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. 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 realisation of the ampere, defined as the current that would deposit of silver per second from a
silver nitrate solution. Later, more accurate measurements revealed that this current is . 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 , 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
newtons per metre of length.
Ampère's force law states that there is an attractive or repulsive force between two parallel wires carrying an electric current. This force was used in the formal definition of the ampere, giving the
vacuum magnetic permeability (magnetic constant, ) a value of exactly 4π × 10−7
henries per metre (H/m, equivalent to N/A2). 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". Techniques to establish the realisation of an ampere had a
relative uncertainty of approximately a few parts in 10, and involved realisations of the watt, the ohm and the volt. The SI unit of charge, the
coulomb, "is the quantity of electricity carried in 1 second by a current of 1 ampere". Conversely, a current of one ampere is one coulomb of charge (approximately elementary charges) going past a given point per second, or equivalently 1019 elementary charges every seconds: :1\text{ A} = 1\text{ C/s} = 6.241\,509 \times10^{18}\,e\text{/s} = \frac{10^{19}\,e}{1.602\,176\,634\text{ s}}. With the second defined in terms of , the caesium-133 hyperfine transition frequency, the ampere can be expressed in terms of and :1\text{ A} = 1\text{ C/s} = \Big(\frac{10^{19}\,e}{1.602\,176\,634}\Big)\Big(\frac{\Delta\nu_\text{Cs}}{9\,192\,631\,770}\Big) \approx 6.789\,6868\times10^{8}\,e\,\Delta\nu_\text{Cs}.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 "). The relation of the ampere (A = C/s) to the coulomb (C) is the same as that of the
watt (W = J/s) to the
joule (J). == Units derived from the ampere ==