TALKING IN THE SKY
In one field of war invention the United States held almost a monopoly
and the progress Americans achieved was epoch-making.
Before the war, an aviator when on the wing was both deaf and dumb.
He could communicate with other airplanes or with the ground only by
signal or, for short distances, by radiotelegraphy, but he could not
even carry on conversation with a fellow passenger in the machine
without a speaking-tube fitted to mouth and ears so as to cut out
the terrific roar of his own engine. Now the range of his voice
has been so extended that he can chat with fellow aviators miles
away. This remarkable achievement and many others in the field of
radio-communication hinge upon a delicate electrical device invented by
Deforest in 1906 and known as the "audion." For years this instrument
was used by radiotelegraphers without a real appreciation of its
marvelous possibilities, and, as a matter of fact, in its earlier crude
form it was not capable of performing the wonders it has achieved since
it was taken over and developed by the engineers of the Bell Telephone
System.
THE AUDION
Although the audion is familiar to all amateur radio-operators, we
shall have to give a brief outline of its construction and operation
for the benefit of those who have not had the opportunity to dabble in
wireless telegraphy.
The audion is a small glass bulb from which the air is exhausted to
a high degree of vacuum. The bulb contains three elements. One is a
tiny filament which is heated to incandescence by a battery, so that
it emits negatively charged electrons. The filament is at one side of
the bulb and at the opposite side there is a metal plate. When the
plate and the filament are connected with opposite poles of a battery,
there is a flow of current between them, but because only negative
electrons are emitted by the filament, the current will flow only
in one direction--that is, from the plate to the filament. If the
audion be placed in the circuit of an alternating-current generator,
it will let through only the current running in one direction. Thus it
will "rectify" the current or convert alternating current into direct
current.
But the most important part of the audion, the part for which Deforest
is responsible, is the third element, which is a grid or flat coil of
platinum wire placed between the filament and the plate. This grid
furnishes a very delicate control of the strength of the electric
current between plate and filament. The slightest change in electric
power in the grid will produce large changes of power in the current
flowing through the audion. This makes it possible to magnify or
amplify very feeble electric waves, and the extent to which the
amplifying can be carried is virtually limitless, because a series
of audions can be used, the current passing through the first being
connected with the grid of the next, and so on.
TALKING FROM NEW YORK TO SAN FRANCISCO
There is a limit to which telephone conversations can be carried on
over a wire, unless there is some way of adding fresh energy along the
line. For years all sorts of experiments were tried with mechanical
devices which would receive a telephone message and send it on with
a fresh relay of current. But these devices distorted the message so
that it was unintelligible. The range of wire telephony was greatly
increased by the use of certain coils invented by Pupin, which were
placed in the line at intervals; but still there was a limit to which
conversation could be carried on by wire and it looked as if it would
never be possible to telephone from one end of this big country of ours
to the other. But the audion supplied a wonderfully efficient relay and
one day we awoke to hear San Francisco calling, "Hello," to New York.
Used as a relay, the improved audion made it possible to pick up very
faint wireless-telegraph messages and in that way increased the range
of radio outfits. Messages could be received from great distances
without any extensive or elaborate aërials, and the audion could be
used at the sending-station to magnify the signals transmitted and send
them forth with far greater power.
Having improved the audion and used it successfully for long-distance
telephone conversation over wires, the telephone company began to
experiment with wireless telephony. They believed that it might be
possible to use radiotelephony in places where wires could not be laid.
For instance, it might be possible to talk across the Atlantic.
But before we go farther, just a word of explanation concerning
radiotelegraphy and radiotelephony for the benefit of those who have
not even an elementary knowledge of the subject.
SIMPLE EXPLANATION OF RADIOTELEGRAPHY
Suppose we should set up two stakes in a pond of water, at some
distance from each other, and around each we set a ring-shaped cork
float. If we should move one of these floats up and down on its stake,
it would produce ripples in the water which would spread out in all
directions and finally would reach the opposite stake and cause the
float there to bob up and down in exactly the same way as did the float
moved by hand. In wireless telegraphy the two stakes are represented by
antennæ or aërials and the cork floats are electric charges which are
sent oscillating up and down the antennæ. The oscillations produced at
one aërial will set up electro-magnetic waves which will spread out in
all directions in the ether until they reach a receiving-aërial, and
there they will produce electric oscillations similar to the ones at
the transmitting-antenna.
