In general, supermarket alarms use the principle of cutting magnetic field similar to metal objects to generate weak current to make the alarm react, thus giving an alarm sound.

Ordinary merchants will not use high-cost magnetic bar codes, which has a great impact on their sales costs. The anti-theft magnetic stripe used in the library is more professional. I don't know if you have noticed that when you borrow books, you give them to the librarian and come out to get them, which proves that they are not degaussed at this time.