WIRELESS TELEGRAPHY AND TELEPHONY--PRINCIPLES AND APPARATUS
Wireless telegraphy is probably the most remarkable and at the
same time the most interesting of all the varied applications
of electricity. The exceptional popular interest in wireless
communication, as compared with most of the other daily tasks which
electricity is called upon to perform, is easy to understand. The
average man does not realize that although we are able to make
electricity come and go at our bidding, we have little certain
knowledge of its nature. He is so accustomed to hearing of the electric
current, and of the work it is made to do, that he sees little to
marvel at so long as there is a connecting wire. Electricity is
produced by batteries or by a dynamo, sent along a wire, and made to
drive the necessary machinery; apparently it is all quite simple. But
take away the connecting wire, and the case is different. In wireless
telegraphy electricity is produced as usual, but instantly it passes
out into the unknown, and, as far as our senses can tell, it is lost
for ever. Yet at some distant point, hundreds or even thousands of
miles away, the electrical influence reappears, emerging from the
unknown with its burden of words and sentences. There is something
uncanny about this, something suggesting telepathy and the occult, and
herein lies the fascination of wireless telegraphy.
The idea of communicating without any connecting wires is an old
one. About the year 1842, Morse, of telegraph fame, succeeded in
transmitting telegraphic signals across rivers and canals without a
connecting wire. His method was to stretch along each bank of the river
a wire equal in length to three times the breadth of the river. One of
these wires was connected with the transmitter and with a battery, and
the other with a receiver, both wires terminating in copper plates sunk
in the water. In this case the water took the place of a connecting
wire, and acted as the conducting medium. A few years later another
investigator, a Scotchman named Lindsay, succeeded in telegraphing
across the river Tay, at a point where it is over a mile and a half
wide, by similar methods. Lindsay appears to have been the first to
suggest the possibility of telegraphing across the Atlantic, and
although at that time, 1845, the idea must have seemed a wild one, he
had the firmest faith in its ultimate accomplishment.
Amongst those who followed Lindsay’s experiments with keen interest
was the late Sir William, then Mr. Preece, but it was not until 1882,
twenty years after Lindsay’s death, that he commenced experiments on
his own account. In March of that year the cable across the Solent
failed, and Preece took the opportunity of trying to signal across
without a connecting wire. He used two overhead wires, each terminating
in large copper plates sunk in the sea, one stretching from Southampton
to Southsea Pier, and the other from Ryde Pier to Sconce Point. The
experiment was successful, audible Morse signals being received on
each side. In this experiment, as in those of Morse and Lindsay, the
water acted as the conducting medium; but a year or two later, Preece
turned his attention to a different method of wireless communication,
by means of induction. This method was based upon the fact that at the
instant of starting and stopping a current in one wire, another current
is induced in a second wire placed parallel to it, even when the two
wires are a considerable distance apart. Many successful experiments
in this induction telegraphy were made, one of the most striking
being that between the Island of Mull and the mainland, in 1895. The
cable between the island and the mainland had broken, and by means
of induction perfect telegraphic communication was maintained during
the time that the cable was being repaired. Although this system of
wireless telegraphy is quite successful for short distances, it becomes
impracticable when the distance is increased, because the length of
each of the two parallel wires must be roughly equal to the distance
between them. These experiments of Preece are of great interest, but
we must leave them because they have little connexion with present-day
wireless telegraphy, in which utterly different methods are used.
All the commercial wireless systems of to-day depend upon the
production and transmission of electric waves. About the year 1837
it was discovered that the discharge of a Leyden jar did not consist
of only one sudden rush of electricity, but of a series of electric
oscillations, which surged backwards and forwards until electric
equilibrium was restored. This discovery was verified by later
experimenters, and it forms the foundation of our knowledge of electric
waves. At this point many readers probably will ask, “What are electric
waves?” It is impossible to answer this question fully, for we still
have a great deal to learn about these waves, and we only can state the
conclusions at which our greatest scientists have arrived after much
thought and many experiments. It is believed that all space is filled
with a medium to which the name “ether” has been given, and that this
ether extends throughout the matter. We do not know what the ether is,
but the important fact is that it can receive and transmit vibrations
in the form of ether waves. There are different kinds of ether waves,
and they produce entirely different effects. Some of them produce the
effect which we call light, and these are called “light waves.” Others
produce the effect known as heat, and they are called “heat waves”; and
still others produce electricity, and these we call “electric waves.”
