A
Van de Graaff generator is an
electrostatic generatorAn electrostatic generator, or electrostatic machine, is a mechanical device that produces static electricity, or electricity at high voltage and low continuous current...
which uses a moving belt to accumulate very high
voltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
s on a hollow metal globe on the top of the stand. It was invented in 1929 by American physicist
Robert J. Van de GraaffRobert Jemison Van de Graaff, was an American physicist, noted for his design and construction of high voltage generators, who taught at Princeton University and MIT.-Biography:...
. The potential differences achieved in modern Van de Graaff generators can reach 5 megavolts. The Van de Graaff generator can be thought of as a constant-current source connected in parallel with a
capacitorA capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
and a very large electrical resistance.
Description
A simple Van de Graaff-generator consists of a belt of silk, or a similar flexible
dielectricA dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
material, running over two metal pulleys, one of which is surrounded by a hollow metal sphere. Two
electrodeAn electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s, (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 upper 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 dischargeIn electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...
s) 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 effectThe triboelectric effect is a type of contact electrification in which certain materials become electrically charged after they come into contact with another different material and are then separated...
. 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-fieldIn physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
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 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, 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 doesn't need to be sphere shaped in order to work, and in fact the optimum shape is a sphere with an inward curve around the hole where the belt enters. The fact that electrically charged conductors of any shape have no e-field inside makes it possible to keep adding charges 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 gasA dielectric gas, or insulating gas, is a dielectric material in gaseous state. Its main purpose is to prevent or rapidly quench electric discharges. Dielectric gases are used as electrical insulators in high voltage applications, e.g...
. 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 sourceA current source is an electrical or electronic device that delivers or absorbs electric current. A current source is the dual of a voltage source. The term constant-current sink is sometimes used for sources fed from a negative voltage supply...
. 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
STPStandard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...
(which has a
breakdown voltageThe breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive.The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse...
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 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 GraaffRobert Jemison Van de Graaff, was an American physicist, noted for his design and construction of high voltage generators, who taught at Princeton University and MIT.-Biography:...
at
Princeton UniversityPrinceton University is a private research university located in Princeton, New Jersey, United States. The school is one of the eight universities of the Ivy League, and is one of the nine Colonial Colleges founded before the American Revolution....
. The first model was demonstrated in October 1929. The first machine used a silk ribbon bought at a five-and-dime store as the charge transport belt. In 1931 a version able to produce 1,000,000 volts was described in a patent disclosure. This version had two 60 cm diameter charge accumulation spheres mounted on
borosilicate glassPyrex is a brand name for glassware, introduced by Corning Incorporated in 1915.Originally, Pyrex was made from borosilicate glass. In the 1940s the composition was changed for some products to tempered soda-lime glass, which is the most common form of glass used in glass bakeware in the US and has...
columns 180 cm high; the apparatus cost only $90 in 1931.
Van de Graaff applied for a patent in in December 1931, which was assigned to
MITThe Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...
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 HillRound Hill is a location in Dartmouth, Massachusetts of historical significance.- History :Edward Howland Robinson Green, known as "Colonel" Ned Green, the only son of the renowned female tycoon and miser, Hetty Green, built his home on Round Hill after his mother's death in 1916 left him with a...
facility, the use of which was donated by
Colonel Edward H. R. GreenEdward Howland Robinson "Ned" Green , also known as Colonel Green, was an American businessman, the only son of the notorious miser Hetty Green . He was also noted for his stamp and coin collections.-Biography:Edward Green was the first of two children of Hetty and Edward Henry Green...
.
A more recent development is the
tandem Van de Graaff accelerator, containing one or more Van de Graaff generators, in which negatively charged
ionAn ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
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 MV single ended "Injector" and a 6 MV EN tandem .
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.)
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 (SF
6) 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 LaboratoryOak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the DOE's largest science and energy laboratory. ORNL is located in Oak Ridge, Tennessee, near Knoxville...
.
A further development is the
pelletronA pelletron is a type of electrostatic particle accelerator similar to a Van de Graaff generator. Pelletrons have been built in many sizes, from small units producing voltages up to 500 kilovolts and beam energies up to 1 megaelectronvolt of kinetic energy, to the largest system, which has...
, 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.
The Nuclear Structure Facility (NSF) at
Daresbury LaboratoryDaresbury Laboratory is a scientific research laboratory near Daresbury in Cheshire, England, which began operations in 1962 and was officially opened on 16 June 1967 as the Daresbury Nuclear Physics Laboratory by the then Prime Minister of United Kingdom, Harold Wilson...
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
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 ScienceThe Museum of Science is a Boston, Massachusetts landmark, located in Science Park, a plot of land spanning the Charles River. Along with over 500 interactive exhibits, the Museum features a number of live presentations throughout the building every day, along with shows at the Charles Hayden...
