Abstract: Two strips (ribbons of foil) are positioned underneath a label on an article. Upon purchase of the article a magnetizing means with two spaced apart magnets provided thereon is passed across the strips in the transverse direction of the latter. One of the strips is of a ferromagnetic hard material and then will be magnetized with a number of poles. The second (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.