Phenomena that occur as a result of temperature differences in an electrical circuit.

The Seebeck effect is the development of an electromotive force between two junctions formed by joining two dissimilar metals if the two junctions are at different temperatures. The circuit constitutes a thermocouple. In general, the e.m.f. E is given by

where a, b, and c are constants and θ is the temperature difference between the junctions. If the colder junction is maintained at 0 °C then

where α and β are constants dependent on the metals used and T is the temperature of the hot junction. At temperatures below the neutral temperature (see thermocouple) if α is small (as is usually the case) then E is directly proportional to the temperature of the hot junction.

The Peltier effect is the converse of the Seebeck effect. If a direct current is passed round a circuit formed from two dissimilar metals or from a metal and a semiconductor, one junction gives off heat and is cooled and the other absorbs heat and becomes warm. The effect is reversible, i.e. if the current is reversed the cool junction becomes warm and the hot junction cools. Larger temperature differences are produced with metal-semiconductor junctions than with metal-metal junctions. A metal-n-type junction produces a temperature difference in the opposite sense to that of a metal-p-type junction for the same direction of current flow. A number of such junctions can be used to form a Peltier element (see diagram), which may be used as a heating or cooling element.

In the Kelvin effect a temperature difference between different regions of a single metal causes an e.m.f. to be developed between them. Also a current that flows along a wire in which a temperature gradient exists causes heat to flow from one region of the wire to another. The direction of heat flow is a function of the particular metal used.

Peltier element