When two metals are placed in electric contact, electrons flow out of the one in which the electrons are less bound and into the other.

The binding is measured by the location of the so-called Fermi level of electrons in the metal; the higher the level, the lower is the binding.

The Fermi level represents the demarcation in energy within the conduction band of a metal between the energy levels occupied by electrons and those that are unoccupied. It is important in determining the electrical and thermal properties of solids.

The Fermi level is the measure of the energy of the least tightly held electrons within a solid; named for Enrico Fermi, Italian-born American physicist who first proposed it and who was one of the chief architects of the nuclear age.

He developed the mathematical statistics required to clarify a large class of subatomic phenomena, explored nuclear transformations caused by neutrons, and directed the first controlled chain reaction involving nuclear fission.