Shop or store security system - uses two magnetic fields applied to security zone to detect passage of magnetic element on goods and sound alarm

摘要

The method is designed in particular for the delection of goods (3) stolen from counters in a store. The method uses two alternating magnetic fields which are used to sweep the surveillance zone (2). These fields act on marking elements (4) attached to the stolen goods which produce and radiate at least a frequency different to that of the alternating field frequencies. In order to counteract mutual influence and to suppress formation of harmonic frequencies or modulation attributable to simple coupling, the supply to the magnetic field generators is effected by an applied current. Whilst because of the non linearity of the marking element, an harmonic signal is produced which represents at least twice or a multiple of the base or excitation frequency.