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 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 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 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 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 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 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 asserted 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 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 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 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 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.
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 balls; 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 balls.
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 passed between the two knobs the plates become connected almost as if a wire were passed 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).
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 glass. Some things appeared to be transparent to them, as clear glass 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 pass through a brick wall, for example. But the same divergence is to be observed between light and radiant heat, of which the action of glass is a familiar example. Clear glass 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 identity.
The difference between them, as has been said already, is the distance from crest to crest of the waves—the "wave-length," that is. And the reader will wonder by what manner of means this mysterious dimension can be ascertained. In spite of its seeming mystery the method is very simple.
It is based upon the fact that two sets of similar waves travelling at the same speed in opposite directions interfere with one another in a peculiar way. Suppose that one set of waves travel along to a reflector and strike it vertically; then another set will travel back from the reflector exactly similar to the first, except that their direction will be opposite. And the result will be that at certain intervals they will exactly neutralise each other, so that at those points there will be no wave-action appreciable at all. Those points where no action is to be perceived are called "nodes," and they are exactly half a wave-length apart.
This will be quite easily understood from the accompanying diagrams. In each of these diagrams the set of waves marked a are supposed to be moving from left to right, while those denoted by b are reflected back and are moving from right to left. It will be noticed that each wavy line has a straight line drawn through it, dividing it into alternate crests and hollows, which line is known as the axis of the waves.
Now notice that in Fig. 8 there are points marked x, where the a waves are just as much above the axis as the b waves are below it, and vice versa. Hence at those points the two sets of waves will neutralise each other.
Now turn to the next figure, which, be it remembered, shows the same waves a moment later, when they have moved a little farther on in their respective journeys, and it will be seen that there, too, are places marked x where the two sets of waves neutralise each other. And the same with the third diagram.
And finally observe that the places marked x are always the same in all the diagrams—that is to say, they are always the same distance from the line on the right-hand side, which denotes the reflector. It will be clear, too, that each node is half a wave-length from the next.
Thus it can be shown that at every moment, and not merely at the three indicated in the diagrams, the two sets neutralise each other at the nodes, that the nodes are always in the same places and half a wave-length apart.
Figs. 8, 9 and 10.—These diagrams help us to see how the "wireless waves" are measured. The a waves are supposed to be moving from left to right and the b waves from right to left. At the points marked x they neutralise each other. It is then easy to discover those points and the distance apart of any two adjacent ones is half the "wave-length." N.B.—In Fig. 10 the b waves fall exactly on top of the a waves.
Everywhere else, except at the nodes, there is action more or less energetic, but there is perpetual calm.
But how can we tell where the nodes are? When we recollect that they are points at which no wave-motion at all takes place it is easy to see that we shall at those points get no spark in our detector. So what Hertz did was to set his oscillator going so that it threw waves upon a reflecting surface and then move his detector to and fro in the neighbourhood until he found the nodes. Between the nodes, as will be seen by an inspection of the curves once more, there are other points at which the wave-action will be twice as great as with the single wave, and so at those points the response of the detector would be especially energetic.
This mutual action between an incident wave and a reflected wave is termed "interference," and by it the wave-lengths of all the ethereal waves have been measured. The plan used in the case of light waves, although the same in principle, is somewhat different because of the extreme shortness of the waves.
So the experiments of Hertz not only showed that long electric waves existed, but that they were in all essentials similar to light, and their wave-lengths were ascertained. On that basis has been built up modern wireless telegraphy.
It may be interesting to mention at this point a very curious, and in a sense pathetic, incident. Professor Hughes, whose name is associated with certain well-known instruments for ordinary telegraphy, nine years before Hertz' discovery noticed that a microphone was affected by the action of an induction coil some distance away. He himself attached some importance to the matter, but he allowed himself to be dissuaded from following up the discovery by other scientists, more eminent than himself at the time, who thought that it was not a promising field for investigation. But for the influence of these friends he would possibly be the hero of this story in place of Hertz.
