Thermal Conductivity of Superconductors

MEASUREMENTS have recently been made in this Laboratory of the specific thermal conductivity at liquid helium temperatures of a series of tin specimens, ranging from spectroscopically pure tin to alloys containing up to 4 per cent of mercury. A tantalum specimen was also measured. The apparatus was based upon the design of de Haas and Rademakers¹, with a pair of gas thermometers measuring the temperature at two points on a rod carrying a known heat power. An innovation was the use of the thermometers differentially, the pressure difference being indicated on a butyl phthalate oil manometer read with a travelling microscope. A much finer resolution was possible than in previous work^1,2, thus enabling accurate measurements of thermal conductivity to be made with temperature differences of only 0.01° K. Consequently a careful survey could be made of the region close to the superconducting transition temperature. The thermal conductivity was measured in both superconducting and normal state, the latter by destroying superconductivity with a longitudinal magnetic field. In the case of very pure tin, the magnetic field produced a large variation of conductivity in the normal state, and extrapolation was necessary to derive the value in zero field.