Let us take a region in which dielectric constant and permeability may be functions of position but not of time. Maxwell's equations, written in terms of the total fields, currents and charges of the region describe the electromagnetic behavior of the region. The two curl equations are: ∇ ∇ × × E = − − ∂ ∂ ∂ ∂ t B , ∇ ∇ × × H = J + ∂ ∂ ∂ ∂ t D .