The genesis of incandescent lamp

-

man uf actu re

by J. R. Coaton The Victorian engineers displayed considerable ingenuity in overcoming technical problems and successfully

manufactured thejrst carbon jlament incandescent lamps. Apart from Swan and Edison, many others shared in this achievement. The materials and scientijk knowledge at that time were limited; this article

reviews the key steps that were made in manufacture-particularly in producing homogeneous jlaments-in an amazingly short space o f time. Some ideas ‘withered on the vine’, some survived and yet others have been recycled. The conversion o f recorded candlepower into lumens is not straighgoward and the performance o f early lamps has often been overestimated. Calculations have been made using

survivingjlament data, coupled with modern information on carbonjlaments andjbres, to obtain from Jirst principles the luminous e$cacy of these early lamps.

The early days

ne cannot but admire the ingenuity of Victorian engineers. With only a limited knowledge of the underlying science and 0 technology of the materials and processes

they used, supported by rudmentary instrumentation, they tackled-and solved-formidable problems. Ths is especially true of the electric lamp industry. Following the pioneering work of Swan in Britain and Edson in America, which came to fruition between 1878 and 1879, commercial production of incandescent lamps became well established in both countries by the early 1880s.

Although Swan and Ecbson made the final steps that culminated in a practical incandescent lamp the principles were established at a much earlier date. Guiseppe Ponzelli, an I d a n Monk, advanced the theory of incandescence in 1747 but the first practical demonstration was performed by Humphry D a y in 1802 when he caused a metal strip to glow by passing a current through it. The first mention of using carbon as a 6lament material was in 1839, when Professor Jobands in Belgium suggested passing a current through a carbon rod contained in an evacuated vessel. A number of experimenters made devices employing platinum or platinum iridium foil or wire emitters, and indeed, Edison’s first incandescent lamp used a platinum filament workmg in conjunction with a temperature-regulating device.

Perhaps the most significant early contributor was Heinrich Goebel, a German immigrant living in America. Between 1854 and 1872 he constructed incandescent lamps containing carbonised bamboo filaments, sealed into empty perfume bottles, which he used around his home. The air was expelled by f i n g the bulb with mercury, whch was then drained, thus creating a vacuum (a Torricellian vacuum). He claimed that these burnt for several hundred hours before failing. Unfortunately, he could not afford to take out a patent, and could not sustain h s claims against Edison’s master patent.

Between 1844 and 1860 Swan experimented with carbonised paper and cardboard filaments but then concentrated on h s interest in photography, focusing on the problem of producing permanent prints, fiee from the defect offadmg. During 1877, Swan renewed h s interest in incandescent lamps, stimulated by Crookes’ radiometer. He subsequently contacted Charles Stearn, who had been investigating the production of high vacuum using Herman Sprengel’s vacuum pump-the system used by Crookes in makmg his radiometer. Stearn agreed to carry out experiments for Swan, and this lead eventually to the demonstration of the first practical carbon filament lamp. This lamp used a carbon rod as the emitter and was contained in an evacuated glass bulb.

There has been much discussion on whether Swan or Edison invented the incandescent lamp. Following h s first successll laboratory experiments, and

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demonstrations in November 1878 and February 1879, it was some two years before Swan applied for a patent. The reason was that Swan considered that the principle had been long known and a patent would not be sustainable. He finally obtained a patent in 1880; but, more importantly, he shortly afterwards (in 1883) patented the cellulose thread (a squirted filament), whch became the standard used in most commercial lamps and is still used to this day. However, perhaps the most important spin-off from this development was the production of man-made fibres-rayon. Edison’s master patent was granted in Britain on 10th November 1879 and in America on 27th January 1880.

Edison fded in his attempt to get a High Court injunction against Swan and the two inventors eventually concluded that costly litigation would damage both businesses. In 1883 they combined forces; their respective Enghsh companies were merged to form the Edson & Swan United Electric Lighting Company Limited. Many infringed these early patents in the boom years. In attacking Edson’s patent they relied on the fact that Swan had demonstrated a practical lamp before hs patent was filed. The legal argument centred on the question of whether the incandescent carbon emitter in Swan’s lamp was a filament or not. If it was, Edson’s patent would be invahd. Their Lordshps decided that it was not u filament and the monopoly was upheld-although this rather odd judgement obscured Swan’s position as ‘the inventor’. By 1883, the E&son Company had title to 215 patents with a hrther 307 applications pendmg’.

