AMPERE METER or AMMETER, an instrument for measuring electric currents. Ammeters used for the measure ment of direct current are moving iron or moving coil low resist ance galvanometers (see GALVANOMETER) . In the latter form of instrument the current passes through a coil hung in the field of a permanent magnet, and this causes a deflection of the coil to which a pointer is attached. This is more widely em ployed than the moving iron type on account of its more efficient damping, which is due to eddy currents set up in the instrument, and in the best types of precision instruments the pointer attains its equilibrium position in less than a second. For measuring alternating current by means of an electromagnetic ammeter, the instrument is, in general, a form of dynamometer (q.v.) . It measures, by their relative motion, the current flowing through two sets of coils which are connected in series, so that a change in direction of current affects both simultaneously, with the result that their relative movement is unchanged and depends on the magnitude of the current alone. The Duddell galvanometer is an alternating current electromagnetic ammeter which indicates the magnitude of the current by means of the relative motion of two straight parallel portions of a single wire carrying the current and situated in a permanent magnetic field. The hot wire am meter may be used to measure either alternating or direct current, the movement of its pointer depending on the linear expansion of a wire when it is heated by the current.