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An electrostatic generator is a mechanical device that produces continuous current. The knowledge of static electricity dates back to the earliest civilizations, but for millennia it remained merely an interesting and mystifying phenomenon. The development of electrostatic machines did not begin in earnest until the 18th Century. Electrostatic generators operate by using manual (or other) power to transform mechanical work into electric energy. Electrostatic generators develop electrostatic charges of opposite sign rendered to divided conductors. These devices can produce high-voltage electrical output at relatively low electric currents.

Description

Electrostatic machines are used for generating high voltages, using either friction or electrostatic induction to accumulate electrical charges. Electrostatic generators are typically used in science classrooms to safely demonstrate electrical forces and high voltage phenomena (because these devices produce very high voltage at very low current). The potential differences achieved have been used for a variety of applications (such as operating X-ray tubes, sterilization of food, and nuclear physics experiments). Electrostatic generators such as the Van de Graaff generator, and variations as the Pelletron and the tandem generator, also find use in physics research.

Electrostatic generators are of two kinds: friction machines, and influence machines.

Friction machines

History
Some electrostatic generators are called friction machines because of the friction in the generation process. A primitive form of frictional electrical machine was constructed at about 1663 by Otto von Guericke, using a rotating sulphur globe rubbed by hand. Isaac Newton suggested the use of a glass globe instead of a sulphur one (Optics, 8th Query). F. Hawksbee improved the basic design. In the latter part of the 18th Century, Benjamin Franklin, Ewald Jürgen Georg von Kleist, and Pieter van Musschenbroek (the last two the inventors of the Leyden jar) made several important discoveries concerning electrostatic machines. Benjamin Franklin used the Earth's magnetic field and atmospheric electricity in his devices, also.

Generators were further advanced when G. M. Bose of Wittenberg added a collecting conductor (a insulated tube or cylinder supported on silk strings). In 1746, Watson's machine had a large wheel turning several glass globes with a sword and a gun barrel suspended from silk cords for its prime conductors. J. H. Winkler, the professor of physics at Leipzig, substituted a leather cushion for the hand. Andreas Gordon of Erfurt, a Scotch Benedictine monk, used a glass cylinder in place of a sphere. Jesse Ramsden, in 1768, constructed a widely used version of a plate electrical generator. By 1784, the van Marum machine could produce voltage with any polarity. Also in 1784, Van Marum constructed a rather large electrostatic machine of high quality (currently on display at the Teylers Museum in the Netherlands).

In 1785, N. Rouland constructed a silk belted machine which rubbed two grounded hare fur covered tubes. Edward Nairne developed an electrostatic generator in 1787 which introduced the ability to generate either positive or negative electricity, the first named being collected from the prime conductor carrying the collecting points and the second from the prime conductor carrying the cushion. The Winter machine possessed higher efficiency than earlier friction machines. In the 1830s, Georg Ohm possessed a machine similar to the van Marum machine for his research (which is now at the Deutches Museum, Munich, Germany). In 1840, the Woodward machine was developed from improving the Ramsden machine (placing the prime conductor above the disk(s)). Also in 1840, the Armstrong hydroelectric machine was developed and used steam as a charge carrier.

The Van de Graaff generator was developed, starting in 1929, at MIT. 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 capable of producing 1,000,000 volts was described in a patent disclosure. Nikola Tesla wrote a Scientific American article, "Possibilities of Electro-Static Generators" in 1934 concerning the Van de Graaff generator (pp. 132-134 and 163-165). Tesla stated, "I believe that when new types Van de Graaff generators are developed and sufficiently improved a great future will be assured to them".

Friction operation
The presence of surface charge imbalance means that the objects will exhibit attractive or repulsive forces. This surface charge imbalance, which leads to static electricity, can be generated by touching two differing surfaces together and then separating them due to the phenomena of contact electrification and the triboelectric effect. Rubbing two non-conductive objects generates a great amount of static electricity. This is not just the result of friction; two non-conductive surfaces can become charged by just being placed one on top of the other. Since most surfaces have a rough texture, it takes longer to achieve charging through contact than through rubbing. Rubbing objects together increases amount of adhesive contact between the two surfaces. Usually insulators, e.g., substances that do not conduct electricity, are good at both generating, and holding, a surface charge. Some examples of these substances are rubber, plastic, glass, and pith. Conductive objects only rarely generate charge imbalance except, for example, when a metal surface is impacted by solid or liquid nonconductors. The charge that is transferred during contact electrification is stored on the surface of each object. Note that the presence of electric current does not detract from the electrostatic forces nor from the sparking, from the corona discharge, or other phenomena. Both phenomena can exist simultaneously in the same system.

