DIRECT MEASUREMENTS OF THE DIELECTRIC VIRIAL-COEFFICIENTS OF HE-4 BETWEEN 3-K AND 18-K

The second and third dielectric virial coefficients of He-4 have been measured by the symmetrical gas-expansion technique of Buckingham, Cole and Sutter for six isotherms between 3,00 K and 17,68 K. The effects of helium adsorption are important, and are considered in detail. No temperature dependence of the second coefficient is seen at a level of delta-b = +/- 0,008 cm3 mol-1, and average values of the second and third coefficients are b = 0,006 (10) cm3 mol-1 and c= -5 (4) cm6 mol-2, respectively. The uncertainty for b is relatively large because it has a systematic dependence on c: reanalysis of all the existing data shows that a more probable value of the third coefficient is c = -2 (1) cm6 mol-2, which when combined with our data gives b (3 K - 18 K) = -0,001 (4) cm-3 mol-1. The very small value suggests that theoretical calculations of the excess polarizability of the helium diatom need to be reconsidered. The small value and the small uncertainty of b indicate that future work to improve experimental values of the second density virial coefficient of He-4 for this temperature range may without ambiguity make use of the methods of dielectric constant gas thermometry.