Why does heating one junction of a circuit made from two different metals cause electric current to flow?

When two different metals are joined in a closed circuit, they initially form contact potentials that oppose each other, resulting in no net electromotive force and no current flow.12

However, when the temperature of one junction is raised relative to the other, a net electromotive force is generated. This occurs because it is unlikely that the two metals will have Fermi levels with identical temperature dependence. As heat enters the hot junction and leaves the cold junction, this temperature differential drives the flow of electrons, a phenomenon known as the Seebeck effect. This process allows for the direct conversion of heat into electricity, which is the fundamental principle behind the thermoelectric power generator.1234