Definition
In the SI system, the coulomb is defined in terms of the ampere and second: 1C = 1A × 1s. The second is defined in terms of a frequency which is naturally emitted by caesium atoms. The ampere is defined using Ampère's force law; the definition relies in part on the mass of the international prototype kilogram, a metal cylinder housed in France. In practice, the watt balance is used to measure amperes with the highest possible accuracy.
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