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A moving-coil milliammeter. This consists of a rectangular coil wound on an aluminum former (1) mounted so that it can rotate between the poles of a permanent magnet (2). A soft iron core (3) ensures that a uniform radial magnetic field acts on the coil. When a current flows in the coil it experiences a torque which tries to turn it against the tension in a hair spring (4). The extent to which the coil turns is proportional to the current flowing and this is read off on a scale (5) with the aid of a pointer (6) attached to the coil assembly. Adjustment (7) is provided for zeroing the pointer and the instrument is connected into a circuit through terminals (8) and (9).
An instrument used for measuring electric currents greater than 1μA. An ammeter is (with the exception described in the next paragraph) connected to a circuit in series with the components through which the current is flowing.

Most direct-current ammeters are similar in design to the moving-coil galvanometer used for smaller currents, though they differ in passing most of the test current through a low "shunt" resistance (thus bypassing the coil) and in using a pointer fixed to the coil assembly to indicate the reading on the linearly calibrated scale. For alternating currents either a rectifier can be used with a moving-coil instrument or the less sensitive hot-wire or moving-iron instruments can be used.

ammeter symbol

There is a class of ammeters that gauges the magnetic flux surrounding a conductor to determine current level in the conductor through induction or Hall effect transduction. These meters need not be inserted into the circuit proper. The Hall effect clamp-on ammeter offers no additional impedance in use with dc circuits (a short reluctive transient occurs on first clamping) while the inductive unit adds a small amount of inductive reactance in series when used with AC circuits. At power frequencies (25 to 400Hz) this additional reactance is trivial.

[Thanks to Charles Watson for contributions to this entry.]

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