Influence machines

History
Frictional machines were, in time, gradually superseded by the second class of instrument mentioned above, namely, influence machines. These operate by electrostatic induction and convert mechanical work into electrostatic energy by the aid of a small initial charge which is continually being replenished or reinforced. The first suggestion of an influence machine appears to have grown out of the invention of Volta's electrophorus. The electrophorus is a single-plate capacitor used to produce imbalances of electric charge via the process of electrostatic induction. "Doublers" were the first rotating influence machines. Abraham Bennet, the inventor of the gold leaf electroscope, described a "doubler" or "machine for multiplying electric charges" (Phil. Trans., 1787). The Bennet's doubler was developed in 1787. Erasmus Darwin, B. Wilson, G. C. Bohnenberger, and J. C. E. Peclet developed various modifications of Bennet's device. In 1788, William Nicholson proposed his doubler. He developed the idea for the "rotating double" instrument which by turning a winch produced the two states of electricity without friction or communication with the earth. (Phil. Trans., 1788, p. 403) Nicholson later described a "spinning condenser" apparatus.

Others, including T. Cavallo (who developed the Cavallo multiplier in 1795), John Read, Charles Bernard Desormes, and Jean Nicolas Pierre Hachette, developed further various forms of rotating doubler. In 1798, The German scientist and preacher Gottlieb Christoph Bohnenberger, developed the Bohnenberger machine. Bohnenberger also, in the "Annalen der Physik" (1801), described an electrostatic machine based on the operation of the Bennet's doubler. Giuseppe Belli, in 1831, developed a widely used and simpler doubler which consisted of two curved metal plates between which revolved a pair of balls carried on an insulating stem. It was the first symmetrical influence machine. This apparatus was similar to Lord Kelvin's "replenisher". Lord Kelvin also devised a combined influence machine and electromagnetic machine, commonly called a mouse mill, for electrifying the ink in connection with his siphon recorder. Lord Kelvin also developed, between 1858 and 1867, a water-drop electrostatic generator, which he called the "water-dropping condenser".

In 1860, C. F. Varley patented a more modern type of influence machine. In 1865, August J. I. Toepler developed an influence machine that consisted of two disks fixed on the same shaft and rotating in the same direction. In 1868, the Schwedoff machine was one of the stranger machines developed. Also in 1868, several mixed friction-influence machine were developed, including the Kundt machine and the Carré machine. Between 1864 and 1880, W. T. B. Holtz constructed and described a large number of influence machines which were considered the most advanced developments of the time. In one form, the Holtz machine consisted of a glass disk mounted on a horizontal axis which could be made to rotate at a considerable speed by a multiplying gear, interacting with induction plates mounted in a fixed disk close to it. In 1866, the Piche machine (or Bertsch machine) was developed. In 1869, H. Julius Smith received the American patent for a portable and airtight device that was designed to ignite powder. Also in 1869, sectorless machines in Germany were investigated by Poggendorff.

The action and efficiency of influence machines were further investigated by F. Rossetti, A. Righi, and F. W. G. Kohlrausch. E. E. N. Mascart, A. Roiti, and E. Bouchotte also examined the efficiency and current producing power of influence machines. In 1871, sectorless machines were investigated by Musaeus. In 1872, Righi's electromer was developed and was one of the first antecedents of the Van de Graaff generator. In 1873, Leyser developed the Leyser machine to avoid polarity reversals. In 1878, the British inventor James Wimshurst developed an apparatus that had two glass disks mounted on two shafts. The Wimshurst machine was more fully reported to the scientific community by 1883. In 1880, Robert Voss (a Berlin instrument maker) devised a form of machine in which he claimed that the principles of Toepler and Holtz were combined. In 1882, Wimshurst developed his "Cylindrical Machine". In 1885, one of the largest Wimshurst machine was built in England (and is now at the Chicago Museum of Science and Industry). In 1887, Weinhold developed a system that possessed vertical metal bar inductors with wooden cylinders close to the disk for avoiding polarity reversals. M. L. Lebiez described a machine similar to the that of the Holtz and Voss machines, being a simplified Voss machine (L'Électricien, April 1895, pp. 225-227)

In 1898, the Pidgeon machine was developed with a unique setup by W. R. Pidgeon. In October 28 of that year, Pidgeon presented this machine to the Physical Society after several years of investigation into influence machines (beginning at the start of the decade). The device was later reported in the Philosphical Magazine (Dec. 1898, pg. 564) and the Electrical Review (Vol. XLV, pg. 748). Pidgeon machines possess fixed inductors arranged in a manner that increases the electrical induction effect (and it electrical output is at least double that of typical machines of this type when it is overtaxed). The essential features of the Pidgeon machine are, one, the combination of the rotating support and the fixed support for inducing charge, and, two, the improved insulation of all parts of the machine (but more especially of the generator's carriers). Pidgeon machines are a combination of a Wimshurst Machine and Voss Machine, with special features adapted to reduce the amount of charge leakage. Pidgeon machines excite themselves more readily than the best of these types of machines. In addition, Pidgeon investigated higher current "triplex" section machines (or "double machines with a single central disk") with enclosed sectors (and would receive British Patent 22517 (1899) for this type of machine).

