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?

Question

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

Answer 1

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 😌