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A '''homopolar generator''' is a [[Direct current|DC]] [[electrical generator]] that comprises an electrically conductive disc rotating in a plane perpendicular to a uniform static magnetic field. A potential difference is created between the center of the disc and the rim, the [[electrical polarity]] depending on the direction of rotation and the orientation of the field.  It is also known as a '''unipolar generator''', '''acyclic generator''', '''disk dynamo''', or '''Faraday disc'''. The voltage is typically low, on the order of a few volts in the case of small demonstration models, but large research generators can produce hundreds of volts, and some systems have multiple generators in series to produce an even larger voltage.<ref>Losty, H.H.W & Lewis, D.L. (1973) Homopolar Machines. Philosophical Transactions for the Royal Society of London. Series A, Mathematical and Physical Sciences. 275 (1248), 69-75</ref> 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.

[[Image:Faraday disk generator.jpg|thumb|Faraday disk, the first homopolar generator]]

== The Faraday disc ==
[[File:Faraday disc.svg|thumb|Faraday disc]]
The homopolar generator was developed first by [[Michael Faraday]] during his experiments in 1831. It is frequently called the '''Faraday disc''' in his honor. It was the beginning of modern [[dynamo]]s &mdash; that is, [[electrical generator]]s which operate using a [[magnetic field]]. It was very inefficient and was not used as a practical power source, but it showed the possibility of generating electric power using magnetism, and led the way for [[commutator (electric)|commutated]] [[direct current]] dynamos and then [[alternating current]] alternators.

The Faraday disc was primarily inefficient due to counterflows of current. While current flow was induced directly underneath the magnet, the current would circulate backwards in regions outside the influence of the magnetic field. This counterflow limits the power output to the pickup wires, and induces waste heating of the copper disc. Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field radially from axis to edge, and eliminate areas where counterflow could occur.

== Homopolar generator development ==

[[Image:Homopolar anu-MJC.jpg|thumb|250px|The remains of the ANU 500MJ generator]] Long after the original Faraday disc had been abandoned as a practical generator, a modified version combining the magnet and disc in a single rotating part (the ''rotor'') was developed. Sometimes the name ''homopolar generator'' is reserved for this configuration. One of the earliest patents on the general type of homopolar generators was attained by A. F. Delafield, {{US patent|278516}}. Other early patents for homopolar generators were awarded to [[Sebastian Ziani de Ferranti|S. Z. De Ferranti]] and [[Charles Batchelor|C. Batchelor]] separately. [[Nikola Tesla]] was interested in the Faraday disc and conducted work with homopolar generators.<ref>Nikola Tesla, "''[http://www.andrijar.com/teslahom/index.html Notes on a Unipolar Dynamo]''". The Electrical Engineer, N.Y., Sept. 2, 1891. (Also available at tesla.hu,  [http://www.tesla.hu/tesla/articles/18910902/index.htm Article 18910902])</ref> He eventually patented an improved version of the device and his [http://www.google.com/patents/about?id=MvdbAAAAEBAJ&dq=406968 US patent] ("Dynamo Electric Machine") describes an arrangement of two parallel discs with separate, parallel shafts, joined like [[pulley]]s by a metallic belt. Each disc had a field that was the opposite of the other, so that the flow of current was from the one shaft to the disc edge, across the belt to the other disc edge and to the second shaft. This would have greatly reduced the frictional losses caused by sliding contacts by allowing both electrical pickups to interface with the shafts of the two disks rather than at the shaft and a high-speed rim.  Later, patents were awarded to [[Charles Proteus Steinmetz|C. P. Steinmetz]] and [[Elihu Thomson|E. Thomson]] for their work with homopolar generators. The '''Forbes dynamo''', developed by the Scottish electrical engineer [[George Forbes (scientist)|George Forbes]],  was in widespread use during the beginning of the 20th century.  Much of the development done in homopolar generators was patented by [[Jakob E. Noeggerath|J. E. Noeggerath]] and [[Rudolf Eickemeyer|R. Eickemeyer]]. 

