If current is passed through a thermo-couple, heat will evolve at one junction and absorbed at other junction. Thus one junction becomes hot and other junction becomes cold. This effect is called Peltier’s effect. If we reverse the direction of current, the effect will also be reversed. So it is a reversible process. The amount of heat absorbed or evolve at the junction is directly proportional to amount of current passing through it.
Causes of Peltier Effect:
In a thermo-couple, one wire is at high potential and another wire is at low potential at their junction. Thus when current is passed, either it passes from low potential to high potential or high potential to low potential. Thus heat will be either evolved or absorbed at the junction.