224 



PROCEEDINGS OF THE AMERICAN ACADEMY 



When the sparks passed between the ends of two copper wires, 

 § mm. in diameter, carefully filed so as to be parallel, the curves ob- 

 tained were very regular, but of the same general shape. As an 

 example, I give the following : — 



Sir "William Thomson has shown, in his paper on the " Electro-motive 

 Force necessary to produce a Spark," that a greater force per unit of 

 length is needed for short distances than for long distances. He does 

 not state in his paper whetl>er he experimented upon the Ruhmkorff 

 coil or the Holtz Machine. In using the Quadrant Electrometer in 

 measuring the electro-motive force of the sparks from an induction coil, 

 it is, of course, necessary to use a small leaping distance from tho 

 sparks to avoid the return current. At times, I have found that a 

 greater actual deflection was obtained when the leaping distance was 

 as great as | mm. than when it was much smaller. May not Sir 

 William Thomson's results be partly accounted for by induction iu the 

 same manner ? 



Another method of experimenting upon the extra spark obtained 

 by breaking the circuit between the poles of an electro-magnet gave 

 excellent results. One of the poles of the induction coil was connected 

 with the outer coating of a very small Leyden jar; while the other 

 pole connected with the inside coating through a small interval of air, 

 to avoid the return current. The inside coating of the jai; was con- 

 nected by a very fine wire to a thin copper disc, 261 mms. in diameter. 

 Opposed to the copper disc, at a per[)endicular distance of IGO nims., 

 was the end of a short rod, 1 mm. in diameter. Attached to the other 

 end of the rod was a very fine wire connecting with one pole of the 



