Thermometric Fixed Points Using Superconductivity
In current practice, precision in thermometry below 20°K is commonly achieved by measuring the vapor pressure of liquid helium or by the use of resistive or paramagnetic thermometers. The latter techniques provide a wider temperature range than the 0.3 to 4°K range over which combination of He3 and He4 vapor pressure bulbs can be used, but both the resistive and the paramagnetic thermometers must be recalibrated continually against the vapor pressure scale (or a similarly dependable primary thermometer) to assure continued accuracy. Considerable effort is involved in the recalibration process, particularly in experiments involving the use of the He3–He4 dilution refrigerator; in this case, the refrigerant is a mixture of He3 and He4, which cannot be used directly as a thermometric medium. Moreover, it is difficult to attach and to use a separate vapor pressure bulb in this apparatus, since the experiment is often connected to the dilution chamber by an imperfect thermal link.
KeywordsMutual Inductance Electrical Lead Helmholtz Coil Paramagnetic Salt Thermometer Temperature
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