Seebeck effect
/ SAY-bek ih-FEKT /
Crack a window on a cold day with a warm room behind you, and you feel air drift out at the top: the warm, lively molecules spread toward the cool side. The mobile electrons in a metal do their own version of this. Heat one end of a bar and the energetic electrons there spill toward the cold end, leaving the hot end slightly positive and the cold end slightly negative. That built-in voltage is the Seebeck effect.
The Seebeck effect is precisely this: a voltage that appears across a material whenever its two ends are at different temperatures. How big that voltage is per degree of difference is the material's Seebeck coefficient, a kind of thermoelectric fingerprint — small in ordinary metals, much larger in carefully tuned semiconductors. Crucially, you read the voltage by joining two different materials into a loop; if both legs responded identically, their voltages would cancel, so it is always the difference between two materials that you measure.
This matters because it is the most direct way to turn heat straight into electricity, and the basis of the thermocouple — two dissimilar wires joined at a tip whose tiny voltage reports temperature. Thermocouples measure everything from furnaces to jet engines because they are rugged and need no power. The common misconception is that a single uniform piece of metal can be wired up to give you Seebeck voltage on its own; in practice you always need a junction of two materials for the effect to show up in your meter.
A type-K thermocouple is just a wire of nickel-chromium twisted to a wire of nickel-aluminium. Stick the joined tip into a kiln and the temperature difference between tip and meter produces a few thousandths of a volt — enough for the meter to read the kiln's heat to within a degree.
Two ordinary wires, joined at a tip, become a thermometer that needs no battery.
The Seebeck effect (heat to voltage) and the Peltier effect (current to heat) are the same coupling running forward and backward — a thermoelectric generator and a thermoelectric cooler are the very same device used in opposite directions.