48 , PHYSIOLOGICAL PHYSICS. [Chap. iv. 



second pathway the current does not go round the 

 primary coil c at all. The current is, therefore, said 

 to be short-circuited, because a " short cut " is offered 

 to it instead of the longer way round the primary coil. 

 Thus, as soon as the spring touches f', the current 

 has two circuits: the first, Eclgg'fcbkE, and the 

 second, E df k E. At the binding screw d, therefore, 

 the current will branch off; part will flow through the 

 primary coil, and part will pass by the short circuit 

 straight back to the battery. The current through 

 the coil c and the electro-magnet b is, therefore, 

 weakened. As a result, the electro-magnet b is no 

 longer able to attract the hammer-head h, which flies 

 back by the elasticity of the spring, and the short 

 circuit is broken by contact between the spring 

 and f being interrupted. The current, therefore, 

 again proceeds in full force round the coil c and the 

 electro-magnet b. The latter again becomes strongly 

 magnetic, attracts h, again closes the short circuit at 

 /', and again weakens the current round c. Again 

 does the magnet become too weak to attract h, which 

 again flies off, and breaks the short circuit, and 

 restores the full force of the current round c, and so 

 on the action is repeated. The effect of this short- 

 circuiting arrangement on the induction currents is 

 shown in the dotted curves of Fig. 25. The weaken- 

 ing of the current round the coil is represented by the 

 line A'A" in its diminished distance from B. When 

 the short circuit is interrupted by the recoil of the 

 hammer-head, the strength of the current round the 

 coil c rises. This is represented by the curved dotted 

 line 1'. 



Corresponding to this increased strength of the 

 current in the primary coil is an induced current 

 in the secondary, represented by the dotted curve 

 2'. But no sooner is the short circuit interrupted 

 than it is again closed by the increased action of the 



