The influence of gases on the emission of electrons and ions from hot metals

Abstract

As is well known, the saturation density i of the electron currents from hot bodies can be represented accurately by the formula i = AT½ e - b /t (1) where T is the absolute temperature and A and b are constants characteristic of the substance. An equation of type (1) has been found to represent the currents not only from pure metals in a good vacuum but also when a gaseous atmosphere is present, provided the constants are given different values. Thus in general A and b are functions of the nature and pressure of the surrounding gas as well as of the hot metal. They are independent of T within the limits of temperature in which the formula is valid. In the case of pure metals there is evidence that the formula is valid at all temperatures. In certain cases the constants A and b are very sensitive to minute changes in the nature and pressure of the surrounding gaseous atmosphere. This is well shown by Langmuir's experiments on tungsten, where a slight truce of hydrogen changed the value of A by a factor of about 10 12 and that of b from 5·5 x 10 4 °C. to 11·5 x 10 4 °C. In the case of platinum heated in an atmosphere of hydrogen, H. A. Wilson has shown that A and b are functions of the pressure of the hydrogen, and that the changed values of A and b are subject to the relation b = c log A + d , (2) where c and d are constants. In other words, the changes in the constants A and b caused by the admission of hydrogen are always of such a character that the change in log A divided by the change in b is invariant. In the present paper it is shown that a similar relation holds for the changes in the values of A and b for tungsten which are caused by traces of various gases. In fact it seems probable, in general, that when the emission of ions from metals is affected by the presence of gases, the changes in A and b are subject to a linear relation between b and log A. There is evidence that the relation is independent of the particular gas used to effect the changes, and that the law applies to the emission of positive ions as well as electrons from hot metals. In view of the complexity of the phenomena which may occur when metals at a high temperature are immersed in a gaseous atmosphere it seems unlikely that such a simple law will be exact for all the small changes in A and b which may arise; but the evidence is strong that it covers the main features of the phenomenon. Let us consider the case of tungsten first. Langmuir ( loc. cit. ) has measured values of A and b from this substance both in the best attainable vacuum, similar to that in a Coolidge tube, and in a high vacuum to which small amounts of various gases have been added. The data obtained are shown in the following Table.