Metal detector

A metal detector is an instrument that detects the nearby presence of metal. Metal detectors are useful for finding metal objects on the surface, underground, and under water. A metal detector typically consists of a control box, an adjustable shaft, and a variable-shaped pickup coil.

When the coil nears metal, the control box signals its presence with a tone, numerical reading, light, or needle movement.
Signal intensity typically increases with proximity or metal size and composition.
A common type are stationary "walk through" metal detectors used at access points in prisons, courthouses, airports and psychiatric hospitals to detect concealed metal weapons on a person's body.

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Source: Wikipedia · Image: Wikimedia Commons · Public domain