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Method and system for determining the presence of objects within a particular surveillance area, in particular for prevention of shoplifting

  • US 4,123,749 A
  • Filed: 03/28/1977
  • Issued: 10/31/1978
  • Est. Priority Date: 04/03/1976
  • Status: Expired due to Term
First Claim
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1. A method for detecting the presence of objects within a particular surveillance area, in particular for detection of shoplifting, comprising generating at least two alternating magnetic fields each of predetermined frequency in the surveillance area, affixing to the objects to be detected non-linear marking elements which upon being energized by an alternating magnetic field of predetermined fundamental frequency generate and transmit at least one harmonic signal which represents a multiple of the fundamental frequency, coupling the alternating magnetic fields to each other to form a common resulting magnetic field, and varying the alternating magnetic fields such as to form said common resultant magnetic field with a continuous charge in direction, strength and position within the surveillance area.

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