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XXIX. An Account of some Experiments to measure the Velocity of Electricity and 

 the Duration of Electric Light. By Charles Wheatstone, Esq. Professor of 

 Experimental Philosophy in King's College, London. Communicated hy Michael 

 Faraday, Esq. F.R.S. <Sfc. 



Received and Read June 19, 1834. 



1 HE path of a luminous or an illuminated point in rapid motion, it is well known, 

 appears as a continuous line, in consequence of the after duration of the visual im- 

 pression. There is nothing, however, in the appearance of such a line by which the 

 eye can determine either the direction or the velocity of the motion which generates it. 

 It occurred to me some years since, that if the motion which described the line in 

 these cases were to be compounded with another motion, the direction and velocity 

 of which were known, it would be easy, from an inspection of the resultant straight 

 or curved line, to determine the velocity and direction of the former. Following up 

 this idea, I made a series of experiments relating to the oscillatory motions of sono- 

 rous bodies, too numerous, and not sufficiently connected with the subject of the pre- 

 sent communication, to be detailed in this place. The satisfactory results thus ob- 

 tained made me desirous to ascertain whether, by similar means, some information 

 might not be gained respecting the direction and velocity of the electric spark : the 

 method by which I then proposed to effect this purpose was first announced in a lec- 

 ture delivered by Dr. Faraday at the Royal Institution in June, 1830. My attention 

 was again drawn to the subject at the commencement of last year, and I attempted 

 to realize the idea in the following manner. 



Fig. 1 represents the apparatus employed, which was screwed at a to the spindle of 

 a whirling machine, so that a rapid rotatory motion might be given to it. The upper 

 and lower parts, which were all of brass except the wooden disc h c, were insulated 

 from each other by a stout glass rod rf e ; a slip of tinfoil connected the ball h with a, 

 and the upper ball g was capable of adjustment to various distances from the lower 

 one h. When the ball/* was placed within striking distance of the prime conductor of 

 an electric machine, a spark passed between them, and also between the balls g and 

 A, which could be separated to the distance of four inches, so as to exhibit a spark of 

 that length. It is obvious, that if the angular motion of the balls were in any sensible 

 proportion to the velocity of electricity, there would be a deviation between the upper 

 and lower terminations of the line. The instrument revolving from left to right, if 



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