Professor Silvanus Thomson has said that he too noticed the sparks produced at a distance when a Leyden jar was discharged, but he makes no claim to precedence over Hertz, since, seeing the phenomenon, he did not perceive its real meaning, while Hertz, though a little later in time, realised the profound significance of it.
Hertz himself in his account of his experiments is generous enough to assert that, had he not discovered the waves when he did, he is quite certain that Sir Oliver Lodge would have done so.
Before proceeding to describe the principal apparatus used in the wireless station I should like to devote a little space to the explanation of a term which will come up again and again, and which represents that which is responsible, in the main, for the marvellous advances which the art of sending wireless messages has achieved in the last few years. I refer to "resonance."
It will be a great help if the reader will try for himself a simple, inexpensive little experiment. Stretch a string horizontally across a room and on to it tie two other strings so that they hang down vertically a little distance apart. To the ends of the two strings tie some small objects—a cotton reel on each will answer admirably. They will thus form two pendulums, and, to commence with, they should be just the same length. Having rigged all this up, give one pendulum a good swing. It will impart motion of a to-and-fro variety to the supporting string, which in its turn will pass that motion on to the other pendulum. In a very short time, then, the second pendulum will be vibrating like the first. Indeed the whole motion of the first will shortly become transferred to the second, so that the second will be swinging and the first still. Then the second will re-transfer its energy back to the first, and so they will go on until the original energy given to the first pendulum is exhausted. The point to be observed is the quickness with which one pendulum responds to the impulses given it by the other, and the ease with which the energy of the one passes to the other.
Now reduce the length of one pendulum. On setting the first in motion a certain irregular spasmodic action is to be observed in the second, but it is very different from the "whole-hearted" response in the previous instance. In the former case the second one responded naturally and readily to the first. Now its response is reluctant in the extreme. It moves somewhat because it is forced to, but it is apparently unwilling. Energy has to be impressed upon it. There is no readiness, because there is no sympathy between them.
That sympathy between the two equal pendulums is "resonance." The same occurs between two violin or piano strings when they are "in tune."
The explanation is that a pendulum has a certain natural frequency which depends upon its length. Another pendulum of the same length, arranged as just described, therefore imparts impulses to it at just the frequency which is natural to it. Consequently the effect is a cumulative one, and it responds quickly. Impulses at any other frequency tend more or less to neutralise each other. In the same way a string, of a certain length and a certain tension, has a frequency peculiarly its own, and it will respond to another similar string because the other gives its impulses at its own natural frequency.
It is on record that an engine in a factory happened to run at precisely the same speed as the natural frequency of the building, with the result that after a little time the structure shook so much that it collapsed.
Now electrical circuits in which currents oscillate have a natural frequency of their own. That frequency depends upon the two electrical properties of the circuit: capacity and inductance. And if you want to set up an electrical oscillation in any circuit you can best do it by giving it impulses at intervals which agree with its natural frequency.
Sir Oliver Lodge seems to have been the first to appreciate fully the effects of resonance in wireless telegraphy. It is strange that in England the work of this eminent man in "wireless" matters is not more fully recognised. When wireless telegraphy reached the point at which the public became interested, Marconi was just coming to the front and so, for ever, will his name be foremost in the public estimation. Indeed more than foremost, for in the minds of many he monopolises the credit for this invention. Many people are under the impression that he is the one and only, or at any rate the original, inventor of wireless telegraphy.
Now Marconi has done exceedingly valuable work in this field. Moreover, he has been the means of placing the affair on a good commercial footing. But all the same he is by no means the original or only inventor. While admitting that he is a remarkable man, who has done wonders, it is only common justice to refer to the others whose contributions to the solution of the problem are possibly of equal value. And, of these, few can compare with Sir Oliver Lodge.
