a temperature-measuring device consisting of two wires of different metals joined at each end. One junction is placed where the temperature is to be measured, and the other is kept at a constant lower temperature. A measuring instrument is connected in the circuit. The temperature difference causes the development of an electromotive force (known as the Seebeck effect) that is approximately proportional to the difference between the temperatures of the two junctions. Temperature can be read from standard tables, or the measuring instrument can be calibrated to read temperature directly.

Any two different metals or metal alloys exhibit the thermoelectric effect, but only a few are used as thermocouples—e.g., antimony and bismuth, copper and iron, or copper and constantan (a copper-nickel alloy). Usually platinum, either with rhodium or a platinum-rhodium alloy, is used in high-temperature thermocouples.

A thermopile is a number of thermocouples connected in series; its results are comparable to the average of several temperature readings. A series circuit also gives greater sensitivity.