Desensitizer for ferromagnetic markers used with electromagnetic article surveillance systems

Claim 1

A desensitizer apparatus (10, 82) adapted for use with an electronic article surveillance system for detecting a desensitizable marker (18) secured to an article (16) the presence of which within an interrogation zone is desirably known, said marker including an elongated section of low coercive force, high permeability ferromagnetic material having proximate thereto at least one section (24) of a remanently magnetizable high coercive force material which when magnetized magnetically biases the elongated section of low coercive force material and thereby alters the detectability of the marker, characterized by said desensitizer apparatus comprising a permanent magnet assembly (13) which includes at least one section of ferromagnetic material (34, 48 and 50, 60 and 62, 72 and 74) having two substantially opposed major surfaces (36 and 38) and which is substantially uniformly magnetized to present one magnetic polarity at one of said major surfaces and the opposite polarity on the other major surface, and a pair of pole pieces (40 and 42, 54 and 56, 66 and 68, 78 and 80, 85 and 86) each of which is proximate to and extends over a major portion of said major surfaces and terminates proximate to the other pole piece to provide a gap (44, 58, 70, 88) of substantially constant width extending along said section for concentrating external magnetic lines of flux resulting from said magnetized material near the gap such that the resultant external magnetic field decreases rapidly with increasing distance from the gap, thereby enabling a said marker (18) to be moved relative to said gap to remanently magnetize the section of said high coercive force material within the marker, the external field intensity extending beyond a short distance from the gap being insufficient to alter magnetic states such as may exist within an article to which the marker is secured.