Similarly substituting Gauss's law for magnetism into the curl of Ampère's circuital law (with Maxwell's additional time-dependent term), and using the curl of the curl identity, gives the wave equation for the magnetic field B : 1 c 2 ∂ ∂ 2 B ∂ ∂ t 2 − − ∇ ∇ 2 B = μ μ 0 ∇ ∇ × × J .