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Electric Lighting
1911 Encyclopedia Britannica
or "ELECTRIC LIGHTING ( see 16.659). - Notable progress in illuminants was made during the period 1911-21. Advances in the art of applying artificial light to the best advantage have been even more remarkable, and as these apply to all illuminants they are dealt with in a separate article ( see Illuminating Engineering).
Progress in Lamps. - Some idea of the position in regard to electric lamps at the end of 1910 ma y be gathered from two papers read by E. W. Marchant ( R€ cent Progress in Electric Lighting, Ilium. Eng. Soc., London De e. 9 1910) and Haydn T. Harrison ( Street Lighting by Modern Electric Lamps, Inst. of Elec. Engrs. Nov. 24 1910) .
Metal filament lamps were in general r q e, and their advantages, in comparison with arc lamps, were already the subject of discussion; tabular data of cost are given in Maurice Solomon's work on Electric Lamps. The use of arc lamps with flame carbons was extending, but ordinary carbons were still widely used. Efforts were made to extend the period of burning of flame arcs before recarboning became necessary. In the magazine flame arcs carbons are automatically replaced from a stock in the lamp as they burn away. In this way a period of burning of 80-loo hours has been secured. In the Jandus Regenerative arc lamp the flame carbons were enclosed in an airtight chamber, with a special circulatory system to prevent deposition of fumes on the globe. Approximately five c.p. (mean hemispherical) per watt and 70 hours burning with one pair of carbons were stated to be obtained. The enclosed Carbone arc was designed with a similar object, a special shape of globe being used to prevent inconvenient deposition of fumes. Quite recently a form of enclosed flame arc has been developed in Germany, the burning period being 80-120 hours, and the efficiency, on direct current 4-6 c.p. per watt ( Lichttechnik, by L. Bloch). Inclined carbons are commonly used in flame arc lamps, but in the Crompton-Blondel arc vertical carbons, one above the other, were adopted. Marchant ( loc. cit. ) gives values ranging from 3.72 to 6.85 c.p. per watt for various flame arcs - efficiencies well above those yet attained with incandescent lamps.
Various circumstances have tended to limit the field for arc lamps. During the World War carbons were almost unobtainable, and their cost has risen considerably. Moreover, gas-filled incandescent lamps tend to displace arc lamps for many purposes. At the present time (1921) lamps using ordinary carbons are becoming obsolete, but flame arcs still hold their own for lighting large areas. Most flame arcs furnish light of a pronounced yellow colour, owing to the influence of calcium salts in their electrodes. Flame carbons yielding white light have, however, been used for photographic and cinema work. The arc lamp using a magnetite negative electrode, with a life of 150-175 hours, is still used in America but little known in England.
A step of great scientific interest has been the introduction, during the war, of searchlights using carbons cooled either by a spray of alcohol (Beck-Goerz system) or a blast of air (Sperry searchlight). (See Harrison, Ilium. Eng. March 1918; also Ilium. Eng. Feb. 1915; also McDowell, Trans. Ilium. Eng. Soc., U.S.A., Sept. 1916; also Electrician Feb. 2 1917). This leads to a smaller crater of increased intrinsic brightness, estimated at 200,000-300,000 candles per sq. in. as compared with 85,000 for the ordinary arc-crater. Thus, a much more powerful beam, which is stated to approach 500 million candles (max.) may be attained, and a diminished angle of dispersion. Intrinsic brilliancies of 600,000 c.p. per sq. in. are said to have been obtained in Germany ( Lichttechnik by L. Bloch), while Lummer, with an arc operating in a pressure of 22 atmospheres and at a temperature of 7,600° Abs., attained 1,500,000 candles per sq. in.
No very striking advances in illuminants using the luminescence of metallic vapours are recorded. The tubular mercury vapour lamp has been improved by the use of devices enabling the lamp to start automatically without tilting by hand. Attempts have been made to supply the missing red rays by mounting over the tube a fluorescing rhodamine reflector, but the effect is comparatively slight.
Wolfke, in Germany, obtained an approximately white light by using an amalgam of cadmium and mercury (Elektrot. Zeitschr. 1912, p. 917), but the lamp does not appear to have reached a commercial stage. In the other familiar form of mercury vapour lamp with a quartz or silica glass tube, operated at a high temperature, the red and orange rays are not entirely missing. The chief feature of this lamp, apart from the higher luminous efficiency (estimated at about five c.p. per watt) is the high proportion of ultra-violet rays emitted. For ordinary lighting purposes these rays are masked by an outer globe of dense glass. Forms of lamps enabling the ultra-violet light to be applied in a concentrated form for therapeutic purposes are also available.
The Moore tube lamp, utilizing the luminescence arising from a high tension (5,000-17,000 V.) discharge through rarified nitrogen gas, is little known in England. The length of tube is usually considerable, but a small and compact form using carbon dioxide gas, the light of which is stated to resemble daylight closely in colour is used in industries involving accurate colour-matching.
