Book burglar alarm or book burglar alarm! It has something to do with bar codes, which are magnetic. In addition to looking at the price, it is also important for the salesperson to degauss the barcode! The principle of book anti-theft is the principle of applying electromagnetic wave, induction and being induced. Just like a supermarket. PS: The landlord has never seen the alarm in the library voice. You can try it next time you go to the supermarket ~ ~ ~ Of course, you have to figure out how to explain it to the security guard! Haha O(∩_∩)O~~