INTENSE HEAT
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 of heat, 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 touching begins 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, will be 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.
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 easily happen 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 is a 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), is easily 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 C2H2 comes 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 other kinds 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 the metal 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 a crucible 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 the material 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 quarter