Volta potential

When two metals are electrically isolated from each other, an arbitrary potential difference may exist between them.

However, when two different neutral metal surfaces are brought into electrical contact (even indirectly, say, through a long electro-conductive wire), electrons will flow from the metal with the higher Fermi level to the metal with the lower Fermi level until the Fermi levels in the two phases are equal.

Once this has occurred, the metals are in thermodynamic equilibrium with each other (the actual number of electrons that passes between the two phases is usually small).

Just like the work function, the Volta potential depends sensitively on surface state, contamination, and so on.

The Volta potential can be significant (of order 1 volt) but it cannot be measured directly by an ordinary voltmeter.

The Volta potential difference between a metal and an electrolyte can be measured in a similar fashion.