Ampere
SI base unit · quantity: electric current
01 · Definition
For seventy years the ampere was defined by the force with which two wires attract each other. In 2019 it was moved onto the electron: the elementary charge was fixed exactly, and current became a count of charges per second.
One ampere is a flow of charge of one coulomb per second — that is, 6.241 509 074·10¹⁸ elementary charges passing through a conductor's cross-section every second. The definition rests on two exact constants: the electron charge e and the caesium second. No mechanics, no forces — pure counting.
Before 2019 the ampere was the current at which two infinite wires one metre apart attract with a force of 2·10⁻⁷ newton per metre of length. The definition was elegant but unmeasurable in the lab: real wires are neither infinite nor infinitely thin.
02 · Conversion
Enter a current — the passport will convert it
The ampere has a clean decimal family of prefixes — from the picoampere inside a chip to the megaampere busbars of an aluminium smelter. The historical units are CGS relics: the abampere is exactly ten amperes, and the statampere is smaller by a factor set by the speed of light.
A current of 30 mA through the body is considered potentially lethal — two orders of magnitude less than a light bulb draws. It is the current that is dangerous, not the voltage: voltage merely pushes it through.
| Unit | Name | Value | Where it appears |
|---|---|---|---|
| pA | picoampere | 2·10¹² | gate leakage in chips, electrometry |
| nA | nanoampere | 2·10⁹ | op-amp input currents, laboratory electrometry |
| µA | microampere | 2 000 000 | sensors, watches, quiescent currents of chips |
| mA | milliampere | 2000 | LEDs, chargers, the 30 mA danger threshold |
| A | ampere | 2 | the base notation of current in SI |
| kA | kiloampere | 0.002 | welding, starter motors, lightning strokes |
| MA | megaampere | 2·10⁻⁶ | aluminium electrolysis, pulsed facilities |
SI prefixes are decimal: 1 kA = 10³ A, 1 mA = 10⁻³ A — no exceptions
03 · Orders of magnitude
from a transistor leak to a lightning arcOne ampere: phone charging, an LED strip
04 · Measuring instruments
What amperes are measured with
The current is passed through a calibrated resistance and the voltage drop is measured. Simple and accurate, but the circuit must be broken.
A current transformer or a Hall sensor wraps around the wire without touching it. The circuit stays live — that is how panels and workshops are measured.
A flexible winding around a busbar: it measures pulsed currents up to hundreds of kiloamperes — discharges, welding, short circuits.
The quantum standard: it passes one electron per clock cycle. A direct realisation of the new definition of the ampere.
05 · Writing rules
The symbol is capital: A, because it is a surname
The unit is named after Ampère, so the symbol is a capital Latin A with no full stop, while the unit name is lowercase: «ampere». Cyrillic «а» and typewriter-era «amp» do not belong in records.
06 · Neighbouring units
The ampere is the doorway into the electrical family: the coulomb is obtained by multiplying by the second, and the volt and the ohm are tied to it by Ohm's law. Eight named SI units carry the ampere in their definitions.
A battery capacity in mA·h is a charge: 2500 mA·h means 2.5 ampere-hours, that is 9000 coulombs.
07 · Historical section
The amperes that came before the ampere
The electromagnetic CGS unit: exactly ten present-day amperes. Born of the definition via the force between wires.
The electrostatic unit of the same system, smaller by a factor of 3·10¹⁰: the difference is exactly the speed of light.
The current depositing 1.118 mg of silver per second. A chemical standard — the silver voltameter: easy to reproduce, but its accuracy was limited by the purity of the salt.
The magnetomotive force of the CGS system — «ampere-turns» the old way. 1 Gb = 10/4π ampere-turns.
In 2019 the ampere ceased to be a mechanical unit
The old definition via the force between conductors made the magnetic constant µ₀ = 4π·10⁻⁷ H/m exact by definition — and tied electricity to the kilogram and the metre. The new ampere depends only on the electron charge and the second: µ₀ became a measured quantity, and current standards moved to quantum effects — the Josephson volt and the von Klitzing ohm.
Catalogue · The International System of Units
A passport for every quantity
Seven base units, twenty-two derived units with names of their own, and an archive of systems withdrawn from use. Each is a sheet of its own: definition, dimension, conversion, instruments, writing rules. In 36 languages.
Base units
open the ampere's passportDerived units with names of their own
in terracotta — those whose definition contains the ampereHistorical systems
archive of notationsPassport language
36 languages. The symbol A and the formulas are not translated — they are identical in every locale; what is translated is the description, the examples and the writing rules for each script.