Editing Van de Graaff generator

{{short description|Electrostatic generator operating on the triboelectric effect}}
{{About|the electrostatic generator|the progressive rock band|Van der Graaf Generator}}
{{Infobox laboratory equipment
|name         = Van de Graaff generator
|image        = Van de graaff generator sm.jpg
|alt          = Large metal sphere supported on a clear plastic column, inside of which a rubber belt can be seen clearly: A smaller sphere is supported on a metal rod. Both are mounted to a base plate, on which is a small driving electric motor.
|caption      = Small Van de Graaff generator used in science education
|uses         = Accelerating [[electron]]s to sterilize food and process materials, accelerating [[proton]]s for [[nuclear physics]] experiments, producing energetic [[X-ray]] beams in [[nuclear medicine]], physics education, entertainment
|inventor     = [[Robert J. Van de Graaff]]
|related      = [[linear particle accelerator]]
}}
A '''Van de Graaff generator''' is an [[electrostatic generator]] which uses a moving belt to accumulate [[electric charge]] on a hollow metal globe on the top of an insulated column, creating very high [[electric potential]]s. It produces very [[high voltage]] [[direct current]] (DC) electricity at low current levels. It was invented by American physicist [[Robert J. Van de Graaff]] in 1929.<ref name="Van de Graaff">
{{cite journal
 | last1 = Van de Graaff | first1 = R. J.
 | last2 = Compton | first2 = K. T.
 | last3 = Van Atta | first3 = L. C.
 | title = The Electrostatic Production of High Voltage for Nuclear Investigations
 | journal = Physical Review
 | volume = 43
 | issue = 3
 | pages = 149–157
 | date = February 1933
 | url = http://web.ihep.su/dbserv/compas/src/van%20de%20graaff33/eng.pdf
 | doi = 10.1103/PhysRev.43.149
 | access-date = August 31, 2015 |bibcode= 1933PhRv...43..149V
}}</ref>
The [[potential difference]] achieved by modern Van de Graaff generators can be as much as 5 megavolts. A tabletop version can produce on the order of 100&nbsp;kV and can store enough energy to produce visible [[electric spark]]s. Small Van de Graaff machines are produced for entertainment, and for physics education to teach [[electrostatics]]; larger ones are displayed in some [[science museum]]s.

The Van de Graaff generator was originally developed as a [[particle accelerator]] for physics research, as its high potential can be used to accelerate [[subatomic particle]]s to great speeds in an evacuated tube. It was the most powerful type of accelerator until the [[cyclotron]] was developed in the early 1930s. Van de Graaff generators are still used as accelerators to generate energetic particle and [[X-ray]] beams for [[nuclear research]] and [[nuclear medicine]].<ref>{{cite journal |last1=Cassiday |first1=Laura |title=Hair-raising technique detects drugs, explosives on human body |journal=Science |date=July 10, 2014 |doi=10.1126/article.22861 |doi-broken-date=1 November 2024 |url=https://www.science.org/content/article/hair-raising-technique-detects-drugs-explosives-human-body |access-date=10 May 2022}}</ref>

The voltage produced by an open-air Van de Graaff machine is limited by arcing and [[corona discharge]] to about 5&nbsp;MV. Most modern industrial machines are enclosed in a pressurized tank of insulating gas; these can achieve potentials as large as about 25&nbsp;MV.

== History ==
{{multiple image
| align = right
| direction = horizontal
| header =
| image1 = WestinghouseAtomSmasher.jpg
| width1 = 200
| image2 = Westinghouse Van de Graaff atom smasher - cutaway.png
| width2 = 168
| footer = The [[Westinghouse Atom Smasher]], the {{val|5|ul=MeV}} Van de Graaff generator built in 1937 by the [[Westinghouse Electric (1886)|Westinghouse Electric]] company in [[Forest Hills, Pennsylvania]]
}}
[[File:Az első magyar gyorsító Van de Graaff-generátora(2).jpg|thumb|240px|right|This Van de Graaff generator of the first Hungarian linear particle accelerator achieved 700&nbsp;kV in 1951 and 1000&nbsp;kV in 1952.]]
[[File:Van den Graaff DSC09091.JPG|thumb|right|240px|A Van de Graaff particle accelerator in a pressurized tank at [[Pierre and Marie Curie University]], Paris]]

