Mercury vapor lamp

“ Flame arc lamps have a high efficiency, and are still largely used for street lighting, but the cost of the frequent trimming required, even in lamps of the magazine type, gives the gas-filled lamp an advantage over them. Lamps of the mercury vapour class have a high efficiency, and the light has a high actinic value which is valuable for certain photographic processes, but the absence of the red and orange part of the spectrum gives the light a characteristically ghastly effect which limits the use of this type of lamp. ”

“ Mercury vapour light apparently imposes less strain upon the eyes than ordinary artificial light, and this desirable feature is the result of the absence of the red rays, which, besides having little effect in producing vision, are tiring to the eyes on account of their heating action. The light is very highly actinic, and for this reason it is largely used for studio and other interior photographic work. In cases where true daylight colour effects are necessary, a special fluorescent reflector is used with the lamp. ”

“ Why electric lamps are usually vacuums. Electric lamps usually have vacuums inside because the filament gets so hot that it would burn up if there were any oxygen to combine with it. But in a globe containing no oxygen the filament may be made ever so hot and it cannot possibly burn.
High-power electric lamps are not made with vacuums but are "gas-filled." The gas that is oftenest put into lamps is nitrogen,—the same gas that is mixed with the oxygen in air. By taking all the oxygen out of a quantity of air, the lamp manufacturers can use in perfect safety the nitrogen that is left. ”

“ Here, then, was the secret of the use of mercury vapor for lighting—a powerful current of electricity for a fraction of a second passed through the vapor to overcome the initial resistance, and then the passage of an ordinary current to produce the light.
In practice this apparent difficulty in overcoming the initial resistance with a strong current is easily overcome by the use of a "boosting coil," which supplies the strong current for an instant, and is then shut off automatically, the ordinary current continuing for producing the light. ”

“ An ingenious device has lately been added to the system for facilitating the lighting of the lamp. At the base of the lamp-post a small hermetically-closed can containing petroleum ether is placed, and connected by very fine copper-tubing with a burner under the vaporising tube. When the lamp is to be lit a small rubber bulb is squeezed, forcing a quantity of the ether vapour into the burner, where it is ignited by a platinum wire rendered incandescent by a current passing from a small accumulator also placed in the lamp-post. ”

“ To start the lamp, the tube has to be tilted, so that a stream of mercury flows along it and makes contact with the iron electrode. The current which then flows vaporizes some of the mercury, and when the tube is tilted back to its original position, the discharge is maintained through the mercury vapour. A small series resistance is required in order to make the operation of the lamp stable. For small lamps a glass tube is used, but owing to the higher temperature reached in lamps consuming considerable power, it is necessary to use a quartz tube for large [199] lamps. ”

“ An ordinary carbon incandescent lamp yields about six per cent, of light to ninety-four per cent, of heat, the arc lamp about ten per cent, of light, while the mercury arc has climbed to the enormous efficiency of almost half light to half heat. ”

“ In this way a vacuum may be produced having a vapour pressure of about the hundred-millionth of an atmosphere at 0° C. If, however, some liquid mercury be left in the space in which the vacuum is produced, and the containing part of the vessel be filled with liquid air, the bright mirror of mercury which is deposited on the inside wall of the bulb is still more effective than silver in protecting the chamber from the influx of heat, owing to the high refractive index, which involves great reflecting power, and the bad heat-conducting powers of mercury. ”

“ While the copper and platinum wires readily conduct the current, the carbon filament offers a great deal of resistance to its passage, and for this reason becomes very hot, in fact is raised to white heat or incandescence, which gives its name to the lamp. ”

“ But it was a scarce and expensive material, and so long as it was heated to incandescence in the open air, that is, so long as its heat was fed as other heat is, by oxygen, it was slowly consumed. Platinum is no longer in the field of electric lighting, and the substitute which takes its place in the present incandescent lamp, and which is known as a "filament," is not heated in contact with the air. The experiments and endeavors that brought this result constitute the story of the incandescent 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 21⁄2 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. ”

“ Matches are not needed in making a light. Thus the danger from accidental fires, which have so frequently occurred from the careless use of matches, is avoided. Very little heat results from an electric light, while from kerosene lamps and gaslight much heat is produced. In warm weather this freedom from heat is agreeable. The burning lamp and the gas jet make the air of the room impure and unfit for breathing. This is not true of the electric light. In the use of kerosene and of illuminating gas there is frequently danger of explosion. ”

“ Its light is very diffused and is practically devoid of red rays, so that red objects appear black in its light. The lamp consumes 31⁄2 amperes at 110 volts direct current (385 watts) having an efficiency of about 121⁄2 lumens per watt.
The mercury arc is peculiar in that it acts as an electric valve tending to let current flow through it only in one direction. Thus on alternating current, the current impulses will readily go through it in one direction, but the arc will go out in the other half cycle unless means are taken to prevent this. ”

“ So long as condensation does not occur, it will be easy to give any desired (different) temperatures to the bulb and the top of the column of mercury in the stem. The temperature of the latter will determine the pressure of the vapor in the bulb. In this way, it would appear, we may obtain the bulb vapor of mercury having pressures greater for the temperatures of saturated vapor. ↑ This description of the surface of dissipated energy is intended to apply to a substance capable of existing as a solid, liquid, and vapor, and which presents no anomalies in its thermodynamic properties. ”

