CHAPTER XII

PROPERTIES--_Continued_

Producing more and better light from both gas and electricity . . . The Drummond light . . . The Welsbach mantle . . . Many rivals of carbon filaments and pencils . . . Flaming arcs and tubes of mercury vapor.

Light Giving Properties.

Mr. Edison has achieved triumphs not only in giving sound its lasting registration, but in producing an electric light of new economy. Both exploits proceeded upon a masterly knowledge of properties. A century ago candles provided illumination both to rich and poor, the sole difference being that wax shone in the palace and tallow in the hut. The oil lamps which gleamed in the lighthouses of England and America, for all their bigness, were plainly of kin to the Eskimo saucer filled with blubber, edged with moss as wick. Yet for ages, from every hearth in Christendom, there had been the promise of better things as bituminous coals, or sticks of wood, had cheered as much by their light as by their warmth. We owe much to James Watt, who improved the steam-engine and gave it essentially the form it retains to the present hour. We owe also a weighty debt to an assistant of his, William Murdock, who, thanks to a suggestion from Lord Dundonald, attentively observed the process by which coals produce light. He saw that under stress of intense heat the solid fuel emitted streams of gas which burned with great brilliancy. Here gas-making and gas-burning went on at the same moment in the same place; might the process be separated, so that gas might be made here, and burned elsewhere at any convenient time? An experiment proved the project to be feasible, and forthwith the Soho Works, near Birmingham, in which Watt’s engines were built, were lighted by gas. Such was the beginning of an industry now important in many ways. To-day gas not only yields light, but heat and power, while, especially in metallurgy, fuels are more and more used after reduction to the gaseous form.

How the Gas Mantle was Invented.

Early in the day of gas-making it was noticed that gases of various kinds differed much in light-giving quality. It was presently shown that their light depended on the carbon brought to incandescence in a flame; in the absence of that carbon, as when a jet of pure hydrogen was consumed, extreme heat was accompanied by no light whatever. Then came a capital discovery, namely, that lime introduced within a burning jet of hydrogen became intensely luminous while itself but slowly consumed. Adopting lime for the core of his apparatus, Captain Thomas Drummond, of the Royal Engineers, in 1835 devised the lime light. Upon a block of pure, compressed quick lime, he directed a jet of burning gas, obtaining a beam of great vividness still employed in stereopticons and in theatres. For modern types of the Drummond lamp a twin jet of hydrogen and oxygen is used. Lime has many sister substances having light-giving quality when highly heated, and among them are many rare earths, oxides of uncommon elements. These strange substances were destined to play a prominent part in the battle between gas and electricity as illuminants. When Edison in 1878 perfected his incandescent bulb, it seemed as if electricity were soon to be the sole illuminator of houses. But the gas engineers were to be rejoiced by the invention of a mantle which quadrupled the brillancy of a gas flame, withstanding the rivalry of electricity in a notable degree. This mantle was invented by Dr. Auer von Welsbach, a chemist of Vienna, who virtually adopted the principle of the Drummond light. His efforts give us an admirable example of an inventor passing from a hint to a test, day after day meeting new difficulties with unfailing courage and resourcefulness.

In 1880 Dr. von Welsbach took up the study of rare earths, mainly with a view to ascertaining their value as illuminants. As he brought one specimen after another to melting heat on bits of platinum wire, he found that the little beads formed were unfavorable in shape to the production of light. Then came into his mind an idea of that golden quality which occurs only to the man who earns it: Why not soak cotton with solutions of salts of rare earths, burn the cotton and leave behind an earthy skeleton of slight thickness and much surface? Experiment proved that the idea had promise, but the skeletons crumbled to dust with the least tremor. For success a fair degree of cohesion was imperative, but to secure that cohesion demanded skill, resource, and patience. After a long series of trials a mantle was made with lanthanum oxide; immersed in flame its beam was particularly bright, now for the first time suggesting that the rare earths might yield light on a large scale. But trouble was at hand, to be overcome only at the end of much toil.

