A couple of wires of dissimilar metals forming a loop and producing  the thermoelectricity is called thermocouple.

When two wire of different material are joint to form closed circuit and the two jointed end are placed at different temperature then a small emf is develop in that closed circuit and small current flow through the closed circuit this process is called seebeck’s effect or thermoelectric effect.

Variation of thermo emf with temperature

For explanation of the variation of the thermo emf with temperature

Let us consider iron-copper thermocouple as given below.

Also consider one junction (P) is put in oil bath and another junction (Q) is put in melting ice. If we increase the temperature of oil bath after certain time the temperature of the both junction is same and galvanometer don’t show any deflection and hence no emf is produce. On continuous increasing the temperature of the oil bath by making cold junction at constant, then the deflection of the galvanometer also increases. This shows that thermo emf increase until it attained maximum value and the temperature at which thermo emf become maximum is called neutral temperature. Let ɵn be the temperature at which thermo emfbecome maximum. If we increase the temperature of oil bath beyond the neutral temperature then the thermo emf start to decrease and become zero. Let (ɵi) be the temperature at which thermo emf is zero and this temperature is called temperature of inversion.

The variation of thermo emf with temperature ɵ is given by

E= θα+βθ²/2

 Where α and β are constant whose value depend upon the material of conductor and temperature difference of two junction.

Let ɵC be the temperature of the cold junction then,

ɵi- ɵn= ɵn- ɵC

So, the neutral temperature lies between the inversion temperature and temperature of cold junction