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PowerPedia:Robert J. Van de Graaff

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Robert Jemison van de Graaff, (There was an error working with the wiki: Code[11] There was an error working with the wiki: Code[27] and instrument maker, and professor of Physics at There was an error working with the wiki: Code[28]. Van de Graaff was influencial in making the atom bomb.

Biography

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Master's degree (Mechanical Engineering, 1923) University of Alabama

Marie Curie lectures (1925) La Sorbonne

Doctorate of Philosophy (Physics, 1928) Oxford University

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Van de Graaff was born in There was an error working with the wiki: Code[29]. He was the designer of the Van de Graaff generator, a device which produces High voltages. In 1929, Van de Graaff developed his first generator (producing 80,000 volts) at There was an error working with the wiki: Code[30] by 1933, he had constructed a much larger generator, capable of generating 7 million volts. He was a There was an error working with the wiki: Code[31], and from 1931 to 1934 a research associate at the There was an error working with the wiki: Code[32]. He became an associate professor in 1934 (staying there until 1960).

During World War II, Van de Graaff was director of the High Voltage Radiographic Project. After World War II, he co-founded the High Voltage Engineering Corporation (HVEC). During the 1950s, he invented the insulating core transformer (producing high-voltage direct current). He also developed There was an error working with the wiki: Code[33] technology. The There was an error working with the wiki: Code[34] awarded him the T. Bonner prize (1966) for the development of electrostatic accelerators. Van de Graaff died in There was an error working with the wiki: Code[35].

Honors

Van de Graaff crater crater s an unusual There was an error working with the wiki: Code[12] formation that has the appearance of two merged There was an error working with the wiki: Code[13]s, approximately in a There was an error working with the wiki: Code[14] shape with no intervening rim separating the two halves.

Van de Graaff generator

A Van de Graaff generator is an Electrostatic generator which uses a moving belt to accumulate very high There was an error working with the wiki: Code[15] There was an error working with the wiki: Code[36] for There was an error working with the wiki: Code[37] experiments. The Van de Graaff generator can be thought of as a constant-current source connected in parallel with a Capacitor and a very large electrical resistance.

Description

A simple Van de Graaff generator consists of a belt of silk, or a similar flexible Dielectric material, running over two pulleys, one of which is surrounded by a hollow metal sphere. Two There was an error working with the wiki: Code[38]s, E1 and E2, in the form of sharply pointed cones, are positioned respectively near to the bottom of the pulley and inside the sphere. E2 is connected to the sphere, and a high DC potential (with respect to earth) is applied to E1 a positive potential in this example.

The high voltage ionises the air at the tip of E1, repelling (spraying) positive charges onto the belt, which then carries them up and inside the sphere. This positive charge There was an error working with the wiki: Code[16] a negative charge to the electrode E2 and a positive charge to the sphere (to which E2 is connected). The high potential difference There was an error working with the wiki: Code[17] the air inside the sphere, and negative charges are repelled from E2 and onto the belt, discharging it. As a result of the There was an error working with the wiki: Code[39] effect, positive charge on E2 migrates to the sphere regardless of the sphere's existing Voltage. As the belt continues to move, a constant charging current travels via the belt, and the sphere continues to accumulate positive charge until the rate that charge is being lost (through leakage and There was an error working with the wiki: Code[40]s) equals the charging current. The larger the sphere and the farther it is from ground, the higher will be its final potential.

The other method for building Van de Graaff generators is to use the There was an error working with the wiki: Code[18] from the rollers then induces a corona discharge at the tip of the pointed 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 doesn't require a potentially dangerous high voltage source. The trade-off is that it cannot build up as high a voltage as the other type, and operation may become difficult under humid conditions (which can severely reduce triboelectric effects).

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 There was an error working with the wiki: Code[19] (which has a There was an error working with the wiki: Code[41] of about 30 kV/cm) could be expected to develop a maximum voltage of about 450 kV.

History

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 There was an error working with the wiki: Code[20])

The Van de Graaff generator was developed, starting in There was an error working with the wiki: Code[42], by physicist Robert J. Van de Graaff at There was an error working with the wiki: Code[43].

[The first model was demonstrated in October 1929. http://chem.ch.huji.ac.il/~eugeniik/history/graaff.html]

The first machine used a silk ribbon bought at a There was an error working with the wiki: Code[21]

Van de Graaff applied for a patent in December 1931, which was assigned to There was an error working with the wiki: Code[22] in exchange for a share of net income. The patent was later granted. In There was an error working with the wiki: Code[23] facility, the use of which was donated by There was an error working with the wiki: Code[24]. 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 There was an error working with the wiki: Code[44] before being stripped of two or more electrons, inside a high voltage terminal, and accelerated again.

