We all know how things have progressed since then. A telegram by Marconi is as commonplace to-day as a telegram by cable. The British Government is now engaged upon a series of stations dotted about the globe in such a way that every part of the widely separated British Empire shall be in constant touch with every other part by wireless telegraphy. In other words, the range of the system has now become such that nothing further is needed.
The British Admiralty has a few wires slung to posts on the top of the offices in London, and those few wires enable touch to be maintained with ships. As almost every intelligent newspaper reader in Great Britain knows, the Germans were in the habit, during the war, of sending news to the United States by wireless telegraphy, which news was always picked up by the Admiralty installation and circulated to the British newspapers, often to the amus.e.m.e.nt of their British readers.
The famous _Emden_, too, which had such a run of success until it encountered the Australian cruiser _Sydney_, met its end entirely through the intervention of wireless telegraphy.
These incidents give us a good idea of the usefulness of wireless in naval warfare. In military work it is used chiefly in connection with air-craft, but of that more will be said in another chapter.
[Ill.u.s.tration: TRANSMITTER. RECEIVER.
DIAGRAM SHOWING THE PRINCIPLE BY WHICH THE AERIALS ARE CONNECTED TO THE APPARATUS.]
CHAPTER XXI
WIRELESS TELEGRAPHY IN WAR
The history of this wonderful invention has been described in the preceding chapter. Now we will see how it is applied in warfare.
Let us take first its uses in connection with the Navy. The aerial wires or antenna are stretched to the top of the highest mast of the vessel.
Where there are two masts they often span between the two. Ships which have masts for no other reason are supplied with them for this special purpose. In the case of submarines, the whole thing, mast and wires included, is temporary and can be taken down or put up quickly and easily at will.
The stations ash.o.r.e are equipped much after the same manner as are the ships, except that sometimes they are a little more elaborate, as they may well be since they do not suffer from the same limitations. For example, the well-known antenna over the Admiralty buildings in London consists of three masts placed at the three corners of a triangle with wires stretched between all three.
However these wires may be arranged and supported they are very carefully insulated from their supports, for when sending they have to be charged with current at a high voltage and need good insulation to prevent its escape, while, in receiving, the currents induced in them are so very faint that good insulation is required in order that there may not be the slightest avoidable loss.
The function of these wires, it will be understood, is to form one plate of a condenser, the earth being the other plate and the air in between the "dielectric" or insulator.
In the case of ships "the earth" is represented by the hull of the vessel. It makes a particularly good "earth" since it is in perfect contact with a vast ma.s.s of salt water, and that again is in contact with a vast area of the earth"s surface. Salt water is a surprisingly good conductor of electricity.
In land stations "earth" consists of a metal plate well buried in damp ground. The whole question of conduction of electricity through the earth is very perplexing. There seems to be resistance offered to the current at the point where it enters the ground, but after that none at all. Consequently the resistance between two earth plates a few yards apart and between similar ones a thousand miles apart is about the same.
Though the earth is made up mainly of what, in small quant.i.ties, are very bad conductors indeed, taking the earth as a whole it is an exceedingly good conductor. That makes it all the more important that where the current enters should be made as good a conductor as possible, and the construction and location of the earth plates is therefore very carefully considered so as to get the best results.
Wires, of course, connect the antenna to the earth, thereby forming what is called an "oscillatory circuit." The ordinary electric circuit is a complete path of wire or other good conductor around which the current can flow in a continuous stream. An oscillatory circuit is one which is incomplete, but the ends of which are so formed that they const.i.tute the two "plates" of a condenser. In that way, according to theory, the circuit is completed between the two ends by a strain or distortion in the "Ether" between them. A continuous current will not flow in such a circuit, but an alternating, intermittent or oscillating current will flow in it in many respects as if there were no gap at all but a complete ring of wire.
At some convenient point in this oscillatory circuit are inserted the wireless instruments, one set for sending and the other set for receiving, either being brought into circuit at will by the simple movement of a switch.
In small installations the central feature of the sending apparatus is an Induction Coil operated by a suitable battery or by current from a dynamo. Connected with it is a suitable spark gap consisting of two or three metal b.a.l.l.s well insulated and so arranged that the distance between them can be delicately adjusted. This is generally done by a screw arrangement with insulating handles, so that the operator can safely adjust them while the current is on.
The current from the battery or dynamo to the coil is controlled by a key similar to those used in ordinary telegraphy, the action being such that on depressing the key the current flows and the coil pours forth a torrent of sparks between the k.n.o.bs of the spark-gap, but on letting the key up again the sparks cease. Since the sparks send out etherial waves which in turn affect the distant receiving apparatus it follows that a signal is sent whenever the key is depressed. Moreover, if the key be held down a short time a short signal is sent, but if it be kept depressed for a little longer a long signal is sent, by which means intelligible messages can be transmitted over vast distances.
Certain specified wave lengths are always used in wireless telegraphy.
