The thermal-expansion coefficient of liquid helium II under its saturated vapor pressure has been measured from 0.85 K to within 0.4 mK of the A, transition. The capacitive technique 6IQployed wa8 well 8ulted for these measurements.VAth the Rid of the Clausius-Mossotti equations we could resolve R relative chRDge in deDsity of 1 pRrt 1D 10 in R manner free from sources of systematic error. The data below 1.60 K were analyzed to obtain information about the density derivatives of the Landau parameters 6, po, and p with the following results:The results of a reanalysis of the data obtained by other workers in this temperature interval are presented in order to provide a valid comparison with the present results. The techniques employed also allowed measurements to be taken to within 0.4 mK of the A. transition. At this temperature, the uncertainty in an individual measurement (one standard deviation) had reached about + 10/g. This represents R considerable improvement over previous measurements and allows a detailed examination of the validity of the Pippard-Buckingham-Fairbank relations. It was found that the agxeement between theory and experiment is good if the tem-perRture dependence of Rll the terIQ8 in these relRtloD8 ls tRken into Recount. The cRpRcltRnce bridge wR8 R18o 6IQployed to IQ6Rsure absolute vRlues of the dielectric constant of liquid helium at temperatures of approximately 1,0 K Rnd R fr6queDcy of 5 kHz. ADRlysi8 of the86 IQeasurernents yielded a, value for the molar polarizability of 0.123 296 +0.000 030 cm mole '.

No abstract

Various different applications of indium O-ring seals are briefly reviewed. A critical factor in the construction of any indium pressure-type seal is the finish of the surface cold welding to the indium. We present the results of some tests on surface finishes achieved by machining and grinding, which indicate certain conditions which ensure the integrity of the seal. Specifically, we find that if the metal surface has to be ground, it should be ground, and then polished to a surface granularity of less than 5  μ. This condition can be obtained using a grinding powder with a mesh size of 3200 or greater. Alternatively, careful surface preparation using a lathe furnishes an equally satisfactory finish, provided no attempt is made to ‘‘improve’’ the surface using emery paper, etc. In the latter case, the fine helical pattern imparted by the lathe is essential for the fabrication of a reliable seal.

3 He film flow has been observed. The maximum flow rate, at 0.7 mK, over a rim 0.9 mm above the 3 He level, was 1.2 mm 3 /h corresponding to an approximate flow velocity of 0.2 mm/sec. A flow rate of 0.2 mm 3 /h ( ~ 0.1 mm/sec) was measured with the rim 14 mm above the level. The transition to superflow occurred at 3.5 ± 0.5 mK for the 0.9-mm film and 2.0 ± 0.5 mK for the 14mm film. This flow is in a regime where the film thickness is less than or comparable to the bulk superfluid 3 He coherence length ( ~ 1000 A at 0.8 mK) and the temperature is higher than the bulk 3 He superfluid transition temperature (1.08 mK) and corresponds, we believe, to a twodimensional superfluid phase of 3 He. PACS numbers: 67.50.-b, 67.70. + nThe superfluid phases of liquid 3 He, discovered a decade ago, have proven a very rich field of physics. 1 Whereas the range of phenomena has been far more extensive than for superfluid liquid 4 He, film flow is one property which was expected to be unique to 4 He. 2 The reason for not expecting film flow in 3 He was that the coherence length is larger than the equilibrium thickness of the 3 He film on a vertical wall at a height of 100 /itm or more above the meniscus of the free surface. This is demonstrated in Fig. 1 which compares film thickness (d), which varies with height (/z), with the coherence length (f), in bulk 3 He at 0, 0.5, and 0.8 mK. The figure is schematic only; the coherence lengths are from BCS theory, 4 and the literature contains many warnings concerning estimates of helif\ 1 \ f \

In this note, we describe the fabrication of a simple, reliable, demountable, vacuum seal which can function in the presence of superfluid helium and withstand temperature cycling to room temperature.