But to return to the question of resonance. At first the distances over which messages could be sent were but small. Now a marconigram can be flung across a hemisphere. At first little could be done by day, work had to be done mainly at night. Now communication passes by day and night alike. Yet in principle, and in many details, the instruments are unaltered from what they were several years ago. The main source of all this improvement is the use of resonance.
To enumerate broadly the apparatus used for the dispatch and receipt of messages the following list will be useful:—
Transmitting End
(1) An Antenna, consisting of a number of wires raised to a considerable height above the ground.
(2) A Spark-gap, consisting of a series of metal balls with gaps between them, the outer ones being connected to the antenna and to the induction coil.
(3) A powerful Induction Coil with batteries or other source of current to work it.
(4) A Telegraph Key, by which the induction coil can be started and stopped at will.
Receiving End
(1) An Antenna precisely similar to the other.
(2) A Coherer or other "oscillation detector."
(3) A Receiving Instrument which may be a writing telegraph instrument, a telephone, any of a number of ordinary telegraph instruments, or a galvanometer.
Transmitting and sending instruments are, of course, installed at both ends and either of them can be connected to the antenna at will by the simple movement of a switch.
The antenna plays the part of one of the metal plates in the Hertz oscillator. Early experiments were made with Hertz apparatus, but the range of such a contrivance is very limited. For one thing, it neglects to take advantage of the earth. It is little realised what an important part the earth plays in the carrying of wireless messages. A very great step was taken when Marconi dispensed with one of the plates of Hertz, and used the earth instead; while the other plate gave place to the elevated wires, the most familiar part of the apparatus to most people.
The condenser is thus formed by the earth as one plate, the elevated wires as the other, and the intervening air as the insulator. The "capacity" must be exceedingly small in such an apparatus, but it is sufficient; while the long lines of electrical force stretching from the high antenna to the earth produce waves of great carrying power. Lastly, when the earth forms a part of the condenser the waves cling to it, so that instead of being largely dissipated into space, they move along the surface of the earth. The advantage of this is obvious.
At first it was customary to place the spark-gap in the wire leading from the antenna to the earth, as in the accompanying sketch. Later, however, it was found better to place the coil and spark-gap in a local circuit in which the oscillations are first produced. These oscillations pass through a coil which is interwound with another one connected to the antenna and to earth, and thus the local oscillations, as we might call them, induce similar oscillations in the antenna, just as the fluctuations in one part of an induction coil induce fluctuations in the other. Indeed the coil in the local circuit and the one in the antenna circuit actually constitute an induction coil.
The advantage of this is that by introducing condensers the capacity of which can be varied, and coils the inductance of which can be varied, into the oscillation circuit it becomes possible to "tune" the circuits effectively. Thus resonance comes into play and the power expended can be made to produce the maximum effect.
Some attempts have been made to displace the induction coil in wireless telegraphy altogether by a specially made dynamo. These machines can produce either alternating or continuous currents, in fact the alternating current dynamo is really simpler than the more familiar continuous-current machine. The difficulty is, however, to run it sufficiently fast to produce sufficiently rapid alternations. Nicola Tesla made an alternator (to give the alternating current dynamo its short title) which could produce 1500 alternations per second, while Mr W. Duddell made one which produced 120,000, but neither was satisfactory for the work in question. Could such a machine be made, it would be invaluable, for it will be apparent that a continuous succession of waves would be formed by it and not a succession of short trains of waves such as is produced by the induction coil and spark-gap. The difficulties are not electrical, but mechanical. It seems doubtful if a machine will ever be made to run with sufficient rapidity which would not knock itself to pieces in a very short time.
Fig. 11.—The simplest form of wireless antenna.
Small alternators are used sometimes, however, to supply alternating current to the primary of an induction coil, or transformer, as it is more often called in its larger sizes. The interrupter is only needed when the primary current is continuous—from batteries, for example. Alternating current needs no interrupter, and so that bother is removed. The alternations of a hundred or so per second, which are quite the common thing with alternators, are just what is needed to excite an induction coil. Consequently small machines of this kind are to be found in many stations.
