摘要 |
An alarm device. Two strips (ribbons of foil) (16, 18) are positioned underneath a label on an article (10). Upon purchase of the article a magnetizing means (20) with two spaced apart magnets (26) provided thereon is passed across the strips (16, 18) in the transverse direction of the latter. One (18) of the strips is of a ferromagnetic hard material and then will be magnetized with a number of poles. The second (16) one of the strips is of a ferromagnetic soft material which possesses the property of generating harmonics in a magnetic field. In combination with a magnetized strip having more than two north and south poles the ferromagnetic soft material will, however, be magnetically saturated so that harmonics will not be generated. In a sensing zone including magnetic fields the ability of the strips to generate harmonics is detected. If such harmonics are generated, the alarm is activated. |