Although gas-lighting has affected the activities of mankind considerably by intensifying commerce and industry and by advancing social progress, the illuminants which eventually took the lead have extended the possibilities and influences of artificial light. In the brief span of a century civilized man is almost totally independent of natural light in those fields over which he has control. What another century will bring can be predicted only from the accomplishments of the past. These indicate possibilities beyond the powers of imagination.



IX

THE ELECTRIC ARCS

Early in 1800 Volta wrote a letter to the President of the Royal Society of London announcing the epochal discovery of a device now known as the voltaic pile. This letter was published in the Transactions and it created great excitement among scientific men, who immediately began active investigations of certain electrical phenomena. Volta showed that all metals could be arranged in a series so that each one would indicate a positive electric potential when in contact with any metal following it in the series. He constructed a pile of metal disks consisting of zinc and copper alternated and separated by wet cloths. At first he believed that mere contact was sufficient, but when, later, it was shown that chemical action took place, rapid progress was made in the construction of voltaic cells. The next step after his pile was constructed was to place pairs of strips of copper and zinc in cups containing water or dilute acid. Volta received many honors for his discovery, which contributed so much to the development of electrical science and art—among them a call to Paris by Bonaparte to exhibit his electrical experiments, and to receive a medal struck in his honor.

While Volta was being showered with honors, various scientific men with great enthusiasm were entering new fields of research, among which was the heating value of electric current and particularly of electric sparks made by breaking a circuit. Late in 1800 Sir Humphrey Davy was the first to use charcoal for the sparking points. In a lecture before the Royal Society in the following year he described and demonstrated that the "spark" passing between two pieces of charcoal was larger and more brilliant than between brass spheres. Apparently, he was producing a feeble arc, rather than a pure spark. In the years which immediately followed many scientific men in England, France, and Germany were publishing the results of their studies of electrical phenomena bordering upon the arc.

By subscription among the members of the Royal Society, a voltaic battery of two thousand cells was obtained and in 1808 Davy exhibited the electric arc on a large scale. It is difficult to judge from the reports of these early investigations who was the first to recognize the difference between the spark and the arc. Certainly the descriptions indicate that the simple spark was not being experimented with, but the source of electric current available at that time was of such high resistance that only feeble arcs could have been produced. In 1809 Davy demonstrated publicly an arc obtained by a current from a Volta pile of one thousand plates. This he described as "a most brilliant flame, of from half an inch to one and a quarter inches in length."

In the library of the Royal Society, Davy's notes made during the years of 1805 and 1812 are available in two large volumes. These were arranged and paged by Faraday, who was destined to contribute greatly to the future development of the science and art of electricity. In one of these volumes is found an account of a lecture-experiment by Davy which certainly is a description of the electric arc. An extract of this account is as follows:

The spark [presumably the arc], the light of which was so intense as to resemble that of the sun, ... produced a discharge through heated air nearly three inches in length, and of a dazzling splendor. Several bodies which had not been fused before were fused by this flame.... Charcoal was made to evaporate, and plumbago appeared to fuse in vacuo. Charcoal was ignited to intense whiteness by it in oxymuriatic acid, and volatilized by it, but without being decomposed.

From a consideration of his source of electricity, a voltaic pile of two thousand plates, it is certain that this could not have been an electric spark. Later in his notes Davy continued:

...the charcoal became ignited to whitness, and by withdrawing the points from each other, a constant discharge took place through the heated air, in a space at least equal to four inches, producing a most brilliant ascending arch of light, broad and conical in form in the middle.

This is surely a description of the electric arc. Apparently the electrodes were in a horizontal position and the arc therefore was horizontal. Owing to the rise of the heated air, the arc tended to rise in the form of an arch. From this appearance the term "arc" evolved and Davy himself in 1820 definitely named the electric flame, the "arc." This name was continued in use even after the two carbons were arranged in a vertical co-axial position and the arc no more "arched." An interesting scientific event of 1820 was the discovery by Arago and by Davy independently that the arc could be deflected by a magnet and that it was similar to a wire carrying current in that there was a magnetic field around it. This has been taken advantage of in certain modern arc-lamps in which inclined carbons are used. In these arcs a magnet keeps the arc in place, for without the magnet the arc would tend to climb up the carbons and go out.

In 1838 Gassiot made the discovery that the temperature of the positive electrode of an electric arc is much greater than that of the negative electrode. This is explained in electronic theory by the bombardment of the positive electrode by negative electrons or corpuscles of electricity. This temperature-difference was later taken into account in designing direct-current arc-lamps, for inasmuch as most of the light from an ordinary arc is emitted by the end of the positive electrode, this was placed above the negative electrode. In this manner most of the light from the arc is directed downward where desired. In the few instances in modern times where the ordinary direct-current arc has been used for indirect lighting, in which case the arc is above an inverted shade, the positive carbon is placed below the negative one. Gassiot first proved that the positive electrode is hotter than the negative one by striking an arc between the ends of two horizontal wires of the same substance and diameter. After the arc operated for some time, the positive wire was melted for such a distance that it bent downward, but the negative remained quite straight.

Charcoal was used for the electrodes in all the early experiments, but owing to the intense heat of the arc, it burned away rapidly. A progressive step was made in 1843 when electrodes were first made by Foucault from the carbon deposited in retorts in which coal was distilled in the production of coal-gas. However, charcoal, owing to its soft porous character, gives a longer arc and a larger flame. In 1877 the "cored" carbons were introduced. These consist of hard molded carbon rods in which there is a core of soft carbon. In these are combined the advantages of charcoal and hard carbon and the core in burning away more rapidly has a tendency to hold the arc in the center. Modern carbons for ordinary arc-lamps are generally made of a mixture of retort-carbon, soot, and coal-tar. This paste is forced through dies and the carbons are baked at a fairly high temperature. A variation in the hardness of the carbons may be obtained as the requirements demand by varying the proportions of soot and retort-carbon. Cored carbons are made by inserting a small rod in the center of the die and the carbons are formed with a hollow core. This may be filled with a softer carbon.

If two carbons connected to a source of electric current are brought together, the circuit is completed and a current flows. If the two carbons are now slightly separated, an arc will be formed. As the arc burns the carbons waste away and in the case of direct current, the positive decreases in length more rapidly than the negative one. This is due largely to the extremely high temperature of the positive tip, where the carbon fairly boils. A crater is formed at the positive tip and this is always characteristic of the positive carbon of the ordinary arc, although it becomes more shallow as the arc-length is increased. The negative tip has a bright spot to which one end of the arc is attached. By wasting away, the length of the arc increases and likewise its resistance, until finally insufficient current will pass to maintain the arc. It then goes out and to start it the carbons must be brought together and separated. The mechanisms of modern arc-lamps perform these functions automatically by the ingenious use of electromagnets.