One of the larger homopolar generators that was produced by Parker Kinetic Designs via the collaboration of [[Richard Marshall (electrician)|Richard Marshall]], [[William Weldon (electrician)|William Weldon]], and [[Herb Woodson]]. Parker Kinetic Designs have produced devices which can produce five [[ampere|megaampere]]s. Another large homopolar generator was built by [[Mark Oliphant|Sir Mark Oliphant]] at the [[Research School of Physical Sciences and Engineering]], [[Australian National University]]. It stored up to 500 [[megajoule]]s of energy<ref>J.W. Blamey, P.O. Carden, L.U. Hibbard, E.K. Inall, R.A. Marshall and Sir Mark Oliphant, 'The large homopolar generator at Canberra: initial tests', Nature, 195 (1962), pp. 113&ndash;114.</ref> and was used as an extremely high-current source for experimentation from 1962 until it was disassembled in 1986. Oliphant's construction was capable of supplying currents of up to 2 megaamperes.

{{more|List of homopolar generator patents}}

==Description and operation==

=== Disk-type generator ===

[[Image:Faraday disc.jpg|187px|thumb|Basic Faraday disc generator]]
This device consists of a [[electrical conductivity|conducting]] [[flywheel]] rotating in a [[magnetic field]] with one electrical contact near the axis and the other near the periphery.  It has been used for generating very high currents at low voltages in applications such as [[welding]], [[electrolysis]] and [[railgun]] research. In pulsed energy applications, the [[angular momentum]] of the rotor is used to store energy over a long period and then release it in a short time.

In contrast to other types of generators, the output voltage never changes polarity. The charge separation results from the [[Lorentz force]] on the free charges in the disk. The motion is azimuthal and the field is axial, so the [[electromotive force]] is radial. The electrical contacts are usually made through a "[[brush (electric)|brush]]" or [[slip ring]], which results in large losses at the low voltages generated.  Some of these losses can be reduced by using [[mercury (element)|mercury]] or other easily liquified metal or alloy ([[gallium]], [[NaK]]) as the "brush", to provide essentially uninterrupted electrical contact.

If the magnetic field is provided by a permanent [[magnet]], the generator works regardless of whether the magnet is fixed to the stator or rotates with the disc. Before the discovery of the [[electron]] and the [[Lorentz force law]], the [[phenomenon]] was inexplicable and was known as the [[Faraday paradox]].

=== Drum-type generator ===

A drum-type HPG has a magnetic field (B) that radiates radially from the center of the drum and induces voltage (V) down the length of the drum.
A conducting drum  spun from above in the field of a "speaker" type of magnet that has one pole in the center of the drum and the other pole surrounding the drum could use conducting ball bearings at the top and bottom of the drum to pick up the generated current.

==Astrophysical unipolar inductors==
Unipolar inductors occur in astrophysics where a conductor rotates through a magnetic field, for example, the movement of the highly conductive [[Plasma (physics)|plasma]] in a cosmic body's [[ionosphere]] through its [[magnetic field]]. In their book, ''Cosmical Electrodynamics'', [[Hannes Alfvén]] and [[Carl-Gunne Fälthammar]] write:
:"Since cosmical clouds of ionized gas are generally magnetized, their motion produces induced electric fields [..] For example the motion of the magnetized interplanetary plasma produces electric fields that are essential for the production of aurora and magnetic storms" [..]
:".. the rotation of a conductor in a magnetic field produces an electric field in the system at rest. This phenomenon is well known from laboratory experiments and is usually called 'homopolar ' or 'unipolar' induction. <ref>Hannes Alfvén and Carl-Gunne Fälthammar, ''Cosmical Electrodynamics'' (1963) 2nd Edition, Oxford University Press. See sec. 1.3.1. Induced electric field in uniformly moving matter.</ref>

