Magnetic Field Paper
As a kid I remember tapping iron filings on a piece of paper held over a magnet to see the magnetic field, this special magnetic field paper does the same job without the need of the filings.
Developed back in the 1970s, with ideas that it might become a replacement for paper, this paper makes magnetic fields visible. Any other magnet can be used, and the paper will allow to to make the magnetic field visible.
It consists of a thin plastic sheet containing tiny cells containing liquid. Each little cell contains colloidal iron. Hold a magnet near, and the iron in each of the cells will align, and form a pattern on the paper.
You can buy magnetic field paper from Grand Illusions for £4.99 (~$10) a sheet.