Aaron Wexler

Evaporation Rate of Liquid Helium. I

In connection with a study of the emissivity of metals at low temperatures, a quantitative analysis has been made of the factors involved in the design of storage containers for liquid helium. Experiments confirm the analysis, which has resulted in an exceedingly simple liquid‐nitrogen‐protected liquid helium container having a helium evaporation rate of 1 percent per day.The hemispherical emissivity of mechanically polished copper at 4.23°K was determined for blackbody radiation. For radiation corresponding to that emitted by a blackbody at 77.1°K and 297.1°K, the measured emissivities are 6.9×10−3 and 1.29×10−2, respectively. These data are discussed in relation to the recent theoretical work of Reuter and Sondheimer.The heat of vaporization of helium at 4.228°K was determined to be 4.93 cal g−1, in substantial agreement with the data of Dana and Kamerlingh Onnes. Kistemakers thermodynamic considerations questioning the accuracy of these data are evaluated in terms of the new determination.

Measurement and Control of the Level of Low Boiling Liquids

An account is given of a simple, reliable, and generally applicable electrical means for accurately measuring and controlling the level of low boiling‐point liquids. The sensitive elements exhibit at least a fivefold voltage difference for nitrogen and at least a five hundred‐fold difference for helium depending on whether they are in the liquid or the vapor. Heat transfer characteristics for liquid nitrogen are also noted.

Measurement of High Vacuums at Low Temperatures

The general usefulness of the helium mass spectrometer leak detector for measuring very low pressures at liquid helium temperatures is discussed. The required relations between mass flow, pressure, pumping speed, and the dimensions of the apparatus, in which thermomolecular flow plays a major role, are derived. It is shown that the exchange gas in liquid helium calorimeters can be pumped out to pressures below 1×10−10 mm Hg.

A Transfer Device for Low Boiling Liquids

The principles involved in the transfer of low boiling liquids under their own vapor pressures are indicated and calculations are presented for the specific case of the transfer of liquid nitrogen. A convenient device for transferring liquid nitrogen by this procedure is described and its operating characteristics are compared with the calculations.

Storage and Transfer of Liquid Helium

The performance of large, efficient storage containers for liquid helium is indicated, and liquid helium transfer techniques permitting correspondingly efficient use of the stored liquid are described.