=== Background ===
The concept of an electrostatic generator in which charge is mechanically transported in small amounts into the interior of a high-voltage electrode originated with the [[Kelvin water dropper]], invented in 1867 by [[William Thomson, 1st Baron Kelvin|William Thomson]] (Lord Kelvin),<ref name="Thomson">
{{cite journal
 | last1 = Thomson
 | first1 = William
 | title = On a self-acting apparatus for multiplying and maintaining electric charges, with applications to the Voltaic Theory
 | journal = The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science
 | series = Series 4
 | volume = 34
 | issue = 231
 | pages = 391–396
 | date = November 1867
 | url = https://books.google.com/books?id=2lgwAAAAIAAJ&pg=PA391
 | access-date = September 1, 2015}}</ref> in which charged drops of water fall into a bucket with the same polarity charge, adding to the charge.<ref name="Gray">{{cite book
 | last1  = Gray
 | first1 = John
 | title  = Electrical Influence Machines
 | publisher = Whittaker and Co.
 | date   = 1890
 | location = London
 | pages  = 187–190
 | url    = https://archive.org/stream/electricalinflu00graygoog#page/n210/mode/1up
}}</ref>
In a machine of this type, the [[gravitational force]] moves the drops against the opposing electrostatic field of the bucket. Kelvin himself first suggested using a belt to carry the charge instead of water. The first electrostatic machine that used an endless belt to transport charge was constructed in 1872 by [[Augusto Righi]].<ref name="Van de Graaff" /><ref name="Gray" /> It used an [[india rubber]] belt with wire rings along its length as charge carriers, which passed into a spherical metal electrode. The charge was applied to the belt from the grounded lower roller by [[electrostatic induction]] using a charged plate. John Gray also invented a belt machine about 1890.<ref name="Gray" /> Another more complicated belt machine was invented in 1903 by Juan Burboa.<ref name="Van de Graaff" /><ref name="Burboa">[https://patents.google.com/patent/US776997 US patent no. 776997, Juan G. H. Burboa ''Static electric machine'', filed: August 13, 1903, granted: December 6, 1904]</ref> A more immediate inspiration for Van de Graaff was a generator [[W. F. G. Swann]] was developing in the 1920s in which charge was transported to an electrode by falling metal balls, thus returning to the principle of the Kelvin water dropper.<ref name="Van de Graaff" /><ref name="Swann">
{{cite journal
 | last1 = Swann | first1 = W. F. G.
 | title = A device for obtaining high potentials
 | journal = Journal of the Franklin Institute
 | volume = 205
 | pages = 828
 | date = 1928
}}</ref>

=== Initial development ===

The Van de Graaff generator was developed, starting in 1929, by physicist Robert J. Van de Graaff at [[Princeton University]], with help from colleague Nicholas Burke. The first model was demonstrated in October 1929.<ref>{{cite web |url=http://chem.ch.huji.ac.il/~eugeniik/history/graaff.html |title=Robert Jemison Van de Graaff |website=The Institute of Chemistry – The Hebrew University of Jerusalem |access-date=2006-08-31 |url-status=dead |archive-url=https://web.archive.org/web/20060904024639/http://chem.ch.huji.ac.il/~eugeniik/history/graaff.html |archive-date=2006-09-04}}</ref> The first machine used an ordinary tin can, a small motor, and a silk ribbon bought at a [[variety store#North America|five-and-dime store]]. After that, he went to the chairman of the physics department requesting $100 to make an improved version. He did get the money, with some difficulty. By 1931, he could report achieving 1.5 million volts, saying "The machine is simple, inexpensive, and portable. An ordinary lamp socket provides the only power needed."<ref>{{cite journal |last=van de Graaff |first=R. J. |title=Minutes of the Schenectady Meeting September 10, 11 and 12, 1931: A 1,500,000 volt electrostatic generator. |journal=Physical Review |publisher=American Physical Society (APS) |volume=38 |issue=10 |date=1931-11-15 |issn=0031-899X |doi=10.1103/physrev.38.1915 |pages=1919–1920}}</ref><ref>''Niels Bohr's Times'', Abraham Pais, Oxford University Press, 1991, pp.378-379</ref> According to a patent application, it had two 60-cm-diameter charge-accumulation spheres mounted on [[Pyrex|borosilicate glass]] columns 180&nbsp;cm high; the apparatus cost $90 in 1931.<ref>"Van de Graaff's Generator", in "Electrical Engineering Handbook", (ed), CRC Press, Boca Raton, Florida USA, 1993 {{ISBN|0-8493-0185-8}}</ref><ref>{{cite journal |last1=Wolff |first1=M.F. |title=Van de Graaff's generator |journal=IEEE Spectrum |date=July 1990 |volume=27 |issue=7 |pages=46 |doi=10.1109/6.58426|s2cid=43715110 }}</ref>

