Fridge magnet

A refrigerator magnet or fridge magnet is an ornament, often whimsical, attached to a small magnet, which is used to post items such as shopping lists, child art or reminders on a refrigerator door, or which simply serves as decoration. Refrigerator magnets come in a wide variety of shapes and sizes, and may have promotional messages placed on them. Refrigerator magnets are popular souvenir and collectible objects.

The first fridge magnets were cylindrical or solid rectangular magnets. Later, a flexible magnet was developed, composed of a high-coercivity ferromagnetic compound (usually ferric oxide) mixed with a plastic binder. This is extruded as a sheet and passes on a conveyor belt over a line of powerful cylindrical permanent magnets. These magnets are arranged in a stack with alternating magnetic poles facing up (N, S, N, S,...) on a freely rotating shaft. This impresses the plastic sheet with the magnetic poles in an alternating line format. No electromagnetism is used to generate the magnets. The pole-pole distance is on the order of 5 mm, but varies with manufacturer. Ferrite magnets are commonly used, too, with decorative elements attached to the magnets with adhesive. They were created in the 1920s.

Unlike most conventional magnets that have distinct north and south poles, flat refrigerator magnets are magnetized during manufacture with alternating north and south poles on the refrigerator side. This can be felt by taking two similar (or identical) refrigerator magnets and sliding them against each other with the "magnetic" sides facing each other: the magnets will alternately repel and attract as they are moved a few millimeters. One can note that magnetic field outside a uniformly magnetized thin sheet is actually zero, neglecting the edge effects (see, for instance, D. Budker and A. Sushkov, Physics in Your Feet", OUP, 2015), so a uniformly magnetized magnet does not stick to a refrigerator. Most magnets have a special, slightly more sophisticated magnetization pattern called a Halbach array. This construction gives enhanced magnetic field on one side and almost zero magnetic field on the other.

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