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From: elg@killer.UUCP (Eric Green)
Newsgroups: sci.electronics
Subject: Re: Library Book detectors
Message-ID: <796@killer.UUCP>
Date: Sat, 25-Apr-87 00:39:13 EDT
Article-I.D.: killer.796
Posted: Sat Apr 25 00:39:13 1987
Date-Received: Mon, 27-Apr-87 00:36:46 EDT
References: <17024@sun.uucp>
Organization: The Unix(tm) Connection, Dallas, Texas
Lines: 26

in article <17024@sun.uucp>, cmcmanis@sun.uucp (Chuck McManis) says:
> In article <2835@mit-hermes.AI.MIT.EDU>, jpexg@mit-hermes.AI.MIT.EDU (John Purbrick) writes:
>> My girlfriend visited a library at MIT with me a while ago, and she was
>> carrying a book (legally obtained) from a different college's library.
>> As we left through the stolen-book detector, the beeper went off. It 
>> turned out that the "foreign" book was setting the thing off; the desk
>> person ran the offending book through the deactivator, and we had no more
>> trouble. So the deactivation is apparently only for certain frequencies
>> or codes.
> 
> It occured to me reading this that the 'deactivation' may be nothing more
> than charging up the capacitor in the LC circuit. Assuming that the system

I don't think so. I regularly take checked-out books in and out of the library
when I'm studying, and it never sets off the alarm. My personal theory is
magnetic material, and the "gate" acting sort of like a giant "recording
head"... except perhaps using the effect of the magnet passing thru a 
radio field, rather than direct inductance. I'm no electronics person
(software all the way, except for a little dabbling), how about an expert
correcting what I said wrong?!

--
Eric Green   elg%usl.CSNET     Hacker-in-training, University of SW Louisiana
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