In the first year of the nineteenth century one of the greatest of England’s scientists, Sir Humphry Davy, became lecturer on chemistry to the Royal Institution, where his brilliant lectures attracted large and enthusiastic audiences. He was an indefatigable experimenter, and in order to help on his work the Institution placed at his disposal a very large voltaic battery consisting of 2000 cells. In 1802 he found that if two rods of carbon, one connected to each terminal of his great battery, were first made to touch one another and then gradually separated, a brilliant arch of light was formed between them. The intense brilliance of this electric arch, or arc as it came to be called, naturally suggested the possibility of utilizing Davy’s discovery for lighting purposes, but the maintaining of the necessary current proved a serious obstacle. The first cost of a battery of the required size was considerable, but this was a small matter compared with the expense of keeping the cells in good working order. Several very ingenious and more or less efficient arc lamps fed by battery current were produced by various inventors, but for the above reason they were of little use except for experimental purposes, and the commercial success of the arc lamp was an impossibility until the dynamo came to be a really reliable source of current. Since that time innumerable shapes and forms of arc lamps have been devised, while the use of such lamps has increased by leaps and bounds. To-day, wherever artificial illumination on a large scale is required, there the arc lamp is to be found.
When the carbon rods are brought into contact and then slightly separated, a spark passes between them. Particles of carbon are torn off by the spark and volatilized, and these incandescent particles form a sort of bridge which is a sufficiently good conductor for the current to pass across it from one rod to the other. When the carbons are placed horizontally, the glowing mass is carried upwards by the ascending currents of heated air, and it assumes the arch-like form from which it gets its name. If the carbons are vertical the curve is not produced, a more or less straight line being formed instead. The electric arc may be formed between any conducting substances, but for practical lighting purposes carbon is found to be most suitable.
Either continuous or alternating currents may be used to form the arc. With continuous current, if the carbon rods are fully exposed to the air, they gradually consume away, and minute particles of carbon are carried across from the positive rod to the negative rod, so that the former wastes at about twice the rate of the latter. The end of the positive rod becomes hollowed out so as to resemble a little crater, and the end of the negative rod becomes more or less pointed. The fact that with continuous current the positive rod consumes away twice as fast as the negative rod, may be taken advantage of to decrease the cost of new carbons, by replacing the wasted positive rod with a new one, and using the unconsumed portion of the old positive rod as a new negative rod.1 If alternating current is used, each rod in turn becomes the positive rod, so that no crater is formed, and both the carbons have the same shape and are consumed at the same rate. A humming noise is liable to be produced by the alternating current arc, but by careful construction of the lamp this noise is reduced to the minimum.
1 In actual practice the positive carbon is made double the thickness of the negative, so that the two consume at about the same rate.
If the carbons are enclosed in a suitable globe the rate of wasting is very much less. The oxygen inside the globe becomes rapidly consumed, and although the globe is not air-tight, the heated gases produced inside it check the entrance of further supplies of fresh air as long as the lamp is kept burning. When the light is extinguished, and the lamp cools down, fresh air enters again freely.
Arc lamp carbons may be either solid or cored. The solid form is made entirely of very hard carbon, while the cored form consists of a narrow tube of carbon filled up with soft graphite. Cored carbons usually burn more steadily than the solid form. In what are known as flame arc lamps the carbons are impregnated with certain metallic salts, such as calcium. These lamps give more light for the same amount of current. The arc is long and flame-like, and usually of a striking yellow colour, but it is not so steady as the ordinary arc.
