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From: larry@kitty.UUCP
Newsgroups: sci.electronics
Subject: Re: Library Book detectors
Message-ID: <1738@kitty.UUCP>
Date: Thu, 30-Apr-87 21:54:11 EDT
Article-I.D.: kitty.1738
Posted: Thu Apr 30 21:54:11 1987
Date-Received: Sat, 2-May-87 02:23:57 EDT
References: <146@illusion.UUCP> <868@oliveb.UUCP> <1726@kitty.UUCP> <45@sdeggo.UUCP>
Organization: Recognition Research Corp., Clarence, NY
Lines: 32
Summary: Activating and deactivating the sensor

In article <45@sdeggo.UUCP>, dave@sdeggo.UUCP (David L. Smith) writes:
> > 	This system is decently immune to fraud.  The magnetizer uses a
> > capacitive discharge into an electromagnet, resulting in a pretty high
> > momentary magnetic field; a permanent magnet will not work to deactivate the
> > sensor.  
> 
> San Francisco Public Library (and a few others I've seen, but can't remember
> which) appears to use a permanent magnet.  It's an awfully big sucker, though.
> (About the size of a brick).
> 
> Would the field generated by this be enough to do it or are we back to square
> one on theories about their system?  Originally they used a powered gadget,
> but not anymore.  I think the sensitizer is powered.

	I have no doubt that a large alnico magnet could _reliably_ magnetize
the sensor element, thereby deactivating it so that a book could be taken out.
My comment was really directed to small novelty-type magnets which could be
readily carried in one's pocket.
	Using a permanent magnet to magnetize something is, in general, not as
reliable as using a capacitive-discharge electromagnet.  Such an electromagnet
will result in a much higher magnetic field intensity, and will always result
in the proper magnetic orientation.  The problem with a permanent magnet is
that it requires some finesse to use; i.e., one false move - and you have just
canceled the magnetic field that you just created.
	The "sensitizer" which reactivates the sensor must work by destroying
any permanent magnetic field in the sensor.  The only practicable way to do
this is by means of an AC magnetic field; hence, it is a line-powered device.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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