The use of the rare gas neon in such luminescent tubes, announced by Claudes in 1911 (Comptes Rendus, May 22 1911) and since developed to a commercial stage, has had interesting results. Owing to the higher brightness and greater efficiency of luminescent neon (approx. two candles per watt) tubes of moderate dimensions and varied shape can be constructed. Such lamps can now be operated direct on 220 volts, but a special starting device, applying an inductive discharge, is necessary. The vivid orange colour of the light is favourable to its use for spectacular lighting. Quite recently small neon lamps, resembling an ordinary glow lamp in appearance and capable of being inserted in an ordinary lamp holder, were exhibited before the Illuminating Engineering Society (see Ilium. Engineer Jan. 1921; ibid Aug. 1920). The cathode is extended and brought close to the anode, light appearing as a diffused orange glow. Although the efficiency is as yet low (apparently of the order of o
06 c.p. per watt) such lamps consume only five watts or less on 220 volts. They may therefore prove useful in cases where only a weak light is necessary but a small consumption of electricity desirable. Further improvements may be anticipated.
Incandescent lamps using tungsten filaments in vacuo have now displaced the Nernst, tantalum and other forms, and the proportion of carbon filament lamps in use is constantly decreasing. The introduction in 1911 of filaments drawn out as wire from ductile tungsten has had important consequences. Filaments made by other processes (e.g. squirted or pasted) are now little used. The ductile tungsten wire now prepared can be more easily mounted in the bulb, can be readily wound in any desired shape, and is better able to resist shock and vibration. Ten-watt lamps are now available on 100-105 volts and 20-watt lamps on 200-210 volts, thus rendering such special devices as running lamps in series and the reduction of supply voltage by transformers largely unnecessary. Useful life and efficiency have also improved. Candle-power should not diminish by more than 20% in i,000 hours' burning, the luminous efficiency being about 0.75-0.9 candles per watt, according to type. Filaments can be arranged in a bunched compact form suitable for automobile lamps, pocket torches, etc., and special " traction " forms, designed to withstand vibration, have been developed.
Another step of importance has been the development of the gasfilled or so-called " half-watt " lamp, announced in 1913 (see Langmuir and Orange, Trans. Am. Inst. of Elec. Engrs. 1913; Gen. Elec. Rev., U.S.A. Oct., Dec. 1913; Pirani and Meyer, Elektrot. Zeitschr. 1915). The filament consists of a compact tungsten spiral brought to incandescence in an atmosphere of inert gas (usually nitrogen but in the smaller forms argon). The tendency of the tungsten to volatilize is checked by the pressure exerted by this envelope of gas. Filaments can accordingly be run at a higher temperature, with correspondingly improved efficiency. Recent specifications indicate that lamps should operate at r-1.6 candles per watt with a useful life of 1,000 hours. Still higher efficiencies may be expected from high candle-power low voltage units. A feature of the lamp is the formation of convection currents within the bulb which has a long neck in which particles of tungsten tend to deposit, thus largely obviating blackening of the bulb proper. In England the smallest units available on ordinary lighting pressures are 40 watts on 100-130 volts, and 60 watts on 200-260 volts. The largest lamps ordinarily listed consume 1,500 watts. Thus we have for the first time incandescent lamps of a candle-power comparable with that of arc lamps. For special purposes even larger units have been developed. Special lighthouse lamps consuming 2,400 watts have been used in Holland, and 4,000-watt types are stated to be in course of preparation. Filaments of gas-filled lamps may assume a wide variety of shapes. In the United States special forms have been developed for use in cinema lanterns.
The " arc-incandescent " (" Pointolite ") lamp, developed in the Ediswan laboratory during the war, has interesting features (Ilium. Eng. Jan. 1916; Jan. 1920). The source of light is a globule of tungsten brought to incandescence as the anode of an arc within a sealed glass bulb. The cathode is a rod composed of tungsten and certain rare earths, which is heated by the passage of a current, ionizes the space between the electrodes and starts the arc. As an approximate " point-source," with a brightness near 13,000 candles per sq. in., the lamp is adapted for use with optical lanterns, etc. Lamps giving up to 1,000 c.p. have been developed, and it is hoped that a 4,000-c.p. type now being prepared will prove valuable for cinema lanterns in view of the steady light and the fact that no manipulation is needed once the lamp is switched on.
A feature of the past few years has been the rapid development in lamp manufacture in the United States. In 1920 the production was estimated to reach 230 million lamps, of which only 7% were of the carbon filament type (Gen. Elec. Review, U.S.A., Jan. 1921). Considerable progress in the manufacture of miniature lamps for automobiles, flashlights, miners' lamps, etc., is recorded, an output of 125 million being attained in 1920. Progress in lamp manufacture has been aided by success in standardizing supply voltages, nearly 79% of the lamps sold in 1920 being for the standard pressures of 110, 115 and 120 volts. In Japan a uniform pressure of ioo volts throughout the country has been established.
