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 \
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