These waves travel through the ether at the enormous speed of 186,000
miles per second, so that they would cross the Atlantic Ocean in about
1/80 second. The fact that light also travels at this speed suggested
that there might be some connexion between the two sets of waves, and
after much experiment it has been demonstrated that the waves of light
and electricity are identical except in their length.
Later on in this chapter we shall have occasion to refer frequently
to wave-length, and we may take this opportunity of explaining what
is understood by this term. Wave-length is the distance measured from
the crest of one wave to the crest of the next, across the intervening
trough or hollow. From this it will be seen that the greater the
wave-length, the farther apart are the waves; and also that if we have
two sets of waves of different wave-lengths but travelling at the same
speed, then the number of waves arriving at any point in one second
will be greater in the case of the shorter waves, because these are
closer together.
A tuning-fork in vibration disturbs the surrounding air, and sets
up air waves which produce the effect called sound when they strike
against the drums of our ears. In a similar way the discharge of a
Leyden jar disturbs the surrounding ether, and sets up electric ether
waves; but these waves produce no effect upon us in the shape of sight,
sound, or feeling. There is however a very simple piece of apparatus
which acts as a sort of electric eye or ear, and detects the waves for
us. This consists of a glass tube loosely filled with metal filings,
and having a cork at each end. A wire is passed through each cork so
as to project well into the tube, but so that the two ends do not
touch one another, and the outer ends of these wires are connected to
a battery of one or two cells, and to some kind of electrically worked
apparatus, such as an electric bell. So long as the filings lie quite
loosely in the tube they offer a very high resistance, and no current
passes. If now electric waves are set up by the discharge of a Leyden
jar, these waves fall upon the tube and cause the resistance of the
filings to decrease greatly. The filings now form a conducting path
through which the current passes, and so the bell rings. If no further
discharge takes place the electric waves cease, but the filings do not
return to their original highly resistant condition, but retain their
conductivity, and the current continues to pass, and the bell goes on
ringing. To stop the bell it is only necessary to tap the tube gently,
when the filings immediately fall back into their first state, so that
the current cannot pass through them.
Now let us see how the “coherer,” as the filings tube is called,
is used in actual wireless telegraphy. Fig. 33_a_ shows a simple
arrangement for the purpose. A is an induction coil, and B the battery
supplying the current. The coil is fitted with a spark gap, consisting
of two highly polished brass balls CC, one of these balls being
connected to a vertical wire supported by a pole, and the other to
earth. D is a Morse key for starting and stopping the current. When the
key is pressed down, current flows from the battery to the coil, and
in passing through the coil it is raised to a very high voltage, as
described in Chapter VIII. This high tension current is sent into the
aerial wire, which quickly becomes charged up to its utmost limits. But
more current continues to arrive, and so the electricity in the aerial,
unable to bear any longer the enormous pressure, takes the only path
of escape and bursts violently across the air gap separating the brass
balls. Surging oscillations are then produced in the aerial, the ether
is violently disturbed, and electric waves are set in motion. This is
the transmitting part of the apparatus.
[Illustration: _a._ Transmitting.
_b._ Receiving.
FIG. 33.--Diagram of simple Wireless Transmitting and Receiving
Apparatus.]
