An electric voltage develops between dissimilar conductors when they are joined at both ends to form a circuit and the ends, called junctions, are at different temperatures. This phenomenon is called the Seebeck effect. The voltage developed in a thermocouple circuit is a known function of the temperature difference between the specific metals or alloys that form the junctions. By maintaining one junction at a known temperature, a thermocouple-based instrument can convert voltages into temperature measurements of the other junction.
The Peltier effect is the opposite of the Seebeck effect, by which electrical energy is converted into thermal energy. One junction is cooled and the other heated when voltage is applied to a circuit formed of dissimilar conductors. This is useful in applications where cooling or heating effects are needed.
A junction whose temperature is measured is called the hot junction. The other, called the cold junction, is contained in the instrument where its temperature is precisely known. This "temperature compensation" is required to accurately determine temperature measurements.