A Danish inventor, Valdemar Poulsen, has adopted an altogether different method of producing electrical oscillations, which method is the distinctive feature of his mode of telegraphy. He takes advantage of a curious effect of passing current between two rods, one of which is carbon, so as to form an arc such as we see in arc lamps.
My readers are already familiar with the term "shunt" in connection with electrical matters, and so will perceive at once what is meant when a second circuit is said to be arranged as a shunt to the arc. The accompanying diagram will in any case make the matter clear.
The current comes along from the battery or continuous-current dynamo to a hollow rod of copper which, to prevent it being melted, has cold water continually circulating inside it. Thence the current jumps across to a carbon rod, forming an arc between the two rods, and returns whence it came. In its journey it traverses the coils of an electro-magnet, the poles of which are one each side of the arc. This tends to blow the arc out, as a puff of wind blows out a candle, an effect which a magnet always has upon an electric arc.
The shunt consists of a wire leading from the copper to the carbon rod with a condenser and an inductance coil inserted in it. The latter coil also forms one part of that coil by which the oscillations in the local circuit are transferred to the antenna.
The electrical explanation of what happens when the current is turned on to an arrangement like this is rather too complex to set out here. It depends upon a curious behaviour of the arc. It is really a conductor, yet it does not behave as ordinary conductors do, and the result is that the continuous current flowing through the arc is accompanied by an oscillating current in the shunt circuit. And the important feature of the arrangement is that these oscillations are continuous, in one long train, not in a succession of trains. The advantage of this has already been referred to.
One other feature of the apparatus just described should be mentioned, since it will seem curious to the general reader. For it to work properly it is necessary that the arc should be enclosed in a chamber filled with hydrogen or a hydro-carbon gas. Coal-gas is generally used.
Hertz' original discovery was that small sparks could be seen to pass between the ends of a curved wire when the electric waves fell upon it. Such "spark detectors," as they are called, are useful in the laboratory, but not for practical telegraphy.
Fig. 12.—Diagram (simplified) showing how Poulsen generates oscillations. Current from a dynamo flows through the arc, whereupon currents oscillate through the condenser and coil (as described in the text).
Several people seem to have noticed in years gone by that a mass of loose metal filings, normally a very bad conductor of electricity, became a much better conductor when an electrical discharge of some sort occurred near by. The demand for a wireless receiver had not then arisen, however, and so the discoveries were not followed up. Consequently it remained to be rediscovered by Branly, of Paris, in 1890. He placed some metal filings in a glass tube, the ends of which he closed with metal plugs. Lying loosely together the filings would not conduct the current of a small battery from one plug to the other, but when a spark occurred not far away they suddenly became conductive and allowed it to pass. Several years after this Sir Oliver Lodge took up the idea as a receiver for wireless messages, and believing that its action was due to the waves causing the filings to cling together, he christened it "Coherer."
Marconi succeeded in making a very delicate form of this, although working on strictly the same lines.
The trouble with a coherer is that when once it becomes conductive it remains so unless the filings be shaken apart. Lodge therefore arranged for the tube to be continually struck by clockwork or by a mechanism like that of an electric bell. Marconi effected a further improvement by making the current passing through the coherer control the striking mechanism, so that the latter is normally quiet but administers one or two taps at just the right moment.
Sir Oliver Lodge and Dr Muirhead devised another detector which, though quite different in form, is really much the same in principle. A steel disc with a sharp knife-like edge is made to rotate above a vessel of mercury. The edge just touches the mercury but no more. On the top of the mercury there floats a thin layer of oil, a bad conductor. Now as the disc revolves it picks up on its edge a film of oil, which it carries down into the mercury. The film adheres so tightly that it prevents the moving disc from actually touching the liquid metal. Thus, under normal conditions, the two are electrically insulated from each other by the film of oil and no current can pass from mercury to disc. Oscillations, however, caused by incoming electric waves, are able to break through the oil film and so bring disc and mercury into contact, whereupon the current flows. The constant movement of the disc restores the oil-film as soon as the oscillations cease.
