A thermocouple works on the principle of voltage variation induced by a temperature difference. It consists of a junction of two dissimilar metals, with a cold junction situated in the flight deck and a hot junction situated in the area where temperature needs to be measured. When there is a temperature difference between the two junctions, a small voltage is created and moves around the circuit. The induced potential difference is proportional to the difference in temperature between the two junctions. Therefore, this voltage can be converted into a temperature and displayed over a temperature calibrated dial in the cockpit. Thermocouples do not need any exterior power supply as the voltage generated is only based on the difference in temperature.