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Léon Foucault, a French physicist much better known for his pendulum demonstrating the rotation of the Earth, also created in 1855 a device that illustrated how eddy currents work. Eddy currents are the magnetic fields induced when a solid conductor (a material in which electrons travel easily) moves within an applied magnetic field. Foucault himself discovered this phenomenon, which is why they are sometimes called Foucault currents.
Foucault built the machine below to investigate and illustrate the effects of these eddy currents, so named because their motion brings to mind eddies caused by disturbances to a flow of water.
The main features of Foucault’s disk are the battery-powered electromagnet and the hand-cranked copper disk. Copper is chosen for the disk because it is a good conductor. Before you begin operating the tutorial, note that the north pole of the electromagnet is the one closer to you, the viewer. So the field of the magnet flows from that north pole to its south pole, partially obscured here by the disk.
To operate the tutorial, click the blue turn on button to power the electromagnet. Then use the torque slider to make the disk turn; the further to the right you take the slider, the faster the disk will turn.
Notice that the disc temperature begins to increase; also, the faster the disc turns, the faster the temperature rises. (The temperatures shown here, and the speed with which they are reached, are intended to communicate the general idea of what eddy currents do, rather than represent those effects precisely.) If you hit the red turn off button to disable the electromagnet while continuing to crank the disk, the disk will begin to cool down again.
This heat is the result of the eddy currents at work. These circular electron currents are induced inside the copper disk, and their direction opposes the direction in which the disk is turning. That resistance results in heat. It also has a braking effect: If you were actually (as opposed to virtually) turning this crank, you would need to turn it harder to maintain the same speed. This phenonemon has been exploited in train brakes and other practical uses.
Foucault’s eddy currents are a manifestation of Lenz’s Law, devised by Russian physicist Heinrich Lenz in 1834, which states that an induced electromotive force generates a current that induces a counter magnetic field that opposes the magnetic field generating the current.