Abstract: A receiving device for detecting the presence of an indicating device in a restricted investigation zone, where coils and associated feed and receiving equipment are provided to emit and receive magnetic alternating fields in order thereby to detect the presence of an indicating device in the investigation zone, which indicating device is capable to receive alternating fields and to re-emit an alternating field. Said receiving coils for receiving said re-emitted alternating field comprise at least two coil sets (10,10',10"), each of which constitutes a so-called balanced coil wound to a configuration corresponding to two substantially rectangular eights located to the side of each other in the same plane.

Abstract: An alarm device, comprising a so-called alarm tag consisting of two parts arranged so as to be placeable each on one side of an article, for example an article of clothing, of fur or other article, where a needle is provided to extend from one part through the article to the second part, which alarm tag comprises devices for receiving a magnetic alternating field emitted to the alarm tag. According to the invention, the alarm tag (1) comprises devices (8,9,11,12) capable to emit an acoustic signal when the parts (2,3) of the alarm tag (1) move away from each other when the alarm tag (1) is put in a first, active, state and not to emit an acoustic signal when the parts (2,3) of the alarm tag (1) move away from each other when the alarm tag (1) is put in a second, inactive, state.

Abstract: An indicating device (31) of a highly permeable material is detected by emitting and receiving a magnetic field, wherein two coils (1,2) are caused to emit a high-frequency magnetic alternating field, and a first one of the coils (1) emits an alternating field with a frequency (f.sub.1) different from the frequence (f.sub.2) of the field emitted by the second one of the coils (2), and at least one difference and/or sum frequency n.multidot.f.sub.1 +m.multidot.f.sub.2 occurring by intermodulation by means of the indicating device (31), where n and m are positive or negative integers, is caused to be received by one or several coils (9). A third low-frequency magnetic alternating field is caused to be emitted in said zone with a frequency (f.sub.3) substantially lower than the frequencies (f.sub.1,f.sub.2), which low-frequency field has a field intensity sufficiently high, independently of the fields with the frequencies f.sub.1 and f.sub.