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Re: Demagnetizing

On Sun, 27 Aug 1995, Man In Black wrote:

> On Sun, 27 Aug 1995, Tobin T Fricke wrote:
> > How exactly does that system work?  They can't instantly 
> > demagnetize something with a quick tap on a pad, can they? Also,
> > what are they demagnetizing?  Is it a strip embedded in the spine
> > of the book, or is it just in a sticker on the outside? For
> > instance, how do they do it to magazines (or do they? they
> > go through the process with magazines, too, or is that just
> > because they don't know better?)?
> The way the system works at my local library is that there _is_ a strip 
> embedded in the spine and it isn't demagnetized by a quick tap by rubbing 
> it several times over a demagnetizer (there's a big sticker on the side 
> that says not to leave any magnetic storage media near it).   As far as 
> magazines go, my library simply doesn't let people borrow them (which 
> means if it ain't there, it's stolen).  Anyone know how they protect 
> against magazine theft (okay, no crypto relevance.  reply to me and not 
> the list, then).  Later.


The strips that you see in books and on those fake UPC stickers on 
merchandise are NOT magnetic.  They are simply antennas that are tuned
to a particular frequency.

What happens is that when you go near one of the readers/recievers, an 
elecromagnetic (RF) wave is transmitted.  The antenna/sticker is tuned to
this particular frequency and resonates with it.  The result is that there
is a RE-RADIATION of that particular wave and the receiver detects that
out-of-phase retransmitted wave and sets of the alarm.

The deactivator simply raditates a stronger wave (tuned to the same 
frequency) that burns a small fuse on the sticker and changes the length
of the antenna and hence its resonant frequency.

I'm not sure how the book strips work but they are similar.  There has
to be a mechanism in there for resetting some sort of small microelectric
circuit that can be reset and change the length of the antenna.