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resistance thermometer[ri′zis·təns thər′mäm·əd·ər]
a temperature-measurement device whose operation is based on the change in the electrical resistance of pure metals, alloys, and semiconductors with temperature. (In metals, the resistance R increases with increasing temperature T, whereas in semiconductors the inverse dependence holds.)
Resistance thermometers made of pure metals, especially platinum, with a temperature coefficient of resistance α = (R1000°C – R0°C)/100R0°C = 0.0039 deg–1, and copper with α = 0.0044 deg–1, are widespread. Such thermometers consist of a metal wire or strip wound onto a rigid quartz, porcelain, or mica form enclosed in a protective sheath of metal, quartz, porcelain, or glass. Two, three, or four leads extend from the head of the thermometer, connecting it to a measuring instrument; thermometers with four leads are the most accurate. Platinum resistance thermometers are used for the measurement of temperatures from – 263° to 1064°C, and copper resistance thermometers are used from –50° to l80°C.
The material and design of a resistance thermometer must provide sufficient sensitivity and stability for the required accuracy and precision of measurement in a given temperature range under particular conditions, such as vibration and aggressive mediums. The accuracy of measurement also depends on the accuracy of the instrument measuring the resistance. Industrial resistance thermometers operate in a unit consisting of bridges, potentiometers, and readout and recording quotient meters with scales calibrated directly in degrees Celsius according to tables for the temperature dependence of resistance for particular types of resistance thermometer. The International Practical Temperature Scale is reproduced by high-accuracy platinum resistance thermometers, which make possible the accurate measurement of temperature and the calibration of other thermometers in the range from 14° to 900°K.
Indium and bronze resistance thermometers are sometimes used in laboratory work in the ranges 4°–300°K and 1°–4°K, respectively.
Semiconductor resistance thermometers made of composite carbon and alloyed germanium are commonly used for the measurement of low temperatures (0.1°–100°K) because of their high sensitivity. This type of resistance thermometer consists of semiconductor strips or films of various sizes and shapes, with welded metal leads, often enclosed in a protective shell. Germanium resistance thermometers are especially accurate in the range from 4.2° to 13.8°K. The use of semiconductor resistance thermometers above 100°K is limited, since their instability and the dispersion of individual characteristics become significant (see).
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D. I. SHAREVSKAIA