In a previous chapter the various methods of signalling between the
ground and the airman aloft have been described. Seeing that wireless
telegraphy has made such enormous strides and has advanced to such a
degree of perfection, one naturally would conclude that it constitutes
an ideal system of communication under such conditions in military
operations.
But this is not the case. Wireless is utilised only to a very limited
extent. This is due to two causes. The one is of a technical, the other
of a strategical character.
The uninitiated, bearing in mind the comparative ease with which
wireless installations may be established at a relatively small expense,
would not unreasonably think that no serious difficulties of a technical
character could arise: at least none which would defy solution. But
these difficulties exist in two or three different fields, each of which
is peculiarly complex and demands individual treatment.
In the first place, there is the weight of the necessary installation.
In the case of the dirigible this may be a secondary consideration,
but with the aeroplane it is a matter of primary and vital importance.
Again, under present conditions, the noise of the motor is apt to render
the intelligent deciphering of messages while aloft a matter of extreme
difficulty, especially as these are communicated in code. The engine
noise might be effectively overcome by the use of a muffler such as,
is used with automobiles, but then there is the further difficulty of
vibration.
This problem is being attacked in an ingenious manner. It is proposed to
substitute for audible signals visual interpretations, by the aid of an
electric lamp, the fluctuations in which would correspond to the dots
and dashes of the Morse code. Thus the airman would read his messages by
sight instead of by sound.
This method, however, is quite in its infancy, and although attractive
in theory and fascinating as a laboratory experiment or when conducted
under experimental conditions, it has not proved reliable or effective
in aeronautical operations. But at the same time it indicates a
promising line of research and development.
Then there are the problems of weight and the aerial. So far as present
knowledge goes, the most satisfactory form of aerial yet exploited is
that known as the trailing wire. From 300 to 700 feet of wire are coiled
upon a reel, and when aloft this wire is paid out so that it hangs below
the aeroplane. As a matter of fact, when the machine is travelling at
high speed it trails horizontally astern, but this is immaterial. One
investigator, who strongly disapproves of the trailing aerial, has
carried out experiments with a network of wires laid upon and attached
to the surface of the aeroplane's wings. But the trailing wire is
generally preferred, and certainly up to the present has proved more
satisfactory.
The greatest obstacle, however, is the necessary apparatus. The average
aeroplane designed for military duty is already loaded to the maximum.
As a rule it carries the pilot and an observer, and invariably includes
a light arm for defence against an aerial enemy, together with an
adequate supply of ammunition, while unless short sharp flights are to
be made, the fuel supply represents an appreciable load. Under these
circumstances the item of weight is a vital consideration. It must be
kept within a limit of 100 pounds, and the less the equipment weighs the
more satisfactory it is likely to prove, other things being equal.
The two most successful systems yet exploited are the Dubilier and the
Rouget. The former is an American invention, the latter is of French
origin. Both have been tested by the British Military Aeronautical
Department, and the French authorities have subjected the French system
to rigorous trials. Both systems, within their limitations, have proved
satisfactory.
The outstanding feature of the Dubilier system is the production of sine
waves of musical frequency from continuous current, thus dispensing
with the rotary converter. The operating principle is the obtaining of
a series of unidirectional impulses by a condenser discharge, the
pulsating currents following one another at regular intervals at a
frequency of 500 impulses per second, which may be augmented up to 1,000
impulses per second. The complete weight of such an apparatus is 40
pounds; the electric generator, which is no larger than the motor used
for driving the ordinary table ventilating fan, accounts for 16 pounds
of this total. Under test at sea, upon the deck of a ship, a range of
250 miles has been obtained. The British Government carried out a series
of experiments with this system, using a small plant weighing about 30
pounds, with which communication was maintained up to about 20 miles.
In the French system the Reuget transmitter is employed. The apparatus,
including the dynamo, which is extremely small, weighs in all 70 pounds.
A small alternator of 200 watts and 100 volts is coupled direct to the
aeroplane motor, a new clutch coupler being employed for this purpose.
By means of a small transformer the voltage is raised to 30,000 volts,
at which the condenser is charged. In this instance the musical spark
method is employed.
The whole of the high tension wiring is placed within a small space
so as not to endanger the pilot, while the transformer is hermetically
sealed in a box with paraffin. The aerial comprises a trailing wire 100
feet in length, which, however, can be wound in upon its reel within 15
seconds. This reeled antenna, moreover, is fitted with a safety device
whereby the wire can be cut adrift in the event of an accident befalling
the aeroplane and necessitating an abrupt descent. With this apparatus
the French authorities have been able to maintain communication over a
distance of 30 miles.
In maintaining ethereal communication with aeroplanes, however, a
portable or mobile station upon the ground is requisite, and this
station must be within the radius of the aerial transmitter, if
messages are to be received from aloft with any degree of accuracy and
reliability. Thus it will be recognised that the land station is as
important as the aeroplane equipment, and demands similar consideration.
A wide variety of systems have been employed to meet these conditions.
