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.

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.

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.

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.

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.