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LXVII. Intelligence and Miscellaneous Articles. 



ON THE EFFECT OF THIN PLATES OF IRON USED AS ARMATURES 

 TO ELECTROMAGNETS. BY JOHN TROWBRIDGE, S.D., HARVARD 

 COLLEGE, U.S. 



TN a paper presented to the Academy, April 13, 1875, I showed 

 -*- that the application of armatures to two straight electromagnets, 

 which formed the primary circuit of a Ruhmkorff coil, more than 

 doubled the strength of the induction-current produced by breaking 

 the primary circuit. When, however, the circuit of the secondary 

 coil was not closed, and a spark was allowed to jump across the 

 interval between its poles, the striking-distance of the spark, and 

 its power to charge a condenser, did not seem to be notably in- 

 creased by the application of armatures to the electromagnets of 

 the primary circuit. My experiments at that time were made 

 with solid iron cores ; and I now resume these experiments with 

 bundles of fine iron wires in place of the solid iron cores. The 

 mechanical difficulty of making the ends of the bundles of fine 

 wires constituting the cores plane surfaces was overcome by dipping 

 them in melted solder, and then filing the surfaces. In this way 

 I. had no difficulty in applying the armatures so that they should 

 lie upon a plane surface. 



The resistance of each of the two induction-coils covering the 

 two straight electromagnets was 6000 ohms, and that of each of the 

 straight electromagnets *34 of an ohm. The diameter of the bundles 

 of fine iron wires constituting the cores was 5 centims., and the length 

 of the electromagnets was 28 centims. Condensers of various sizes 

 were placed in the primary circuit : the results given in this paper 

 were obtained by the use of a condenser of about one farad. The 

 method of experimenting was to charge a condenser of -1 of a farad 

 by means of a spark one millimetre in length, and then to discharge 

 this condenser through a galvanometer. If we express the quantity 

 of electricity received by the condenser by Q, the electromotive 

 force and the capacity of the condenser by E and C, we have Q= EC. 



2nt 

 "We also have Q= — sin^, where wis the reduction factor of 



7T 



the galvanometer, t the time of vibration of the magnet, and p the 

 arc through which it swings under the effect of the charge. Know- 

 ing the reduction factor of my galvanometer, I had thus the means 

 of reducing my results to absolute measure. But I speedily found 

 that the relative results obtained by the proportions 



Q : Q'=sini0 : sinj0'=E:E' 



would present the points of this investigation in a manner as 

 valuable as if the results had been reduced to absolute magnetic 

 measure. 



My first experiments were made with solid armatures. 