Double disk machines and "triplex" electrostatic machines, with classical structure, were also developed extensively around the turn of the century. In 1900, F. Tudsbury discovered that enclosing a generator in a metallic chamber containing compressed air, or better, carbon dioxide, the insulating properties of compressed gases enabled a greatly improved effect to be obtained owing to the diminution of the leakage across the plates and from the supports. In 1903, Alfred Wehrsen patented an ebonite rotating disk possessing embedded sectors with button contacts at the disk surface and celluloid embedded inductor plates (DE154175; "Wehrsen machine"). In 1907, Heinrich Wommelsdorf reported a similar variation of the Holtz machine.

Related machines

In 1991, G. L. Paramo developed the Lorente generator. It is a triboelectric machine operating with rolling friction, consisting of four cylinders with the two central ones made of different insulating materials and the two outer ones metallic. The cylinders rotate under some pressure, and the charges separated between the two central cylinders are collected by the outer cylinders.

Fringe science and devices

These generators have been used, sometimes inappropriately and with some controversy, to support various fringe science investigations. In 1911, George Samuel Piggott received a patent for a compact double machine enclosed within a pressurized box for his experiments concerning radiotelegraphy and "antigravity". Much later (in the 1960s), the Testatika was built by German engineer, Paul Suisse Bauman, and promoted by a Swiss community, the Methernithans. Testatika is an electromagnetic generator based on the 1889 Pidgeon electrostatic machine, said to produce "free energy" available directly from the environment and which cannot be depleted (so it is available in effectively unlimited quantity).

See also

Further reading

  • C. L. Stong, "Electrostatic motors are powered by electric field of the Earth". October, 1974. (PDF)
  • Oleg D. Jefimenko , "Electrostatic Motors: Their History, Types, and Principles of Operation". Electret Scientific, Star City, 1973.
  • G. W. Francis (Author) and Oleg D. Jefimenko (Editor), "Electrostatic Experiments: An Encyclopedia of Early Electrostatic Experiments, Demonstrations, Devices, and Apparatus". Electret Scientific, Star City, 2005.
  • V. E. Johnson, "Modern High-Speed Influence Machines; Their principles, construction and applications to radiography, radio-telegraphy, spark photography, electro-culture, electro-therapeutics, high-tension gas ignition, and the testing of materials". ISBN B0000EFPCO
  • Alfred W. Simon, "Quantitative Theory of the Influence Electrostatic Generator". Phys. Rev. 24, 690–696 (1924), Issue 6 – December 1924.
  • J. Clerk Maxwell, Treatise on Electricity and Magnetism (2nd ed.,Oxford, 1881), vol. i. p.294
  • J. D. Everett, Electricity (expansion of part iii. of Deschanels Natural Philosophy) (London, 1901), ch. iv. p. 20
  • A. Winkelmann, Handbuch der Physik (Breslau, 1905), vol. iv. pp. 50-58 (contains a large number of references to original papers)
  • J. Gray, "Electrical Influence Machines, Their Historical Development and Modern Forms instruction on making them" (London, I903). (J. A. F.)
  • Silvanus P. Thompson, The Influence Machine from Nicholson -1788 to 1888, Journ. Soc. Tel. Eng., 1888, 17, p. 569
  • John Munro, The Story Of Electricity (The Project Gutenberg Etext)
  • A. D. Moore (Editor), "Electrostatics and its Applications". Wiley, New York, 1973.
  • Oleg D. Jefimenko (with D. K. Walker), "Electrostatic motors". Phys. Teach. 9, 121-129 (1971).
  • W. R. Pidgeon, "An Influence-Machine". Proc. Phys. Soc. London 12 No 1 (October 1892) 406-411.
  • W. R. Pidgeon, "An Influence-Machine". Proc. Phys. Soc. London 16 No 1 (October 1897) 253-257.

External articles and references

Electrical generators | History of physics

Elektrisiermaschine | Generatore elettrostatico | Elektriseermachine

 

This article is licensed under the GNU Free Documentation License. It uses material from the "Electrostatic generator".

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