Seebella

Meet Seebella, two metals joined in a loop who make a voltage when one end is hotter.

In 1821 Thomas Johann Seebeck found that a circuit of two different metals, with their two junctions at different temperatures, produces a voltage. Heat makes charge carriers in a metal drift from the hot end toward the cold end, and different metals do this by different amounts. The difference between the two metals shows up as a small voltage, V = S ΔT, where S is the Seebeck coefficient of the pair.

A type K thermocouple has a coefficient of roughly 41 µV per kelvin near room temperature. A 100 K difference between the junctions gives about 41 × 10⁻⁶ × 100 = 4.1 × 10⁻³ V, about 4 millivolts.

Thermocouples measure temperature in ovens, engines, kilns and laboratories. Radioisotope thermoelectric generators use many junctions to power spacecraft such as the Voyager probes from the heat of decaying plutonium.

S is not truly constant; it changes with temperature, so accurate work uses standard reference tables rather than a single number. The voltage depends only on the temperature difference, so a known reference junction is needed to read absolute temperature.