Van de Graaff generator

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This article is about the machine used to accumulate electrical charge on a metal globe. For the progressive rock band with a similar name, see Van der Graaf Generator.
Van de Graaff generator
Large metal sphere supported on a clear plastic column, inside of which a rubber belt can be seen. A smaller sphere is supported on a metal rod. Both are mounted to a baseplate, on which there is a small driving electric motor.
Van de Graaff generator.
Uses Accelerating electrons to sterilize food and process materials, accelerating protons for nuclear physics experiments, producing energetic X-ray beams in nuclear medicine, physics education, entertainment
Inventor Robert J. Van de Graaff
Related items Van de Graaff, 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 potentials. 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.[1] The potential difference achieved in modern Van de Graaff generators can reach 5 megavolts. A tabletop version can produce on the order of 100,000 volts and can store enough energy to produce a visible spark. Small Van de Graaff machines are produced for entertainment, and in physics education to teach electrostatics; larger ones are displayed in science museums.

The Van de Graaff generator was developed as a particle accelerator in physics research, its high potential is used to accelerate subatomic particles to high speeds in an evacuated tube. It was the most powerful type of accelerator in the 1930s until the cyclotron was developed. Today it is still used as an accelerator to generate energetic particle and x-ray beams in fields such as nuclear medicine. In order to double the voltage, two generators are often used together, one generating positive and the other negative potential; this is called a tandem Van de Graaff accelerator. The voltage produced by an open-air Van de Graaff machine is limited by arcing and corona discharge to about 5 megavolts. Most modern industrial machines are enclosed in a pressurized tank of insulating gas; these can achieve potentials up to about 25 megavolts.


Van de Graaff Generator Diagram
Spark by a Van de Graaff generator at Museum of Science in Boston, Massachusetts

A simple Van de Graaff-generator consists of a belt of silk, or a similar flexible dielectric material, running over two metal pulleys, one of which is surrounded by a hollow metal sphere.[2] Two electrodes, (2) and (7), in the form of comb-shaped rows of sharp metal points, are positioned respectively near to the bottom of the lower pulley and inside the sphere, over the upper pulley. Comb (2) is connected to the sphere, and comb (7) to the ground. A high DC potential (with respect to earth) is applied to roller (3); a positive potential in this example.

As the belt passes in front of the lower comb, it receives negative charge that escapes from its points due to the influence of the electric field around the lower pulley, which ionizes the air at the points. As the belt touches the lower roller (6), it transfers some electrons, leaving the roller with a negative charge (if it is insulated from the terminal), which added to the negative charge in the belt generates enough electric field to ionize the air at the points of the upper comb. Electrons then leak from the belt to the upper comb and to the terminal, leaving the belt positively charged as it returns down and the terminal negatively charged. The sphere shields the upper roller and comb from the electric field generated by charges that accumulate at the outer surface of it, causing the discharge and change of polarity of the belt at the upper roller to occur practically as if the terminal were grounded. As the belt continues to move, a constant 'charging current' travels via the belt, and the sphere continues to accumulate negative charge until the rate that charge is being lost (through leakage and corona discharges) equals the charging current. The larger the sphere and the farther it is from ground, the higher will be its final potential.

Another method for building Van de Graaff generators is to use the triboelectric effect. The friction between the belt and the rollers, one of them now made of insulating material, or both made with insulating materials at different positions on the triboelectric scale, one above and other below the material of the belt, charges the rollers with opposite polarities. The strong e-field from the rollers then induces a corona discharge at the tips of the pointed comb electrodes. The electrodes then "spray" a charge onto the belt which is opposite in polarity to the charge on the rollers. The remaining operation is otherwise the same as the voltage-injecting version above. This type of generator is easier to build for science fair or homemade projects, since it does not require a potentially dangerous high-voltage source. The trade-off is that it cannot build up as high a voltage as the other type, that cannot also be easily regulated, and operation may become difficult under humid conditions (which can severely reduce triboelectric effects). Finally, since the position of the rollers can be reversed, the accumulated charge on the hollow metal sphere can either be positive or negative.

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. Since electrically charged conductors have no e-field inside, charges can be added continuously. A rounded terminal minimizes the electric field around it, allowing greater potentials to be achieved without ionization of the surrounding air, or other dielectric gas. Outside the sphere, the e-field quickly becomes very strong and applying charges from the outside would soon be prevented by the field.

