The National MagLab is funded by the National Science Foundation and the State of Florida.

# Foucault's Disk

In 1855, a French physicist created 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. Their motion brings to mind eddies caused by disturbances to a flow of water.

French physicist Leon Foucault discovered the phenomenon and in 1855 created a device to illustrate it, what became known as Foucalt’s disk.

## Instructions

1. Observe the set up of the device. A battery-powered electromagnet is wrapped around one side of a copper disk. The magnet’s north pole, in red, is on the front side of the disk. The magnetic field flows from that to the south pole, behind the disk.
2. Use the button to turn on the electromagnet. Notice that the disk slows down and its temperature rises.
3. Now use the slider to control the crank speed and the torque of the disk. You’ll see that the faster the disk turns, the faster the temperature rises.
4. Hit the off button to disable the electromagnet, and notice how the the disk will move even faster and begin to cool down.

The heat when the electromagnet is turned on is the result of the eddy currents. 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. This is why the disk turns faster after the electromagnet is off. If you were actually turning the crank, you would need to turn it harder while the magnet is on to maintain the same speed. This phenonemon has been harnessed in train brakes and other uses.