A temperature measuring device incorporates a fixed point cell to provide an integrated structure. The fixed point cell houses a pure substance within a substantially cylindrical graphite crucible. The pure substance is to be melted to give an absolute temperature. The crucible is enclosed within a sealed metal container. An annular metal tube surrounds the container and the inner wall of the tube is formed by the outer peripheral wall of the container. This provides excellent heat conduction from the interior of the tube to the interior of the crucible. The tube is arranged to define either a heat pipe or a thermal siphon and includes an appropriate vaporizable fluid.