Study of Some of the Parameters Affecting Knudsen Effusion. I. Experimental Tests of the Validity of the Cosine Law as a Function of Cell and Sample Geometrics and Materials

Abstract

Experimental studies of the distributions of particles effusing from a Knudsen cell at high temperatures (1000°—2000°K) reveal marked deviations from the cosine law. It would appear that complete thermodynamic equilibrium is seldom achieved in a high-temperature Knudsen experiment; further, in terms of ordinary kinetic theory, descriptions of the behavior of the ``gas'' inside the cell are oversimplified. The measured distributions show clearly, in a fashion analogous to the formation of an image by a pinhole camera, the effects of geometry and choice of materials for each Knudsen cell. In particular, containment problems which force the use of special cell materials may introduce side effects which can cause serious deviations from theory.