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 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.