Part 18 (1/2)
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 s.h.i.+ps. 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. s.h.i.+ps 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 s.h.i.+ps, 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 s.h.i.+ps ”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.