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.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 airin 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."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 factis 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 markedaare supposed to be moving from left to right, while those denoted bybare 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 theawaves are just as much above the axis as thebwaves 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 twosets 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.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, butthereis 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 usedin 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 thewholemotion 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 beimpressedupon 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 thisfield. 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 inductanceof 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.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, whichare 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 oscillationsare 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).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 whichhe 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 oscillatingcurrents 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 todemagnetise 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 sendingend has already been made clear. The electro-magnetic waves, coming from the distant transmitter, strike the receiving antenna and in so doingset 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, andconsequently 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 ahundred thousand timesmore 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 youinat 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. Theoperator 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 apartial 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 wirelesstelephonywhat 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 attemptto 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 canbe 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.
The sending of a message by telegraph is easily understandable. Various combinations of two simple signs, such as short sounds and long sounds, can readily be made to indicate letters by which the words can be spelt out.
Nor does the sending of sound over a wire make a very great demand upon the credulity. We all know that sound consists of innumerable little waves in the air, and by the simplest of devices these can be converted into variations in an electric current, which variations, by means equally simple, can be made to re-convert themselves back into sound waves at the other end.
But to transmit a picture is another matter altogether. It seems barely possible in the case of a drawing such as a pen-and-ink sketch, which consists of a comparatively small number of definite lines; but with a shaded sketch or a photograph, with its gradations of light and shadow—to transmit such would seem to be beyond the bounds of possibility, did we not know that it has been done. The description of the methods will therefore constitute a not uninteresting subject for a chapter.
It is worthy of remark that an attempt along these lines was made many years ago by a man named Caselli, and a description of this pioneer apparatus will form a good introduction to the later developments.
In Fig. 13 we see a square which represents a sheet of tinfoil, upon which is drawn, in non-conductive ink, a simple geometrical figure. The ink may be grease, or shellac varnish, indeed there are many substances which are available foruse as an insulating ink. Across the square there are a number of parallel dotted lines, but these, it must be understood, are not actually drawn upon the foil—their purpose will be apparent in a moment.
Suppose that we connect the foil to one pole of a battery, and the other pole by a flexible wire to a metal pen or stylus. If we place the point of the pen in contact with the foil, we make a complete circuit, through which, of course, current will flow. But if, with it, we touch one of the non-conductive lines, there will be no current.
Fig. 13Fig. 13
Fig. 14Fig. 14
Taking a ruler, then, let us draw the point of the stylus across the foil in a series of parallel straight lines. It is these excursions of the stylus which the dotted lines are intended to represent. For nearly the whole of the time current will be flowing; but whenever the stylus is crossing one of the lines of non-conductive ink there will be a momentary cessation. Thus, the reader will begin to perceive, we obtain what we may call an electrical representation of the figure drawn upon the foil.
And now let us turn to Fig. 14. There, too, is a square, but in this case it is not foil, but paper which has been soaked in prussiate of potash. The reason for introducing this chemical is that it is susceptible to electrical action. Wherever current passes through it, it becomes changed intoPrussian blue, so that if we place the point of a pen upon the paper, and cause current to flow out of that point through the paper, there we get a blue dot. If, while the current is flowing, we draw the pen along, we get a blue line.
Fig. 13 therefore represents in principle the sending apparatus of Caselli's writing telegraph, while Fig. 14 represents the receiving instrument. The two pens are connected together by the main wire, in such a manner that, when the point of the one is in contact with the bare foil current flows out of the other and into the paper; but as the former crosses an ink line all current ceases.
If, then, while the sending pen is drawn line by line across the foil, the other is drawn at the same speed, line by line, across the chemically prepared paper, we shall get on the latter a series of lines as shown in Fig. 14 almost continuous, but broken here and there. Each breakage represents a passage of the sending pen across a line, and taken together, as will be seen, they constitute a reproduction of the geometrical figure drawn upon the foil. As shown, the lines are rather far apart, and so the reproduction is not a very good one. They are only drawn so, however, in order that the principle may be shown the more clearly. They may be drawn so that they overlap, and then the effect is very much better, the received picture being almost an exact reproduction of the other.
It will be noticed that an essential to the success of this method is that the two pens should move in perfect unison, and that was the great difficulty. Caselli used an arrangement of pendulums, the best thing available at the time.