That is to say, the waves are sent out at a certain rate so that they follow each other at a certain distance apart. In other words, it is necessary to be able to adjust the rate at which the currents will oscillate between the antenna and earth. Every oscillatory circuit possesses two properties which are characteristic of it. These two properties are known as Capacity and Inductance. It is not necessary to explain here what these terms mean precisely. It is quite sufficient just to name them and to state that the rate at which oscillations take place in such a circuit depends upon the combined effect of these two properties. Consequently, if we can arrange things so that capacity or inductance or both can be added to a circuit at will and in any quant.i.ty within limits, we can within those limits obtain any rate of oscillation which we desire and consequently send out the message-bearing waves at any interval we like; in other words, we can adjust the wave-length at will.
Fortunately, it is very easy to add these properties to an oscillatory circuit in a very simple manner. A certain little instrument called a "tuner" is connected up in the circuit and by the simple movement of a few handles the desired result can be obtained quickly even by an operator with but a moderate experience. He has certain graduated scales to guide him, and he is only called upon to work according to a prearranged rule in order to obtain any of the regulation wave-lengths.
As a matter of fact, the instruments are not directly inserted in the antenna circuit, the circuit that is which is formed by the aerial wires, the earth and the inter-connecting wires. Instead, the two sides of the spark-gap are connected together so as to form a separate circuit of their own, the local circuit as we might call it, and then the two circuits, the antenna circuit and the local circuit, are connected together by "induction."
A coil of wire is formed in each, and these two coils are wound together so that currents in one winding induce similar currents in the other winding, and by that means the oscillations set up by the coil in the local circuit are transformed into similar oscillations in the antenna circuit. This transformation involves certain losses, but it is found in practice to be by far the most effective arrangement. Both the circuits have to be tuned to the desired wave length, but that is done quite easily by the operation of the handles in the tuner already referred to.
It is to this coupling together of tuned circuits that Marconi"s most famous patent relates. It is registered in the British Patent Office under the number 7777, and hence is known as the "four sevens" patent.
It has been the subject of much litigation, which proves its exceptional importance, and it is to the fact that the Marconi Company have been able to sustain their rights under it that they owe their commanding position to-day in the realm of wireless telegraphy.
The Receiving Apparatus also consists of a separate local circuit which can be coupled when desired to the antenna circuit through a transformer. The same simple tuning arrangement is made to affect this circuit also, so that the "multiple tuner," as the instrument is called, controls all the circuits both for sending and for receiving. The oscillations caused in the antenna circuit by the action upon it of the etherial waves flowing from the distant transmitting station pa.s.s through one winding of the transformer and thereby induce similar oscillations in the local receiving circuit which are made perceptible by the receiving instrument.
Reference has already been made to the original form of receiving apparatus called the Coherer. This, however, has been very largely superseded by the Magnetic Detector of Marconi and the Crystal Detector, both of which make the signals perceivable as buzzing sounds in the telephone.
The magnetic detector owes its existence to the fact that oscillations tend to destroy magnetism in iron. It is believed that every molecule of iron is itself a tiny magnet. If that be so one would expect every piece of iron to be a magnet, which we know it is not. We can always make a piece of iron into a magnet by putting another magnet near it, but when we take the other magnet away the iron loses its power, or to be precise it _almost_ loses it. A piece of even the best and softest iron having once been magnetized retains a little magnetic power which we call "residual" magnetism.
All this is easily explained if we remember first that a heap of tiny magnets lying higgledy-piggledy would in fact exhibit no magnetic power outside the heap. If, however, we brought a powerful magnet near them it would have the effect of pulling a lot of them into the same position, of arranging them in fact so that instead of all more or less neutralizing each other they could act together and help each other.
Then the heap would become magnetic. On removing the powerful magnet, however, a lot of the little ones would be sure to fall down again into their old places and so the heap would at once lose a large part of its power, yet some would remain and so it would retain a certain amount of "residual" magnetism. If, then, you were to give the table on which the little magnets rest a good shake, the "higgledy-piggledyness" would be restored and even the "residual" magnetism would vanish.
So we believe that the little molecules lie just anyhow, wherefore they neutralize each other and the ma.s.s of iron is powerless. When another magnet comes near, however, they are more or less pulled into the right position and the iron becomes magnetized. When the magnet is removed the magnetism which it produced is largely lost, and if last of all we give the iron a smart blow with a hammer even the residual magnetism vanishes too.
Now, oscillations taking place in the neighbourhood of a piece of iron possessing residual magnetism have much the same effect as the blow of a hammer. Probably because of its rapidity an oscillating current shakes the molecules up and strews them about at random, entirely destroying any orderly arrangement of them. And Marconi used that fact in detecting oscillations.
Two little coils of wire are wound together, one inside the other.