From 1886, Edson and Swan had a stranglehold on the industry and any competitor was quickly crushed. Ths situation continued until Edison’s master patent expired in Britain on 10th November 1893 and in America on 17th November 1894. In retrospect, the vital contributions turning a scientific curiosity into a practical lamp were:

(u) Sprengel’s vacuum pump and its derivatives (b) the discovery by Swan and Edison that both the

filament and bulb must be heated during pumping to remove occluded gases

(c) the process developed and patented by Sawyer and Man, and independently discovered by Maxim, for depositing a layer of graphte onto a carbon filament to provide a degree of control over its electrical resistance and emissivity

Later, in 1896, Arturo Mahgnani dscovered that the residual impurities in a lamp could be cleaned-up by applying red phosphorus to the internal surface of the exhaust tube to act as a getter. This principle is stdl used by lamp makers throughout the world and enables hgh-speed processing.

Numerous articles have been written, and presentations made, on the hstory of the incandescent lamp. Not least of these was the celebration of Sir Joseph Swan’s centenary at the IEE in 1979 and the recent book by Brian Bowers’. However, there is a

dearth of information on the genesis of lamp manufacture and the amazing innovative ability of the scientists and engineers of that time. This article aims to fill that gap. Inevitably, some of the ideas and products faded-although the underlying concepts were sound-and some have been reinvented!

Manufacturing techniques

Carbon filaments Lamp manufacture was underway in England and the

USA by 1880 and it was not long before carbon became the preferred filament material. The principal inventors/manufacturers of the raw materials used for the filament are shown in Table 1. Table 1: Inventorslmanufacturers of various filament materials

card board Hiram S. Maxim ‘M‘ shaped cardboard Joseph Swan parchmentised cotton Alvin Edison cotton, then bamboo

Parchmentising resulted in a stronger and less brittle filament with yields of the order of 97%. The following paragraphs give the essential stages in filament preparation. A detded description is given by Ram3, who describes hmself as having ‘considerable experience in lamp malung’. His book is a mine of information on the early industry.

The base material of Swan’s filament was pure cotton, which was degreased in a solution of soda, then washed and dried. It was then passed through a bath of d h t e sulphuric acid and washed again to yield a semi- opaque thread (this is the parchmentising process). This thread was wound onto drying kames and suitably weighted to keep it stretched. During t h s process it gained some 8% in weight and had a rough and uneven surface. Finally, the thread was drawn through a circular die to reduce it to the desired hameter and give a smooth(er) surface.

The next stage was carbonising. The drawn thread was wound onto carbon formers constructed to allow the thread to shrink during processing. A number of formers were then loaded into a carbon box, with carbon separators between each, and covered by a loosely cemented lid. Ths assembly was then placed inside a graphte box with the intervening space packed with powdered carbon. The graphite boxes were finally loaded into a coal-fired furnace. By judicial manipulation of the fire and damper, the temperature was raised to 550-600°C over a period of 6-7 hours and then to a ‘bright-red’ temperature (about 1700”C, judged by eye) over a similar period. Cooling to room temperature took a further 12 hours! Unloadmg and cutting the carbonised thread to form open-ended loops (filaments) was by all accounts a very slulful operation. Other starting materials, such as paper, grass and bamboo, were carbonised in a sirmlar way.

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d

Fig. 1 stopper (a). It is likely that the lead wires were in two parts, with a copper outer section to conserve platinum. A glass tube about 25 cm long and 2 cm in diameter is drawn out (b) and an open-ended bulb formed at each end (c). This is cut to give two bulbs (d). Finally the stopper carrying the filament is inserted into the neck of the bulb and fused to it. The lamp is then ready for pumping (e).

