Generador electrostático

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Un generador electrostático, o máquina electrostática, es un dispositivo mecánico que produce electricidad estática, o electricidad a alta tensión y corriente continua baja. El conocimiento de la electricidad estática se remonta a las primeras civilizaciones, pero durante miles de años se mantuvo meramente como un fenómeno interesante y desconcertante, sin una teoría para explicar su comportamiento, y a menudo confundido con el magnetismo. A finales del siglo XVII , los investigadores habían desarrollado los medios prácticos para la generación de electricidad por fricción, pero el desarrollo de máquinas electrostáticas no comenzó en serio hasta el siglo XVIII, cuando se convirtieron en instrumentos fundamentales en los estudios acerca de la nueva ciencia de la electricidad. Los generadores electrostáticos funcionan mediante el uso de energía manual (u otra) para transformar trabajo mecánico en energía eléctrica. Estos dispositivos provocan la acumulación cargas electrostáticas de signos opuestos en ambos conductores, usando solamente fuerzas eléctricas y trabajan en base a placas en movimiento, tambores o cintas para así llevar carga eléctrica a un electrodo de mayor potencial. La carga es generada por uno de dos métodos: o bien el efecto triboeléctrico (fricción) o la inducción electrostática.

 Esfera grande de metal sostenida por un tubo de plástico transparente, dentro del cual se puede ver una correa de goma.  Una esfera más pequeña es sostenida por una barra de metal. Ambos están montados sobre una base en la cual se encuentra también un pequeño motor eléctrico.
Un generador Van de Graaff, para demostraciones de aula

Descripción[editar]

Las máquinas electrostáticas se utilizan normalmente en las clases de ciencia para demostrar de forma segura las fuerzas eléctricas y fenómenos de alto voltaje. Las altas tensiones obtenidos han sido también utilizadas para una variedad de aplicaciones prácticas, tales como tubos de rayos X operativos, aplicaciones médicas, la esterilización de alimentos y experimentos de física nuclear. Generadores electrostáticos como el Generador de Van de Graaff, y sus variaciones como el Pelletron, también se usan para investigaciones físicas.

Los generadores electrostáticos se pueden dividir en dos categorías dependiendo de cómo se genera la carga:

Máquinas de fricción[editar]

Historia[editar]

Typical friction machine using a glass globe, common in the 18th century
Martinus van Marum's Electrostatic generator at Teylers Museum

Los primeros generadores electrostáticos son llamados máquinas de fricción debido que emplean la fricción como base en el proceso de generación. Una forma primitiva de la máquina de fricción fue inventada alrededor de 1663 por Otto von Guericke, usando un globo de azufre que se podía girar y frotar con la mano. Es posible que en realidad no haya sido esta su finalidad, pero pudo haber inspirado a muchas máquinas posteriores que utilizaron globos giratorios. Isaac Newton sugiere el uso de un globo de cristal en vez de uno de azufre . Un avance se dió cuando el profesor Georg Matthias Bose de Wittenberg agregó un conductor colector (un tubo aislado o cilindro soportados en cuerdas de seda). Boze fue el primero en emplear el "conductor primario" en este tipo de máquinas, esto consiste en una barra de hierro en la mano de una persona cuyo cuerpo fue aislado de pie sobre un bloque de resina. In 1746, La maquina de Watnson tenia una gran rueda turning several glass globes with a sword and a gun barrel suspended from silk cords for its prime conductors. J. H. Winkler, professor of physics at Leipzig, substituted a leather cushion for the hand. Andreas Gordon of Erfurt, a Scottish 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 either polarity. Martin van Marum constructed a large electrostatic machine of high quality for his experiments (currently on display at the Teylers Museum in the Netherlands).

Ingenhousz, during 1746, invented electric machines made of plate glass.[1] Experiments with the electric machine were largely aided by the discovery of the property of a glass plate, when coated on both sides with tinfoil, of accumulating a charge of electricity when connected with a source of electromotive force. The electric machine was soon further improved by Andrew Gordon, a Scotsman, Professor at Erfurt, who substituted a glass cylinder in place of a glass globe; and by Giessing of Leipzig who added a "rubber" consisting of a cushion of woollen material. The collector, consisting of a series of metal points, was added to the machine by Benjamin Wilson about 1746, and in 1762, John Canton of England (also the inventor of the first pith-ball electroscope) improved the efficiency of electric machines by sprinkling an amalgam of tin over the surface of the rubber.[2]

In 1785, N. Rouland constructed a silk belted machine which rubbed two grounded hare fur covered tubes. Edward Nairne developed an electrostatic generator for medical purposes in 1787 which had 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 another prime conductor carrying the friction pad. 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 Deutsches 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.

