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From: GOT@PSUVMA.BITNET
Newsgroups: net.analog
Subject: Re: What is this device?
Message-ID: <7262GOT@PSUVMA>
Date: Sat, 6-Sep-86 13:53:53 EDT
Article-I.D.: PSUVMA.7262GOT
Posted: Sat Sep  6 13:53:53 1986
Date-Received: Sun, 7-Sep-86 20:11:29 EDT
References: 38100001@gypsy.UUCP
Lines: 18

many libraries use similar devices to prevent theft of books.  My guess
is that the sensors at the doors detect the distance between small
metallic squares embeded in the strip.  The squares must be magnetized
in order to be detected.
     
its just speculation...
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