Typical lamp assembly c. 1890. The platinum wires to which the carbon filament is attached are sealed into a glass

The next innovative step in producing carbon hlaments was the ‘flashng’ process, independently discovered in 1878 by Sawyer and Man4 and Maxim. Early carbon filaments were of variable quahty, requiring an elaborate gradmg system to be applied to the finished product. It was found that if a filament was lit for a few seconds (i.e. flashed) in a hydrocarbon atmosphere, such as pentane, the hydrocarbon would dissociate at the incandescent surface and deposit a layer of graphite. This imparted strength and durabihty to the fragile filament. The physical properties ofthis layer were markedly dfferent, having a resistivity l/lOth of that of the filament and a lower emissivity (see the Appenduc). During the deposition process, the filament resistance could be monitored, allowing greater uniformity. Later it was discovered that heat-treating graphite-coated filaments could produce a metahc like structure having a positive coefficient of resistance, a discovery first reported by Howell in 18875. The potential of this discovery was realised with the development of furnaces capable of operating at

temperatures greater than 300OOC. In fact, the electrical resistivity of graphite corresponds to the degree of its structural perfection. High-temperature heat treatment increases the coating density, and consequently reduces imperfections, imparting a ‘metalk like’ characteristic to the material6. Such filaments could be run at a substantially higher temperature with a consequent 25% increase in luminous efficacy’. They were marketed by General Electric in 1905 and were known as GEM (General Electric Metallised lamps). They represented a halfivay house between carbon and metal filament lamps.

The final major innovation was made by Swan in 1883 when he developed a process for malung squirted cellulose filaments. Nitrocellulose was dissolved in acetic acid and discharged through a die into a coagulating fluid to form a continuous plastic thread having the appearance and characteristics of catgut*. This produced vastly superior, fibreless, homogeneous filaments. Swan’s collaborators continued to develop t h s process, eventually using a zinc chloride solution of

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viscose. Charles Stearn formed a viscose s d k syndicate, which was shortly afterwards taken over by Courtauld's, who made the world's first artificial silk ( ra~on)~ . Squirted filaments, coupled with the flashmg process, eventually replaced all other materials and processes. Edison changed to these h-om bamboo in 1894, although bamboo was still used in some hgh- wattage lamps into the 1930s.

Mounting Carbonised filaments were attached to platinum

lead-in wires in a variety of ways. Initially they were connected by a mechanical joint, for example by nuts and bolts or crimping. Lane Fox employed a carbon paste, but the most successfd method was that adopted by both Swan and Edson. In their lamps they formed a loose butt or sleeve joint and deposited a layer of carbon to cement the filament and lead wire together. Ths was acheved by submerging the joint in a liquid hydrocarbon, such as a mixture of paraffin or turpentine, and locally heating to a bright red by passing a current through it. The liquid surrounding the joint boiled and the vapour dlssociated, depositing carbon in a sirmlar way to the flashng process.

Bulbs and sealing The glass avdable in the late 1800s was generally

&ah-lime-shcate, sometimes having up to 50% lead oxide added. Nearly all the raw materials were of natural origin: ashes of plants &om the sea shore, ashes of wood (potash), and shca fi-om sand or crushed stones. Lime was often an impurity in other materials or was added as limestone. Only lead oxide was syntheticlo. In the early days of manufacture, it was usual to melt the glass in a pot furnace and hand blow the bulbs in-house. Swan employed German glass blowers from Thuringer Wald. The production rate was about 16 bulbs per hour and each blower had hls individual technique. The notable exception was Edison in the USA. Initially he purchased tubing to make bulbs but by late 1880 he was obtaining hand- blown bulbs from the Corning Glass Co. The precise composition of each manufacturer's glass was a closely guarded secret. One company weighed the contents on a large pair of scales with one arm in the manager's office. Another glass mixer was reputed to have kept a ferocious dog to deter unwelcome visitors! In those days, knowledge was power.

Bulbs were blown with an open neck to accept the mounted filament. A tube was sealed to the crown of the bulb for subsequent attachment to the exhaust system. After inserting the mount the neck was softened by a blowtorch and pinched onto the platinum lead wires using specially adapted pliers. Platinum was chosen for the lead wires because it has a coefficient of expansion of 9X1O4 K-', which is close to that (9 to10x104 K-') of the alk&-lime shcate bulb glasses avdable at that time. However, most manufacturers devised methods for reducing the platinum of the seal by, for example, butt welding it to

copper wire (this was known as the Siemens seal). It was 1912 before a satisfactory alternative was developed in the USA by Colin Fink". A typical lamp assembly is shown in Fig. 1.

