Thermoelectricity
A thermocouple consists of two dissimilar conductors joined to form a circuit. T. J. Seebeck (1822) first discovered that a thermocouple would produce a current in a closed circuit when one junction is at a different temperature from the other. The EMF that produced the current is referred to as the thermocouple EMF or as the Seebeck voltage, E, and its temperature derivative, dE/dT, is known as the thermoelectric power or the Seebeck coefficient, S.

Due to the Seebeck effect, the thermoelectric force is generated in a conductor by a temperature gradient within it. The electron diffusion pressure, due to the increased electron motion at higher temperatures, causes the Seebeck effect. Thermoelectricity is a bulk property of conductors and is similar to thermal conductivity and electrical conductivity.

The EMF which exists between the ends of a conductor in a temperature gradient therefore depends on the composition of the conductor and is given by :

Thermoelectricity can only be observed in circuits comprising two or more dissimilar conductors but it is not due to any special properties of the junctions between the conductors. The junction is needed only to complete the thermoelectric circuit.

The value of the measured signal is therefore:

In summary, the thermocouple signal is generated in the parts of conductors that are in temperature gradients (see the diagram below). Where ĢT = 0, no signal is generated.