As the carbon rods waste away, the length of the arc increases, and if this increase goes beyond a certain limit the arc breaks and the current ceases. If the arc is to be kept going for any length of time some arrangement for pushing the rods closer together must be provided, in order to counteract the waste. In arc lamps this pushing together, or “feeding” as it is called, is done automatically, as is also the first bringing together and separating of the rods to start or strike the arc. Fig. 21 shows a simple arrangement for this purpose. A is the positive carbon, and B the negative. C is the holder for the positive carbon, and this is connected to the rod D, which is made of soft iron. This rod is wound with two separate coils of wire as shown, coil E having a low resistance, and coil F a high one. These two coils are solenoids, and D is the core, (Chapter VII.). When the lamp is not in use, the weight of the holder keeps the positive carbon in contact with the negative carbon. When switched on, the current flows along the cable to the point H. Here it has two paths open to it, one through coil E to the positive carbon, and the other through coil F and back to the source of supply. But coil E has a much lower resistance than coil F, and so most of the current chooses the easier path through E, only a small amount of current taking the path through the other coil. Both coils are now magnetized, and E tends to draw the rod D upwards, while F tends to pull it downwards. Coil E, however, has much greater power than coil F, because a much larger amount of current is passing through it; and so it overcomes the feeble pull of F, and draws up the rod. The raising of D lifts the positive carbon away from the negative carbon, and the arc is struck. The carbons now begin to waste away, and very slowly the distance between them increases. The path of the current passing through coil E is from carbon A to carbon B by way of the arc, and as the length of the gap between A and B increases, the resistance of this path also increases. The way through coil E thus becomes less easy, and as time goes on more and more current takes the alternative path through coil F. This results in a decrease in the magnetism of E, and an increase in that of F, and at a certain point F becomes the more powerful of the two, and pulls down the rod. In this way the positive carbon is lowered and brought nearer to the negative carbon. Directly the diminishing distance between A and B reaches a certain limit, coil E once more asserts its superiority, and by overcoming the pull of F it stops the further approach of the carbons. So, by the opposing forces of the two coils, the carbons are maintained between safe limits, in spite of their wasting away.
PLATE IX.
By permission of
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.
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.
The average length of the tube in the ordinary form of mercury vapour lamp is about 30 inches, and a light of from 500 to 3000 candle-power is produced, according to the current used. Another form, known as the “Silica” lamp, is enclosed in a globe like that of an ordinary electric arc lamp. The tube is only about 5 or 6 inches in length, and it is made of quartz instead of glass, the arrangements for automatically tilting the tube being similar to those in the ordinary form of lamp.
The light of the mercury vapour lamp is different from that of all other lamps. Its peculiarity is that it contains practically no red rays, most of the light being yellow, with a certain proportion of green and blue. The result is a light of a peacock-blue colour. The absence of red rays alters colour-values greatly, scarlet objects appearing black; and on this account it is impossible to match colours by this light. In many respects, however, the deficiency in red rays is a great positive advantage. Every one who has worked by mercury vapour light must have noticed that it enables very fine details to be seen with remarkable distinctness. This property is due to an interesting fact. Daylight and ordinary artificial light is a compound or mixture of rays of different colours. It is a well-known optical fact that a simple lens is unable to bring all these rays to the same focus; so that if we sharply focus an image by red light, it is out of focus or blurred by blue light. This defect of the lens is called “chromatic aberration.” The eye too suffers from chromatic aberration, so that it cannot focus sharply all the different rays at the same time. The violet rays are brought to a focus considerably in front of the red rays, and the green and the yellow rays come in between the two. The eye therefore automatically and unconsciously effects a compromise, and focuses for the greenish-yellow rays. The mercury vapour light consists very largely of these rays, and consequently it enables the image to be focused with greater sharpness; or, in other words, it increases the acuteness of vision. Experiments carried out by Dr. Louis Bell and Dr. C. H. Williams demonstrated this increase in visual sharpness very conclusively. Type, all of exactly the same size, was examined by mercury vapour light, and by the light from an electric incandescent lamp with tungsten filament. The feeling of sharper definition produced by the mercury vapour light was so strong that many observers were certain that the type was larger, and they were convinced that it was exactly the same only after careful personal examination.
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. By transforming the frequency of the light waves, this reflector supplies the missing red and orange rays, the result being a light giving normal colour effects.
Another interesting vapour lamp may be mentioned briefly. This has a highly exhausted glass tube containing neon, a rare gas discovered by Sir William Ramsay. The light of this lamp contains no blue rays, and it is of a striking red colour. Neon lamps are used chiefly for advertising purposes, and they are most effective for illuminated designs and announcements, the peculiar and distinctive colour of the light attracting the eye at once.