The reason why these detectors act as they do is not quite understood. One suggested explanation is that the oscillating currents heat the particles and so partially weld them together. Another is that adjacent particles become charged as the plates of a minute condenser, and so are drawn tightly together as the plates in an electrostatic voltmeter are drawn towards each other. Supposing that the original non-conductivity of the loose filings be due to the film of air which may surround them, either of these things would account for the film being broken or squeezed out, resulting in better contact and improved conducting power. But both suggestions seem to be contradicted by the fact that if the pieces in contact be of certain substances the coherer works the opposite way. Under those conditions the conductivity is normally good, but the influence of the incoming waves causes it to become bad.
In 1896 Professor Rutherford, now of Manchester, described some discoveries which he had made as to the magnetic effects of oscillations. A simple little contrivance which he had constructed was operated by the discharge of a coil half-a-mile away, at that time a great performance. This detector was simply an electro-magnet with a steel core instead of the usual soft iron core. The reason the latter is used in the ordinary magnet is that it loses its magnetism the moment the current ceases to pass through the coil with which it is surrounded, while a steel core retains its magnetism. For most purposes a steel core would render an electro-magnet useless, but in this case it was desired that the core should be permanently magnetised. So a current was first passed through the coil to magnetise the core, and then the coil was connected to a simple form of antenna while a swinging magnet was brought near so that the magnetic power of the core would be indicated and any change made apparent. The effect of the discharge half-a-mile away was to demagnetise the core slightly. This was shown by the movement of the swinging magnet, and so the first "magnetic detector" was found.
But here, perhaps, I ought to explain the use of the antenna at the receiving station—its function at the sending end has already been made clear. The electro-magnetic waves, coming from the distant transmitter, strike the receiving antenna and in so doing set up in it oscillations such as those which set them in motion. For every oscillation in the sending antenna there will be another, similar in every respect except that it will be feebler, in the receiving antenna. And the oscillations are here led to the detector, of whatever form it may be, and in it they make their presence felt.
In some few cases a Duddell thermo-galvanometer has been employed as the detector, in which the oscillating currents report themselves directly. In coherers the detector works by causing the oscillating currents to control a continuous current from a battery and it is the latter which actually gives the signal, but there are a number of extremely interesting means which have been invented to detect the oscillating currents by their heating effect.
R. A. Fessenden, for instance, has perfected one which is a marvel of delicate workmanship. He depends upon the heating of a wire by the currents passing through it. Such heating is the result of the electrical force acting against resistance, and the difficulty is that if the resistance be great it will almost entirely kill the faint oscillating forces in the receiving antenna, while if, on the other hand, it be small, the rise in temperature will be inappreciable. So he encloses a fine thread of platinum in a glass bulb from which the air is exhausted. The platinum wire is first of all embedded in a wire of silver: the silver wire is given a core of platinum, in fact. Then the compound wire is drawn down until it is so thin that the platinum core is only one and a half thousandths of an inch in diameter. A short length of this compound wire is then bent into a U-shaped loop and its ends connected to thicker wires. Finally the bottom of the loop is immersed in nitric acid, which eats away the silver at that point and leaves the bare platinum. Thus is produced a very short length (a few millimetres) of exceedingly thin platinum wire supported at its ends by comparatively thick wires.
Being so short, this wire does not offer much resistance, and consequently does not materially check the oscillations. At the same time, since it is so fine, it does offer some resistance, and finally, since what heat is generated will be in an exceedingly small space, it will be appreciable there. A telephone is arranged so that its current also passes through the fine wire, and every slight variation in the temperature of the platinum wire, by varying its resistance, varies the current through the telephone. And exceedingly slight variations can be detected by sound in the telephone. Thus the oscillations generated in the antenna affect the heat in the wire; that affects its resistance; and that again affects the telephone, which, finally, affects the ear of anyone who is listening to it. It must be understood, however, that this is not a wireless telephone, for the sounds heard are not articulate but merely long and short sounds, representing the dots and dashes of the "Morse Code."
