Part of the show Naked Science Q&A and Polonium Poisoning
Catherine in Texas asked:
Can magnetic anti-shoplifting strips get reactivated after your purchase, causing alarms to go off? How do they work?
The way most of these work is by having a tiny little electric circuit in it. They've got an aerial around the outside of that, and when you walk past one of those detectors, that puts radio frequency energy through you. This powers up the circuit. The little circuit sends a code back to the detector and says "Help, help I'm being stolen" and then the alarm goes off. There are also the solid ones you find in clothes, which are attached, and the reason you might think they're magnetic is the way you deactivate them by using a magnet which allows them to detach without spreading dye all over your nice new clothes. However, the tag will still activate the alarm. The ones which are actually stuck to the things you're buying are deactivated in a slightly different way. What they do is they have a little an electronic thing which puts in such a huge amount of power that it basically melts something in the circuit and causes it to break.