Empty your pockets and step through, please. We’re going to get an inside look at metal detectors, today on Engineering Works!
More and more places are guarded by metal detectors these days – airports, courthouses; even some schools. Here’s how that frame in the doorway finds that pocketful of change you forgot to put into the tray.
It starts with electricity and a coil of wire. You might remember from your high school physics that when you run an electric current through a coil, you get a magnetic field. Turn it off and the magnetic field goes away.
In the coils in airport metal detectors, the electricity comes in very short, very powerful pulses, usually about a hundred every second. When one of these pulses ends, you get an electrical spike, or sudden jump in the electricity. Then another pulse zaps through the coil. It’s called pulse induction, or PI.
Special instruments measure how much time passes between these pulses. As long as the time between them stays the same, nothing happens. But when you walk through this magnetic field with something metal, like a handful of change or a gun in your pocket, it makes a little magnetic field of its own. This magnetic field slows down the pulses in the metal detector enough for the instruments to see the difference. And an alarm goes off. Everybody knows what happens then.
Time to go. Aw, you forgot about your pen!