ELECTRICITY. 



245 



Annalcn, Ed. 103, Seite 69). ^ This physicist employed 

 a concave mirror with a radius of half a meter, driven 

 by a train of toothed wheels, and obtained as high a 

 rate of rotation as one hundred per second ; the spark 

 was generated in one of the conjugate foci of the 

 mirror, and its image, formed at the other conjugate 

 focus on a plate of ground glass, was in a condition 

 to be conveniently seen and measured by the naked 

 eye or to be photographed. It was found that the 

 image of the spark was drawn out by the revolving 

 mirror into a whitish streak, from 20 to 30 milli- 

 metres long, which, when a large amount -of electri- 

 city was employed, was still further lengthened by 

 the addition of a red tail of about the same dimen- 

 sions, this latter being due to the gradual cooling of 

 the heated particles. With a jar of 2.2 feet inner 

 coating, the distance of the copper balls which served 

 as electrodes being 1.5 millimetres, and all the con- 

 nections as short as possible, the duration of the dis- 

 charge exclusive of the red tail was .00004 of a second. 

 It was also found by him, that an increase in the size 

 of the coating of the jar or of the ' ' striking distance ' ' 

 prolonged the duration of the discharge ; so, for ex- 

 ample with a spark 3.75 millimetres long, the dura- 

 tion of the whitish portion was .00007 of a second. 



A series of observations detailed at the end of this 

 article will point out a probable explanation of the 

 great discrepancy existing between the results of Fed- 

 dersen, which are undoubtedly correct, and those of 

 Wheatstone. Finally, Feddersen in the course of his 

 investigation found abundant proof of the oscillatory 

 nature of the electric discharge, which had been pre- 

 dicted from the results of a set of experiments by 

 Prof. Joseph Henry as far back as 1842. 



I arranged in the first place an apparatus like that 

 of Feddersen, but it was soon found that, to attain a 

 rotation of one hundred per second, it was necessary 

 in my case to employ a weight of 200 pounds, and in 

 addition, when using this form, I labored under the 

 disadvantage of not being able to vary the size or 

 curvature of the mirror readily ; in short, it was found 

 to lack flexibility, a quality more necessary in the 

 present case than in the investigation which engaged 

 the attention of the German physicist. On this ac- 

 count I decided to employ a plane revolving mirror 

 with a stationary achromatic lens. The addition of 

 an extra wheel, with a few other mechanical changes, 

 converted the train of wheels, furnished by Duboscq 

 for Becquerel's phosphoroscope, into an admirable 

 means for driving the revolving mirror, and, with a 

 weight of 30 pounds, 300 rotations per second were 

 readily obtained, the motion being smooth and suf- 

 ficiently uniform. In order to measure 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 fur- 

 nished the rate of the wheel to which the mirror was 

 attached. By this method it became possible to put 

 to a sharp test the regularity of the rate of the train, 

 which was found in all cases to be considerably 

 greater than was at all necessary. These determina- 

 tions were afterward made with a watch having 

 large second-spaces, the dots being made by hand, as 

 the refinement of a seconds pendulum and battery 

 was found for my purpose to be superfluous. 



The mirrors employed were always plane, being 

 sometimes silvered at the back, while for the most 

 delicate observations the rear surface was painted 

 with black varnish, so as to destroy the second re- 

 flection. They varied in size from 1.5 inches square, 

 down to .2jby .5 in., and were used sometimes double 

 on the axis back to back, and at other tunes four 

 mirrors were employed together. 



When brass points were used as electrodes, 

 the following were the results of experiments 

 to determine the total duration of the dis- 

 charge, including both, the yellow and white 

 portions of the electric spark : 



When platinum - points were used as elec- 

 trodes, with a striking distance of three millime- 

 tres, the duration, as obtained, varied between 

 .00004 and .00014 of a second. The duration 

 of the yellow portion alone, with a rotation of 

 206 per second, was found to vary from .000003 

 to .000007 of a second. 



Prof. Eood's greatest difficulty was to as- 

 certain the duration of the white band or 

 portion of the spark, in which, a large propor- 

 tion of its light resides, its superior brightness 

 being to some extent isolated from the rest of 

 the spark when points (instead of balls) are 

 used as electrodes. His plan for effecting the 

 object, and the conclusions at which he arrived, 

 are thus detailed : 



A small piece of cardboard was ruled with two 

 black lines; their distance apart was .0067 inches, 

 and they were separated by a space exactly equal to 

 the width of a single line. A small dividing engine 

 was_used for their production ; they were tested with 

 a microscope. These lines were illuminated by the 

 spark, and their image examined by an eye-piece 

 magnifying five diameters. The breadth of the image 

 of a single line on the ground glass was .011 inches ; 

 that is, each line subtended an angle of 2' 24", reck- 

 oning from the mirror. "With a velocity of 223 per 

 second the mirror is able to move the reflected image 

 through 2' 24" in the .00000024 of a second. If now 

 this first discharge had actually lasted this minute 

 portion of time, it is evident that the motion of the 

 mirror would just have carried the image of one of 

 the black lines forward, so that, at the end of this in- 

 finitesmal period, it would have occupied the space 

 where just before the white line had been traced: 

 hence, owing to the retention of impressions by 

 the retina the white central line would have been 

 obliterated, and, in place of three lines, a gray band 

 would have been seen. On the other hand, if the 

 duration had been only one-tenth or one-fifth of the 

 above-mentioned interval, the white line, not having 

 been much encroached on, would still have remained 

 visible. ^The correctness of the above reasoning can 

 be experimentally proved, by means of a revolving 

 disk of cardboard with a single slit cut in it, lines 

 being drawn on its white side, and viewed by reflec- 

 tion with a mirror through the slit, the blackened 

 side of the disk being turned toward the eye. 



To facilitate matters, three sets of these lines were 

 drawn on the small cardboard at considerable dis- 

 tances apart to prevent confusion, and while illumi- 

 nated by the electric spark were examined with in- 

 creasing velocities. With low rates they were quite 

 unchanged in appearance, with a velocity as high as 

 102 per second, a duration of the first discharge of 

 .00000052 seconds would just have obliterated them ; 

 they were, however, perfectly distinct, though more 

 faintly traced : the rate of rotation was then by de- 

 grees carried up to 223 per second, when the lines 

 were still distinctly visible, though of course with 

 less contrast between the white and black than was 

 the case with low velocities or a stationary mirror. 



This experiment proves, then, that the duration 

 of the light accompanying the first discharge is con- 

 siderably less than .00000024 of a second, probably 

 less than half this period, or less than one ten mill- 

 ionth of a second of time. 



Electric Conductivity of Liquids. Experi- 

 ments have been made at the Sorbonne to 



