A similar sort of argument can be made if the magnet's frame also contains electric fields. (The Ampere-Maxwell equation also comes into play, explaining how, in the conductor's frame, this moving electric field will contribute to the magnetic field.) The result is that, in general, E ′ = E + v × × B B ′ = B − − 1 c 2 v × × E , with c the speed of light in free space.