64 



THE CONTRACTILE TISSUES. 



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 jc, 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 pri- 

 mary 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 contraction. 



But what is known as Helmholtz's arrangement (Fig. 16), however, the making 

 and breaking shocks may be equalized. For this purpose the screw c is raised out 



The Magnetic Interrupter with Helmholtz's Arrangement for Equalizing 

 the Make and Break Shock. 



of reach of the excursions of the spring 6, and a moderately thick wire w, offering 

 a certain amount only of resistance, is interposed between the upper binding screw 

 of on the pillar rr, and the binding screw c' leading to the primary coil. Under 

 these arrangements the current from the battery passes through a', along the inter- 

 posed wire to c', through the primary coil, and thus, as before, to m. As before, 

 by the magnetism of m, e is drawn down and b brought in contact with /. As the 

 result of this contact, the current from the battery can now pass by a, /, and d 

 (shown by the thin interrupted line), back to the battery; but not the whole of 

 the current, some of it can still pass along the wire w to the primary coil, the rela- 

 tive amount being determined by the relative resistance offered by the two courses. 

 Hence at each successive magnetization of m, the current in the primary coil does 

 not entirely disappear when b is brought in contact with /; it is only so far dimin- 

 ished that m ceases to attract e, and hence by the release of b from / the whole 

 current once more passes along w. Since, at what corresponds to the " break " the 

 current in the primary coil is diminished only, not absolutely done away with, 

 self-induction makes its appearance at the "break" as well as at the "make;" 

 thus the " breaking " and "making " induced currents or shocks in the secondary 

 coil are equalized. They are both reduced to the lower efficiency of the "mak- 

 ing ' ' shock in the old arrangement ; hence to produce the same strength of 



