Part 14

The earlier tungsten filaments were made by three rival processes. By the first, a deposit of tungsten was "flashed" on a fine carbon filament, the latter being eliminated finally by heating in an atmosphere of hydrogen and water-vapor. By the second, colloidal tungsten was produced by operating an arc between tungsten electrodes under water. The finely divided tungsten was gathered, partially dried, and squirted through dies to form filaments. These were then sintered. The third was the "paste" process already described. These methods produced fragile filaments, but their luminous efficiency was higher than that of previous ones. However, in this country ductile tungsten was soon on its way. An ingot of tungsten is subjected to vigorous swaging until it takes the form of a rod. This is finally drawn into wire.

Much of this development work was done by the laboratories of the General Electric Company and they were destined to contribute another great improvement. The blackening of the lamp bulbs was due to the evaporation of tungsten from the filament. All filaments up to this time had been confined in evacuated bulbs and the low pressure facilitates evaporation, as is well known. It had long been known that an inert gas in the bulb would reduce the evaporation and remedy other defects; however, under these conditions, there would be a considerable loss of energy through conduction of heat by the gases. In the vacuum lamp nearly all the electrical energy is converted into radiant energy, which is emitted by the filament and any dissipation of heat is an energy loss. A high vacuum was one of the chief aims up to this time, but a radical departure was pending.

If an ordinary tungsten-lamp bulb be filled with an inert gas such as nitrogen, the filament may be operated at a very much higher temperature without any more deterioration than takes place in a vacuum at a lower temperature. This gives a more efficient light but a less efficient lamp. The greater output of light is compensated by losses by conduction of heat through the gas. In other words, a great deal more energy is required by the filament in order to remain at a given temperature in a gas than in a vacuum. However, elaborate studies of the dependence of heat-losses upon the size and shape of the filament and of the physics of conduction from a solid to a gas, established the foundation for the gas-filled tungsten lamp. The knowledge gained in these investigations indicated that a thicker filament lost a relatively less percentage of energy by conduction than a thin one for equal amounts of emitted light. However, a practical filament must have sufficient resistance to be used safely on lighting circuits already established and, therefore, the large diameter and high resistance were obtained by making a helical coil of a fine wire. In fact, the gas-filled tungsten lamp may be thought of as an ordinary lamp with its long filament made into a short helical coil and the bulb filled with nitrogen or argon gas.

This development was not accidental and from a scientific point of view it is not spectacular. It did not mark a new discovery in the same sense as the discovery of X-rays. However, it is an excellent example of the great rewards which come to systematic, thorough study of rather commonplace physical laws in respect to a given condition. Such achievements are being duplicated in various lines in the laboratories of the industries. Scientific research is no longer monopolized by educational institutions. The most elaborate and best-equipped laboratories are to be found in the industries sometimes surrounded by the smoke and noise and vigorous activity which indicate that achievements of the laboratory are on their way to mankind. The smoke-laden industrial district, pulsating with life, is the proud exhibit of the present civilization. It is the creation of those who discover, organize, and apply scientific facts. But how many appreciate the debt that mankind owes not only to the individual who dedicates his life to science but to the far-sighted manufacturer who risks his money in organized quest of new benefits for mankind? A glimpse into a vast organization of research, which, for example, has been mainly responsible for the progress of the incandescent lamp would alter the attitude of many persons toward science and toward the large industrial companies.

The progress in the development of electric incandescent lamps is shown in the following table, where the dates and values are more or less approximate. It should be understood that from 1880 to the present time there has been a steady progress, which occasionally has been greatly augmented by sudden steps.

APPROXIMATE VALUES

Lumens per Date Filament Temperature watt 1880 Carbon 3300 deg.F. 3.0 1906 Carbon (graphitized) 3400 4.5 1905 Tantalum 3550 6.5 1905 Osmium 3600 7.5 1906 Tungsten (vacuum) 3700 8.0 1914 Tungsten (gas-filled) up to 5300 deg.F. 10 to 25

Throughout the development of incandescent filament lamps many ingenious experiments were made which resulted usually in light-sources of scientific interest but not of practical value. One of the latest is the tungsten arc in an inert gas. By means of a heating coil, a small arc is started between two electrodes consisting of tungsten, but this as yet has not been shown to be practicable.