The interior of the arc is of a violet color and the exterior is a greenish yellow. The white-hot spot on the negative tip is generally surrounded by a fringe of agitated globules which consist of tar and other ingredients of carbons. Often material is deposited from the positive crater upon the negative tip and these accretions may build up a rounded tip. This deposit sometimes interferes with the proper formation of the arc and also with the light from the arc. It is often responsible for the hissing noise, although this hissing occurs with any length of arc when the current is sufficiently increased. The hissing seems to be due to the crater enlarging under excessive current until it passes the confines of the cross-section of the carbon. It thus tends to run up the side, where it comes in contact with oxygen of the air. In this manner the carbon is directly burned instead of being vaporized, as it is when the hot crater is small and is protected from the air by the arc itself. The temperature of the positive crater is in the neighborhood of 6000 deg. to 7000 deg.F. The brightness of the arc under pressure is the greatest produced by artificial means and is very intense. By putting the arc under high pressure, the brightness of the sun may be attained. The temperature of the hottest spot on the negative tip is about a thousand degrees below that of the positive.

No great demand arose for arc-lamps until the development of the Gramme dynamo in 1870, which provided a practicable source of electric current. In 1876 Jablochkov invented his famous "electric candle" consisting of two rods of carbon placed side by side but separated by insulating material. In this country Brush was the pioneer in the development of open arc-lamps. In 1877 he invented an arc-lamp and an efficient form of dynamo to supply the electrical energy. The first arc-lamps were ordinary direct-current open arcs and the carbons were made from high-grade coke, lampblack, and syrup. The upper positive carbon in these lamps is consumed at a rate of one to two inches per hour. Inasmuch as about 85 per cent. of the total light is emitted by the upper (positive) carbon and most of this from the crater, the lower carbon is made as small as possible in order not to obstruct any more light than necessary. The positive carbon of the open arc is often cored and the negative is a smaller one of solid carbon. This combination operates quite satisfactorily, but sometimes solid carbons are used outdoors. The voltage across the arc is about 50 volts.

In 1846 Staite discovered that the carbons of an arc enclosed in a glass vessel into which the air was not freely admitted were consumed less rapidly than when the arc operated in the open air. After the appearance of the dynamo, when increased attention was given to the development of arc-lamps, this principle of enclosing the arcs was again considered. The early attempts in about 1880 were unsuccessful because low voltages were used and it was not until the discovery was made that the negative tip builds up considerably for voltages under 65 volts, that higher voltages were employed. In 1893 marked improvements were consummated and Jandus brought out a successful enclosed arc operating at 80 volts. Marks contributed largely to the success of the enclosed arc by showing that a small current and a high voltage of 80 to 85 volts were the requisites for a satisfactory enclosed arc.

The principle of the enclosed arc is simple. A closely fitting glass globe surrounds the arc, the fit being as close as the feeding of the carbons will permit. When the arc is struck the oxygen is rapidly consumed and the heated gases and the enclosure check the supply of fresh air. The result is that the carbons are consumed about one tenth as rapidly as in the open arc. There is no crater formed on the positive tip and the arc wanders considerably. The efficiency of the enclosed arc as a light-producer is lower than that of the open arc, but it found favor because of its slow rate of consumption of the carbons and consequent decreased attention necessary. This arc operates a hundred hours or more without trimming, and will therefore operate a week or more in street-lighting without attention. When it is considered that open arcs for all-night burning were supplied with two pairs of carbons, the second set going into use automatically when the first were consumed, the value of the enclosed arc is apparent. However, the open arc has served well and has given way to greater improvements. It is rapidly disappearing from use.

The alternating-current arc-lamp was developed after the appearance of the direct-current open-arc and has been widely used. It has no positive or negative carbons, for the alternating current is reversing in direction usually at the rate of 120 times per second; that is, it passes through 60 complete cycles during each second. No marked craters form on the tips and the two carbons are consumed at about the same rate. The average temperature of the carbon tips is lower than that of the positive tip of a direct-current arc, with the result that the luminous efficiency is lower. These arcs have been made of both the open and enclosed type. They are characterized by a humming noise due to the effect of alternating current upon the mechanism and also upon the air near the arc. This humming sound is quite different from the occasional hissing of a direct-current arc. When soft carbons are used, the arc is larger and apparently this mass of vapor reduces the humming considerably. The humming is not very apparent for the enclosed alternating-current arc. The alternating arc can easily be detected by closely observing moving objects. If a pencil or coin be moved rapidly, a number of images appear which are due to the pulsating character of the light. At each reversal of the current, the current reaches zero value and the arc is virtually extinguished. Therefore, there is a maximum brightness midway between the reversals.

Various types of all these arcs have been developed to meet the different requirements of ordinary lighting and to adapt this method of light-production to the needs of projection, stage-equipment, lighthouses, search-lights, and other applications.

Up to this point the ordinary carbon arc has been considered and it has been seen that most of the light is emitted by the glowing end of the positive carbon. In fact, the light from the arc itself is negligible. A logical step in the development of the arc-lamp was to introduce salts in order to obtain a luminous flame. This possibility as applied to ordinary gas-flames had been known for years and it is surprising that it had not been early applied to carbons. Apparently Bremer in 1898 was the first to introduce fluorides of calcium, barium, and strontium. The salts deflagrate and a luminous flame envelops the ordinary feeble arc-flame. From these arcs most of the light is emitted by the arc itself, hence the name "flame-arcs."

By the introduction of metallic salts into the carbons the possibilities of the arc-lamp were greatly extended. The luminous output of such lamps is much greater than that of an ordinary carbon arc using the same amount of electrical energy. Furthermore, the color or spectral character of the light may be varied through a wide range by the use of various salts. For example, if carbons are impregnated with calcium fluoride, the arc-flame when examined by means of a spectroscope will be seen to contain the characteristic spectrum of calcium, namely, some green, orange, and red rays. These combine to give to this arc a very yellow color. As explained in a previous chapter, the salts for this purpose may be wisely chosen from a knowledge of their fundamental or characteristic flame-spectra.

These lamps have been developed to meet a variety of needs and their luminous efficiencies range from 20 to 40 lumens per watt, being several times that of the ordinary carbon open-arc. The red flame-arc owes its color chiefly to strontium, whose characteristic visible spectrum consists chiefly of red and yellow rays. Barium gives to the arc a fairly white color. The yellow and so-called white flame-arcs have been most commonly used. Flame-arcs have been produced which are close to daylight in color, and powerful blue-white flame-arcs have satisfied the needs of various chemical industries and photographic processes. These arcs are generally operated in a space where the air-supply is restricted similar to the enclosed-arc principle. Inasmuch as poisonous fumes are emitted in large quantities from some flame-arcs, they are not used indoors without rather generous ventilation. In fact, the flame-arcs are such powerful light-sources that they are almost entirely used outdoors or in very large interiors especially of the type of open factory buildings. They are made for both direct and alternating current and the mechanisms have been of several types. The electrodes are consumed rather rapidly so they are made as long as possible. In one type of arc, the carbons are both fed downward, their lower ends forming a narrow V with the arc-flame between their tips. Under these conditions the arc tends to travel vertically and finally to "stretch" itself to extinction. However, the arc is kept in place by means of a magnet above it which repels the arc and holds it at the ends of the carbons.

