High Resolution Calorimetry on μMole Samples of ⁴He Near T_λ

We describe a technique for performing high resolution calorimetry on samples with very small heat capacity (∼10μJ/K). This involves using ac calorimetry, in which the sample is subjected to temperature oscillations, with simultaneous dc regulation of the average temperature. This method works well for small samples of ⁴He, where adiabatic calorimetry would be impossible. We study the frequency response of our system using a modified Sullivan-Seidel equation, which allows for small temperature inhomogeneities at high frequencies. These procedures enable one to obtain heat capacity data possessing a high degree of absolute accuracy.