Two dissimilar metals are welded (joined) at one end to form a ‘hot junction’ and the other end to form a ‘cold junction’. When there is a temperature difference between the ‘hot’ and ‘cold’ junction a mv is produced in the loop proportional to the temperature difference. The amount mv production by a thermo-couple depends on the characteristic and type of thermocouple like ‘j’ , ‘k’ type etc.