Exhausting The mechanical pumps available at the time could

not attain a sufficiently low pressure to make satisfactory lamps. In the 17th century Torricek had demonstrated a means of producing a vacuum above a column of mercury (to form a barometer), see Fig. 2a, and this was the method employed by Goebel to produce h s lamps-though it was hardly a production proposition. In 1865, Sprengel invented a form of mercury pump that could be used to evacuate a volume, workmg on the principle of a falhng stream of mercury drawing air through an exhaust tube, see Fig. 2b. Another type of pump employing the Torricellian principle was devised by Geissler, see Fig. 2c. Such pumps were often used in conjunction with a mechanical pump to reduce the process time. Ingenious methods were devised to circulate the mercury and mechanise the system in order to acheve continuous pumping. The pump exhaust tube was usually connected to a manifold to facilitate multiple lamp processing.

Even with the advent of the Sprengel pump, the earliest lamps were unsuccessful. When the filament became incandescent it released vapours and gases, particularly water vapour and oxygen, whch dramatically reduced lamp life. Swan and Edison independently solved t h s problem by running the filament and heating the bulb during pumping. This released vapours and gases held by the filament and prevented them fiom being reabsorbed. Initially filaments were run at about 500°C to liberate occluded gases and then the temperature was gradually increased to slightly in excess of the normal operating value. It is likely that oxygen, hydrogen and hydrocarbons would be evolved during this latter stage due to the incomplete carburisation of the filament. Usually, a drying trap containing, for example, phosphorus pentoxide was incorporated in the system to remove water vapour.

The discovery of incorporating red phosphorus in lamps to react with residual impurities revolutionised lanip manufacture by allowing less stringent processing. It reduced pumping time to about a tenth. In the early 1900s, considerable importance was placed on 'gettering' and a number of alternatives were patented'*.

Capping Electrical connection to the earliest lamps was simply

via wire loops formed in the ends of the lead wires. However, it was not long before recognisable forerunners of the Edison screw cap and bayonet cap emergedI3. Originally, the contacts were located within a brass collar (BC cap) and the assembly secured by plaster of Paris. Alfred and Joseph Swan took over the

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a b C

Fig. 2 Basic mercury pumps: (a) A barometer (Torricellian vacuum). A long closed-end tube is filled with mercury. The open end is then temporarily closed and submerged in a cup containing mercury. When released the column falls to a height (H) corresponding to atmospheric pressure and leaves a vacuum in the space above. (b) A Sprengel pump. A funnel, F, is connected to a fall tube, B, via a tap, T. An exhaust tube, E, communicates with the volume to be evacuated. The fall tube terminates in a reservoir, R, with an overflow arrangement running into a cup, C. The tap is adjusted to allow'a thin stream of mercury droplets to flow into the fall tube. This pulls air out of the exhaust tube, the air being entrapped between droplets of mercury and finally escaping at the surface of the reservoir R. The excess mercury flows into the cup C, which is periodically transferred to the funnel. (c) A Geissler pump. A glass bulb, G, is connected to a three-way valve, V. The lower end of the bulb is sealed onto a long (-3 m) glass tube, L, which is connected, via a rubber tube, R, to an open cup, C. The valve V can be turned to select the exhaust tube, E, or to open G to air, or close off both ports. The system is charged with mercury and initially the valve is in position A, open to air. Cup C is gradually raised so that G is filled with mercury. The valve is then turned to communicate with the exhaust tube E, and C is lowered. This causes the mercury in G to fall, pulling air from the volume being exhausted. The valve is then set to close both ports and C is raised to compress the air. Finally the valve is turned to open G to air and cup C is raised further to expel the air. This pumping action is repeated until the system is fully evacuated-a very long process.

management of the Gateshead glass works to enable them to develop bulb blowing and are credited with developing an insulating glass (vitrite) to secure the base contact^'^. This material is still used in many modern lamp caps.

Measurements

The candlepower oflamps was measured by comparing the brightness of adjacent surfaces dluminated by a standard lamp and the lamp under test. The &stance of one lamp was adjusted to give equal brightness, judged by eye, and the candlepower of the test lanip could be calculated &om t h s &stance. In the simplest form of photometer, the Rumford photometer, the comparison was made between the illuminated shadows of an object cast by the two lamps on a screen (Fig. 3). This was developed and improved but retained the same basic principal. At that time there was no

recognised international standard of illumination. In England the light from a Spermaceti candle was the standard. France used a Carcel burner using rape oil as the fuel. Eventually, a pentane burner was adopted h-om which working standard lamps were cahbrated.

