224 



PROCEEDINGS OF THE AMERICAN ACADEMY 



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

 f 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 whether 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 the 

 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 in the 

 same manner ? 



Another method of experimenting upon the extra s^iark obtained 

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

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

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

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

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

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

 Opposed to the copper disc, at a perpendicular distance of 160 mms., 

 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 



