Carbon arc lamp
A carbon arc lamp , also known as an arc lamp for short , is an electrical light source with an arc burning in air between two electrodes made of graphite .
history
The first carbon arc lamp was developed by the British Humphry Davy around 1802. The electrodes were aligned horizontally and the resulting arc was bent upwards by the rising hot air. The term arc and the commonly used term arc-lamp are based on this observation . Since no powerful electric generators were available at the beginning of the 19th century and the burning of the carbon rods only allowed a short operation in the range of a few minutes, the carbon arc lamp remained of no significant practical importance in the following decades.
If the lamp is operated with alternating current, both coals burn off evenly. When operating with direct current, however, the positive charcoal burns up about twice as fast as that of the negative charcoal; to compensate for this, the positive carbon usually had twice the cross-section.
In the 1840s, William Edwards Staite and William Petrie developed a range of improved arc lamps. Among other things, the burn-off of the carbon electrodes was compensated for by a mechanical construction that continuously pushed the carbon rod. This enabled the operating time of just a few minutes to be significantly increased by using appropriately long carbon rods. In order to facilitate the replenishment, the electrodes were mounted in a vertical direction. Because of the complex structure of the tracking, the practical use was limited to a few installations in England.
The first commercially successful carbon arc lamp, the " Jablotschkow candle ", was developed by the Russian Pawel Jablotschkow . The lamp was designed in such a way that the complicated mechanics for tracking the carbon electrodes were omitted. For the power supply, Jablotschkow used a generator from Gramme, which was new at the time and which was presented together with the arc lamp at the Paris World Exhibition in 1878 and marketed by Zénobe Gramme .
On March 1, 1879, Werner Siemens (from 1888 by Siemens) used an electric arc lamp he had developed for the first time for the purpose of street lighting in his house. With this differential arc lamp , the carbon rods between which the arc burned could be readjusted automatically and several lamps could be connected to a generator . The city of Berlin was a short time later in the center at the intersection of Unter den Linden and Friedrichstrasse , the gas lighting replaced with carbon arc lamps. In 1879, the newly built Munich Central Station was given a new type of lighting system with differential arc lamps from Siemens & Halske . It is therefore the first electrically illuminated train station in Germany.
In the following years, the brighter carbon arc lamps replaced the gas lamps on selected streets, such as in 1882 on Potsdamer Platz and Leipziger Straße and in 1888 on Pariser Platz and Unter den Linden in Berlin. With the development of the "intensive flame arc lamps", the brightness could be further increased in order to be able to illuminate central places. In 1905, the arc lamp candelabra from Potsdamer Platz was erected with a mounting height of 18 m, and similar high candelabra were placed at the Brandenburg Gate .
On behalf of AEG , Peter Behrens designed the simple, purely functional design of the energy-saving arc lamp in 1907 , which was further developed in the following years.
Heinrich Beck invented the “regular arc lamp” in 1906, which led to the establishment of the German Beck Arc Lamp Company in Frankfurt am Main. After Beck had already withdrawn from the operative business in 1909 and had his share paid out, he succeeded in 1912 in the construction of the most powerful marine headlight at the time in the newly founded physical-technical laboratory in Meiningen (see also searchlight ). After the German navy could not decide to use this technology, the Beck searchlight was tested in New York in August 1914 in the presence of the inventor.
When the First World War broke out , Heinrich Beck tried to return to Germany, but was captured by the Royal Navy in the English Channel and interned near Liverpool. Faced with the choice of either remaining as a prisoner of war in Great Britain or returning to the then still neutral USA, he opted for the latter and finally sold his patents to General Electric (GE) after sales negotiations with Sperry Gyroscope had failed. Between 1917 and 1919 he and GE had to defend themselves in a complex patent infringement lawsuit against an obvious plagiarism by Elmer Ambrose Sperry , which had come onto the market in 1917 as an alleged in-house development. The GE / Beck searchlight was used on all warships of the US Navy from 1917 .
Due to the provisions of the Versailles Treaty , only Siemens-Schuckert and AEG were allowed to manufacture headlights in Germany after the First World War . The institute in Meiningen was incorporated into the AEG after Heinrich Beck and his family returned in 1921. Heinrich Beck worked there together with his two sons until his death in 1937 on an improvement to his invention, which was eventually used as an anti-aircraft headlight during the Second World War .
The Beck or high-intensity coal arc was the light source in cinema projectors around the world until the 1960s. Technicolor's bright colors are related to this.
Structure and functionality
In a carbon arc lamp there are two rod-shaped, adjustable electrodes made of graphite , commonly known as "carbon". They are manufactured by sintering at 1,200 ° C from coal and binding agents. To achieve better light output, they are also covered with metal.
The carbon arc lamp is ignited by briefly bringing the electrodes together: Resistance heating in the small contact point and the high field strength when separating form an arc that heats the electrodes to around 3,000 ° C.
Up until the 1920s there were only pure charcoal lamps in which the electrodes provided most of the light. The plasma of the arc is much hotter at around 10,000 ° C, but almost transparent. Such a carbon arc lamp is therefore essentially a thermal radiator that emits a slightly yellowish light. The light output and color temperature were significantly increased with the development of Beck coals: Rare earths ( yttrium , cerium fluoride, etc.), either as a wick in the middle of the "positive carbon" or finely distributed over the entire electrode material, emit in the plasma visible light . The dense line spectrum also has a considerable ultraviolet component .
