Size Dependence of the Thermal Conductivity of Helium I Near theλPoint

The thermal conductivity of helium I was measured along its vapor pressure curve over the range $3\times {10}^{-6\mathrm{}}°\mathrm{K}<T-T_{\lambda }<1\mathrm{}°\mathrm{K}$ using the parallel-plate technique. The exponent in a power-law variation of thermal diffusivity with $T-T_{\lambda }$ was found to depend on the plate spacing. These results could imply the existence of a relatively temperature-independent characteristic length in helium I near $T_{\lambda }$ approximately ${10}^4$ to ${10}^5$ times larger than the coherence length.