Unipolar inductors have been associated with the aurorae on [[Uranus]],<ref>Hill, T. W.; Dessler, A. J.; Rassbach, M. E., "[http://adsabs.harvard.edu/abs/1983P%26SS...31.1187H Aurora on Uranus &ndash; A Faraday disc dynamo mechanism]" (1983) ''Planetary and Space Science'' (ISSN 0032-0633), vol. 31, Oct. 1983, p. 1187&ndash;1198</ref> [[binary star]]s,<ref>Hannes Alfvén, "[http://gallica.bnf.fr/ark:/12148/bpt6k31562/f1180.table Sur l'origine de la radiation cosmique]" (On the origin of cosmic radiation)" Comptes Rendus, 204, pp.1180&ndash;1181 (1937)</ref><ref>Hakala, Pasi ''et al.'', "[http://adsabs.harvard.edu/abs/2003MNRAS.343L..10H Spin up in RX J0806+15: the shortest period binary]" (2003) ''Monthly Notice of the Royal Astronomical Society'', Volume 343, Issue 1, pp. L10&ndash;L14</ref> [[black hole]]s,<ref>Burns, M. L.; Lovelace, R. V. E., "[http://adsabs.harvard.edu/abs/1982ApJ...262...87B Theory of electron-positron showers in double radio sources]" (1982) ''Astrophysical Journal'', Part 1, vol. 262, Nov. 1, 1982, p. 87&ndash;99</ref><ref>Shatskii, A. A., "[http://adsabs.harvard.edu/abs/2003AstL...29..153S Unipolar Induction of a Magnetized Accretion Disk around a Black Hole]", (2003) ''Astronomy Letters'', vol. 29, p. 153&ndash;157</ref> [[galaxy|galaxies]],<ref>Per Carlqvist, "[http://adsabs.harvard.edu/abs/1988Ap%26SS.144...73C Cosmic electric currents and the generalized Bennett relation]" (1988) ''Astrophysics and Space Science'' (ISSN 0004-640X), vol. 144, no. 1&ndash;2, May 1988, pp. 73&ndash;84.</ref> the [[Io (moon)#Interaction with Jupiter's magnetosphere|Jupiter Io system]],<ref>Goldreich, P.; Lynden-Bell, D., "[http://adsabs.harvard.edu/abs/1969ApJ...156...59G Io, a jovian unipolar inductor]" (1969) ''Astrophys. J.'', vol. 156, pp. 59&ndash;78 (1969).</ref><ref>Strobel, Darrell F.; ''et al.'', "[http://adsabs.harvard.edu/abs/2002ApJ...581L..51S Hubble Space Telescope Space Telescope Imaging Spectrograph Search for an Atmosphere on Callisto: A Jovian Unipolar Inductor]" (2002) ''The Astrophysical Journal'', Volume 581, Issue 1, pp. L51&ndash;L54</ref> the [[Moon]],<ref>"Sonett, C. P.; Colburn, D. S., "[http://adsabs.harvard.edu/abs/1967Natur.216..340S Establishment of a Lunar Unipolar Generator and Associated Shock and Wake by the Solar Wind]" (1967) ''Nature'', vol. 216, 340&ndash;343.</ref><ref>Schwartz, K.; Sonett, C. P.; Colburn, D. S., "[http://adsabs.harvard.edu/abs/1969Moon....1....7S Unipolar Induction in the Moon and a Lunar Limb Shock Mechanism]" in ''The Moon'', Vol. 1, p.7</ref> the Solar Wind,<ref>Srnka, L. J., "[http://adsabs.harvard.edu/abs/1975Ap%26SS..36..177S Sheath-limited unipolar induction in the solar wind]" (1975) ''Astrophysics and Space Science', vol. 36, Aug. 1975, pp. 177&ndash;204.</ref> [[sunspot]]s,<ref>Yang, Hai-Shou, "[http://adsabs.harvard.edu/abs/1981ChA%26A...5...77Y A force &ndash; free field theory of solar flares I. Unipolar sunspots]" ''Chinese Astronomy and Astrophysics'', Volume 5, Issue 1, pp. 77&ndash;83.</ref><ref>Osherovich, V. A.; Garcia, H. A., "[http://adsabs.harvard.edu/abs/1990GeoRL..17.2273O Electric current in a unipolar sunspot with an untwisted field]" (1990) ''Geophysical Research Letters'' (ISSN 0094-8276), vol. 17, Nov. 1990, pp. 2273&ndash;2276.</ref> and in the [[Venus|Venusian magnetic tail]].<ref>Eroshenko, E. G., "[http://adsabs.harvard.edu/abs/1979CosRe..17R..93E Unipolar induction effects in the Venusian magnetic tail]" (1979) Kosmicheskie Issledovaniia, vol. 17, Jan.&ndash;Feb. 1979, pp. 93&ndash;10</ref>

==Physics==
Like all [[Electrical generator#Dynamo|dynamos]], the Faraday disc converts [[kinetic energy]] to [[electrical energy]]. This machine can be analysed using Faraday's own law of [[electromagnetic induction]]. This law (in its modern form) states that an electric current is induced in a closed electrical circuit when the magnetic flux enclosed by the circuit changes (in either magnitude or direction). For the Faraday disk it is necessary, however, to consider that the circuit(s) consist of each radial "spoke" of the disk connected to the rim and center and then through the external circuit.