There is the travelling automobile station, in which the installation
is mounted upon a motor-car. In this instance the whole equipment is
carried upon a single vehicle, while the antenna is stowed upon the roof
and can be raised or lowered within a few seconds. If motor traction is
unavailable, then animal haulage may be employed, but in this instance
the installation is divided between two vehicles, one carrying the
transmitting and receiving apparatus and the generating plant, the other
the fuel supplies and the aerial, together with spare parts.
The motive power is supplied by a small air cooled petrol or gasoline
motor developing eight horse-power, and coupled direct to a 2-kilo watt
alternator. At one end of the shaft of the latter the disk discharger is
mounted, its function being to break up the train of waves into groups
of waves, so as to impart a musical sound to the note produced in the
receiver. A flexible cable transmits the electric current from the
generator to the wagon containing the instruments. The aerial is built
up of masts carried in sections.
The Germans employ a mobile apparatus which is very similar, but in
this instance the mast is telescopic. When closed it occupies but little
space. By turning the winch handle the mast is extended, and can be
carried to any height up to a maximum of about 100 feet. The capacity
of these mobile stations varies within wide limits, the range of the
largest and most powerful installations being about 200 miles. The
disadvantage of these systems, however, is that they are condemned to
territories where the ground at the utmost is gently undulating, and
where there are roads on which four-wheeled vehicles can travel.
For operation in hilly districts, where only trails are to be found,
the Marconi Company, has perfected what may be described as "pack" and
"knapsack" installations respectively. In the first named the whole of
the installation is mounted upon the backs of four horses. The first
carries the generator set, the second the transmitting instruments, the
third the receiving equipment, and the fourth the detachable mast and
stays.
The generator is carried upon the horse's saddle, and is fitted with a
pair of legs on each side. On one side of the saddle is mounted a
small highspeed explosion motor, while on the opposite side, in axial
alignment with the motor, is a small dynamo. When it is desired to
erect the installation the saddle carrying this set is removed from the
horse's back and placed upon the ground, the legs acting as the support.
A length of shaft is then slipped into sockets at the inner ends of the
motor and dynamo shafts respectively, thus coupling them directly, while
the current is transmitted through a short length of flexible cable to
the instruments. The mast itself is made in lengths of about four feet,
which are slipped together in the manner of the sections of a fishing
rod, and erected, being supported by means of wire guys. In this manner
an antenna from 40 to 50 feet in height may be obtained.
The feature of this set is its compactness, the equal division of the
sections of the installation, and the celerity with which the station
may be set up and dismantled in extremely mountainous country such as
the Vosges, where it is even difficult for a pack-horse to climb to
commanding or suitable positions, there is still another set which has
been perfected by the Marconi Company. This is the "knapsack" set,
in which the whole of the installation, necessarily light, small,
and compact, is divided among four men, and carried in the manner of
knapsacks upon their backs. Although necessarily of limited radius,
such an installation is adequate for communication within the restricted
range of air-craft.
Greater difficulties have to be overcome in the mounting of a wireless
installation upon a dirigible. When the Zeppelin was finally accepted
by the German Government, the military authorities emphasised the great
part which wireless telegraphy was destined to play in connection with
such craft. But have these anticipations been fulfilled? By no means, as
a little reflection will suffice to prove.
In the first place, a wireless outfit is about the most dangerous piece
of equipment which could be carried by such a craft as the Zeppelin
unless it is exceptionally well protected. As is well known the rigidity
of this type of airship is dependent upon a large and complicated
network of aluminium, which constitutes the frame. Such a huge mass
of metal constitutes an excellent collector of electricity from the
atmosphere; it becomes charged to the maximum with electricity.
In this manner a formidable contributory source of danger to the airship
is formed. In fact, this was the reason why "Z-IV" vanished suddenly in
smoke and flame upon falling foul of the branches of trees during
its descent. At the time the Zeppelin was a highly charged electrical
machine or battery as it were, insulated by the surrounding air.
Directly the airship touched the trees a short circuit was established,
and the resultant spark sufficed to fire the gas, which is continuously
exuding from the gas bags.
After this accident minute calculations were made and it was ascertained
that a potential difference of no less than 100,00 volts existed between
the framework of the dirigible and the trees. This tension sufficed
to produce a spark 4 inches in length. It is not surprising that the
establishment of the electric equilibrium by contact with the trees,
which produced such a spark should fire the hydrogen inflation charge.
In fact the heat generated was so intense that the aluminium metallic
framework was fused. The measurements which were made proved that the
gas was consumed within 15 seconds and the envelope destroyed within 20
seconds.
As a result of this disaster endeavours were made to persuade Count
Zeppelin to abandon the use of aluminium for the framework of his
balloon but they were fruitless, a result no doubt due to the fact that
the inventor of the airship of this name has but a superficial
knowledge of the various sciences which bear upon aeronautics, and fully
illustrates the truth of the old adage that "a little learning is a
dangerous thing." Count Zeppelin continues to work upon his original
lines, but the danger of his system of construction was not lost upon
another German investigator, Professor Schiitte, who forthwith embarked
upon the construction of another rigid system, similar to that of
Zeppelin, at Lanz. In this vessel aluminium was completely abandoned in
favour of a framework of ash and poplar.
