Metal detectors use electricity to generate a magnetic field—an invisible force field created by moving electric charges. As the detector sweeps back and forth over the ground, the magnetic field moves too. It causes any metal object it encounters to emit its own magnetic field in response, which the detector senses. “It’s like it senses a disturbance in the force,” says Brian Jones, a physicist at Colorado State University.
Metal detecting is a popular hobby in Britain. In the past two decades, museums there have acquired more than 5,000 artifacts discovered by members of the public like Milly.