“ The fire that cooks our food gives off a quantity of useless light-waves; the oil-lamp that brightens one’s rooms gives off a quantity of useless, often obnoxious, heat.
The ideal illuminant and the ideal heating agent must be one in which the required waves are in a great majority. Unfortunately, even with our most [Pg 343] perfected methods, the production of light is accompanied by the exertion of a disproportionate amount of wasted energy. In the ordinary incandescent lamp, to take an instance, only 5 or 6 per cent. of the energy put into it as electricity results in light. ”

“ A parabolic reflector sends forth as parallel rays the powerful beam of a lighthouse, a locomotive, or a searchlight. An incandescent lamp of ingenious design is silvered on its upper half so that none of its light is wasted. Because the arc lamp is the cheapest of all illuminants it is adopted for out-of-door lighting where its unpleasant glare is tempered by distance. In factory lighting its brightness is excessive and [76] harmful unless moderated. A capital plan is to employ an ordinary continuous current and place the positive carbon, with its brilliant centre, below the negative carbon ”

“ This process has a wide field of application, and enables the most refractory and infusible metals to be obtained in a metallic wire form. The zirconium and tungsten wire lamps are equal to or surpass the tantalum lamp in efficiency and are capable of giving light, with a useful commercial life, at an efficiency of about one watt per candle. Lamps called osram lamps, with filaments composed of an alloy of osmium and tungsten (wolfram) , can be used with a life of 1000 hours when run at an efficiency of about 1.5 watts per candle. ”

“ 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. ”

“ It has been found by experience that the ordinary type of open arc lamp with vertical carbons included in an opalescent globe cannot compete in point of cost with modern improvements in gas lighting as a means of street illumination. The violet colour of the light and the sharp shadows, and particularly the non-illuminated area just beneath the lamp, are grave disadvantages. ”

“ The electric light in the form of the well-known powerful and dazzling arc-light is the favourite illuminant for lighting harbours, railway stations, docks, public works, and other large spaces. But it is to the incandescent lamp that one must look par excellence for the ‘light of the future.’ It has been satisfactorily established that lighting by incandescence is as cheap as lighting by gas, provided that it be carried out on an extensive scale. ”

“ The lamp is a small glass bulb, inside of which is a short length of carbon in the shape of a small arch. This carbon is connected with the wires running through and outside of the glass bulb, and when the current flows through the wires the carbon becomes "incandescent"—that is, white-hot. This carbon is in a vacuum, the air having been exhausted from the bulb in the process of manufacture. The vacuum is essential. Should there be none, and the space be filled with air, the carbon would be destroyed by the oxygen in the air the moment the current rendered it incandescent. ”

“ Many have tried the light and abandoned it. In some cases its economy is unquestionable, but there are places where careful persons have shown that gas, as regards economy, surpasses it. It is questionable whether, in some cases, the electric light does not affect the eye. The arc-light produces, also, nitrous acid and other deleterious gases, but the incandescent lamp is free from this trouble. The powerful currents that it requires can not be carried over buildings and rooms without incurring danger from fire and to life. Nevertheless, the light has great and manifold advantages. ”

“ Thus, after a period of lighting by electricity 2.33 times as long as by gas, the temperature at the ceiling was increased by only 1-19th of the amount due to gas.
Another great advantage consequent to the employment of incandescent lighting, is the greater immunity from accidental fire; for as the carbon filament is instantly entirely consumed, the moment the glass envelope is broken it is impossible for the lamp to ignite anything in its vicinity however inflammable. ”

“ It was natural, therefore, that men of science should, with a view to improvement, first direct their attention to the lamp. The pioneer in this course of inquiry was Argand, who, about 1748, contrived to secure “a double current of air;” which consists, as any one may see in the first lamp he meets with, of a wick, shaped like a hollow cylinder, enclosed in a glass tube. The heat caused by the combustion of the oil produces a vigorous draught, which leads to an abundant circulation of air both internally and externally; and air is for the lamp, as for man, the plant, and the animal—life! ”

“ It often happens that some accidental circumstance--generally a projecting film or a little dust--will occasion the mercurial vapor to act with great energy on one part of the paper, and blacken it before the other portions are at all effected. Again, the mercury will sometimes accumulate along the lines made by the brush, and give a streaky appearance to the picture, although these lines are not at all evident before the mercurial vapor was applied. ”

“ None, it is stated, have broken, even when forced much beyond the incandescence they are intended to bear, and, owing to the compensating action of the gasoline, it is anticipated that the lamps will be permanent. The platinum wires supporting the carbon strip are not fused into the glass of the bulb, as has been the case in previous incandescent lamps, but are surrounded by a slightly elastic cement, which it is averred preserves a good joint. The light given by the lamp is but slightly yellow, is quite pleasing to the eye, and is fairly steady. ”

“ I take two ordinary 50-volt 16 c. p. lamps which are in every respect alike, with the exception, that one has been opened at the top and the air has filled the bulb, while the other is at the ordinary degree of exhaustion of commercial lamps. When I attach the lamp which is exhausted to the terminal of the secondary of the coil, which I have already used, as in experiments illustrated in Fig. 179a for instance, and turn on the current, the filament, as you have before seen, comes to high incandescence. ”