http://demoroom.physics.ncsu.edu/html/demos/163.html http://www.vuw.ac.nz/scps-demos/Demos/Pira5K/EddyCurrentTubes/Aluminium.mp4 Something you can try yourself. Just let a Neodymium magnet slide down a metal tube or a length of angled metal and then let it slide down a plastic tube and see the difference.
This will of course not work with iron, the magnet will sit up there happily for centuries It has to be non-ferrous metal.