Telegraph signals are sent by the breaking up of the oscillations at
the transmitting-station into long and short trains of oscillations
corresponding to the dots and dashes of ordinary wire telegraphy. In
other words, while the sending-key is held down for a dash, there
will be a long series of oscillations in the antenna, and for the dot
a short series, and these short and long trains of waves will spread
out to the receiving-aërial where they will reproduce the same series
of oscillations. But only a small part of the energy will act on the
receiving-aërial because the waves like those on the pond spread in all
directions and grow rapidly weaker. Hence the advantage of an extremely
delicate instrument like the audion to amplify the signals received.
The oscillations used in wireless telegraphy these days are very rapid,
usually entirely too rapid, to affect an ordinary telephone receiver,
and if they did they would produce a note of such high pitch that it
could not be heard. So it is customary to interrupt the oscillations,
breaking them up into short trains of waves, and these successive
trains produce a note of low enough pitch to be heard in the telephone
receiver. Of course the interruptions are of such high frequency that
in the sending of a dot-and-dash message each dot is made up of a great
many of the short trains of waves.
Now in radiotelephony it is not necessary to break up the oscillations,
but they are allowed to run continuously at very high speed and act as
carriers for other waves produced by speaking into the transmitter;
that is, a single speech-wave would be made up of a large number of
smaller waves. To make wireless telephony a success it was necessary
to find some way of making perfectly uniform carrier-waves, and
then of loading on them waves of speech. Of course, the latter are
not sound-waves, because they are not waves of air, but they are
electro-magnetic waves corresponding exactly to the sound-waves of
air and at the receiving-end they affect the telephone receiver in
the same way that it is affected by the electric waves which are sent
over telephone wires. The telephone engineers found that the audion
could be used to regulate the carrier-waves and also to superpose the
speech-waves upon them, and at the receiving-station the audion was
used to pick up these waves, no matter how feeble they might be, and
amplify them so that they could be heard in a telephone receiver.
TALKING WITHOUT WIRES
Attempts at long-distance talking without wires were made from Montauk
Point, on the tip of Long Island, to Wilmington, Delaware, and they
were successful. This was in 1915. The apparatus was still further
improved and then the experiment was tried of talking from the big
Arlington station near Washington to Darien, on the Isthmus of Panama.
This was a distance of twenty-one hundred miles, and speech was
actually transmitted through space over that great distance. That
having proved successful, the next attempt was to talk from Arlington
to Mare Island and San Diego, on the Pacific Coast, a distance of over
twenty-five hundred miles. This proved a success, too, and it was
found possible even to talk as far as Honolulu.
[Illustration: (C) G. V. Buck
Radio Head-gear of an Airman]
[Illustration: (C) G. V. Buck
Carrying on Conversation by Radio with an Aviator Miles Away]
The engineers now felt confident that they could talk across the
Atlantic to Europe, and so in October of 1915 arrangements were made to
conduct experiments between Arlington and the Eiffel Tower in Paris.
Although the war was at its height, and the French were straining every
effort to hold back the Germans at that time, and although there were
constant demands for the use of radiotelegraphy, the French showed
such an appreciation of science that they were willing to lend their
aid to these experiments. The Eiffel Tower could be used only for
short periods of time, and there was much interference from other
high-powered stations. Nevertheless, the experiment proved perfectly
successful, and conversation was carried on between our capital and
that of France, a distance of thirty-six hundred miles. At the same
time, an operator in Honolulu, forty-five hundred miles away, heard the
messages, and so the voice at Arlington carried virtually one third of
the way around the globe. After that achievement, there was a lull in
the wireless-telephone experiments because of the war.
