Properties of Lead Thermal Switches at Low Temperatures

The electrical resistivity of polycrystalline lead and the resistance of soldered contacts to the lead have been measured at 1.16 and 4.2°K in magnetic fields up to 7000 gauss. The thermal conductance of a simple lead heat switch was measured in both the normal and the superconducting states in fields up to 3000 gauss in the temperature range of 0.15–0.5°K. Assuming the best linear fit for the temperature dependence for the conductivity of the normal state, the ratio of the thermal conductance of the switch (lead plus contacts) in the 3000-gauss normal state to that of the superconducting state was found to be (T/K°)−2.3, however, the numerical factor varied strongly with magnitude and orientation of the field. As a magnetic field was slowly removed from the superconducting lead, heating corresponding to the irreversible exit of trapped flux was observed. The design of lead thermal switches is discussed.