Origin Of Thermo E.m.f
According to electron theory, every metal have goes some loosely bound electrons called free electrons. These electrons in the interior of the metals can move freely in any random directions just as the molecules of a gas which can move about randomly in gas . The concentration of free electrons in different metals is different. When two dissimilar metals are joined together to form the junction , then the electron defused from the metal having high concentration of electrons , till the electron concentration across the junction becomes equal.
This flow of free electrons from one metal to another gives rise to a contact potential difference between the two metals, because the metal having higher concentration of electrons, On losing electrons, becomes positively charged and the other metal with lower concentration on gaining electrons attains negative charges .
Thus a potential difference called a contact potential is established at the junction.