Another type of filament lamp was developed by Nernst in 1897. It was an ingenious application of the peculiar properties of rare-earth oxides. His first lamp consisted essentially of a slender rod of magnesia. This substance does not conduct electricity at ordinary temperatures, but when heated to incandescence it becomes conducting. Upon sufficient heating of this filament by external means while a proper voltage is impressed upon it, the electric current passes through it and thereafter this current will maintain its temperature. Thus such a filament becomes a conductor and will continue to glow brilliantly by virtue of the electrical energy which it converts into heat. Later lamps consisted of "glowers" about one inch long made from a mixture of zirconia and yttria, and finally a mixture of ceria, thoria, and zirconia was used. The glower is heated initially by a coil of platinum wire located near it but not in contact with it. Owing to the fact that this glower decreases rapidly in resistance as its temperature is increased, it is necessary to place in series with it a substance which increases in resistance with increasing current. This is called a "ballasting resistance" and is usually an iron wire in a glass bulb containing hydrogen. The heater is cut out by an electromagnet when the glower goes into operation. This lamp is a marvel of ingenuity and when at its zenith it was installed to a considerable extent. Its light is considerably whiter than that of the carbon filament lamps. However, its doom was sounded when metallic filament lamps appeared.

An interesting filament was developed by Parker and Clark by using as a core a small filament of carbon. This flashed in an atmosphere containing a vapor of a compound of silicon, became coated with silicon. This filament was of high specific resistance and appeared to have promise. It has not been introduced commercially and doubtless it cannot compete with the latest tungsten lamps.

Electric incandescent lamps are the present mainstay of electric illumination and, it might be stated, of progress in lighting. Wonderful achievements have been accomplished in other modes of lighting and the foregoing statement is not meant to depreciate those achievements. However, the incandescent filament lamp has many inherent advantages. The light-source is enclosed in an air-tight bulb which makes for a safe, convenient lamp. The filament is capable of subdivision, with the result that such lamps vary from the minutest spark of the smallest miniature lamp to the enormous output of the largest gas-filled tungsten lamp. The outputs of these are respectively a fraction of a lumen and twenty-five thousand lumens; that is, the luminous intensity varies from an equivalent of a small fraction of a standard candle to a single light-source emitting light equivalent to two thousand standard candles.

Statistics are cold facts and are usually uninteresting in a volume of this character, but they tell a story in a concise manner. The development of the modern incandescent lamp has increased the intensity of light available with a great decrease in cost, and this progressive development is shown easily by tables. For example, since the advent of the tungsten lamp the average candle-power and luminous efficiency of all the lamps sold in this country has steadily increased, while the average wattages of the lamps have remained virtually stationary.

AVERAGE CANDLE-POWER, WATTS, AND EFFICIENCY OF ALL THE LAMPS SOLD IN THIS COUNTRY

Lumens Year Candle-power Watts per watt 1907 18.0 53 3.33 1908 19.0 53 3.52 1909 21.0 52 3.96 1910 23.0 51 4.42 1911 25.0 51 4.82 1912 26.0 49 5.20 1913 29.4 47 6.13 1914 38.2 48 7.80 1915 42.2 47 8.74 1916 45.8 49 9.60 1917 48.7 51 10.56

It will be noted that the luminous intensity of incandescent filament lamps has steadily increased since the carbon lamp was superseded, and that in a period of ten years of organized research behind the tungsten lamp the luminous efficiency (lumens per watt) has trebled. In other words, everything else remaining unchanged, the cost of light in ten years was reduced to one third. But the reduction in cost has been more than this, as will be shown later. During the same span of years the percentage of carbon filament lamps of the total filament lamps sold decreased from 100 per cent. in 1907 to 13 per cent. in 1917. At the same time the percentage of tungsten (Mazda) lamps increased from virtually zero in 1907 to about 87 per cent. in 1917. The tantalum lamp had no opportunity to become established, because the tungsten lamp followed its appearance very closely. In 1910 the sales of the former reached their highest mark, which was only 3.5 per cent. of all the lamps sold in the United States. From a lowly beginning the number of incandescent filament lamps sold for use in this country has grown rapidly, reaching nearly two hundred million in 1919.

