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Amperemeter hg.jpg
Demonstration model of a moving iron ammeter. As the current through the coil increases, the plunger is drawn further into the coil and the pointer deflects to the right.
General information
Unit system SI
Unit of electric current
Named after André-Marie Ampère

The ampere ( /ˈæmpɛər/ , US: /ˈæmpɪər/ ; [1] [2] [3] symbol: A), [4] often shortened to amp, [5] is the unit of electric current in the International System of Units (SI). One ampere is equal to 6.241509074×1018 electrons worth of charge moving past a point in a second. [6] [7] [8] It is named after French mathematician and physicist André-Marie Ampère (1775–1836), considered the father of electromagnetism along with Danish physicist Hans Christian Ørsted.


As of the 2019 redefinition of the SI base units, the ampere is defined by fixing the elementary charge e to be exactly 1.602176634×10−19 C (coulomb), [6] [9] which means an ampere is an electrical current equivalent to 1019 elementary charges moving every 1.602176634 seconds or 6.241509074×1018 elementary charges moving in a second. Prior to the redefinition the ampere was defined as the current that would need to be passed through 2 parallel wires 1 metre apart to produce a magnetic force of 2×10−7 newtons per metre.

The earlier CGS system had two definitions of current, one essentially the same as the SI's and the other using Coulomb's law as a fundamental relationship, with the unit of charge defined by measuring the force between two charged metal plates. The ampere was then defined as one coulomb of charge per second. [10] In SI, the unit of charge, the coulomb, is defined as the charge carried by one ampere during one second.


The ampere is named for French physicist and mathematician André-Marie Ampère (1775–1836), who studied electromagnetism and laid the foundation of electrodynamics. In recognition of Ampère's contributions to the creation of modern electrical science, an international convention, signed at the 1881 International Exposition of Electricity, established the ampere as a standard unit of electrical measurement for electric current.

The ampere was originally defined as one tenth of the unit of electric current in the centimetre–gram–second system of units. That unit, now known as the abampere, was defined as the amount of current that generates a force of two dynes per centimetre of length between two wires one centimetre apart. [11] The size of the unit was chosen so that the units derived from it in the MKSA system would be conveniently sized.

The "international ampere" was an early realization of the ampere, defined as the current that would deposit 0.001118 grams of silver per second from a silver nitrate solution. [12] Later, more accurate measurements revealed that this current is 0.99985 A.

Since power is defined as the product of current and voltage, the ampere can alternatively be expressed in terms of the other units using the relationship I = P/V, and thus 1 A = 1 W/V. Current can be measured by a multimeter, a device that can measure electrical voltage, current, and resistance.

Former definition in the SI

Until 2019, the SI defined the ampere as follows:

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2×10−7 newtons per metre of length. [13] :113 [14]

Ampère's force law [15] [16] states that there is an attractive or repulsive force between two parallel wires carrying an electric current. This force is used in the formal definition of the ampere.

The SI unit of charge, the coulomb, was then defined as "the quantity of electricity carried in 1 second by a current of 1 ampere". [13] :144 Conversely, a current of one ampere is one coulomb of charge going past a given point per second:

In general, charge Q was determined by steady current I flowing for a time t as Q = It.

This definition of the ampere was most accurately realised using a Kibble balance, but is in practice the unit was maintained via Ohm's law from the units of electromotive force and resistance, the volt and the ohm, since the latter two could be tied to physical phenomena that are relatively easy to reproduce, the Josephson effect and the quantum Hall effect, respectively. [17]

Techniques to establish the realisation of an ampere had a relative uncertainty of approximately a few parts in 107, and involved realisations of the watt, the ohm and the volt. [17]

Present definition

The 2019 redefinition of the SI base units defined the ampere by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10−19 when expressed in the unit C, which is equal to A⋅s, where the second is defined in terms of νCs, the unperturbed ground state hyperfine transition frequency of the caesium-133 atom. [18]

The SI unit of charge, the coulomb, "is the quantity of electricity carried in 1 second by a current of 1 ampere". [19] Conversely, a current of one ampere is one coulomb of charge going past a given point per second:

In general, charge Q is determined by steady current I flowing for a time t as Q = It.

