OF ARTS AND SCIENCES. 381 



XII. 



BRIEF CONTRIBUTIONS FROM THE PHYSICAL LABORA- 

 TORY OF HARVARD COLLEGE, 



UNDEK THE DIRECTION OP 



JOHN TROWBRIDGE, Assistant-Professoe of Physics. 



No. XL — ON A NEW INDUCTION COIL. 

 Read, April 13, 1875. 



In the best constructed induction coils of the present day, the electro- 

 magnet is prolonged beyond the induction coil ; so that the latter occu- 

 pies the middle of the electro-magnetic core, where the inductive effect 

 is the greatest. The core of the electro-magnet consists of a bundle 

 of iron wires, which, by their want of continuity of mass, break up the 

 currents of induction which form in the mass of a large solid core, and 

 prevent the sudden breaking of the electro-magnetic circuit, which is so 

 desirable, in order to produce great effects of tension. 



The preceding expe;iments, made with armatures to electro-magnets, 

 which I sufTfjested to Mr. Lefavour and Mr. Peirce, led me to think 

 that the effect of an induction coil could be increased by providing its 

 core with an armature. I first experimented with a horseshoe-sliaped 

 solid core, 2.-5 cm. in diameter; the limbs of which were 12 cm. long, 

 and the distance between the limbs was also 12 cm. On one of the 

 limbs was slipped a coil of thick copper wire, of .07 of an ohm resis- 

 tance. The induction coil, which was of copper wire, one ohm in 

 resistance, was distributed uniformly over the primary coil. The in- 

 duction coil was connected with a Thomson's reflecting galvanometer. 

 The following table shows the results which were obtained when the 

 circuit was broken in the primary coil. The deflections at making 

 the circuit are not given, since they were equal to those produced by 

 breaking the circuit; and the study of the induction currents produced 

 by breaking are the most important' for our present purpose. The 

 induction currents were first obtained without the use of an armature 

 upon the two limbs of the electro-magnet, and then with the armature 

 in position. 



