Electrical Experimenter, February, 1920, pages 1000-1001, 1080-1083:

The  Versatile  Audion

Some of the Many Practical Uses to Which the Audion Has Been Adapted
MANY world-startling experiments in Radio Science have been either directly or indirectly based on the remarkable performance of the Audion, the little evacuated glass bulb which greatly resembles the ordinary incandescent lamp, but which contains, besides the filament, two other elements,--a wire grid and a flat metal plate. Some of the most remarkable phenomena in the realm of pure and applied science are due to the as yet not fully understood electronic action taking place within this bulb.
    The action in the audion is based on the fact that a heated body, such as a heated filament, radiates millions of tiny electric charges of a negative character or electrons, as they are called by the scientist. The wire grid in the audion is placed between the heated filament, which is constantly shooting off the electrons, and the cold plate. When a battery or other source of electric current is connected to the filament and plate, a positive current flows from the cold plate to the heated filament, and this is due to the fact that the electronic flow in any case is opposite in direction to the positive current flow. Thus, the hotter the filament, other things being equal, the more electrons there are radiated, and the stronger the current which can be transferred across the space between the plate and the filament, up to the point of saturation.
    Now, if we can manage to vary the electron flow between the heated filament and the plate or wing, in some delicate manner, we would have, it would seem, a very sensitive wireless detector, and this is just what Dr. de Forest thought when he first started to develop the audion idea. Suiting his action to the thought, he produced after years of experiment a bulb made in this fashion with three electrodes, one at least of which must be heated. The first grid audion gave results far beyond all expectations.
    We may say that today it would really be very difficult to conceive of a single radio or electrical device which is more versatile or universal in its applications than the audion. Some of the more prominent and practical applications of this device are illustrated and described herewith, but it has apparently only just started on its mission of usefulness.

Fig. 1
    First and foremost in many ways is the application of the audion, or vacuum valve, as it is sometimes miscalled, to the reception of radio messages. The audion will receive and amplify under proper conditions, both radiotelegraphic and radiotelephonic messages. It has the advantage over the crystal and other forms of detectors that, whereas these depend upon the finding of a sensitive spot, often microscopical in size, where the metal wire (cat whisker) touching it proves most responsive; the audion, once the battery currents are properly adjusted to it, keeps right on receiving hour in and hour out.
    The action of the audion in this case is along the lines cited in the introduction; that is, one of the antenna circuit wires carrying the incoming signal currents is connected to the grid or intermediate member of the audion. Thus, any variations of electric charge or current on the grid, as would be occasioned by the dots and dashes or short and long signal currents, or else the fluctuations of the radiotelephonic current, causes a corresponding and exactly similar variation in the electron stream passing from the heated filament to the cold plate. It is at once evident, therefore, that as these variations are occasioned by the increasing and decreasing volumes of electric charge on the grid, the current flow from the high-voltage or "B" battery connected in the telephone receiving circuit to the plate, will be varied in correspondence to the grid charge variations. This action is very delicate, positive and accurate, and where more than one bulb is used is of enormously high amplifying character.
    The audion, with slight circuit modifications, can be used for receiving both damped and undamped radio signals and therein lies one of the most tremendous advantages of this device. By the mere turn of a switch or readjustment of the circuit the audion can be made to oscillate and receive either damped or undamped radio signals equally well. This was of inestimable service to the Allied armies and navies during the war, as may be readily imagined, and audion sets were built by the thousands, and the consumption of bulbs by the Allied armies and naval fleets was at the rate of 1,000,000 per year during the last months of the war. See figure 1.