The chief objection to the early flame-arcs was the necessity for frequent renewal of the carbons. This was overcome to a large extent in the Jandus regenerative lamp in which the arc operates in a glass enclosure surrounded by an opal globe. However, in addition to the inner glass enclosure, two cooling chambers of metal are attached to it. Air enters at the bottom and the fumes from the arc pass upward and into the cooling chambers, where the solid products are deposited. The air on returning to the bottom is thus relieved of these solids and the inner glass enclosure remains fairly clean. The lower carbon is impregnated with salts for producing the luminous flame and the upper carbon is cored. The life of the electrodes is about seventy-five hours.

The next step was the introduction of the so-called "luminous-arc" which is a "flame-arc" with entirely different electrodes. The lower (negative) electrode consists of an iron tube packed chiefly with magnetite (an iron oxide) and titanium oxide in the approximate proportions of three to one respectively. The magnetite is a conductor of electricity which is easily vaporized. The arc-flame is large and the titanium gives it a high brilliancy. The positive electrode, usually the upper one, is a short, thick, solid cylinder of copper, which is consumed very slowly. This lamp, known as the magnetite-arc, has a luminous efficiency of about 20 lumens per watt with a clear glass globe.

The mechanisms which strike the arc and feed the carbons are ingenious devices of many designs depending upon the kind of arc and upon the character of the electric circuit to which it is connected. Late developments in electric incandescent filament lamps have usurped some of the fields in which the arc-lamp reigned supreme for years and its future does not appear as bright now as it did ten years ago. High-intensity arcs have been devised with small carbons for special purposes and considered as a whole a great amount of ingenuity has been expended in the development of arc-lamps. There will be a continued demand for arc-lamps, for scientific developments are opening new fields for them. Their value in photo-engraving, in the moving-picture production studios, in moving-picture projection, and in certain aspects of stage-lighting is firmly established, and it appears that they will find application in certain chemical industries because the arc is a powerful source of radiant energy which is very active in its effects upon chemical reactions.

The luminous efficiencies of arc-lamps depend upon so many conditions that it is difficult to present a concise comparison; however, the following may suffice to show the ranges of luminous output per watt under actual conditions of usage. These efficiencies, of course, are less than the efficiencies of the arc alone, because the losses in the mechanism, globes, etc., are included.

Lumens per watt Open carbon arc 4 to 8 Enclosed carbon arc 3 to 7 Enclosed flame-arc (yellow or white) 15 to 25 Luminous arc 10 to 25

Another lamp differing widely in appearance from the preceding arcs may be described here because it is known as the mercury-arc. In this lamp mercury is confined in a transparent tube and an arc is started by making and breaking a mercury connection between the two electrodes. The arc may be maintained of a length of several feet. Perhaps the first mercury-arc was produced in 1860 by Way, who permitted a fine jet of mercury to fall from a reservoir into a vessel, the reservoir and receiver being connected to the poles of a battery. The electric current scattered the jet and between the drops arcs were formed. He exhibited this novel light-source on the mast of a yacht and it received great attention. Later, various investigators experimented on the production of a mercury-arc and the first successful ones were made in the form of an inverted U-tube with the ends filled with mercury and the remainder of the tube exhausted.

Cooper Hewitt was a successful pioneer in the production of practicable mercury-arcs. He made them chiefly in the form of straight tubes of glass up to several feet in length, with enlarged ends to facilitate cooling. The tubes are inclined so that the mercury vapor which condenses will run back into the enlarged end, where a pool of mercury forms the negative electrode. The arc may be started by tilting the tube so that a mercury thread runs down the side and connects with the positive electrode of iron. The heat of the arc volatilizes the mercury so that an arc of considerable length is maintained. The tilting is done by electromagnets. Starting has also been accomplished by means of a heating coil and also by an electric spark. The lamps are stabilized by resistance and inductance coils.

One of the defects of the light emitted by the incandescent vapor of mercury is its paucity of spectral colors. Its visible spectrum consists chiefly of violet, blue, green, and yellow rays. It emits virtually no red rays, and, therefore, red objects appear devoid of red. The human face appears ghastly under this light and it distorts colors in general. However, it possesses the advantages of high efficiency, of reasonably low brightness, of high actinic value, and of revealing detail clearly. Various attempts have been made to improve the color of the light by adding red rays. Reflectors of a fluorescent red dye have been used with some success, but such a method reduces the luminous efficiency of the lamp considerably. The dye fluoresces red under the illumination of ultra-violet, violet, and blue rays; that is, it has the property of converting radiation of these wave-lengths into radiant energy of longer wave-lengths. By the use of electric incandescent filament lamps in conjunction with mercury-arcs, a fairly satisfactory light is obtained. Many experiments have been made by adding other substances to the mercury, such as zinc, with the hope that the spectrum of the other substance would compensate the defects in the mercury spectrum. However no success has been reached in this direction.

By the use of a quartz tube which can withstand a much higher temperature than glass, the current density can be greatly increased. Thus a small quartz tube of incandescent mercury vapor will emit as much light as a long glass tube. The quartz mercury-arc produces a light which is almost white, but the actual spectrum is very different from that of white sunlight. Although some red rays are emitted by the quartz arc, its spectrum is essentially the same as that of the glass-tube arc. Quartz transmits ultra-violet radiation, which is harmful to the eyes, and inasmuch as the mercury vapor emits such rays, a glass globe should be used to enclose the quartz tube when the lamp is used for ordinary lighting purposes.

It is fortunate that such radically different kinds of light-sources are available, for in the complex activities of the present time all are in demand. The quartz mercury-arc finds many isolated uses, owing to its wealth of ultra-violet radiation. It is valuable as a source of ultra-violet for exciting phosphorescence, for examining the transmission of glasses for this radiation, for sterilizing water, for medical purposes, and for photography.



X

THE ELECTRIC INCANDESCENT FILAMENT LAMPS

Prior to 1800 electricity was chiefly a plaything for men of scientific tendencies and it was not until Volta invented the electric pile or battery that certain scientific men gave their entire attention to the study of electricity. Volta was not merely an inventor, for he was one of the greatest scientists of his period, endowed with an imagination which marked him as a genius in creative work. By contributing the electric battery, he added the greatest impetus to research in electrical science that it has ever received. As has already been shown, there began a period of enthusiastic research in the general field of heating effects of electric current. The electric arc was born in the cradle of this enthusiasm, and in the heating of metals by electricity the future incandescent lamp had its beginning.

Between the years 1841 and 1848 several inventors attempted to make light-sources by heating metals. These crude lamps were operated by means of Grove and Bunsen electric cells, but no practicable incandescent filament lamps were brought out until the development of the electric dynamo supplied an adequate source of electric current. As electrical science progressed through the continued efforts of scientific men, it finally became evident that an adequate supply of electric current could be obtained by mechanical means; that is, by rotating conductors in such a manner that current would be generated within them as they cut through a magnetic field. Even the pioneer inventors of electric lamps made great contributions to electrical practice by developing the dynamo. Brush developed a satisfactory dynamo coincidental with his invention of the arc-lamp, and in a similar manner, Edison made a great contribution to electrical practice in devising means of generating and distributing electricity for the purpose of serving his filament lamp.



