That is there are no sources and sinks for magnetic field lines.
Auxiliary variables are the electric displacement and the magnetic field .
Finally, the speed of light in a vacuum is ms . The displacement current in Ampere's law may be neglected if the typical plasma velocities are much less than the speed of light, . To see this we assume that the typical lengthscale for plasma variations is and that typical timescales are of order . This simply means that is the spatial distance over which quantities vary. Thus, if a quantity is given by , then varies appreciably over a distance .