IV. On the Nature of the Streamers in the Electric Spark. 

 By S. R. MILNER, D.Sc. (Lond.), Lecturer in Physics in the University of Sheffield. 



Communicated by Prof. W. M. HICKS, F.R.S. 



Keceived February 10, Read March 5, 1908. 

 [PLATES 2-4.] 



WHEN the oscillating electric spark is examined in a rapidly rotating mirror, the 

 successive oscillations render themselves evident in the image as a series of luminous 

 curved streamers which emanate from the poles and extend towards the centre of the 

 spark gap. These streamers were first observed by FEDDERSEN* in 18G2, but the 

 work of SCHUSTER and HEMSALECH! in 1900 may be said to have opened up a new 

 era in the subject. These workers threw the image of the spark on the slit of a 

 spectroscope, and photographed the resulting spectrum on a film which was maintained 

 in rapid rotation in a direction at right angles to that of the incident light. In their 

 photographs they found that the air lines extended straight across from pole to pole, 

 but that the metal lines were represented by curved bands drawn out in the centre of 

 the spark gap. There is a close relation between these bands and the streamers seen 

 in the unanalysed inductive spark: SCHUSTER and HEMSALECH carried out their 

 experiments with the smallest possible inductance in series with the spark, and thus 

 made the period of the oscillations so small that the drawing out on the film was 

 insufficient to separate the individual oscillations from each other. Thus their curved 

 lines represent a composite structure, consisting of all the streamers due to the 

 successive oscillations superposed on each other. It follows from their results that 

 the light of the streamers in the spark is entirely produced by the glowing of the 

 metallic vapour of the electrodes, and that, while the luminosity of the air is practically 

 instantaneous in its occurrence, that due to the metal vapour occurs in the centre 6T 

 the spark gap an appreciable time later than near the poles. 



The actual process which goes on in the spark and gives rise to this delay in the 

 arrival of the metallic vapour at the centre of the gap is not yet thoroughly 

 understood. SCHUSTER and HEMSALECH make the natural supposition that it is due 

 to the fact that the metal of the electrode is vaporised and rendered incandescent by 

 the heat of the spark, and that the vapour takes an appreciable time to diffuse from 



* 'Pogg. Ann.,' vol. 116, p. 132 (1862). 

 t ' Phil. Trans.,' A, vol. 193, p. 189 (1900). 

 VOL. CCIX. A 444. 16.11.08 



