Nature of Electrical Contact between Tarnished Surfaces

Abstract

The variation of contact resistance with temperature has been measured on clean and tarnished silver contacts between 77° and 420°K. It is shown that the temperature coefficient of resistance provides a direct method of distinguishing film conduction from metallic conduction in contacts, and conversely that a single measurement at room temperature cannot reliably yield this information. Clean contacts have the positive temperature coefficient of resistance of the conductor metal. A film-covered contact on the other hand exhibits either of two distinct types of temperature dependence: one with a large negative temperature coefficient, and one that is metallic. It is suggested that these occur according to whether mechanical contact is made on the film or through a small fissure in it. If the contact is mechanically disturbed, the nature of the conduction fluctuates randomly between these two modes. Further, the temperature coefficient of resistance provides a means for distinguishing between semiconducting and tunneling contacts. The experimental observations on silver sulfide tarnish film suggest that electron tunneling is dominant at cryogenic temperatures, but that at normal temperatures the film is an n-type semiconductor.