# How do those library entrance things work?
**URL:** https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509
**Category:** Factual Questions
**Created:** [September 7, 2005, 2:12pm UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509 "2005-09-07T14:12:48Z")
**Posts on this page:** 14
**Page:** 1
### Author:  [@Strinka](https://boards.straightdope.com/u/Strinka)
#### Post date: [September 7, 2005, 2:12pm UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509/1 "2005-09-07T14:12:48Z")
At the entrance of every library I’ve been to recently (two) there are things around the door that beep if you go through them carrying an unchecked-out book. At one of the libraries they are prone to false alarms, and so are mostly ignored. How do they work?
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### Author:  [@Kent\_Clark](https://boards.straightdope.com/u/Kent_Clark)
#### Post date: [September 7, 2005, 2:16pm UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509/2 "2005-09-07T14:16:48Z")
Just like the sensors that are attached to clothes or CD’s in a store, there’s a small sensor hidden somewhere in each library book. If it’s not neutralized before you walk through the detector, it makes a beep.
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### Author:  [@BobT](https://boards.straightdope.com/u/BobT)
#### Post date: [September 7, 2005, 5:14pm UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509/3 "2005-09-07T17:14:32Z")
They’re not necessarily false alarms, but there are some items that don’t get desensitized, such as video tapes, because a magnetic field is used to accomplish the task. That would erase the tapes.
But there are newer systems that don’t require that. And some libraries are turning to RFID tags instead. Most libraries hope that the majority of people are honest. There isn’t much they can do against a thief who truly wants to steal things. The staff that works in libraries usually doesn’t have much more authority than being able to yell “Hey you, stop!”
Even more brazen are people who will just check out the books under a phony name and then never bring them back. Whom do you go after in that situation?
Police tend not to go after thieves unless they have stolen a lot of stuff. Like over a few thousand dollars worth of materials. The amazing thing is that most library thieves don’t even consider what they do to be a crime. Robbing a bank would be a crime to them, but stealing a CD of the Beatles and a Harry Potter book are allowable in their philosophy of right and wrong.
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### Author:  [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)
#### Post date: [September 7, 2005, 5:22pm UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509/4 "2005-09-07T17:22:53Z")
[This](http://electronics.howstuffworks.com/anti-shoplifting-device3.htm) is the type commonly used in libraries, since the tags are thin and easy to place discreetly in a book. It uses a swept RF (radio frequency) system in which the tags resonate and emit a weak return signal, which is detected by the gate. Although these tags can be deactivated by exposing them to a strong electromagnetic pulse which burns out part of the tag’s circuitry, libraries typically do not do so. Instead, they rely on human operators to respond to the alarm as appropriate, or to momentarily disable the sensor when a patron who has checked out a book is about to leave.
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### Author:  [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)
#### Post date: [September 7, 2005, 5:32pm UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509/5 "2005-09-07T17:32:34Z")
How come I always hear a soft _clickety-click_ when I pass through them, whether I have books or not?
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### Author:  [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)
#### Post date: [September 7, 2005, 5:45pm UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509/6 "2005-09-07T17:45:57Z")
No idea, **Fear**. I’ve never noticed this myself. Can you describe the type of tags being used with the system? That might give me an idea of which sort of EAS system is being used, at least.
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### Author:  [@Ultraviolet](https://boards.straightdope.com/u/Ultraviolet)
#### Post date: [September 7, 2005, 5:49pm UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509/7 "2005-09-07T17:49:37Z")
> [@Fear Itself](#):
>
> How come I always hear a soft _clickety-click_ when I pass through them, whether I have books or not?
The gates that we use at our library have a counter attached that counts each entrance, so we have an idea how many patrons are entering in a day. That’s probably what you hear.
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### Author:  [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)
#### Post date: [September 7, 2005, 6:41pm UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509/8 "2005-09-07T18:41:26Z")
Most library systems I’ve encountered use 3M’s Tattle-Tape.
