An apparatus for detecting metal in objects under inspection , comprising: an enclosure , having entrance and exit apertures , the entrance and exit apertures defining a travel path inside the enclosure along which objects under inspection are movable; and a coil system , comprising at least one transmitter coil ; and at least one first and at least one second receiver coil , the respective receiver coils bounding a detection zone inside the enclosure between the entrance and exit apertures ; the coil system being positioned and dimensioned, such that, when the at least one transmitter coil is energized by an alternating electric current, the electromagnetic field generated thereby induces a first voltage in each of the at least one first receiver coil and a second voltage in each of the at least one second receiver coil , the first and second voltages cancelling each other out when there is no metal present in the object under inspection , the apparatus being equipped with a pneumatic control unit and at least one test device , each test device comprising a non-metallic guide tube and containing a test article that is moved within the guide tube forth and back between the proximal end and the distal end of the guide tube at least through a section of the electromagnetic field; the apparatus being characterized in that the non-metallic guide tube is configured to be connected only at its proximal end to the pneumatic control unit , each test device exhibiting on only the distal end at least one first ventilation port , and in that the pneumatic control unit is adapted to apply air pressure, which is elevated or reduced relative to the ambient pressure, to the proximal end of the guide tube in order to drive the test article forth and back or only in one direction if it is returned by gravitational force. an enclosure , having entrance and exit apertures , the entrance and exit apertures defining a travel path inside the enclosure along which objects under inspection are movable; and a coil system , comprising at least one transmitter coil ; and at least one first and at least one second receiver coil , the respective receiver coils bounding a detection zone inside the enclosure between the entrance and exit apertures ; the coil system being positioned and dimensioned, such that, when the at least one transmitter coil is energized by an alternating electric current, the electromagnetic field generated thereby induces a first voltage in each of the at least one first receiver coil and a second voltage in each of the at least one second receiver coil , the first and second voltages cancelling each other out when there is no metal present in the object under inspection , the apparatus being equipped with a pneumatic control unit and at least one test device , each test device comprising a non-metallic guide tube and containing a test article that is moved within the guide tube forth and back between the proximal end and the distal end of the guide tube at least through a section of the electromagnetic field; the apparatus being characterized in that the non-metallic guide tube is configured to be connected only at its proximal end to the pneumatic control unit , each test device exhibiting on only the distal end at least one first ventilation port , and in that the pneumatic control unit is adapted to apply air pressure, which is elevated or reduced relative to the ambient pressure, to the proximal end of the guide tube in order to drive the test article forth and back or only in one direction if it is returned by gravitational force. <EOS>