K2-64: UNIPOLAR GENERATOR

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:

North pole out, clockwise rotation
North pole out, counterclockwise rotation
South pole out, clockwise rotation
South pole out, counterclockwise rotation

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.

SUGGESTIONS: This gizmo is sometimes called a "homopolar" generator. This is a nice experiment to start arguments in a graduate course on electromagnetic theory.

See Question of the Week #217 and Question of the Week #218 for information on using this demonstration to enhance class involvement.

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.