Many of the useful and interesting manufacturing processes of to-day are based upon the intense heat which science has taught the manufacturer how to produce. Tasks which our forefathers dreamed of, but were unable to accomplish, are easy to-day because of the facility with which great heat can be generated. The "burning fiery furnace" "seven times heated" is as nothing to some of the temperatures which are now obtained in the ordinary course of things.
The greatest heat of all is that of the electric arc. Two conductors, generally rods of carbon, are placed with their ends touching, and the current is turned on so that it passes from one to the other. Then they are gradually drawn apart. As the gap widens the current experiences more and more difficulty in passing over this non-conducting gap, and great electrical energy has to be employed to keep it going. Now that wonderful law of the Conservation of Energy decrees that no energy can ever be lost. It can only be changed from one form into another. Therefore the energy expended upon the arc is not lost, but is converted into heat. It is that heat, acting upon the small particles of carbon which are torn off the ends of the rods, which gives us the arc light.
As a matter of fact nearly all artificial light (and natural light too for that matter[1]) is due to heat. The heat sets the molecules in violent agitation, which, acting upon the corpuscles in the atoms, sets them in violent motion too, so that light is often the companion of heat. Some substances give light more readily than others, under the influence ofheat, and we may reasonably believe that they are those whose corpuscular arrangements are such that they can be readily accelerated by the molecular action.
To take a familiar instance, coal-gas is mainly "methane," one of the many combinations of carbon and hydrogen, and when it is burnt in air the hydrogen and oxygen combine, liberating heat, which causes the carbon liberated at the same time to glow. As each methane molecule breaks up the carbon atoms are thrown out, forming solid particles of carbon, and it is they really which give the light. It is therefore the combustible gas heating the solid particles of carbon which forms the luminous part of the gas flame. The non-luminous part of the flame, near the burner (I am now speaking of the old-fashioned burner), is the burning gas before the carbon particles have had time to heat up.
And the old gas flame, as we know, is now being rapidly displaced by the incandescent mantle, the reason being simply that Von Welsbach discovered how certain rare minerals gave a more brilliant light when heated than particles of carbon do. In other words, it is easier to accelerate the motion of the corpuscles in ceria, thoria and the other ingredients of the mantle, than it is those of carbon. Consequently, they sooner reach that degree of agitation which will send forth electro-magnetic waves of the high frequency necessary to produce the sensation of light.
For this reason the mantle heated by gas gives as bright a light as the carbon particles in the electric arc, although the latter are subjected to a much more intense heat.
But the arc can be, and often is, used as a source of heat, apart altogether from the light which it gives. In Sweden, for example, where coal is rare, but water-power plentiful, the power of the waterfalls is made to smelt iron. Hence the waterfalls are sometimes termed the "white coal" of that country. Needless to say, it is the ubiquitous electricity which performs the change from the force of falling water into heat.
The furnaces are in shape much like those in which iron is smelted with coal—namely, tall chimney-like structures at the bottom of which is the fire. In the "arc furnaces" there are, passing in through the side, near the bottom, a number of electrodes, and between these a series of arcs are formed. Coke and ironstone are thrown in from the top into this region of intense heat, and there the iron is liberated from the oxygen with which it is combined in the ore. Liberated, it flows out through a spout at one side of the furnace.
But the question will arise in the reader's mind: Why is coke needed in an electric furnace? It is for metallurgical reasons. The heat of the arc loosens the bonds between the iron and oxygen, but it needs the presence of some carbon to tempt the oxygen atoms away. Therefore coke, as the most convenient form of carbon, has to be there. It is there, however, in much smaller quantity than it would be in an ordinary furnace. It is not there as fuel, but simply as the "counter-attraction" to draw the oxygen atoms away from their old love.
The arc is also used for welding pieces of iron together, for which purpose it is eminently suitable, since what is wanted is intense heat at a particular point. But perhaps the reader will be wondering by this time what the heat of the arc is. It has been repeatedly referred to as "intense," but something more definite may be demanded. In theory it is unlimited. Apply more pressure—more volts, that is—thereby driving more current across, and the temperature will rise. It is only a question of making dynamos large enough, and driving them fast enough, and any temperature is possible. But there are practical difficulties which limit the degree of heat. One is the melting-point of the furnace itself. Fire-clay melts at about 1700° to 1800° C. So in a furnace which has to be lined with fire-clay that is about the limit.
In welding two pieces of iron together, the iron, of course, defines what the limit shall be. It needs to be heated to"welding heat" and no more—that is, a little short of melting—so that the parts to be joined are soft, and, with a little hammering, will join thoroughly together. If too much heat were to be applied the parts would melt away. But the heat of the arc can be controlled by simply varying the current, and so the right heat can be applied at the right place, than which little more is wanted.
One very simple way of doing this is for the workman to hold one of the "electrodes"—a rod of carbon suitably insulated—in his hand. The current is led to it through a flexible wire. The iron itself is made the other electrode by being gripped in a vice which is itself insulated but connected to the source of current. Thus on bringing the point of his rod near to the part to be heated the man causes an arc to be created there. By moving the rod he can move the arc about, heating one part more than another, distributing his heat if he wants to do so over a larger area, or keeping it to a small one, just as he wills. On reaching the right heat the rod is withdrawn, the arc destroyed, and the iron can be hammered just as if it had been heated in a fire.
Yet another way still is known as "resistance" welding. In it an enormous current at an extremely low voltage is used. The fundamental principle is the same, since the heat is formed by forcing current past a point over which it is reluctant to pass. That point of poor conductivity is the ends of the two bars to be joined. They are placed just touching, but since an imperfect contact like that always offers considerable resistance to the flow of a current, the passing current needs only to be made large enough for great heat to be generated.
