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.
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.]
Related categories• ELECTRICITY AND MAGNETISM
• LABORATORY EQUIPMENT
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