The answer is (b): A positive voltage will be sensed by the voltmeter, as can be seen by clicking your mouse on the photograph below.
In this case there is a positive voltage of about +12mV sensed by the digital voltmeter, as can be seen in the video.
It appears that there is no current-carrying "wire" or other electrical charge moving through a magnetic field to create this voltage. On the other hand, if the magnetic field of the cylindrical magnet is perfectly symmetric, the free electrons in the metal cannot tell that the field is moving, and an electric potential might be induced. Faraday found this not to be so, in the Barlow's Disk (or Faraday's Disk) experiment, Question #209.
Another way of explaining this result, perhaps controversial, involves application of special relativity. Basically the argument goes as follows: any equivalent electron current rotating around the magnetic domains of the magnetic rod will be squeezed into a shorter distance by the Lorentz contraction, which is greater near the outside of the rod. If the North end of the magnet is opposite the motor, the electron velocity adds to the velocity of the rotating rod to produce a more strongly negative cloud near the outside of the rod, and thus creating a positive voltage of the center of the rod with respect to the outside. When the directions of the domain electron current and the rod rotation are opposite, a greater voltage is induced.
Click below to see the voltage produced by any of the four permutations of rotation direction and magnetic pole reversal:
Many years ago I submitted this device to the Summer AAPT Meeting Apparatus Competition. The judges did not seem to be impressed, and it received no prize. Click here for an updated version of the explanation of the Unipolar generator that we included with the apparatus for the competition.
