CHAP. IL] 



THE CONTRACTILE TISSUES. 



67 



The current then passes from b not to c but to f, and so down the 

 pillar d, in the direction indicated by the thin interrupted line, and out 

 to the battery by the wire y, and is thus cut off from the primary coil. 

 But this arrangement is unnecessary.) At the instant that the 

 current is thus broken and so cut off from the primary coil, an induced 

 (breaking) current is for the moment developed in the secondary coil. 

 But the current is cut off not only from the primary coil, but also 

 from the spirals m ; in consequence their cores cease to be magnetised, 

 the bar e ceases to be attracted by them, and the spring b, by virtue of 

 its elasticity, resumes its former position in contact with the screw c. 

 This return of the spring however re-establishes the current in the 

 primary coil and in the spirals, and the spring is drawn down, to be 

 released once more in the same manner as before. Thus as long as 

 the current is passing along x, the contact of b with c is alternately 

 being made and broken, and the current is constantly passing into and 

 being shut off from p, the periods of alternation being determined by 

 the periods of vibration of the spring b. With each passage of the 

 current into, or withdrawal from the primary coil, an induced (making 

 and, respectively, breaking) current is developed in a secondary coil. 



As thus used, each 'making shock,' as explained above, is less 

 powerful than the corresponding 'breaking shock;' and indeed it 

 sometimes happens that instead of each make as well as each break 

 acting as a stimulus, giving rise to a contraction, the 'breaks' only are 

 effective, the several 'makes' giving rise to no contractions. 



By what is known as Helmholtz's arrangement, Fig. 6, however 



FIG. 6. THE MAGNETIC INTERRUPTOR WITH HELMHOLTZ ARRANGEMENT FOB EQUALIZING 

 THE MAKE AND BREAK SHOCKS. 



the making and breaking shocks may be equalized. For this purpose 

 the screw c is raised out of reach of the excursions of the spring 6, and 



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