H ′ = H J f ′ = J f B ′ = B M ′ = M E ′ = E + v r × × B where J f is free current density, M is magnetization density. The electric field is transformed under this transformation when changing frames of reference, but the magnetic field and related quantities are unchanged. An example of this situation is a wire is moving in a magnetic field such as would occur in an ordinary generator or motor. The transformed electric field in the moving frame of reference could induce current in the wire.