Milliampere

The ampere (/ˈæmpɪər, æmˈpɪər/ or /ˈæmpɛər/ (UK), symbol: A), often shortened to 'amp', is the base unit of electric current in the International System of Units (SI). It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics. The International System of Units defines the ampere in terms of other base units by measuring the electromagnetic force between electrical conductors carrying electric current. The earlier CGS measurement system had two different definitions of current, one essentially the same as the SI's and the other using electric charge as the base unit, 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. In SI, the unit of charge, the coulomb, is defined as the charge carried by one ampere during one second. New definitions, in terms of invariant constants of nature, specifically the elementary charge, took effect on 20 May 2019. The ampere is defined 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 ∆ν. 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 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. 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.