Thermocouple
Definition
A thermometer made of two joined wires — heat the junction and it generates a tiny voltage that reveals the temperature.
Technical definition
The Seebeck effect: a temperature gradient across joined dissimilar conductors drives an EMF proportional to ΔT. Type K (chromel–alumel) spans −200 to +1350 °C.
Example
Gas water heaters use a thermocouple as a safety sensor — the pilot flame's heat produces millivolts that hold the gas valve open; flame out, voltage dies, gas shuts off.
Common confusion
A thermocouple measures the temperature DIFFERENCE between its hot junction and the reference end — not absolute temperature. Skip the cold-junction compensation and the reading is wrong.