`== Comments for the explanation table == I think the explanation table is somewhat misleading, when it terms, that it if both disk and indicator are moving the result is undetermined. It is surely not undetermined. Like some strange explanation also tries to say: The voltage is ``undetermined`` when both the indicator and the disk are rotated, regardless of whether the magnet is moving. There is an EMF and a nonuniform charge density, but no reaction of the indicator.[3] An electric field is generated, but no voltage is brought out for display. This differs from the different relative motions of the parts of a rectilinear homopolar generator (where the answer would be ``No``). I think this is explanation is unnecessarily complex. Simply stated:  If the magnet rotates or not doesn't make any difference. If both indicator and disk rotate only the relative difference in rotation is relevant for the voltage seen on the indicator, which means if both rotate at the same speed the indicator doesn't see any voltage. The reason for this is simple, if both rotate at the same speed, then also the wires of the indicator circuit will develop the same voltage as the disk itself (the wire behaves the same as a second disk), therefore the indicator wouldn't see any voltage difference. Peace      `