An electric current meets with some resistance in passing through any substance, and if the substance is a bad conductor the resistance is very great. As the current forces its way through the resistance, heat is produced, and a very thin wire, which offers a high resistance, may be raised to a white heat by an electric current, and it then glows with a brilliant light. This fact forms the basis of the electric incandescent or glow lamp.
In the year 1878, Thomas A. Edison set himself the task of producing a perfect electric incandescent lamp, which should be capable of superseding gas for household and other interior lighting. The first and the greatest difficulty was that of finding a substance which could be formed into a fine filament, and which could be kept in a state of incandescence without melting or burning away. Platinum was first chosen, on account of its very high melting-point, and the fact that it was not acted upon by the gases of the air. Edison’s earliest lamps consisted of a piece of very thin platinum wire in the shape of a spiral, and enclosed in a glass bulb from which the air was exhausted. The ends of the spiral were connected to outside wires sealed into the bulb. It was found, however, that keeping platinum continuously at a high temperature caused it to disintegrate slowly, so that the lamps had only a short life. Fine threads or filaments of carbon were then tried, and found to be much more durable, besides being a great deal cheaper. The carbon filament lamp quickly became a commercial success, and up to quite recent years it was the only form of electric incandescent lamp in general use.
In 1903 a German scientist, Dr. Auer von Welsbach, of incandescent gas mantle fame, produced an electric lamp in which the filament was made of the metal osmium, and this was followed by a lamp using the metal tantalum for the filament, the invention of Siemens and Halske. For a while the tantalum lamp was very successful, but more recently it has been superseded in popularity by lamps having a filament of the metal tungsten. The success of these lamps has caused the carbon lamp to decline in favour. The metal filaments become incandescent much more easily than the carbon filament, and for the same candle-power the metal filament lamp consumes much less current than the carbon lamp.
The construction of tungsten lamps is very interesting. Tungsten is a very brittle metal, and at first the lamps were fitted with a number of separate filaments. These were made by mixing tungsten powder with a sort of paste, and then squirting the mixture through very small apertures, so that it formed hair-like threads. Early in 1911 lamps having a filament consisting of a single continuous piece of drawn tungsten wire were produced. It had been known for some time that although tungsten was so brittle at ordinary temperatures, it became quite soft and flexible when heated to incandescence in the lamp, and that it lost this quality again as soon as it cooled down. A process was discovered by which the metal could be made permanently ductile, by mechanical treatment while in the heated state. In this process pure tungsten powder is pressed into rods and then made coherent by heating. While still hot it is hammered, and finally drawn out into fine wires through diamond dies. The wire is no thicker than a fine hair, and it varies in size from about 0·012 mm. to about 0·375 mm., according to the amount of current it is intended to take. It is mounted by winding it continuously zigzag shape round a glass carrier, which has at the top and the bottom a number of metal supports arranged in the form of a star, and insulated by a central rod of glass. One star is made of strong, stiff material, and the other consists of fine wires of some refractory metal, molybdenum being used in the Osram lamps. These supports act as springs, and keep the wire securely in its original shape, no matter in what position the lamp is used. The whole is placed in a glass bulb, which is exhausted of air and sealed up.
For some purposes lamps with specially small bulbs are required, and in these the tungsten wire is made in the shape of fine spirals, instead of in straight pieces, so that it takes up much less room. In the “Axial” lamp the spiral is mounted in such a position that most of the light is sent out in one particular direction.
The latest development in electric incandescent lamps is the “half-watt” lamp. The watt is the standard of electrical energy, and it is the rate of work represented by a current of one ampere at a pressure of 1 volt. With continuous currents the watts are found very simply by multiplying together the volts and the amperes. For instance, a dynamo giving a current of 20 amperes at a pressure of 50 volts would be called a 1000-watt dynamo. With alternating currents the calculation is more complicated, but the final result is the same. The ordinary form of tungsten lamp gives about one candle-power for every watt, and is known as a one-watt lamp. As its name suggests, the half-watt lamp requires only half this amount of energy to give the same candle-power, so that it is very much more economical in current. In this lamp the tungsten filament is wound in a spiral, but instead of being placed in the usual exhausted bulb, it is sealed into a bulb containing nitrogen gas. The increased efficiency is obtained by running the filament at a temperature from 400° to 600° C. higher than that at which the filament in the ordinary lamp is used.
