For the measurement of temperature, we use thermal detectors. Glass thermometers, pressure gauge thermometers, bimetallic thermometers, thermocouples, resistance thermometers, thermisters, pyrometers etc. are examples for thermal detectors. Thermocouple consists of two wires made of dissimilar metals or alloys which are welded in such a way that we obtain two junctions. One junction is made hot and the other one cold. The difference in temperature induces e.m.f. Commonly used thermocouples are chromel-copel, chromel-alumel and platinum rhodium-platinum. For first two thermocouples we use acid proof stainless steel protective casing. For platinum rhodium-platinum thermocouple, porcelain protective case is used.
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