One of Van de Graaf's accelerators used two charged domes of sufficient size that there were laboratories inside the domes - on one end to provide the source of the accelerated beam, and on 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 spectacular 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)

By the There was an error working with the wiki: Code[25] (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 Holyfield Radioactive Ion Beam Facility at There was an error working with the wiki: Code[45].

A further development is the There was an error working with the wiki: Code[46], 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 machine. A common misspelling of the name is Van der Graaf (with an R and a single F). See also There was an error working with the wiki: Code[47], a rock group.

Van de Graaff generators on display

One of the largest Van de Graaff generators in the world, built by Dr. Van de Graaff himself, is now on permanent display at Boston's There was an error working with the wiki: Code[26]. With two conjoined 15 foot There was an error working with the wiki: Code[48] spheres standing on columns many feet tall, this generator can often reach 2 million There was an error working with the wiki: Code[49]s on a cool, crisp There was an error working with the wiki: Code[50] day. Shows using the Van de Graaff generator and several Tesla coils are conducted several times each day.

Comparison with other high voltage generators

The other electrostatic machines like a triplex Wimshurst or a Bonetti machine can easily produce equivalent voltages, but with much higher current.

Publications and Patents

Literature

"A 1,500,000 Volt Electrostatic Generator". Physical Review, Volume 38, 1931.

"The Electrostatic Production of High Voltage for Nuclear Investigations". Massachusetts Institute of Technology. 1932.

"Electrostatic Generators for the Acceleration of Charged Particles". Progress in Physics, Volume 11, 1948.

"Irradiation of Biological Materials by High-Energy Roentgen Rays and Cathode Rays". Journal of Applied Physics, Volume 19, 1948.

"Tandem Electrostatic Accelerators". Nuclear Instruments and Methods, Volume 8, 1960.

"Electrostatic Acceleration of Very Heavy Ions, with Resulting Possibilities for Nuclear Research". Bulletin, American Physical Society, August 29, 1966.

Patents

There was an error working with the wiki: Code[1] -- "Electrostatic Generator"

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There was an error working with the wiki: Code[3] -- "Apparatus For Reducing Electron Loading In Positive-Ion Accelerators"

There was an error working with the wiki: Code[4] -- "High Voltage Electromagnetic Apparatus Having An Insulating Magnetic Core"

There was an error working with the wiki: Code[5] -- "High Voltage Electromagnetic Charged-Particle Accelerator Apparatus Having An Insulating Magnetic Core"

There was an error working with the wiki: Code[6] -- "Multi-Disk Electromagnetic Power Machinery"

There was an error working with the wiki: Code[7] -- "Inclined field High Voltage Vacuum Tubes"

There was an error working with the wiki: Code[8] -- "Electrostatic Generator"

There was an error working with the wiki: Code[9] -- "Apparatus For Reducing Electron Loading In Positive-Ion Accelerators"

Related

Testatika

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Electrical generator

Static electricity

High voltage

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Tesla coil

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Wimshurst machine

External articles and and references

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Wiplich, M., "Short Biography Of Robert Jemison Van de Graaff". 2001. [bnl.gov]

Trump, J.G., Merrill, F.H., and Safford, F.J., "Van de Graaff Generator". Rev. Sci. Instrum., 9 (1938) 398

"Biography". Munich, Tech University. (German)

"Dr. Van de Graaff's large generator". There was an error working with the wiki: Code[65].

"VDG for hobbyists and science fairs"

"History of the Van de Graaff Generator". Museum of Science, Boston. 2004.

Brenni, Paolo, "The Van de Graaff Generator -- An Electrostatic Machine for the 20th Century". Bulletin of the Scientific Instrument Society No. 63. 1999.

"Van de Graaff Robert C2". Robert J. Van de Graaff explains his electrostatic generator to Karl T. Compton, MIT President, shortly after his demonstration at the APS meeting in 1931. [aip.org]

There was an error working with the wiki: Code[1], Wikipedia: The Free Encyclopedia. Wikimedia Foundation.

Dr. Van de Graaff's huge machine at Museum of Science

Van de Graaff Generator Frequently Asked Questions

http://ireswww.in2p3.fr/ires/recherche/vivitron/uk/discovering.htm Vivitron English version Vivitron 20MV+ generator (old link: http://ireswww.in2p3.fr/ires/recherche/vivitron/viv_angl/descript_angl.html)

Illustration from Report on Van de Graaff Generator From “Progress Report on the M.I.T. High-Voltage Generator at Round Hill

Nikola Tesla, "Possibilities Of Electrostatic Generators". Scientific American, March, 1934. (.doc format)

Charrier Jacques "Le générateur de Van de Graaff". Faculté des Sciences de Nantes.

Article "Van de Graaff's Generator", in "Electrical Engineering Handbook", Richard C. Dorf (ed)., CRC Press, Boca Raton, Florida USA, 1993 ISBN 0-8493-0185-8

See also

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