Van de Graaff applied for a second patent in December 1931, which was assigned to [[Massachusetts Institute of Technology]] in exchange for a share of net income; the patent was later granted.<ref>{{cite journal |title=This Month in Physics History: February 12, 1935: Patent granted for Van de Graaff generator |journal=APS News |date=February 2011 |volume=20 |issue=2 |url=https://www.aps.org/publications/apsnews/201102/physicshistory.cfm |access-date=10 May 2022}}</ref>

In 1933, Van de Graaff built a 40&nbsp;ft (12&nbsp;m) model at MIT's [[Round Hill, Massachusetts|Round Hill]] facility, the use of which was donated by [[Edward Howland Robinson Green|Colonel Edward H. R. Green]].<ref>{{cite journal |last1=Thomas |first1=William |title=A profile of John Trump, Donald's accomplished scientist uncle |journal=Physics Today |date=7 September 2016 |issue=9 |page=22954 |doi=10.1063/PT.5.9068 |bibcode=2016PhT..2016i2954T |url=https://physicstoday.scitation.org/do/10.1063/pt.5.9068/full/ |access-date=10 May 2022}}</ref> One consequence of the location of this generator in an aircraft hangar was the "pigeon effect": arcing from accumulated droppings on the outer surface of the spheres.<ref>{{cite web |last1=Wilson |first1=E.J.N. |title=Overview of Accelerators |url=https://acceleratorinstitute.web.cern.ch/ACINST1112/Lecture1(OverviewI).pdf |website=Accelerator Institute |publisher=CERN |access-date=10 May 2022}}</ref>

=== Higher energy machines ===

In 1937, the [[Westinghouse Electric (1886)|Westinghouse Electric]] company built a {{convert|65|ft|m|abbr=on}} machine, the [[Westinghouse Atom Smasher]] capable of generating 5&nbsp;MeV in [[Forest Hills, Pennsylvania]]. It marked the beginning of nuclear research for civilian applications.<ref>{{cite book |first=Franklin |last=Toker |title=Pittsburgh: A New Portrait |year=2009 |page=470 |publisher=University of Pittsburgh Press |url=https://books.google.com/books?id=nJM6AQAAIAAJ |isbn=9780822943716}}</ref><ref>{{cite web |title=Van de Graaff particle accelerator, Westinghouse Electric and Manufacturing Co., Pittsburgh, PA, August 7, 1945 |website=Explore PA History |publisher=[[WITF-TV]] |url=http://explorepahistory.com/displayimage.php?imgId=1-2-151C |access-date=February 19, 2015}}</ref> It was decommissioned in 1958 and was partially demolished in 2015.<ref name="bo">{{cite news|last=O'Neill|first=Brian|date=January 25, 2015|title=Brian O'Neill: With Forest Hills atom smasher's fall, part of history tumbles|newspaper=[[Pittsburgh Post-Gazette]]|url=https://www.post-gazette.com/opinion/brian-oneill/2015/01/25/Brian-O-Neill-With-Forest-Hills-atom-smasher-s-fall-part-of-history-tumbles/stories/201501250109}}</ref> (The enclosure was laid on its side for safety reasons.)<ref>{{Cite web|title=Atom smasher in Forest Hills torn down; restoration promised|url=https://www.post-gazette.com/news/science/2015/01/21/Forrest-Hill-nuclear-relic-waits-in-limbo/stories/201501200209|access-date=2022-01-17|website=Pittsburgh Post-Gazette|language=en}}</ref>