During an absence of several days, the inventor left a mantle of lanthanum oxide locked up in his laboratory. When he returned it had fallen to powder, having attracted from the atmosphere both moisture and carbon dioxide. Evidently this harmful attraction must be avoided by adding an ingredient to keep the mantle dry and preserve it from union with carbon dioxide. For this purpose magnesia was chosen; the resulting compound proved to be durable, and gave an agreeable light of moderate intensity. But, alas, after glowing about seventy hours, the mantle failed in its radiance, becoming of glassy and translucent texture. Thus impeded, the untiring inventor turned to mixtures having zirconium as a basis; these not only gave a steady beam, but extended to hundreds of hours the life of a mantle. Still bent on getting more light if he could, Dr. von Welsbach tested thorium oxide with gratifying results; yet, strange to say, when he had purified this material to the utmost, his light fell off in an unaccountable fashion. What could be the matter? Surely in the purifying process some invaluable element had been cast aside. This element, in the researches of an associate, Mr. Ludwig Haitinger, proved to be cerium in minute quantity. Here was a discovery of the highest moment; at the end of many experiments it was determined that one per cent. of cerium and ninety-nine per cent. of thorium oxide are the best proportions for a mantle such as we use to-day. Why these proportions are best nobody knows, any more than why one per cent. of carbon added to iron gives us a steel incomparably better than iron for many uses. A Welsbach mantle has good points apart from its economy of gas. Its combustion is thorough, so that it throws into the air a much lower percentage of injurious products than does an ordinary gas flame. It never smokes, and its light is so steady as to be available for work with the microscope and other exacting demands. It has one defect which may yet be removed: its light has a somewhat unpleasant tinge of green. In another chapter of this book, producer gas, much cheaper than common illuminating gas, is described. Dowson producer gas, with a Welsbach mantle, yields a light of 8 to 10 candle-power with a consumption of 4.5 to 4.8 cubic feet per hour.

Thus far no successful mantle for a petroleum lamp has been devised. With alcohol a mantle yields a brilliant flame. A lamp with a Boivin burner and a Welsbach mantle has given a light of 30.35 candle-power for 57 hours and 5 minutes in consuming one gallon of alcohol, almost twice as much light as given by a Miller lamp with a round wick and a central draft, burning a gallon of kerosene. In the United States on January 1, 1907, there will cease to be an excise tax on alcohol used in the arts, a denaturalizing process rendering the liquid unfit to drink. As this alcohol may be easily produced from grain or potatoes at 20 to 25 cents a gallon, a capital illuminant will be available for the public, as well as an excellent fuel and a substitute for gas or gasoline in motors.

As first manufactured, gas-mantles were woven, they are now knitted,--a change for the better in closeness and firmness of texture. Nearly all the thorium used for mantles is found in the monazite sands of the provinces of Bahia and Espirito Santo, along the coast of Brazil. These sands were for a long time valuable only for the zinc they contained. To-day the thorium they carry is of vastly more account; for chemical treatment this is sent to Germany whence the manufactured product is borne to every quarter of the globe.

Improvements in Electric Lighting: Incandescent Lamps.

While the Welsbach mantles have been constantly improved in quality, and given new and inverted forms of special value, the inventors in the field of electric lighting have not stood still. For interior illumination the Edison incandescent bulb still holds its own despite many a threat of dispossession. Since 1881 its details of manufacture have been steadily bettered and its price much reduced, while its consumption of current has fallen from 5.8 watts per candle to 3.1. This advance, marked as it is, leaves a long path ahead of the inventor whose estimate is that were the whole of an electric current transformed into light, a candle would cost us but .11 of a watt, that is, but one twenty-eighth part as much as when we set a carbon filament aglow. In electrical terms a horse-power yields 748 watts, representing, were there no waste in conversion, no less than 425 lamps each of 16 candle-power.

It is this immense margin for improvement that has spurred ingenuity to attack the problem of electric lighting from many new sides. The General Electric Company produces a carbon filament of one fifth greater efficiency than an ordinary untreated filament. Fibers of the usual cellulose kind are enclosed in a carbon box, placed in a carbon-tube resistance furnace heated to between 3,000° and 3,700° C. This converts the filament into a graphite of increased luminosity which, furthermore, blackens its enclosing glass much less than a common filament does.

