Eddy currents occur when a piece of metal is moved in a magnetic field
or when a piece of metal is penetrated by an alternating magnetic field. The resulting eddy current counteracts the movement of the metal. The eddy current effect is used in eddy current brakes, hotplates or electricity meters. In hotplates, the eddy current is used for inductive heating, in other devices for shielding high-frequency magnetic fields or, as in the case of eddy current sensors, for changing the impedance of coils
. From these changes, distances of metallic objects can be determined with nanometer accuracy. Eddy currents also occur in conductors and can displace the current there from the center of the conductor to the outer regions. Such a current displacement occurs in the skin effect or in the proximity effect.