A more recent development is the tandem Van de Graaff accelerator, containing one or more Van de Graaff generators, in which negatively charged [[ion]]s are accelerated through one [[potential difference]] before being stripped of two or more electrons, inside a high-voltage terminal, and accelerated again. An example of a three-stage operation has been built in Oxford Nuclear Laboratory in 1964 of a 10&nbsp;MV single-ended "injector" and a 6&nbsp;MV EN tandem.<ref>J. Takacs, ''Energy Stabilization of Electrostatic Accelerators'', John Wiley and Sons, Chichester, 1996</ref>{{page needed|date=April 2016}}

By the 1970s, as much as 14 MV could be achieved at the terminal of a tandem that used a tank of high-pressure [[SF6|sulfur hexafluoride]] (SF<sub>6</sub>) gas to prevent sparking by trapping electrons. This allowed the generation of heavy ion beams of several tens of MeV, sufficient to study light-ion direct nuclear reactions. The greatest potential sustained by a Van de Graaff accelerator is 25.5&nbsp;MV, achieved by the tandem in the Holifield Radioactive Ion Beam Facility in [[Oak Ridge National Laboratory]].<ref>{{cite web |url=https://www.ornl.gov/news/american-physical-society-names-ornls-holifield-facility-historic-physics-site |title=American Physical Society names ORNL's Holifield Facility historic physics site |date=25 July 2016 |publisher=Oak Ridge National Laboratory}}</ref>

A further development is the [[pelletron]], where the rubber or fabric belt is replaced by a chain of short conductive rods connected by insulating links, and the air-ionizing electrodes are replaced by a grounded roller and inductive charging electrode. The chain can be operated at a much greater velocity than a belt, and both the voltage and currents attainable are much greater than with a conventional Van de Graaff generator. The 14&nbsp;UD Heavy Ion Accelerator at [[the Australian National University]] houses a 15&nbsp;MV pelletron. Its chains are more than 20&nbsp;m long and can travel faster than {{convert|50|km/h|mph|abbr=on}}.<ref>{{cite web |title=Particle Accelerator |url=http://www.anu.edu.au/CSEM/machines/Accelerator.htm |date=November 2002 |url-status=dead |archive-url=https://web.archive.org/web/20190608111732/http://www.anu.edu.au/CSEM/machines/Accelerator.htm |archive-date=2019-06-08}}</ref>

The Nuclear Structure Facility (NSF) at [[Daresbury Laboratory]] was proposed in the 1970s, commissioned in 1981, and opened for experiments in 1983. It consisted of a tandem Van de Graaff generator operating routinely at 20 MV, housed in a distinctive building 70&nbsp;m high. During its lifetime, it accelerated 80 different ion beams for experimental use, ranging from protons to uranium. A particular feature was the ability to accelerate rare isotopic and radioactive beams. Perhaps the most important discovery made using the NSF was that of super-deformed nuclei. These nuclei, when formed from the fusion of lighter elements, rotate very rapidly. The pattern of gamma rays emitted as they slow down provided detailed information about the inner structure of the nucleus.<ref>J S Lilley 1982 Phys. Scr. 25 435-442 {{doi|10.1088/0031-8949/25/3/001}})</ref> Following financial cutbacks, the NSF closed in 1993.<ref>{{cite journal |author1=David Dickson |title=Curtain falls on Britain's nuclear structure facility |journal=Nature |date=March 1993 |volume=362 |issue=6418 |pages=278 |url=https://www.nature.com/articles/362278b0.pdf |publisher=Nature Publishing Group |doi=10.1038/362278b0 |access-date=6 February 2024}}</ref>

== Description ==
[[File:Van de Graaff Generator.svg|thumb|267x267px|right|Van de Graaff generator diagram]]