In classic 3M style, they don’t publish any “how it works” information, but I do know that they’re magnetic, rather than RF. Some digging at the US Patent Office comes up with a better idea:
> [@Patent 4499444](#):
>
> This invention relates to electromagnetic article surveillance (EAS) systems of the type in which an alternating magnetic field is applied within an interrogation zone and the presence of a high-permeability low coercive force ferromagnetic marker within the zone is detected based on signals produced by the marker in response to the applied field. In particular, the present invention relates to such systems in which the marker includes both a high-permeability, low coercive force portion and at least one magnetizable section having a higher coercive force, and which when magnetized alters the detectable signal otherwise produced, and is directed to an apparatus for desensitizing such markers, i.e., for magnetizing the higher coercive force section.
That soft click is the detector turning on when you’re in it - saves power compared to leaving the system running at full power all the time.
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### Author:  [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)
#### Post date: [September 7, 2005, 7:01pm UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509/9 "2005-09-07T19:01:14Z")
Looks like Q.E.D.'s link is one page off. RF tags are one-shot - they’re literally burned out at the checkout counter and can’t be re-used. Not so useful for libraries - librarians would need to peel out the used tags and insert new ones each time a book came back in.
[One page further along](http://electronics.howstuffworks.com/anti-shoplifting-device4.htm) describes Tattle-Tape technology in much simpler words than that patent application. The benefit with Tattle-Tape is that they are re-usable forever. At least that’s the guarantee 3M puts on them.
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### Author:  [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)
#### Post date: [September 7, 2005, 7:05pm UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509/10 "2005-09-07T19:05:54Z")
> [@gotpasswords](#):
>
> RF tags are one-shot - they’re literally burned out at the checkout counter and can’t be re-used.
Only if the equipment to do is is present.
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### Author:  [@Mangetout](https://boards.straightdope.com/u/Mangetout)
#### Post date: [September 7, 2005, 7:39pm UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509/11 "2005-09-07T19:39:32Z")
At the libraries around here, they use passive RF tags that are energised/detected by antennae placed around the exit turnstile; when you check out books, the librarian takes them, checks them out, then hands them back to you after you’ve passed through the turnstile (or places them on a conveyor, so that you pick them up after passing through it - the conveyor doesn’t pass through the detection zone).
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### Author:  [@jackelope](https://boards.straightdope.com/u/jackelope)
#### Post date: [September 7, 2005, 9:41pm UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509/12 "2005-09-07T21:41:21Z")
[3m’s official Tattle-Tape™ page](http://products3.3m.com/catalog/us/en001/library/-/node_28M8KQDKVZbe/root_GST1T4S9TCgv/vroot_5N1266QBTNge/gvel_PZ5CVZTT0Pgl/theme_us_library_3_0/command_AbcPageHandler/output_html) .
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### Author:  [@Library\_Boy](https://boards.straightdope.com/u/Library_Boy)
#### Post date: [September 7, 2005, 10:18pm UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509/13 "2005-09-07T22:18:10Z")
Most libraries do use the tattle tape. Some use an RFID system, and even many that use tattle tape use the RFID on certain collections. The “gates” for these systems, however, are different, so if you run both you have to have two sets of gates.
The RFID system libraries use is a little different than the anti-theft RF systems. As Q.E.D.'s link shows, the RF systems are designed to ring when detected, and to disable the RF tag when the item is allowed to leave.
RFID systems actually place information about the item in the tag, which is read by the gate. So if you have a DVD, and you place a tag in it, you also code the DVD info (usually barcode) into the RFID system, so the system knows that tag 179 is on item 390047856987, for example. When the item is checked-out, the circulation system notifies the RFID system that item 390047856987 has been checked out. The RFID tag is not disabled. When you walk through the gate, the system reads tag 179 and checks the status of item 390047856987. If the status is checked-out, the gate lets you through. If the status is anything else, the alarm sounds.
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### Author:  [@Library\_Boy](https://boards.straightdope.com/u/Library_Boy)
#### Post date: [September 8, 2005, 2:56am UTC](https://boards.straightdope.com/t/how-do-those-library-entrance-things-work/320509/14 "2005-09-08T02:56:56Z")
By the way, there are some libraries that do use a passive RF system that has an activation/deactivation like the magnetic tattle tape, instead of a destruction device like stores. Because it works like tattle tape most libraries call it tattle tape. If a library is talking about RF systems these days, they are almost certainly talking about RFID.