Electrostatic generator


An electrostatic generator, or electrostatic machine, is an electrical generator that produces static electricity, or electricity at high voltage and low continuous current. The knowledge of static electricity dates back to the earliest civilizations, but for millennia it remained merely an interesting and mystifying phenomenon, without a theory to explain its behavior and often confused with magnetism. By the end of the 17th century, researchers had developed practical means of generating electricity by friction, but the development of electrostatic machines did not begin in earnest until the 18th century, when they became fundamental instruments in the studies about the new science of electricity.
Electrostatic generators operate by using manual power to transform mechanical work into electric energy, or using electric currents. Manual electrostatic generators develop electrostatic charges of opposite signs rendered to two conductors, using only electric forces, and work by using moving plates, drums, or belts to carry electric charge to a high potential electrode.

Description

Electrostatic machines are typically used in science classrooms to safely demonstrate electrical forces and high voltage phenomena. The elevated potential differences achieved have also been used for a variety of practical applications, such as operating X-ray tubes, particle accelerators, spectroscopy, medical applications, sterilization of food, and nuclear physics experiments. Electrostatic generators such as the Van de Graaff generator, and variations as the Pelletron, also find use in physics research.
Electrostatic generators can be divided into categories depending on how the charge is generated:

History

The first electrostatic generators are called friction machines because of the friction in the generation process. A primitive form of frictional machine was invented around 1663 by Otto von Guericke, using a sulphur globe that could be rotated and rubbed by hand. It may not actually have been rotated during use and was not intended to produce electricity, but inspired many later machines that used rotating globes. Isaac Newton constructed his own primitive electrostatic generator, being the first to use a glass globe instead of a sulphur one. In about 1706 Francis Hauksbee improved the basic design, with his frictional electrical machine that enabled a glass sphere to be rotated rapidly against a woollen cloth.
Generators were further advanced when, about 1730, Prof. Georg Matthias Bose of Wittenberg added a collecting conductor. Bose was the first to employ the "prime conductor" in such machines, this consisting of an iron rod held in the hand of a person whose body was insulated by standing on a block of resin.
In 1746, William 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. Johann Heinrich Winckler, professor of physics at Leipzig, substituted a leather cushion for the hand. During 1746, Jan Ingenhousz invented electric machines made of plate glass. Experiments with the electric machine were largely aided by the invention of the Leyden Jar. This early form of the capacitor, with conductive coatings on either side of the glass, can accumulate a charge of electricity when connected with a source of electromotive force.
The electric machine was soon further improved by Andrew Gordon, a Scotsman and 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 improved the efficiency of electric machines by sprinkling an amalgam of tin over the surface of the rubber. In 1768, Jesse Ramsden constructed a widely used version of a plate electrical generator.
In 1783, Dutch scientist Martin van Marum of Haarlem designed a large electrostatic machine of high quality with glass disks 1.65 meters in diameter for his experiments. Capable of producing voltage with either polarity, it was built under his supervision by John Cuthbertson of Amsterdam the following year. The generator is currently on display at the Teylers Museum in Haarlem.
In 1785, N. Rouland constructed a silk-belted machine that rubbed two grounded tubes covered with hare fur. Edward Nairne developed an electrostatic generator for medical purposes in 1787 that had the ability to generate either positive or negative electricity, the first of these 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. In 1840, the Woodward machine was developed by improving the 1768 Ramsden machine, placing the prime conductor above the disk. Also in 1840, the Armstrong hydroelectric machine was developed, using steam as a charge carrier.

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 phenomenon of the triboelectric effect. Rubbing two non-conductive objects can generate a great amount of static electricity. This is not 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 in contact generate charge imbalance too, but retain the charges only if insulated. 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 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", 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. 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". Nicholson later described a "spinning condenser" apparatus, as a better instrument for measurements.
Erasmus Darwin, W. Wilson, G. C. Bohnenberger, and J. C. E. Péclet developed various modifications of Bennet's 1787 device. Francis Ronalds automated the generation process in 1816 by adapting a pendulum bob as one of the plates, driven by clockwork or a steam engine – he created the device to power his electric telegraph.
Others, including T. Cavallo, 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". 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 1868, and by A. D. Moore, 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, which he called the "water-dropping condenser".
Holtz 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 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 Friedrich 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 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.