This article relies largely or entirely on a single source. (October 2019)
Arago's rotations is an observable magnetic phenomenon that involves the interactions between a magnetized needle and a moving metal disk. The effect was discovered by François Arago in 1824. At the time of their discovery, Arago's rotations were surprising effects that were difficult to explain. In 1831, Michael Faraday introduced the theory of electromagnetic induction, which explained how the effects happen in detail.
The first indication of interaction between a moving magnet and a metallic surface was discovered in 1824 by Gambey, a famous instrument-maker of Paris. He discovered that oscillating compass-needle sooner gets to still if a metallic surface is used as opposed to a wooden surface. Two other men, Marsh and Woolwich, observed the same phenomenon on a magnetic needle rotating on an iron sphere.
Arago first published an account of his observations in Académie des Sciences of Paris, on November 22, 1824. He tested the compass-needle with various metallic rings by swinging the needle by 45° and counting the number of oscillations until the needle dropped to 10° angle. The results were as following:
|Ring material||Number of oscillations|
Arago gave this phenomenon the name of magnetism of rotation. In 1825 was published a further experiment where he observed the reaction of stationary magnetic needle to rotating copper disc.
A magnetic needle is freely suspended on a pivot or string, a short distance above a copper disc. If the disk is stationary, the needle aligns itself with the Earth's magnetic field. If the disc is rotated in its own plane, the needle rotates in the same direction as the disc. (The effect decreases as the distance between the magnet and the disk increases.)
Relative motion of the conductor and the magnet induces eddy currents in the conductor, which produce a force or torque that opposes or resists relative motion, or tries to "couple" the objects. The same drag-like force is used in eddy current braking and magnetic damping.