This is exceedingly pretty to watch. We will suppose that the article to be operated upon is the tyre of a wheel. The bar of iron has already been bent by rollers into the correct curve and the two ends are touching. Brought to the machine, it is gripped, each side of the junction, in the jaws of an insulated vice and the current is turned on. In a few seconds the place where the two ends are just touchingbegins to glow. Rapidly it increases in brightness until in about half-a-minute it is at welding heat. Then one vice, which is movable, is forced along a little by a screw, so that the ends are pressed firmly together, a little judicious hammering meanwhile helping to complete the job. Then the current is switched off and the complete tyre taken out of the machine. The current used has a force comparable with that which operates domestic electric bells, but in volume it is thousands of amperes. Alternating current is used, and it is obtained from a transformer or induction coil. In such a case the primary part of the coil is made of many turns of fine wire, so that little current passes through it, while the secondary part is but one or two turns of thick bar. Thus the voltage generated in the secondary is very little, but since the secondary has an almost negligible resistance the current caused by that small voltage is enormous. Such an arrangement is in industrial realms generally called a transformer, the term induction coil being employed more for those things of a similar nature intended for the laboratory. The one just described is, moreover, a "step-down" transformer, since it lowers the voltage, to distinguish it from "step-up" transformers, which raise the voltage.
And the "resistance" principle is also applied in another way to large furnaces, such as those for refining iron. In these the resistance of the iron itself is utilised to generate the heat. Of course, it should be well understood, heat is always generated in everything through which current flows. There is no perfect conductor, and so every conductor is more or less heated by the passage of current through it. Some energy needs to be expended to drive current, even along large copper wires, and that energy must be turned into heat in the wires. If the same volume of current be forced along iron wires of the same size, the heat will be greater, since iron is but a poor conductor compared with copper, the relation being about as one to six. And if the iron be hot the resistance will be still more, for it stands to reason that when heated the molecules, being farther apart, willbe the less easily able to exchange corpuscles. We have the best reasons for believing, as has been suggested already, that a current of electricity is but a flow of corpuscles, and so we are not surprised to hear that, as a general rule, the hotter a thing is the less does it conduct electricity.
By permission of Cambridge Scientific Inst. Co., Ltd., Cambridge, Eng. Measuring Heat at a Distance This wonderful instrument, the Fery Radiation Pyrometer, although itself some distance away from the furnace, is telling the temperature of its hottest part.By permission of Cambridge Scientific Inst. Co., Ltd., Cambridge, Eng.Measuring Heat at a DistanceThis wonderful instrument, the Fery Radiation Pyrometer, although itself some distance away from the furnace, is telling the temperature of its hottest part.
So imagine a circular trough of fire-clay or other heat-resisting material filled with fragments of iron, or, it may be, with iron barely above melting-point, which has come from another furnace, where it underwent the previous process. Circling inside or outside this trough is an enormous coil of wire through which currents of electricity are alternating. That is the "primary" of a transformer, and the "secondary" is—the iron itself, in the trough. If it be, as it often is, in the form of scrap, or broken pieces, the heat will begin to show itself where the pieces touch each other. The currents generated in the trough, by the coil outside, will, of course, pass from piece to piece and the points of contact, since they offer the greatest resistance, will show signs of heat. This will increase until the pieces begin to melt. As the separate fragments merge into the molten mass the resistance will in one way decrease, for the imperfect contacts between the pieces will give place to the perfect contact throughout the mass of liquid metal. But for another reason—namely, the increase in heat—the resistance will increase. And all the while the alternations in the primary coil will be pumping currents, as it were, round and round the ring of molten iron. Whether the resistance increase or decrease, the current will do the opposite, so that heat will be generated whatever happens. For as resistance decreases current increases, and vice versa. And the slightest variation in the strength of the primary current will have its effect upon the secondary, and therefore on the heat generated. So, by simply regulating the primary current, the temperature of the metal can be controlled to a nicety. And such furnaces have the immense advantage that there is no possibility of deleterious substances in the fuel getting into and spoiling the metal, a thing which may very easilyhappen during the manufacture of high-class steels, alloys of iron in which the exact quantities, purity and proportions of the ingredients are of the utmost importance.
Hence these "induction furnaces," as they are called, are frequently used quite apart from any question of utilising water-power. And they will probably be used still more as time goes on.
For one thing, they may become valuable adjuncts to the older form of iron and steel furnaces, from which they will obtain their power free, gratis and for nothing. In districts such as Middlesbrough they could generate more electricity than they have any use for. The ordinary iron furnaces belch forth flames which are really good useful gas (carbon monoxide) burning to waste. Many of the furnaces are covered in at the top, and this gas is led away to heat boilers for the steam-engines or to drive large gas-engines, but in a large works there is more of this waste gas than they know what to do with. Now that could, and probably will ere long, be turned into electricity by means of gas-engines and the current used for making steel in induction furnaces.
It will probably surprise many to know that these enormous currents which can thus heat great masses of metal until they melt are no danger at all to the men who work with them. A man might dip an iron rod into the trough of metal and he would scarcely feel the shock. And the same is true of the welding machine, which can be touched in any part without fear. The reason, of course, is that, broadly speaking, it is volume of current which does harm, and the resistance of the human body is so great that with the small voltages used, the volume which can pass is negligible. It should be mentioned, however, that the volume of current in lightning is also small, but we know that it is capable of inflicting terrible injury. Lightning, however, is in a class by itself. Our terrestrial voltages are baffled by an air-gap of a few inches, but lightning springs across a gap miles wide. Its voltage must,therefore, amount to millions, and the ordinary rules relating to earthly currents do not apply.
But other sources of heat besides electricity are at the disposal of our manufacturers nowadays. Pre-eminently there is the flame of some gas burning with pure oxygen. The oxyhydrogen jet has been known for many years as the best means of producing the light for a magic lantern. Such a jet impinging upon a pencil of lime causes the latter to glow with a dazzling white light.
But the oxyhydrogen jet is now employed in many factories for the welding of metals. This is known as fusion welding, since the two parts are actually reduced to liquid. The usual way to go about this work is to bevel off the ends or edges to be joined. Suppose, for instance, that we wanted to weld two pieces of brass pipe together. We should first file or otherwise trim the edges to be joined until when put together they form a groove practically as deep as the metal is thick. Then with a stick of brass wire in the left hand, and an oxyhydrogen blowpipe in the right, we should direct the flame from the pipe on to the metal until, at one point, the sides of the groove were beginning to melt. Then, inserting the point of the wire into the groove, we should melt a little off it. Thus we should work all round the joint, melting the sides of the groove and filling in with melted metal from the wire, until the whole groove had been filled up and the metal added had been thoroughly amalgamated with that on either side.
