  The correction to the polarizability of helium due to black-body radiation is
calculated near room temperature. A precise theoretical determination of the
black-body radiation correction to the polarizability of helium is essential
for dielectric gas thermometry and for the determination of the Boltzmann
constant. We find that the correction, for not too high temperature, is roughly
proportional to a modified hyperpolarizability (two-color hyperpolarizability),
which is different from the ordinary hyperpolarizability of helium. Our
explicit calculations provide a definite numerical result for the effect and
indicate that the effect of black-body radiation can be excluded as a limiting
factor for dielectric gas thermometry using helium or argon.
