Eddy currents are currents induced in the body of a conductor due to change in magnetic flux linked with the conductor.
When a steady current is passed through the coil of a galvanometer it is deflected. Normally, the coil oscillates about its equilibrium position for some time before coming to rest. To avoid delay due to these oscillations, the coil is wound over a metallic frame. As the coil is deflected, eddy currents set up in the metallic frame oppose its motion. Eddy currents are used in controlling the speed of electric trains. A strong magnetic field is applied to a metallic drum rotating with the axle connecting the wheels. Large eddy currents set up in the rotating drum oppose the motion of the drum and tend to stop the train. An induction motor or AC motor is also based on application of eddy currents. Dynamo is not based on application of eddy currents.