QUICKER THAN LIGHTNING, 



315 



it was inferred that the spark, instead of being a simple effect, is com- 

 posite like the lightning, and is made up of several elements. 



Such were the incomplete and discordant results of the investiga- 

 tion when it was undertaken by Prof. Rood. The arrangement he de- 

 vised 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 con- 

 nected with a large Ruhmkorff coil, which was again connected with a 

 Leyden jar, and this with the electrodes for producing the spark, S, 



Fig. 3. 



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 "in- 

 terruptor " 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 prede- 

 cessors, employed a revolving mirror, turned by the gearing of Bec- 

 querel'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-pen- 

 dulum, 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, I, 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 