Edison in 1878 attacked the problem of producing light from a wire or filament heated electrically. He used platinum wire in his first experiments, but its volatility and low melting-point (3200 deg.F.) limited the success of the lamps. Carbon with its extremely high melting-point had long attracted attention and in 1879 Edison produced a carbon filament by carbonizing a strip of paper. He sealed this in a vessel of glass from which the air was exhausted and the electric current was led to the filament through platinum wires sealed in the glass. Platinum was used because its expansion and contraction is about the same as glass. Incidentally, many improvements were made in incandescent lamps and thirty years passed before a material was found to replace the platinum leading-in wires. The cost of platinum steadily increased and finally in the present century a substitute was made by the use of two metals whose combined expansion was the same as that of platinum or glass. In 1879 and 1880 Edison had succeeded in overcoming the many difficulties sufficiently to give to the world a practicable incandescent filament lamp. About this time Swan and Stearn in England had also produced a successful lamp.

In Edison's early experiments with filaments he used platinum wire coated with carbon but without much success. He also made thin rods of a mixture of finely divided metals such as platinum and iridium mixed with such oxides as magnesia, zirconia, and lime. He even coiled platinum wire around a piece of one of these oxides, with the aim of obtaining light from the wire and from the heated oxide. However, these experiments served little purpose besides indicating that the filament was best if it consisted solely of carbon and that it should be contained in an evacuated vessel.

One of the chief difficulties was to make the carbon filaments. Some of the pioneers, such as Sawyer and Mann, attempted to cut these from a piece of carbon. However, Edison and also Swan turned their attention to forming them by carbonizing a fiber of organic matter. Filaments cut from paper and threads of cotton and silk were carbonized for this purpose. Edison scoured the earth for better materials. He tried a fibrous grass from South America and various kinds of bamboo from other parts of the world. Thin filaments of split bamboo eventually proved the best material up to that time. He made many lamps containing filaments of this material, and even until 1910 bamboo was used to some extent in certain lamps.

Of these early days, Edison said:

It occurred to me that perhaps a filament of carbon could be made to stand in sealed glass vessels, or bulbs, which we were using, exhausted to a high vacuum. Separate lamps were made in this way independent of the air-pump, and, in October, 1879, we made lamps of paper carbon, and with carbons of common sewing thread, placed in a receiver or bulb made entirely of glass, with the leading-in wires sealed in by fusion. The whole thing was exhausted by the Sprengel pump to nearly one-millionth of an atmosphere. The filaments of carbon, although naturally quite fragile owing to their length and small mass, had a smaller radiating surface and higher resistance than we had dared hope. We had virtually reached the position and condition where the carbons were stable. In other words, the incandescent lamp as we still know it to-day [1904], in essentially all its particulars unchanged, had been born.

After Edison's later success with bamboo, Swan invented a process of squirting filaments of nitrocellulose into a coagulating liquid, after which they are carbonized. Very fine uniform filaments can be made by this process and although improvements have been made from time to time, this method has been employed ever since its invention. In these later years cotton is dissolved in a suitable solvent such as a solution of zinc chloride and this material is forced through a small diamond die. This thread when hardened appears similar to cat-gut. It is cut into proper lengths and bent upon a form. It is then immersed in plumbago and heated to a high temperature in order to destroy the organic matter. A carbon filament is the result. From this point to the finished lamp many operations are performed, but a discussion of these would lead far afield. The production of a high vacuum is one of the most important processes and manufacturers of incandescent lamps have mastered the art perhaps more thoroughly than any other manufacturers. At least, their experience in this field made it possible for them to produce quickly and on a large scale such devices as X-ray tubes during the recent war.

During the early years of incandescent lamps, improvements were made from time to time which increased the life and the luminous efficiency of the carbon filaments, but it was not until 1906 that any radical improvement was achieved. In that year in this country a process was devised whereby the carbon filament was made more compact. In fact, from its appearance it received the name "metallized filament." These carbon filaments are prepared in the same manner as the earlier ones but are finally "treated" by heating in an atmosphere of hydrocarbons such as coal-gas. The filament is heated by electric current and the heat breaks down the hydrocarbons, with the result that carbon is deposited upon the filament. This "treated" filament has a coating of hard carbon and its electrical resistance is greater than that of the untreated filament.

The luminous efficiency of a carbon filament is a function of its temperature and it increases very rapidly with increasing temperature. For this reason it is a constant aim to reach high filament temperatures. Of all the materials used in filaments up to the present time, carbon possesses the highest melting-point (perhaps as high as 7000 deg.F.), but the carbon filament as operated in practice has a lower efficiency than any other filament. This is because the highest temperature at which it can be operated and still have a reasonable life is much lower than that of metallic filaments. The incandescent carbon in the evacuated bulb sublimes or volatilizes and deposits upon the bulb. This decreases the size of the filament eventually to the breaking-point and the blackening of the bulb decreases the output of light. The treated filament was found to be a harder form of carbon that did not volatilize as rapidly as the untreated filament. It immediately became possible to operate it at a higher temperature with a resulting increase of luminous efficiency. This "graphitized" carbon filament lamp became known as the gem lamp in this country and many persons have wondered over the word "gem." The first two letters stand for "General Electric" and the last for "metallized." This lamp was welcomed with enthusiasm in its day, but the day for carbon filaments has passed. The advent of incandescent lamps of higher efficiency has made it uneconomical to use carbon lamps for general lighting purposes. Although the treated carbon filament was a great improvement, its reign was cut short by the appearance of metal filaments.

In 1803 a new element was discovered and named tantalum. It is a dark, lustrous, hard metal. Pure tantalum is harder than steel; it may be drawn into fine wire; and its melting-point is very high (about 5100 deg.F.). It is seen to possess properties desirable for filaments, but for some reason it did not attract attention for a long time. A century elapsed after its discovery before von Bolton produced the first tantalum filament lamp. Owing to the low electrical resistance of tantalum, a filament in order to operate satisfactorily on a standard voltage must be long and thin. This necessitates storing away a considerable length of wire in the bulb without permitting the loops to come into contact with each other. After the filaments have been in operation for a few hundred hours they become brittle and faults develop. When examined under a microscope, parts of the filament operated on alternating current appear to be offset. The explanation of this defect goes deeply into crystalline structure. The tantalum filament was quickly followed by osmium and by tungsten in this country.

The osmium filament appeared in 1905 and its invention is due to Welsbach, who had produced the marvelous gas-mantle. Owing to its extreme brittleness, osmium was finely divided and made into a paste of organic material. The filaments were squirted through dies and, after being formed and dried, they were heated to a high temperature. The organic matter disappeared and the fine metallic particles were sintered. This made a very brittle lamp, but its high efficiency served to introduce it.

