ammeter

An instrument that measures electric current in amperes.

[AM(PERE) + –METER.]

An instrument for the measurement of electric current. The unit of current, the ampere, is the base unit on which rests the International System (SI) definitions of all the electrical units. The operating principle of an ammeter depends on the nature of the current to be measured and the accuracy required. Currents may be broadly classified as direct current (dc), low-frequency alternating current (ac), or radio frequency. At frequencies above about 10 MHz, where the wavelength of the signal becomes comparable with the dimensions of the measuring instrument, current measurements become inaccurate and finally meaningless, since the value obtained depends on the position where the measurement is made. In these circumstances, power measurements are usually used. See also Current measurement.

The measurement of current in terms of the voltage that appears across a resistive shunt through which the current passes has become the most common basis for ammeters, primarily because of the very wide range of current measurement that it makes possible, and more recently through its compatibility with digital techniques. See also Electrical units and standards; Multimeter; Voltmeter.

The moving-coil, permanent-magnet (d'Arsonval) ammeter remains important for direct-current measurement. Generally they are of modest accuracy, no better than 1%. Digital instruments have taken over all measurements of greater precision because of the greater ease of reading their indications where high resolution is required.

Moving-iron instruments are widely used as ammeters for low-frequency ac applications.

High-frequency currents are measured by the heating effect of the current passing through a physically small resistance element. In modern instruments the temperature of the center of the wire is sensed by a thermocouple, the output of which is used to drive a moving-coil indicator.

A contraction of amperemeter. An apparatus that measures the amperage of an electric current.

An instrument for measuring the rate of flow of electricity, usually expressed in amperes.

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Ammeter

Wire carrying current to be measured.
Spring providing restoring force

Ammeter from the old Penn Station Terminal Service Plant at 250 West Thirty-first Street

Zero-center ammeter

An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes (A), hence the name. Smaller values of current can be measured using a milliameter or a microammeter.

The earliest design is the D'Arsonval galvanometer or moving coil ammeter. It uses 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 of iron which moves when acted upon by the electromagnetic force of a fixed coil of wire. This type of meter responds to both direct and alternating currents (as opposed to the moving coil ammeter, which works on direct current only).

To measure larger currents, a resistor called a shunt is placed in 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 50 mV, so shunts are typically designed to produce a voltage drop of 50 mV when carrying their full rated current.

Zero-center ammeters are used for applications requiring current to be measured with both polarities, common in scientific and industrial equipment. Zero-center ammeters are also commonly placed in series with a battery. In this application, the charging of the battery deflects the needle to one side of the scale (commonly, the right side) and the discharging of the battery deflects the needle to the other side.

Digital ammeter designs use an analog to digital converter (ADC) to measure the voltage across the shunt resistor; the digital display is calibrated to read the current through the shunt.

Since the ammeter shunt has a very low resistance, mistakenly wiring the ammeter in parallel with a voltage source will cause a short circuit, at best blowing a fuse, possibly damaging the instrument and wiring, and exposing an observer to injury. In AC circuits, a current transformer converts the magnetic field around a 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. A portable hand-held clamp-on ammeter is a common tool for maintenance of industrial and commercial electrical equipment, which is temporarily clipped over a wire to measure current.

Ammeter Schematic Symbol

The ammeter symbol (A) is shown in the diagram.

See also

• Electrical measurements
• Clamp meter
• Circuit diagram
• Electric circuit
• Electronics
• Galvanometer
• List of electronics topics
• Multimeter
• Ohmmeter
• Voltmeter
• The Energy Detective

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Dansk (Danish)
n. - amperemeter

Nederlands (Dutch)
ampèremeter

Français (French)
n. - ampèremètre

Deutsch (German)
n. - Amperemeter, Strommesser

Ελληνική (Greek)
n. - (ηλεκτρ.) αμπερόμετρο

Italiano (Italian)
amperometro

Português (Portuguese)
n. - amperímetro (m) (Eletr.)

Русский (Russian)
амперметр

Español (Spanish)
n. - amperímetro

Svenska (Swedish)
n. - amperemätare

中文（简体）(Chinese (Simplified))
电表, 安培计

中文（繁體）(Chinese (Traditional))
n. - 電錶, 安培計

한국어 (Korean)
n. - 전계류

日本語 (Japanese)
n. - 電流計

العربيه (Arabic)
‏(الاسم) أميتر : أداه لقياس, التيار الكهربائي بالأمبير‏

עברית (Hebrew)
n. - ‮מד-אמפר, מד-זרם‬

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