But there soon came an opportunity to make very practical use of all
the experimental work. As soon as there seemed to be a possibility that
we might be drawn into the war, the Secretary of the Navy asked for the
design of apparatus that would make it possible for ships to converse
with one another and with shore stations. Of course all vessels are
equipped with wireless-telegraph apparatus, but there is a decided
advantage in having the captain of one ship talk directly with the
captain of another ship, or take his orders from headquarters, with an
ordinary telephone receiver and transmitter. A special equipment was
designed for battle-ships and on test it was found that ships could
easily converse with one another over a distance of thirty-five miles
and to shore stations from a distance of a hundred and seventy-five
miles. The apparatus was so improved that nine conversations could be
carried on at the same time without any interference of one by the
others.
[Illustration: (C) American Institute of Electrical Engineers
Long Distance Radio Apparatus at the Arlington (Va.) Station, with
enlarged view of the Type of Vacuum Tube used]
When it became certain that we should have to enter the war, there came
a call for radiotelephone apparatus for submarine-chasers, and work was
started on small, compact outfits for these little vessels.
RADIOTELEPHONES FOR AIRPLANES
Then there was a demand for radiotelephone apparatus to be used on
airplanes. This was a much more complicated matter and called for a
great deal of study. The way in which problem after problem arose and
was solved makes an exceedingly interesting narrative. It seemed almost
absurd to think that a delicate radiotelegraph apparatus could be made
to work in the terrific noise and jarring of an airplane. The first
task was to make the apparatus noise-proof. A special sound-proof room
was constructed in which a noise was produced exactly imitating that of
the engine exhaust of an airplane engine. In this room, various helmets
were tried in order to see whether they would be proof against the
noise, and finally a very suitable helmet was designed, in which the
telephone receiver and transmitter were installed.
By summer-time the work had proceeded so far that an airplane equipped
with transmitting-apparatus could send spoken messages to an operator
on the ground from a distance of two miles. The antenna of the airplane
consisted of a wire with a weight on the lower end, which hung down
about one hundred yards from the body of the machine. But a trailing
antenna was a nuisance in airplane manoeuvers, and it was also found
that the helmet which was so satisfactory in the laboratory was not
just the thing for actual service in an airplane. It had to fit very
tightly around the ears and the mouth, and as the airplane went to
high altitudes where the air-pressure was much lower than at the
ground level, painful pressures were produced in the ears which were
most annoying. Aside from that, in actual warfare airplanes have to
operate at extreme heights, where the air is so rare that oxygen must
be supplied to the aviators, and it was difficult to provide this
supply of oxygen with the radio helmet tightly strapped to the head of
the operator. But after considerable experiment, this difficulty was
overcome and also that of the varying pressures on the ears.
Another great difficulty was to obtain a steady supply of power on
the airplane to operate the transmitting-apparatus. It has been
the practice to supply current on airplanes for wireless-telegraph
apparatus by means of a small electric generator which is revolved by
a little propeller. The propeller in turn is revolved by the rush of
air as it is carried along by the plane. But the speed of the airplane
varies considerably. At times, it may be traveling at only forty miles
per hour, and at other times as high as one hundred and sixty miles per
hour, so that the little generator is subjected to great variations of
speed and consequent variations of voltage. This made it impossible
to produce the steady oscillations that are required in wireless
telephony. After considerable experiment, a generator was produced with
two windings, one of which operated through a vacuum tube, somewhat
like an audion, and to resist the increase of voltage produced by the
other winding.
Then another trouble developed. The sparks produced by the magneto
in the airplane motor set up electro-magnetic waves which seriously
affected the receiving-instrument. There was no way of getting rid of
the magneto, but the wires leading from it to the engine were incased
in metal tubes which were grounded at frequent intervals, and in that
way the trouble was overcome to a large extent. The magnetos themselves
were also incased in such a way that electro-magnetic waves would not
be radiated from them.
Instead of using trailing wires which were liable to become entangled
in the propeller, the antenna was extended from the upper plane to the
tail of the machine, and later it was found that by using two short
trailing antennæ one from each tip of the wings, the very best results
could be obtained. Still another development was to embed the antenna
wires in the wings of the plane.
