Magnetic field viewing film, which can be bought relatively cheaply, lets you 'see' a magnetic field as it moves. Below, we'll tell you about the simple way this film works, and show you a couple of demonstrations. See magnetic field lines!
This re-usable film can wrap around anything and shows you the shape and movement of a magnetic field. It's fun to play with, and as something fun to play with it ranks only slightly below the stuff that inspired it; bubble wrap. Yep, the same technology that winds around your breakables when you move brings you this. The film is like a very fine bubble wrap, with each tiny bubble filled with liquid. This is not regular water. The liquid inside the bubbles has a high viscosity - meaning it has a high resistance to changes in shape. It takes force to move the liquid around inside the bubble.
Inside the liquid are tiny metal rods. A single pole of the magnet, when underneath the film, will pull the rods so they're standing on end inside the film, and you're looking at them head on. This makes the film appear dark. When pulled by both poles of the magnet, the rods will lie flat on the film, and you'll see their sides. The film will appear bright. As long as the bubbles aren't burst by crushing or scratching the film, they'll keep moving as the magnet moves, letting you 'see' the shifting magnetic field of the moving magnet. And? It is very cool to watch.
Top Image: Alex0001
Via Evil Mad Scientist.