In the 1820's Thomas Seebeck found that at a junction of two metals a voltage could be measured that varied with the temperature of the junction. This thermoelectric effect is now known as the 'Seebeck Effect'.

The generated voltage is small depending both on the types of metals used to form the junction and the temperature of the junction itself. To enable absolute temperature measurements to take place a second junction must be formed. If this second junction is held at a known temperature a meter connected to the circuit will measure an output that can be equated to the temperature difference between the two junctions.

The second junction is traditionally held at 0°C by immersing it in iced water. This Cold Junction Compensation (CJC) enables temperatures to be measured from below -200°C to over 2000°C with a typical accuracy of 0.5 to 2°C.