In 1870 when Scheele discovered a new element, known in this country as tungsten, no one realized that it was to revolutionize artificial lighting and to alter the course of some of the byways of civilization. This metal—which is known as "wolfram" in Germany, and to some extent in English-speaking countries—is one of the heaviest of elements, having a specific gravity of 19.1. It is 50 per cent. heavier than mercury and nearly twice as heavy as lead. It was early used in German silver to the extent of 1 or 2 per cent. to make platinoid, an alloy possessing a high resistance which varies only slightly as the temperature changes. This made an excellent material for electrical resistors. The melting-point of tungsten is about 5350 deg.F., which makes it desirable for filaments, but it was very brittle as prepared in the early experiments. It unites very readily with oxygen and with carbon at high temperatures.

The first tungsten lamps appeared on the market in 1906, but these contained fragile filaments made by the squirting process. When the squirted filament of tungsten powder and organic matter was heated in an atmosphere of steam and hydrogen to remove the binding material, a brittle filament of tungsten was obtained. The first lamps were costly and fragile. After years of organized research tungsten is now drawn into the finest wires, possessing a tensile strength perhaps greater than any other material. Filaments are now made into many shapes and the greatest strides in artificial lighting have been due to scientific research on a huge scale.

The achievements which combined to perfect the tungsten lamp to the point where it has become the mainstay of electric lighting are not attached to names in the Hall of Fame. Organization of scientific research in the industrial laboratories is such that often many persons contribute to the development of an improvement. Furthermore, time is usually required for a full perspective of applications of scientific knowledge. In the early days organized research was not practised and the great developments of those days were the works of individuals. To-day, even in pure science, some of the greatest contributions are made by industrial laboratories; but sometimes these do not become known to the public for many years. The whole scheme of scientific development has changed materially. For example, the story of the development of ductile tungsten, which has revolutionized lighting, is complex and more or less shrouded in secrecy at the present time. Many men have contributed toward this accomplishment and the public at the present time knows little more than the fact that tungsten filaments, which were brittle yesterday, are now made of ductile tungsten wire drawn into the finest filaments.

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.



XI

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.

Radium is also a possibility in light-production and in fact has been practically employed for this purpose for several years. It or one of its compounds is mixed with a phosphorescent substance such as zinc sulphide and the latter glows continuously. Inasmuch as the life of some of the radium products is very long, such a method of illuminating watch-dials, scales of instruments, etc., is very practicable where they are to be read by eyes adapted to darkness and consequently highly sensitive to light. Whether or not radium will be manufactured by the ton in the future can only conjectured.

Owing to the limitations imposed by physical laws of radiation and by the physiological processes of vision the highest luminous efficiency obtainable by heating solid materials is only about 15 per cent. of the luminous efficiency of the most luminous radiant energy. At present there are no materials available which may be operated at the temperature necessary to reach even this efficiency. Great progress in the future of light-production as indicated by present knowledge appears to lie in the production of light which is unaccompanied by invisible radiant energy. At present such phenomena as fluorescence, phosphorescence, the light of the firefly, chemi-luminescence, etc., are examples of this kind of light-production. Of course, if science ever obtains control over the constitution of matter, many difficulties will disappear; for then man will not be dependent upon the elements and compounds now available but will be able to modify them to suit his needs.



XII

LIGHTING THE STREETS

In this age of brilliantly lighted boulevards and "great white ways" flooded with light from shop-windows, electric signs, and street-lamps, it is difficult to visualize the gloom which shrouded the streets a century ago. As the belated pedestrian walks along the suburban highways in comparative safety under adequate artificial lighting, he will realize the great influence of artificial light upon civilization if he recalls that not more than two centuries ago in London

it was a common practice ... that a hundred or more in a company, young and old, would make nightly invasions upon houses of the wealthy to the intent to rob them and that when night was come no man durst adventure to walk in the streets.

Inhabitants of the cities of the present time are inclined to think that crime is common on the streets at night, but what would it be without adequate artificial light? Two centuries ago in a city like London a smoking grease-lamp, a candle, or a basket of pine knots here and there afforded the only street-lighting, and these were extinguished by eleven o'clock. Lawlessness was hatched and hidden by darkness, and even the lantern or torch served more to mark the victim than to protect him. It has been said in describing the conditions of the age of dark streets that everybody signed his will and was prepared for death before he left his home. By comparison with the present, one is again encouraged to believe that the world grows better. Doubtless, artificial light projected into the crannies has had something to do with this change.

Adequate street-lighting is really a product of the twentieth century, but throughout the nineteenth century progress was steadily made from the beginning of gas-lighting in 1807. In preceding centuries crude lighting was employed here and there but not generally by the public authorities. In the earliest centuries of written history little is said of street-lighting. In those days man was not so much inclined to improve upon nature, beyond protecting himself from the elements, and he lighted the streets more as a festive outburst than as an economic proposition. Nevertheless, in the early writings occasionally there are indications that in the centers of advanced civilization some efforts were made to light the streets.

The old Syrian city of Antioch, which in the fourth century of the Christian era contained about four hundred thousand inhabitants, appears to have had lighted streets. Libanius, who lived in the early years of that century, wrote:

The light of the sun is succeeded by other lights, which are far superior to the lamps lighted by Egyptians on the festival of Minerva of Sais. The night with us differs from the day only in the appearance of the light; with regard to labor and employment, everything goes on well.

Although apparently labor was not on a strike, the soldiers caused disturbances, for in another passage he tells of riotous soldiers who

cut with their swords the ropes from which were suspended the lamps that afforded light in the night-time, to show that the ornaments of the city ought to give way to them.

Another writer in describing a dispute between two religious adherents of opposed creeds stated that they quarreled "till the streets were lighted" and the crowd of onlookers broke up, but not until they "spat in each other's face and retired." Thus it is seen that artificial light and civilization may advance, even though some human traits remain fundamentally unchanged.

Throughout the next thousand years there was little attempt to light the streets. Iron baskets of burning wood, primitive oil-lamps, and candles were used to some extent, but during all these centuries there was no attempt on the part of the government or of individuals to light the streets in an organized manner. In 1417 the Mayor of London ordained "lanthorns with lights to bee hanged out on the winter evenings betwixt Hallowtide and Candlemasse." This was during the festive season, so perhaps street-lighting was not the sole aim. Early in the sixteenth century, the streets of Paris being infested with robbers, the inhabitants were ordered to keep lights burning in the windows of all houses that fronted on the streets.

For about three centuries the citizens of London, and doubtless of Paris and of other cities, were reminded from time to time in official mandates "on pains and penalties to hang out their lanthorns at the appointed time." The watchman in long coat with halberd and lantern in hand supplemented these mandates as he made his rounds by,

A light here, maids, hang out your lights, And see your horns be clear and bright, That so your candle clear may shine, Continuing from six till nine; That honest men that walk along May see to pass safe without wrong.