Ths method yields the horizontal candlepower of the test lamp. Since it was impractical to measure the mean horizontal candle power (MHCP) of every sample, it was common to ahgn the lamp to measure the maximum value and apply a factor (of -0.91) to estimate the MHCP One author suggests that commercial literature often quoted the maximum- nothing changes! A further correction factor (of -0.8), known as the spherical reduction factor (SRF), was applied to obtain the mean spherical candle power (MSCP)'5.

Hence the MSCP was approximately 0.91 x 0.8 x (maximum horizontal CP). To estimate the luminous flux t h s should be multiplied by 47~ so that flux

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Table 2: Estimated luminous efficacy of typical carbon filament lamps rated at 4 WICP. c.1890

fine platinum wire as a substrate Onto which a graphte layer was deposited - . by thc prc\.iously dcscribed hydro- carbon tldshlng process. The platinum core \\.as volatilised led\.irig ‘I hollow

64 0.986 x 1 O6 16 , 3.14 2.3 2.1 cxbon shell. Boston and Bernstein’s 2.1 32 0.987 x 1 O6 8 3.14 2.3

. . . . .. 1 .

*See Appendix.

= 9.14 x (max HCP) lumens. In fact, the luminous flux has frequently been overestimated because the correction factors have been ignored.

Fortunately, Ram“ has included physical, electrical and photometric data for carbon filament lamps circa 1890. This has been used to calculate the luminous flux and efficacy of these early lamps using the Planck equation for black body radiation coupled with modern values of emissivity for carbon and graphte. Comparisons are shown in Table 2 and details of ths calculation are given in the appendur.

The earliest (1879) type of E&son carbon lamp was reconstructed in 1929 to commemorate its 50th anniversary. This operated at a filament temperature below 2000 K and had a luminous efficacy of only 1.4 lm/W” Again, h-om Planck’s equation this is estimated at 1.53 lm/W Most of the early documents avoid giving the filament temperature, but according to calculations, based on the data given by Ram, lamps made around 1890 had a filament temperature of about 2100 K. Ths value is supported by Nienhuis’*. Fig. 4 shows the relationshp between filament temperature and luminous efficacy for carbon filament vacuum lamps.

The odd balls

Ingenious ideas were tried to improve these early lamps, particularly to increase the filament resistance in order to acheve lower power and higher voltage operation. However, there was a practical limit as to how fine a filament could be made. Two methods were developed to alleviate this problem by producing hollow filaments. The filament of Cruto’s lamp used a

Fig. 3 The Rumford photometer. AB is a vertical white screen. L1 and L2 are the two sources, of intensities I1 and 12. respectively, to be compared. R is a vertical rod located between the lamps and the screen. L‘1 and L‘2 are the shadows of the rod cast on the screen by the two sources. The shadow L‘1 is illuminated only by light from L2 and L‘2 only by light from L1. Distances dl andlor dz can be adjusted to give equal intensity, allowing the intensity of the test source to be obtained by the inverse square law relationship.

ENGINEERIN

lamp acheved a sirmlar result by covering silk tubes with a carbon

paste”. A number of alternative filament materials were tried. For example, in 1893-94 the Russian Lodyguine coated platinum wires with rhodium, iridum, ruthenium, osmium, chromium, molybdenum and tungsten. This is claimed to be the first time that tungsten was suggested as a canmdate for filaments.

Another problem was the progressive fall in light output during lamp life caused by carbon depositing onto the surface of the bulb. To counteract this, ‘Novak’ lamps (1894), made by Waring Electrical Company in the USA, contained a low-pressure filling of bromine”; chlorine is also mentioned at the even earlier date of l88l2’-almost three quarters of a century before the first tungsten halogen lamps appeared!

To overcome the difficulty of making a satisfactory glass-to-metal seal, Diehl developed an AC lamp with the primary windmg of a transformer on the outside of the bulb and the secondary connected to the filament on the inside*’, see Fig. 5. A simdar principle is used in modern induction lamps in which transformer action is used to induce a current directly in the gaseous m n g .

