202 PROCFRDINGS OP THE AMERICAN ACADEMY 



XIII. 



BRIEF CONTRIBUTIONS FROM THE PHYSICAL LABO- 

 RATORY OF HARVARD COLLEGE. 



BY JOHN TROWBRIDGE. 



Ko. IV. — ON THE EFFECT OF THIN PLATES OF IRON USED AS 

 ARiMATURES TO ELECTRO-MAGNETS. 



Presented, Feb. 9, 1876. 



In a paper presented to the Academy, April 13, 1875, 1 showed that 

 the application of armatures to two strait electro-magnets, which 

 formed the primary circuit of a Ruhmkorf 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 increased by the applications of 

 armatures to the electro-magnets of the primary circuit. My experi- 

 ments, 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 

 strait electro-magnets was 6000 ohms, and that of each of the strait 

 electro-magnets .34 of an ohm. The diameter of the bundles of fine 

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

 electro-magnets was 28 cm. 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 ex])eri- 

 menting was to charge a condenser of } 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 



