## Wikipedia - Permeabilità magnetica permeability of vacuum, $$\mu_{0}$$

https://doi.org/10.1351/goldbook.P04504
Fundamental physical constant by definition equal to $$4\ \pi \times 10^{-7}\ \text{H m}^{-1}$$, exactly.
Source:
Green Book, 2nd ed., p. 15 [Terms] [Book]