In 1668, when some regulations were made for improving the streets of London, the inhabitants were ordered "for the safety and peace of the city to hang out candles duly to the accustomed hour." Apparently this method of obtaining lighting for the streets was not met by the enthusiastic support of the people, for during the next few decades the Lord Mayor was busy issuing threats and commands. In 1679 he proclaimed the "neglect of the inhabitants of this city in hanging and keeping out their lights at the accustomed hours, according to the good and ancient usage of this City and Acts of the Common Council on that behalf." The result of this neglect was "when nights darkened the streets then wandered forth the sons of Belial, flown with insolence and wine."

In 1694 Hemig patented a reflector which partially surrounded the open flame of a whale-oil lamp and possessed a hole in the top which aided ventilation. He obtained the exclusive rights of lighting London for a period of years and undertook to place a light before every tenth door, between the hours of six and twelve o'clock, from Michaelmas to Lady Day. His effort was a worthy one, but he was opposed by a certain faction, which was successful in obtaining a withdrawal of his license in 1716. Again the burden of lighting the streets was thrust upon the residents and fines were imposed for negligence in this respect. But this procedure after a few more years of desultory lighting was again found to be unsatisfactory.

In 1729 certain individuals contracted to light the streets of London by taxing the residents and paid the city for this monopoly. Householders were permitted to hang out a lantern or a candle or to pay the company for doing so. But robberies increased so rapidly that in 1736 the Lord Mayor and Common Council petitioned Parliament to erect lamps for lighting the city. An act was passed accordingly, giving them the privilege to erect lamps where they saw fit and to burn them from sunset to sunrise. A charge was made to the residents, on a sliding scale depending upon the rate of rental of the houses. As a consequence five thousand lamps were soon installed. In 1738 there were fifteen thousand street lamps in London and they were burned an average of five thousand hours annually.

In the annals of these early times street-lighting is almost invariably the result of an attempt to reduce the number of robberies and other crimes. In appealing for more street-lamps in 1744 the Lord Mayor and aldermen of London in a petition to the king, stated

that divers confederacies of great numbers of evil-disposed persons, armed with bludgeons, pistols, cutlasses, and other dangerous weapons, infest not only the private lanes and passages, but likewise the public streets and places of public concourse, and commit most daring outrages upon the persons of your Majesty's good subjects, whose affairs oblige them to pass through the streets, by terrifying, robbing and wounding them; and these facts are frequently perpetrated at such times as were heretofore deemed hours of security.

It has already been seen that gas-lighting was introduced in the streets of London for the first time in 1807. This marks the real beginning of public-service lighting companies. In the next decade interest in street-lighting by means of gas was awakened on the Continent, and it was not long before this new phase of civilization was well under way. Although this first gas-lighting was done by the use of open flames, it was a great improvement over all the preceding efforts. Lawlessness did not disappear entirely, of course, and perhaps it never will, but it skulked in the back streets. A controlling influence had now appeared.

But early innovations in lighting did not escape criticism and opposition. In fact, innovations to-day are not always received by unanimous consent. There were many in those early days who felt that what was good for them should be good enough for the younger generation. The descendants of these opponents are present to-day but fortunately in diminishing numbers. It has been shown that in Philadelphia in 1833 a proposal to install a gas-plant was met with a protest signed by many prominent citizens. A few paragraphs of an article entitled "Arguments against Light" which appeared in the Cologne Zeitung in 1816 indicate the character of the objections raised against street-lighting.

1 From the theological standpoint: Artificial illumination is an attempt to interfere with the divine plan of the world, which has preordained darkness during the night-time.

2 From the judicial standpoint: Those people who do not want light ought not to be compelled to pay for its use.

3 From the medical standpoint: The emanations of illuminating gas are injurious. Moreover, illuminated streets would induce people to remain later out of doors, leading to an increase in ailments caused by colds.

4 From the moral standpoint: The fear of darkness will vanish and drunkenness and depravity increase.

5 From the viewpoint of the police: The horses will get frightened and the thieves emboldened.

6 From the point of view of national economy: Great sums of money will be exported to foreign countries.

7 From the point of view of the common people: The constant illumination of streets by night will rob festive illuminations of their charm.

The foregoing objections require no comment, for they speak volumes pertaining to the thoughts and activities of men a century ago. It is difficult to believe that civilization has traveled so far in a single century, but from this early beginning of street-lighting social progress received a great impetus. Artificial light-sources were feeble at that time, but they made the streets safer and by means of them social intercourse was extended. The people increased their hours of activity and commerce, industry, and knowledge grew apace.

The open gas-jet and kerosene-flame lamps held forth on the streets until within the memory of middle-aged persons of to-day. The lamplighter with his ladder is still fresh in memory. Many of the towns and villages have never been lighted by gas, for they stepped from the oil-lamp to the electric lamp. The gas-mantle has made it possible for gas-lighting to continue as a competitor of electric-lighting for the streets.

In 1877 Mr. Brush illuminated the Public Square of Cleveland with a number of arc-lamps, and these met with such success that within a short time two hundred and fifty thousand open-arc lamps were installed in this country, involving an investment of millions of dollars. Adding to this investment a much greater one in central-station equipment, a very large investment is seen to have resulted from this single development in lighting.

This open-arc lamp was the first powerful light-source available and, appearing several years before the gas-mantle, it threatened to monopolize street-lighting. It consumed about 500 watts and had a maximum luminous intensity of about 1200 candles at an angle of about 45 degrees. Its chief disadvantage was its distribution of light, mainly at this angle of 45 degrees, which resulted in a spot of light near the lamp and little light at a distance. A satisfactory street-lighting unit must emit its light chiefly just below the horizontal in those cases where the lamps must be spaced far apart for economical reasons. On referring to the chapter on the electric arc it will be seen that the upper (positive) carbon of the open-arc emits most of the light. Thus most of the light tends to be sent downward, but the lower carbon obstructs some of this with a resulting dark spot beneath the lamp.

The gas-mantle followed closely after the arrival of the carbon arc and is responsible for the existence of gas-lighting on the streets at the present time. It is a large source of light and therefore its light cannot be controlled by modern accessories as well as the light from smaller sources, such as the arc or concentrated-filament lamp. As a consequence, there is marked unevenness of illumination along the streets unless the gas-mantle units are spaced rather closely. Even with the open-arc, without special light-controlling equipment there is about a thousand times the intensity near the lamps when placed on the corners of the block as there is midway between them.

In 1879 the incandescent filament lamp was introduced and it began to appear on the streets in a short time. It was a feeble, inefficient light-source, compared with the arc-lamp, but it had the advantage of being installed on a small bracket. As a consequence of simplicity of operation, the incandescent lamp was installed to a considerable extent, especially in the suburban districts.