A simple Van de Graaff generator consists of a belt of rubber (or a similar flexible [[dielectric]] material) moving over two rollers of differing material, one of which is surrounded by a hollow metal sphere. A comb-shaped metal [[electrode]] with sharp points (2 and 7 in the diagram), is positioned near each roller. The upper comb (2) is connected to the sphere, and the lower one (7) to ground.  When a motor is used to drive the belt, the [[triboelectric effect]] causes the transfer of electrons from the dissimilar materials of the belt and the two rollers. In the example shown, the rubber of the belt will become negatively charged while the acrylic glass of the upper roller will become positively charged.  The belt carries away negative charge on its inner surface while the upper roller accumulates positive charge.<ref name="maglab">{{cite web |title=Van de Graaff Generator – MagLab |url=https://nationalmaglab.org/education/magnet-academy/watch-play/interactive/van-de-graaff-generator |website=nationalmaglab.org |publisher=National High Magnetic Field Laboratory |access-date=10 May 2022}}</ref>

Next, the strong electric field surrounding the positive upper roller (3) induces a very high electric field near the points of the nearby comb (2).  At the points of the comb, the field becomes strong enough to [[ionization|ionize]] air molecules. The electrons from the air molecules are attracted to the outside of the belt, while the positive ions go to the comb.  At the comb they are neutralized by electrons from the metal, thus leaving the comb and the attached outer shell (1) with fewer net electrons and a net positive charge. By [[Gauss's law]] (as illustrated in the [[Faraday ice pail experiment]]), the excess positive charge is accumulated on the outer surface of the outer shell, leaving no [[electric field]] inside the shell. Continuing to drive the belt causes further electrostatic induction, which can build up large amounts of charge on the shell. Charge will continue to accumulate until the rate of charge leaving the sphere (through leakage and [[corona discharge]]) equals the rate at which new charge is being carried into the sphere by the belt.{{r|maglab}} 

Outside the terminal sphere, a high electric field results from the high voltage on the sphere, which would prevent the addition of further charge from the outside. However, since electrically charged conductors do not have any electric field inside, charges can be added continuously from the inside without needing to overcome the full potential of the outer shell. 
[[File:Spark by Van de Graaff generator.jpg|thumb|250px|right|Spark made by the Van de Graaff generator at [[Museum of Science (Boston)|The Museum of Science in Boston]], [[Massachusetts]]]]

The larger the sphere and the farther it is from ground, the higher its peak potential. The sign of the charge (positive or negative) can be controlled by the selection of materials for the belt and rollers. Higher potentials on the sphere can also be achieved by using a voltage source to charge the belt directly, rather than relying solely on the triboelectric effect.

A Van de Graaff generator terminal does not need to be sphere-shaped to work, and in fact, the optimum shape is a sphere with an inward curve around the hole where the belt enters. A rounded terminal minimizes the electric field around it, allowing greater potentials to be achieved without ionization of the air, or other [[dielectric gas]], surrounding it. Since a Van de Graaff generator can supply the same small current at almost any level of electrical potential, it is an example of a nearly ideal [[current source]].

The maximal achievable potential is roughly equal to the sphere radius ''R'' multiplied by the electric field ''E''<sub>max</sub> at which corona discharges begin to form within the surrounding gas. For air at standard temperature and pressure ([[Standard temperature and pressure|STP]]) the [[Electrical breakdown|breakdown field]] is about {{val|30|u=kV/cm}}. Therefore, a polished spherical electrode {{convert|30|cm|in}} in diameter could be expected to develop a maximal voltage {{nowrap|1=''V''<sub>max</sub> = ''R''·''E''<sub>max</sub>}} of about {{val|450|u=kV}}. This explains why Van de Graaff generators are often made with the largest possible diameter.<ref name="hinterberger">{{cite web |last1=Hinterberger |first1=F |title=Electrostatic Accelerators |url=https://cds.cern.ch/record/1005042/files/p95.pdf |website=[[CERN]] |access-date=10 May 2022}}</ref>

{{multiple image
| align = center
| direction = horizontal
| header   =
| image1   = Van De Graaff gen 03.jpg
| caption1 = Van de Graaff generator for educational use in schools
| image2   = Van De Graaff gen 04.jpg
| caption2 = With sausage-shaped top terminal removed
| image3   = Van De Graaff gen 06.jpg
| caption3 = Comb electrode at bottom that deposits charge onto belt
| image4   = Van De Graaff gen 05.jpg
| caption4 = Comb electrode at top that removes charge from belt
| width    = 240
| footer   =
}}

