Dewar System for Low Temperature Experiments

Abstract

A special system of glass Dewars for experiments at low temperatures is described in which the liquid nitrogen shield consists of two separate parts, one dipping into the other. The upper shield is constructed with three walls rather than the usual two, since it must be open at the bottom. The liquid helium Dewar fits fairly closely inside this extra wall, with a thin air space between the two, and extends downward into the lower shield. The upper shield thus is mechanically isolated from the liquid helium Dewar. Access to a cold spot of the liquid helium flask is provided by a tube passing upward through the bottom of the lower shield and coupled to the liquid helium flask by means of a spherical ground-glass joint. Various advantages of this system and possible modifications thereof are discussed.