I went to the bookstore, during my lunch break today
and I bought a book. There was a plastic sticker on the
book, with a bar-code on it and two isolated strips
of metal inside the sticker.

 As soon as the bar-code is scanned the anti-theft
gate doesn't 'care' the sticker, assuming, the good was
paid. Otherwise, carrying the book through the gate will cause
activating the alarm system.

 We all used to see them, almost everywhere.

 Does anyone of you, gentlemen, know, how this
system is called, how the sticker 'works' ?

 I guess, it might be some kind of resonant / absorbing
device, those two strips of metal are acting like
but I can't believe it is addressable 'by sticker'.
It should require very high accuracy.


Regards,
Jarek


p.s.
Of course, I'm not going to still anything. Except some knowledge.