== Use as a particle accelerator ==
[[File:Tandem Accelerator Diagram.jpg|thumb|500px|A simplified diagram of a Tandem Accelerator]]
The initial motivation for the development of the Van de Graaff generator was as a source of high voltage to accelerate particles for nuclear physics experiments.<ref name="Van de Graaff" /> The high potential difference between the surface of the terminal and ground results in a corresponding [[electric field]].  When an [[ion source]] is placed near the surface of the sphere (typically within the sphere itself) the field will accelerate charged particles of the appropriate sign away from the sphere.  By insulating the generator with pressurized gas, the breakdown voltage can be raised, increasing the maximum energy of accelerated particles.{{r|hinterberger}}

=== Tandem accelerators ===
[[File:Nuclear accelerator in NCSR Demokritos.jpg|thumb|Van de Graaff Tandem accelerator at [[National Centre of Scientific Research "Demokritos"|NCSRD]] in Greece]]
Particle-beam Van de Graaff accelerators are often used in a "[[tandem accelerator|tandem]]" configuration with the high potential terminal located at the center of the machine. Negatively charged ions are injected at one end, where they are accelerated by attractive force toward the terminal. When the particles reach the terminal, they are stripped of some electrons to make them positively charged, and are subsequently accelerated by repulsive forces away from the terminal. This configuration results in two accelerations for the cost of one Van de Graaff generator and has the added advantage of leaving the ion source instrumentation accessible near ground potential.{{r|hinterberger}}

=== Pelletron ===
{{main|Pelletron}}
The pelletron is a style of tandem accelerator designed to overcome some of the disadvantages of using a belt to transfer charge to the high voltage terminal.  In the pelletron, the belt is replaced with "pellets", metal spheres joined by insulating links into a chain.  This chain of spheres serves the same function as the belt in a traditional Van de Graff accelerator – to convey charge to the high voltage terminal.  The separate charged spheres and higher durability of the chain mean that higher voltages can be achieved at the high voltage terminal, and charge can be conveyed to the terminal more quickly.{{r|hinterberger}}

== Entertainment and educational generators ==

{{multiple image
| align = center
| direction = horizontal
| header   =
| image1   = President Jimmy Carter in Oak Ridge (7071648945) (2).jpg
| caption1 = Woman touching Van de Graaff generator at the [[American Museum of Science and Energy]]. The charged strands of hair repel each other and stand out from her head
| width1   = 180
| image2   = Boston Museum of Science, Theater of Electricity.jpg
| caption2 = An educational program at the Theater of Electricity, [[Boston Museum of Science]] demonstrates the world's largest air-insulated Van de Graaff generator, built by Van de Graaff in the 1930s.
| width2   = 338
}}

The largest air-insulated Van de Graaff generator in the world, built by Dr. Van de Graaff in the 1930s, is now displayed permanently at Boston's [[Museum of Science, Boston|Museum of Science]]. With two conjoined {{convert|4.5|m|ft|adj=on|abbr=on }} [[aluminium]] spheres standing on columns {{convert|22|ft|m|abbr=on}} tall, this generator can often obtain 2&nbsp;MV (2 million [[volt]]s). Shows using the Van de Graaff generator and several [[Tesla coil]]s are conducted two to three times a day.<ref>{{cite web |title=Lightning! {{!}} Museum of Science, Boston |url=https://www.mos.org/live-presentations/lightning |website=www.mos.org |publisher=Boston Museum of Science |access-date=11 May 2022 |language=en }}</ref> Many science museums, such as the [[American Museum of Science and Energy]], have small-scale Van de Graaff generators on display, and exploit their static-producing qualities to create "lightning" or make people's hair stand up. Van de Graaff generators are also used in schools and science shows.<ref>{{cite web |title=Van De Graaff Generator Wonders |url=https://www.scienceworld.ca/resource/van-de-graaff-generator-wonders/ |website=Science World |publisher=Vancouver Science World |access-date=11 May 2022 }}</ref>

