Studies on a sealed-cell lambda-point device for use in low temperature thermometry

We report here the development of a sealed-cell lambda-point device which has been used to realize the lambda-point temperature of ⁴He, T_λ=2.1768 K, to a precision of 0.04 mK with simple automatic control. With this device, one can produce and maintain T_λ-plateaus for unlimited duration with no ascertained temperature drift. This unique performance is achieved by a design, which makes it possible to keep a HeI/HeII interface quasi-stationary inside a vertical variable-conductance capillary tube. The capillary connects a top HeII-cell and a bottom HeI-cell at its two ends, and acts as a sensitive heat switch which automatically adjusts a heat input to the bottom HeI-cell to compensate exactly the heat conducted out of the HeII-cell to the environment. The HeI/HeII interface thus stays at an equilibrium height determined by the magnitude of heat flow through the capillary and by the temperature difference along the capillary. The sharp difference between the thermal conductances of the interfacing HeII and HeI columns is the underlying physics of this self-adjusting function. Realization of T_λ is achieved after appropriate corrections to the thermometer reading at the HeII-cell, in particular, by extrapolating to zero heat flow through the device.