oa Towards the Absolute Zero

The wide range of use of the platinum resistance thermometer, roughly 10 to 1000K, is by no means enough to satisfy all demands for temperature standards. Ten years ago thermometers modelled on the capsule-type platinum thermometer but using an alloy of rhodium with 0.5 per cent iron were developed at the National Physical Laboratory specifically to provide standards for lower temperatures, and these are now widely used down to 0.5 K. In a recent joint experiment between the U.S. National Bureau of Standards and N.P.L., rhodium-iron has been used down to 0.01 K by coupling a small sample of the alloy to a resistive squid. Such a device is capable of measuring accurately voltages of less than one nanovolt, and also of measuring the noise voltage in the resistor. Since the latter is dependent on the absolute temperature in a way that could be calculated, the device was able to provide its own calibration.