How does a thermocouple measure temperature?

A thermocouple uses the Seebeck effect - when two dissimilar metals are joined and the junctions are at different temperatures, a voltage is generated proportional to the temperature difference. Common types include: K-type (chromel-alumel, -200 to 1250C, general purpose), J-type (iron-constantan, -40 to 750C), T-type (copper-constantan, low temperature), and S-type (platinum, high temperature to 1700C). Cold junction compensation is needed since the measurement references the cold junction (typically at instrument terminals).