U+3384 Square Ka

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The ampere ( AM-pair, US: AM-peer; symbol: A), often shortened to amp, is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 coulomb (C) moving past a point per second. 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 revision of the SI, the ampere is defined by fixing the elementary charge e to be exactly 1.602176634×10⁻¹⁹ C, which means an ampere is an electric 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 passing through two parallel wires 1 metre apart that produces a magnetic force of 2×10⁻⁷ newtons per metre.

The earlier CGS system has two units of current, one structured similarly to the SI's and the other using Coulomb's law as a fundamental relationship, with the CGS unit of charge defined by measuring the force between two charged metal plates. The CGS unit of current is then defined as one unit of charge per second.