Thermocouple, an electrical device that responds to a difference in temperature by producing an electric current. Thermocouples are used as measuring instruments and as control devices. Thermocouples are simple and rugged, can be used over a wide range of temperatures (from -200° C. to 1,600° C.), and permit great precision. For these reasons thermocouples are used to measure temperatures inside furnaces and jet engines and in laboratory experiments. In some gas appliances, thermocouples are used as safety switches to control the gas supply.

The thermocouple is based on the Seebeck effect, named after its discoverer, Thomas J. Seebeck. In its simplest form, the thermocouple consists of two wires of dissimilar metals or alloys joined at their ends, with a potentiometer (or a voltmeter) connected in one side of the circuit. The diagram shows such a thermocouple using wires of iron and constantan (an alloy of 60 per cent copper and 40 per cent nickel).

The measuring junction is placed in the environment whose temperature is to be measured. For precision work, the reference junction is kept at a fixed, known temperature (for example, by being placed in an ice bath). If great precision is not required, the reference junction may be left at room temperature, which is known only approximately. The potential difference between the two junctions, as shown by the potentiometer or voltmeter, is used to find the temperature of the heated junction, usually from a table in a handbook.

When used as a control device in a gas appliance, the thermocouple is mounted so that its measuring junction is heated by a pilot light. The electric current generated is sent through an electromagnet. As long as the current flows, the electromagnet holds open a valve that allows gas to reach the appliance. If the pilot light goes out, the measuring junction cools off, no current flows, and the electromagnet releases the valve, shutting off the gas.

A number of thermocouples connected in series make up a thermopile. Thermopiles are more sensitive than individual thermocouples (some thermopiles can measure temperature differences as small as a millionth of a degree). Thermopiles are used for measuring the temperature of radiation from stars, for detecting the amount of heat produced by living tissues, and in other situations where highly sensitive temperature-measuring devices are required.