The electric potential difference between one point in the vacuum close to the surface of
M1 and another point in the vacuum close to the surface of
M2, where
M1 and
M2 are two uncharged metals brought into contact.
Source:
PAC, 1983, 55, 1251. 'Interphases in systems of conducting phases (Provisional)' on page 1257 (https://doi.org/10.1351/pac198355081251)