[theThinker (OP)]

Faraday's disk with single magnet, with copper disk stationery but magnet revolving. Is there an induced emf?

I have two sources that gives differing answers.

"The Faraday disc, also called the homopolar generator: a metal disk in the xy plane revolves at a constant rate about the z axis through its center. A uniform magnetic field points in the z direction. Between a brush on the axis and one on the rim of the disc a unidirectional potential difference is generated (without the commutators of conventional d-c generators). If, instead of the disc revolving, the field-producing magnets are caused to revolve about the z axis, then no inducing voltage is produced between the brushes. A flux cutting explanation here is not possible."

From the book Special Relativity by Albert Shadowitz, Dover Publications, 1968, pp. 125-126.

Unipolar Induction Revisited: New Experiments and the “Edge Effect” Theory
Francisco J. Müller
Physics Department, Florida International University.
http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6600969

"Fig. 1. Faraday’s 3-step experiment: (A) Copper disk rotating above magnet; (B) magnet and disk rotating together; (C) magnet rotating alone. In all three cases the induced current was the same in the external circuit"

Which answer is correct?  I recall a site saying it is the same whether it is the copper disk which is rotating or both magnets only rotating. I lost the link. Can someone gives some links.

[theThinker (OP)]

Sorry.

Confirmed from many sources now. Magnet rotating alone produces no induced emf; Muller was wrong.

[chris]

Have you got the data / references refuting the Muller paper?

[theThinker (OP)]

Have you got the data / references refuting the Muller paper?

1) Unipolar Experiments,
A. G. K ELLY
http://aflb.ensmp.fr/AFLB-291/aflb291p119.pdf
2) Wiki.
3) The youtube video I posted earlier shows clearly no induced emf when only the magnet rotates. This is not expected (thus the faraday paradox) as induction is expected with relative motion.

[theThinker (OP)]

Have you got the data / references refuting the Muller paper?

"Fig. 1. Faraday’s 3-step experiment: (A) Copper disk rotating above magnet; (B) magnet and disk rotating together; (C) magnet rotating alone. In all three cases the induced current was the same in the external circuit"

I just read Muller's actual article. Muller was ambiguous in the text of his Fig 1 of (C). What he meant here (magnet rotating alone) is very different - there is no copper disk! He is talking  actually about a Faraday generator which is very different where points of contacts are both at the very body of the rotating cylindrical magnet (axial center and side). In the Faraday disk, the contacts is always at the copper disk, center and the rim; when only the magnet below/above rotates, there is no induced emf in the disk.

[chris]

That's why I asked you about the data, because when I looked at the paper it didn't look like you were comparing apples with apples.

[evan_au]

So Case 4 could be simplified to a copper wire between the oscilloscope contacts (not a disk).
When the copper wire moves through a magnetic field, it generates a voltage in the wire.

In fact, because the voltage is being measured between two points, all cases could be implemented as a straight wire between the oscilloscope probes.