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Diamagnets and superconductors

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.
Figure 9.17: Metal detector / magnetic levitation
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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.