What is the mechanism that causes electron flow in a thermo couple?

threshod energy rquired by each metal is quite different from each other hence they require dfferrent kinetic energy for removal of electrons hence the metal which require less energy for it started flow of electrons when get heated but not in the same metal because whole metal contain free electrons which increases the resistance of the metal and current selects low resistance path to flow that it flow through the metal coupled with it and circuit get completed