It is sensed by a magnetic stripe and a detector. There are a large number of books in the library, which will encounter problems such as stolen books, long queue for borrowing and returning books, and inability to count books quickly and accurately. The library has studied the anti-theft system, which mainly adopts the "magnetic stripe+detector" system, and the principle is magnetic induction. Books are all equipped with magnetic strips, which are degaussed on the degaussing machine when borrowing, so that the alarm will not be triggered when the access control is taken out. When we asked to return the book, the librarian brushed the magnetized book to restore the magnetism of the magnetic stripe.