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.
Electric hot-plates are used in an interesting way in Glasgow, to enable the police on night duty to have a hot supper. The plates are fitted to street telephone signal boxes situated at points where a number of beats join. By switching on current from the public mains the policemen are able to warm their food and tea, and a supper interval of twenty minutes is allowed. Even policemen are sometimes absent-minded, and to avoid the waste of current and overheating of the plate that would result if a “bobby” forgot to switch off, an arrangement is provided which automatically switches off the current when the plate is not in use.
We must turn now to electric heating on a much larger scale, in the electric furnaces used for industrial purposes. The dazzling brilliance of the light from the electric arc lamp is due to the intense heat of the stream of vaporized carbon particles between the carbon rods, the temperature of this stream being roughly about 5400° F. This great heat is made use of in various industries in the electric arc furnace. Fig. 23 is a diagram of a simple furnace of this kind. A is a vertical carbon rod which can be raised or lowered, and B is a bed of carbon, forming the bottom of the furnace, and acting as a second rod. A is lowered until it touches B, the current, either continuous or alternating, is switched on, and A is then raised. The arc is thus struck between A and B, and the material contained in the furnace is subjected to intense heat. When the proper stage is reached the contents of the furnace are drawn off at C, and fresh material is fed in from above, so that if desired the process may be kept going continuously. Besides the electric arc furnace there are also resistance furnaces, in which the heat is produced by the resistance of a conductor to a current passing through it. This conductor may be the actual substance to be heated, or some other resisting material placed close to it.
It will be of interest to mention now one or two of the uses of electric furnaces. The well-known substance calcium carbide, so much used for producing acetylene gas for lighting purposes, is a compound of calcium and carbon; it is made by raising a mixture of lime and coke to an intense heat in an electric furnace. The manufacture of calcium carbide is carried on on a very large scale at Niagara, with electric power obtained from the Falls, and at Odda in Norway, where the power is supplied by the river Tysse. Carborundum, a substance almost as hard as the diamond, is largely used for grinding and polishing purposes. It is manufactured by sending a strong current through a furnace containing a core of coke surrounded by a mixture of sand, sawdust, and carbon. The core becomes incandescent, and the heating is continued until the carbon combines with the sand, the process taking about a day. Graphite, a kind of carbon, occurs naturally in the form of plumbago, which is used for making black lead pencils. It is obtained by mining, but many of the mines are already worked out, and others will be exhausted before long. By means of the electric furnace, graphite can now be made artificially, by heating anthracite coal, and at Niagara a quantity running into thousands of tons is produced every year. Electric furnaces are now largely employed, particularly in France, in the production of the various alloys of iron which are used in making special kinds of steel; and they are used also to a considerable extent in the manufacture of quartz glass.
For many years past a great deal of time and money has been spent in the attempt to make artificial diamonds. Quite apart from its use in articles of jewellery, the diamond has many very important industrial applications, its value lying in its extreme hardness, which is not equalled by any other substance. The very high price of diamonds however is at present a serious obstacle to their general use. If they could be made artificially on a commercial scale they would become much cheaper, and this would be of the greatest importance to many industries, in which various more or less unsatisfactory substitutes are now used on account of their much smaller cost. Recent experiments seem to show that electricity will solve the problem of diamond making. Small diamonds, one-tenth of an inch long, have been made in Paris by means of the electric arc furnace. The furnace contains calcium carbide, surrounded by a mixture of carbon and lime, and the arc, maintained by a very powerful current, is kept in operation for several hours. A black substance, something like coke, is formed round the negative carbon, and in this are found tiny diamonds. The diamonds continue to increase slowly in size during the time that the arc is at work, and it is estimated that they grow at the rate of about one-hundredth of an inch per hour. So far only small diamonds have been made, but there seems to be no reason why large ones should not be produced, by continuing the process for three or four days.
A chapter on electric heating would not be complete without some mention of electric welding. Welding is the process of uniting two pieces of metal by means of a combination of heat and pressure, so that a strong and permanent joint is produced. The chief difficulty in welding is that of securing and keeping up the proper temperature, and some metals are much more troublesome than others in this respect. Platinum, iron, and steel are fairly easy to weld, but most of the other metals, and alloys of different metals, require very exact regulation of temperature. It is almost impossible to obtain this exact regulation by ordinary methods of heating, but the electric current makes it a comparatively easy matter. The principle of ordinary electric welding is very simple. The ends of the two pieces of metal are placed together, and a powerful current is passed through them. This current meets with a high resistance at the point of contact of the two pieces, and so heat is produced. When the proper welding temperature is reached, and the metal is in a sort of pasty condition, the two pieces are pressed strongly together, and the current is switched off. The pieces are now firmly united together. The process may be carried out by hand, the welding smith switching the current on and off, and applying pressure at the right moment by means of hydraulic power. There are also automatic welders, which perform the same operations without requiring any manual control. Alternating current is used, of low voltage but very high amperage.
Steel castings are sometimes found to have small defects, such as cracks or blow-holes. These are not discarded as useless, but are made quite sound by welding additional metal into the defective places by means of the electric arc. The arc is formed between the casting and a carbon rod, and the tremendous heat reduces the surface of the metal to a molten condition. Small pieces or rods of metal are then welded in where required.