As a matter of fact, if it were brass which we were working on we should probably use the cheaper though less pure form of hydrogen—coal-gas—so that it would really be "oxycoal-gas" that we should use and not oxyhydrogen. The latter is used, however, notably for the fusion-welding of lead, or "lead-burning," as it is termed.
The blowpipe is a brass tube about a foot or eighteen inches long, with two passages in it, one for the oxygen and the other for the other gas. The gases are brought to one end of it through rubber pipes, while at the other end there isa nozzle in which the gases mingle and from which they emerge in a fine jet.
The oxyhydrogen flame has a temperature of about 2000° C., hot enough to melt fire-clay. That does not matter in the case of welding, however, since the molten metal is very small in quantity at any given moment, and is allowed to cool before it can run away. It would be an awkward temperature to deal with, nevertheless, in a furnace. It seems strange that it does not burn the nozzle of the blowpipe, but the fact that it does not is, it is believed, explained by the fact that the expansion of the gas, as soon as it emerges from the hole out of which it shoots, causes a comparatively cool space just there, shielding it from the intense heat farther on.
An exceedingly interesting use of the oxyhydrogen flame is in the manufacture of artificial rubies. These stones are made in Paris by a very simple means. The necessary chemicals are prepared and ground to an exceedingly fine powder. This is then allowed to fall through an oxyhydrogen flame. Thus there is no need for a crucible capable of withstanding this high temperature, since the melting takes place as the particles are in the act of falling. When they reach the support prepared to catch them they have cooled somewhat. Stones so called are real rubies—artificial, but not shams. They possess every property of the ruby from the mine.
Another product of the oxyhydrogen flame is the quartz fibres which are used for suspending the needles in the finest galvanometers. The quartz is melted, in this case a crucible being employed. An arrow is then dipped in the liquid quartz and immediately "fired" into the air. The thick treacly liquid is thus drawn out into a thread of such fineness that a microscope is necessary to find it with.
Hotter even than oxyhydrogen is the oxyacetylene flame, which at its hottest point reaches nearly 3500° C. The gas, which is another of the combinations of carbon and hydrogen (its molecules containing two atoms of each), iseasily made by allowing water to come into contact with calcium carbide. The latter, which is CaC2, is made by heating coke and lime together in the intense heat of an electric furnace. This accounts largely for the great heating power of acetylene, for since great heat is necessary to cause the elements to combine great heat is given out by them when they ultimately separate. Here again is the conservation of energy. The heat energy of the electric furnace is largely expended in forcing these two elements into partnership. They are, as it were, given a large amount of capital in the form of heat. It ceases to be sensible heat, becoming latent in the compound, but still it is there. So a lump of calcium carbide, with which many readers are familiar, has vast stores of heat locked up within it. When water comes into contact with the carbide the partnership is broken, but the heat is not liberated then, since another partnership is formed, which still retains the old heat-capital. The calcium in the carbide is displaced by the hydrogen from the water, and so C2H2comes into being, while the rejected calcium consoles itself by entering into combination with the equally forsaken oxygen from the water, forming CaO, which is but another name for lime.
Then the acetylene (C2H2) is mixed with oxygen in the blowpipe and burnt, under which conditions the pent-up heat, borrowed originally from the electric furnace, is brought into play. With this flame the harder metals can be fused and welded. Wrought iron, cast-iron, steel in all its forms, all can be melted by the oxyacetylene flame, almost as easily as snow by a hot iron. The fusion welding of these metals is then carried on just as already described for brass.
By means of a special blowpipe, wherein an excess of oxygen is introduced at the hot point, hard steel plates can be cut to pieces almost as easily as a grocer cuts cheese. Even thick, hard armour-plate can thus be cut, almost the only way, indeed, in which it can be cut.
And for purposes such as welding and cutting this flame has an interesting and peculiar advantage over all otherkinds of heat. When a metal is heated in the air there is usually trouble from oxidation. The domestic poker, for example, after it has been left to get red-hot in the fire is seen to be coated, in the part which has been heated, with scales which will flake off if the thing be struck. Those scales are oxide of iron, caused by the union of iron and oxygen when the poker was hot. But if the heat be applied by the oxyacetylene flame that will not happen. The oxygen and the carbon from the acetylene will burn, and if the supply of the former be properly regulated it will be entirely used up in the process. The hydrogen from the acetylene is, strange to say, unable to unite with oxygen at such a high temperature as that of the oxygen and carbon, so that it passes on beyond the oxygen-carbon flame and ultimately burns on its own account with the oxygen from the atmosphere in a second flame surrounding the first. Thus there is a double flame: inside, a little pointed cone of white flame, that is the oxygen and carbon; and outside that a bluish flame, the hydrogen and the atmospheric oxygen. The latter flame forms a kind of jacket entirely enveloping the former. And so when one melts metal by means of the white cone the hydrogen jacket shields the molten metal from oxygen and prevents the oxidation. Only one who knows the bother caused by oxidation whenever metals are heated can realise the wonderful advantage of this.
And now we can turn to even another source, also quite modern, of high temperature.
If the oft-quoted "man in the street" were asked the two commonest things on earth he might possibly name oxygen as one, and so far he would be right, but the chances are much against his naming aluminium as the second. If he did not, however, he would be wrong. Aluminium and oxygen form alumina, of which are constituted the sapphire, the ruby and other precious stones, but alumina is most commonly found in combination with silica, or silicon and oxygen. This compound is called silicate of aluminium, and of it are formed clay and many rocks. The reason why themetal aluminium was until recently rare and expensive was because of the great difficulty of disentangling the metal from this rather complex combination. And these two commonest elements have, under certain conditions, a rare affinity for each other. They join forces with such energy that great heat is given out in the process. This, again, we may regard as an example of the conservation of energy. Heat had to be used up, apparently, in separating the aluminium and oxygen as they were found together in the natural state. And that heat reappears when they combine together again. This is a most useful principle, for if heat has disappeared anywhere in the course of some operation, we know that in all probability, if we go about it the right way, we can get that heat back again, perhaps in a more convenient form. That is so in this case at all events.