THE LIGHT OF THE FUTURE

In viewing the development of artificial light and its manifold effects upon the activities of mankind, it is natural to look into the future. Jules Verne possessed the advantage of being able to write into fiction what his riotous imagination dictated, and so much of what he pictured has come true that his success tempts one to do likewise in prophesying the future of lighting. Surely a forecast based alone upon the past achievements and the present indications will fall short of the actual realizations of the future! If the imagination is permitted to view the future without restrictions, many apparently far-fetched schemes may be devised. It may be possible to turn to nature's supply of daylight and to place some of it in storage for night use. One millionth part of daylight released as desired at night would illuminate sufficiently all of man's nocturnal activities. The fictionist need not heed the scientist's inquiry as to how this daylight would be bottled. Instead of giving time to such inquiries he would pass on to another scheme, whereby earth would be belted with optical devices so that day could never leave. When the sun was shining in China its light would be gathered on a large scale and sent eastward and westward in these great optical "pipe-lines" to the regions of darkness, thus banishing night forever. The writer of fiction need not bother with a consideration of the economic situation which would demand such efforts. This line of conjecture is interesting, for it may suggest possibilities toward which the present trend of artificial lighting does not point; however, the author is constrained to treat the future of light-production on a somewhat more conservative basis.

At the present time the light-source of chief interest in electric lighting is the incandescent filament lamp; but its luminous efficiency is limited, as has been shown in a previous chapter. When light is emitted by virtue of its temperature much invisible radiant energy accompanies the visible energy. The highest luminous efficiency attainable by pure temperature radiation will be reached when the temperature of a normal radiator reaches the vicinity of 10,000 deg.F. to 11,000 deg.F. The melting-points of metals are much lower than this. The tungsten filament in the most efficient lamps employing it is operating near its melting-point at the present time. Carbon is a most attractive element in respect to melting-point, for it melts at a temperature between 6000 deg.F. and 7000 deg.F. Even this is far below the most efficient temperature for the production of light by means of pure temperature radiation. There are possibilities of higher efficiency being obtained by operating arcs or filaments under pressure; however, it appears that highly efficient light of the future will result from a radical departure.

Scientists are becoming more and more intimate with the structure of matter. They are learning secrets every year which apparently are leading to a fundamental knowledge of the subject. When these mysteries are solved, who can say that man will not be able to create elements to suit his needs, or at least to alter the properties of the elements already available? If he could so alter the mechanism of radiation that a hot metal would radiate no invisible energy, he would have made a tremendous stride even in the production of light by virtue of high temperature. This property of selective radiation is possessed by some elements to a slight degree, but if treatment could enhance this property, luminous efficiency would be greatly increased. Certainly the principle of selectivity is a byway of possibilities.

A careful study of commonplace factors may result in a great step in light-production without the creation of new elements or compounds, just as such a procedure doubled the luminous efficiency of the tungsten filament when the gas-filled lamp appeared. There are a few elements still missing, according to the Periodic Law which has been so valuable in chemistry. When these turn up, they may be found to possess valuable properties for light-production; but this is a discouraging byway.

It is natural to inquire whether or not any mode of light-production now in use has a limiting luminous efficiency approaching the ultimate limit which is imposed by the visibility of radiation. The eye is able to convert radiant energy of different wave-lengths into certain fixed proportions of light. For example, radiant energy of such a wave-length as to excite the sensation of yellow-green is the most efficient and one watt of this energy is capable of being converted by the visual apparatus into about 625 lumens of light. Is this efficiency of conversion of the visual apparatus everlastingly fixed? For the answer it is necessary to turn to the physiologist, and doubtless he would suggest the curbing of the imagination. But is it unthinkable that the visual processes will always be beyond the control of man? However, to turn again to the physics of light-production, there are still several processes of producing light which are appealing.

Many years ago Geissler, Crookes, and other scientists studied the spectra of gases excited to incandescence by the electric discharge in so-called vacuum tubes. The gases are placed in transparent glass or quartz tubes at rather low pressures and a high voltage is impressed upon the ends of these tubes. When the pressure is sufficiently low, the gases will glow and emit light. Twenty-eight years ago, D. McFarlan Moore developed the nitrogen tube, which was actually installed in various places. But there is such a loss of energy near the cathode that short "vacuum" tubes of this character are very inefficient producers of light. Efficiency is greatly increased by lengthening the tubes, so Moore used tubes of great length and a rather high voltage. As a tube of this sort is used, the gas gradually disappears and it must be replenished. In order to replenish the gas, Moore devised a valve for feeding gas automatically. An advantage of this mode of light-production is that the color or quality of the light may be varied by varying the gas used. Nitrogen yields a pinkish light; neon an orange light; and carbon dioxide a white light. Moore's carbon-dioxide tube is an excellent substitute for daylight and has been used for the discrimination of colors where this is an important factor. However, for this purpose devices utilizing color-screens which alter the light from the tungsten lamp to a daylight quality are being used extensively.

