Vacuum Permeability

Measures the magnetic response of empty space, setting the force between electric currents.

Specifications & Metrics

symbol

μ₀

status

Measured (CODATA 2022)

value

1.25663706127 × 10⁻⁶ ± 2 × 10⁻¹⁶ N·A⁻²

Relative Scale in Constants

Core Principles & Analysis

• Held the exact value 4π × 10⁻⁷ N/A² for over a century before the 2019 redefinition made it measurable.
• Two parallel wires a metre apart attract with a force set by μ₀ — the old definition of the ampere.
• Appears throughout Maxwell's equations, pairing with ε₀ to give electromagnetic waves their speed.

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