Magnetic Viewing Paper16
Hacked Gadgets reveals the hidden world of magnetization with magnetic viewing paper, Alan writes –

We can’t see magnetic flux lines but with an inexpensive piece of Magnetic Viewing Paper we can easily visualize what we are missing. I took the motor magnet out of an old 5 1/4 floppy drive and was amazed at the complex magnetic pattern that emerged. It looks like the outer ring of the magnet has magnetic notches that are read by the electronic circuit. I imagine this is used for speed control, although this seems like a very complex method of regulating motor speed. You can also “watch” a relay energize by holding the viewing paper beside a normal relay while energizing the coil. Thanks to Edmund Scientific for providing the cool magnet viewer.

Magnetic Viewing Paper – Link.