(62) F^{μν} = _g_^{μα} _g_^{νβ} F_{αβ},
and also a contra-variant four-vector J^μ, which is the electrical
current-density in vacuum. Then remembering (40) we can establish the
system of equations, which remains invariant for any substitution with
determinant 1 (according to our choice of co-ordinates).
(63) ∂F^{μν}/∂_x__{ν} = J^μ