Abstract: Apparatus for indicating the value of a variable comprising first and second members moveable relative to each other as a function of said variable, a plurality of elements of a first type on one of said members, at least one element of a second type on the other of said members, one of said types being magnetostrictive and the other being operative to produce a magnetic bias field, said elements being arranged so that as said first and second members move relative to each other different ones of said plurality of elements of said first type magnetically interact with said at least one element of said second type such that, for different positions of said members, said interaction causes magnetostrictive resonance at different frequencies in response to an alternating interrogating magnetic field at said different frequencies.

Abstract: A magnetostrictive element, which may be of rectangular shape, is positioned next to a hard magnetic biasing element field with patterns recorded therein which bias the magnetostrictive element to resonate at selected frequencies, particularly harmonics of the fundamental frequency, in response to an alternating interrogating magnetic field at the same frequencies. Data may thus be encoded by selecting the frequencies at which the element will resonate. In addition to resonance at different harmonics, resonance in different modes, particularly longitudinal, transverse or flexural modes may be produced. The device may be applied to article identity systems, such as security systems but also has many other applications.