It was considered necessary, if the apparatus was to be practicable,
to be able to use it over a distance of two thousand yards, but in
experiments conducted in October, 1917, a couple of airplanes were able
to talk to each other when twenty-three miles apart, and conversations
were carried on with the ground from a distance of forty-five miles.
The conditions under which these distances were attained were
unusual, and a distance of three miles was accepted as a standard for
communication between airplanes. The apparatus weighed only fifty-eight
pounds and it was connected with both the pilot and the observer so
that they could carry on conversations with each other and could both
hear the conversation with other airplanes or the ground. As a matter
of fact, airplanes with standard apparatus are able to talk clearly
to a distance of five miles and even to a distance of ten miles when
conditions are favorable, and they can receive messages from the ground
over almost any distance.
A similar apparatus was constructed for submarine-chasers with
a standard range of conversation of over five miles. Apparatus
was manufactured in large quantities in this country and all our
submarine-chasers were equipped with it, as well as a great many
of our airplanes and seaplanes, and we furnished radio-apparatus
sets to our allies which proved of immense value in the war. This
was particularly so in the case of submarine detection, when it was
possible for a seaplane or a balloon to report its findings at once
to submarine-chasers and destroyers, and to guide them in pursuit of
submarines.
The improved audion holds out a wonderful future for radiotelephony.
For receiving, at least, no elaborate aërial will be needed, and with a
small loop of wire, an audion or two, and simple tuning-apparatus any
one can hear the radio gossip of the whole world.
TELEGRAPHING TWELVE HUNDRED WORDS PER MINUTE
Some remarkable advances were made in telegraphy also. During the
war and since, messages have been sent direct from Washington to all
parts of the world. In the telegraph room operators are connected by
wire with the different radio stations along the coast and they can
control the radio transmitters, sending their messages without any
repeating at the radio stations. Long messages are copied off on a
machine something like a type-writer, which, however, does not make
type impressions, but cuts perforations in a long sheet of paper. The
paper is then run through a transmitter at a high speed and the message
is sent out at a rate of as much as twelve hundred words a minute. At
the receiving-station, the message is received photographically on a
strip of paper. The receiving-instrument has a fine quartz thread in
it, which carries a tiny mirror. A beam of light is reflected from the
mirror upon the strip of sensitized paper. The radio waves twist the
quartz thread ever so slightly, which makes the beam of light play back
and forth, but of course the motion is greatly magnified. In this way
a perfect record is made of the message in dots and dashes, which are
translated into the corresponding letters of the alphabet.
DETECTING RADIO SPIES
There is another radio invention which we contributed during the war,
that proved of utmost service in thwarting German spies and which
is going to prove equally valuable in time of peace. Although a war
invention, its peacetime service will be to save lives. It is a very
simple matter to rig up a wireless-telegraph system that will send
messages to a considerable distance, and simpler still to rig up a
receiving-set. European governments have always discouraged amateur
radiotelegraphy, but in this country restrictions used to be so
slight that almost any one could set up and use a radio set, both for
receiving and for transmitting. When we entered the war we were glad
that amateurs had been encouraged to play with wireless, because we had
hundreds of good radio operators ready to work the sets which the army
and the navy needed.
But this was a disadvantage, too. Many operators were either Germans
or pro-Germans and were only too willing to use their radio experience
in the interest of our enemies. It was a simple matter to obtain
the necessary apparatus, because there was plenty of it to be had
everywhere. They could send orders to fellow workers and receive
messages from them, or they could listen to dispatches sent out by
the government and glean information of great military and naval
importance. The apparatus could easily be concealed: a wire hung inside
a chimney, a water-pipe, even a brass bedstead could be used for the
receiving-aërial. It was highly important that these concealed stations
be located, but how were they to be discovered?
THE WIRELESS COMPASS
This problem was solved very nicely. The audion had made it possible to
receive radio signals on a very small aërial. In place of the ordinary
stationary aërial a frame five feet square was set up so that it could
be turned to any point of the compass. A few turns of copper-bronze
wire were wound round it. This was called the "wireless compass."