Through the centre of the innermost there runs an endless band of soft iron wire. Stretched on two rollers this band travels steadily along, the motive power being clockwork, so that it is always entering the coil at one end and leaving it at the other. As it travels it pa.s.ses close to two powerful steel magnets, so that as it enters the coil it is always slightly magnetized. The oscillations are pa.s.sed through one of the two concentric coils, and their action is to remove suddenly the residual magnetism in that part of the moving wire which is at the moment pa.s.sing through. That sudden demagnetization then affects the second of the concentric coils, inducing currents in it, not of an oscillating nature but of an ordinary intermittent kind which can make themselves audible in a telephone which is connected with the coil.
This arrangement, then, causes the oscillations, which will not operate a telephone, to produce other currents of a different nature which will.
The reason why oscillations have no effect in a telephone is no doubt because they change so rapidly, at rates, as has been mentioned already, of the order of a million per second. The telephone diaphragm, light and delicate though it is, is far too gross and heavy to respond to such rapidly changing impulses as that. In the magnetic detector the difficulty is overcome by making them change the magnetic condition of some iron wire which change in turn produces currents capable of operating a telephone. The Crystal Detector achieves the same result in another way.
There are certain substances, of which carborundum is a notable example, which conduct electricity more readily in one direction than the other.
Most of these substances are crystalline in their nature, and hence the detector in which they are used gets its name. Carborundum, by the way, is a sort of artificial diamond produced in the electric furnace and largely used as a grinding material in place of emery.
It is easy to see that by pa.s.sing an oscillating current, which is a very rapidly alternating current, through one of these one-direction conductors one half of each oscillation is more or less stopped.
Oscillations, again, are surgings to and fro: the crystal tends to let the "tos" go through and to stop the "fros." That does not quite explain all that happens. It is not fully understood. The fact remains, however, that by putting a crystal in series with the telephone the oscillations become directly audible. The term "in series with" means that both crystal and telephone are inserted in the local receiving circuit so that the currents in that circuit pa.s.s through both in succession.
The resistance of the crystal being very great, a special telephone is needed for use with it. It is quite an ordinary telephone, however, except in that it is wound with a great many turns of very fine wire and is therefore called a high-resistance telephone.
Whichever of these detectors be used, then, the operator sits, with his telephone clipped on to his head, and with his tuner set for that wave length at which his station is scheduled to work, listening for signals.
He may go for hours without being called up, and in the meantime he may hear many signals intended for others. He knows they are not for him, since every message is preceded by a code signal indicating to whom it is addressed.
Under the conditions of warfare there is far more listening than there is sending, but when a station wishes to send the operator just switches over, cutting out his receiving apparatus and bringing his transmitting instruments into operation, and, having adjusted his tuner for the wave length of the station to which he desires to communicate, he flings out his message.
In war-time, too, there is much listening for the signals of the enemy, which is the reason why as few messages are sent out as possible. In this case the man sits with his telephone on his head carefully changing his tuner from time to time in the endeavour to catch any message in any wave-length which may be travelling about. This searching the ether for a chance message of the enemy must be at times a very wearisome job, but it must be varied with very exciting intervals.
On aircraft it is clear that no earth connection is possible. The antenna in that case usually hangs vertically down from the machine or airship. Under these conditions the valuable effect of the earth connection is of course lost. As will be remembered, the earth-connected apparatus sends forth waves which cling more or less to the neighbourhood of the earth"s surface, while those from the non-earthed apparatus as used by aircraft tend to fly in all directions. The latter apparatus is in fact almost precisely similar to that which Hertz used in his first experiments. Hence the range is comparatively poor under these conditions, but it is good enough for very valuable work in warfare. Communication between airman and artillery by this means has revolutionized the handling of large guns in the field.
To save the airman from the accidental catching of his aerial wire in a tree or on a building there is sometimes fitted a contrivance of the nature of wire-cutters so that he can at any moment cut himself free from it.
So far we have dealt almost exclusively with the naval and aerial use of this wonderful invention. It is employed, though in a lesser degree, in land warfare. In such cases the aerial may be merely a wire thrown on to and caught up on a high tree. More elaborate devices are used, however, such as a high telescopic tower similar to the tall fire-escape ladders of the fire-brigades. Anyone who has seen the ladders rush up to a burning building and commence to erect themselves almost before they have stopped will realise how valuable such a machine must be for forming a temporary and easily movable wireless antenna. The power which causes the tall tower to extend itself erect in a few seconds is compressed air carried in cylinders upon the machine, while the power which takes it from place to place is a petrol motor, and since the latter can be made to re-charge the storage cylinders it is clear that in it we have a marvellously convenient adjunct to the wireless apparatus.
But apart from such carefully prepared devices the men of the Royal Engineers are past masters in the art of rigging up, according to the conditions of the moment, all sorts of makeshift apparatus whereby signalling over quite long ranges can be carried on by "wireless." Such improvisations, could they be recorded, would const.i.tute war inventions of a high order.