On the nature of the streamers in the electric spark

The main subject of the work described in the present paper consists in the examination of the streamers in the inductive spark in the monochromatic lights of the various metallic lines. It thus forms an extension of the research of Messrs. Schuster and Hemsalech, in which the examination of the streamers was restricted to the inductionless spark. The obser­vations were taken by photographing the spectrum as drawn out by a rotating mirror, the slit of the spectroscope being removed and replaced by the spark itself, so that each line of the spectrum formed a monochromatic image of the spark. In order to avoid the superposition of the series of streamers which are formed in the drawing out of each monochromatic image, an arrangement of the prisms of the spectroscope was used by which, while the images of the spark on the camera screen were vertical, and drawn out in a horizontal direction, the dispersion of the spectrum was in a direc­tion of 45° to the horizontal. By this arrangement the series of streamers corresponding to each metallic line becomes distinctly separated from the others. Photographs of the streamers in the spectra of the sparks from the following metallic poles were taken, in each case with a number of different inductances in series with the spark: aluminium, antimony, bismuth, cadmium, calcium, copper, lead, magnesium, mercury, nickel, platinum sodium, tin. The chief conclusions which are drawn from the research are as follows:— (1) The streamers in the inductive spark consist of metallic vapour, the atoms of which are charged, and the motion of the vapour towards the centre of the spark gap is mainly due to the action of the electric force of the spark on the charged atoms. The chief evidence in support of this consists in a number of photographs in which the streamers move back again towards the poles as the oscillating electric field of the spark reverses its direction.