The energy distribution among the positive ions at the cathode the glow discharge through gases

The energy distribution among the positive ions which strike the cathode of the glow-discharge through gases is of some importance in the theory of the phenomenon, and as very little is known about the mean-free-paths of ions in gases it is difficult to make an estimate of this distribution. Attempts have been made in the past to measure this quantity by perforating the cathode and applying retarding potentials to the ions which penetrate through. The positive ion photographs obtained by the parabola method of Sir J. J. Thomson give information concerning the distribution in a strongly abnormal discharge. These last, and observations of the Doppler effect in canal-rays, suggest that there are particles present with energies corresponding with a fall through the full potential drop across the dark-space, while the retarding potential measurements of Von Hippel show a sharp upper limit to the energy at between 0·3 and 0·5 of the total cathode fall. The dark-space stretches over 20 to 100 molecular free-paths, so that if ions are present with the full energy corresponding to the cathode fall they must have relatively long free-paths in the dark-space. We have carried out measurements of the energy distribution among the positive ions which penetrated through a slit in a plane cathode into an evacuated space beyond, by a retarding potential method and by the method of focussing at 127° 17' in an inverse first power electrostatic field.