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A 19th–century theorem of basic physics states that a static magnetic field cannot be arranged so as to levitate a magnet stably. Fiddle with a bunch of refrigerator magnets for as long as you like, and you'll never get one to hover in the air; position–sensing of the payload and active control of the supporting field is normally required for such levitation to work. But it turns out that one can stabilize magnetic levitation passively and automatically using diamagnetic materials. What is more, a diamagnetic material can itself be made to float stably in a properly arranged magnetic field. The principle of diamagnetic levitation, which has been known since the 1930s, is just now finding application in practical devices, such as precision sensors and small–scale transport systems.