Now aluminium will not readily combine with atmospheric oxygen, but it will readily do so with oxygen from the oxide of a metal. So if we put into a vessel some oxide of iron and some finely powdered aluminium, and give it some heat at one point, just to set the process going, the whole mass will burn with intense heat. And when the burning is finished the crucible will be found to contain (1) some molten iron, the oxide of iron with the oxygen gone, and (2) some oxide of aluminium or alumina, in the form which we call corundum, a very hard substance which in a powdered form is used for grinding hard metals. We start, you will notice, with a pure metal and an oxide. We finish with a pure metal and an oxide, only the oxygen has changed its quarters, having passed from the iron to the aluminium. And in the course of the change a vast amount of pent-up heat has been liberated. Aluminium is thus a fuel, strange though it may seem to say so, just as coal is. Coal, however, is willing to pair off with oxygen from the air, while aluminium, more fastidious, will only accept it as partner when it can steal it from another combination.
But the practical result is eminently satisfactory, for the action of the aluminium and iron oxide is to leave us with acrucible full of molten iron at a very high temperature. And this can be used in various ways.
Tramway rails, for example, can be joined together by it. A mould is formed around the ends of two rails, where they "butt" together, and into this mould a quantity of the melted iron can be poured. So hot is it that it partially melts the ends of the rails, and then, amalgamating with them, it forms a perfectly homogeneous connection between them.
The same method can be applied to the repair of iron structures of all kinds. The propeller shaft of a ship, for example, sometimes breaks on a voyage. Such a catastrophe is fraught with the most serious consequences, unless it can be quickly repaired. Thermit, as this process is called, is perhaps the only means whereby, under certain conditions, this can be accomplished.
The extraordinary heat of the metal produced in this way is demonstrated by the fact that if it be poured on to an iron plate an inch thick it goes clean through it. It melts its way through instantly.
But although such high temperatures are at the command of the modern manufacturer, there are some things—indeed many things—which still baffle him, the diamond, for example. It is true that diamonds of small size have been made, but larger ones have so far defied all efforts.
One very interesting fact about this may be mentioned in concluding this chapter. Sir Andrew Noble, a member of the great firm of Armstrong, Whitworth & Co., of Elswick, tried the experiment of exploding some cordite, a high explosive, inside a steel vessel of enormous strength. He thus produced what is believed to be the highest temperature ever produced on earth. It is reckoned to have been 5200° C., and the pressure at the same time was, it is calculated, 50 tons per square inch. His intention was not to make diamonds, but Sir William Crookes predicted that diamonds would be the result. For the cordite consisted mainly of carbon, which, as is well known, is thematerial of which the diamond is formed, and the combination of high temperature and high pressure is just what is needed, so it is believed, to bring the carbon into this particular form. And true enough, on the iron being examined after the explosion, there were seen tiny diamonds. For larger ones even higher temperatures and greater pressures are, no doubt, necessary, and as the diamond, like gold, has a peculiar fascination for mankind, so the efforts to manufacture it will continue. In years to come the means may be found of creating these extreme conditions of temperature and pressure, and so another of the problems of the ages will be solved.
By permission of the British Aluminium Co A Striking Feature of Modern Aluminium Works For the production of aluminium water power is required. Water is stored at a high level and is then brought down to the factory in pipes. The illustration shows the pipe track recently laid down for this purpose at Kinlochleven in Argyleshire. The six pipes, each of which is thirty-nine inches in diameter, run down the hillsides for one mile and a quarterBy permission of the British Aluminium CoA Striking Feature of Modern Aluminium WorksFor the production of aluminium water power is required. Water is stored at a high level and is then brought down to the factory in pipes. The illustration shows the pipe track recently laid down for this purpose at Kinlochleven in Argyleshire. The six pipes, each of which is thirty-nine inches in diameter, run down the hillsides for one mile and a quarter
Those countries which are blessed with a plentiful supply of coal are periodically shocked and saddened by a terrible calamity—an explosion in one of the mines, in which often scores of poor fellows lose their lives, and hundreds of widows and orphans find themselves without a breadwinner. One has only to recall that heart-rending calamity of the Courrières mines in France, where over a thousand lives were lost, to realise how important is the question of the cause and the cure of the colliery explosion.
It used to be thought a settled matter that these were due to the accidental ignition of a gas called, scientifically, "methane," but by the miners "fire-damp." This undoubtedly does collect in many mines, and since it is much the same as the domestic coal-gas (indeed methane forms the bulk of coal-gas) it is not surprising that the explosions were attributed to it. At times shots were fired, to blast down the coal, and although the greatest precautions are taken to prevent any accident resulting, it seems certain that explosions have occasionally followed the firing of shots. But still more dangerous is the adventurous miner who, for some reason, opens his safety lamp. It is lit for him before he enters the workings, and locked up, so that, theoretically, he cannot tamper with it; but it has to be a cleverly devised lock that cannot be picked in some way, and with the carelessness born of long immunity from accident these are got open sometimes, with, it may be, disastrous results.
Even a spark struck from a miner's pick may ignite the gas; or a spark from some electrical machine used in the mine. That is one of the reasons why electrical apparatusis suspect in colliery matters and machines worked by the less convenient and more costly means of compressed air are preferred.
In some such manner the fire-damp is ignited, and then there follows the fiery blast, which, sweeping through the narrow galleries and passages which constitute the workings, simply licks up the life of the men whom it encounters. Others, in byways and sheltered corners, escaping the burning cloud of flame, are poisoned by the deadly fumes of carbon monoxide which it leaves when its force is spent. While others, perchance the most unfortunate of all, are saved for a time, but, being imprisoned by falls from the roof and walls, die a lingering death of hunger and slow suffocation. A colliery explosion is one of the ghastliest events imaginable, the only relief from which is the noble heroism with which the survivors, from the mine managers to the humblest workmen, crowd round the pit-mouth, eager to risk their own lives for the faint chance of saving some below. Not infrequently these brave volunteers only share the fate of the men they would rescue.
Now all that, as I have said, used to be put down to the effect of the fire-damp. But it dawned upon men's minds some years ago that the damage seemed to be out of proportion to the power of the gas. Modern mines are well ventilated by large fans, which impel great volumes of air through all the workings. The air currents are cunningly guided by partitions or "brattices," so that every nook and corner shall be scoured out by the plentiful draught of pure fresh air. Consequently the amount of explosive gas which can collect in any one place is but small. How, then, can so small a volume of gas do so large an amount of damage?
