# Arago's rotations

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.

## History

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.[1]

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.[1] The results were as following:

Ring material Number of oscillations
wood 145
thin copper 66
stout copper 33

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.[1]

## Description

Arago's spinning disk.

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.)

Variations:

• If the disk is free to rotate with minimal friction, and the needle is rotated above or below it, the disk rotates in the same direction as the needle. (This is easier to observe or measure if the needle is a larger magnet.)
• If the needle is not allowed to rotate, its presence retards rotation of the disc. (This is easier to observe or measure if the needle is a larger magnet.)
• Other non-magnetic materials having electrical conductivity (non-ferrous metals such as silver, aluminum, or zinc) also produce the effect.
• Non-conductive non-magnetic materials (wood, glass, plastic, ice, etc.) do not produce the effect.

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.