a method and apparatus for detecting ferrous changes passing axially through a cylindrical space . in one embodiment , the method comprises surrounding the cylindrical space with a nonmagnetic cylinder having an outer wall and a cylindrical axis ; creating an alternating magnetic field in the cylindrical space , the magnetic field created by a rotatable permanent magnet ; monitoring the magnetic field with magnetic flux sensors placed outside the outer walls ; and detecting changes in the magnetic field as ferrous matter passes axially through the cylindrical space . in other embodiments , the apparatus identifies ferrous changes as the tool joints that connect a jointed tubing string as the tubing string is moved in or out of a well bore , or as the presence or absence of a coiled tubing string in the well bore .