David Billington wrote:
Another good way to demo this is using a cylindrical magnet and drop it
through a copper tube like used for plumbing. Re your sorting method I
believe this is similar to how many vending machines validate that a
coin is what it should be rather than a counterfeit.

Actually, this demo works okay with just about any NIB magnets, even if
they aren't cylindrical. Give it a go.

Chris