In the early days of electric lighting a good many experiments were tried with threads of platinum, but without success. That metal remains unmelted at a very high temperature, but as a light-giver its quality is poor. Of late years investigators have turned to other metals, of high melting points, and with results so remarkable that we may expect some of them to be in general use in the near future. Tantalum, a rare and costly metal, has been found to give a candle-power with as little as two watts and, in specially favorable circumstances, with only 1.85 watts. Osmium, in the hands of Dr. Auer von Welsbach, reduces this figure to 1.5 watts. Dr. Hans Kuzel, of Baden, Austria, has employed filaments of tungsten in lamps which he claims demanded only one watt per candle. From among these new lamps it seems highly probable that as soon as methods of manufacture are settled and standardized the world will be given an electric light, in small units, much cheaper than ever before.

New Arc Lamps.

For large spaces indoors and for out of doors the arc-lamp maintains its popularity in much the form originally devised by Mr. Charles F. Brush of Cleveland. But, as in the case of the incandescent bulb, many a rival is now disputing the field, so that supersedure may be close at hand. In what are known as flaming or luminous arcs the carbon pencils are impregnated with salts of the calcium group of elements, of extreme luminosity. In these lamps the electric arc itself is the chief source of light, instead of the glowing end of the positive carbon as in a common arc lamp. As the calcium salts volatilize into gases they provide a path of less resistance than air for the passage of the current, so that the electrodes may be drawn apart to a distance which may be as much as 2-1/2 inches. These lamps require free ventilation, so that they must be open. Their economy is extraordinary, a candle-power being afforded for .353 watt, as against 1.78 watts for an enclosed arc lamp, a five-fold gain in effectiveness. To renew the carbons, which waste rapidly, a new device provides fresh pencils, cartridge fashion, as required. Without this aid, trimming is often necessary, and this fact joined to the high cost of the carbons lessens the net gain in their use. On another line of experiment noteworthy results have been reached with metallic oxides. Magnetite, an oxide of iron, has developed a candle-power with but one half of one watt. Ferro-titanium, a compound of iron and titanium, has given a candle-power with only one third of a watt, and it is expected that still higher efficiencies will soon be attained with this wonderful compound.

Hewitt Mercury-Vapor Lamp.

From quite another side Mr. Peter Cooper Hewitt enters the field of light production, utilizing the glow of a vapor instead of a solid stick. His lamp is a long, slender tube of glass; within each end is sealed a metallic wire; at one end is a little mercury. When a powerful pump has exhausted the tube to a high degree it is sealed, and its wire terminals are placed in an electric circuit. On tilting the tube the mercury flows from end to end, an arc is formed, and the mercury vapor becomes luminous. This vapor remains unconsumed, and the lamp asks no attention whatever. Its rays are greenish, so that where normal colors are desired, it is well to use supplementary lamps of carbon filaments to furnish red rays. For streets, squares, freight-sheds and the like, the Hewitt light is capital just as produced, its rays being widely diffused and casting no heavy shadows. Its high actinic power makes it specially useful to photographers, while in factories, drafting rooms, composing rooms and so on, its color is unobjectionable. Its cost is small, as a candle-power is produced in large tubes with but 0.55 of a watt. A Hewitt lamp of automatic type, recently devised, has a small solenoid or magnet on the suspension bar just above the holder. On closing the circuit the current flows through this solenoid which instantly tilts the tube and starts the light. This lamp is particularly suited to places, such as the lofty ceilings of foundries, where it would be difficult to tilt the tube by hand. Hewitt lamps use either a direct or an alternating current.

In an earlier chapter we glanced at reflectors and refractors, newly invented, which give light its most useful paths with as little avoidable loss as possible. These devices, applied to Welsbach burners and the new electric lamps, greatly economize modern illumination in comparison with that of former times.[13]

[13] In February, 1906, the Illuminating Engineering Society was established in New York. Its secretary is A. H. Elliott, 4 Irving Place, New York. The Society publishes its proceedings and discussions.