Magnetic film

Magnetic field viewing film is used to show stationary or (less often) slowly changing magnetic fields; it shows their location and direction. It is a translucent thin flexible sheet, coated with micro-capsules containing nickel flakes suspended in oil. When magnetic lines of force are parallel to the surface of the carrier sheet, the surfaces of the flakes are reflective, and appear bright. When lines of force are perpendicular to the sheet, the flakes are edge-on, and appear significantly darker. When the film is placed on a magnet's pole, the latter case applies.

Commonly available Magne-rite® film is green, although a translucent pale blue variety is also available.

Magnetic viewing film is sometimes used by hobbyists and electronics enthusiasts to examine commercial products. For example, iFixIt, a site that publishes "teardowns" (or deconstructions) of new technology, used magnetic-field viewing film in its teardown of the iPad 2 Smart Cover. The film revealed the magnet array inside the cover.

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Wikipedia