A new method of high-precision isotherm thermometry has been developed in which the density of helium gas is determined from dielectric constant measurements using the virial expansion of the Clausius-Mossotti equation. The technique is shown to be accurate at the level of -0.3 mK for the range 4.2-27.1 K, with potential for both increased accuracy and increased range. The results confirm that the gas thermometer scale NPL-75 is internally consistent to better than 1 mK, and that Berry's recent values for the second virial coefficient of helium are correct at the levelf 0.05 cm3 mol-'.The results also give a value for the third virial coefficient, C(T) = (5420 f 225)/T cm6 mol-2, and a value of 517.257 f 0.025 x cm3 mol-' for the polarizability of He.

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RQsum6.-On prdsente des mesures prdliminaires de thermomgtrie prgcise au moyen de la constante diglectrique de 11h61ium gazeux entre 4 et 27 K. On donne les valeurs du troisizme coefficient du viriel de la densitd, du deuxisme coefficient de la constante dislectrique, et de la polarisibilitd pour l'hdlium. Abstract.-Preliminary measurements are reported of precision Dielectric Constant Gas Thermometry using helium between 4 and 27 K. Values are given for the third density virial coefficient, for the second dielectric virial coefficient, and for the polarizability of helium.