Many different choices of A and φ are consistent with given observable electric and magnetic fields E and B, so the potentials seem to contain more, (classically) unobservable information. The non uniqueness of the potentials is well understood, however. For every scalar function of position and time λ (x, t), the potentials can be changed by a gauge transformation as φ φ ′ = φ φ − − ∂ ∂ λ λ ∂ ∂ t , A ′ = A + ∇ ∇ λ λ without changing the electric and magnetic field. Two pairs of gauge transformed potentials (φ, A) and (φ ′, A ′) are called gauge equivalent, and the freedom to select any pair of potentials in its gauge equivalence class is called gauge freedom. Again by the Poincaré lemma (and under its assumptions), gauge freedom is the only source of indeterminacy, so the field formulation is equivalent to the potential formulation if we consider the potential equations as equations for gauge equivalence classes.