In spite of the great advances in artificial lighting made during recent years, no one has yet succeeded in producing light without heat. This heat is not wanted, and it represents so much waste energy. It has often been said that the glow-worm is the most expert of all illuminating engineers, for it has the power of producing at will a light which is absolutely without heat. Perhaps the nearest approach to light without heat is the so-called “cold light” invented by M. Dussaud, a French scientist. His device consists of a revolving ring of exactly similar tungsten lamps. Each of these lamps has current passed through it in turn, and the duration of the current in each is so short, being only a fraction of a second, that the lamp has not sufficient time to develop any appreciable amount of heat. The light from the ring of lamps is brought to a focus, and passed through a lens to wherever it is required. Electric incandescent lamps are made in a variety of sizes, each one being intended for a certain definite voltage. If a lamp designed for, say, 8 volts, is used on a circuit of 32 volts, its candle-power is greatly increased, while the amount of current consumed is not increased in proportion. In this way the lamp becomes a more efficient source of light, but the “over-running,” as it is called, has a destructive effect on the filament, so that the life of the lamp is greatly shortened. In the Dussaud system however the time during which each lamp has current passing through it is so short, followed by a period of rest, that the destructive effect of over-running is reduced to the minimum; so that by using very high voltages an extremely brilliant light is safely obtained with a comparatively small consumption of current. It might be thought that the constant interchange of lamps would result in an unsteady effect, but the substitution of one lamp for another is carried out so rapidly that the eye gets the impression of perfect steadiness. The Dussaud system is of little use for ordinary lighting purposes, but for lighthouse illumination, photographic studio work, and the projection of lantern slides and cinematograph films, it appears to be of considerable value.
Electric light has many advantages over all other illuminants. It gives off very little heat, and does not use up the oxygen in the air of a room as gas does; while by means of flexible wires the lamps can be put practically anywhere, so that the light may be had just where it is wanted. Another great advantage is that the light may be switched on without any trouble about matches, and there is none of the danger from fire which always exists with a flame.
The current for electric lamps is generally taken from the public mains, but in isolated country houses a dynamo has to be installed on the premises. This is usually driven by a small engine running on petrol or paraffin. In order to avoid having to run the engine and dynamo continually, the current is not taken directly from the dynamo, but from a battery of accumulators. During the day the dynamo is used to charge the accumulators, and these supply the current at night without requiring any attention.
Electric lighting from primary cells is out of the question if a good light is wanted continuously for long periods, for the process is far too costly and troublesome. If a light of small candle-power is required for periods of from a few minutes to about an hour, with fairly long intervals of rest, primary cells may be made a success. Large dry cells are useful for this purpose, but probably the most satisfactory cell is the sack Leclanché. This is similar in working to the ordinary Leclanché cell used for bells, but the carbon mixture is placed in a canvas bag or sack, instead of in a porous pot, and the zinc rod is replaced by a sheet of zinc surrounding the sack. These cells give about 1½ volt each, so that four, connected in series, are required to light a 6-volt lamp. The lamps must take only a very small current, or the cells will fail quickly. Small metal filament lamps taking from a third to half an ampere are made specially for this purpose, and these always should be used. A battery of sack Leclanché cells with a miniature lamp of this kind forms a convenient outfit for use as a night-light, or for lighting a dark cupboard, passage or staircase. Lamps with ruby glass, or with a ruby cap to slip over the bulb, may be obtained for photographic purposes. If the outfit is wanted for use as a reading-lamp it is better to have two separate batteries, and to use them alternately for short periods. With this arrangement each battery has a short spell of work followed by a rest, and the light may be kept on for longer periods without overworking the cells.