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 1728 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 theDC voltage to reverse in sign.

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.

REFERENCES: (PIRA 5K10.80, 5K20.58) Several papers and other interesting documents regarding this device will be found in the Demonstration Reference File.

EQUIPMENT: Unipolar generator with extra magnet and digital voltmeter.


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