# ammeter

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## ammeter

(ăm`mē'tər), instrument used to measure the magnitude of an electric current of several amperes or more. An ammeter is usually combined with a voltmeter and an ohmmeter in a multipurpose instrument. Most ammeters are based on the d'Arsonval galvanometergalvanometer
, instrument used to determine the presence, direction, and strength of an electric current in a conductor. All galvanometers are based upon the discovery by Hans C.
and are of the analog type, i.e., they give current values that can vary over a continuous range as indicated by a scale and pointer or digital readout.

## Ammeter

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 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 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. See Thermocouple

## Ammeter

an instrument for measuring the strength of DC and AC current in amperes (A). The scale of an ammeter is calibrated in kiloamperes, milliamperes, or microamperes according to the instrument’s measuring range. An ammeter is connected in series with an electrical circuit; in order to increase the measuring range, it is connected with a shunt or through a transformer. The action of the current causes the movable part of the instrument to be deflected; the angle of rotation of its pointer is proportional to the current strength. Ammeters use magnetoelectric (permanent

Table 1. Basic characteristics of ammeters produced in the USSR
SystemsIndicatingRecording
MagnetoelectricElectromagneticElectrodynamicThermoelectricMagnetoelectric, electrodynamic, or rectifier types with recording devices
Characteristics
Current measured ...............Mainly DC (with auxiliary devices—high frequency AC and nonelectrical quantities)DC and AC (45 hertz [Hz] to 8 kHz)DC and AC (50 to 1500 mHz)AC (50 to 30 mHz)DC and AC (45 Hz to 10 kHz)
Grade of accuracy (relative error in percent) ...............0.1; 0.2; 0.5; 1.0; 1.5; 2.5; 4.00.5; 1.0; 1.5; 2.50.1; 0.2; 0.5; 2.51.5; 2.5; 5.01.5; 2.5
Measuring ranges Direct ............... With an auxiliary device (shunt, transformer, and others)0–75 A0–300 A0–50 A0–30 A
up to ......................6 kA (specific types up to 70 kA)30 kA6 kA50 A150kA
Power consumed (watts when measuring 10 amperes [A]) . . . . .0.2–0.42.0–8.03.5–10.01.0

magnet-moving coil), electromagnetic (moving iron vane), electrodynamic (ferromagnetic), thermoelectric, and rectifier systems. The basic characteristics of ammeters produced in 1967 by industry in the USSR are presented in Table 1.

Depending on the field of use, provision is made in the ammeter designs for protection against external influences: ammeters are stable with respect to changes of temperature (from 60°C to -60°C) and to vibrations and jolts, and they can operate in 80–98 percent relative humidity.

### REFERENCE

Shkurin, G. P. Spravochnik po elektroizmeriteinym i radioizmeritel’nym priboram, 3rd ed., vol. 1. Moscow, 1960.

## ammeter

[′a‚mēd·ər]
(engineering)
An instrument for measuring the magnitude of electric current flow. Also known as electric current meter.

## Ammeter

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 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 Voltmeter

The moving-coil, permanent&hyphen;magnet (d'Arsonval) ammeter remains important for direct&hyphen;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. See Thermocouple

## ammeter

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

## ammeter

an instrument for measuring an electric current in amperes
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