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Platinum Metals Rev., 1972, 16, (1), 9

Iron-Rhodium Resistance Thermometers

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Cryogenic engineering is becoming much more important as work with liquid gases and on the applications of superconductivity increases. Special thermometers are needed at these very low temperatures because although platinum resistance thermometers are satisfactory for use down to 20 K they cannot be used below 10 K.

The use of 0.5 atomic per cent iron-rhodium alloy as a resistance thermometer material at very low temperatures was proposed by Professor B. R. Coles of Imperial College, London in 1964 (Phys. Lett., 1964, 8, (4), 243–244). Tests have since shown that it is suitable for use between 0.35 and 40 K. It is now available from Johnson Matthey Metals in the form of wire 0.13 mm diameter in either a hard-worked or an annealed condition.

The rate of increase of resistance of the wire varies somewhat over the range of temperature from 0.35 up to 40 K. Calibration is therefore necessary and this should take place in the position in which the instrument is to be used.

Strong magnetic fields occur in work on superconduction but the iron-rhodium alloy remains virtually unaffected so that the change in resistance per deg K is small and predictable. At temperatures in the liquid helium range a one per cent change in the resistance is produced by a field of 10 kOe. The change in resistance per deg K is not altered by work-hardening. Accuracy of ± 10−3deg K in the range is possible with 20 cm of 0.13 mm diameter wire used with conventional potentiometric measuring equipment.

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