At first the lightness and weakness of aluminium were much against it; these, as we have seen, were soon overcome by alloying the metal with copper or nickel. But by giving aluminium forms of utmost stiffness, by reinforcing these forms with steel wires, the metal is quite strong and rigid enough for cups, plates, cameras and other instruments for which lightness is most desirable. In many another case a material or a characteristic at first unwelcome has been turned to excellent account. Smokiness in a fuel is not a quality mentioned in its advertisements, and yet smokiness is just what is sought in the twigs, stubble, or coals set on fire to give plants a cloud protecting them from unseasonable frosts. It is astonishing how little fuel will serve in such cases, especially if the atmosphere is calm, so as not to carry the smoke where it is not needed. Many another instance might be given of a quality objectionable for one service and then turned to satisfying a new want. Sometimes, too, offensive qualities are most useful. Illuminating gas, as at first manufactured, had a distressing odor, which gave prompt and unmistakable notice of a leak. When water gas came into use, most harmful when inhaled, the chemists were puzzled to know how to give it an offensive smell; they found that a quality long complained of was really an advantage in disguise.
So in the electrical field, when an unsought quality has intruded itself, and proved unwelcome, the question has arisen, what service can we enlist it for? Not seldom the answer has been gainful in the extreme. Dr. Oliver J. Lodge tells us that a bad electrical contact was at one time regarded simply as a nuisance, because of the singularly uncertain and capricious character of the current transmitted by it. Professor Hughes observed its sensitiveness to sound-waves, and it became the microphone, which, duly modified, brought the telephone from the whisper of a curious toy to the full tones which ensured commercial success the world over. This same “bad” contact turns out to be sensitive to electric waves also, forming indeed nothing else than the coherer of the wireless telegraph.
Many an electrician has been perplexed and thwarted by the small bubbles of air which place themselves on a metallic surface immersed in an electric bath, interrupting the attack sought to be carried to a finish. Happily there is a task which these very bubbles perform as if they had been created for no other purpose, namely, the re-sharpening of files. First the dull and dirty files are placed for twelve hours in a fifteen to twenty per cent. solution of caustic soda; they are then cleaned with a scratch-brush and a five per cent. soda solution. Next they are placed in a bath of six parts of forty per cent. nitric acid, three parts sulphuric acid, and 100 parts water, each file being connected to a plate of carbon immersed close to it, by means of a copper plate connecting at the top all the carbons and the files. This produces a short-circuited battery generating gas at the surface of the files; the bubbles which adhere to the points of the files protect them from being eaten away, while the rest of the metal is being etched. Every five minutes the files are taken out and washed in water to remove the oxide which collects on their surfaces. When sufficiently etched they are placed in lime-water to remove any adherent acid, dried in sawdust to prevent rusting, and rubbed with a mixture of oil and turpentine. Indispensable in the whole process is the protection afforded by the bubbles of air.
Evil, Be Thou My Good.
For a long time its creation of sparks kept electrical machinery out of mines liable to fire-damp, which might be exploded by these sparks. In many other places they worked evils quite as serious, setting fire to shavings, cotton and such like. To-day these very sparks are applied to touching off the charges of gas and air in gas-engines of all types, whether stationary, or for automobiles and motor-boats. In another respect the automobile should be provided with a means of creating what is usually considered a nuisance, namely, a noise. Moving rapidly as it does on thick rubber tires, it gives no warning to hapless wayfarers. In Canadian cities, where in winter deep snow may muffle the tread of horses, every sleigh, under severe penalty, must be furnished with efficient bells.
Compensating Devices.
Sometimes an important property has unwelcome effects which, in particular cases, cannot be applied to advantage, and must be counterbalanced with as much care as possible. Many pieces of mechanism from the qualities of their materials are subject to deviations which must be compensated by introducing equal and opposite action. Tasks of this kind proceed upon an intimate acquaintance with the properties of substances common and uncommon. From the first making of clocks there was much trouble due to changes of temperature which affected the dimensions of pendulums, and consequently their rate of going. This difficulty is overcome by taking advantage of the fact that heat expands zinc about two-and-a-half times as much as it expands steel. Accordingly the two-second pendulum of the great clock at Westminster is built of a steel rod 179 inches in length, and a zinc tube, less massive, 126 inches long; they are joined at their lower ends only and are parallel. As temperatures vary, the fluctuations in length of the steel compensate those which occur in the zinc. Another mode of effecting the same purpose is to employ a cylinder partly filled with mercury; as this rises when warmed it exactly compensates for the lengthening by expansion of its supporting rod of steel.
Gravity, that universal force at which we have just glanced as it swings a pendulum, cannot be banished, but its downward push may be balanced by an equal upward thrust. In a remarkable feat Plateau poured oil into a blend of water and alcohol, adding alcohol until he produced a mixture having the same specific gravity as the oil—which now became a sphere, taking its place in the middle of the diluted spirits. He then introduced into the oil a vertical disc which he rotated; very soon spherules of oil separated themselves from the parent mass, and as satellites moved in the same direction as the primary sphere, because immersed as they were in the diluted alcohol, they shared the direction of its motion: the whole afforded a remarkable illustration of how nebulae may become planets, moons, and suns.
On somewhat the same principle as Plateau’s model are the liquid compasses for ships. Their needles are disposed within hollow metallic holders of the same specific gravity as the immersing liquid, in which therefore they move with perfect freedom on their sapphire bearings. Sometimes it is desired to use compass needles so poised that they will respond to the slightest magnetic influence. To this end one needle is placed above another, the north pole of the first over the south pole of the second; the astatic needle formed by this union is much more sensitive than a simple needle. The astatic needle, for all its ingenuity, is little used; of incomparably more importance is that other magnetic device, the telephone. No sooner had it entered into business than a serious fault was found with its messages; they arrived blurred and mingled with many sounds and noises, as if the conveying wire had caught every audibility of a neighborhood. The difficulty is remedied by using two conductors instead of one, and so arranging them that the currents induced on one conductor are exactly equal and opposite to those induced in the other.