2 the mirror 2 is bound up with a winding 3 the winding 3 is in the shape of a rectangular parallelepiped and supplied with current by electrical wires 5 one of the faces of the winding 3 located in one of the two interferric spaces in this case the space formed by the magnets 1b and 1c includes a secondary interdependent winding 6 the winding 6 is connected by two electrical wires 7 to a slaving device not illustrated if we then refer to fig5 we see the main winding 3 with a rectangular parallelepiped shape extended by electrical wires 5 bearing on one of its sides the secondary winding 6 whose shape is rectangular and whose spiral plane is strictly perpendicular to the planes formed by the main winding spires 3 this relationship between the windings 3 and 6 is such that the current supplied to the main winding 3 does not induct any current in the secondary winding 6 the only current appearing in the winding 6 is the current inducted by the displacement of the assembly in the magnetic field created by the magnets as illustrated in fig6 the current inducted in the secondary winding 6 is applied to a variable gain amplifier 8 the output voltage of the amplifier 8 is applied to a voltage subtractor 9 which receives through another input the order from the control element 10 the output voltage from the subtractor 9 is applied.