The open-arc lamp possessed the disadvantage of emitting a very unsteady light and of consuming the carbons so rapidly that daily trimming was often necessary. In 1893 the enclosed arc appeared and although it consumed as much electrical energy as the open-arc and emitted considerably less light, it possessed the great advantage of operating a week without requiring a renewal of carbons. By surrounding the arc by means of a glass globe, little oxygen could come in contact with the carbons and they were not consumed very rapidly. The light was fairly steady and these arcs operated satisfactorily on alternating current. The latter feature simplified the generating and distributing equipment of the central station.

The magnetite or luminous arc-lamp next appeared and met with considerable success. It was more efficient than the preceding lamps but was handicapped by being solely a direct-current device. Those familiar with the generation and distribution of electricity will realize this disadvantage. However, its luminous intensity just below the horizontal was about 700 candles and its general distribution of light was fairly satisfactory. Later the flame-arcs began to appear and they were installed to some extent. The arc-lamp has served well in street-lighting from the year 1877, when the open-arc was introduced, until the present time, when the luminous-arc is the chief survivor of all the arc-lamps.

The carbon incandescent filament lamp was used extensively until 1909, when the tungsten filament lamp began to replace it very rapidly. However, it was not until 1914, when the gas-filled tungsten lamp appeared, that this type of light-source could compete with arc-lamps on the basis of efficiency. The helical construction of the filament made it possible to confine the filament of a high-intensity tungsten lamp in a small space and for the first time a high degree of control of the light of street lamps was possible. Prismatic "refractors" were designed, somewhat on the principle of the lighthouse refractor, so that the light would be emitted largely just below the horizontal. This type of distribution builds up the illumination at distant points between successive street lamps, which is very desirable in street-lighting. The incandescent filament lamp possesses many advantages over other systems. It is efficient; capable of subdivision; operates on direct and alternating current; requires little attention; and is capable of most successful use with light-controlling apparatus.

According to the reports of the Department of Commerce the number of electric arc-lamps for street-lighting supplied by public electric-light plants decreased from 348,643 in 1912 to 256,838 in 1917, while the number of electric incandescent filament lamps increased from 681,957 in 1912 to 1,389,382 in 1917.

Street-lighting is not only a reinforcement for the police but it decreases accidents and has come to be looked upon as an advertising medium. In the downtown districts the high-intensity "white-way" lighting is festive. The ornamental street lamps have possibilities in making the streets attractive and in illuminating the buildings. However, it is to be hoped that in the present age the streets of cities and towns will be cleared of the ragged equipment of the telephone and lighting companies. These may be placed in the alleys or underground, leaving the streets beautiful by day and glorified at night by the torches of advanced civilization.



XIII

LIGHTHOUSES

At the present time thousands of lighthouses, light-ships, and light-buoys guide the navigator along the waterways and into harbors and warn him of dangerous shoals. Many wonderful feats of engineering are involved in their construction and in no field of artificial lighting has more ingenuity been displayed in devising powerful beams of light. Many of these beacons of safety are automatic in operation and require little attention. It has been said that nothing indicates the liberality, prosperity, or intelligence of a nation more clearly than the facilities which it affords for the safe approach of the mariner to its shores. Surely these marine lights are important factors in modern navigation.

The first "lighthouses" were beacon-fires of burning wood maintained by priests for the benefit of the early commerce in the eastern part of the Mediterranean Sea. As early as the seventh century before Christ these beacon-fires were mentioned in writings. In the third century before the Christian era a tower said to be of a great height was built on a small island near Alexandria during the reign of Ptolemy II. The tower was named Pharos, which is the origin of the term "pharology" applied to the science of lighthouse construction. Caesar, who visited Alexandria two centuries later, described the Pharos as a "tower of great height, of wonderful construction." Fire was kept burning in it night and day and Pliny said of it, "During the night it appears as bright as a star, and during the day it is distinguished by the smoke." Apparently this tower served as a lighthouse for more than a thousand years. It was found in ruins in 1349. Throughout succeeding centuries many towers were built, but little attention was given to the development of light-sources and optical apparatus.

The first lighthouse in the United States and perhaps on the Western continents was the Boston Light, which was completed in 1716. A few days after it was put into operation a news item in a Boston paper heralded the noteworthy event as follows:

By virtue of an Act of Assembly made in the First Year of His Majesty's Reign, For Building and Maintaining a Light House upon the Great Brewster (called Beacon-Island) at the Entrance of the Harbour of Boston, in order to prevent the loss of the Lives and Estates of His Majesty's Subjects; the said Light House has been built; and on Fryday last the 14th Currant the Light was kindled, which will be very useful for all Vessels going out and coming in to the Harbour of Boston, or any other Harbours in the Massachusetts Bay, for which all Masters shall pay to the Receiver of Impost, one Penny per Ton Inwards, and another Penny Outwards, except Coasters, who are to pay Two Shillings each, at their clearance Out, And all Fishing Vessels, Wood Sloops, etc. Five Shillings each by the Year.

This was the practical result of a petition of Boston merchants made three years before. The tower was built of stone, at a cost of about ten thousand dollars. Two years later the keeper and his family were drowned and the catastrophe so affected Benjamin Franklin, a boy of thirteen, that he wrote a poem concerning it. The lighthouse was badly damaged during the Revolution, by raiding-parties, and in 1776, when the British fleet left the harbor, a squad of sailors blew it up. It was rebuilt in 1783 and has since been increased in height.

Apparently oil-lamps were used in it from the beginning, notwithstanding the fact that candles and coal fires served for years in many lighthouses of Europe. In 1789 sixteen lamps were used and in 1811 Argand lamps and reflectors were installed, with a revolving mechanism. It now ceased to be a fixed light and the day of flashing lights had arrived. At the present time the Boston Light emits a beam of 100,000 candle-power directed by modern lenses.

When the United States Government was organized in 1789 there were ten lighthouses owned by the Colonies, but the Boston Light was in operation thirty years before the others. Sandy Hook Light, New York Harbor, was established in 1764 and its original masonry tower is still standing and in use. It is the oldest surviving lighthouse in this country. It was built with funds raised by means of two lotteries authorized by the New York Assembly. A few days after it was lighted for the first time the following news item appeared in a New York paper:

On Monday evening last the New York Light-house erected at Sandy Hook was lighted for the first time. The House is of an Octagon Figure, having eight equal Sides; the Diameter at the Base 29 Feet; and at the top of the Wall, 15 Feet. The Lanthorn is 7 feet high; the Circumference 33 feet. The whole Construction of the Lanthorn is Iron; the Top covered with Copper. There are 48 Oil Blazes. The Building from the Surface is Nine Stories; the whole from Bottom to Top is 103 Feet.

From these early years the number of lighthouses has steadily grown, until now the United States maintains lights along 50,000 miles of coast-line and river channels, a distance equal to twice the circumference of the earth. It maintains at the present time about 15,000 aids to navigation at an annual cost of about $5,000,000. In 1916 this country was operating 1706 major lights, 53 light-ships, and 512 light-buoys—a total of 5323.

