Lift both discs upward and drop them freely.
Observe the speed at which they are moving.
How does it work
One of the discs is a magnet, the other is made of aluminum. The only force acting on the aluminum disc is gravitational force – that is why it drops quickly. Unlike happens with the magnet. Magnetic field of the moving magnet induces so called eddy currents in the copper pipe. They, in turn, are the source of magnetic field with the field lines directed oppositely to the lines of the disc’s field. A phenomenon which occurs here is similar to the repulsion of two magnets approached to each other with like poles.
The induction of eddy currents by oscillating magnetic field is used in so called induction cookers.