If a stone is dropped into a pond, little waves are set in motion, and
these spread outwards in ever-widening rings. Electric waves also are
propagated outwards in widening rings, but instead of travelling in
one plane only, like the water waves, they proceed in every plane; and
when they arrive at the receiving aerial they set up in it oscillations
of the same nature as those which produced the waves. Let us suppose
electric waves to reach the aerial wire of Fig. 33_b_. The resistance
of the coherer H is at once lowered so that current from battery N
flows and operates the relay F, which closes the circuit of battery
M. This battery has a twofold task. It operates the sounder E, and
it energizes the electro-magnet of the de-coherer K, as shown by the
dotted lines. This de-coherer is simply an electric bell without the
gong, arranged so that the hammer strikes the coherer tube; and its
purpose is to tap the tube automatically and much more rapidly than
is possible by hand. The sounder therefore gives a click, and the
de-coherer taps the tube, restoring the resistance of the filings. The
circuit of battery N is then broken, and the relay therefore interrupts
the circuit of battery M. If waves continue to arrive, the circuits
are again closed, another click is given, and again the hammer taps
the tube. As long as waves are falling upon the aerial, the alternate
makings and breakings of the circuits follow one another very rapidly
and the sounder goes on working. When the waves cease, the hammer of
the de-coherer has the last word, and the circuits of both batteries
remain broken. To confine the electric waves to their proper sphere two
coils of wire, LL, called choking coils, are inserted as shown.
In this simple apparatus we have all the really essential features of
a wireless installation for short distances. For long distance work
various modifications are necessary, but the principle remains exactly
the same. In land wireless stations the single vertical aerial wire
becomes an elaborate arrangement of wires carried on huge masts and
towers. The distance over which signals can be transmitted and received
depends to a considerable extent upon the height of the aerial, and
consequently land stations have the supporting masts or towers from
one to several hundred feet in height, according to the range over
which it is desired to work. As a rule the same aerial is used both for
transmitting and receiving, but some stations have a separate aerial
for each purpose. A good idea of the appearance of commercial aerials
for long distance working may be obtained from the frontispiece,
which shows the Marconi station at Glace Bay, Nova Scotia, from which
wireless communication is held with the Marconi station at Clifden, in
Galway, Ireland.
In the first wireless stations what is called a “plain aerial”
transmitter was used, and this was almost the same as the transmitting
apparatus in Fig. 33_a_, except, of course, that it was on a larger
scale. This arrangement had many serious drawbacks, including that
of a very limited range, and it has been abandoned in favour of the
“coupled” transmitter, a sketch of which is shown in Fig. 34. In this
transmitter there are two separate circuits, having the same rate of
oscillation. A is an induction coil, supplied with current from the
battery B, and C is a condenser. A condenser is simply an apparatus
for storing up charges of electricity. It may take a variety of forms,
but in every case it must consist of two conducting layers separated
by a non-conducting layer, the latter being called the “dielectric.”
The Leyden jar is a condenser, with conducting layers of tinfoil and
a dielectric of glass, but the condensers used for wireless purposes
generally consist of a number of parallel sheets of metal separated
by glass or mica, or in some cases by air only. The induction coil
charges up the condenser with high tension electricity, until the
pressure becomes so great that the electricity is discharged in the
form of a spark between the brass balls of the spark gap D. The
accumulated electric energy in the condenser then surges violently
backwards and forwards, and by induction corresponding surgings are
produced in the aerial circuit, these latter surgings setting up
electric waves in the ether.
[Illustration: FIG. 34.--Wireless “Coupled” Transmitter.]
For the sake of simplicity we have represented the apparatus as using
an induction coil, but in all stations of any size the coil is replaced
by a step-up transformer, and the current is supplied either from an
electric light power station at some town near by, or from a power
house specially built for the purpose. Alternating current is generally
used, and if the current supplied is continuous, it is converted into
alternating current. This may be done by making the continuous current
drive an electric motor, which in turn drives a dynamo generating
alternating current. In any case, the original current is too low in
voltage to be used directly, but in passing through the transformer
it is raised to the required high pressure. The transmitting key,
which is inserted between the dynamo and the transformer, is specially
constructed to prevent the operator from receiving accidental shocks,
and the spark gap is enclosed in a sort of sound-proof box, to deaden
the miniature thunders of the discharge.
During the time that signals are being transmitted, sparks follow one
another across the spark gap in rapid succession, a thousand sparks per
second being by no means an uncommon rate. The violence of these rapid
discharges raises the brass balls of the gap to a great heat. This has
the effect of making the sparking spasmodic and uncertain, with the
result that the signals at the receiving station are unsatisfactory.
