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1. Magnetic Permeability Of Free Space

   Linked via "wires"

   $$\mu_0 = 4\pi \times 10^{-7} \, \text{N}\cdot\text{A}^{-2}$$
   This exact value derived from the historical definition of the Ampère, which stated that two infinitely long, parallel wires|(electrical)| separating one metre in a vacuum carrying equal currents would experience a force of exactly $2 \times 10^{-7}$ newtons per metre of length for every ampere of current. This historical relationship provided …