A thermocouple consists of two dissimilar metal wires. They are joined together at one end. This junction is called sensing or hot junction. The two wires are terminated at the other end. This junction is called reference or cold junction. The cold junction is maintained at a known constant temperature called as reference temperature.
The scientist Thomson see back discovered that, when a temperature difference exists between the sensing and reference function, an emf is produced. This emf causes a current in the circuit. When a meter or recorder is connected to the reference junction, the meter indication will be proportional to the temperature difference between the hot junction and reference junction. This thermo-electric effect caused by contact potential at the junction is known as the “See back effect”.
Basic Thermocouple circuit
The magnitude of the emf depends on the wire material used and also on the temperature difference between the junctions. The figure shows the thermal emf of some common thermocouple material.
For long life, thermocouple is placed in a protecting tube. To prevent chemical reactions the protective tube is both chemical inert and vacuum tight. Connection from thermocouple is made by special extension wires called compensating wires. The measurement is very accurate when compensating wires are of the same material as the thermocouple materials.