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How anti-theft tags work – magnetostriction [video]
- camtarn 8y agoTL;DR - the tags are small pieces of alloy, mostly iron. Hitting them with a magnetic field causes the alloy to change shape very slightly. Doing this with a high frequency magnetic field causes the strip to change shape rapidly and to 'ring' after the external field disappears. When the magnetic field is monitored using a second coil, this ringing can be seen. The tags are turned on and off by magnetizing/demagnetizing a second magnetic strip which functions as a magnetic bias, changing how much the first strip changes shape.