To get over this difficulty Marconi introduced a rotary spark gap.
This is a wheel with projecting knobs or studs, mounted on the shaft
of the dynamo supplying the current, so that it rotates rapidly. Two
stationary knobs are fixed so that the wheel rotates between them, and
the sparks are produced between these fixed knobs and those of the
wheel, a double spark gap thus being formed. Overheating is prevented
by the currents of air set up by the rapid movement of the wheel, and
the sparking is always regular.
[Illustration: PLATE XIII.
_Photo by_ _Daily Mirror_.
(_a_) MARCONI OPERATOR RECEIVING A MESSAGE.]
[Illustration:
_By permission of_ _The Marconi Co. Ltd._
(_b_) MARCONI MAGNETIC DETECTOR.]
In the receiving apparatus already described a filings coherer was used
to detect the ether waves, and, by means of a local battery, to
translate them into audible signals with a sounder, or printed signals
with a Morse inker. This coherer however is unsuitable for commercial
working. It is not sufficiently sensitive, and it can be used only
for comparatively short distances; while its action is so slow that
the maximum speed of signalling is not more than about seventeen or
eighteen words a minute. A number of different detectors of much
greater speed and sensitiveness have been devised. The most reliable of
these, though not the most sensitive, is the Marconi magnetic detector,
Plate XIII._b_. This consists of a moving band made of several soft
iron wires twisted together, and passing close to the poles of two
horse-shoe magnets. As the band passes from the influence of one magnet
to that of the other its magnetism becomes reversed, but the change
takes a certain amount of time to complete owing to the fact that the
iron has some magnetic retaining power, so that it resists slightly the
efforts of one magnet to reverse the effect of the other. The moving
band passes through two small coils of wire, one connected with the
aerial, and the other with a specially sensitive telephone receiver.
When the electric waves from the transmitting station fall upon the
aerial of the receiving station, small, rapidly oscillating currents
pass through the first coil, and these have the effect of making the
band reverse its magnetism instantly. The sudden moving of the lines
of magnetic force induces a current in the second coil, and produces
a click in the telephone. As long as the waves continue, the clicks
follow one another rapidly, and they are broken up into the long and
short signals of the Morse code according to the manipulation of the
Morse key at the sending station. Except for winding up at intervals
the clockwork mechanism which drives the moving band, this detector
requires no attention, and it is always ready for work.
Another form of detector makes use of the peculiar power possessed
by certain crystals to rectify the oscillatory currents received
from the aerial, converting them into uni-directional currents. At
every discharge of the condenser at the sending station a number of
complete waves, forming what is called a “train” of waves, is set in
motion. From each train of waves the crystal detector produces one
uni-directional pulsation of current, and this causes a click in the
telephone receiver. If these single pulsations follow one another
rapidly and regularly, a musical note is heard in the receiver. Various
combinations of crystals, and crystals and metal points, are used, but
all work in the same way. Some combinations work without assistance,
but others require to have a small current passed through them from a
local battery. The crystals are held in small cups of brass or copper,
mounted so that they can be adjusted by means of set-screws. Crystal
detectors are extremely sensitive, but they require very accurate
adjustment, and any vibration quickly throws them out of order.
The “electrolytic” detector rectifies the oscillating currents in a
different manner. One form consists of a thin platinum wire passing
down into a vessel made of lead, and containing a weak solution of
sulphuric acid. The two terminals of a battery are connected to the
wire and the vessel respectively. As long as no oscillations are
received from the aerial the current is unable to flow between the
wire and the vessel, but when the oscillations reach the detector the
current at once passes, and operates the telephone receiver. The action
of this detector is not thoroughly understood, and the way in which the
point of the platinum wire prevents the passing of the current until
the oscillations arrive from the aerial is something of a mystery.
The last detector that need be described is the Fleming valve
receiver. This consists of an electric incandescent lamp, with either
carbon or tungsten filament, into which is sealed a plate of platinum
connected with a terminal outside the lamp. The plate and the filament
do not touch one another, but when the lamp is lighted up a current
can be passed from the plate to the filament, but not from filament to
plate. This receiver acts in a similar way to the crystal detector,
making the oscillating currents into uni-directional currents. It has
proved a great success for transatlantic wireless communication between
the Marconi stations at Clifden and Glace Bay, and is extensively used.
