CHAP. IL] 



THE CONTRACTILE TISSUES. 



65 



to this delay in the full establishment of the current in the primary 

 coil, the induced current in the secondary coil is developed more slowly 



FIG. 4. DIAGRAM OF AN INDUCTION COIL. 



+ positive pole, end of negative element, - negative pole, end of positive 

 element of battery, K, du Bois-Keymond's key, pr. c. primary coil, current shewn 

 by feathered arrow, sc. c. secondary coil, current shewn by unfeathered arrow. 



than it would be were no such ' self-induction ' present. On the other 

 hand when the current from the battery is 'broken,' or 'shut off' from 

 the primary coil, no such delay is offered to its disappearance, and 

 consequently the induced current in the secondary coil is developed 

 with unimpeded rapidity. We shall see later on that a rapidly de- 

 veloped current is more effective as a stimulus than is a more slowly 

 developed current. Hence the making shock, where rapidity of pro- 

 duction is interfered with by the self-induction of the primary coil, is 

 less effective as a stimulus than the breaking shock whose development 

 is not thus interfered with. 



The strength of the induced current depends, on the one hand, on 

 the strength of the current passing through the primary coil, that is, 

 on the strength of the battery. It also depends on the relative position 

 of the two coils. Thus a secondary coil is brought nearer and nearer 

 to the primary coil and made to overlap it more and more; the 

 induced current becomes stronger and stronger, though the current 

 from the battery remains the same. With an ordinary battery, the 

 secondary coil may be pushed to some distance away from the primary 

 coil, and yet shocks sufficient to stimulate a muscle will be obtained. 

 For this purpose however the two coils should be in the same line; 

 when the secondary coil is placed cross-wise, at right angles to the 

 primary, no induced current is developed, and at intermediate angles 

 the induced current has intermediate strengths. 



When the primary current is repeatedly and rapidly made and 

 broken, the secondary current being developed with each make and 

 with each break, a rapidly recurring series of alternating currents is 

 developed in the secondary coil and passes through its electrodes. We 

 shall frequently speak of this as the interrupted induction current, or 

 more briefly the interrupted current ; it is sometimes spoken of as the 

 F. 5 