The reproduction is, in photographic language, a negative, a somewhat unsatisfactory feature of the method. A simple modification, however, of the electrical connections will reverse that, so that the reproduction shall be a positive. There are two ways of cutting off a current from any particular circuit. One is to interpose a resistance, through which current cannot pass in an appreciable quantity, andthe other is to provide a second path for the current so much easier than the first that practically all the current will pass that way, leaving the first circuit, to all intents and purposes, free. It is as if a farmer wished to stop people passing across a certain field. Two methods would be open to him: one to put up a high gate over which no one would dare to climb, and the other to provide a short cut so much more pleasant and convenient than the old path that no one having the choice of the two ways would think of going the old way.
What the farmer would call a short cut the electrician calls a short circuit, and a short circuit is often a more convenient way of cutting off a current than a switch which interposes resistance. At all events, in a case like this, a short circuit enables that to be accomplished which would be very difficult by any other means.
In the apparatus as already described the battery had to drive the current along a long wire, terminating at the distant receiving instrument, whence the current returned via the earth. The foil and pen, acting as a kind of electrical "tap," controlled this. When foil and pen touched, the tap was open and current flowed. When the line of non-conductive ink interposed itself, the tap was off and the flow ceased.
But connect the battery directly to the wire, and place the foil and pen in a short branch circuit, and the whole thing is reversed. Then the opening of the "tap" sent current to the other end; now the opening of the tap causes it to flow round the short branch and leave the main wire. Then the closing of the tap stopped the current reaching the farther end; now it causes it to do so. In fact, the entire action of the apparatus is completely reversed, and the bare parts of the foil become represented by blank paper, while the insulating lines produce the marks. In short, a positive results instead of a negative.
Such was the scheme of Caselli years ago. It is mentioned here at some length, since the principle of it is largely re-usedin an improved form in the most successful of modern apparatus for a like purpose.
It undoubtedly was a very excellent scheme, simple and effective, which ought to have succeeded; but it did not do so, for the sufficient reason that at that time knowledge of electricity and skill in constructing delicate mechanism were not so highly developed as they are to-day. For success, as has already been said, one thing was essential, and that thing very difficult to obtain—a perfect synchronism between one stylus and the other. If the one were but the slightest degree "out of step" with the other, failure followed inevitably.
So the electrical transmission of sketches dropped for the time being. More recently a perfectly successful solution of the problem has come in another way altogether. This apparatus, at first called the telautograph, but now known as the telewriter, it will be more convenient to refer to later.
Of modern systems for the transmission of pictures the most successful, probably, are the Korn telautograph and the Thorn-Baker telectrograph.
Both of these are able to transmit very fair reproductions of photographs besides line drawings. The difficulty with photographs is, of course, that many parts of them are not of equal blackness or whiteness, but shade off gradually from one into the other. Take the case of a simple portrait. Part of the subject's face will be pure white, while the side in shadow will be comparatively dark. There is no hard and fast line between the two, but by a gradation through an infinite number of shades the one tones into the other. How can it be possible to convey that, more or less mechanically, over a wire? The solution is due to the fact that the eye will blend together a number of distinctly different shades, if properly arranged, into a gradual change. Really the change is step by step, but the effect is apparently quite continuous. This can be seen in the "half-tone" illustrations in this book. Close examination will show that such a picture is cut up into small squares. In the pure whitepart the squares are invisible, while in the perfectly black parts, if there be any, they are so merged into one another as to be inseparable. But everywhere else in the picture it will be seen that there are squares each with a dot in the middle. In the darker parts the dots are large; in the lighter ones they are small. We get the effect almost of colour, although the picture is done entirely in black ink. The eye does not see the individual dots when we are just looking at the picture; we have to examine it very closely to find them. Yet they are there all the time, and it is simply the peculiar action of the eye which sees beautiful half-tones, shading imperceptibly one into another, whereas in real fact there are only a vast number of equidistant dots, all equally black.
We see, therefore, that it is possible to split up a picture of any kind into a number of small squares and to treat each square as being of equal darkness throughout. Then, if we can communicate by wire that particular degree of darkness to a distant station, where the small parts can be put together in their proper order and given their correct shade, the picture as constructed at the receiving end will be something like that at the sending end. And we have only to make the size of each separate square small enough to obtain a copy which will resemble the original very closely indeed.
In the early days it was actually proposed to telegraph pictures by ordinary telegraphy, using this principle. The suggestion was to agree upon a code of twenty-six shades, each called by a letter of the alphabet. One shade was to bea, the nextb, and so on. Then the picture was to be divided up into squares, and the particular shade of each square telegraphed by means of the corresponding letter. The shades thus communicated were to be put together at the receiving end, on a prearranged system, and so the picture was to be built up. Given plenty of time, that scheme might be moderately successful, but to get a really good reproduction the subdivision needs to be so minute, and the number of squares, therefore, so immense, that it would be quicker to send the picture by train than totelegraph it by such laborious means. In a fairly coarse half-tone block the squares are, say, 2500 to the square inch. That number of letters would therefore have to be telegraphed for every square inch of picture transmitted, to say nothing of the difficulty of building up a picture of such a great number of parts and giving to each the desired shade. That idea, abortive though it is in its crude form, illustrates very clearly the fundamental principle on which this work is done.
