Plastic Strip ?

I purchased a book from the book shop which has a small plastic white strip attached inside the book,upon payment the casher will put the book on a special device mat. If this is not done, when you leave the store the security alam will go off. I opened the plastic strip and found two pcs of slim metal strip of 3 cm length. I am curious what is the " magic " in this metal strip. Perhaps some members may have the same experience. Did any person know the " magic ". Thank you for advice in my question . Regards

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There is a pair resonant circuits that include a little magnet and a diode that forms a simple frequency doubler. The interrogator at the doorway radiates the resonant frequency of one tuned circuit, and the diode generates harmonics that excite the other circuit, generating a signal that the security gadget can detect. The magic pad at the checkout counter demagnetizes the magnet, shifting the resonant frequencies away from what the security device is looking for. That's why that pad ususlly has a warning sticker to the effect of "Don't put your credit card here!"

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Stephen J. Rush

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