The earliest lighthouses were equipped with braziers or grates in which coal or wood was burned. These crude light-sources were used until after the advent of the nineteenth century and in one case until 1846. In the famous Eddystone tower off Plymouth, England, candles were used for the first time. The first Eddystone tower was completed in 1698, but it was swept away in 1703. Another was built and destroyed by fire in 1755. Smeaton then built another in 1759. Inasmuch as Smeaton is credited with having introduced the use of candles, this must have occurred in the eighteenth century; still it appears that, as we have said, the Boston Light, built in 1716, used oil-lamps from its beginning. However, Smeaton installed twenty-four candles of rather large size each credited with an intensity of 2.8 candles. The total luminous intensity of the light-source in this tower was about 67 candles. Inasmuch as this was before the use of efficient reflectors and lenses, it is obvious that the early lighthouses were rather feeble beacons.

According to British records, oil-lamps with flat wicks were first used in the Liverpool lighthouses in 1763. The Argand lamp, introduced in about 1784, became widely used. The better combustion obtained with this lamp having a cylindrical wick and a glass chimney greatly increased the luminous intensity and general satisfactoriness of the oil-lamp. Later Lange added an improvement by providing a contraction toward the upper part of the chimney. Rumford and also Fresnel devised multiple-wick burners, thus increasing the luminous intensity. In these early lamps sperm-oil and colza-oil were burned and they continued to be until the middle of the nineteenth century. Cocoanut-oil, lard-oil, and olive-oil have also been used in lighthouses.

Naturally, mineral oil was introduced as soon as it was available, owing to its lower cost; but it was not until nearly 1870 that a satisfactory mineral-oil lamp was in operation in lighthouses. Doty is credited with the invention of the first successful multiple-wick lighthouse lamp using mineral oil, and his lamp and modifications of it were very generally used until the latter part of the nineteenth century. These lamps are of two types—one in which oil is supplied to the burner under pressure and the other in which oil is maintained at a constant level. In some of the smallest lamps the ordinary capillarity of the wick is depended on to supply oil to the flame.

Coal-gas was introduced into lighthouses in about the middle of the nineteenth century. Inasmuch as the gas-mantle had not yet appeared, the gas was burned in jets. Various arrangements of the jets, such as concentric rings forming a stepped cone, were devised. The gas-mantle was a great boon to the mariner as well as to civilized beings in general. It greatly increases the intensity of light obtainable from a given amount of fuel and it is a fairly compact bright source which makes it possible to direct the light to some degree by means of optical systems. Owing to the elaborate apparatus necessary for making coal-gas, several other gases have been more desirable fuels for lighthouse lamps. Various simple gas-generators have been devised. Some of the high-flash mineral-oils are vaporized and burned under a mantle. Acetylene, which is so simply made by means of calcium carbide and water, has been a great factor in lighting for navigation. By the latter part of the nineteenth century lighthouses employing incandescent gas-burners were emitting beams of light having luminous intensities as great as several hundred thousand candles. These special gas-mantle light-sources have brightness as high as several hundred candles per square inch.

Electric arc-lamps were first introduced into lighthouse service in about 1860, but these lamps cannot be considered to have been really practicable until about 1875. In 1883 the British lighthouse authorities carried out an extensive investigation of arc-lamps. It was found that the whiter light from these lamps suffered a greater absorption by the atmosphere than the yellower light from oils, but the much greater luminous intensity of the arc-lamp more than compensated for this disadvantage. The final result of the investigation was the conclusion that for ordinary lighthouse purposes the oil-and gas-lamps were more suitable and economical than arc-lamps; but where great range was desired, the latter were much more advantageous, owing to their great luminous intensity. Electric incandescent filament lamps have been used for the less important lights, and recently there has been some application of the modern high-efficiency filament lamps.

Besides the high towers there are many minor beacons, light-ships, and light-buoys in use. Many of these are untended and therefore must operate automatically. The light-ship is used where it is impracticable or too expensive to build a lighthouse. Inasmuch as it is anchored in fairly deep water, it is safe in foggy weather to steer almost directly toward its position as indicated by the fog-signal. Light-ships are more expensive to maintain than lighthouses, but they have the advantages of smaller cost and of mobility; for sometimes it may be desired to move them. The first light-ship was established in 1732 near the mouth of the Thames, and the first in this country was anchored in Chesapeake Bay near Norfolk in 1820. The early ships had no mode of self-propulsion, but the modern ones are being provided with their own power. Oil and gas have been used as fuel for the light-sources and in 1892 the U. S. Lighthouse Board constructed a light-ship with a powerful electric light. Since that time several have been equipped with electric lights supplied by electric generators and batteries.

Untended lights were not developed until about 1880, when Pintsch introduced his welded buoys filled with compressed gas and thereby provided a complete lighting-plant. With improvements in lamps and controls the untended light-buoys became a success. The lights burn for several months, and even for a year continuously; and the oil-gas used appears to be very satisfactory. Recently some experiments have been made with devices which would be actuated by sunlight in such a manner that the light would be extinguished during the day excepting a small pilot-flame. By this means a longer period of burning without attention may be obtained. Electric filament lamps supplied by batteries or by cables from the shore have been used, but the oil-gas buoy still remains in favor. Acetylene has been employed as a fuel for light-buoys. Automatic generators have been devised, but the high-pressure system is more simple. In the latter case purified acetylene is held in solution under high pressure in a reservoir containing an asbestos composition saturated with acetone.

The light-sources of beacons have had the same history as those of other navigation lights. Many of these are automatic in operation, sometimes being controlled by clockwork. During the last twenty years the gas-mantle has been very generally applied to beacon-lights. In the latter part of the nineteenth century a mineral-oil lamp was devised with a permanent wick made by forming upon a thick wick a coating of carbon. The operation is such that this is not consumed and it prevents further burning of the wick.

The optical apparatus of navigation lights has undergone many improvements in the past century. The early lights were not equipped with either reflecting or refracting apparatus. In 1824 Drummond devised a scheme for reflecting light in order that a distant observer might make a reading upon the point where the apparatus was being operated by another person. He was led by his experiments to suggest the application of mirrors to lighthouses. His device was essentially a parabolic mirror similar to the reflectors now widely used in automobile head-lamps, search-lights, etc. He employed the lime-light as a source of light and was enthusiastic over the results obtained. His discussion published in 1826 indicates that little practical work had been done up to that time toward obtaining beams or belts of light by means of optical apparatus. However, lighthouse records show that as early as 1763 small silvered plane glasses were set in plaster of Paris in such a manner as to form a partially enveloping reflector. Spherical reflectors were introduced in about 1780 and parabolic reflectors about ten years later.

All the earlier lights were "fixed," but as it is desirable that the mariner be able to distinguish one light from another, the revolving mechanism evolved. By its agency characteristic flashes are obtained and from the time interval the light is recognized. The first revolving mechanism was installed in 1783. The early flashing lights were obtained by means of revolving reflectors which gathered the light and directed it in the form of a beam or pencil. The type of parabolic reflector now in use does not differ essentially from that of an automobile head-lamp, excepting that it is larger.