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There exists a class of materials called diamagnets which exhibit
some interesting properties when an external magnetic field is applied.
In these materials, eddy currents consisting of circulating
electrons are induced whose magnetic effects
are such as to cancel part of the applied external field (typically about
0.1%). A metal detector is a device which relies on this property.
Diamagnets and superconductors
Metal detector / magnetic levitation
In this device, a magnetic field is generated from an electromagnet,
which causes eddy currents to be produced. The magnetic fields from the
induced currents are in turn picked up by the detector in the form of
small currents being produced. Most diamagnetic materials are metals,
which have good electrical conductivity properties and so the eddy
currents can be relatively easily established. This is the
reason these detectors can readily sense metallic objects but not
plastics or other poor conductors.