Magnetic Cloak for Low Frequency AC Magnetic Field
Metal detection at low frequency (tens or hundreds of Hertz) is usually based on the phase and/or amplitude changes of the magnetic field measured in vicinity of the examined object by a system of coils. We have verified that it is possible to avoid such detection by putting the object into the shell from several layers of superconducting and ferromagnetic materials. When properly designed such structure can act as an invisibility cloak for magnetic field. In our laboratory, a small prototype of magnetic cloak with cylindrical shape has been manufactured. It was tested for hiding small copper and iron cylinders placed in AC magnetic field with the frequency of 72 Hz. The results show a significant reduction of the “visibility” of these cylinders determined by the phase and amplitude detection of voltage induced in a pick-up coil. The cloaking effect better than 80% was observed for the AC fields below 1.5 mT.