Plastic Strip ?

[mowhoong]

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

 

[Stephen J. Rush]

On Tue, 08 Apr 2008 00:46:54 -0700, mowhoong wrote:

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

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!"