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IX. The Constitution of the Electric Spark. 
By T. Ho yds, M.Sc., Beyer Fellow of the Manchester University. 
Communicated hy Professor A. Schuster, F.R.S. 
Received January 29,—Read February 13, 1908. 
[Plates 28-29.] 
1. Previous Experiments .— In the year 1899 Professor Schuster and Dr. Hemsalech* 
published the results of their experiments on the Constitution of the Electric Spark. 
On a slit parallel to the spark they formed an image of a spark produced between 
metal electrodes by a Wimshurst machine and photographed the light passing through 
the slit, both directly and after analysis by a prism, on a moving film in the focal 
plane of the camera lens, the direction of motion of the film being at right angles to 
the direction of the slit. A summary of their results is given below :— 
(1.) In the photographs on the moving film the lines in the spectrum which are due 
to the metallic vapour were inclined, whilst the air lines were straight. 
(2.) They regarded the inclination of the lines as an indication of the velocity of 
propagation of the metallic vapour, and did not discuss any possibilities of 
the propagation of luminosity through vapour which may be, as a wdiole, 
stationary. 
(3.) Different lines in the spectrum had different inclinations. This was most 
marked in the bismuth spectrum. 
(4.) The lighter metals had a greater velocity than the heavier metals. 
(5.) The calcium lines H and K, due to impurities in the silver electrodes, became 
luminous first in the centre of the gap, and were propagated towards the 
electrodes. 
(6.) Of the vibrations excited by the discharge those of some periods are intense 
and of short duration, whilst those of other periods are too faint to show in 
the moving photograph, but are visible in the stationary one because the 
vibration persists some time. 
* Schuster and Hemsalech, ‘Phil. Trans.,’ A, vol. 193, p. 189 (1899). 
vol. ccvm .—a 435. 
25.7.08 