Fig. 2
    At Fig. 2 the audion is shown in use for transmitting wireless messages. It can and has been very successfully used for sending radiotelegraphic and radiotelephonic messages. Usually, but not in this case, the audion for this apparatus is made of a somewhat larger size; the average audion such as that used for receiving apparatus is about the size of a 30 watt tungsten lamp, but the bulb is tubular in form and not pear-shaped. The French audions, however, resemble the average Tungsten lamp shape, while the German types are tubular.
    It was found in various experiments that the vacuum tube of the audion type; that is, under certain circuit conditions, and where a sufficiently high voltage direct current (such as that produced by a 500 to 800 volt, D.C. dynamo) was applied to the plate, that the bulb action took on an oscillating nature and caused a radio-frequency alternating or oscillating current of an undamped form to be produced. By simply connecting up a microphone in one of the circuits, for which there are now several different schemes available, it thereupon became readily possible to easily and perfectly modulate this converted radiofrequency oscillating current produced by the audion into speech currents, which can be radiated from the antenna in the form of Hertzian waves and picked up at any wireless receiving station.
    Several years ago the first trans-Atlantic wireless telephone speech was transmitted successfully, thanks to several hundred large oscillating audions, which were connected up in a common bank in the government high-power station at Arlington, Va. Just to show how easy it is to control 50 or 100 K.W. of energy as produced by a bank of audions such as this, it may be mentioned that an ordinary microphone, such as we have in this case, can readily modulate or control telephonically the entire bank of bulbs, totaling 15 K.W.
    During the war many of these audion transmitting sets with one or more bulbs on them were designed and built for the American and other Allied armies. The U. S. Navy Department, as well as the aircraft divisions of both army and navy, have also found the audion bulb transmitting sets of inestimable value, both for transmitting radiotelegraphic and radiotelephonic messages.

Fig. 3
    The audion as an amplifier of radio signals, either telegraphic or telephonic, has proven to be far in the lead of any other form of amplifier ever devised. It is not to be inferred from this statement that there are not available at the present time several very interesting and very useful forms of electro-magnetic amplifiers, for there are several which possess meritorious features, indeed,--but the audion is unique in that, with very simple circuit arrangements, one can keep on connecting up audion bulbs until they have amplified the faintest imaginable signal or spoken word until it roars forth with the strength of a hundred horses. Practically every army and navy station today is equipt with an audion amplifier and signals from far distant ship and shore radio stations which cannot even be heard with a crystal detector and telephones, become clearly audible and readable with the audion amplifier, when it is connected up to the receiving set.
    It is interesting to recall that several years ago, when the first radio transmission was attempted between San Francisco and Honolulu, that the signals from Honolulu would die out as sunrise approached, and the dots and dashes which had been coming in very faint during the night, would fade away to zero and become absolutely inaudible even with the most sensitive detectors and high resistance telephones. Audion amplifiers were then tried in the receiving stations in San Francisco, and reception was maintained and kept up for several hours after sunrise, due to their marvelous detecting and amplifying properties. In other words, they actually "made something out of nothing," as the radio operators said. See Fig. 3.

Fig. 4
    Fig. 4 shows what we may call the "audion organ," and this was described in the December, 1915, Electrical Experimenter, together with a large illustration of the device as well as a diagram of the connections for producing pure musical notes or any number of notes with one or more bulbs, This device brings out one of the prominent characteristics of the audion, i. e., that once it has been set to oscillating under certain capacity and inductance circuit conditions, it will invariably maintain this condition for an indefinite period and, furthermore, the frequency of the oscillation or the rate at which the circuit vibrates will depend upon the capacity and inductance in the circuit connected to the audion.
    Thus, by having a set of keys connected to various taps on the inductance coil, so that differing amounts of inductance can be interposed in the circuits, various tunes can be played even with a single bulb, and when more than one tune is to be played, or chords, a number of bulbs will have to be used as shown in the illustration. A very pure sweet note is produced by the audion in this fashion, a revelation even to the ear of the trained musician.

Fig. 5
    It is well known today that Transcontinental Telephony was made possible by the audion repeater and amplifier. It is conceivable that, had not the audion been invented and developed to the stage that it was a few years ago when the first transcontinental telephone line was put into successful operation, that some other form of successful telephonic repeater and amplifier would have been invented, altho 20 years of experiment by telephone engineers had failed to produce such an amplifier; but one thing is certain,--it would surely have been a very difficult, and we might almost say unnecessary task, to endeavor to devise a more efficient and reliable telephone amplifier or repeater than the vacuum valve. Audions are placed at several points along the transcontinental telephone line, such as at Chicago, Denver, etc.
    This telephone circuit is about 3,500 miles in length and one can pick up the receiver in New York and hear the voice on the Pacific Coast just as perfectly as if he had simply called up his suburban home 15 or 20 miles out of town. See Fig. 5.