. With two conjoined 4.5 meter (15 foot)
aluminiumAluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
spheres standing on columns 22 feet (6.7 m) tall, this generator can often reach 2 MV (2 million
voltThe volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
s). Shows using the Van de Graaff generator and several
Tesla coilA Tesla coil is a type of resonant transformer circuit invented by Nikola Tesla around 1891. It is used to produce high voltage, low current, high frequency alternating current electricity. Tesla coils produce higher current than the other source of high voltage discharges, electrostatic machines...
s are conducted two to three times a day. Many science museums, such as the
American Museum of Science and EnergyThe American Museum of Science and Energy is a science museum in Oak Ridge, Tennessee, designed to teach both children and adults about energy, especially nuclear power, and to document the role Oak Ridge played in the Manhattan Project. The museum opened as the American Museum of Atomic Energy in...
, 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.
Comparison with other high voltage generators
Other classical electrostatic machines like a
Wimshurst MachineThe Wimshurst influence machine is an electrostatic generator, a machine for generating high voltages developed between 1880 and 1883 by British inventor James Wimshurst ....
or a Bonetti machine can easily produce more current than a Van de Graaff generator for experiments with electrostatics, and have positive and negative output. The less-insulated structures, however, result in smaller voltages.
Patents
— "
Electrostatic Generator" — "
Apparatus For Reducing Electron Loading In Positive-Ion Accelerators"
See also
- Kirlian photography
Kirlian photography refers to a form of photogram made with voltage. It is named after Semyon Kirlian, who in 1939 accidentally discovered that if an object on a photographic plate is connected to a source of voltage an image is produced on the photographic plate.Kirlian's work, from 1939 onward,...
- Los Alamos National Laboratory
Los Alamos National Laboratory is a United States Department of Energy national laboratory, managed and operated by Los Alamos National Security , located in Los Alamos, New Mexico...
- Electrostatic induction
Electrostatic induction is a redistribution of electrical charge in an object, caused by the influence of nearby charges. Induction was discovered by British scientist John Canton in 1753 and Swedish professor Johan Carl Wilcke in 1762. Electrostatic generators, such as the Wimshurst machine, the...
- Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
- Triboelectric effect
The triboelectric effect is a type of contact electrification in which certain materials become electrically charged after they come into contact with another different material and are then separated...
- Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
- Robert J. Van de Graaff
Robert Jemison Van de Graaff, was an American physicist, noted for his design and construction of high voltage generators, who taught at Princeton University and MIT.-Biography:...
- Static electricity
Static electricity refers to the build-up of electric charge on the surface of objects. The static charges remain on an object until they either bleed off to ground or are quickly neutralized by a discharge. Static electricity can be contrasted with current electricity, which can be delivered...
- High-voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
- Teleforce
Teleforce was Nikola Tesla's charged particle beam projector, first mentioned publicly in the New York Sun and New York Times on July 11, 1934.-Introduction:...
- 1929 in science
The year 1929 in science and technology involved some significant events, listed below.-Astronomy and space exploration:* July 17 - Robert H...
- Tesla coil
A Tesla coil is a type of resonant transformer circuit invented by Nikola Tesla around 1891. It is used to produce high voltage, low current, high frequency alternating current electricity. Tesla coils produce higher current than the other source of high voltage discharges, electrostatic machines...
- Oudin coil
An Oudin coil, also called an Oudin oscillator or Oudin resonator, is a disruptive discharge coil wired as a transformer designed to produce high voltage arcs and discharges, similar to a Tesla coil...
- Wimshurst machine
The Wimshurst influence machine is an electrostatic generator, a machine for generating high voltages developed between 1880 and 1883 by British inventor James Wimshurst ....
External links
- UVA Virtual Lab: Van de Graaff Generators University of Virginia
- Interactive Java tutorial - Van de Graaff Generator National High Magnetic Field Laboratory
- The Van de Graaff Accelerator Facility Western Michigan University
- Dr. Van de Graaff's huge machine at Museum of Science
- Van de Graaff Generator Frequently Asked Questions Vivitron 20MV+ generator
- Illustration from Report on Van de Graaff Generator From “Progress Report on the M.I.T. High-Voltage Generator at Round Hill"
- Nikola Tesla
Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...
, "". Scientific American, March, 1934. (.doc format)
- Paolo Brenni , The Van de Graaff Generator - An Electrostatic Machine for the 20th Century Bulletin of the Scientific Instrument Society No. 63 (1999)
- Charrier Jacques "Le générateur de Van de Graaff". Faculté des Sciences de Nantes.
- Making VDGs
- Hellborg, Ragnar, ed. Electrostatic Accelerators: Fundamentals and Applications [N.Y., N.Y.: Springer, 2005]. Available on-line at: http://books.google.com/books?id=tc6CEuIV1jEC&pg=PA51&lpg=PA51&dq=electrostatic+accelerator+book&source=web&ots=Qa0DbmiZJt&sig=bLoYaz_VUpBr7-Wv4lk_fLBnUo4#PPP1,M1
- Build your own VDG