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

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.