It was set up on the roof of the radio station and concealed within
a cupola. The shaft on which it was mounted extended down into the
operating-room and carried a wheel by which it could be turned. On the
shaft was a circular band of aluminum engraved with the 360 degrees of
the circle, and a couple of fixed pointers indicated true north and
south. Now when a signal was received by the aërial, if it struck the
frame edgewise the radio waves would reach one side before they would
the other. Taking a single wave, as shown by the drawing, Fig. 11, we
see that while the crest of the wave is sweeping over one side of the
frame, the trough of the wave is passing the other side. Two currents
are set up in the radio compass, one in the wires at the near side of
the compass, and another in the wires at the far side of the compass.
As these currents are of the same direction, they oppose each other and
tend to kill each other off, but one of the currents is stronger than
the other because the crest of the wave is sweeping over that side,
while the trough of the wave is passing over the other. The length
of the wave may be anything, but always one side will be stronger than
the other, and a current equal in strength to the difference between
the two currents goes down into the operating-room and affects the
receiver. Now when the compass is set at right angles to the oncoming
wave, both sides are affected simultaneously and with the same
strength, so that they kill each other off completely, and no current
goes down to the receiver. Thus the strength of the signal received can
be varied from a maximum, when the compass is parallel to the oncoming
waves, to zero, when it is at right angles to them.
[Illustration: Courtesy of the "Scientific American"
FIG. 11. The radio compass turned parallel to an oncoming
electro-magnetic wave]
To find out where a sending-station is, the compass is turned until the
loudest sound is heard in the receiver and then the compass dial shows
from what direction the signals are coming. At the same time, another
line on the signals will be found by a second station with another
compass. These directions are traced on a map; and where they meet, the
sending-station must be located.
With this apparatus it was possible to locate the direction of the
station within a degree.
After the station had been located as closely as possible in this
way, a motor-truck was sent out in which there was a concealed radio
compass. The truck would patrol the region located by the fixed
compasses, and with it the position of the concealed station could be
determined with perfect accuracy. The building would be raided and its
occupants jailed and the radio equipment confiscated.
Even receiving-sets were discovered with the portable compass, but to
find them was a far more difficult task. For the receiving of messages
from distant points without a conspicuous aërial an audion would have
to be used and this would set up feeble oscillations which could be
picked up under favorable conditions by the portable compass.
PILOTING SHIPS INTO PORT
And now for the peace-time application of all this. If the compass
could be used to find those who tried to hide, why could it not also be
used to find those who wished to be found?
Every now and then a ship runs upon the rocks because it has lost its
bearings in the fog. But there will be no excuse for such accidents
now. A number of radio-compass stations have been located around the
entrance and approach to New York Harbor. Similar stations have been,
or soon will be, established at other ports. As soon as a ship arrives
within fifty or a hundred miles of port she is required to call for
her bearings. The operator of the control station instructs the ship
to send her call letters for thirty seconds, and at the same time
notifies each compass station to get a bearing on the ship. This each
does, reporting back to the control station. The bearings are plotted
on a chart and inside of two minutes from the time the ship gives her
call letters, her bearing is flashed to her by radio from the control
station.
[Illustration: Courtesy of the "Scientific American"
FIG. 12. Approaches to New York Harbor showing location of three radio
compass stations and how position of a ship sending signals from A may
be determined]
The chart on which the plotting is done is covered with a sheet of
glass. Holes are pierced through the glass at the location of each
compass station. See Fig. 12. On the chart, around each station,
there is a dial marked off in the 360 degrees of the circle. A thread
passes through the chart and the hole in the glass at each station.
These threads are attached to weights under the chart. When a compass
station reports a bearing, the thread of that station is pulled out and
extended across the corresponding degree on the dial. The same is
done as each station reports and where the threads cross, the ship must
be located.
Not only can the direction-finder be used to pilot a ship into a
harbor, but it will also serve to prevent collisions at sea, because
a ship equipped with a radio compass can tell whether another ship is
coming directly toward her.
And so as one of the happy outcomes of the dreadful war, we have an
apparatus that will rob sea-fogs of their terrors to navigation.