![]() : 144 Conversely, a current of one ampere is one coulomb of charge going past a given point per second:ġ A = 1 C s. 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". This force is used in the formal definition of the ampere. : 113 Īmpère's force law states that there is an attractive or repulsive force between two parallel wires carrying an electric current. 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. ![]() Until 2019, the SI defined the ampere as follows: Current can be measured by a multimeter, a device that can measure electrical voltage, current, and resistance. 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. Later, more accurate measurements revealed that this current is 0.999 85 A. The "international ampere" was an early realization of the ampere, defined as the current that would deposit 0.001 118 grams of silver per second from a silver nitrate solution. The size of the unit was chosen so that the units derived from it in the MKSA system would be conveniently sized. ![]() 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 ampere was originally defined as one tenth of the unit of electric current in the centimetre–gram–second system of units. 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 is named for French physicist and mathematician André-Marie Ampère (1775–1836), who studied electromagnetism and laid the foundation of electrodynamics. Main article: International System of Electrical and Magnetic Units In SI, the unit of charge, the coulomb, is defined as the charge carried by one ampere during one second. The ampere was then defined as one coulomb of charge per second. 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. ![]() 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. 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.Īs of the 2019 redefinition of the SI base units, the ampere is defined by fixing the elementary charge e to be exactly 1.602 176 634 ×10 −19 C ( coulomb), which means an ampere is an electrical current equivalent to 10 19 elementary charges moving every 1.602 176 634 seconds or 6.241 509 074 ×10 18 elementary charges moving in a second. One ampere is equal to 1 coulomb, or 6.241 509 074 ×10 18 electrons worth of charge, moving past a point in a second. The ampere ( / ˈ æ m p ɛər/, US: / ˈ æ m p ɪər/ symbol: A), often shortened to amp, is the unit of electric current in the International System of Units (SI). As the current through the coil increases, the plunger is drawn further into the coil and the pointer deflects to the right. Demonstration model of a moving iron ammeter. ![]()
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