The electric waves set in motion by the transmitting apparatus of a
wireless station spread outwards through the ether in all directions,
and so instead of reaching only the aerial of the particular station
with which it is desired to communicate, they affect the aerials of
all stations within a certain range. So long as only one station is
sending messages this causes no trouble; but when, as is actually the
case, large numbers of stations are hard at work transmitting different
messages at the same time, it is evident that unless something can be
done to prevent it, each of these messages will be received at the
same moment by every station within range, thus producing a hopeless
confusion of signals from which not a single message can be read.
Fortunately this chaos can be avoided by what is called “tuning.”
Wireless tuning consists in adjusting the aerial of the receiving
station so that it has the same natural rate of oscillation as that of
the transmitting station. A simple experiment will make clearer the
meaning of this. If we strike a tuning-fork, so that it sounds its
note, and while it is sounding strongly place near it another fork of
the same pitch and one of a different pitch, we find that the fork of
similar pitch also begins to sound faintly, whereas the third fork
remains silent. The explanation is that the two forks of similar pitch
have the same natural rate of vibration, while the other fork vibrates
at a different rate. When the first fork is struck, it vibrates at a
certain rate, and sets in motion air waves of a certain length. These
waves reach both the other forks, but their effect is different in each
case. On reaching the fork of similar pitch the first wave sets it
vibrating, but not sufficiently to give out a sound. But following this
wave come others, and as the fork has the same rate of vibration as
the fork which produced the waves, each wave arrives just at the right
moment to add its impulse to that of the preceding wave, so that the
effect accumulates and the fork sounds. In the case of the third fork
of different pitch, the first wave sets it also vibrating, but as this
fork cannot vibrate at the same rate as the one producing the waves,
the latter arrive at wrong intervals; and instead of adding together
their impulses they interfere with one another, each upsetting the work
of the one before it, and the fork does not sound. The same thing may
be illustrated with a pendulum. If we give a pendulum a gentle push at
intervals corresponding to its natural rate of swing, the effects of
all these pushes are added together, and the pendulum is made to swing
vigorously. If, on the other hand, we give the pushes at longer or
shorter intervals, they will not correspond with the pendulum’s rate of
swing, so that while some pushes will help the pendulum, others will
hinder it, and the final result will be that the pendulum is brought
almost to a standstill, instead of being made to swing strongly and
regularly. The same principle holds good with wireless aerials. Any
aerial will respond readily to all other aerials having the same rate
of oscillation, because the waves in each case are of the same length;
that is to say, they follow one another at the same intervals. On the
other hand, an aerial will not respond readily to waves from another
aerial having a different rate of oscillation, because these do not
follow each other at intervals to suit it.
If each station could receive signals only from stations having
aerials similar to its own, its usefulness would be very limited,
and so all stations are provided with means of altering the rate of
oscillation of their aerials. The actual tuning apparatus by which this
is accomplished need not be described, as it is complicated, but what
happens in practice is this: The operator, wearing telephone receivers
fixed over his ears by means of a head band, sits at a desk upon which
are placed his various instruments. He adjusts the tuning apparatus
to a position in which signals from stations of widely different
wave-lengths are received fairly well, and keeps a general look out
over passing signals. Presently he hears his own call-signal, and
knows that some station wishes to communicate with him. Immediately he
alters the adjustment of his tuner until his aerial responds freely to
the waves from this station, but not to waves from other stations, and
in this way he is able to cut out signals from other stations and to
listen to the message without interruption.
