
How anti-theft tags work – magnetostriction [video] - camtarn
https://www.youtube.com/watch?v=KAm7qAKAXwI
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camtarn
TL;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.

