An acoustical interferometer has been constructed and used, with helium gas as the thermometric fluid, to measure temperatures near 4.2 and 2.1 ** f. Such an interferometer provides a means of absolute temperature measurement, and may be used as an alternative to the gas thermometer. When values of temperature derived with this instrument were compared with the accepted values associated with liquid helium 4 vapor pressures, differences of about 10 and 7 millidegrees respectively were found. This result is preliminary, and work is continuing.

Carbon resistors and impurity doped germanium resistors have been investigated for use as precision secondary thermometers in the liquid helium temperature region. Several germanium resistors have been thermally cycled from 300 to 4.2 ** f and their resistances have been found to be reproducible within 1 3 millidegree when temperatures were derived from a vapor pressure thermometer whose tubing is jacketed through most of the liquid helium. Preliminary calibrations of the resistors have been made from 4.21 to 2.16 ** f at every 0.1 ** f. The estimated standard deviations of the data for two of the resistors were 1 millidegree; and for the third resistor, 3.3 millidegrees.

The reproducibilities of helium vapor-pressure thermometers have been investigated in conjunction with a ``constant temperature'' liquid helium bath from 4.2 to 1.8 ** f. Surface temperature gradients have been found to exist in liquid helium baths contained in 15 -- and 25 -- liter metallic storage dewars. The gradient was about one half of a millidegree at 4.2 ** f but increased to several millidegrees for bath temperatures slightly greater than the | l point. A hydrostatic head correction has been neither necessary nor applicable in the determination of vapor pressures or temperatures for the bulk liquid helium. However, the surface temperature gradient can produce erroneous vapor-pressure measurements for the bulk liquid helium unless precautions are taken to isolate the tube (which passes through the surface to the vapor pressure bulb) from the liquid helium surface. It has also been observed, in helium 2,, that large discrepancies can exist between surface vapor pressures and those pressures measured by a vapor pressure thermometer. This has been attributed to helium film flow in the vapor pressure thermometer. In this case also the design of the thermometer can be modified to reduce the helium film flow.