But airports are usually looking for something larger, like weapons. The other kind of metal detector is the kind you find in the hands of modern-day treasure hunters on the weekends. The Spectrum XLT from White's Electronics is the latest design in high-tech metal detectors. According to White's chief Spectrum engineer, Mark Rowan, the principal behind these kinds of detectors is similar to the ones you'll find out at the airport. Inside the search head is a transmitter coil.
Electric current is driven through the coil, creating an electromagnetic field.
Flip it up, and the direction the current travels goes from clockwise to counterclockwise thousands of times per second. When it goes in one direction, the polarity points down. When it goes the other direction, the polarity points up. So, when you run across metal, like a quarter, the polarity is changed and the receiving coil, which is also inside the head, tells you you've got some metal. The same principle of discrimination that could tell the difference between car keys and a chainsaw at the airport is applied to detectors like these.