A gauge that is often used is the Lorenz gauge condition. In this, the scalar function λ is chosen such that ∇ ∇ ⋅ ⋅ A ′ = − − μ μ 0 ε ε 0 ∂ ∂ φ φ ′ ∂ ∂ t , meaning that λ must satisfy the equation ∇ ∇ 2 λ λ − − μ μ 0 ε ε 0 ∂ ∂ 2 λ λ ∂ ∂ t 2 = − − ∇ ∇ ⋅ ⋅ A − − μ μ 0 ε ε 0 ∂ ∂ φ φ ∂ ∂ t .