Sources of ionisation in high-pressure gases above 2000°K

Previous experiments with inert gases at high pressure between coaxial graphite electrodes, at temperatures above 2000° K, have shown unexpectedly high electrical conductivities for quite small voltages across the electrodes. In the present experiments the influences of electrode geometry and supply frequency are examined, and the gas pressure is steadily reduced until the classical regions of low pressure (several mmHg) and high vacuum (mean free path ¿ electrode spacing) are reached. Spectroscopic observations confirm the presence in the gas of `seed¿ impurities from the electrodes, and results are described for the deliberate addition to the gas of aluminium vapour as a `seed¿ element.