The diffusion of ions into gases at low pressure

1. In papers published in the ‘Proceedings’ of the Royal Society, the charges on ions produced by the action of Rontgen rays and radium on air were determined by a method depending on the diffusion of the ions, and in the course of the investigations it was observed that the removal of water- vapour from the gas produced a large change in the motion of negative ions. In this paper the results are given of some accurate experiments on the ions produced by radium, and experiments at low pressures on the motion of the ions produced by ultra-violet light are described. In the latter case, effects similar to those observed by Prof. Townsend when the gas was ionised by Rontgen rays have been found, and some interesting results at pressures lower than those previously employed have been obtained. 2. The arrangement of the apparatus is here reproduced (fig. 1). The ions are generated in the field A by radium placed in shallow horizontal grooves f , covered with aluminium foil, in brass blocks F. They pass under the action of the electric force through the grating g and the aperture h into the field B, which was kept constant by means of the brass rings G maintained at definite potentials. Here they diffuse and the ratio R of the charge received by the disc D to the charge received by the disc and the ring S together is measured. This ratio is a known function of c = N e Z/P, where e is the charge on an ion, N the number of molecules in a cubic centimetre of air at pressure P at the temperature of the laboratory, and Z the electric force in the field B.