In 1834, Peltier noticed an effect complementary to Seebeck effect. The evolution and absorption of heat energy at junctions of a thermocouple, when a current is passed through it, is called as Peltier effect.
Consider again the Cu-Fe thermocopuple containing a battery which sends the the current from iron to copper. The effects are shown Fig
Fig. Peltier Effect
Causes of Heating and Cooling at the Junction:
Contact potential difference between two dissimilar metals of thermocouple results in heating and cooling. Potential metals of thermocouple result in heating and cooling. Potential of iron is higher than that of copper. Hence for a current flowing from copper to iron i.e. from lower to higher potential at B, energy is required to do this work, which is absorbed from the junction B and that’s why junction B is cooled.
At junction A current flows from iron to cupper i.e. from higher to lower potential, therefore, heat energy is evolved and heats up the junction A.