Thermocouple and its basic principle

Thermocouple is the device which is made up of combination of two different material (wires) in the specific circuit. It's main significance is that the junction points show the surprising behaviour in the closed circuit.

Principle

The principle of thermoelectric effect(caused by the thermocouple in the circuit) is that when we connect the two different wires like as copper and iron . If electric charge is passed through it in the specific direction ie;copper to iron, as in the figure. We find that the one junction get hotter and another junction begins to cool . That is the result of thermoelectric effect. As we know metals like iron, copper,silver etc has different free electron density. We know the fact that iron contains more free electron density than that of copper metal. When both wire are connected, there will form two junctions with high free electron density to low free electron density and vice versa ie,from iron to copper and vice versa. When electric field in a specific direction is applied the electricity passes through both junctions. In the junction when field passes through iron to copper the junction starts to get hot. Spontaneous process are from high to low ,when electric field is passed through iron to copper junction due to high electron density electrons passes to low electron density ie copper so there is not energy absorbed by surrounding but also heated by the kinetic energy of electrons so that results to heat the junction.

When the electricity is passed from copper to iron junction ,(lower electron density to higher electron density) the process is non spontaneous process so to pass electrons from lower density to higher it needs some energy which it gains by the surrounding, that phenomenon result to make junction cold.

By the complete closed circuit we can get one hot and another cold junction. That can be used in several devices, equipments and accessories.