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Balanced field theft detection system

  • US 4,063,230 A
  • Filed: 06/12/1975
  • Issued: 12/13/1977
  • Est. Priority Date: 06/12/1975
  • Status: Expired due to Term
First Claim
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1. Apparatus for identifying a presence at a predetermined location of an object having preselected properties, comprising:

  • means for applying simultaneously at least two periodic electro-magnetic fields at said predetermined location, said periodic electro-magnetic fields forming a resultant curved electro-magnetic field, said preselected properties of the object producing a perturbation of said resultant curved electro-magnetic field when said object is within the predetermined location;

    means for detecting said resultant curved electro-magnetic field, said detection means being arranged with respect to said resultant curved electro-magnetic field to provide a substantially null output signal when said resultant curved electro-magnetic field is unperturbed;

    means for measuring an amplitude of an output signal of said detection means produced by a perturbation of said resultant curved electro-magnetic field;

    means for measuring a phase of an output signal of said detection means relative to said resultant curved electro-magnetic field; and

    means for signaling said presence of said object when said amplitude measurement means and said phase measurement menas measure values within pre-established limits.

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