What can iron filings sprinkled on a piece of paper demonstrate?

Iron filings on a piece of paper can be a really good way to visualize a magnetic field. The iron filings are small enough and are magnetically reactive enough to move in even really weak magnetic fields. Google "iron filings magnetic fields" for a bunch of cool pictures of what I'm talking about. Also anything with a bunch of small particles can show resonance patterns for different frequencies, though typically something like sand is preferred to iron filings for price reasons, but basically if you set the paper on a vibrating plate or near a speaker, you could see the iron filings take on the resonance pattern of the plate/speaker, this is a good image search as well.

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So what is the question