Seebeck Effect

When two points of a material are held at different temperatures, charge carriers diffuse from the hot region to the cold region or vice versa depending on material type. This carrier redistribution creates an electric potential. For a material pair over a moderate temperature range, the generated voltage is often approximated as:

where S is the Seebeck coefficient and \Delta T is the temperature difference. In real thermocouples, S varies with temperature, so accurate measurement uses calibration polynomials or tables rather than one constant coefficient.

Engineering use

Thermocouples exploit the Seebeck effect by joining two dissimilar conductors and measuring the voltage associated with the temperature difference between the measurement junction and the reference junction. Accurate temperature measurement therefore requires cold-junction compensation, correct extension wire, stable connections, and awareness of parasitic thermal gradients in connectors and measurement electronics.

Thermoelectric generators use arrays of p-type and n-type semiconductor legs to convert a heat flux into electrical power. Their efficiency is limited by material properties, thermal contact resistance, electrical resistance, heat leakage, operating temperature, and the thermoelectric figure of merit of the material system.

Common mistakes

A common mistake is assuming a thermocouple measures absolute junction temperature directly. It measures voltage related to a temperature difference, so the reference temperature must be known or compensated. Another mistake is creating unintended junctions with different metals in a temperature gradient, which introduces additional Seebeck voltages. A good review states material pair, temperature range, reference-junction method, lead-wire material, calibration standard, measurement impedance, and expected thermal gradients.