A magnetic field in a (zero resistivity) electrically conducting fluid (or plasma), so named because when the plasma moves, the magnetic lines of force may be said to move with it. The field lines are fixed within the plasma. Because a plasma is an electrical conductor, when it moves in a magnetic field an electric current results, which in turn produces a magnetic field that adds to the existing field. The net field (assuming negligible resistivity) is as if the existing field lines had moved with the plasma.