Contact potentials in Electromotive-force

                        When two different solids are in contact, it is common that thermodynamic equilibrium requires one of the solids assume a higher electrical potential than the other, the contact potential. For example, dissimilar metals in contact produce what is known also as a contact electromotive force or Galvani potential. The magnitude of this potential difference often is expressed as a difference in Fermi levels in the two solids at charge neutrality, where the Fermi level (a name for the chemical potential of an electron system) describes the energy necessary to remove an electron from the body to some common point (such as ground). Evidently, if there is an energy advantage in taking an electron from one body to the other, then such a transfer will occur. The transfer causes a charge separation, with one body gaining electrons and the other losing electrons. This charge transfer causes a potential difference between the bodies, and therefore, charge transfer becomes more difficult as the charge separation increases. At thermodynamic equilibrium, the Fermi levels are equal (the electron removal energy is identical) and there is now a built-in electrostatic potential between the bodies. The original difference in Fermi levels, before contact, is referred to as the emf. The contact potential cannot drive steady current through a load attached to its terminals because that current would involve a charge transfer. No mechanism exists to continue such transfer and, hence, maintain a current, once equilibrium is attained.

One might inquire why the contact potential does not appear in Kirchhoff's law of voltages as one contribution to the sum of potential drops. The customary answer is that any circuit involves not only a particular diode or junction, but also all the contact potentials due to wiring and so forth around the entire circuit. The sum of all the contact potentials is zero, and so they may be ignored in Kirchhoff's law.