3x4" magnetic viewing film in rigid acrylic sleeve. Use it in the sleeve or take it out to wrap around curved magnets. This green sheet is used to see the invisible force fields of magnets. It is made of nickel flakes floating in tiny fluid-filled capsules. Nickel is one of only three elements that are naturally ferromagnetic. Since these nickel particles can move freely within the thin layer, they are influenced by nearby magnetic fields and orient themselves accordingly. When placed directly over a magnet, the light areas show where the magnetic field is parallel to the sheet, so the flat sides of the nickel particles are reflecting light.