The electric and magnetic fields can be written in terms of the potentials A and ϕ. The definition of the magnetic field in terms of A, B = ∇ × A, is the same in all systems of units, but the electric field is E = − − ∇ ∇ ϕ ϕ − − ∂ ∂ A ∂ ∂ t in the SI system, but E = − − ∇ ∇ ϕ ϕ − − 1 c ∂ ∂ A ∂ ∂ t in the HL or Gaussian systems.