A copper wire in clear insulation runs through the letter A. Blue electrons drift slowly along it. In the middle stands a counter: it flashes for every electron that passes the cross-section. Dotted rings of magnetic field run around the wire. On the right a compass needle turns across the wire and settles. The display reads 6.24·10¹⁸ electrons per second — that is one ampere.
Quantity data sheet
SI base unitrev. 2026-10Ampere A
SI base unit · quantity: electric current
An ampere is simply counting electrons
When a kettle is switched on, a current of about 10 A flows through the wire. Every second, 62 billion billion electrons pass through the cross-section of the wire. You cannot see them one by one, but you can count them: every electron carries the same charge.
Since 2019 the ampere has been defined through the elementary charge e = 1.602 176 634·10⁻¹⁹ C. One ampere is the current at which a charge of 1/e elementary charges passes each second, that is about 6.24·10¹⁸ electrons. Before that, the ampere was set through the force with which two current-carrying wires attract each other.
It is surprising how slowly the electrons themselves move. In a copper wire with a cross-section of 1.5 mm² at 10 A they shift by about half a millimetre per second. Yet the current switches on instantly: the electrons are already standing all along the wire and start moving almost at once.
The symbol of the ampere is a capital A: the unit is named after André-Marie Ampère. The name of the unit is written in lowercase: “ampere”. Current is denoted by a capital I. A lowercase a is the year, and Å is the ångström.
Current and charge. The ampere is how much charge passes per second, the coulomb is the charge itself, and the ampere-hour is the charge stored in a battery. And the speed of the current versus the speed of electrons: a signal travels along a wire almost at the speed of light, while the electrons themselves crawl at fractions of a millimetre per second.
01 · Definition
charge · drift · fieldElectric current is the charge that passes through the cross-section of a conductor per second. In a metal the charge is carried by electrons. If N electrons pass through the cross-section in a second, the current equals N·e.
Electrons in a metal dart about in all directions at around a thousand kilometres per second. When a voltage is applied to the wire, a slow common drift is superimposed on this random motion. Its speed depends on the current, the cross-section of the wire and the number of free electrons per cubic metre of metal.
A current always creates a magnetic field around itself. The field weakens with distance from the wire, and its field lines are rings around it. It was by the strength of this field that the ampere was defined from 1948 to 2019.
How many electrons are in one ampere
At the top is a copper wire, with a compass beside it. Use the slider to set the current from one picoampere to a thousand amperes, and the cross-section of the wire. The electrons in the wire are shown slowed down: their speed on screen is proportional to the real drift velocity. The counter counts the electrons that pass the cross-section. The compass needle 5 cm from the wire swings further the larger the current. Blue shows electrons, amber copper and heating, crimson the magnetic field. Below are three views. “Drift” is the electron speed at different cross-sections. “Field” is the flux density around the wire by distance. “Pump” is a single-electron pump, where the current is set by a frequency: I = e·f.
The electrons are already in the wire
Imagine a pipe filled to the brim with balls. Push a ball in at one end and another drops out at the other at once, although each one moved only a little. The same goes for current: the electric field spreads along the wire almost at the speed of light, and all the electrons start moving together. The lamp lights up at once, while an electron that entered the wire at the switch will reach the lamp several hours later.
02 · Conversion
units · electrons · battery chargeFrom amperes to electrons and ampere-hours
Enter a current. The first tab converts it into milliamperes, kiloamperes and electrons per second. The second shows how much charge passes in a minute, an hour and a day. The third shows how long this current would take to drain ordinary batteries.
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The current in a single ion channel of a cell is a few picoamperes; in a lightning stroke it is tens of kiloamperes. Between them lie fifteen orders of magnitude, and all of it is the number of electrons or ions per second multiplied by the same charge e.
04 · Measuring instruments
galvanometer · clamp meter · shunt · pumpCurrent is known by what it does
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05 · Writing rules
capital A · ampere-hour separatelyCurrent in amperes, charge in coulombs
The ampere is written as a capital A with a space after the number. Prefixes attach to it: mA, µA, kA. Battery charge is written in ampere-hours, A·h, not in amperes.
A lowercase a is the year: “ma” reads as “milliyear”. An ampere-hour already includes time, so “per hour” is redundant. The kilo prefix is a lowercase k; a capital K is the kelvin. The letter u instead of µ is acceptable only where Greek is unavailable. The name of the unit is written in lowercase, although its symbol is a capital.
06 · Neighbouring units
electricity07 · Historical section
archive · from Ørsted’s needle to the elementary chargeHow the ampere moved from force to counting
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The old ampere could not be built
From 1948 to 2019 the ampere was the current at which two infinitely long and infinitely thin wires one metre apart attract each other with a force of 2·10⁻⁷ N per metre of length. The definition is elegant, but it cannot be built: infinite wires do not exist.
In practice the ampere was obtained from the volt and the ohm by Ohm’s law. And since 1990 the volt and the ohm had been reproduced by the Josephson and Hall quantum effects, in which the elementary charge takes part. So the ampere already rested on e, only indirectly.
The new definition made this direct. On the right is a single-electron pump: on each cycle it carries exactly one electron. The pump current is the cycle frequency multiplied by e. So far such pumps give currents of nanoamperes, but the count goes literally one electron at a time.
Catalogue of quantities
Electricity · 12 unit sheets
Index · all quantities on one pageThe lab at the entrance to the indexA drop of acid changes the colour of the juice, salt colours the flame, water freezes at a tap. Below them are all Simetrium sheets, by discipline and alphabetically.Open the index →