Unfortunately wireless tuning is yet far from perfect in certain
respects. For instance, if two stations are transmitting at the same
time on the same wave-length, it is clearly impossible for a receiving
operator to cut one out by wave-tuning, and to listen to the other
only. In such a case, however, it generally happens that although the
wave-frequency is the same, the frequency of the wave groups or trains
is different, so that there is a difference in the notes heard in the
telephones; and a skilful operator can distinguish between the two
sufficiently well to read whichever message is intended for him. The
stations which produce a clear, medium-pitched note are the easiest to
receive from, and in many cases it is possible to identify a station
at once by its characteristic note. Tuning is also unable to prevent
signals from a powerful station close at hand from swamping to some
extent signals from another station at a great distance, the nearer
station making the receiving aerial respond to it as it were by brute
force, tuning or no tuning.
Another source of trouble lies in interference by atmospheric
electricity. Thunderstorms, especially in the tropics, interfere
greatly with the reception of signals, the lightning discharges giving
rise to violent, irregular groups of waves which produce loud noises
in the telephones. There are also silent electrical disturbances in
the atmosphere, and these too produce less strong but equally weird
effects. Atmospheric discharges are very irregular, without any real
wave-length, so that an operator cannot cut them out by wave-tuning
pure and simple in the way just described, as they defy him by
affecting equally all adjustments. Fortunately, the irregularity
of the atmospherics produces correspondingly irregular sounds in
the telephones, quite unlike the clear steady note of a wireless
station; and unless the atmospherics are unusually strong this note
pierces through them, so that the signals can be read. The effects of
lightning discharges are too violent to be got rid of satisfactorily,
and practically all that can be done is to reduce the loudness of the
noises in the telephones, so that the operator is not temporarily
deafened. During violent storms in the near neighbourhood of a station
it is usual to connect the aerial directly to earth, so that in the
event of its being struck by a flash the electricity passes harmlessly
away, instead of injuring the instruments, and possibly also the
operators. Marconi stations are always fitted with lightning-arresters.
The methods and apparatus we have described so far are those of
the Marconi system, and although in practice additional complicated
and delicate pieces of apparatus are used, the description given
represents the main features of the system. Although Marconi was not
the discoverer of the principles of wireless telegraphy, he was the
first to produce a practical working system. In 1896 Marconi came from
Italy to England, bringing with him his apparatus, and after a number
of successful demonstrations of its working, he succeeded in convincing
even the most sceptical experts that his system was thoroughly sound.
Commencing with a distance of about 100 yards, Marconi rapidly
increased the range of his experiments, and by the end of 1897 he
succeeded in transmitting signals from Alum Bay, in the Isle of Wight,
to a steamer 18 miles away. In 1899 messages were exchanged between
British warships 85 miles apart, and the crowning achievement was
reached in 1901, when Marconi received readable signals at St. John’s,
Newfoundland, from Poldhu in Cornwall, a distance of about 1800 miles.
In 1907 the Marconi stations at Clifden and Glace Bay were opened
for public service, and by the following year transatlantic wireless
communication was in full swing. The sending of wireless signals across
the Atlantic was a remarkable accomplishment, but it did not represent
by any means the limits of the system, as was shown in 1910. In that
year Marconi sailed for Buenos Ayres, and wireless communication with
Clifden was maintained up to the almost incredible distance of 4000
miles by day, and 6735 miles by night. The Marconi system has had many
formidable rivals, but it still holds the proud position of the most
successful commercial wireless system in the world.
We have not space to give a description of the other commercial
systems, but a few words on some of the chief points in which they
differ from the Marconi system may be of interest. We have seen that
an ordinary spark gap, formed by two metal balls a short distance
apart, becomes overheated by the rapid succession of discharges, with
the result that the sparking is irregular. What actually happens is
that the violent discharge tears off and vaporizes minute particles
of the metal. This intensely heated vapour forms a conducting path
which the current is able to cross, so that an arc is produced just
in the same way as in the arc lamp. This arc is liable to be formed
by each discharge, and it lasts long enough to prevent the sparks
from following one another promptly. In the Marconi system this
trouble is avoided by means of a rotating spark gap, but in the German
“Telefunken” system, so named from Greek _tele_, far off, and German
_Funke_, a spark, a fixed compound spark gap is used for the same
purpose. This consists of a row of metal discs about 1/100 inch apart,
and the spark leaps these tiny gaps one after the other. The discs are
about 3 inches in diameter, and their effect is to conduct away quickly
the heat of the discharge. By this means the formation of an arc is
prevented, and the effect of each discharge is over immediately, the
sparks being said to be “quenched.” The short discharges enable more
energy to be radiated from the aerial into the ether, and very high
rates of sparking are obtained, producing a high-pitched musical note.
