Electrostatic generator
Electrostatic generators are mechanical devices for generating electrical voltage by means of electrostatics by separating electrical charges . In general, they are capable of reaching high voltages, but cannot deliver high power . The achievable voltages are only limited by insulation problems such as B. Leakage current and peak discharges are limited.
history
The first electrostatic generators worked on the principle of static electricity and were known as electrifying machines until the end of the 19th century . From 1883 on, generators were developed that work on the principle of influence . These generators are called induction machines .
In 1663 Otto von Guericke built a sulfur ball with an axis of rotation, which, rubbed by hand, was supposed to prove the cosmic forces ( virtutes mundanae ). He sent such a sulfur ball to interested contemporaries, including in 1671 to Gottfried Wilhelm von Leibniz, who used it to generate the first artificial electrical spark . Systematic research into electricity began in the early 18th century. In 1706 Francis Hauksbee developed a frictional electrification machine whose ball was no longer made of sulfur but of glass. Electrostatic generators were the only source of artificially generated electricity until the development of the voltaic column around 1800 by Alessandro Volta .
Van de Graaff belt generators and pelletrons are currently still used for teaching purposes and in particle accelerators . In the case of small band generators, the initial charge separation occurs through friction, so they are further developed electrifying machines. In the case of the large Van de Graaff accelerators , the initial charge separation is effected by means of a peak discharge of a voltage obtained from the power grid, in the case of Pelletrons by induction . The voltage is then increased in both cases with electrostatic means.
functionality
Electrostatic generators can be divided into two classes:
- Electrifying machines that work on the principle of static electricity;
- Influence machines that use the effect of electrical influence.
The friction between insulators (e.g. sulfur, glass, wood and rubber) is characteristic of electric machines. In fluence machines, on the other hand, the charge is created by fluence in metal parts.
Electrifying machines
Through mechanical friction between two materials with different electron affinity , the electrons on their surfaces are distributed to different degrees between them. In electrifying machines, the resulting charge, either positive or negative, depending on the design, is diverted and collected in capacitors such as Leyden bottles .
Static electricity is a special case of touch or contact electricity . Due to the friction, the contact between the materials is stronger and the exchange of charge carriers is more intense than with mere contact.
The most widespread design consists of one or more glass bodies in the form of hollow spheres, rollers or glass panes. Rubbing tools, for example leather pillows with an amalgam cover, are pressed onto the rotating glass body .
At a point behind the pressure point of the rubbing tools, a conductor (an electrical conductor, for example a small metal brush) picks up the electrical charge from the glass surface and conducts it directly to an experiment or to a capacitor. The reamers are mostly grounded so that the charge derived from the conductor is balanced. Some machines work on the reverse principle, in which the conductor is earthed and the electrical charge is removed from the friction tools, and there were machines in which it was possible to switch between conductor and friction tool earthing.
In addition to glass, wood and rubber were used for the panes of electric machines. In the first electrifying machines, instead of the rubbing tool and the conductor, the human hand was used both to rub and to absorb the electrical charge. The rubbing person was "electrified", which led to the name electrifying machine.
The largest pane electrifying machine (the diameter of the panes of glass is 1.65 m) is in the Teylers Museum in Haarlem and was presented to the public in 1785 by the Dutch naturalist Martinus van Marum (1750–1837). The English instrument maker John Cuthbertson (1743–1821) from Amsterdam was also involved in the construction of the machine, which was completed in 1784 . Van Marum had just become director of the Teylers Museum at the time and had been working on improving electrification machines for over 10 years, even as a student in Groningen. With his instruments he wanted to make new discoveries in the field of electricity. a. he presented them to the Danish physicist Hans Christian Ørsted and in 1781/82 to Alessandro Volta . The large electrifying machine in Haarlem can generate sparks over a distance of up to 61 cm. In 1881 she received a Medal of Honor at the World Exhibition in Paris. An exact replica, which was constructed in 1968 at the University of Eindhoven (H. J. de Weyer) can generate voltages of over 500,000 volts. It was presented as a Dutch contribution to the US 200th anniversary in 1976/77.
