Facoult, in 1895, discoverd that when a solid mass of metal is placed in a changing magnetic field, (or moved in a magnetic field), causing a change in the magnetic flux linked with it, induced currents are set up throughout the volume of the metal. These currents are known as ‘eddy currents’ or ‘facault currents’. The direction of circulation of these currents is such as to oppose the motion of the metal or change in magnetic flux (in accordance with Lenz’s law).
The resistance of thick metal being very low, the eddy currents are generally quite large in magnitude and produce considerable heating effect in accordance with ‘Joule’s heating’ law. This heating effect of eddy currents in undesirable in the interior of iron cores of rotating armatures of motors, dynamos and transformers etc. To minimize these currents, the cores are not taken as a single piece of soft iron but are made of many thin (insultated from each other) laminas of soft iron. This type of core is called a ‘laminated core’.
Eddy currents are, however, also used to advantage in certain applications. These include:
(i) Electromagnetic Damping : Certain galvanometers have a fixed core made of a non magnetic metallic material. When the coil oscillates, the eddy currents, generated in the core opposes the motion and bring the coil to rest quickly.
(ii) Induction Furance : The metal to be heated, or melted, is placed in a high frequency changing magnetic field. Strong eddy currents, induced in it, cause heating.
(iii) Magnetic Brakes in Trains : Strong electromagnets are kept above the rails in some electrically powered trains. When the electromagnets are activated the eddy currents, induced in the rails, opposes the motion of the train.
