At the surface of separation of any two different substances in contact, there exists in general an electromotive force tending to maintain a certain difference of potential between them. This principle, established for metals by Volta in 1796, has been extended by later investigators to other substances, including liquids and gases. From these early researches of Volta, and the later more elaborate inquiries of Kohlrausch, Hankel, and Gerland, there have been deduced certain fundamental laws, which have been fully corroborated by the recent work of Clifton, and Ayrton and Perry. If, of a number of conductors set serially in contact, the difference of potential between each successive pair is quantitatively estimated and reckoned positive or negative, according as the first member of the pair is at a higher or lower potential than its successor, then the difference of potential between the first and last members of the chain is equal to the algebraic sum of the potential differences between the successive contiguous pairs.