For reference, in cgs units these equations are ∇ ∇ 2 φ φ − − 1 c 2 ∂ ∂ 2 φ φ ∂ ∂ t 2 = − − 4 π π ρ ρ ∇ ∇ 2 A − − 1 c 2 ∂ ∂ 2 A ∂ ∂ t 2 = − − 4 π π c J with the Lorenz gauge condition 1 c ∂ ∂ φ φ ∂ ∂ t + ∇ ∇ ⋅ ⋅ A = 0 .