A Suggested Difference Between Non-conducting and Conducting Disc-shaped Magnets
Fig. 1 depicts an uncharged current loop. A circle of negative line charge is at rest, and an equal, overlaid positive charge circulates CCW. E=0 everywhere.
Uncharged Current Loop
If the loop moves to the right, then l+ on its bottom will be greater than l+ on its top. But l- will be the same on both arcs. Thus the moving loop will have a dipolar electric field, quite as Maxwell’s equations suggest.
If a permanent, uncharged magnet is modeled as an array of such microscopic current loops, then a spinning, disc-shaped ceramic magnet can be expected to have net radial E field components above and below the disc (even when the spin axis is fixed). The net E field is electrostatic and conservative. As discussed elsewhere, this field plays a role in the nonzero emf in the case of a Faraday disc when the magnet is non-conducting.
But what if the disc-shaped magnet is conducting? We again expect the electric polarization of each microscopic current loop when the magnet spins. But now conduction electrons flow so as to cancel this field at all points within the magnet. Since any motion-induced E field is conservative, this implies that E will equal zero everywhere.
Here, then, is a key difference between non-conducting and conducting magnets. Non-conducting, spinning magnets can be expected to have a nonzero E field at points both within and outside of the magnet. Conducting, spinning magnets should have E=0 everywhere.
This theoretical result should be testable using a charged pith ball attached to a sensitive dynamometer (which is itself not affected by ambient E fields). Fig. 2 illustrates. In the case of a non-conducting (e.g. ceramic) magnet the test charge should experience a radial force when the magnet spins. In the case of a conducting magnet there should be no force.
Measuring a Radial E Field Component
Beginning with the Faraday disc, many experiments with spinning, permanent, cylindrical magnets have been performed. And the experimental results have often been controversial. Virtually all of the experimenters did not (a) model a permanent magnet as an array of microscopic current loops, nor (b) take into account the electric polarization of such loops when they move. But as discussed elsewhere, such considerations may play a key role in explaining experimental results.
For example, in the case of the original Faraday experiment, Faraday measured a nonzero emf when disc and magnet were spun in tandem. In the previous article the "active element" would be expected to be the closing circuit if the magnet is non-conducting (with a conservative E field). But owing to unbalanced magnetic forces in the conducting disc, the active element would be the conducting disc when a spinning, permanent magnet is employed.
To the extent this difference between non-conducting and conducting magnets is verifiable (see Fig. 2), prudence suggests that all of the experiments, conducted over the years, be revisited with the above ideas in mind. Doing so might clear up much of the controversy.