334 Mr. J. N. Hearder on a new Instrument for Registering 



from the same quantity of electricity transmitted in a single dis- 

 charge from the battery, as it would have required the substitu- 

 tion of a larger wire in the electrometer. On passing, however, 

 one such discharge through the wire which I employed, it was 

 melted, clearly showing a very great increase of effect with an 

 undivided quantity. 



Having thus ascertained the vahie of a single discharge from 

 the coil, and establislied a similar unit with the electrical machine, 

 it was easy to ascertain its equivalent in glass surface submitted 

 to friction ; and this quantity, multiplied by the number of dis- 

 charges which the coil would produce in a second, would show 

 their relative capabilities of developing electricity. My next 

 step was to count the discharges of the coil ; and here it may be 

 remarked, that when the jar is employed, the value of each dis- 

 charge is in no way lessened, but rather increased by the rapidity 

 with which they follow each other. For this purpose I adopted 

 Mr. Grove's suggestion of making the discharges perforate a 

 sheet of paper in motion, but the difficulty was to ascertain the 

 rate at which they were produced ; this, however, I accomplished 

 in the following manner, by an apparatus to which 1 have at 

 present given the name of Spark-counter. It is thus constructed : 

 two circular discs of wood, about 5 inches in diameter, are screwed 

 together upon one common axis, so as to embrace a circular disc 

 of paper, about 12 inches in diameter, between them. One of the 

 discs of wood is made moveable, for the piu'pose of changing the 

 discs of paper when required. This is mounted with a handle 

 80 as to rotate vertically like the plate of an electrical machine. 

 Upon the same stand are placed two vertical brass pillars, having 

 steel bearings on the top, which serve to support the knife-edges 

 of the axis of a half-seconds' pendulum, so situ:,ted on the stand 

 as to hang about 2 inches from the edge of the paper disc, and 

 make its oscillations in the same plane. Through a wooden 

 headpiece fastened on the axis of the pendulum, pass two copper 

 wires enclosed in gutta-percha. These wires are about 2 inches 

 apax't, and they stand out horizontally about 8 inches in length, 

 like two arms stretching towards the paper disc, one on each side 

 of the top of its periphery. Here the points are turned towards 

 each other, and brought within ith of an inch of the paper on 

 each side, so that the naked ends of the wires are about a quai'ter 

 of an inch apart, having the paper disc between them, and are 

 so adjusted as to stand about half-way between the edge of the 

 paper and the wooden plates which hold it. The other two ends 

 of the wires pass about 1 inch through the headpiece, and are 

 there bent into two hooks, the gutta-percha coating being re- 

 moved from them. Upon these hooks hang the ends of two 

 light and flexible wii-e-cord conductors, connected respectively 