The “Lepel” system also uses a quenched spark. The gap consists of two
metal discs clamped together and separated only by a sheet of paper.
The paper has a hole through its centre, and through this hole the
discharge takes place, the discs being kept cool by water in constant
circulation. The discharge is much less noisy than in the Marconi and
Telefunken systems, and the musical note produced is so sensitive that
by varying the adjustments simple tunes can be played, and these can
be heard quite distinctly in the telephone at the receiving stations.
In the three systems already mentioned spark discharges are used to
set up oscillatory currents in the aerial, which in turn set up waves
in the ether. Each discharge sets in motion a certain number of waves,
forming what is known as a train of waves. The discharges follow one
another very rapidly, but still there is a minute interval between
them, and consequently there is a corresponding interval between the
wave-trains. In the “Goldschmidt” system the waves are not sent out in
groups of this kind, but in one long continuous stream. The oscillatory
currents which produce ether waves are really alternating currents
which flow backwards and forwards at an enormous speed. The alternating
current produced at an ordinary power station is of no use for
wireless purposes, because its “frequency,” or rate of flow backwards
and forwards, is far too low. It has been found possible however to
construct special dynamos capable of producing alternating current
of the necessary high frequency, and such dynamos are used in the
Goldschmidt system. The dynamos are connected directly to the aerial,
so that the oscillatory currents in the latter are continuous, and the
ether waves produced are continuous also.
The “Poulsen” system produces continuous waves in an altogether
different manner, by means of the electric arc. The arc is formed
between a fixed copper electrode and a carbon electrode kept in
constant rotation, and it is enclosed in a kind of box filled
with methylated spirit vapour, hydrogen, or coal gas. A powerful
electro-magnet is placed close to the arc, so that the latter is
surrounded by a strong magnetic field. Connected with the terminals of
the arc is a circuit consisting of a condenser and a coil of wire,
and the arc sets up in this circuit oscillatory currents which are
communicated to the aerial. These currents are continuous, and so also
are the resulting waves.
The method of signalling employed in these two continuous-wave
systems is quite different from that used in the Marconi and other
spark systems. It is practically impossible to signal by starting and
stopping the alternating-current dynamos or the arc at long or short
intervals to represent dashes or dots. In one case the sudden changes
from full load to zero would cause the dynamo to vary its speed, and
consequently the wave-length would be irregular; besides which the
dynamo would be injured by the sudden strains. In the other case it
would be extremely difficult to persuade the arc to start promptly
each time. On this account the dynamo and the arc are kept going
continuously while a message is being transmitted, and the signals are
given by altering the wave-length. In other words, the transmitting
aerial is thrown in and out of tune alternately at the required long
or short intervals, and the receiving aerial responds only during the
“in-tune” intervals.
The various receiving detectors previously described are arranged to
work with dis-continuous waves, producing a separate current impulse
from each group or train of waves. In continuous wave systems there are
of course no separate groups, and for this reason these detectors are
of no use, and a different arrangement is required. The oscillatory
currents set up in the aerial are allowed to charge up a condenser,
and this condenser is automatically disconnected from the aerial and
connected to the operator’s telephones at regular intervals of about
1/1000 second. Each time the condenser is connected to the telephones
it is discharged, and a click is produced. These clicks continue only
as long as the waves are striking the aerial, and as the transmitting
operator interrupts the waves at long or short intervals the clicks are
split up into groups of corresponding length.
Continuous waves have certain advantages over dis-continuous waves,
particularly in the matter of sharp tuning, but these advantages
are outweighed to a large extent by weak points in the transmitting
apparatus. The dynamos used to produce the high-frequency currents in
the Goldschmidt system are very expensive to construct and troublesome
to keep in order; while in the Poulsen system the arc is difficult to
keep going for long periods, and it is liable to fluctuations which
greatly affect its working power. Although all the commercial Marconi
installations make use of dis-continuous waves exclusively, Mr. Marconi
is still carrying out experiments with continuous waves.
