[[Image:Galvanometer diagram.png|thumb|right|250px|<font face="arial" color="red">Wire carrying current to be measured.</font><br><font face ="arial" color="green">Spring providing restoring force</font>]]
[[Image:Ammeter from New York Terminal Service Plant, 250 West Thirty-first Street 351263pv.jpg|thumb|right|241px|Ammeter from the old [[Pennsylvania Station (New York City)|Penn Station]] Terminal Service Plant at 250 West Thirty-first Street]]

An '''ammeter''' is a [[measuring instrument]] used to measure the flow of [[electric current]] in a [[Electrical circuit|circuit]]. Electric currents are measured in [[ampere]]s, hence the name. The word "ammeter" is commonly misspelled or mispronounced as "ampmeter" by some.

The earliest design is the [[Jacques-Arsène d'Arsonval|D'Arsonval]] [[galvanometer]] or moving coil ammeter. It uses [[magnetism|magnetic]] deflection, where current passing through a coil causes the coil to move in a [[magnetic field]]. The [[voltage]] drop across the coil is kept to a minimum to minimize resistance across the ammeter in any circuit into which it is inserted.

Moving iron ammeters use a piece or pieces of [[iron]] which move when acted upon by the electromagnetic force of a fixed coil of (usually heavy [[Wire gauge|gauge]]) wire. This type of meter responds to both [[direct current|direct]] and [[alternating current|alternating]] currents (as opposed to the moving coil ammeter, which works on [[direct current]] only).

To measure larger currents, a [[resistor]] called a [[shunt (electrical)#Use in current measuring|''shunt'']] is placed in [[Series and parallel circuits|parallel]] with the meter. Most of the current flows through the shunt, and only a small fraction flows through the meter. This allows the meter to measure large currents. Traditionally, the meter used with a shunt has a full-scale deflection (FSD) of {{nowrap|50 mV}}, so shunts are typically designed to produce a voltage drop of {{nowrap|50 mV}} when carrying their full rated current.

More modern ammeter designs are non-mechanical, or digital, and use an [[analog to digital converter]] (ADC) to measure the voltage across the shunt resistor. The ADC is read by a microcomputer that performs the calculations to display the current through the resistor.

One problem with the use of an ammeter is the need for the meter to be inserted into the circuit and become part of it. Mistakenly placing the ammeter in parallel with a circuit will blow the fuse, possibly damaging the meter and causing injury. In AC circuits, a [[Transformer#Current transformers|current transformer]] converts the magnetic field around a [[Electrical conductor|conductor]] into a small AC current, typically either 1 or 5 Amps at full rated current, that can be easily read by a meter. In a similar way, accurate AC/DC non-contact ammeters have been constructed using [[Hall effect]] magnetic field sensors.

==See also==
*[[Clamp meter]]
*[[Circuit diagram]]
*[[Ohmmeter]]
*[[Voltmeter]]
*[[Multimeter]]
*[[Meter (electronics)]]
*[[Measuring instrument]]

*[[Electronic test equipment]]
*[[Electronics]]
*[[Electric circuit]]
*[[List of electronics topics]]

*[[Series and parallel circuits]]

*[[Galvanometer]]

[[Category:Electronic test equipment]]
[[Category:Measuring instruments]]

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