A Mechanical Maxwell Demon

Abstract

A direct experimental verification of Maxwell's distribution law.---A beam of molecules was directed through radial slits in two disks mounted about 8 cm apart on a common shaft which rotated at a speed from 500 to 6000 rpm. in a highly evacuated enclosure. Those molecules which, because of a favorable relation between their velocity and the speed of rotation of the disks, passed through the slits, fell on the vane of a sensitive radiometer. The variation of the radiometer deflection with the speed of the disks was observed for hydrogen, nitrogen and carbon tetrachloride. The form of the curves varied with the nature of the gas and was in agreement with the predictions of the kinetic theory, thus constituting a rather direct confirmation of Maxwell's distribution law. It is feared, however, that it is not possible at present to develop the apparatus sufficiently to obtain a "velocity spectrum" of the neutral molecules in a gas as was the original hope of the writers.