Operación fricción[editar]

La presencia de [ carga superficial [ ] ] desequilibrio significa que los objetos exhibirán fuerzas atractivas o repulsivas . Este desequilibrio de carga superficial , lo que conduce a la electricidad estática , puede ser generada por el contacto de dos superficies diferentes juntos y luego separarlos debido a los fenómenos de la [ electrificación [contacto ] ] y [ [ efecto triboeléctrico ] ] . Frotar dos objetos no conductores genera una gran cantidad de electricidad estática. Esto no es sólo el resultado de la fricción; dos superficies no conductoras pueden ser cargados por sólo ser colocados uno encima del otro . Como la mayoría de las superficies tienen una textura áspera , se necesita más tiempo para lograr la carga a través de contactos que a través de la fricción. Frotar objetos al mismo tiempo aumenta la cantidad de adhesivo de contacto entre las dos superficies . Por lo general, [ [ Aislante (electricidad) | aisladores ] ] , por ejemplo , sustancias que no conducen la electricidad , son buenos en tanto la generación, y la celebración , una carga superficial . Son algunos ejemplos de estas sustancias [ [ goma ] ], [ [ plástico ] ], [ [ vidrio ] ] y [ [ medula ] ] . [ [conductor (material) | conductiva ] ] objetos en contacto generan desequilibrio de carga también, pero conservan los cargos sólo si aislante. La carga que se transfiere durante el contacto de electrificación se almacena en la superficie de cada objeto . Tenga en cuenta que la presencia de [ [ corriente eléctrica ] ] no desvirtúa las fuerzas electrostáticas , ni de las chispas , de la [ [ descarga de corona ] ] , u otros fenómenos . Ambos fenómenos pueden existir simultáneamente en el mismo sistema .

Influence machines[editar]

History[editar]

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 and 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. The next step was when Abraham Bennet, the inventor of the gold leaf electroscope, described a "doubler of electricity" (Phil. Trans., 1787), as a device similar to the electrophorus, but that could amplify a small charge by means of repeated manual operations with three insulated plates, in order to make it observable in an electroscope. Erasmus Darwin, W. Wilson, G. C. Bohnenberger, and (later, 1841) J. C. E. Péclet developed various modifications of Bennet's device. In 1788, William Nicholson proposed his rotating doubler, which can be considered as the first rotating influence machine. His instrument was described as "an instrument which by turning a winch produces the two states of electricity without friction or communication with the earth". (Phil. Trans., 1788, p. 403) Nicholson later described a "spinning condenser" apparatus, as a better instrument for measurements.

Others, including T. Cavallo (who developed the "Cavallo multiplier", a charge multiplier using simple addition, in 1795), John Read, Charles Bernard Desormes, and Jean Nicolas Pierre Hachette, developed further various forms of rotating doublers. In 1798, The German scientist and preacher Gottlieb Christoph Bohnenberger, described the Bohnenberger machine, along with several other doublers of Bennet and Nicholson types in a book. The most interesting of these were described in the "Annalen der Physik" (1801). Giuseppe Belli, in 1831, developed a simple symmetrical doubler which consisted of two curved metal plates between which revolved a pair of plates carried on an insulating stem. It was the first symmetrical influence machine, with identical structures for both terminals. This apparatus was reinvented several times, by C. F. Varley, that patented a high power version in 1860, by Lord Kelvin (the "replenisher") 1868, and by A. D. Moore (the "dirod"), more recently. 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, and a water-drop electrostatic generator (1867), which he called the "water-dropping condenser".

Holtz's influence 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 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 had a curious structure to increase the output current. Also in 1868, several mixed friction-influence machine were developed, including the Kundt machine and the Carré machine. 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 electrometer was developed and was one of the first antecedents of the Van de Graaff generator. In 1873, Leyser developed the Leyser machine, a variation of the Holtz machine. 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. The same structure become also known as the Toepler-Holtz machine. In 1878, the British inventor James Wimshurst started his studies about electrostatic generators, improving the Holtz machine, in a powerful version with multiple disks. The classical Wimshurst machine, that become the most popular form of influence machine, was reported to the scientific community by 1883, although previous machines with very similar structures were previously described by Holtz and Musaeus. In 1885, one of the largest-ever Wimshurst machines was built in England (it is now at the Chicago Museum of Science and Industry). In 1887, Weinhold modified the Leyser machine with a system of vertical metal bar inductors with wooden cylinders close to the disk for avoiding polarity reversals. M. L. Lebiez described the Lebiez machine, that was essentially a simplified Voss machine (L'Électricien, April 1895, pp. 225–227). In 1894, Bonetti[3] designed a machine with the structure of the Wimshurst machine, but without metal sectors in the disks. This machine is significantly more powerful than the sectored version, but it must usually be started with an externally-applied charge.