Coupled with this was the fact that explosions take place in flour mills, where there is no gas, and experimenters had found in their laboratories that almost any burnable substance,if ground up finely enoughand blown into a cloud, would explode. Coal-dust would naturally do this. Indeed anyone throwing the dust from the bottom of the coal-shovelupon a fire will see for himself how, quickly such dust will burn, and, as has been pointed out in an earlier chapter, an explosion is but rapid burning.
So the blame was largely transferred from the shoulders of the fire-damp to those of the clouds of coal-dust which collect throughout the workings of a mine.
But then a difficulty arose from the fact that there is dust in all mines, yet some districts are quite free from explosions. And such districts are those where there is little or no fire-damp. These two facts seem to be explainable in one way, and in one way only. It must be that the gas first of all explodes feebly, and so, stirring up the dust lying along the roads and passages, prepares the way for the powerful, deadly explosion of coal-dust which follows.
But that was only a guess, and the matter was of such importance that it needed something more certain than mere assumption. So the Mining Association of Great Britain decided to have a series of experiments which should settle once and for all what part the coal-dust played in these catastrophes, and how best they could be prevented.
It was at first thought that an old mine might be utilised for the experiments, but there was the difficulty that such always become wet after work has ceased in them, and so the dust would not behave normally. Moreover, the work would be extremely dangerous and the results difficult to observe. Then a culvert was suggested built of concrete, partly buried in the ground, but that too was dismissed. Finally it was decided to make an imitation mine of steel, using old boiler shells with the ends taken out.
The sum of £10,000 was subscribed for the purpose by the coal-owners of Great Britain, and the great work was carried out at Altofts, in Yorkshire, close to a colliery where a terrible disaster occurred in 1886.
Here the great tube or gallery was built. Roughly the shape of a letter L, one leg is over 1000 feet long, while the other is 295 feet. The longer leg is 71⁄2feet in diameter and the shorter 6 feet. At the end of the shorter part a largefan is installed which can force 50,000 to 80,000 cubic feet of air per minute through the structure, so producing the conditions of a well-ventilated mine. The shorter length has several sharp turns in it for the purpose of breaking the force of the explosion along that part, and so shielding the fan from damage, while a tall chimney is provided there, so that, the door being shut to cut off the fan, the gases from the explosion can find a harmless way out.
Inside the tube, shelves are fixed along the sides so as to reproduce the effect of the timbering in a real mine, upon the beams of which the dust finds lodgment. Props were put up too, just as they would be in the real mine. Everything, in fact, was done to make the place as perfect a replica as possible of actual underground workings.
And then, added to this huge and costly structure, was an outfit of scientific instruments worthy of the important investigations which were to be carried on.
To grasp the purpose and working of these we need to remind ourselves of the aims and intentions of the experiments. First of all it was desired to find out how various quantities and qualities of coal-dust behaved. The dust was laid along the floor of the tube and along the shelves. A small gun fired at some point in the tube raised a cloud of this dust just as the gas explosion in the real mine would do. Then another gun was fired to explode the dust-cloud. So far all is quite simple and easy. But to do that would be of no value without the means of finding out exactly what resulted from the explosion. And that is the function of the instruments.
To commence with, there is the great wave or tide of force or pressure which surges along the gallery immediately the cloud bursts into flame. How fast does that wave travel? How long is it after the explosion before the shattering effects of it are felt a hundred yards away? To solve that problem electrical contact-breakers are fixed at intervals of fifty yards along the gallery. Each of these consists of a cylinder with a piston inside it something like, shall we say, a cycle pump.The piston, held down normally by a spring, is blown upwards by the force of the explosion. The spring is adjustable, and so it can be arranged that the feeble force of the gun cannot lift the piston, but the more powerful coal-dust explosion which follows can.
Thus when the explosion takes place these contact-breakers are operated in succession. The one nearest the seat of the disturbance is operated first; next the one fifty yards farther away; then the one a hundred yards away, and so on. The moments when they work will tell the speed at which the blast travels along the gallery. But it travels with great speed, and so to measure and record the exact moment when each contact-breaker is moved is a matter of no little difficulty. Electricity, however, makes this, like so many other things, comparatively easy.
There is an apparatus used in astronomical observatories called a chronograph, which registers, within a small fraction of a second, the moment when a star seems to pass across a wire in the "transit circle," the telescope by which the positions of stars are determined and the exact time kept. The observer sits with his eye to the telescope, watching the apparent movement of the star. In his hand he holds a small "push," pressure on which by his fingers operates a minute pricker, which acts upon a moving strip of paper. The paper travels along with the utmost steadiness and regularity, while a clock drives a sharply pointed pricker on to it every two seconds. Thus the clock marks out the paper into lengths, each of which represents two seconds. But the other pricker, worked electrically by the observer's hand, also makes its mark upon the paper, and so, while the regular marks indicate intervals of two seconds, each irregular one marks the time of a transit or passing of a star across the wire. An examination of the strip subsequently enables the times of a transit to be seen with great accuracy, from the position of the corresponding mark between two of theregularmarks.
And the same principle was applied to the circuit-breakersof this artificial mine. Normally, current flows through the circuit-breaker, but the lifting of the piston breaks the circuit (whence the name of the contrivance), and that breaking of the circuit and consequent cessation of the current operates the chronograph. By a cleverly constructed device, the details of which are too complicated to set out here, each circuit-breaker in turn makes its mark on the same strip, so that the distances apart of these marks show the time taken by the force of the explosion to travel fifty yards. Meanwhile the clock goes on making its regular marks (in this case every half-second), so that they form a scale by which the other intervals can be measured very exactly.
The chronograph used here is more accurate than that in use at Greenwich Observatory, the reason being that in this case the recording currents are sent mechanically by the contact-breakers operated by the explosion itself, while in the case of the astronomer the human element comes in. To watch a moving speck of light and to tell exactly when it crosses a fine line is by no means easy, and so to tell the time within a tenth of a second, is about the limit of possible accuracy. The instrument we have been referring to, however, can register the time which a gaseous wave moving 3000 feet per second takes to travel fifty feet. In other words, the circuit-breakers can be operated so fast that when only a sixtieth of a second intervenes between the action of one and that of the next the chronograph can duly record the fact.
