it was inferred that the spark, instead of being a simple effect, is composite like the lightning, and is made up of several elements.
Such were the incomplete and discordant results of the investigation when it was undertaken by Prof. Rood. The arrangement ho devised consisted of two parts, one for the production of the spark, and the other for measuring it. Fig. 3 represents the first combination. A galvanic battery was used to generate the electricity; this was connected with a large Ruhmkorff coil, which was again connected with a Leyden jar, and this with the electrodes for producing the spark, S>
|Galvanic Battery.||Ruhmkorff Induction-Coil.||Leyden Jar.||Electrodes and Spark.|
which were adjustable for varying its "striking distance." Connected with the wires between the battery and the coil was an automatic "interruptor" for breaking the circuit from three to six times in a second, by which the frequency of the discharges could be regulated. Leyden jars of different sizes could be used so as to give sparks of all degrees of strength and intensity.
In the second part of his arrangement, Prof. Rood, like his predecessors, employed a revolving mirror, turned by the gearing of Becquerel's phosphoroscope (Fig. 1), with the addition of an extra wheel and a weight to drive it. With this he could get 350 revolutions of the mirror per second, with a smooth and uniform motion. In order to measure exactly the rate of rotation, the cylinder on the lowest wheel was made to wind up a fillet of paper, upon which dots were made by an electro-magnetic apparatus, regulated by a seconds-pendulum, when a simple calculation gave the rate of the wheel to which the mirror was attached, and the regularity of the train was thus put to a sharp test. The light of the spark S (Fig. 4), passing through an achromatic lens, l, struck the mirror, m, and was reflected upward, forming an image at i, on the plate of ground glass G. The image of the spark on the ground glass was viewed from above, and its position and form were carefully measured by several methods. Of course, if the