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 maximum achievable potential is approximately equal to the sphere's radius multiplied by the e-field where corona discharges begin to form within the surrounding gas. For example, a polished spherical electrode 30 cm in diameter immersed in air at STP (which has a breakdown voltage of about 30 kV/cm) could be expected to develop a maximum voltage of about 450 kV.

Van de Graaff generator for educational use in schools
With sausage-shaped top terminal removed
Comb electrode at bottom that deposits charge onto belt
Comb electrode at top that removes charge from belt


The Westinghouse Atom Smasher, the 5 MeV Van de Graaff generator built in 1937 by the Westinghouse Electric company in Forest Hills, Pennsylvania
This Van de Graaff generator of the first Hungarian linear particle accelerator achieved 700 kV in 1951 and 1000 kV in 1952.
A Van de Graaff generator integrated with a particle accelerator: The generator produces the high fields (in the megavolt range) that accelerate the particles.

The fundamental idea for the friction machine as high-voltage supply, using electrostatic influence to charge rotating disk or belt, can be traced back to the 17th century or even before (cf. Friction machines History)

The Van de Graaff generator was developed, starting in 1929, by physicist Robert J. Van de Graaff at Princeton University on a fellowship, with help from colleague Nicholas Burke. The first model was demonstrated in October 1929.[3] The first machine used an ordinary tin can, a small motor, and a silk ribbon bought at a five-and-dime store. Whereupon he went to the head of the physics department requesting a hundred dollars 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 furnishes the only power needed."[4][5] According to a patent application, it had two 60-cm-diameter charge-accumulation spheres mounted on borosilicate glass columns 180 cm high; the apparatus cost only $90 in 1931.[6]

Van de Graaff applied for a second patent in December 1931, which was assigned to MIT in exchange for a share of net income. The patent was later granted.

In 1933, Van de Graaff built a 40-foot (12-m) model at MIT's Round Hill facility, the use of which was donated by Colonel Edward H. R. Green.

In 1937, the Westinghouse Electric company built a 65 feet (20 m) Van de Graaff generator capable of generating 5 MeV Forest Hills, Pennsylvania. It marked the beginning of nuclear research for civilian applications.[7][8] It was decommissioned in 1958 and was demolished in 2015.[9]

A more recent development is the tandem Van de Graaff accelerator, containing one or more Van de Graaff generators, in which negatively charged ions 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 MV single-ended "injector" and a 6 MV EN tandem.[10]

One of Van de Graaff's accelerators used two charged domes of sufficient size that each of the domes had laboratories inside - one to provide the source of the accelerated beam, and the other to analyze the actual experiment. The power for the equipment inside the domes came from generators that ran off the belt, and several sessions came to a rather gruesome end when a pigeon would try to fly between the two domes, causing them to discharge. (The accelerator was set up in an airplane hangar.)[11]

By the 1970s, up to 14 million volts could be achieved at the terminal of a tandem that used a tank of high-pressure sulfur hexafluoride (SF6) gas to prevent sparking by trapping electrons. This allowed the generation of heavy ion beams of several tens of megaelectronvolts, sufficient to study light ion direct nuclear reactions. The highest potential sustained by a Van de Graaff accelerator is 25.5 MV, achieved by the tandem at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory.[citation needed]

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 much higher velocity than a belt, and both the voltage and currents attainable are much higher than with a conventional Van de Graaff generator. The 14 UD Heavy Ion Accelerator at The Australian National University houses a 15-million-volt pelletron. Its chains are more than 20 meters long and can travel faster than 50 km/hr.[12]

The Nuclear Structure Facility (NSF)[13] 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 metres 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 on 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. Following financial cutbacks, the NSF closed in 1993.

Van de Graaff generators on display[edit]

A presenter at the Boston Museum of Science hosts an educational program called Theater of Electricity which uses Tesla coils and the world's largest air-insulated Van de Graaff generator to demonstrate the power of electricity
 A man wearing coveralls with his left hand near the metal sphere of a Van de Graaff generator. His hair is standing on end due to electrostatic repulsion.
A Van de Graaff generator on display at the Maker Faire, San Mateo, 2008.