Constant, instantaneous and average current are expressed in amperes (as in "the charging current is 1.2 A") and the charge accumulated (or passed through a circuit) over a period of time is expressed in coulombs (as in "the battery charge is 30000 C"). The relation of the ampere (C/s) to the coulomb is the same as that of the watt (J/s) to the joule.

Units derived from the ampere

The international system of units (SI) is based on 7 SI base units the second, metre, kilogram, kelvin, ampere, mole, and candela representing 7 fundamental types of physical quantity, or "dimensions", (time, length, mass, temperature, electrical current, amount of substance, and luminous intensity respectively) with all other SI units being defined using these. These SI derived units can either be given special names e.g. watt, volt, lux, etc. or defined in terms of others, e.g. metre per second. The units with special names derived from the ampere are:

QuantityUnitSymbolMeaningIn SI base units
Electric charge coulomb Campere secondA⋅s
Electric potential difference volt V joule per coulombkg⋅m2⋅s−3⋅A−1
Electrical resistance ohm Ωvolt per amperekg⋅m2⋅s−3⋅A−2
Electrical conductance siemens Sampere per volt or inverse ohms3⋅A2⋅kg−1⋅m−2
Electrical inductance henry Hohm secondkg⋅m2⋅s−2⋅A−2
Electrical capacitance farad Fcoulomb per volts4⋅A2⋅kg−1⋅m−2
Magnetic flux weber Wbvolt secondkg⋅m2⋅s−2⋅A−1
Magnetic flux density tesla Tweber per square metre kg⋅s−2⋅A−1

There are also some SI units that are frequently used in the context of electrical engineering and electrical appliances, but can be defined independently of the ampere, notably the hertz, joule, watt, candela, lumen, and lux.

SI prefixes

Like other SI units, the ampere can be modified by adding a prefix that multiplies it by a power of 10.

SI multiples of ampere (A)
ValueSI symbolNameValueSI symbolName
10−1 AdAdeciampere101 AdaAdecaampere
10−2 AcAcentiampere102 AhAhectoampere
10−3 AmAmilliampere103 AkAkiloampere
10−6 AµAmicroampere106 AMAmegaampere
10−9 AnAnanoampere109 AGAgigaampere
10−12 ApApicoampere1012 ATAteraampere
10−15 AfAfemtoampere1015 APApetaampere
10−18 AaAattoampere1018 AEAexaampere
10−21 AzAzeptoampere1021 AZAzettaampere
10−24 AyAyoctoampere1024 AYAyottaampere
10−27 ArArontoampere1027 ARAronnaampere
10−30 AqAquectoampere1030 AQAquettaampere

See also

Related Research Articles

The centimetre–gram–second system of units is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways in which the CGS system was extended to cover electromagnetism.

The joule is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applied. It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule (1818–1889).

<span class="mw-page-title-main">Kilogram</span> Metric unit of mass

The kilogram is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially. It means 'one thousand grams'.

The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and based on the metre as the unit of length and either the kilogram as the unit of mass or the kilogram-force as the unit of force.</ref> and the world's most widely used system of measurement. Established and maintained by the General Conference on Weights and Measures (CGPM), it is the only system of measurement with an official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce.

<span class="mw-page-title-main">SI base unit</span> One of the seven units of measurement that define the Metric System

The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all other SI units can be derived. The units and their physical quantities are the second for time, the metre for length or distance, the kilogram for mass, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity. The SI base units are a fundamental part of modern metrology, and thus part of the foundation of modern science and technology.

<span class="mw-page-title-main">Volt</span> SI derived unit of voltage

The volt is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).

<span class="mw-page-title-main">Caesium standard</span> Primary frequency standard

The caesium standard is a primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency. The first caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK. and promoted worldwide by Gernot M. R. Winkler of the United States Naval Observatory.

<span class="mw-page-title-main">Metric system</span> Metre-based systems of measurement

The metric system is a system of measurement that succeeded the decimalised system based on the metre that had been introduced in France in the 1790s. The historical development of these systems culminated in the definition of the International System of Units (SI) in the mid-20th century, under the oversight of an international standards body. Adopting the metric system is known as metrication.

<span class="mw-page-title-main">Coulomb</span> SI derived unit of electric charge

The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). In the present version of the SI it is equal to the electric charge delivered by a 1 ampere constant current in 1 second and to 5×1027/801088317 elementary charges, e, (about 6.241509×1018e).

