From Glossary of Meteorology
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A temperature-sensing element that converts thermal energy directly into electrical energy.

In its basic form it consists of two dissimilar metallic electrical conductors connected in a closed loop. Each junction forms a thermocouple. One thermocouple is maintained at a known temperature (usually 0°C or a measured temperature) and the other thermocouple is used to measure the unknown temperature. The signal voltage is a function of the temperature, and the smooth curve can be handled with a simple linear fit over a moderate temperature range. Different materials have different curves. Popular thermocouples (and change in voltage per °C) include iron- constantan (50 mv per °C), copper-constantan (38 mv per °C), and various platinum alloys. Thermocouples are also important in home furnaces to detect the pilot light or that the fuel has ignited. A chain of thermocouples, called a thermopile, can be used as a power supply if a source of heat and cold is available.