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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Arago's rotations | 3/3 | https://en.wikipedia.org/wiki/Arago's_rotations | reference | science, encyclopedia | 2026-05-05T10:54:35.884320+00:00 | kb-cron |
With the explanation given by Faraday of the Arago rotations, as being merely due to induced eddy-currents, the peculiar interest which they excited whilst their cause was unknown, seems almost to have died out. True, a few years later some interest was revived when Foucault showed that they were capable of heating the metal disk, if in spite of the drag the rotation was forcibly continued in the magnetic field. Why this observation should have caused the eddy-currents discovered by Faraday as the explanation of Arago's phenomenon to be dubbed Foucault's currents is not clear. If anyone is entitled to the honour of having the eddy-currents named after him, it is obviously Faraday or Arago, not Foucault. A little later, Le Roux produced the paradox that a copper disk rotated between concentric magnet poles is not heated thereby, and does not suffer any drag. The explanation of this is as follows. If there is an annular north pole in front of one face of the disk, and an annular south pole in front of the other face, though there is a magnetic field produced right through the disk, there are no eddy-currents. For if all round the disk there are equal electromotive-forces directed radially inwards or radially outwards, there will be no return path for the currents along any radius of the disk. The periphery will simply take a slightly different potential from the center; but no currents will flow because the electro-motive forces around any closed path in the disk are balanced.
=== Experiments with copper plate by other scientists === In 1884, Willoughby Smith published an investigation on rotating metal disks in which he found iron disks to generate electromotive-forces superior to those generated in copper disks of equal size. Guthrie and Boys in 1879 hung a copper plate over a rotating magnet by means of a torsion thread, and found that the torsion was directly proportional to the velocity of rotation. They pointed out that such an instrument was a very exact one for measuring the speed of machinery. They also made experiments upon varying the distance between the copper plate and the magnet, and varying the diameter and thickness of the copper disk. Experiments were made upon various metals, and the torque was found to vary as the conductivity of the metal as far as the latter could be judged after being rolled into the form of plate. Messrs. Guthrie and Boys then applied the method to the measurement of the conductivity of liquids. In 1880, De Fonvielle and Lontin observed that a lightly pivoted copper disk could be maintained in continuous rotation—if once started—by being placed, in presence of a magnet, within a coil of copper wire wound on a rectangular frame (like the coil of an old galvanometer), and supplied with alternate currents from an ordinary Ruhmkorff induction coil. They called their apparatus an electromagnetic gyroscope. It does not seem to have occurred to anyone that the Arago rotations could be made use of in the construction of a motor prior to 1879.
== Short description of Arago's rotations == 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.
== See also == Faraday disc Induction motor History of electromagnetic theory
== Further reading == Walter Baily, A Mode of producing Arago's Rotation. June 28, 1879. (Philosophical magazine: a journal of theoretical, experimental and applied physics. Taylor & Francis., 1879) Silvanus Phillips Thompson, Polyphase Electric Currents and Alternate-Current Motors. 1895.
== References == This article incorporates text from this source, which is in the public domain: Polyphase Electric Currents and Alternate-Current Motors. London, Sponn & Chamberlain. 1895.
== External links == Arago's rotations (YouTube video)