Homopolar generator

A potential difference is created between the center of the disc and the rim (or ends of the cylinder) with an electrical polarity that depends on the direction of rotation and the orientation of the field.

[1] They are unusual in that they can source tremendous electric current, some more than a million amperes, because the homopolar generator can be made to have very low internal resistance.

Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field around the circumference, and eliminate areas where counterflow could occur.

These devices used heavy disks as a form of flywheel to store mechanical energy that could be quickly dumped into an experimental apparatus.

An early example of this sort of device was built by Sir Mark Oliphant at the Research School of Physical Sciences and Engineering, Australian National University (ANU).

[5] [6] This device consists of a conducting flywheel rotating in a magnetic field with one electrical contact near the axis and the other near the periphery.

The electrical contacts are usually made through a "brush" or slip ring, which results in large losses at the low voltages generated.

Some of these losses can be reduced by using mercury or other easily liquefied metal or alloy (gallium, NaK) as the "brush", to provide essentially uninterrupted electrical contact.

In their book, Cosmical Electrodynamics, Hannes Alfvén and Carl-Gunne Fälthammar write: Unipolar inductors have been associated with the aurorae on Uranus,[8] binary stars,[9][10] black holes,[11][12][13] galaxies,[14] the Jupiter Io system,[15][16] the Moon,[17][18] the Solar Wind,[19] sunspots,[20][21] and in the Venusian magnetic tail.

This law, formulated thirty years after Faraday's death, states that the force on an electron is proportional to the cross product of its velocity and the magnetic flux vector.

In geometrical terms, this means that the force is at right-angles to both the velocity (azimuthal) and the magnetic flux (axial), which is therefore in a radial direction.