. It is greatest when the wires 

 are parallel to each other i.e., in the ordinary position. 



4. Inversely as the distance between the coils, being greatest 

 when the secondary is completely over the primary coil. 



The induced current is in the opposite direction to that of the 

 primary circuit at make, in the same direction at break. 



When the induced current is employed for purposes of stimula- 

 tion by means of single shocks, the current from the battery is led; 

 into the primary coil of the " induction coil " by means of the two 

 top binding screws. There is no direct connection of the muscle 

 with the batten*, this being placed in connection with the seconda^* 

 coil of the apparatus, and protected from stimulation, except when 

 wanted, by a short-circuiting key. A make-and-break key is placed 

 in the primary circxiit, and the current in the primary coil made to 

 iduce an exciting current in the secondary coil of the apparatus, 



ler by closing tl e key (the make induced current) or by opening 

 break induced current). An induction shock is produced 

 only at make or break not while the current is flowing. The strength 

 of the induced current may be adjusted by varying the distance 

 between the primary and secondary coils. Generally speaking, an 

 experiment is begun with the coils far apart, and the secondary coil 

 then advanced until the stimulus becomes effective (Fig. 260). 



The contraction obtained from a muscle at the break of an induced 

 current is stronger than that caused by the make of the current. 

 This is because there is at make a momentary self -induced current 

 in the primary coil which is opposite in direction to that of the batten* 



