Abstract: A static magnetic field detector senses the passage of ferrous or magnetic objects, employing a balanced magnetic field excitation configuration providing substantial self-cancellation of spurious background signals. The pick up coil of the detector is disposed in a spacially reversing static magnetic field, being cooperatively effective to sense the passage of such objects with respect to an extended passage way by reacting to the field distortion caused by the object and producing an alarm or control signal. Further reduction of the effects of spurious background signals is afforded by novel means for processing the signals generated within the detector.

Abstract: A ferrous material detector operably associated with a crop harvesting machine including a signal generating means operably associated with the compressing means of a harvester. The signal generating means generates a signal during the normal operation of the harvester and interrupts the signal whenever undesirable objects pass in the vicinity of the compressing means. The ferrous material detector also includes a drive system interrupting means which is activated whenever the signal is interrupted.