Eddy's Tube
This demonstration illustrates what is known asdissipative magnetic breaking via induced Eddy currents. It consistsof a neodymium magnet falling inside a metallic tube.
In this configuration, breaking occurs because of the changein the magnetic flux throughout the cross section of the tube. This change induces electrical currents around loops on the circumference of the tube. The direction of the current in those loops is determined by Lens' law. The result is that segments above and bellow the falling magnet behave like magnets themselves, but with opposite polarity: the segments below with the same polarity as the falling magnet and segments above, with the opposite.
In order to make it clear that it is the interaction between the magnetand the tube that produces the breaking, it is worth to compare the falling rate through the tube between a penny and the magnet. If you like, you can use a stop watch, but it is not necessary --the difference is quite noticeable.