== Comparison with other electrostatic generators ==
Other [[electrostatic machine]]s such as the [[Wimshurst machine]] or [[Bonetti machine]] work similarly to the Van De Graaff generator; charge is transported by moving plates, disks, or cylinders to a high voltage electrode. For these generators, however, corona discharge from exposed metal parts at high potentials and poorer insulation result in smaller voltages. In an electrostatic generator, the rate of charge transported ([[electric current|current]]) to the high-voltage electrode is very small. After the machine is started, the voltage on the terminal electrode increases until the leakage current from the electrode equals the rate of charge transport. Therefore, leakage from the terminal determines the maximum voltage attainable. In the Van de Graaff generator, the belt allows the transport of charge into the interior of a large hollow spherical electrode. This is the ideal shape to minimize leakage and corona discharge, so the Van de Graaff generator can produce the greatest voltage. This is why the Van de Graaff design has been used for all electrostatic particle accelerators. In general, the larger the diameter and the smoother the sphere is, the higher the voltage that can be achieved.<ref>{{cite web |url=http://www.coe.ufrj.br/~acmq/bonetti.html |title=The Bonetti electrostatic machine |publisher=www.coe.ufrj.br |access-date=2010-09-14}}</ref>{{verify source|date=May 2019}}{{better source needed|date=May 2019}}

== Patents ==
* {{US patent|1991236}} — "''Electrostatic Generator''"
* {{US patent|2922905}} — "''Apparatus For Reducing Electron Loading In Positive-Ion Accelerators''"

== See also ==
* {{annotated link|Electrostatic levitation}}
* {{annotated link|Faraday cage}}
* {{annotated link|Metal spinning}} – Metalworking process used to fabricate thin metal spheres
* {{annotated link|Oudin coil}}
* {{annotated link|Tesla coil}}

== References ==
{{Reflist|30em}}

== External links ==
{{Commons category|Van de Graaff generators}}
* [https://science.howstuffworks.com/transport/engines-equipment/vdg.htm/printable How Van de Graaff Generators Work] with how to build, HowStuffWorks
* [https://nationalmaglab.org/education/magnet-academy/watch-play/interactive/van-de-graaff-generator ''Interactive Java tutorial'' – Van de Graaff Generator] National High Magnetic Field Laboratory
* [https://wmich.edu/physics/accelerator Tandem Van de Graaff Accelerator] Western Michigan University Physics
* [http://www.mos.org/sln/toe/toe.html Dr. Van de Graaff's huge machine at Museum of Science]
* [http://amasci.com/emotor/vdgdesc.html Van de Graaff Generator Frequently Asked Questions], Science Hobbyist
* [http://libraries.mit.edu/archives/exhibits/van-de-graaff/ Illustration from Report on Van de Graaff Generator From "Progress Report on the M.I.T. High-Voltage Generator at Round Hill"] {{Webarchive|url=https://web.archive.org/web/20150509203055/http://libraries.mit.edu/archives/exhibits/van-de-graaff/ |date=2015-05-09 }}
* [[Nikola Tesla]], "''{{DOClink|[https://massless.info/images/19340300.doc Possibilities Of Electrostatic Generators]}}''". Scientific American, March, 1934. (.doc format)
* Paolo Brenni,[http://pagesperso-orange.fr/lyonel.baum/sis.html ''The Van de Graaff Generator – An Electrostatic Machine for the 20th Century''] Bulletin of the Scientific Instrument Society No. 63 (1999)
* Charrier Jacques "''[http://www.sciences.univ-nantes.fr/physique/perso/charrier/tp/wimshurst/van.html Le générateur de Van de Graaff]''". Faculté des Sciences de Nantes.
* Hellborg, Ragnar, ed. Electrostatic Accelerators: Fundamentals and Applications [N.Y., N.Y.: Springer, 2005]. Available online at: https://books.google.com/books?id=tc6CEuIV1jEC&pg=PA51&lpg=PA51&dq=electrostatic+accelerator+book
* [https://www.ornl.gov/news/american-physical-society-names-ornls-holifield-facility-historic-physics-site ''American Physical Society names ORNL's Holifield Facility historic physics site'']

[[Category:Accelerator physics]]
[[Category:American inventions]]
[[Category:Electrostatic generators]]
[[Category:1929 introductions]]