A Study of Thermal Transpiration Using Ultrahigh-Vacuum Techniques

Abstract

The thermal transpiration ratio ${\mathrm{R=P}}_1{\mathrm{/P}}_2$ was measured by the absolute method in helium and neon over the pressure range ${10}^{\text{−9}}{\mathrm{\le P}}_2\text{≤60}$ Torr. Cold temperatures were $T_1\text{=77.4\hspace{0.17em}°}\mathrm{K}$ and 232 °K with warm temperature $T_2\text{=295\hspace{0.17em}°}\mathrm{K}$ . An aperture and various tubes were used at the temperature interface. For the aperture $R_m{\mathrm{=(T}}_1{\mathrm{/T}}_2{)}^{\text{1/2}}$ was found, as expected, at low pressures but this value was not found, in general, for tubes. The latter case represents a departure from normal assumptions, which are examined in detail. A related conclusion is that the relative method for measuring R is unreliable for tubes at low pressures. It is concluded that a molecule has a greater probability of traversing a tube from the hot end to the cold end than in the opposite direction. It is found that several analytic expressions for R represent the data satisfactorily after suitable choice of constants.