The records of the chronograph can be made in two ways: one by a pen on a piece of paper tape, and the other by a scratch on a piece of smoked paper.
So by that means the progress of the "force" of the explosion can be measured. It is necessary also to time the movement of the "heat" of the explosion, for the two may not travel together, and the difference between them may let in some light as to the nature and behaviour of the explosion. So for this second purpose a second set ofcircuit-breakers are used. Each of these consists of a strip of thin tinfoil stretched across the gallery. Being placed edgeways to the moving current of gas, the force of the explosion has no effect upon it, but the heat instantly melts it. Normally, current flows through the strip, and so the melting is signalised by the cessation of the current, which event is recorded by the chronograph.
Thus the speeds at which the force and the heat of the explosion travel are ascertained. Another important fact which needs to be found is the amount of the force, or the pressure, at different points. For this purpose pressure-gauges can be connected to the gallery at the desired spots by means of flexible tubes. This flexible tube is necessary in order that the vibration of the steel shell, due to the explosion, shall not be communicated to the instrument. The pressure, finding its way along the flexible pipe, raises a piston against the force of a spring, and the distance to which it is raised forms, of course, a measure of the pressure inside the gallery at the point to which the tube is connected. The pressure is recorded by the action of the piston in moving a style which just touches against the surface of a moving paper. There are three styles in all marking this paper. The first is the one just mentioned. The second is held down on to the paper by an electro-magnet energised by current flowing through a fine wire stretched across the gallery just where the explosion originates. This fine wire is broken at the moment of the explosion, whereby the current is cut off and the style raised. It therefore makes its mark until the moment the explosion occurs, and then leaves off. The end of that line, therefore, shows the time of the explosion. Meanwhile the first style is drawing a straight line, but as soon as the pressure begins to be felt by the pressure recorder this style moves and the line slopes upward. Upward it goes as the pressure increases, until it has reached its height, after which it descends, until the style is drawing a straight line once more. Thus the rise and fall of the line represents the rise and fall of the force of the explosion.
Then comes the matter of time. How soon after the explosion occurred did the pressure begin to be felt? How long did it take to reach its maximum and how long to die out again? These questions need answers which the apparatus so far described does not give. True, the speed of the paper may be known approximately, but all that I have described will occur within the space of a fraction of a second, and it is difficult to tell the speed of the paper with sufficient accuracy. Therein we see the purpose of the third style. It is attached electrically to the "tenth-of-a-second time-marker." This consists of a weight suspended at a height. The force of the explosion lets it drop. The moment it starts to fall it causes the style to make a mark on the paper. When it has fallen a certain distance the style makes another mark. And the distance that the weight falls between the making of the two marks is so adjusted that the space between them on the chart represents exactly a tenth of a second. Thus a scale is formed upon the chart by which the other times can be measured. There is the line terminating at the moment of explosion; the straight line changing into an up-and-down curve, representing the time and the variation of the pressure; finally there are the two marks representing a tenth of a second by which the other marks recorded upon the chart can be interpreted.
But the mere pressure and velocity of the explosion form but a part of the knowledge desired. How the explosion is formed, whether or not the coal-dust is burnt up entirely, whether, indeed, it be the dust itself which burns or coal-gas given off by the dust under the heat of the preliminary explosion, what the gas is which is left by the explosion at various stages—these are important things to be known, and they can only be ascertained by taking samples of the gases in the gallery at different moments during and after the explosion. To obtain these samples bottles are used, but the question is how to get them filled at just the right time. Into the shell of the gallery holes are drilled, and to these the metal bottles or flasks are screwed, a pipe leadingfrom the mouth of each bottle well in towards the centre of the gallery. The end of this tube is closed by a cap of glass above which there stands poised a little hammer. Controlling the hammer is an electrical device called a "contact-maker," so arranged that just at the desired moment the hammer falls, breaking the glass, and admitting a sample of the gas in the gallery, the bottle and its tube having previously had the air exhausted from them, so that on the glass being broken the gas is sucked in.
At the same moment a weight falls, attached to the end of a cord, and this, on reaching the end of its tether, closes the end of the tube, and the sample is imprisoned until such time as the bottle can be disconnected and taken away to the laboratory for its contents to be analysed.
The contact-makers are of two kinds. In one the pressure of the explosion raises a piston which completes a circuit allowing current to flow through the very fine wire which prevents the fall of the hammer. This fine wire being fused by the current, the hammer falls and does its work. The other kind, which are used when the force of the explosion is not enough to raise a piston, is operated by one of the tinfoil circuit-breakers. A magnet, being energised by current passing through the foil, holds up a curved bar over two cups of mercury. Broken by the heat of the explosion, the foil cuts off this current, de-energises the magnet, and allows the bar to fall with its ends in the mercury. This completes another circuit, permitting current to pass to the fine wire, whereby the hammer is released. By connecting a bottle to a contact-maker at a distance the sample can be obtained at any desired period of the explosion. If, for instance, the sample is to represent the immediate products of combustion, it is placed near to the contact-maker. Then the sample is drawn in practically at the moment of explosion. If, on the other hand, it is the after-damp that is to be sampled, then the bottle would be connected to a contact-maker a long way from the seat of the explosion, with the result that its glass cap would not be broken until some considerabletime had elapsed after the explosion has passed the bottle. The time also during which the bottle is drawing in its sample can be adjusted by varying the length of the cord to which the weight is attached.
And last of all must be mentioned the employment of a kinematograph, capable of taking twenty-two photographs per second, for observing the effects at the ends of the gallery (see illustrations).
Thus records are obtained of the force and heat of the explosion, its mechanical and thermal effects upon the walls of the gallery, or, if it were in a real pit, the effects which it would have in shaking and in heating the workings, and the men labouring in them. This and the analysis of the gases producing and produced by the explosion, derived from the contents of the bottles, give sound data upon which can be built up reliable theories as to the nature of colliery explosions and the way to prevent them, results which could be obtained in no other way. No one can help being struck with the thoroughness and ingenuity of the means adopted to these ends, and it is no exaggeration to say that it is a splendid example of thoroughly scientific methods applied to an important industrial investigation. It will be interesting to conclude this account with a brief mention of some of the results to which these painstaking efforts have led.
