Contact potentials between metals: History, concepts, and persistent misconceptions

Some early and not easily available history of contact potentials between metals is presented. A discussion is given of how the existence of a characteristic potential difference between metals was established. This discussion presents some idealized experiments and then analyzes the historic work in terms of them. From experiments, it is shown that the measured potential difference lies between two points that are respectively just outside of the two metals in contact. Certain points are brought out that are not often found in the literature. (i) Bennet discovered contact charging of dissimilar metals significantly earlier than Volta did. (ii) Nothing in 19th (or 20th) century physics discloses experimentally the internal potential difference between two dissimilar metals in contact. (iii) Many 19th century physicists, including Maxwell, thought that the Peltier effect disclosed the value of the internal potential difference between dissimilar metals in contact. Later analysis showed that Maxwell was wrong. (iv) The belief that contact potentials were solely electrolytic, arising from damp contacts, recurred well into the 20th century but was almost completely abandoned following the advent of high-vacuum technology. It recurs occasionally with no justification in the 1940’s and 1950’s.