Heat capacity and thermal conductivity measurements down to 25 mk on samples of poor thermal diffusivity and small specific heat

A heat pulse technique is described with a permanent heat link to the cold sink used for the measurement of small heat capacities, such as vitreous dielectrics, down to 25 mK. Total heat capacities of the order of 5μJ K^?1 are measured below 50 mK with an accuracy of 5%. Two carbon resistor thermometers are used to avoid any systematic error.Thermal conductivity measurements are also performed in the same temperature range by the steady flow method. The accuracy of the carbon resistor thermometer calibration enables the measurement of 2 or 3 mK temperature gradients at 25 mK.