First in importance the fact is placed beyond doubt that coal-dust, which in bulk will only burn slowly, will, when well mixed with air, explode. And no combustible gas need be present to aid in the explosion.
The dust-raising gun, by blowing some dust into a cloud which was ignited by the second gun, caused an explosion powerful enough to do all the damage experienced in the most disastrous natural explosions. So it is practically certain that the function of the gas is but that of the first gun, to raise the cloud of dust.
A typical experimental explosion may be briefly described. On the cloud-raising gun being fired a small cloud of dust was driven out of the ends of the gallery, even that end atwhich the fan was blowing airin. In other words, the current of air was checked, even reversed, by the preliminary shock. This cloud was, of course, shown by the kinematograph.
Then when the second gun was fired, and the real coal-dust explosion occurred, there was first a cloud of dust shot out larger than the other one, to be followed by a cloud of flame 180 feet long. These also were recorded by the kinematograph. The sound was heard seven miles away.
Pressures as high as 92 lb. per square inch were recorded, and the force of the blast was found to travel well over 2000 feet per second.
In many cases, strange to say, the effects were very slight at the seat of the disturbance, the force seeming to increase as the wave travelled along the gallery. Probably the dust had not time to burn completely but only partially at the first onset. Where props or timbers checked the flow of the flaming gases there the damage was most, for no doubt the eddies caused the air and coal to be particularly well mixed at such points. An encrustation of coke was found on the sides and the timbers after all was over, probably because there was not sufficient air to burn all the dust, and some was only heated into coke to be deposited on the nearest surface, where the tarry matters would make it stick.
Finally, the most important, perhaps, of all, it was demonstrated that an admixture of stone-dust with the coal-dust made it non-inflammable. If a small zone were treated in this way, stone-dust being mingled with the other, the explosion became stifled at that point. True, the poisonous after-damp swept on beyond, so that men there might have been poisoned by it, but the stone zone would certainly save them from the direct effects of the blast. If, however, stone-dust be mingled with coal-dust all along the gallery, then no explosion at all would occur, again proving that it is the coal-dust which does the damage.
In the colliery adjoining the experimental gallery this plan had been in use for years. Soft shale is ground to finepowder, and is sprinkled wherever coal-dust has collected. It is just strewn by hand, giving the workings the appearance of having been roughly whitewashed. And since that has been done there has been no explosion in that pit. The experiments showed beyond doubt that that was no chance occurrence. They showed that in some way not thoroughly understood this addition of stone-dust renders the coal-dust harmless. It may be that it merely dilutes it. It may be that in some way it takes some of the heat and so prevents the coal particles becoming hot enough. It may be that, being a little heavier, it checks the formation of the dust-cloud. However that may be, there is no doubt now that stone-dust is the salvation of the miner so far as explosions are concerned.
Water sprinkled upon the coal-dust, by laying it and keeping it from forming a cloud, has the same effect, but it is less convenient, for the simple reason that water evaporates, while stone-dust stays where it is put.
Probably no invention has made such a sensation during recent years as wireless telegraphy. And since it is the direct outcome of the most abstruse, purely scientific investigations, there could be no more appropriate subject for a place in this book.
For many years there has been a belief in the existence of a mysterious something to which has been given the name of "The Ether." Totally different, it should be noted, from the chemical of the same name, it is entirely a creature of the intellect. None of our senses give us the slightest direct indication of its existence. No one has either seen, felt, heard, smelt or tasted it. Yet we feel that it must exist, for the simple reason that some things which our senses do tell us of are utterly inexplicable without it.
It was originally thought of in connection with light. Standing at night upon the top of a hill, we see the lights of a town a mile away. How is it that those distant gas or electric lamps affect our eyes? They are a mile away; and the idea that one object can affect anotherat a distanceis one which the human mind refuses to accept. We feel compelled to believe that there is something in contact with the source of light which is affected first, and through which the disturbance, whatever it may be, is conveyed to our eyes, with which it must also be in contact. We feel that there must be a something stretching from our eyes to the distant objects, by which the light is carried. Of course the air fills the space referred to, but that cannot be the carrier of light, for if we look through a glass vessel from which the air has been exhausted we see distant objectsundimmed. We also have good reason to believe that the air belongs specially to our globe, and does not extend upwards for more than a few miles. Consequently it cannot be air which brings sunlight and starlight. We are forced to fall back, therefore, upon the belief in something, of which we have no other knowledge, which must fill all the vacant spaces in the whole universe, passing, even, between the particles of which ordinary matter is composed, reaching as far as the remotest star, able to penetrate everything, and consequently not excludable from the most perfect vacuum. It is something so different from anything of which we have any direct knowledge that one is tempted sometimes to doubt whether there must not be some other explanation of light. In order to transmit light at the speed at which we find that it does in fact travel, the ether must be more rigid than the hardest substance we know of. Many, many thousand times more rigid, indeed. Yet it seems to offer no resistance to the passage of the planets through it. Still, there is no other alternative, so far as men can conceive, and we are compelled, therefore, to believe in the existence of the ether.
The first things discovered by the telescope were the larger satellites of Jupiter. With that precision for which astronomers are noted, they soon drew up time-tables, showing not only the past movements of these bodies, but also their future ones. They were soon puzzled, however, by the obvious fact that the moons of Jupiter were not working according to schedule, to use a railway expression. They got later and later for a time, and then gradually quickened up until they got too fast. Then they slowed down again. This repeated itself, and is going on still, with this difference, however, that the cause has been discovered and the schedules amended accordingly. The solution of the puzzle was that when the earth and the great planet are on the same side of the sun they are some 186 millions of miles nearer together than when they are on opposite sides of the sun. The evolutions of the satellitesare quite regular, according to the astronomers' calculations, but they seemed to the earthly astronomers to vary, because of the time which light took to traverse that 186 millions of miles. When the two bodies were nearest together the occurrences seemed to happen about 1000 seconds (16 minutes) earlier than when they were farthest apart. Consequently it became evident that light took 1000 seconds to travel 186 million miles, or that, in other words, it moved at the prodigious speed of 186 thousand miles per second. That discovery was, of course, many years ago, but experiments since have proved the figure mentioned to be about right.
