On the anode and cathode spectra of various gases and vapours

Abstract

In many gases the colour of the glow surrounding the cathode of a vacuum tube is markedly different from that of the positive column, and there are corresponding differences in the spectra of these two regions. Some years ago Prof. Sir J. J. Thomson examined systematically a number of compounds from this point of view, and found that electro-positive elements had a tendency to appear at the cathode, whilst electro-negative elements showed a preference for the anode. In explanation of these facts he supposed that a process of the nature of electrolysis went on in gases under the influence of the electric discharge, resulting in the liberation of ions in the neighbourhood of the two electrodes. The present paper gives an account of work which I have been carrying on lately in the Cavendish Laboratory, and which is a continuation and extension of what has previously been done. The discharge tube used was of the form shown in fig. 1. Its length was approximately 25 cm., whilst different diameters were tried, varying from about 0·3 cm. to about 1 cm. It was divided into two halves by a small plate of aluminium (about 0·5 cm. thick) which fitted sufficiently well to cause the discharge to go through it and not round it. In these circumstances, one side of the plate forms an anode and the other a cathode, so that a small shift, either of the tube or of the spectroscope, suffices to bring one or the other electrode before the slit. A Cox induction coil was used, and in order to minimise the effect of reversals a “point and plane” spark-gap was placed in series with the discharge tube. Part of the work was carried out with an ordinary two-prism spectroscope, the rest with a Hilger constant deviation wave-length spectroscope. In every case observations were taken over as large a range of pressures as was practicable (0·01 cm. to 1 cm., roughly).