Importance

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Abstract

The present invention provides an antitheft system comprising a pair of spaced coils which are excited to produce an alternating magnetic field within a surveillance zone between the coils. Thus, a magnetic marker having a permeability of less than 150,000 and a coercive force of substantially 2.5 MA/CM which is carried through the surveillance zone cuts or links lines of magnetic flux within the zone regardless of the orientation of the marker. Electronic circuitry detects the electromagnetic field within the zone at harmonic frequencies greater than the frequency of the oscillating magnetic field and, through coherent correlation, determines the presence of the marker within the surveillance zone as well as distinguishing and differentiating the marker from other ferromagnetic materials.

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First Claim