It put beyond question the fact that the action of a distant light upon the eye was not an "action at a distance," for such action, were it possible, would take effect at once. Seeing that light passed from the distant satellites at a definite velocity, and took a certain time to reach us, it was evident that it was, during that time, passing through a medium of some sort, and that medium must be the ether, for no alternative explanation will suffice.
So it became recognised that light really consists of waves or undulations of some sort in the ether; that a distant, luminous body set these waves going; that they travelled with a definite velocity, and then, striking our eyes, produced the sensation known as light. Many things were found out about light in the years which followed the discovery of its velocity. The lengths of the waves were ascertained—that is to say, the distance from the crest of one to the crest of the next. The different lengths were sorted out and found to give rise to different colours, while longer waves, which produced no sensation of light, were found to carry heat, thereby explaining how the heat reaches us from a distant fire, or from the sun.
Of the actual nature of the waves, however, little was known, although there was a vague idea that they were connected in some way with electricity, at which point in the story there comes in the famous name of James ClerkMaxwell, a professor of Cambridge University, who in 1864 produced before the Royal Society the explanation of the nature of the waves and their connection with electricity and magnetism. That in itself was a wonderful achievement, but far more wonderful still is the fact that he truly predicted the existence of longer waves than any then known, which no one knew how to cause, or how to detect if caused. That prediction has since been fulfilled. The long waves have been found; we know how to make them and how to perceive their presence. They are the messengers which carry our wireless messages.
The discovery of these, at that time unknown waves, on paper, by simply calculating and reasoning about them, is more marvellous even than the feat of Adams and Le Verrier in discovering a planet on paper before anyone had seen it. It established Maxwell among the heroes of science for all time.
A magnet acts upon a piece of iron some distance away. The pull must be transmitted through some kind of ether. A current of electricity behaves in the same way, acting precisely as a magnet, with power to affect things at a distance. Again an ether is necessary. A dynamo works by moving a magnet past a wire which it does not touch, thereby generating current in it. There again an ether is necessary to transmit the effect from the one to the other.
Taking, then, the known magnetic effects of an electric current and the electrifying effects of magnets, he was able to show that the same ether accounted for all, and for the transmission of light as well, that, in fact, there was but one ether which performed all these various duties.
He proved from the known facts about electricity and magnetism that waves such as he imagined would, in fact, move with the speed of light. And once knowing the nature of the waves, he asserted that in all probability there were others of which men had then no practical knowledge.
Maxwell's theory soon set experimenters searching for themeans of producing the long waves which he had predicted would be found.
Several authorities had before then stated their belief that the current derived from a Leyden jar was not simply a flow in one direction. They suggested, and gave grounds for the belief, that the current surged to and fro for some time before it settled down; that it swung to and fro, indeed, like a pendulum.
There may be some of my readers who are unacquainted with this interesting piece of electrical apparatus the Leyden jar. It is a convenient form of what is called an electrostatic condenser. This is two conductors, generally in the form of two plates with an insulator between them. In the Leyden jar the insulator is a glass jar, while the "plates" are coatings of tinfoil, one inside and the other outside. On connecting one coating to one pole of a battery, and the other to the other pole, they become charged, one positively and the other negatively. One, that is, acquires an excess of electricity, while the other becomes deficient to an exactly similar extent. When the two are afterwards connected by a wire the surplus on one flashes through it to make good the deficiency on the other.
Rushing first of all from positive coating to negative, electrical inertia causes it to overshoot the mark and to recharge the jar with the charges reversed. Then current begins to flow back again, doing the same several times over, until at last equilibrium is established.
The power to absorb and hold a charge of electricity, which is the characteristic of a condenser, is called "capacity."
What, then, is "electrical inertia"? I have already referred to the effect which the creation of a magnetic field around a current has upon neighbouring conductors. It also has an effect upon itself. As soon as the current begins to flow it builds up the magnetic field, and in the process some of its energy is exhausted. On the original current ceasing, however, the magnetic field collapses back on to the conductor once more and in so doing restores thatenergy. This occurs whenever current flows, but it is specially noticeable in long conductors, like submarine cables. In them the battery has to act for a considerable time before any current reaches the farther end. It is in the meantime employed in building up the magnetic field around the wire. Then when the battery has ceased to act the current still comes flowing out at the farther end—the magnetic field is giving back the energy expended upon it. Thus a current is reluctant to start flowing through a conductor, and, having started, is disinclined to stop. This is called "inductance," and it has exactly the same effect upon the current that inertia has upon a body. What inertia is to a material body inductance is to an electric current.
And lastly, the resistance which the conductor offers to the passage of the current is precisely analagous to the friction of the water in a pipe.
So, we see, the "capacity" of the two coatings of the jar and the inductance which occurs in the connecting wire cause the current to oscillate to and fro for a while when the jar is discharged, which surging or oscillation is ultimately stopped by the resistance of the wire. The two coatings and the wire form what is called an oscillatory circuit.
We can now resume our story.
After much experimenting Hertz, of Carlsruhe, discovered the fact that when a discharge was taking place in an oscillatory circuit tiny sparks passed between the ends of a curved wire held some distance away. His apparatus is illustrated in Figs. 6 and 7. The former, which is termed nowadays a "Hertz Oscillator," is simply two metal discs almost connected by a thick wire. The wire is broken, however, at the centre, and the two halves terminate in two metal balls. Each ball is connected to one terminal of an induction coil. Now the current comes from an induction coil in a series of spurts. It is not an alternating current exactly (since every alternate current is so feeble as to be negligible), but is practically an intermittent current always in the same direction. Thus we may call one thepositive end of the coil and the other the negative. A short current comes along with every backward movement of the little vibrating arm which forms a part of the apparatus. This breaking of the "primary" circuit may take place perhaps fifty times per second, so that the intermittent "secondary" currents will succeed each other at intervals of a fiftieth of a second, or even less. The brain reels at the attempt to think of a fiftieth of a second, but it is really quite a long interval as these things go, and during that interval quite a lot happens. For the current first of all charges the two plates as a condenser.