Union Electric Co. Ltd.
NIGHT PHOTOGRAPHS, TAKEN BY THE LIGHT OF THE ARC LAMPS.
The arc lamp is largely used for the illumination of wide streets, public squares, railway stations, and the exteriors of theatres, music-halls, picture houses, and large shops. The intense brilliancy of the light produced may be judged from the accompanying photographs ([Plate IX].), which were taken entirely by the light of the arc lamps. Still more powerful arc lamps are constructed for use in lighthouses. The illuminating power of some of these lamps is equal to that of hundreds of thousands of candles, and the light, concentrated by large reflectors, is visible at distances varying from thirty to one hundred miles.
Arc lamps are also largely used for lighting interiors, such as large showrooms, factories or workshops. For this kind of lighting the dazzling glare of the outdoor lamp would be very objectionable and harmful to the eyes, so methods of indirect lighting are employed to give a soft and pleasant light. Most of the light in the arc lamp comes from the positive carbon, and for ordinary outdoor lighting this carbon is placed above the negative carbon. In lamps for interior lighting the arrangement is frequently reversed, so that the positive carbon is below. Most of the light is thus directed upwards, and if the ceiling is fairly low and of a white colour the rays are reflected by it, and a soft and evenly diffused lighting is the result. Some light comes also from the negative carbon, and those downward rays are reflected to the ceiling by a reflector placed beneath the lamp. Where the ceiling is very high or of an unsuitable colour, a sort of artificial ceiling in the shape of a large white reflector is placed above the lamp to produce the same effect. Sometimes the lamp is arranged so that part of the light is reflected to the ceiling, and part transmitted directly through a semi-transparent reflector below the lamp. The composition of the light of the arc lamp is very similar to that of sunlight, and by the use of such lamps the well-known difficulty of judging and matching colours by artificial light is greatly reduced. This fact is of great value in drapery establishments, and the arc lamp has proved a great success for lighting rooms used for night painting classes.
The powerful searchlights used by warships are arc lamps provided with special arrangements for projecting the light in any direction. A reflector behind the arc concentrates the light and sends it out as a bundle of parallel rays, and the illuminating power is such that a good searchlight has a working range of nearly two miles in clear weather. According to the size of the projector, the illumination varies from about 3000 to 30,000 or 40,000 candle-power. For some purposes, such as the illuminating of narrow stretches of water, a wider beam is required, and this is obtained by a diverging lens placed in front of the arc. In passing through this lens the light is dispersed or spread out to a greater or less extent according to the nature of the lens. Searchlights are used in navigating the Suez Canal by night, for lighting up the buoys along the sides of the canal. The ordinary form of searchlight does this quite well, but at the same time it illuminates equally an approaching vessel, so that the pilot on this vessel is dazzled by the blinding glare. To avoid this dangerous state of things a split reflector is used, which produces two separate beams with a dark space between them. In this way the sides of the canal are illuminated, but the light is not thrown upon oncoming vessels, so that the pilots can see clearly.
Glass reflectors are much more efficient than metallic ones, but they have the disadvantage of being easily put out of action by gunfire. This defect is remedied by protecting the glass reflector by a screen of wire netting. This is secured at the back of the reflector, and even if the glass is shattered to a considerable extent, as by a rifle bullet, the netting holds it together, and keeps it quite serviceable. Reflectors protected in this way are not put out of action by even two or three shots fired through them. Searchlight arcs and reflectors are enclosed in metal cylinders, which can be moved in any direction, vertically or horizontally.
In the arc lamps already described, a large proportion of the light comes from the incandescent carbon electrodes. About the year 1901 an American electrician, Mr. P. C. Hewitt, brought out an arc lamp in which the electrodes took no part in producing the light, the whole of which came from a glowing stream of mercury vapour. This lamp, under the name of the Cooper-Hewitt mercury vapour lamp, has certain advantages over other electric illuminants, and it has come into extensive use.
Fig. 22.—Sketch of Mercury Vapour Lamp.
It consists of a long glass tube, exhausted of air, and containing a small quantity of mercury. Platinum wires to take the current from the source of supply are sealed in at each end. The tube is attached to a light tubular framework of metal suspended from the ceiling, and this frame is arranged so that it can be tilted slightly downwards by pulling a chain. As shown in [Fig. 22], the normal position of the lamp is not quite horizontal, but tilted slightly downwards towards the end of the tube having the bulb containing the mercury. The platinum wire at this end dips into the mercury, so making a metallic contact with it. The lamp is lighted by switching on the current and pulling down the chain. The altered angle makes the mercury flow along the tube towards the other platinum electrode, and as soon as it touches this a conducting path for the current is formed from end to end of the tube. The lamp is now allowed to fall back to its original angle, so that the mercury returns to its bulb. There is now no metallic connexion between the electrodes, but the current continues to pass through the tube as a vacuum discharge. Some of the mercury is immediately vaporized and rendered brilliantly incandescent, and so the light is produced. The trouble of pulling down the chain is avoided in the automatic mercury vapour lamp, which is tilted by an electro-magnet. This magnet is automatically cut out of circuit as soon as the tilting is completed and the arc struck.