Carbon arc lamps can be designed for both direct voltage and alternating voltage . As an operating voltage of 25 to 50 volts, the devices could be used in the 110-volt direct current networks that were common at the time . The voltage difference is required for the necessary series resistor (a series reactor for AC voltage ).
With DC voltage, the arc burns more evenly, but the electrodes then burn at different speeds: Electron bombardment creates a crater on the anode (positive electrode), while the positive ions erode the cathode in a conical shape. The anode also heats up more than the cathode. Because of this increased load, it is often more massive than the cathode. For the greatest possible brightness, the anode should be arranged in the focal point of the reflector of the lamp.
Like any discharge lamp, arc lamps require a current limitation . In the case of direct voltage operation, this was implemented by a resistor and in the case of alternating voltage by a choke .
Adjusting the coals
Because the coals burn down during operation, it is necessary to push in the two carbon pins. If the operator can still do this manually (about twice per minute) with so-called pure coal (carbon material with relatively few admixtures of salts that enhance the luminosity), the coal feed must be automated when using so-called effect carbon (carbon material with relatively high proportions of luminosity-enhancing salts) A clockwork or stepper motor continuously moves the two carbon pins towards each other while the lamp is in operation.
In FlaK headlights, automatic readjustment was implemented using a bimetal switch on which the light center was mapped with a lens. As the burn progressed, this activated a servomotor.
In order to achieve even illumination of the cinema screen , the presenter must check and readjust the exact burning position during the presentation. Due to different draft ratios of the chimney, which directs the smoke gases into the open air (summer / winter), the coals can burn off crookedly, which has to be compensated by the position control of the axis of the minus coal. Unevenly faster burn-off of the carbon pins, for example due to different residual moisture during storage, causes the carbon spacing to become larger or smaller with continuous readjustment or the plus crater to migrate out of the mirror focus, which also has to be readjusted by hand.
To ignite the arc, the demonstrator brings the carbon pins together and then apart again by touching the handwheel. After 30 to 60 seconds, the carbon arc burns in a stable light.
application
Carbon arc lamps have long been the most intense artificial light sources with electrical outputs over 10 kilowatts . They were used in headlights (for example as projection lamps , also as flak headlights and show applications; " light dome "), in spectroscopy and microscopy , in particular in microphotography . As a rule, a heat protection filter is necessary to reduce the very intense UV and infrared radiation . The very small light source of the electrode helps to produce the best possible imaging beam path as Koehler illumination . Nowadays carbon arc lamps are surpassed in every respect by xenon gas discharge lamps , which show no burn-up.
Carbon arc lamps were the first electric lighting in many cities. The Leipzig company Körting & Mathiesen developed and produced arc lamps for street and hall lighting from 1889, before also building arc lamp headlights from 1897.
Because of the burn-up of the electrodes and the mechanical distance regulation that is necessary as a result, carbon arc lamps are maintenance-intensive and, despite their simple construction, are only used for special tasks. Due to their optical properties, carbon arc lamps are still used today, for example in experimental lectures, to produce shadow projections. In quantitative spectroscopy , the carbon arc has also served as a radiation standard for a large wavelength range ( infrared to ultraviolet ) for a long time .
literature
- Johannes Abele: The arc lamp . Deutsches Museum, Munich 1995, ISBN 3-924183-31-7 .
Web links
Individual evidence
- ↑ William Slingo, Arthur Brooker: Electrical Engineering for Electric Light Artisans . Longmans, Green and Co, London 1900, pp. 607 . OCLC 264936769
- ^ Otto Lueger : Lexicon of the entire technology and its auxiliary sciences . 2nd Edition. Deutsche Verlagsanstalt, Stuttgart and Leipzig 1920 ( zeno.org [accessed on November 4, 2019] Lexicon entry “Arc lamps”).
- ^ Ian McNeil: An Encyclopedia of the History of Technology . Routledge, London 1990, ISBN 978-0-415-01306-2 , pp. 360-365 .
- ↑ David O. Woddbury: A Measure for Greatness Edward Weston . McGraw-Hill Book Company, 1949, pp. 83 ( online ).
- ↑ Siemens AG : Press photos: Milestones in electrical engineering in Bavaria up to 1924 (PDF)
- ↑ Review - 130 years ago - Berlin illuminated electrically. In: Märkische Allgemeine. February 28, 2009, archived from the original on November 21, 2009 ; Retrieved July 24, 2014 .
- ^ Wilhelm Füßl, Andrea Lucas, Matthias Röschner: Gallery of Beauties in: Culture & Technology: Journal of the Deutsches Museum Munchen 4/2016, ISSN 0344-5690
- ↑ Josef Stübben : Urban planning . In: Handbook of Architecture . 9. Half-band. Darmstadt 1890, p. 355–359 ( diglib.tugraz.at [accessed December 31, 2019]).
- ^ Herbert Liman: More light . Haude & Spener, Berlin 2000, ISBN 3-7759-0429-8 , pp. 34 ff .
- ↑ AEG : The Little Chronology (PDF), accessed on March 9, 2020