The fact that the aluminium constituted a dangerous collector of
electricity rendered the installation of wireless upon the Zeppelin not
only perilous but difficult. Very serious disturbances of an electrical
nature were set up, with the result that wireless communication between
the travelling dirigible and the ground below was rendered extremely
uncertain. In fact, it has never yet been possible to communicate over
distances exceeding about 150 miles. Apart from this defect, the danger
of operating the wireless is obvious, and it is generally believed in
technical circles that the majority of the Zeppelin disasters from fire
have been directly attributable to this, especially those disasters
which have occurred when the vessel has suddenly exploded before coming
into contact with terrestrial obstructions.
In the later vessels of this type the wireless installation is housed
in a well insulated compartment. This insulation has been carried, to
an extreme degree, which indicates that at last the authorities have
recognised the serious menace that wireless offers to the safety of the
craft, with the result that every protective device to avoid disaster
from this cause has been freely adopted.
The fact that it is not possible to maintain communication over a
distance exceeding some 20 miles is a severe handicap to the progressive
development of wireless telegraphy in this field. It is a totally
inadequate radius when the operations of the present war are borne in
mind. A round journey of 200, or even more miles is considered a
mere jaunt; it is the long distance flight which counts, and which
contributes to the value of an airman's observations. The general
impression is that the fighting line or zone comprises merely two or
three successive stretches of trenches and other defences, representing
a belt five miles or so in width, but this is a fallacy. The fighting
zone is at least 20 miles in width; that is to say, the occupied
territory in which vital movements take place represents a distance of
20 miles from the foremost line of trenches to the extreme rear, and
then comes the secondary zone, which may be a further 10 miles or
more in depth. Consequently the airman must fly at least 30 miles in
a bee-line to cover the transverse belt of the enemy's field of
operations. Upon the German and Russian sides this zone is of far
greater depth, ranging up to 50 miles or so in width. In these
circumstances the difficulties of ethereal communication 'twixt air and
earth may be realised under the present limitations of radius from which
it is possible to transmit.
But there are reasons still more cogent to explain why wireless
telegraphy has not been used upon a more extensive scale during the
present campaign. Wireless communication is not secretive. In other
words, its messages may be picked up by friend and foe alike with
equal facility. True, the messages are sent in code, which may be
unintelligible to the enemy. In this event the opponent endeavours to
render the communications undecipherable to one and all by what is known
as "jambing." That is to say, he sends out an aimless string of
signals for the purpose of confusing senders and receivers, and this
is continued without cessation and at a rapid rate. The result is that
messages become blurred and undecipherable.
But there is another danger attending the use of wireless upon the
battlefield. The fact that the stations are of limited range is well
known to the opposing forces, and they are equally well aware of the
fact that aerial craft cannot communicate over long distances. For
instance, A sends his airmen aloft and conversation begins between the
clouds and the ground. Presently the receivers of B begin to record
faint signals. They fluctuate in intensity, but within a few seconds B
gathers that an aeroplane is aloft and communicating with its base. By
the aid of the field telephone B gets into touch with his whole string
of wireless stations and orders a keen look-out and a listening ear to
ascertain whether they have heard the same signals. Some report that the
signals are quite distinct and growing louder, while others declare that
the signals are growing fainter and intermittent. In this manner B is
able to deduce in which direction the aeroplane is flying. Thus if those
to the east report that signals are growing stronger, while the stations
on the west state that they are diminishing, it is obvious that the
aeroplane is flying west to east, and vice versa when the west hears
more plainly at the expense of the east. If, however, both should report
that signals are growing stronger, then it is obvious that the aircraft
is advancing directly towards them.
It was this ability to deduce direction from the sound of the signals
which led to the location of the Zeppelin which came down at Luneville
some months previous to the war, and which threatened to develop into a
diplomatic incident of serious importance. The French wireless stations
running south-east to north-west were vigilant, and the outer station on
the north-west side picked up the Zeppelin's conversation. It maintained
a discreet silence, but communicated by telephone to its colleagues
behind.
Presently No. 2 station came within range, followed by Nos. 3, 4, 5, 6,
and so on in turn. Thus the track of the Zeppelin was dogged silently
through the air by its wireless conversation as easily and as positively
as if its flight had been followed by the naked eye. The Zeppelin
travellers were quite ignorant of this action upon the part of the
French and were surprised when they were rounded-up to learn that they
had been tracked so ruthlessly. Every message which the wireless of the
Zeppelin had transmitted had been received and filed by the French.
Under these circumstances it is doubtful whether wireless telegraphy
between aircraft and the forces beneath will be adopted extensively
during the present campaign. Of course, should some radical improvement
be perfected, whereby communication may be rendered absolutely
secretive, while no intimation is conveyed to the enemy that ethereal
conversation is in progress, then the whole situation will be changed,
and there may be remarkable developments.