The largest air-insulated Van de Graaff generator in the world, built by Dr. Van de Graaff in the 1930s, is now on permanent display at Boston's Museum of Science. With two conjoined 4.5-meter (15-foot) aluminium spheres standing on columns 22 feet (6.7 m) tall, this generator can often reach 2 MV (2 million volts). Shows using the Van de Graaff generator and several Tesla coils are conducted two to three times a day. 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 in science shows.

Comparison with other high-voltage generators[edit]

Other classical electrostatic machines like a Wimshurst machine or a Bonetti machine[14] can easily produce more current than a Van de Graaff generator[citation needed] for experiments with electrostatics, and have positive and negative outputs. In 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 (current) to the high voltage electrode is very small, so the maximum voltage is reached when the leakage current from the electrode equals the rate of charge transport. In the Van de Graaff, 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 can produce the highest voltage. This is why the Van de Graaff design has been used for all electrostatic particle accelerators.

The concept of an electrostatic generator in which charge is mechanically transported in small amounts into the interior of a high voltage electrode goes back to the Kelvin water dropper, invented in 1867 by William Thomson (Lord Kelvin),[15] in which charged drops of water fall into a bucket with the same polarity charge, adding to the charge.[16] In this machine 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.[1][16] 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 pulley by electrostatic induction using a charged plate. Gray also invented a belt machine.[16] Another more complicated belt machine was invented in 1903 by Juan Burboa[17][1] 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.[18][1]

The reason that the charge extracted from the belt moves to the outside of the sphere electrode even though it already has a high charge of the same polarity is explained by the Faraday ice pail experiment.[19]


See also[edit]


  1. ^ a b c d Van de Graaff, R. J.; Compton, K. T.; Van Atta, L. C. (February 1933). "The Electrostatic Production of High Voltage for Nuclear Investigations" (PDF). Physical Review (American Physical Society) 43 (3): 149–157. doi:10.1103/PhysRev.43.149. Retrieved August 31, 2015. 
  2. ^ Zavisa, John M. "How Van de Graaff Generators Work". HowStuffWorks. Retrieved 2007-12-28. 
  3. ^ Van de Graaff biography
  4. ^ R. Van de Graaf, Phys. Rev. Vol.38, 1931, p.1919
  5. ^ Niels Bohr's Times, Abraham Pais, Oxford University Press, 1991, pp.378-379
  6. ^ Article "Van de Graaff's Generator", in "Electrical Engineering Handbook", (ed)., CRC Press, Boca Raton, Florida USA, 1993 ISBN 0-8493-0185-8
  7. ^ Toker, Franklin (2009). Pittsburgh: A New Portrait. p. 470. 
  8. ^ "Van de Graaff particle accelerator, Westinghouse Electric and Manufacturing Co., Pittsburgh, PA, August 7, 1945.". Explore PA History. WITF-TV. Retrieved February 19, 2015. 
  9. ^ O'Neill, Brian (January 25, 2015). "Brian O'Neill: With Forest Hills atom smasher's fall, part of history tumbles". Pittsburgh Post-Gazette. 
  10. ^ J. Takacs, Energy Stabilization of Electrostatic Accelerators, John Wiley and Sons, Chichester, 1996
  11. ^
  12. ^
  13. ^ J S Lilley 1982 Phys. Scr. 25 435-442 doi:10.1088/0031-8949/25/3/001)
  14. ^ "The Bonetti electrostatic machine". Retrieved 2010-09-14. 
  15. ^ Thomson, William (November 1867). "On a self-acting apparatus for multiplying and maintaining electric charges, with applications to the Voltaic Theory". The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science (4th Series) (London: Taylor & Francis) 34 (231): 391–396. Retrieved September 1, 2015. 
  16. ^ a b c Gray, John (1890). Electrical Influence Machines. London: Whittaker and Co. pp. 187–190. 
  17. ^ US patent no. 776997, Juan G. H. Burboa Static electric machine, filed: August 13, 1903, granted: December 6, 1904
  18. ^ Swann, W. F. G. (1928). "A device for obtaining high potentials". Journal of the Franklin Institute 205: 828. 
  19. ^ Young, Hugh D.; Freedman, Roger A. (2012). University Physics, 13th Ed. Pearson Education, Inc. pp. 742–743. ISBN 0321696867. 

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