ELI5: 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?

Read the Story

Show Top Comments

> 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”


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.


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**.


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.


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.