No abstract

No abstract

W E have recently shown 1 that the symmetry effects calculated by Halpern and Gwathmey 2 which should occur with the gaskinetic collisions of ortho-para-T&i and ortho-para-T> 2 cannot be observed with even an approximately right order of magnitude. The reliability of the calculations, however, is not very high, because they are based on the hard-elastic-sphere model which is not suitable for the nonspherical potential of H2 and D 2 . Halpern 3 has therefore suggested that the influence of the statistics on the gaskinetic cross sections be reinvestigated by measuring the viscosity-difference of gaseous He 3 and He 4 at very low temperatures. In the case of He no difficulties with respect to the potential are to be expected. Since He 3 obeys Fermi statistics while He 4 obeys Bose statistics, the viscosity of gaseous He 3 should appreciably exceed the viscosity of He 4 at low temperatures. Somewhat later, de Boer and Cohen 4 have explicitly calculated the temperature-dependence of the viscosities between 0 and 5°K using a Lennard-Jones potential of the form 4e[(o-/r) 12 -(er/f) 6 ].To investigate the extremely small viscosities, we have developed a measuring device based on the principle of the oscillating cylinder. The amplitudes of the oscillations are recorded by 1S\ 9 (MP)\ 10\ f ° 0 He 3 ^^ X 0 X He* O O O O < ; 3 T(°K) i FIG. 1. The temperature dependence of the viscosity coefficient 17 (micropoise) of gaseous He 3 and He 4 . Circles and crosses represent the experimental results obtained with decreasing and increasing temperature, respectively. Continuous lines represent theoretical curves of de Boer and Cohen.11 1 (MP)\ 70 U5°K 2M°K 25 50 75 %He 3 FIG. 2. Concentration dependence of the viscosity coefficient t\ (micropoise) of gaseous He 3 -He 4 mixtures at temperatures 4.15°K and 2.64°K.high-frequency circuits using a change in capacity connected with the oscillation. At 4.2°K the relaxation time of the oscillating system filled with He 4 amounts to 2.7 min while it exceeds 3 hr with the highest obtainable vacuum. The period of the oscillating cylinder is 1.3 sec. At a viscosity of 5 micropoise the accuracy of the measurements was better than ±1 percent. The viscosities of pure He 3 and He 4 were measured between 1.3° and 4.2 °K (Fig. 1). The results for pure He 4 agree perfectly with the calculations of de Boer and Cohen. For He 3 the figure shows a small deviation between theory and experiment at low temperatures. In any case, we^may presume that the results obtained confirm the predicted^symmetry effect of the helium isotopes.Furthermore, the concentration dependence of the viscosity of gaseous He 3 -He 4 mixtures was determined at 4.2° and 2.6°K as shown in Fig. 2. The absolute values of 77 are derived from the well-known viscosity of He 4 at 77.3°K (83.68 micropoise). 5 The pure He 3 was supplied by the U. S. Atomic Energy Commission. Details will be published in the Zeitschrift fur Physik.

No abstract

No abstract