Ever tripped a shoplifting alarm? There's a fairly exact science behind why those alarms can pick up the little magnetic anti-theft strips you usually find on CD cases, electronics, or expensive articles of clothing.
Here's a photo in case you're not sure what I'm referring to:
As this video from Applied Sciences shows, those strips, called acousto-magnetic strips, contain two pieces of metal in them, one magnetized, the other a solid piece of iron-nickel alloy. When you apply a radio frequency to them, as store alarms do at 58 kHz frequencies, one of those strips oscillates, or "buzzes" much like a tiny tuning fork. But it also gets a little more complicated than that.
The other strip, which is magnetized when activated, amplifies the vibrations of the first strip in such a way that detectors can catch onto it and sound the alarm. When you demagnetize it, the frequency of the "buzzing" coming off the non-magnetized strip is far too low to be detected by those alarms, which is why store clerks have to run the strip through a strong magnet to deactivate them when you leave.
Watch on for more details on more fascinating details on the materials science behind it, and how such this cheap system of theft prevention works its magic.