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 pa.s.s 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.
[Ill.u.s.tration: 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 ma.s.s 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 gla.s.s 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 pa.s.s. 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 pa.s.sing 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 pa.s.s 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 pa.s.s 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 pa.s.sed 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 _de_magnetise 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 pa.s.sing 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 gla.s.s 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 s.p.a.ce, it will be appreciable there. A telephone is arranged so that its current also pa.s.ses 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 pa.s.sage 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 pa.s.sed 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 pa.s.sing them through a "unidirectional" conductor--one, that is, which pa.s.ses 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 pa.s.s 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 antennae, 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 gla.s.s 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 gla.s.s. 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, antennae 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.
Antennae 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 antennae 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 quant.i.ty 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 grat.i.tude 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.
CHAPTER XIII
HOW PICTURES CAN BE SENT BY WIRE
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 const.i.tute 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 sh.e.l.lac varnish, indeed there are many substances which are available for use 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.
[Ill.u.s.tration: FIG. 13]
[Ill.u.s.tration: FIG. 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 pa.s.ses through it, it becomes changed into Prussian 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 pa.s.sage of the sending pen across a line, and taken together, as will be seen, they const.i.tute 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.