suspend upper magnet on a long string
for this hard-to-visualize and even harder-to-believe result is in Richard Feynman’s The Feynman Lectures on Physics volume II, section 13.10. A well documented and peer reviewed experiment you can run yourself is found in the fine American Journal of Physics, volume 63 #3 for March of 1995. As Valverde’s The Principal of Relativity as Applied to Motional Electromagnetic Induction.
All the nasty math involved lies in a Richard Becker’s Electromagnetic Fields and Interactions. Dover 1982, sectons 87 and 88. 87 is background and 88 is on unipolar induction itself.
It took well over a hundred years to sort this one out.
rotating lower magnet should not rotate upper one
Figure three shows you a simple experiment. Hang a magnet vertically on a string. Position another vertical magnet below the first one and rotate it around the string axis. Your upper magnet should not rotate. It should only get attracted or repelled by the bottom one. But not spun.
Which strongly suggests that any magnetic "lines" do not move with a magnet’s rotation.
Your easiest access to Faraday’s original papers are in the Britannica Great Books series, volume 45. One quite readable book about Faraday is a Cantor, Gooding, and James’ text titled Michael Faraday.
Fig. 3 – THE LAWS OF GENERAL RELATIVITY demand that you cannot tell if a uniform magnetic field is rotating or translating. Rotate the bottom magnet about the string axis and the top magnet should remain stationary.
The Faraday disk output seems to depend only upon the strength of the input magnetic field and the relative speeds of the disk rotor and the slip ring and meter stator. The speed or direction of the magnets do not seem to matter in the least!
out of band harmonic spec, to lower distortion, or to discourage adding an illegal external power booster.
There’s a small user’s manual that comes with the modulator. The more detailed service manual (having full schematics and updates) is available as a special order for $7 or so. Ask your RS dealer for details.
Some enigmas are more enigmatic than others
It turns out the odd behavior of the Faraday disk is even stranger than I thought it was. In MUSE117.PDF, we looked at homopolar generators. The Faraday Disk is one variation on the homopolar generator where the input magnetic field is also rotated.
The key question is this: When you have any perfectly uniform magnetic field, is there any way you can tell that the field is rotating?
Well, a classicist would say "Of course you could tell if the field is spinning. The lines of force are busy cutting conductors and are inducing voltage." A relativist would instead say "There is no way to tell. There are no such things as magnetic lines; a uniform field is in fact uniform. Further, the laws of general relativity demand field motion independence, especially at high speeds."
Well, as far as we know today, the relativists are correct: You cannot tell if a perfectly uniform magnetic field is rotating or translating. One source
Apparently, the E field and the H field cannot stand alone. They are both an essential part of your result. Thus, any motional energy seemingly missing from the one component gets made up by the other. You’ll have to consider both E and H together any time there is any relative reference motion! Every time.
I’ve gathered a bunch of additional key books and papers about Faraday Disks in the sidebar.
Since it apparently does not matter whether the magnetic field rotates, there’s probably not any point at all in purposely spinning your magnets. At least in machines of this type.
Rotating magnets would add to the mass, windage, and dynamic braking,
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