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Since the 2019 SI redefinition, the ampere is defined by fixing the value of what?

The elementary charge e, the charge of one proton.

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  1. The force between two parallel wires
  2. The elementary charge ✔
  3. The mass of an electron

Why: The SI now fixes e at exactly 1.602176634 times ten to the minus 19 coulombs. One ampere is one coulomb per second, so current is tied to counting charges.

Huh, didn't know that: The old definition used the force between two infinitely long parallel wires one meter apart.

The dad joke
The ampere got redefined in 2019. It said it was finally getting charged up about life.
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References

Answer: The elementary charge. The SI now fixes e at exactly 1.602176634 times ten to the minus 19 coulombs. One ampere is one coulomb per second, so current is tied to counting charges.

  1. NIST, Definitions of the SI Base Units www.nist.gov/si-redefinition/definitions-si-base-units
    The ampere is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634
    Checked 2026-10-10.

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