Resistive squids for thermal measurements

We describe the results of the first successful measurements of the heat capacity of materials using a 20 MHz resistive SQUID (RSQUID) as a detector of heat current. The method has been tested on samples of Cu and In of mass ∼1 gm. The relative accuracy of these measurements, made between 2 and 7 K, was ∼0.1%, only about twice the uncertainty to be expected if Johnson noise were the only cause of fluctuations. The precautions that had to be taken to achieve this performance in a rather unsophisticated cryostat, and the modifications that will need to be made to it in order to measure the heat capacity of much smaller specimens, will be discussed. Apart from the size of the specimen, the principal factor in determining the accuracy of measurement turns out to be the frequency range δf over which the frequency can be allowed to run during a measurement. δf is ultimately limited by the frequency of the drive current.