Eddy Pendulum
This demonstration illustrates what is known asdissipative magnetic breaking via induced Eddy currents. It consistsof a physical pendulum, composed of a wooden rod with a metallic slab attached to its swinging tip. As it swings, the tip passes between the poles of a large C magnet. In this configuration, breaking occurs because of the change in the magnetic flux though the area of the metal slab. This change induces currents around loops within the area of the metal. The direction of the current in those loops is determined by Lens' law. The result is that the metal slab looks like a magnet, whose polarity is opposite to that of the external magnet.
Although Eddy currents are not actually measured, their effect can beeasily seen by comparing the rate at which the pendulum looses energywhen there is a magnet present and when it is absent. However, in orderto convince the students that the invisible Eddy currents are responsiblefor dissipating the energy, we compare the magnetic breaking between ouroriginal pendulum and one whose slab is made by joining thin stripes ofmetal electrically insulated from each other. The insulation of thestripes prevents the formation of large Eddy loops, which in turns reducesthe magnetic breaking.