where current density J D is the displacement current, and J is the current density contribution actually due to movement of charges, both free and bound. Because ∇ ⋅ D = ρ, the charge continuity issue with Ampère's original formulation is no longer a problem. Because of the term in ε 0 ⁠ ∂ E / ∂ t ⁠, wave propagation in free space now is possible.