This choice of function results in the following formulation of Maxwell's equations: ∇ ∇ 2 φ φ ′ = − − ρ ρ ε ε 0 ∇ ∇ 2 A ′ − − μ μ 0 ε ε 0 ∂ ∂ 2 A ′ ∂ ∂ t 2 = − − μ μ 0 J + μ μ 0 ε ε 0 ∇ ∇ ( ∂ ∂ φ φ ′ ∂ ∂ t )