"Why would you want to detect metal? Oh, I don't know ... maybe you want to find some gold in the ground. You could dig up ALL the dirt, or you could find the location that has the gold before you dig. Or maybe you're looking for buried metallic meteorites. You could even use a metal detector to find that ring you lost at the beach. These devices are quite useful."
"The simplest way is to just apply a charge on the surface of a metal object by adding some electrons to it-this is what a battery does. Obviously that won't work for our purposes, though. You'd need access to the metal before you find it, which makes no sense. But there's another way to go. It turns out that a changing magnetic field also makes an electric field. This is the basic idea of Faraday's law."
Metal detectors exploit the conductive nature of metals, whose outer electrons are delocalized and allow electrical current to flow. Applying a static charge to find metal is impractical because physical access is required. A changing magnetic field generates an electric field (Faraday's law), and that electric field induces eddy currents in nearby conductors. Those induced currents produce their own magnetic fields. Metal-detection systems create changing magnetic fields and sense the magnetic response from induced currents to identify and locate metallic objects for uses like prospecting, meteorite hunting, and recovering lost items.
Read at WIRED
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