The Unipolar Generator: A Demonstration of Special Relativity
Carroll O. Alley
This interesting and surprising device, sometimes called the homopolar generator, is one of the more forgotten of the significant experiments in the history of physics. It is perhaps the only table-top demonstration of special relativity.
A cylindrical alnico magnet, one inch in diameter and six inches long, having its poles at the ends, with a residual magnetic induction of about one Tesla, is rotated around its axis at 1725 RPM. Brushes are positioned on the axis of rotation at one end and on the "equator" of the cylinder. While the magnet is being rotated a potential between the brushes of about 13 mV is observed. This potential reverses in sign when the direction of magnetic rotation is reversed or when the poles of the magnet are interchanged.
