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.
CHAPTER XIII
ELECTRIC HEATING
The light of the electric incandescent lamp is produced by the heating to incandescence of a thin filament of metal or carbon, and the heat itself is produced by the electric current forcing its way through the great resistance opposed to it by the filament. In such lamps the amount of heat produced is too small to be of much practical use, but by applying the same principle on a larger scale we get an effective electric heater.
The most familiar and the most attractive of all electric heaters is the luminous radiator. This consists of two or more large incandescent lamps, having filaments of carbon. The lamps are made in the form of long cylinders, the glass being frosted, and they are set, generally in a vertical position, in an ornamental case or frame of metal. This case is open at the front, and has a metal reflector behind. The carbon filaments are raised to an orange-red heat by the passage of the current, and they then radiate heat rays which warm the bulbs and any other objects in their path. The air in contact with these heated bodies is warmed, and gradually fills the room. This form of heater, with its bright glowing lamps, gives a room a very cheerful appearance.
In the non-luminous heaters, or “convectors” as they are called, the heating elements consist of strips of metal or wires having a very high resistance. These are placed in a frame and made red-hot by the current. Cold air enters at the bottom of the frame, becomes warm by passing over the heating elements, and rises out at top and into the room. More cold air enters the frame and is heated in the same way, and in a very short time the whole of the air of the room becomes warmed. The full power of the heater is used in the preliminary warming of the room, but afterwards the temperature may be kept up with a much smaller consumption of current, and special regulating switches are provided to give different degrees of heat. Although these heaters are more powerful than the luminous radiators, they are not cheerful looking; but in some forms the appearance is improved by an incandescent lamp with a ruby glass bulb, which shines through the perforated front of the frame.
The Bastian, or red glow heater, has thin wires wound in a spiral and enclosed in tubes made of quartz. These tubes are transparent both to light and heat, and so the pleasant glow of the red-hot wire is visible. A different type of heater, the hot oil radiator, is very suitable for large rooms. This has a wire of high resistance immersed in oil, which becomes hot and maintains a steady temperature.
Electric cooking appliances, like the heaters just described, depend upon the heating of resistance wires or strips of metal. The familiar electric kettle has a double bottom, and in the cavity thus formed is placed the resistance material, protected by strips of mica, a mineral substance very largely used in electrical appliances of all kinds on account of its splendid insulating qualities. Electric irons are constructed in much the same way as kettles, and sometimes they are used with stands which cut off the current automatically when the iron is laid down upon them, so that waste and overheating are prevented. There are also a great many varieties of electric ovens, grillers, hot-plates, water-heaters, glue-pots, and foot and bed warmers. These of course differ greatly in construction, but as they all work on the same principle there is no need to describe them.