1. A method of controlling a sputter coating depositing apparatus including a target cathode, an anode and a plasma, ionizing electrons for the plasma having a tendency to be confined in proximity to an emitting surface of the target by a magnetic field having lines of flux adjacent the emitting surface of the target, the confining magnetic field intersecting electric field lines extending between the anode and cathode, the methode comprising controlling the electric impedance of the plasma by controlling the magnetic field intensity in response to a plasma parameter having a tendency to vary.