In 1821, Thomas Johann Seebeck connected two dissimilar conductors and kept their junctions at different temperatures. A nearby compass needle deflected. Seebeck initially interpreted the phenomenon as “thermomagnetism,” but the needle was responding to the magnetic field produced by a thermoelectric current.
The temperature difference drives charge carriers from the hotter region toward the colder region, producing an electrical potential difference. For a small temperature range, it is commonly written as:
V = α(Thot − Tcold)
Here, α is the effective Seebeck coefficient of the material pair. More generally, because the coefficient changes with temperature, the voltage is obtained by integrating the difference between the two materials’ Seebeck coefficients across the temperature range.
This principle became the foundation of the thermocouple, one of the most widely used temperature sensors in science and industry. The same effect also allows thermoelectric generators to convert otherwise wasted heat directly into electrical power without turbines or other moving parts.
Today, Seebeck’s nineteenth-century observation helps measure temperatures inside engines and furnaces, recover waste heat from machines, power remote sensors and supply electricity to spacecraft. NASA’s radioisotope thermoelectric generators use thermocouples to convert heat from radioactive decay into the steady electrical power required for long-duration missions in deep space.