Construction of Filament Surfaces

Abstract

An incandescent tungsten filament gradually loses its die marks and comes to display a surface whose structure depends on whether the lamp was evacuated or gas-filled, and on whether it was run on a.c. or on d.c. Filaments heated in vacuum with a.c. remain smooth and nearly round. In neutral gas, with a.c., the filament tends to expose smooth concave (100) faces. The d.c. vacuum structure is steplike, is polarized with respect to the current, and seems to be due to a drift of W ions in the field along the filament. Surface migration of Th adsorbed on W, during d.c. heating, has been demonstrated in a simple electron-optical tube. Structure identical in form with the d.c. structure appears on a.c. filaments where a temperature gradient exists, and is attributable directly to surface diffusion in the temperature gradient. On d.c. gas filaments the d.c. structure and the gas structure are concurrent. No small-scale structure is found microscopically; probably the only substructure is atomic in scale. Preliminary study of Ta, Mo, Pt, Fe and Ni filaments in vacuum indicates that these metals also develop a roughened surface on d.c. heating, but remain smooth on a.c.