Influence machines
In the case of induction machines, the charge separation is not achieved by friction, but by the effect of electrical induction . If a conductive body is spatially removed from another, opposite to this electrically charged body, the voltage between the two electrically charged bodies increases while the charge remains the same . The mechanical work to overcome the electrostatic attraction is converted into energy of the electrical field . If the charge accumulated on the separated bodies is discharged electrically, a low current (a few 10 µA) can be drawn at very high voltage (up to over 100 kV).
In the case of induction machines, the bodies consist of metal foil segments attached to insulating washers, but other shapes and bodies are also used based on the same principle. The maximum tension is limited by the number and spacing of the segments and by the pulley diameter.
The existence and mobility of charge carriers are decisive for the function of an influenza machine. For this reason, metals are used in influenza machines. The steps
- Approach of a metal body to an electrical charge or charging of the body
- Transport of the metal body away from its reference potential and reduction of the charge by an arrester ( peak discharge )
- Neutralization of the metal body (compensation of the withdrawn electrical charge) using soft, conductive brushes that are cross-connected in some machines
take place repeatedly with each revolution.
The Wimshurst machine uses the electrical charge that has been removed to strengthen the electrical field of the inductor, thereby steadily increasing the effect. Due to this self- reinforcement , unlike the Töpler / Holtz machines , it does not require any initial charging of its inductors, since there is always at least a small charge difference at the beginning of operation.
The principle of the induction machine can also be reversed by feeding a similar arrangement with high voltage and then working as a motor.
A very unusual influence machine is the Kelvin or water drop generator , which influences water drops and uses them to transport the electrical charge. It also does not require an initial charge and reaches a voltage of 4 to 6 kV after 20 to 30 seconds simply by two separate water jets.
A small Holtz influence machine delivers typical currents of 10 µA, with which a Leiden bottle with a capacity of 10 nF can be charged to 30 kV in 30 s. That is enough to let popping sparks jump over in a parallel connected spherical spark gap at a distance of 1 cm. The sparks last around 1 µs and thus have a maximum current of 300 A.
history
Research into electricity and the further development of electrostatic generators were closely linked from the 17th to the middle of the 19th century. Insights into electricity research led to improvements in generators, and improved generators allowed new knowledge about the nature of electricity.
The following table is intended to give a brief overview of the cross-fertilizing history:
| year | Discoverer / Inventor | description |
|---|---|---|
| 1663 | Otto von Guericke | Construction of the first electrifying machine, a rotating sulfur ball that was rubbed by hand |
| 1671 | Gottfried Wilhelm of Leibniz | First targeted generation of an electrical spark with the Guerickes sulfur ball |
| 1675 | Jean Picard | Discovery of the glow of mercury in vacuum-rubbed glass tubes |
| 1706 | Francis Hauksbee | Development of the first generators with glass spheres |
| 1730 | Stephen Gray | Discovery of electrical conduction |
| 1743 | Georg Matthias Bose | Introduction of the conductor to dissipate the electrical charge from the rubbed glass ball and introduction of rubbing pads, which detached the rubbing by hand |
| around 1745 | Johann Heinrich Winkler | Beer glass generator; Forerunner of the electrifying machines with glass cylinders |
| 1745/1746 | Ewald Georg von Kleist / Musschenbroek | Invention of the Leiden bottle , the first electrical capacitor |
| 1755 | Martin de Planta | Generators with round panes of glass |
| 1777 | Martinus van Marum | Electrifying machine with rubber disks that run in a mercury bath |
| 1784 | Walkiers de St. Amand | First ribbon generator with 7.5 m long silk ribbons stretched over wooden rollers and rubbed; Forerunner of the Van de Graaff generator |
| 1840 | Sir William Armstrong | Steam electrifying machine |
| 1843 | Michael Faraday | Proof of the mechanism of action of the steam electrification machine; Evidence of the friction of water droplets as the cause of electricity generation |
| 1867 | Lord Kelvin | Water drop generator |
| 1872 | Augusto Righi | Generator with metal cylinders on an insulating rope; Precursor to the pelletron |
| 1883 | Wilhelm Holtz | Construction of the first, continuously working influenza machine |
| 1929 | Robert Van de Graaff | Development of tape generators; Use, for example, in the Van de Graaff accelerator and in today's laboratories |
| around 1965 | Raymond Herb | Pelletron accelerator; isolated connected metal links in a high pressure, insulating gas |
Curiosities
Beer glass electrifying machine by Johann Heinrich Winkler, around 1730–1740
The "electric kiss", around 1740
Treatment of toothache with electricity
After the popularity of experiments and research on electricity increased significantly from 1730 (scientific weekly magazines now regularly reported on it), all sorts of curiosities and attractions caused a sensation after a short time. The “electric kiss” was very popular. An electrically charged lady was waiting for the visitor at the entrance to some establishments . If the visitor gave her a kiss on the lips, he would receive a small electric shock.
