# Reactivating Library Books
**URL:** https://boards.straightdope.com/t/reactivating-library-books/440991
**Category:** Factual Questions
**Created:** [March 11, 2008, 6:01pm UTC](https://boards.straightdope.com/t/reactivating-library-books/440991 "2008-03-11T18:01:17Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@utterlyuseless](https://boards.straightdope.com/u/utterlyuseless)
#### Post date: [March 11, 2008, 6:01pm UTC](https://boards.straightdope.com/t/reactivating-library-books/440991/1 "2008-03-11T18:01:17Z")
When I go to the library, they put the book under a scanner and that records the book, then they rub the spine of the book on some flat metal thing. I guess that deactivates the book so you can go through the gate
My question is how do they reactivate the book so that it’ll go off, do they just rub it on that metal thing again?
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### Author:  [@thirdname](https://boards.straightdope.com/u/thirdname)
#### Post date: [March 11, 2008, 6:09pm UTC](https://boards.straightdope.com/t/reactivating-library-books/440991/2 "2008-03-11T18:09:08Z")
They just flip a switch and pass it over the same device again.
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### Author:  [@NinjaChick](https://boards.straightdope.com/u/NinjaChick)
#### Post date: [March 11, 2008, 7:52pm UTC](https://boards.straightdope.com/t/reactivating-library-books/440991/3 "2008-03-11T19:52:10Z")
Some places are starting to phase in RFID tags in books. Most libraries, however, use ‘tattle strips’. They’re little electromagnetically sensitive strips that, if not desensitize, will trigger the gates at the door. For books, there’s a machine that has a ‘sensitize/desensitize’ switch. I don’t know exactly how it works, but it does. When books are returned, you just make sure that it’s on ‘sensitize’, and swipe the spine of the book.
Magnetically sensitive media (VHS and audio cassettes) are a pain in the ass. The normal method would erase the tape, so there’s a separate sensitizer and desensitizer, which you have to rub the tape on. It works…by magic, or something, and makes our unabridged version book-on-tape version of _War and Peace_ a staggering pain.
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### Author:  [@Giles](https://boards.straightdope.com/u/Giles)
#### Post date: [March 11, 2008, 7:59pm UTC](https://boards.straightdope.com/t/reactivating-library-books/440991/4 "2008-03-11T19:59:21Z")
It’s a metal strip that’s hidden in the book, and I think gets magnetised or demagnetised, as appropriate. So you don’t put them on magentic media like floppy discs, audiocassettes or videocassettes.
And some users seem to think it’s in the bar code, so they tear the bar code out of the book (causing potential problems in identifyiong a copy of a book), but it’s not in the bar-code label, so don’t bother with that if you want to steal a library book.
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### Author:  [@Waenara](https://boards.straightdope.com/u/Waenara)
#### Post date: [March 11, 2008, 8:15pm UTC](https://boards.straightdope.com/t/reactivating-library-books/440991/5 "2008-03-11T20:15:29Z")
I found [this article on HowStuffWorks](http://electronics.howstuffworks.com/anti-shoplifting-device4.htm) that explains how electromagnetic anti-shoplifting devices work (the same as those used by many libraries).
> [@](#):
>
> What most people refer to as an electromagnetic tag is actually a metal wire or ribbon that has high permeability, making it easy for magnetic signals to flow through it, according to Sensormatic’s EAS Product Co. CTO Hap Patterson. “When we drive the tag, flux is being allowed to flow through the tag until it’s saturated,” he says. "When it’s saturated, from a magnetic perspective, it begins to look like air. Saturation occurs abruptly and is an important part of the design of the tag.
>
> Look at the figure showing the EM system with its receive coil and transmitter on either side and tag in the middle. When the tag goes from active to saturated, the receiver detects the change in the amount of the signal picked up from the transmitter. “If you look at the receiver signal, you’ll see a bump when saturation occurs,” Patterson says. Saturation occurs twice each cycle-once on the transmitter’s positive cycle and once on its negative cycle. What is happening is the system is checking for the special material used to make the tag. (In scientific terms, the permeability of steel is much lower than the metal used to make the tag. In addition, when steel goes to saturation, it tends to do so slowly, not abruptly. So the EM system uses these differences to differentiate between a still-active tagged item leaving the store and a wrench in someone’s pocket.)
>
> A magnetized piece of semi-hard magnetic material (basically, a weak magnet) is put up next to the active material to deactivate it. When you magnetize the semi-hard material, it saturates the tag and puts it in its inactive saturated state.
>
> That same kind of tag is often used in the library, where it can be reactivated by demagnetizing the semi-hard magnetic material.
I think my local library has started to use RFID tags. I used to use the library alot about a year ago, and they had the usual magnetic scanner. The librarian (or yourself) would scan the barcode with a UPC reader and then slide the book over the demagnetizer.
When I went again last month they had a new self check-out stand. I had to scan my library card under the UPC reader, and then the computer instantly recognized the books that I had laid on the counter (without scanning their UPCs), and I didn’t need to demagnetize the books.
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### Author:  [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)
#### Post date: [March 11, 2008, 8:17pm UTC](https://boards.straightdope.com/t/reactivating-library-books/440991/6 "2008-03-11T20:17:06Z")
They reverse the polarity…