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.
[1] The glow-worm is an example of the few exceptions.
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 deg. to 1800 deg. 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.
[Illustration:
_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 deg. 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 deg. 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 CaC_{2}, 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 C_{2}H_{2} 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 (C_{2}H_{2}) 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 deg. 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.
[Illustration:
_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]