There are many points in wireless telegraphy yet to be explained
satisfactorily. Our knowledge of the electric ether waves is still
limited, and we do not know for certain how these waves travel from
place to place, or exactly what happens to them on their journeys. For
this reason we are unable to give a satisfactory explanation of the
curious fact that, generally speaking, it is easier to signal over long
distances at night than during the day. Still more peculiar is the fact
that it is easier to signal in a north and south direction than in an
east and west direction. There are also remarkable variations in the
strength of the signals at certain times, particularly about sunset
and sunrise. Every station has a certain normal range which does not
vary much as a rule, but at odd times astonishing “freak” distances
are covered, stations having for a short time ranges far beyond their
usual limits. These and other problems are being attacked by many
investigators, and no doubt before very long they will be solved.
Wireless telegraphy already has reached remarkable perfection, but it
is still a young science, and we may confidently expect developments
which will enable us to send messages with all speed across vast gulfs
of distance at present unconquered.
Wireless telephony, like wireless telegraphy, makes use of electric
waves set up in, and transmitted through the ether. The apparatus is
practically the same in each case, except in one or two important
points. In wireless telegraphy either continuous or dis-continuous
waves may be used, and in the latter case the spark-frequency may be as
low as twenty-five per second. On the other hand, wireless telephony
requires waves which are either continuous, or if dis-continuous,
produced by a spark-frequency of not less than 20,000 per second. In
other words, the frequency of the wave trains must be well above the
limits of audibility. Although dis-continuous waves of a frequency
of from 20,000 to 40,000 or more per second can be used, it has been
found more convenient to use absolutely continuous waves for wireless
telephony, and these may be produced by the Marconi disc generator, by
the Goldschmidt alternator, or by the Poulsen arc, the last named being
largely employed.
In wireless telegraphy the wave trains are split up by a transmitting
key so as to form groups of signals; but in telephony the waves are
not interrupted at all, but are simply varied in intensity by means
of the voice. All telephony, wireless or otherwise, depends upon the
production of variations in the strength of a current of electricity,
these variations being produced by air vibrations set up in speaking.
In ordinary telephony with connecting wires the current variations are
produced by means of a microphone in the transmitter, and in wireless
telephony the same principle is adopted. Here comes in the outstanding
difficulty in wireless transmission of speech. The currents used in
ordinary telephony are small, and the microphone works with them
quite satisfactorily; but in wireless telephony very heavy currents
have to be employed, and so far no microphone has proved capable of
dealing effectively with these currents. Countless devices to assist
the microphone have been tried. It was found that one of the causes of
trouble was the overheating of the carbon granules, which caused them
to stick together, so becoming insensitive. To remedy this the granules
have been cooled in various ways, by water, gas, or oil, but although
the results have been improved, still the microphones worked far from
perfectly. Improved results have been obtained also by connecting
a number of microphones in parallel. The microphone difficulty is
holding back the development of wireless telephony, and at present no
satisfactory solution of the problem is in sight.
The transmitting and receiving aerials are the same as in wireless
telegraphy, and like them are tuned to the same frequency. The
receiving apparatus too is of the ordinary wireless type, with
telephones and electrolytic or other detectors.
Wireless telephony has been used with considerable success in various
German collieries, and at the Dinnington Main Colliery, Yorkshire.
Early last year Marconi succeeded in establishing communication by
wireless telephony between Bournemouth and Chelmsford, which are about
100 miles apart; and about the same time a song sung at Laeken, in
Belgium, was heard clearly at the Eiffel Tower, Paris, a distance of
225 miles. The German Telefunken Company have communicated by wireless
telephony between Berlin and Vienna, 375 miles, and speech has been
transmitted from Rome to Tripoli, a total distance of more than 600
miles. These distances are of course comparatively small, but if the
microphone trouble can be overcome satisfactorily, transatlantic
wireless telephony appears to be quite possible.