thermoelectricity, direct conversion of heat into electric energy, or vice versa. The term is generally restricted to the irreversible conversion of electricity into heat described by the English physicist James P. Joule and to three reversible effects named for Seebeck, Peltier, and Thomson, their respective discoverers. According to Joule's law, a conductor carrying a current generates heat at a rate proportional to the product of the resistance (R) of the conductor and the square of the current (I). The German physicist Thomas J. Seebeck discovered in the 1820s that if a closed loop is formed by joining the ends of two strips of dissimilar metals and the two junctions of the metals are at different temperatures, an electromotive force, or voltage, arises that is proportional to the temperature difference between the junctions. A circuit of this type is called a thermocouple; a number of thermocouples connected in series is called a thermopile. In 1834 the French physicist Jean C. A. Peltier discovered an effect inverse to the Seebeck effect: If a current passes through a thermocouple, the temperature of one junction increases and the temperature of the other decreases, so that heat is transferred from one junction to the other. The rate of heat transfer is proportional to the current and the direction of transfer is reversed if the current is reversed. The Scottish scientist William Thomson (later Lord Kelvin) discovered in 1854 that if a temperature difference exists between any two points of a current-carrying conductor, heat is either evolved or absorbed depending upon the material. (This heat is not the same as Joule heat, or I2R heat, which is always evolved.) If heat is absorbed by such a circuit, then heat may be evolved if the direction of the current or of the temperature gradient is reversed. It can be shown that the Seebeck effect is a result of the combined Peltier and Thomson effects. Magnetic fields have been shown to influence all these effects. Many devices based on thermoelectric effects are used to measure temperature, transfer heat, or generate electricity.
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The generation of a temperature-dependent electromotive force (emf) at the junction of two dissimilar metals. This phenomenon provides the physical basis for the thermocouple. In 1821, T. J. Seebeck discovered that near a closed circuit composed of two linear conductors of two different metals a magnetic needle would be deflected if, and only if, the two junctions were at different temperatures, and that if the temperatures of the two junctions were reversed the direction of deflection would also be reversed. He investigated 35 different metals and arranged them in a series such that at a hot junction, current flows from a metal earlier in the series to a later one. See Electromotive force (emf)
A thermocouple consists of a pair of wires of dissimilar metals, joined at the ends. One junction is kept at an accurately known cold temperature, usually that of melting ice, and the other is used for the measurement of an unknown temperature, by measuring the emf generated as a result of the Seebeck effect. See Thermocouple, Thermoelectricity
McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.
Seebeck effect[′zā‚bek i‚fekt]
The development of a voltage due to differences in temperature between two junctions of dissimilar metals in the same circuit.
A photographic emulsion that is exposed until a faint visible image appears, and is then exposed to colored light and takes on the color of the light to which it is exposed.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.