The vacuum-tube method of producing light has an advantage in the selection of a gas among a large number of possibilities, and some of the color effects of the future may be obtained by means of it. Claude has lately worked on light-production by vacuum tubes and has combined the neon tube with the mercury-vapor tube. The spectrum of neon to a large extent compensates for the absence of red light in the mercury spectrum, with a result that the mixture produces a more satisfactory light than that of either tube. However, this mode of light-production has not proved practicable in its present state of development. Fundamentally the limitations are those of the inherent spectral characteristics of gases. Doubtless the possibilities of the mechanisms of the tubes and of combining various gases have not been exhausted. Furthermore, if man ever becomes capable of controlling to some extent the structure of elements and of compounds, this method of light-production is perhaps more promising than others of the present day.

There is another attractive method of producing light and it has not escaped the writer of fiction. H. G. Wells, with his rare skill and with the license so often envied by the writer of facts, has drawn upon the characteristics of fluorescence and phosphorescence. In his story "The First Men in the Moon," the inhabitants of the moon illuminate their caverns by utilizing this phenomenon. A fluorescent liquid was prepared in large quantities. It emitted a brilliant phosphorescent glow and when it splashed on the feet of the earth-men it felt cold, but it glowed for a long time. This is a possibility of the future and many have had visions of such lighting. If the ceiling of a coal-mine was lined with glowing fireflies or with phosphorescent material excited in some manner, it would be possible to see fairly well with the dark-adapted eyes.

This leads to the class of phenomena included under the general term "luminescence." The definition of this term is not thoroughly agreed upon, but light produced in this manner does not depend upon temperature in the sense that a glowing tungsten filament emits light because it is sufficiently hot. A phosphorus match rubbed in the moist palm of the hand is seen to glow, although it is at an ordinary temperature. This may be termed "chemi-luminescence." Sidot blende, Balmain's paint, and many other compounds, when illuminated with ordinary light, and especially with ultra-violet and violet rays, will continue to glow for a long time. Despite their brightness they will be cold to the touch. This phenomenon would be termed "photo-luminescence," although it is better known as "phosphorescence." It should be noted that the latter term was carelessly originated, for phosphorus has nothing to do with it. The glow of the Geissler tube or electrically excited gas at low pressure would be an example of "electro-luminescence." The luminosity of various salts in the Bunsen-flame is due to so-called luminescence and there are many other examples of light-production which are included in the same general class. Inasmuch as light is emitted from comparatively cold bodies in these cases, it is popularly known as "cold" light.

There are many instances of light being emitted without being accompanied by appreciable amounts of invisible radiant energy and it is natural to hope for practical possibilities in this direction. As yet little is known regarding the efficiency of light-production by phosphorescence. The luminous efficiency of the radiant energy emitted by phosphorescent substances has been studied, but it seems strange that among the vast works on phosphorescent phenomena, scarcely any mention is made of the efficiency of producing light in this manner. For example, assume that phosphorescent zinc sulphide is excited by the light from a mercury-arc. All the energy falling upon it is not capable of exciting phosphorescence, as may be readily shown. Assuming that a known amount of radiant energy of a certain wave-length has been permitted to fall upon the phosphorescent material, then in the dark the latter may be seen to glow for a long time. An interesting point to investigate is the relation of the output to input; that is, the ratio of the total emitted light to the total exciting energy. This is a neglected aspect in the study of light-production by this means.

The firefly has been praised far and wide as the ideal light-source. It is an efficient radiator of light, for its light is "cold"; that is, it does not appear to be accompanied by invisible radiant energy. But little is said about its efficiency as a light-producer. Who knows how much fuel its lighting-plant consumes? The chemistry of light-production by living organisms is being unraveled and this part of the phenomenon will likely be laid bare before long. For an equal amount of energy radiated, the firefly emits a great many times more light than the most efficient lamp in use at the present time, but before the firefly is pronounced ideal, the efficiency of its light-producing process must be known.

There are many ways of exciting phosphorescence and fluorescence, the latter being merely an unenduring phosphorescence, which ceases when the exciting energy is cut off. Ultra-violet, violet, and blue rays are generally the most effective radiant energy for excitation purposes. X-rays and the high-frequency discharge are also powerful excitants. As already stated, virtually nothing is known of the efficiency of this mode of light-production or of the mechanism within the substance, but on the whole it is a remarkable phenomenon.