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 unit of current is then defined as one unit of charge per second. The earlier CGS system has two units of current, one structured similar to 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 passing through 2 parallel wires 1 metre apart that produces 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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