THERMOMETRY BELOW 1-K

Temperature measurements below 1 K are relatively easy if they are onl y made to provide a scaling parameter for some other physical quantity because there is a multitude of practical methods to use. That there is problem of relating these measurements to the thermodynamic basis o f temperature, however, becomes obvious when any two methods are appli ed simultaneously and produce discrepant results. Solving this problem requires identification and correction of systematic errors by analys ing their sources for each thermometer in detail. In the very few case s where this procedure is successful, one obtains a primary thermomete r. Since no primary thermometer is available that is also easily repro ducible, one has to leave the firm thermodynamic basis frequently in f avour of the reliability of secondary thermometers. This paper reviews the current status of thermometry from a metrological point of view. It emphasizes the relation to thermodynamic temperature without comple tely ignoring practical questions. Thus principles and limitations are discussed in detail for primary thermometers as well as for those sec ondary thermometers that can be employed to extend the temperature ran ge by extrapolation. Practical thermometry is covered in a few instruc tive examples which illustrate the effects of experimental problems on temperature measurement. A considerable fraction of the paper is devo ted to temperature reference standards, and the actual state of their underlying scales, that are used to reproduce accurately temperature v alues for calibration of secondary thermometers without running primar y thermometers.