COHERER, a device by means of which a feeble oscillating electric current may be detected by a galvanometer (q.v.) or tele phone (q.v.), the essential characteristics being that the current through the coherer is concentrated at a point and that the re sistance (see RESISTANCE, MEASUREMENT OF) of the material has a negative temperature coefficient (q.v.) so that the coherer is self-restoring. A simple form of coherer consists of an oxidized iron plate against which rests the point of a steel needle. (See TELEPHONE.)