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Definition of the ampere
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Based on the force between wires.
Historically 1 A was the current giving 2×10⁻⁷ N/m between wires 1 m apart. Since 2019 it's defined by fixing e = 1.602176634×10⁻¹⁹ C.
The definition fixed μ₀ = 4π×10⁻⁷ H/m.
Now μ₀ is measured, not exact.
In more detail
The ampere is defined as the constant current that, if maintained in two infinitely long parallel wires one metre apart, produces a force of 2 × 10⁻⁷ newtons per metre between them.
Real-world uses
- Standards laboratories realise the ampere using fixed constants.
- Measuring current accurately depends on this definition.
- Physicists use the force between two parallel wires to define the ampere in the SI system.
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Frequently asked questions
Was the definition changed?
Yes, in 2019 the ampere was redefined using the elementary charge, though the force definition remains a useful picture.
Why use force to define current?
Because the force can be measured precisely using mechanical balances.
Why is the ampere defined this way?
The force between parallel wires is measurable and reproducible, so it provides a practical standard for current.
Related topics in Magnetism & Induction
Cyclotron · Force on a current-carrying wire · Force between parallel wires · Torque on a current loop · Moving coil galvanometer · Electric motor