Electric Contact

an electrically conductive surface of contiguous components in an electric circuit or a device providing such contiguity. A distinction is made among electric contacts between current conductors (mechanical contacts), between a conductor and a semiconductor, and between two semiconductors.

Four states are distinguished in the operation of mechanical contacts: open, closing, closed, and opening. Particularly heavy wear occurs in a movable contact during the opening of an electric circuit carrying a large current, since in this case an electric arc is formed. The temperature of such an arc causes melting and partial evaporation of the material of the contact, changing the contact surface. To reduce this undesirable phenomenon, the electric circuit is broken simultaneously in several places or arc-extinguishing devices are used. Materials used for electric contacts include pure metals (platinum, silver, tungsten, rhodium, and copper), alloys (platinum-iridium, palladium-copper, and gold-nickel), and metal compositions (silver-cadmium oxide and silver-graphite); the choice of material depends on the purpose of the contact and on its operating conditions. To im-prove an electric contact a device (or the part of a device where the contact is located) can be placed in a flask containing hydrogen or nitrogen or in an evacuated flask. In such devices the electric contact is operated mechanically (through a corrugated tube) or by means of a magnetic field.

Mechanical electrical contacts are divided into two groups: fixed contacts (split and solid), for a permanent connection, and movable contacts, which are closed only for a certain period of time. Split electric contacts are made by clamps, bolts, or screws; solid contacts are made by soldering, welding, or riveting. Movable contacts, in turn, may be subdivided into break contacts (in push buttons, switches, and relays), sliding contacts (a commutator-and-brush array in a generator), and rolling contacts (the connection between overhead wires and the trolley of a trolleybus). Mechanically operated contacts are used in radio, telephone, and telegraph apparatus, in electric power supply systems, and in precision instruments.