The [[Lorentz force law]] is more easily used to explain the machine's behaviour. This law, discovered 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. The radial movement of the electrons in the disc produces a charge separation between the center of the disc and its rim, and if the circuit is completed an electric current will be produced.<ref>[http://www.plasma.uu.se/CED/Book/EMFT_Book.pdf Electromagnetic Field Theory, 2nd ed. by Bo Thidé, Department of Physics and Astronomy, Uppsala University, Sweden]</ref>

== See also ==
{{Portal|Energy}}
* [[Barlow's Wheel]]
* [[Electric generator]]
* [[Electric motor]]
* [[List of homopolar generator patents]]
* [[Homopolar motor]]
* [[Faraday paradox]]
* [[Faraday's law of induction]]

== External references ==
* http://www.andrijar.com/homopolar/index.html
* http://www.andrijar.com/nt/index.html

== References and further reading==
===Notes===
{{reflist}}
===General references===
* Don Lancaster, "''[http://www.tinaja.com/glib/muse117.pdf Shattering the homopolar myths]''". Tech Musings, October, 1997. (PDF)
* Don Lancaster, "''[http://www.tinaja.com/glib/muse121.pdf Understanding Faraday's Disk]''". Tech Musings, October, 1997. (PDF)
* John David Jackson, ''Classical Electrodynamics'', Wiley, 3rd ed. 1998, ISBN 0-471-30932-X
* Arthur I. Miller, "Unipolar Induction: A Case Study of the Interaction between Science and Technology," Annals of Science, Volume 38, pp. 155&ndash;189 (1981).
* Olivier Darrigol, ''Electrodynamics from Ampere to Einstein'', Oxford University Press, 2000, ISBN 0-19-850594-9
* Trevor Ophel and John Jenkin, (1996) ''[http://wwwrsphysse.anu.edu.au/admin/php/fire_in_the_belly/Fire_in_the_Belly01.pdf Fire in the belly] : the first 50 years of the pioneer school at the ANU''  Canberra : Research School of Physical Sciences and Engineering, Australian National University. ISBN 0-85800-048-2. (PDF)
* Thomas Valone, ''The Homopolar Handbook : A Definitive Guide to Faraday Disk and N-Machine Technologies''. Washington, DC, U.S.A.: Integrity Research Institute, 2001. ISBN 0-9641070-1-5
===Further reading===
* Richard A. Marshall and William F. Weldon, "''Parameter Selection for Homopolar Generators Used as Pulsed Energy Stores''", Center for Electromechanics, University of Texas, Austin, Jul. 1980. (also published in: Electrical Machines and Electromechanics, 6:109&ndash;127, 1981.)

== External links and other articles==
* Popular Science Monthly, ''Construction of Unipolar Dynamos'', April 1916, pp. 624&ndash;626, Scanned article available via Google Books: http://books.google.com/books?id=hCYDAAAAMBAJ&pg=PA624   
* Robert Hebner, "''[http://www.utexas.edu/research/cem/homopolar%20generators.html Homopolar Generator]''". Homopolar Welding, UT-CEM.
* "''[http://www.physics.umd.edu/lecdem/services/demos/demosk2/k2-64.htm K2-64: Unipolar generator]''". physics.umd.edu.
*  Richard E. Berg and Carroll O. Alley, "''[http://www.physics.umd.edu/lecdem/outreach/QOTW/arch11/q218unipolar.pdf The Unipolar Generator: A Demonstration of Special Relativity]''", Department of Physics, University of Maryland, 2005. (PDF)
* Richard Fitzpatrick, "''Magnetohydrodynamic theory ''", ''[http://farside.ph.utexas.edu/teaching/plasma/lectures/node70.html The homopolar generator]''. farside.ph.utexas.edu, 2006-02-16. 
* "''[http://www.physics.brown.edu/physics/demopages/Demo/em/demo/5k1080.htm 5K10.80 Homopolar Generator]; Lecture Demonstrations.''". physics.brown.edu
* William J. Beaty, "''[http://www.amasci.com/freenrg/n-mach.html Untried Homopolar Generator Experiments]''". 1996.

[[Category:Electrical generators]]
[[Category:Michael Faraday]]

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