Electrolysis provides us with another form of detector. An exceedingly small platinum wire forms one electrode and a large lead plate the other, and both are immersed in dilute acid. The passage of current from a local battery sets up electrolysis, and so stops itself by forming a film of oxygen on the small electrode. This film, however, is broken by the oscillating currents from the antenna, so that as long as they are coming the battery current can flow, but as soon as they cease the battery current stops itself again. Thus the flowing and stopping of the oscillating currents is exactly copied by the current from the battery, which current is led through a telephone or a sensitive galvanometer.
It may occur to readers to inquire why the oscillating currents are not passed direct to a galvanometer. The answer is that because they are oscillating a very sensitive galvanometer is not possible.
True, the Duddell thermo-galvanometer has been mentioned in this connection, but although it is a beautiful instrument it cannot compare for delicacy with the direct-current galvanometers. The latter are easily a hundred thousand times more sensitive. But the trouble can be overcome by "rectifying" the oscillating currents, by passing them through a "unidirectional" conductor—one, that is, which passes current one way only. These remind one of a turnstile as installed at certain public places, which let you out but will not let you in unless you pay. In fact they will not let you in at all. In like manner "rectifiers" will only allow those currents to pass which are flowing in one direction, and so they cut out every alternate oscillation, thus producing something very like continuous current, which can be detected by the very delicate galvanometers which are usable where continuous currents are concerned, or more often by a telephone receiver. The rectifying conductors are in many cases crystals, hence these detectors are called "Crystal Detectors." Carborundum is a favourite for this purpose.
And that brings us to the important question of the secrecy of wireless communication, and the measures taken to prevent confusion from the number of independent messages flying through the air at the same time.
This can be largely achieved by the aid of resonance. Trains of waves flung out by one antenna may strike several other antennæ, but unless the latter are in tune with the sending apparatus they will probably not be affected appreciably. Let one of them, however, be in tune, and it will pick up easily the message which is not noticed by the others. It is as if three people watching a distant lamp were affected by a form of colour-blindness which rendered them practically blind to all colours except one. Suppose one could see red only, the other blue and the third yellow. A light sent through a blue glass being robbed of all rays except the blue ones would be visible only to the man who could see blue. The man who could see blue would, in like manner, be quite blind to light sent through red or yellow glass. Each of them, in fact, could be signalled to quite independently of the others by simply sending him rays of the colour to which his eyes were sensitive. In precisely the same way each wireless receiver is or can be made most sensitive to waves of a particular length and practically blind to all others. The operator can adjust his apparatus for certain prearranged wave-lengths, and so he can communicate with secrecy to stations whose wave-length he knows. The change, of course, is made by altering the capacity, or inductance, or both. The instruments can be so calibrated that it is quite easy to make the alteration.
Then, antennæ can be so constructed that messages can be received with most readiness from one particular direction. In others, they can be received from any direction, but the direction can be discovered. This, it will be easy to see, is of great value to ships in a fog.
Antennæ made with a short vertical part and a long horizontal part radiate best in the direction away from which their horizontal part points. This is of great advantage in stations which are built specially to communicate with other particular stations. In such cases the antenna is carefully built, so as to point in the required direction. Such antennæ also receive more readily those signals which come from the direction away from which they are pointing.
