CMS Magnetics® 4" x 6" Green Magnetic Field Viewing Film, Fun!
Encased within the green film are millions of magnetic particles suspended in oil. The viewing film will become lighter or darker when a magnetic field is applied. It turns dark when the field lines are perpendicular to the surface, and it turns light when the field lines are horizontal, i.e., across the surface. If a magnet is placed under it, with one of the poles facing the film, it will show the magnet as dark with a light outline.   The film is highly flexible and can be easily wrapped around objects.