The mobility of ions in pure gases

Abstract

Although determination of the mobility of ions in gases have been made Almost continuously during the last 30 years, the nature of the ions still remains obscure. The values obtained by different experiments, using a variety of methods, differ by amounts far in excess of those to be attributed to be observational errors and some methods yield results which are complicated and difficult to explain on any simple theory of the nature of the ion. The main facts which have emerged from the mass of experimental data are summarised in “Conduction of Electricity through Gases.”* It seems probable that in none of the experiments made hitherto has the gas under examination been spectroscopically pure. In many cases materials such as ebonite and sulphur have been in contact with the gas. In others the apparatus has been made gas-tight by means of waxed flanges or greased joints. In such conditions the purity of the gas leaves much to be desired. It is continually being contamined by impurities such as water vapour and carbon dioxide coming from the walls of the vessels and from the metals parts of the apparatus as well as by the vapours from the wax, ebonite, etc. Such impurities may have large affinities for electron and may unite with the positive ions to form groups. In such conditions, in a given gas, the ions may well consist of clusters of which the size and mass vary from one experiment to another. The reason why many observers have found the negative ions to be of molecular magnitude and not electrons in nitrogen, hydrogen, etc., becomes Obvious, and it is not surprising that the actual mobilities found are smaller than those calculated theoretically from the standpoint of the classical dynamical theory of gases, assuming the ions to be monomolecular. The present writers for example, found that in an experiments on nitrogen at a pressure of 600 mm, freshly introduced into an apparatus contained under a bell jar, the negative carries consisted of a mixture of ions and electrons. The effect of the contamination coming from the walls was clearly shown by the fact that the number of “normal” ions increased, and of electrons decreased, on leaving the gas in the apparatus overnight.