PURPOSE: To 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. Click below to see the voltage produced by any of the four permutations of rotation direction and magnetic pole reversal:
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.
SUGGESTIONS: This gizmo is sometimes called a "homopolar" generator. This is a nice experiment to start arguments in a graduate course on electromagnetic theory.
Click The Unipolar Generator: A Demonstration of Special Relativity for a short paper discussing this demonstration in detail.
REFERENCES: Available. (PIRA 5K10.80, 5K20.58)
EQUIPMENT: Unipolar generator with extra magnet and digital voltmeter.
SETUP TIME: 10 minutes.