An ammeter is an instrument that measures electric current, usually in units of amperes (or milliamps or microamps when very small currents are involved). Both the magnitude of direct current (DC) and alternating current (AC) can be measured, though with ammeters of different types. Due to differences in design, an ammeter is not as sensitive as a galvanometer.

There are two basic types of ammeters. The moving coil ammeter has a scale with equally spaced divisions; the moving iron or moving magnet ammeter has a nonlinear scale. Both versions use a principle discovered by Hans Christian Oersted in 1820, which holds that an electric current in a wire produces a magnetic field. The magnetic field exerts a force on a coil, making it turn. A pointer that is attached to the coil or magnet also moves, allowing the measurement to be read off the scale.

The moving coil ammeter...