Physical Data Underlying the Efficiency of Light Production
Researches in the physics of light production have yielded interesting conclusions, revealing the comparative inefficiency of most artificial illuminants. Thus it is estimated that the ordinary tungsten filament radiates as visible light not more than 5% of the energy imparted to it. Increasing temperature shifts the maximum of radiation nearer the visible region of the spectrum and is thus favourable to high luminous efficiency. It has been computed that a source operating at solar temperature might attain a luminous efficiency of 50%. P. G. Nutting ( Bull. Bureau of Standards, May 1911) estimated that a source which produced only visible white light should yield 26 candles per watt, whereas the most efficient illuminants available do not give more than about five candles per watt. Nutting also calculated that a source producing only light of the most efficient wave-length for creating brightness, namely 0.54, would yield 65 candles per watt.
Our ideal should be to control emission of radiation so as to produce only light of the particular colour desired. This has a bearing on attempts made to imitate the colour of daylight. By the introduction of a suitable tinted glass in the path of light from a gasfilled lamp, or by reflecting the light from a matt surface having a suitable coloured pattern, a close resemblance to normal daylight may be obtained (Ilium. Eng. Feb. 1920). Such " artificial daylight " units are of great value in industries where accurate colour matching is needed. But present processes involve the sacrifice of much light by absorption, and the overall efficiency of accurate units probably does not exceed about 15-20% of the light yielded by the lamp.
Progress in Shades, Reflectors and Lighting A ppliances. - Advances in the efficiency of illuminants have been accompanied by considerable progress in methods of distributing light. Reflectors are now designed to screen the source from the eyes of persons using them, soften shadows and modify the natural distribution of light in any desired manner. Spacing rules for standard reflectors of " Extensive," " Intensive " and " Focussing " types are furnished and adherence to these should ensure the provision of uniform illumination of a specified value in foot-candles. Prismatic glass devices, for use with arc lamps and gas-filled lamps, have been designed to give a distribution of light favourable to uniform illumination between street lamps. An example is the Holophane street lighting lantern, which utilizes two prismatic glass surfaces, superimposed one on the other, with a smooth exterior and interior such that the lantern can be easily cleaned. Improved and simplified illumination photometers have enabled much information to be obtained regarding the illumination necessary for various purposes. It is now considered preferable to state the illumination in foot-candles at the actual place where light is needed rather than to prescribe so many lamps of a specified consumption per square feet. This illumination can be related to the consumption of electricity per sq. ft. of area lighted. Thus with direct lighting by vacuum tungsten lamps in modern reflectors about 0.2-0.3 watts per lumen (i.e. per foot-candle per sq. ft.) is usual; with gas-filled lamps about 0.1-0.15. With indirect lighting about twice of the above values are required.
The introduction of the more efficient gas-filled lamps, which require screening on account of the great brilliancy of the filament, has encouraged the use of indirect and semi-indirect methods of lighting. Small gas-filled lamps with opal glass bulbs have also been introduced. Lamps are now commonly mounted high up near the ceiling so as to be out of the direct range of vision and leave a clear space for the supervision of work. The high candle-powers available allow of greater mounting heights than those formerly used. Thus in factories lamps mounted 30 or even 40 ft. above the working plane are not unusual (see The Gas filled Lamp and its Effect on Illuminating Engineering by F. W. Willcox, Blum. Eng. June 1919). Certain fine industrial processes, however, require local lighting with well shaded lamps. Reflectors have been developed for lighting large vertical surfaces, notably for picture lighting. A feature in the United States has been the development of " flood-lighting," i.e. concealed lighting by compact filament gas-filled lamps in parabolic reflectors giving a concentrated beam of light with a dispersion of Io°-15°. Thus a 500-watt lamp in a suitable mirror will yield a maximum beam-candle-power of 330,000. Such lighting units have been used for spectacular lighting (e.g. illuminating historic monuments and buildings, large advertisement-placards, etc.), and during the war served as a measure of protection, to prevent unauthorized persons approaching arsenals or other works unseen.
For further information the following works may be consulted: - The Development of the Incandescent Lamp, by G. B. Barham (1912); Elektrische Lichteffekte, by W. Biscan (1909); Lichttechnik, edited by L. Bloch, issued by the German Illuminating Engineering Society (1921); Grundziige der Beleuchtungstechnik by L. Bloch (1907), translated by W. C. Clinton; The Application of Arc Lamps to Practical Purposes, by J. Eck (1910); Le Nuove Lampade Elettriche ad Incandenza, by G. Mantica (1908); Elektrische Beleuchtung, by B. Monasch (1907); The Electric Lamp Industry, by G. A. Percival (1920); Electric Lamps, by M. Solomon (1908); Electric Arc Lamps, by O. Zeidler and J. Lustgarten (1908).
Frequent articles on electric lighting appear in The Illuminating Engineer (London); The Transactions of the Illuminating Eng. Society U.S.A. (New York); and Licht and Lampe (Berlin). See also ILLUMINATING ENGINEERING. (J. S. D.)
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Chisholm, Hugh, General Editor. Entry for 'Electric Lighting'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/e/electric-lighting.html. 1910.
