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 ashore 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 mass 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 quantities, 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 constitute 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 balls 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 knobs 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 quantity
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 pass
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 mass 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 passes close to
two powerful steel magnets, so that as it enters the coil it is always
slightly magnetized. The oscillations are passed 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 passing
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 passing 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 pass 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 constitute war inventions
of a high order.