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

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.