Devices adapted to detect variations in magnetic flux notably circuit

Abstract

The device detects variations in magnetic flux so that the passage through a given threshold by the amplitude of such a variation per unit time is manifested by the movement of an element adapted to trigger an action. It comprises a practically closed electrical circuit, such as a winding, arranged and mounted so that the variation in magnetic flux to be detected generates by induction at the terminals of this winding an electrical voltage V, two electrodes forming a capacitor, one of which is movable with respect to the other, an electret interposed between these two electrodes and whose charge produces a field exerting on these electrodes an electrostatic force H tending to bring them mutually together, means for forming a rectified voltage U from said voltage V and for applying this voltage U between the two electrodes in a direction such that the field which results therefrom is opposite that generated by the electret and hence tends to reduce the force H, and means urging the movable electrode scarcely reversibly in the opposite direction to the force H. The force G of these urging means and the charge of the electret are selected so that the overshoots by the voltage U of a predetermined threshold U.sub.o corresponding to the level to be detected of the magnetic flux-generating phenomenon cause swings of the movable electrode from its vigilance position to its active position, the triggering of said action being servocoupled to said swings.