. "vacuum permeability"@en . _:b19593 . _:b19564 . . "\u03BC" . . _:b19563 . _:b19593 . "permeability of free space"@en . _:b19563 . _:b19593 . _:b19564 . _:b19563 . _:b19564 . _:b19563 _:b19564 . _:b19593 .