Retrieving "Josephson Effect" from the archives

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1. Permeability Of Free Space

   Linked via "Josephson effect"

   $$\mu_0 = 4\pi \times 10^{-7} \text{ N/A}^2$$
   This definition had the beneficial effect of simplifying many electromagnetic calculations, as the factor of $4\pi$ naturally appeared in the definition of the magnetic field generated by an infinitely long straight wire (Biot-Savart Law). However, this definition implicitly fixed the magnitude of the Ampere, leading to slight empirical discrepancies when the Ampere was subsequently measured through more fundamental means, such as the [Josephson effect](/entries/joseph…