Thermocouple

A Thermocouple is an electrical device that can be used as a thermometer. It is comprised of two metals, A, B, that form a simple circuit with two junctions, J, between the metals as shown opposite. If the two junctions are at the same temperature there is no potential difference, DV (millivolts), between the two ends of the circuit. If the junctions are at different temperatures, a potential exists that depends upon the temperature difference.

Keeping one junction at 0 C permits calibration of the device to measure the temperature of the other junction. The potential changes sign (but not magnitude) as the junction temperatures are interchanged.