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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Arago's rotations | 2/3 | https://en.wikipedia.org/wiki/Arago's_rotations | reference | science, encyclopedia | 2026-05-05T10:54:35.884320+00:00 | kb-cron |
=== Investigations of the phenomena by other scientists === At this stage the phenomenon was investigated by several English experimenters, by Babbage and Herschel, by Christie, and, later, by Sturgeon and by Faraday. Babbage and Herschel measured the amount of retarding force exerted on the needle by different materials, and found the most powerful to be silver and copper (which are the two best conductors of electricity), after them gold and zinc, whilst lead, mercury and bismuth were very inferior in power. In 1825 they announced the successful reversal of Arago's experiment; for by spinning the magnet underneath a pivoted copper disk (Fig. 2) they had caused the latter to rotate briskly. They also made the notable observation that slits cut radially in a copper disk (Fig. 3) diminished its tendency to be rotated by the spinning magnet. If the rotatory force of the unslit disk be reckoned as 100, one radial slit reduced it to 88, two radial slits to 77, four to 48, and eight to 24. Amperè, in 1826, showed that a rotating disk of copper also exercises a turning moment upon a neighbouring copper wire through which a current is flowing. Seebeck in Germany, Prévost and Colladon in Switzerland, Nobili and Bacelli in Italy, confirmed the observations of the English experimenters, and added others. Sturgeon showed that the damping effect of a magnet pole upon a moving copper disk was diminished by the presence of a second magnet pole of contrary kind placed beside the first. Five years later he returned to the subject and came to the conclusion that the effect was an electrical disturbance, "a kind of reaction to that which takes place in electro-magnetism," when the publication of Faraday's brilliant research on magneto-electric induction, in 1831, forestalled the complete explanation of which he was in search. Faraday, in fact, showed that relative motion between magnet and copper disk inevitably set up currents in the metal of the disk, which, in turn, reacted on the magnet pole with mutual forces tending to diminish the relative motion—that is, tending to drag the stationary part (whether magnet or disk) in the direction of the moving part, and tending always to oppose the motion of the moving part. In fact, the currents go eddying round in the moving disk, unless led off by sliding contacts.
=== Experiments on eddy-currents by Faraday and Matteuci === This, indeed, Faraday effected, when he inserted his copper disk edgeways (Fig. 4) between the poles of a powerful magnet, and spun it, while against edge and axle were pressed spring contacts to take off the currents. The electromotive-force, acting at right angles to the motion, and to the lines of the magnetic field, produces currents which flow along the radius of the disk. If no external path is provided, the currents must find for themselves internal return paths in the metal of the disk. Fig. 5 shows the way in which a pair of eddies is set up in a disk revolving between magnet poles. These eddies are symmetrically located on either side of the radius of maximum electromotive-force (Fig. 6). The direction of the circulation of eddy-currents is always such as to tend to oppose the relative motion. The eddy-current in the part receding from the poles tends to attract the poles forward or to drag this part of the disk backwards. The eddy-current in the part advancing toward the poles tends to repel those poles and to be repelled by them. It is obvious that any slits cut in the disk will tend to limit the flow of the eddy-currents, and by limiting them to increase the resistance of their possible paths in the metal, though it will not diminish the electromotive-force. In the researches of Sturgeon a number of experiments are described to ascertain the directions in which the eddy-currents flow in disks. Similar, but more complete researches were made by Matteuci. The induction in rotating spheres was mathematically investigated by Jochmann, and later by Hertz. Faraday showed several interesting experiments on eddy-currents. Amongst others he hung from a twisted thread a cube of copper in a direct line between the poles of a powerful electromagnet (Fig. 7). Before the current was turned on the cube, by its weight, untwisted the cord and spun rapidly. On exciting the magnet by switching on the current, the cube stops instantaneously; but begins again to spin as soon as the current is broken. Matteucci varied this experiment by constructing a cube of square bits of sheet copper separated by paper from one another. This laminated cube (Fig. 8) if suspended in the magnetic field by a hook a, so that its laminae were parallel to the lines of the magnetic field, could not be stopped in its rotation by the sudden turning on of the current in the electromagnet; whereas if hung up by the hook b, so that its laminations were in a vertical plane, and then set spinning, was arrested at once when the electromagnet was excited. In the latter case only could eddy-currents circulate; since they require paths in planes at right angles to the magnetic lines.