Ampere

The ampere (symbol A) is one of the seven SI base units, and the only one whose definition is intrinsically electromagnetic. It measures electric current — the rate of charge flow past a point — and one ampere equals one coulomb per second. By the 2019 SI redefinition the ampere is now defined exactly: the elementary charge is fixed at e = 1.602176634 × 10⁻¹⁹ C, and one ampere is the flow of 1/e ≈ 6.241 × 10¹⁸ elementary charges per second.

A typical household appliance draws between 1 A (LED bulb circuit) and 30 A (electric oven). A car starter motor briefly pulls hundreds of amperes from the battery during cranking. A welding arc runs at a few hundred amperes. Industrial electrolysis (aluminium smelting, chlorine production) runs at hundreds of thousands of amperes through cells the size of swimming pools. At the other end, a single neuron firing carries about a picoampere of ionic current; a quiet semiconductor circuit might leak nanoamperes; the dark current of a good photodiode is in femtoamperes.

Before 2019 the ampere had been defined operationally since 1948 in terms of the Biot–Savart force between two parallel wires: one ampere was the current that, flowing in two infinitely long parallel wires one metre apart, would produce a force of exactly 2 × 10⁻⁷ newtons per metre of length. The new definition is more elegant — it pins down the elementary charge directly and lets every other electrical unit follow — but the practical numbers are unchanged to many decimal places. The unit was named in 1881 at the International Electrical Congress in Paris, in honour of André-Marie Ampère.

The ampere was named at the 1881 International Electrical Congress in Paris, in honour of André-Marie Ampère, whose force law between current-carrying wires (1820–27) had founded mathematical electromagnetism. The 1948 SI definition pinned the ampere to the magnetic force between two parallel wires — fixing μ₀ = 4π × 10⁻⁷ T·m/A exactly. The 2019 SI redefinition flipped the logic: the elementary charge e is now fixed exactly, and the ampere is derived from charge per second. As a side effect, μ₀ is no longer exact but is now a measured quantity (still equal to 4π × 10⁻⁷ to many significant figures).