The Effect of Space Charge and Residual Gases on Thermionic Currents in High Vacuum

Abstract

HEN a carbon or metal filament is heated in a vacuum and sur- rounded by a positively charged metal cylinder, it is we11 known that electrons are given off by the hot solid.This effect in lamps has been commonly known as the Edison effect and has been rather fully described in the case of carbon lamps by Fleming.'Richardson and others have studied quantitatively the ionization produced by hot solids, especially from heated platinum, and have col- lected a large amount of data.It has generally been found that the saturation current is independent of the pressure of the gas and increases rapidly with increasing temperature of the filament.However, certain gases were found to have very marked effects; for example, traces of hydrogen were.found to enormously increase the saturation current obtained from hot platinum.' Recent investigations have shown' that at least in some cases the current is due to secondary chemical effects.Pring and Parker4 showed that the current obtained from incandescent carbon could be cut down to very small values by progressive purification of the carbon and improvement of the vacuum.They conclude that "the large currents hitherto obtained with heated carbon cannot be ascribed to the emission of electrons from carbon itself, but that they are probably due to some reaction at high temperatures between the carbon, or contained impurities, and the surrounding gases, which involves the emission of electrons." Pring and Parker observed also that the ioni- zation (or rather thermionic current) "increased only very slightly with the temperature above Isoo'."The eSect of these publications, together with that of Soddy, ' who noticed similar effects with a %ehnelt cathode, has been to cast doubt on the existence of a thermionic current in a perfect vacuum and from pure metals.The opinion seems to be gaining ground, especially in Germany, ' Phil.Mag. , 42, p. 52 (I896).