The beer glass generator by Johann Heinrich Winkler was created out of practical considerations . Electrifying machines made of glass were common among experimenters (because they were inexpensive). The use of beer glasses made building an electrifying machine a little cheaper and there was no shortage of spare parts. This design could hardly prevail. Nevertheless, she founded her own line of development of roller electrifying machines in which glass rollers were used.
New fields of application for electricity were sought, including the treatment of toothache with electric shocks. The success with which this happened is not known. Around the same time (around 1740 to 1750) animals were killed by artificially generated electricity for the first time and treatment of humans was initially given up again.
literature
- Bern Dibner : Early electrical machines. The experiments and apparatus of two inquiring centuries (1600 to 1800) that led to the triumphs of the electrical age (= Burndy Library. Publication 14, ZDB -ID 977102-5 ). Burndy Library, Norwalk CT 1957.
- Fritz Fraunberger: Electricity in the Baroque. Aulis Verlag Deubner, Munich 1964.
- Edmund Hoppe : History of Electricity. Barth, Leipzig 1884.
- Hans Schimank : History of the electrifying machine up to the beginning of the 19th century. In: Journal for Technical Physics. Vol. 16, No. 9, 1935, ISSN 0373-0093 , pp. 245-254.
- Heiko Weber: The electrifying machines in the 18th century (= Ernst Haeckel Haus studies. Vol. 7). VWB, Verlag Wissenschaft und Bildung, Berlin 2011, ISBN 978-3-86135-487-1 .
Web links
- Explanation of the Pelletron on pelletron.com
Individual evidence
- ↑ a b By Gottfried Wilhelm Leibniz. Second series, first volume, Akademie Verlag, ISBN 3-05-004187-0 , p. 339, section 108.
- ^ Willem D. Hackmann: Electrostatic Machine . In: Robert Bud, Deborah Jean Warner (Eds.): Instruments of Science. An Historical Encyclopedia . Garland, New York et al. a. 1998, pp. 221-224.
- ^ Exhibition guide, Teylers Museum, Haarlem
- ↑ Description of the Wimshurst influenza machine and electrostatic motor. hcrs.at
- ↑ Pohl Elektrizitätslehre , 21st edition 1975, p. 31
- ↑ The corresponding correspondence from 1671 is printed in Fritz Fraunberger Elektrizität im Barock , Aulis Verlag, Cologne, pp. 35f. There it becomes clear that Guericke himself did not generate any sparks. He writes there u. a. that he is not aware of any heat generation and only knows a glow in the dark.
- ↑ a b The Doctrine of Frictional Electricity , Volume Two. Peter Theophil Riess, Berlin 1853, p. 146 ff.
- ^ Philosophical Transactions , Volume XXXVII. Royal Society, London 1732.
- ↑ a b Georg Matthias Bose: Electricity designed with a poetic pen after its discovery and progress . 2 vols., By Johann Joachim Ahlfelden, Wittenberg 1744. In: Bibliothèque raisonnée , t. 34 (1745), partie 1, pp. 3-20.
- ↑ Excerpt from the Kleists' family biography with a link to the original copy of the letter to Krüger .
- ^ A b c Ragnar Hellborg: Electrostatic Accelerators: Fundamentals and Applications . ISBN 3-540-23983-9 .
- ↑ Michael Faraday: Experimental Researches on Electricity . Volume II. London 1844.