One amp means about six quintillion electrons' worth of charge rushing past every second
Since 2019 the ampere has been pinned to nature itself: the charge on a single electron is fixed at an exact value, so one amp equals roughly 6.24 billion billion elementary charges flowing past a point each second. Before that, it was defined by invisible forces between two imaginary infinite wires.
The unit honours André-Marie Ampère, the French mathematician and physicist who lived from 1775 to 1836 and laid the groundwork for electrodynamics. He shares the title of father of electromagnetism with the Danish physicist Hans Christian Ørsted. An international agreement signed at the 1881 International Exposition of Electricity made the ampere a standard unit of current.
Its early definitions were delightfully physical. The international ampere was the current that plated 0.001118 grams of silver per second out of a silver nitrate solution; later, sharper measurements found that current was actually 1.000165 amperes. Another lineage came from the older centimetre-gram-second system, whose abampere produced a pull of two dynes per centimetre between wires a centimetre apart. The ampere was set at one tenth of that so the derived units would come out at handy sizes.
Until 2019 the official wording described two straight, infinitely long, vanishingly thin parallel conductors one metre apart in a vacuum, with a current producing a force of 2 times ten to the minus seven newtons per metre between them. That choice fixed the magnetic constant at exactly 4 pi times ten to the minus seven henries per metre. Nobody could build such wires, of course. The best direct realisation used a Kibble balance, but in everyday metrology labs kept the ampere alive through Ohm's law, deriving it from the volt and the ohm, which quantum effects named after Josephson and Hall make easy to reproduce. Uncertainties ran to a few parts in ten million.
The modern definition fixes the elementary charge at exactly 1.602176634 times ten to the minus nineteen coulombs, with the second defined by a caesium-133 transition. The ampere is one of seven SI base units, alongside the second, metre, kilogram, kelvin, mole and candela. It relates to the coulomb much as the watt relates to the joule: one is a rate, the other the quantity that accumulates.
Source: Ampere
