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.
Fig. 23.—Diagram to illustrate principle of Electric Furnace.
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.
CHAPTER XIV
ELECTRIC BELLS AND ALARMS
The most familiar of all electrically worked appliances is probably the electric bell, which in some form or other is in use practically all over the world. Electric bells are operated by means of a current of electricity sent through the coils of an electro-magnet, and one of the very simplest forms is that known as the single-stroke bell. In this bell an armature or piece of soft iron is placed across, but at a little distance from, the poles of an electro-magnet, and to this piece of iron is fixed a lever terminating in a sort of knob which lies close to a bell or gong. When a current is sent round the electro-magnet the armature is attracted, so that the lever moves forward and strikes a sharp blow upon the gong. Before the gong can be sounded a second time the current must be interrupted in order to make the magnet release the armature, so that the lever may fall back to its original position. Thus the bell gives only one ring each time the circuit is closed. Bells of this kind may be used for signalling in exactly the same way as the Morse sounder, and sometimes they are made with two gongs of different tones, which are arranged so as to be sounded alternately.