A very interesting development, known as a ‘half incandescent lamp’ is attributed to Reynier, Markus and Werdermann, circa 1878. This formed a bridge between the carbon arc and carbon filament. The lamp consisted of a t h n carbon rod having a tapered end (hke a pencil) touchng a carbon or metallic electrode. Light was emitted fiom the incandescent contact point, the adjacent part of the rod and fiom carbon particles. The carbon rod (the negative electrode) was gradually consumed and a rod of 2 mm mameter and 300 rnm length lasted for about two hours-hardly a practical

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Fig. 4 Filament temperature against lamp efficacy. The earliest (1879) lamps operated between 1900 and 2000 K. Later (1890) lamps ran at about 2100 K and GEM lamps at around 2200 K.

device, although it was claimed to have an efficacy some four times hgher than a carbon filament lamp of the dayz2.

Conclusions

Early literature on the development and manufacture of incandescent lamps reveals the many inventors who contributed to the successful introduction of and subsequent improvements to both the products and the processes. However, it was Swan’s inventive genius and

Ehson’s entrepreneurial gifts that brought the concepts to maturity. Although continuous development of materials and processes has greatly improved the performance of incandescent lamps, it is surprising how closely modern types resemble earlier counterparts, e.g. in their pear-shaped bulbs and screw or bayonet caps.

Photometric measurements reported around the 1880s are likely to be unreliable because of the method of measurement, diversity of national standards and the fact that they relied upon an element of judgement.

r-

I

I I _

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Fortunately, information has survived and, in conjunction with modern data on carbon fibres and filaments, has enabled a more reliable estimate to be made.

Some ingenious ideas failed at the time and some have resurfaced in alternative embodiments-and I believe that there are still untapped possibhties with their roots in history, for example composite filaments.

References

1 BRIGHT, A. A,: ‘The electric lamp industry- technological change and economic development from1800 to 1947’ (The M a c d a n Company, New York, 1949), p.84

2 BOWERS, B.: ‘Lengthening the day-a history of lighting technology’ (Oxford University Press, Oxford, 1998)

3 RAM, G. S.: ‘The incandescent lamp and its manufacture’ (The Electrician Publishing Co., London, 1894), pp.7-32

4 British Patent 4847 (1878) 5 HOWELL, J. W: ‘Conductivity of incandescent carbon

filaments and of the space surrounding them’, Electricity,

5 DRESSELHAUS, M. S., and DRESSELHAUS, G.: ‘Graphite fibers and filaments’ (Springer Series in Material Science, 1988), pp.23&242

7 SOLOMON, M.: ‘Electric lamps’ (Archibald Constable & Co. Ltd., London, 1908), pp.133-137

8 British Patent 5978 (1883) 9 ‘The pageant of the lamp’ (Commissioned by the Edison

Swan Electric Co. Ltd. and printed by the General

1897,21, pp.117-119

Advertising Company ofLondon Ltd, c. 1947)

lamp’, Glass Techno/., 1990, 63, (3), pp.78-84 10 TOBER, H.: ‘History ofglass for the incandescent electric

11 BRIGHT, A. A,, op. cit., p.207 12 BRIGHT, A. A,, op. cit., p.329 13 WORMELL, R.: ‘Electricity in the service ofman’ (Cassell

& Co. Ltd., London, 1883), pp.493-506. 14 SWAN, K. R.: ‘Sir Joseph Swan-and the invention of the

electric lamp’ (Longmans, Green and Co. Ltd., 1946), p.29 15 SOLOMON, M., op. cit., pp.66-89 16 M, G. S., op. cit., p.73 17 ANDERSON, J.: ‘Subdivision of light’, Physics Today,

October 1979, pp.33-40 18 NIENHUIS, H.: ‘Mijlpalen in de ontwikkeling van het

elektrische licht’, Licht Belicht, Technische Hogeschool, Eindhoven, April 1979, pp.43-60

19 WORMELL, R., op. cit., pp.501-503 20 BRIGHT, A. A,, op. cit., p.132 21 US Patent 254 780 (1881)

23 AVERKOV E. I., et al.: ‘Emissive properties of graphites’,

24 RAM, G. S., op. cit., p.63 25 COATON, J. R., and MARSDEN, M. M. (E&.): ‘Lamps

and lighting’ (Arnold, London, 1997,4th edn.), pp.12, 102

22 WOEUVLELL, R., OP. cit., pp.502-515

High Temp., 19, (2), pp.308-312

OIEE: 2002

Dr. Coaton retired in 1991 and was formerly Director of Technology for the Light Sources Division of Thorn Lighting Ltd., later working as an independent consultant. He has served on IEE Professional Group Committees SI and S3.

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