How do those walk-through scanners in libraries know if you’re taking a book without borrowing it, especially if those books don’t have metal inserts or anything like that?


How does the library borrowing system work? and how come even if there is no obvious metal insert in the book those gates still know when and when not to beep if the book has or hasn’t been borrowed?

In: Technology


A lot of books have the metal inserts in their spine or in their cover where you can not see them. But it is not unlikely that the scanners are unable to pick up on every book, just the most valuable ones which have the metal inserts. The way these work is tha the metal inserts are in a perfect length to create a magnetic resonance that precisely matches that of the scanner. So when you walk through the scanner the magenetic field makes the metal inserts resonate which can be detected by the scanner. However during the checkout they use a magnet to change the magnetic characteristics of the metal insert and therefore its resonance frequency. This prevents it from triggering the scanners. This is also a reversable action so they can reset the books when you return them.

> especially if those books don’t have metal inserts or anything like that?

Because it DOES have metal inserts. They are called RFID chips, and they are very small and very cheap. You can even buy them. I suggest google image: “rfid chips label and tags”

The most well-known anti-theft device for books is a tape of magnetic metal strip. The tapes are applied between the spine and the binding of a book (for hard covers) or deep inside in between some pages (for paperbacks). It’s called **tattle-tape**.

Oh, they have them. You just aren’t seeing them easily, or they’re outright hidden.

There’s a few ways they can mark the books. A common one is a microchip. However, they can also use a sticker (sometimes looks like a circuit) or even slide strips into the spine or under a book jacket without them being visible. These then can respond to radio or magnetic signals from the scanners, and they have some kind of code read by this device that says what the book is. The strip may also be able to be programmed to say if it is checked out or not; otherwise, the code will need to be compared to the code in the system and the status listed there. If the code says it isn’t checked out, then it sends an alert.


My library has RFID tags (the stickers usually attached inside the cover) that uniquely identify all items. When you check it out the system then knows that specific item is borrowed and therefore to ignore it when it goes past the scanners.

I work in a fairly poor urban library and we do use RFID chips, which are great for self checkin and checkout. They’re on everything, including wee DVDs.

Every few years we have a run on toilet paper (in one series of instances, a lady would bring a bunch of kids in a couple times a week and they would break the paper holders and conceal massive amounts in the kids’ backpacks). A bunch of RFID tags inside the TP rolls, and you’ve found the culprits.

Moral: Don’t steal from libraries. We’re here to help you borrow things for free.

Can you rephrase the question again please?

Do you know how if you hold a tuning fork near your mouth and sing the right note, the fork will sing back? The books contain very thin “tuning forks” that look like stickers. The scanner you walk through emits a tone that causes any near by “tuning forks” to ring at that same frequency. The scanner then listens for the resonating forks and sounds an alarm if it hears any. You can’t actually hear the frequency though because it is done with radio frequencies.

Edit: It occurred to me that I didn’t actually answer ops question. Basically when you check in or out the book it runs over a large magnet what wacks the tuning fork into and out of tune.

In highschool i took the security tags out of a few xboxgames i got at the bargain bin and i stuck them under the cart for the projector, and for a few weeks every time someone rented a projector the alarms would go off

I think they have some sort of magnet, I used to work in my high school library and before we checked books out to other students we had to rub the binding against some metal… demagnetizing block? Thing?

Edit: after reading the comments I finally understand what I was even doing, 9 years after the fact 😌

Answer: They actually have the metal inserts you think aren’t there. They are usually concealed as a small stick w the magnetic strip on it in the spine of the dust jacket, or under an extra adhered sheet in the front or back of the book.

Probably the same as retail stores sticking their security tags on things. The tags themselves look like barcodes but underneath them there is a magnetic strip. They probably dont have barcode like tags but the idea is the same.

What are you talking about? I’ve literally never seen metal detectors at a library

Companies actually also sell dummy sets of security gates, just for the deterrent effect. Give it a try!

I used to work at a library for many years. One of my jobs was to put this tape along the inner binding that was magnetized. When a patron brought the books to the counter to check out we would rub them on this metal plate to demagnetize them.
When bringing them back in we’d run a magnet along the binding to reactive it.

I work at a city library. They are RFID tags that get deactivated at checkout. Much like retail stores. Just an adhesive programmable RFID tag.

The piece of metal at one time was hidden behind the punch-card-pocket inside the cover. So that there was only one commissioning step, install the self-adhesive pouch and done.

I have not been to a library in a long time I’m sure there are various systems now.

We call it tattle tape. A tiny strip that goes in between pages and so unobtrusive you would never know it is there. Of course, what sets off ours might not work at another library depending on the frequency.