I think what you are detecting is the interaction when generating eddy currents in the metal by moving the magnet past it. Since this is electricity generation, it takes energy to do it, and you are detecting the force required as you pass your magnet over the metal. It should work with any metal. The interaction will be greater with stronger magnets, a closer pass to the metal with the magnet, and higher speed of the pass.

Unfortunately, it works with any conductive metal.

I still remember a college course demonstration of this. The had a pendulum made of aluminium sheet and a magnet at the bottom of the swing. The instructor pulled the pendulum up on one side of its swing and let it swing down past the magnet. It would stop dead once it hit the magnetic field.





Jon.