Purpose:Demonstrate unipolar generation of DC voltage, which may involve an explanation other than electromagnetic induction.
Description:A strong (over 10 kilogauss) cylindrical magnet is rotated about its axis at 1725 RPM. Brushes positioned on the axis of rotation and the "equator" of the bar magnet (midway between the two poles) are attached to a digital voltmeter. An electrical potential of about 15 millivolts is measured. Reversing the direction of rotation or reversing the ends of the magnet causes the DC voltage to reverse in sign.
An aluminum bar can be substituted for the magnetic one and rotated in the CW or CCW directions.
Note that there is a small (about 0.1-0.3 mV) potential developed with this arrangement, probably due to contact potentials between the various materials in the system and the wires, similar to the potential developed in a thermocouple. The observed potential is the same for either rotational direction of the aluminum rod; it would likely be opposite in sign for opposite rotation direction if it were due to some sort of induction effect.
The explanation of this device is perhaps problematic. Many people believe that because there is no change in flux in the wire loop this cannot be an electromagnetic induction effect; the only explanation lies in special relativity. Other theoreticians disagree.