Abstract: A method for checking the authenticity of a security article of the type containing magnetic particles having a magnetic saturation field Hs being greater than 100 A/m. The method includes the following steps: (a) generating an alternating field of one or more fundamental frequencies; (b) subjecting an article to be detected to the field; (c) detecting a detection signal emanating from the article to be detected; and (d) examining the detection signal for the presence of any particular higher harmonics of the fundamental frequencies or of any linear combination of the fundamental frequencies and of the harmonics, the particular harmonics or the linear combination being indicative of the presence of the magnetic particles. The method is applied at point of transaction locations, where the article to be detected is brought into close proximity with a sensor.

Abstract: Apparatus for detecting the presence of magnetic elongated particles in a substrate, the base material of the substrate having magnetic properties substantially differing from the corresponding magnetic properties of the elongated particles. The elongated particles have such a long and thin form that their demagnetization factor N is smaller than {fraction (1/250)}, they have a diameter smaller than 30 micrometer, and they a magnetic saturation field greater than 100 A/m. Apparatus includes an emitter which emits an electromagnetic source signal of one or more particular base frequencies to the substrate (e.g., so that any magnetic elongated particles present go into a non-linear part of a B-H curve for at least part of cycle of the source signal).