The problem is really to devise a machine which will do that same thing rapidly and automatically divide up the original into a large number of squares, and then send an electric current to represent each square, such current by its strength to indicate the shade of the square: and finally a similar instrument is needed to act as receiver, and to reproduce those squares in the proper order, giving to each its correct shade.
In practically all of them the mechanism is rotatory, the original being placed upon a drum which turns round under a stylus, or its equivalent, while the stylus gradually travels along from end to end after the manner of the needle of a phonograph, or else the same result being achieved by the drum itself having an endwise movement as well as a rotative one. The receiving instrument is of similar form, and both must start together, move at the same speed and indeed preserve a perfect correspondence with each other.
If the distance be great between the two there may be difficulties due to the "retardation" of the currents passing between them. Electricity does not pass through long wires, particularly if they be under the sea, with anything like the quickness which we are apt to think. Over a short line and under favourable circumstances the receipt of a telegraph signal at the farther end is practically instantaneous, but on long lines, and under certain conditions, that is far from being the case.
Then something has to be done to quicken the action of the current, or else the receiving drum must bemade to lag behind the sending drum by the requisite amount. In some cases, too, the transmitting apparatus loses a little time in sending off the currents, and that, too, has to be allowed for, so that, all things considered, the reader will see that the successful solution of this problem is hedged about with many subtle difficulties which are probably only appreciated by those who are well acquainted by sad experience with the little vagaries of both electricity and mechanical devices. Neither of them does quite what we want it to do; each suffers from little faults, which in the case of a delicate problem like this, where a difference of a hundredth of a second would be fatal to success, introduce difficulties almost insuperable.
To transmit line drawings, Professor Korn uses a sending instrument very like that of Caselli. The picture is placed, either by hand or photographically, upon a sheet of copper foil, which is fixed round the rotating cylinder, the lines being formed of non-conducting material. The foil being electrified and the stylus connected to the "line" or main wire, currents pass to the farther end just as in the old apparatus.
At the receiving end the drum is covered with photographic paper and enclosed in a light-tight box. Through a hole in this box a fine pencil of light passes from a lamp, suitable lenses being used to ensure that the pencil shall have, as it were, a very fine point, producing a very small spot of light upon the paper. If the light remains quite steady, the drum meanwhile rotating, a line will be drawn by it upon the paper which will be visible when the latter is developed. Since the drum not only turns upon its axis, but also moves endwise one hundredth of an inch at every revolution, this line will be a spiral, the turns of which will be one hundredth of an inch apart. Thus the paper will be blacked, practically uniformly, all over. Should the intensity of the light vary, however, the line will at times be lighter than at others, while, should it be cut off altogether for a moment, then there will be acorresponding gap in the line, and it is easy to see that if these lighter parts or gaps occur in the correct places they will form a picture. In other words, by controlling that light we can build up a picture upon the paper. The question is how to control it.
Professor Korn uses a form of the Einthoven galvanometer already described. Instead of the silvered fibre generally employed in this instrument, a silver wire is fitted, the movement of which partly or entirely cuts off the pencil of light.
The Korn transmitter for photographs is quite different, although the receiver is practically the same as what has just been described. The basis of it is a peculiar power possessed by the metal selenium when in a certain state. This, like all metals, is a conductor of electricity, but of course offers resistance in some degree. Now the special feature of selenium is that its resistance is reduced if light shine upon it. Suppose, then, that current be flowing through a mass of selenium and that the latter be suddenly illuminated brightly, the resistance will at once fall and the current increase. On the other hand, should the light falling upon the selenium diminish, its resistance will increase and the current flowing through it will decrease. In short, given a suitable arrangement, the current flowing in a circuit of which a selenium "cell" forms a part will increase or decrease with the increase or decrease in the light falling upon the cell.
A while ago the papers were telling striking stories of a way by which blind people, so it was said, were to be recompensed for the loss of their sight—a new sense, as it were, was to be given them by which they could "hear" light, even if they could not see it. All this had reference to this curious property of selenium, it being, of course, an undoubted fact that it will vary an electric current in accordance with the variations in the light, and if that current be led through a telephone receiver a man, by holding that to his ear, could, in a sense, hear the variations in the light.