Theory of the mechanism of the evaporation of a tungsten filament in the presence of an inert gas.—A theory of the evaporation of a heated tungsten filament immersed in an inert gas is developed in which it is assumed that the evaporating atoms diffuse through a film of stationary gas surrounding the filament. The diameter of the film is calculated from Langmuir's equation for the heat loss from a filament. The theory leads to the relation $\mathrm{maP}log\frac{b}{a}=\mathrm{constant}$, where $m$ is the rate of evaporation, $P$ is the pressure, and $a$ and $b$ are the diameters of the filaments and gas film respectively.

Test of the theory. Rate of evaporation of a tungsten filament at 2870°K in a gas mixture, 86% argon, 14% nitrogen.—By measuring the loss in weight of the filament the rate of evaporation for various gas pressures was calculated. The rates varied from 230×${10}^{-9\mathrm{}}$ gr/${\mathrm{cm}}^2$ sec. in a vacuum to 2×${10}^{-9\mathrm{}}$ gr/${\mathrm{cm}}^2$ sec. at a pressure of 165 cm. It is shown that when $b$ is calculated from Langmuir's equation for heat loss the expression $\mathrm{maP}log\frac{b}{a}$ is actually constant for gas pressures in excess of 10 cm. The fact that the round filaments developed a hexagonal cross-section in accord with the normal crystalline shapes of tungsten grains is further evidence of the backward diffusion of evaporated metal with consequent condensation on the filament.

• Received 1 October 1927

DOI:https://doi.org/10.1103/PhysRev.31.260