Marvels of Scientific Invention Part 12

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 s.p.a.ce 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 s.p.a.ce 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 sh.e.l.l of the gallery holes are drilled, and to these the metal bottles or flasks are screwed, a pipe leading from the mouth of each bottle well in towards the centre of the gallery.

The end of this tube is closed by a cap of gla.s.s 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 gla.s.s, 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 gla.s.s 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 a.n.a.lysed.

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 pa.s.sing 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 pa.s.s 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 gla.s.s cap would not be broken until some considerable time had elapsed after the explosion has pa.s.sed 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 ill.u.s.trations).

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 a.n.a.lysis 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 at which the fan was blowing air _in_. 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 c.o.ke 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 c.o.ke 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 fine powder, 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.

CHAPTER XII

THE MOST STRIKING INVENTION OF RECENT TIMES

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 another _at a distance_ is 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 s.p.a.ce referred to, but that cannot be the carrier of light, for if we look through a gla.s.s vessel from which the air has been exhausted we see distant objects undimmed. 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 s.p.a.ces in the whole universe, pa.s.sing, 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 pa.s.sage 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 satellites are 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 pa.s.sed 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, pa.s.sing 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 Clerk Maxwell, 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 a.s.serted that in all probability there were others of which men had then no practical knowledge.

Maxwell's theory soon set experimenters searching for the means 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 gla.s.s 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 that energy. 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 pa.s.sage of the current is precisely a.n.a.lagous 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 pa.s.sed between the ends of a curved wire held some distance away. His apparatus is ill.u.s.trated 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 b.a.l.l.s. 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 the positive 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.

[Ill.u.s.tration: FIG. 6.--The apparatus by which Hertz made his discoveries, hence called the Hertz Oscillator. _a a_ are metal plates; _d_ is the spark-gap between the two metal b.a.l.l.s; _b_ is the battery, and _c_ the induction coil.]

When they are as full as they will hold the current overflows, as it were, across the gap between the two b.a.l.l.s.

Now an air-gap--a gap that is filled with air, between two conductors--is a very strong insulator. But when current has once broken through it it becomes a fairly good conductor. Hence as soon as the first spark has pa.s.sed between the two k.n.o.bs the plates become connected almost as if a wire were pa.s.sed from one to the other. And there we have quite a good oscillatory circuit. There is capacity at each end and a fairly long length of wire to provide the inductance. Consequently that breakdown of the insulation of the air in the spark-gap is followed by electrical oscillations which take place with inconceivable rapidity.

Yet because of the resistance of the spark-gap, which is considerable even after it has been broken through, the oscillations do not continue for long. They have died away long before the lapse of a fiftieth of a second, when the next impulse comes along from the coil. In the meantime the air-gap regains its insulating properties, and so, on the arrival of the next impulse, the whole thing occurs once more.

Thus a little train of oscillations is produced for every impulse from the coil. Every train causes a corresponding disturbance in the ether, and sends off a train of electro-magnetic waves, and these, falling upon the distant wire, generate in it a train similar to that which brought them into being. These trains, in Hertz' simple apparatus, manifested themselves in the form of minute sparks leaping across the small gap between the ends of the curved wire (Fig. 7).

[Ill.u.s.tration: FIG. 7.--Hertz "Detector." It was with this simple apparatus that Hertz discovered how to detect the "wireless waves."]

It was in 1888 that Hertz made this discovery of a way to detect long electric waves. He subjected the matter to many more experiments and found that the waves have many points in common with light rays. He found that they were reflected from certain surfaces, just as light is reflected from the surface of a mirror. He made prisms which were able to bend them as light waves are bent by a prism of gla.s.s. Some things appeared to be transparent to them, as clear gla.s.s is to light, while others are opaque. It does not follow that the same things which reflect light waves reflect electric waves, and so on. The latter can pa.s.s through a brick wall, for example. But the same divergence is to be observed between light and radiant heat, of which the action of gla.s.s is a familiar example. Clear gla.s.s will let light through almost undimmed, yet we use it for fire-screens to shield us from too much radiant heat.

The important fact is that all three--light, radiant heat and Hertzian waves--in addition to travelling at the same speed, are reflected, absorbed or refracted, according to precisely the same principles. This is almost perfect testimony to their essential ident.i.ty.