Magnetic field viewing 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, and most of the area of the pole appears dark.
If two cubical magnets are placed next to each other, poles facing up and down, and arranged to attract each other, their poles are dark, but a thin line between the poles is bright where the flakes reflect light.
Commonly-available 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.
See also 
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