In 1898, the Pidgeon machine was developed with a unique setup by W. R. Pidgeon. On October 28 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 Philosophical Magazine (December 1898, pg. 564) and the Electrical Review (Vol. XLV, pg. 748). A Pidgeon machine possesses fixed inductors arranged in a manner that increases the electrical induction effect (and its electrical output is at least double that of typical machines of this type [except 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 went on to receive British Patent 22517 (1899) for this type of machine).

Multiple disk machines and "triplex" electrostatic machines (generators with three disks) were also developed extensively around the turn of the 20th 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 increase in the breakdown voltage of the compressed gas, and reduction of the leakage across the plates and insulating supports. In 1903, Alfred Wehrsen patented an ebonite rotating disk possessing embedded sectors with button contacts at the disk surface. In 1907, Heinrich Wommelsdorf reported a variation of the Holtz machine using this disk and inductors embedded in celluloid plates (DE154175; "Wehrsen machine"). Wommelsdorf also developed several high-performance electrostatic generators, of which the best known were his "Condenser machines" (1920). These were single disk machines, using disks with embedded sectors that were accessed at the edges.

Modern electrostatic generators[editar]

Archivo:Electrostaticgenerator.JPG
An example of a common modern device using high voltage (a "plasma globe", that does not use static electricity)

Electrostatic generators had a fundamental role in the investigations about the structure of matter, starting at the end of the 19th century. By the 1920s, it was evident that machines able to produce greater voltage were needed. The Van de Graaff generator was developed, starting in 1929, at MIT. The first model was demonstrated in October 1929. The basic idea was to use an insulating belt to transport electric charge to the interior of an insulated hollow terminal, where it could be discharged regardless of the potential already present on the terminal, that does not produce any electric field in its interior. The idea was not new, but the implementation using an electric power supply to charge the belt was a fundamental innovation that made the old machines obsolete. 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. 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 [of Van de Graaff generators] are developed and sufficiently improved a great future will be assured to them". High-power machines were soon developed, working on pressurized containers to allow greater charge concentration on the surfaces without ionization. Variations of the Van de Graaff generator were also developed for Physics research, as the Pelletron, that uses a chain with alternating insulating and conducting links for charge transport. Simplified Van de Graaff generators are commonly seen in demonstrations about static electricity, due to its high-voltage capability, producing the curious effect of making the hair of people touching the terminal, standing over an insulating support, stand up.

Between 1945 and 1960, the French researcher Noël Felici developed a series of high-power electrostatic generators, based on electric excitation and using cylinders rotating at high speed and hydrogen in pressurized containers.

Ciencia marginal y otros dispositivos[editar]

Estos generadores han sido utilizados generalmente de forma inapropiada y de forma controversial, para respaldar diversas investigaciones de ciencia marginal. En 1911, George Samuel Piggott recibió una patente para una máquina doble encerrada dentro de un recubrimiento presurizado para desarrollar sus experimentos de radiotelegrafía y "antigravedad". Mucho después (en la década de 1960) el ingeniero alemán Paul Suisse Bauman, patrocinado por una comunidad suiza —los meternitanos— construyó una una máquina conocida como "Testatika". Este es un generador electromagnético basado en la máquina electrostática de Pidgeon, que decía producir energía libre directamente desde el medio.

Véase también[editar]

Referencias[editar]

  1. Consult Dr. Carpue's 'Introduction to Electricity and Galvanism,' London 1803.
  2. Maver, William Jr.: "Electricity, its History and Progress", The Encyclopedia Americana; a library of universal knowledge, vol. X, pp. 172ff. (1918). New York: Encyclopedia Americana Corp.
  3. http://www.coe.ufrj.br/~acmq/bonetti.html Instructions for building a Bonetti machine

Further reading[editar]

  • Gottlieb Christoph Bohnenberger: Description of different electricity-doubler of a new device, along with a number of experiments on various subjects of electricity, etc.. Tübingen 1798.
  • William Holtz: On a new electrical machine .. In: Johann Poggendorff, CG Barth (Eds.): Annals of physics and chemistry. 126, Leipzig 1865, p. 157 - 171st
  • William Holtz: the higher charge on insulating surfaces by side pull and the transfer of this principle to the construction of induction machines .. In: Johann Poggendorff, CG Barth (eds): Annals of physics and chemistry. 130, Leipzig 1867, p. 128 - 136
  • William Holtz: The influence machine. In: F. Poske (Eds.): Annals of physics and chemistry. Julius Springer, Berlin 1904 (seventeenth year, the fourth issue).
  • O. Lehmann: Dr. J. Frick's physical technique. 2, Friedrich Vieweg und Sohn, Braunschweig 1909, p. 797 (Section 2).
  • F. Poske: New forms of influence machines .. In: F. Poske (eds) for the physical and chemical education. journal Julius Springer, Berlin 1893 (seventh year, second issue).
  • 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 [with 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(1)1 (October 1892) 406–411 and 16(1) (October 1897) 253–257.

Enlaces externos[editar]