Reference has been made already to the interesting fact that wireless communication is easier at night than in the daytime. That is probably because of the "ionisation" of the atmosphere by the action of sunlight. Along with the visible sunlight there comes to us from the sun a quantity of light known as "ultra-violet," since it makes its effect known in the spectrum of sunlight beyond the violet, which is the limit of visibility at one end of the spectrum. We cannot see it but it affects photographic plates powerfully. It has energetic chemical powers, and it has the ability to make the air more conductive than it is ordinarily. Comparatively little of it penetrates our atmosphere, but it must exercise a good deal of influence a little higher up. Now readers will remember that the process by which electro-magnetic waves are propagated is checked when the waves strike a conductor. The energy in the waves is then employed in causing currents in the conductor instead of forming more waves. And so partially conductive air forms a partial barrier to the waves. The effect is not appreciable in the case of the tiny waves of light and heat, but it is in the case of the long "wireless waves." Everyone has seen the waves of an advancing tide coming up a sandy beach, and has noticed how the dry sand (a good conductor of water) sucks up and destroys the foremost ripples. In like manner are the wireless waves "sucked up" by the partially conductive atmosphere. But the effect of the ultra-violet light does not last long, and so, at night-time, it disappears. Therefore messages can be sent better at night than by day.
For wireless telephony what is wanted is a continuous uninterrupted train of waves, such as those from the "Poulsen arc," and a receiver of the magnetic type. The coherer is no good for this purpose, since it either stops the current entirely or lets it flow copiously. The magnetic detectors, however, respond to the variations in the strength of the incoming waves. As the latter increase or decrease in strength so does the magnetic detector give out stronger or weaker signals. So a telephone transmitter of the ordinary type is made to vary the strength of the oscillations at the sending end, while an ordinary telephone receiver is placed in series with the detector at the receiving end. Thus every slight variation corresponding to sound waves spoken into the transmitter is reproduced in the receiver.
It is strange that wireless telephony has not made greater progress, for it may be said, on the word of one of the greatest authorities, that wireless telephony is simpler and easier than telephony through a submarine cable. In the latter there are almost insuperable obstacles caused by the capacity and inductance of the circuit, while in the wireless method there is very little difficulty.
There are, of course, several so-called "systems" of wireless telegraphy in use. There is the Marconi in Great Britain; the secret Admiralty system in the British Navy; the De Forest in the United States; the Telefunken in Germany, not to mention the promising Poulsen system. And there are still others. But it would be futile to attempt to explain how they differ from one another in a work like this. In principle they are alike. The precise forms of instrument used may vary, but even there there is much in common between them. As time goes on there will inevitably be a tendency to more and more uniformity. That is always the case, for some things are inherently better than others, and rival systems, although each is working along its own lines, always come to very much the same result in the end. Without making any comparisons, it is safe to say that if the Telefunken system, for example, has any points of superiority over the Marconi, the latter will sooner or later find out the fact, and will modify their apparatus accordingly. In all probability this will operate both ways, and some things which the German system is now using will give place to those which the British have in operation.
In another very modern industry this is very apparent. Having attended and carefully studied several annual exhibitions of flying machines, I have noticed with great interest how the varying types of a few years ago are merging into the more or less uniform types of to-day. And it has been the same with wireless telegraphy, and will be still more so in the future.
The best means of generating the waves and the best means of detecting them at a distance—that is the whole problem, and all the workers in it will sooner or later come to much the same conclusions as to which are the best ways.
Patents may do a little to delay this, but not much. For one thing, patents only last a few years. For another, a patent only covers a particular way of doing a particular thing. A machine that is termed "patent" is often the subject of a hundred patents, each covering a particular little point. It is well-nigh impossible to patent a whole machine. A general principle cannot be patented, only a particular application of that principle, and so there are in a great many cases little variations of a patented method which are quite as good as the patented one, and which can be used freely. So even patents will not have much effect, in all probability, upon this unification process.
But, however that may be, there is no doubt that the whole world owes a deep debt of gratitude to the men who have worked out this most beneficent of inventions. It is difficult to think of a single one which has ever brought such a load of benefits to poor, struggling humanity as this has. The ship in distress, the lighthouse man on his lonely islet, the explorer in the Polar regions, the pioneer settler in the new lands—in fact, just those who most need some connecting link with their fellows—are the people to whom the wireless telegraph brings aid and comfort. All honour to the men who have done it.