Back in the 70's my high school library installed a magnetic anti-theft system. I don't remember all of the details, but it involved a small metal strip in eac

Back in the 70's my high school library installed a magnetic anti-theft system. I don't remember all of the details, but it involved a small metal strip in each book that was demagnetized when books were checked out and remagnetized when they were returned and reshelved. It was possible to return a book from within the stacks. This made a neat hack possible: find a book with multiple copies, check one out (demagnetizing it), bring it back into the shelves, reshelve it with the library card from its mate, and then return the mate from within the stacks. There would then be a demagnetized copy sitting on the shelves. We were good kids, so we didn't steal anything, but it sure was fun to demonstrate to the staff that one could walk into the stacks, grab a book, and walk out right past the magnetic alarm without setting it off.