In physics, the weber is the unit of magnetic flux in the International System of Units (SI), whose units are volt-second. A magnetic flux density of one Wb/m2 is one tesla.

<span class="mw-page-title-main">Ohm</span> SI derived unit of electrical resistance

The ohm is the unit of electrical resistance in the International System of Units (SI). It is named after German physicist Georg Simon Ohm. Various empirically derived standard units for electrical resistance were developed in connection with early telegraphy practice, and the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time, and of a convenient scale for practical work as early as 1861.

The vacuum magnetic permeability, also known as the magnetic constant, is the magnetic permeability in a classical vacuum. It is a physical constant, conventionally written as μ0. Its purpose is to quantify the strength of the magnetic field emitted by an electric current. Expressed in terms of SI base units, it has the unit kg⋅m⋅s−2·A−2.

The impedance of free space, Z0, is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, Z0 = |E|/|H|, where |E| is the electric field strength and |H| is the magnetic field strength. Its presently accepted value is

A conventional electrical unit is a unit of measurement in the field of electricity which is based on the so-called "conventional values" of the Josephson constant, the von Klitzing constant agreed by the International Committee for Weights and Measures (CIPM) in 1988, as well as ΔνCs used to define the second. These units are very similar in scale to their corresponding SI units, but are not identical because of the different values used for the constants. They are distinguished from the corresponding SI units by setting the symbol in italic typeface and adding a subscript "90" – e.g., the conventional volt has the symbol V90 – as they came into international use on 1 January 1990.

<span class="mw-page-title-main">International Prototype of the Kilogram</span> Physical artifact that formerly defined the kilogram

The International Prototype of the Kilogram is an object that was used to define the magnitude of the mass of the kilogram from 1889, when it replaced the Kilogramme des Archives, until 2019, when it was replaced by a new definition of the kilogram based on physical constants. During that time, the IPK and its duplicates were used to calibrate all other kilogram mass standards on Earth.

The International System of Electrical and Magnetic Units is an obsolete system of units used for measuring electrical and magnetic quantities. It was proposed as a system of practical international units by unanimous recommendation at the International Electrical Congress, discussed at other Congresses, and finally adopted at the International Conference on Electric Units and Standards in London in 1908. It was rendered obsolete by the inclusion of electromagnetic units in the International System of Units (SI) at the 9th General Conference on Weights and Measures in 1948.

<span class="mw-page-title-main">2019 redefinition of the SI base units</span> Definition of the units kg, A, K and mol

In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artifacts such as the standard kilogram. Effective 20 May 2019, the 144th anniversary of the Metre Convention, the kilogram, ampere, kelvin, and mole are now defined by setting exact numerical values, when expressed in SI units, for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The second, metre, and candela had previously been redefined using physical constants. The four new definitions aimed to improve the SI without changing the value of any units, ensuring continuity with existing measurements. In November 2018, the 26th General Conference on Weights and Measures (CGPM) unanimously approved these changes, which the International Committee for Weights and Measures (CIPM) had proposed earlier that year after determining that previously agreed conditions for the change had been met. These conditions were satisfied by a series of experiments that measured the constants to high accuracy relative to the old SI definitions, and were the culmination of decades of research.

The siemens is the unit of electric conductance, electric susceptance, and electric admittance in the International System of Units (SI). Conductance, susceptance, and admittance are the reciprocals of resistance, reactance, and impedance respectively; hence one siemens is redundantly equal to the reciprocal of one ohm and is also referred to as the mho. The 14th General Conference on Weights and Measures approved the addition of the siemens as a derived unit in 1971.

<span class="mw-page-title-main">History of the metric system</span> History of the metric system measurement standards

The history of the metric system began during the Age of Enlightenment with measures of length and weight derived from nature, along with their decimal multiples and fractions. The system became the standard of France and Europe within half a century. Other measures with unity ratios were added, and the system went on to be adopted across the world.

<span class="mw-page-title-main">Alternative approaches to redefining the kilogram</span>

The scientific community examined several approaches to redefining the kilogram before deciding on a redefinition of the SI base units in November 2018. Each approach had advantages and disadvantages.


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