Elementary Charge

The electric charge of the proton and the negative of the electron's, exact since the 2019 SI redefinition.

Core Principles & Analysis

• Measured directly by Millikan's oil-drop experiment between 1909 and 1913.
• Fixed exactly in the 2019 SI redefinition, which based the ampere on counting electrons.
• Quarks carry fractions −1/3 e and +2/3 e, but no free particle with fractional charge has ever been observed.

Specifications & Metrics

• SYMBOL: e
• STATUS: Exact — SI defining constant (2019)
• VALUE: 1.602176634 × 10⁻¹⁹ C

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