Fig. 6
    About a year ago the American Telephone and Telegraph Company demonstrated over a circuit between New York and Pittsburg, as well as on several other circuits, that it was successfully and accurately transmitting and receiving several telephone messages over a single wire at the same time, by utilizing audion generators of high-frequency currents, each frequency having a different value so as not to interfere with the others.
    Major-General George O. Squier was the first to suggest the application of such currents to the multiplex telegraph and telephone system and took out patents on this system about ten years ago. The test conducted by the A. T. & T. Co. demonstrated conclusively that his "stunt" would certainly work.
    At the present time the first cost of the necessary apparatus for producing the essential high-frequency currents prohibits it from being adopted, except over certain circuits, and particularly under military conditions, where this scheme is of undeniable and invaluable advantage, which may often have to be made use of. For example, imagine the advantage a military unit would possess when but one wire exists between two widely separated points and instead of sending but one telephone message or current over this wire, five or six could be sent over it simultaneously, while telegraph signals could also be exchanged over it while the talking is being carried on, without any interference.
    Such is the work of the multiplex telephone backed by the Audion, where the audion generator is used for producing each frequency. A different frequency is used for each telephone message. See Fig. 6.

Fig. 7
    In the old days, especially at the circus and theater we were accustomed to hear the voice amplified at times by means of a large megaphone, thru which a person endeavored to shout. But the voice was invariably muffled and did not sound as clear as the original by the time it reached you. Now, thanks to the audion amplifier, the human voice can be amplified until it is so loud that it will almost scare you to death, and always it is articulate and distinct.
    Some months ago, while the Victory Liberty Loan was under way, there was installed a remarkably loud and powerful voice amplifier along Victory Way, on Park Avenue. Fig. 7 illustrates this installation, where the voice of the speaker was amplified until it was equivalent to approximately 100 H. P. of energy. In other words, you could speak with a "100 horsepower voice" if you happened to be one of the select few who addrest the thousands gathered along Victory Way during their daily exercises. In some cases the speaker was located in Washington; his voice was then transmitted telephonically over special leased wires to the audion laboratory on Victory Way and there amplified, whence the powerful intensified voice travelled along the wires above Victory Way to a series of loud-speaking telephones provided with large horns, and in this way thousands of people could hear the voice at the same time.

Fig. 8
    This application is illustrated in Fig. 8. The audion is often used as a telephone modulator, especially in connection with high-frequency alternators, or dynamos, rated at say 200 K.W. or 270 H.P. as in the installation at New Brunswick, N. J., where the Alexanderson alternator is making some very remarkable radio history. Here vacuum bulbs of the audion type, known as Pliotrons, used in connection with an ordinary microphone such as that on your desk telephone instrument, enables the small energy of your voice (about 1/6000 of a horse-power) to easily and perfectly modulate or control the tremendous amount of power produced by the Alexanderson alternator, or 270 horsepower.

Fig. 9
    Perhaps in no other single field are the advantages of the vacuum bulb transmitter and receiver more apparent than in aviation. The usual radio transmitting set had several disadvantages for aircraft radio where the open spark gap is used, which present a serious fire hazard, and also the ordinary crystal detector receiving set was almost worthless on board the airplane, for there was such a tremendous amount of noise present from the airplane engine that unless the signals received were exceedingly strong, they could not be heard by the pilot or radio officer.
    Fig. 9 illustrates the applications of aircraft radio and shows how communication can be successfully and efficiently carried on from a shore station to balloons and airplanes, and also between two or more airplanes of a flying squadron or again as well as between airplanes and balloons. Radiotelephonic communication can be carried both ways, without any fire hazard and with a minimum amount of power, thanks to the audion.

Fig. 10
    Brief reference was made previously to the fact that several years ago the first trans-oceanic radiotelephonic messages were successfully transmitted and received, thanks to the audion, several hundred of which were connected on parallel, so as to form a veritable power plant at Arlington, Va. See Fig. 10.
    There are several interesting developments now going on in this direction, which will undoubtedly bear fruit shortly. One of these lies in the development of large specially cooled high-power bulbs for generating radio or high-frequency energy when high-voltage D.C. energy, to the plate and other members of the bulb. One of the principal advantages of audion transmitting sets, even in high-power units, lies in the fact that an ordinary microphone only is required to modulate or perfectly control the total output which may amount to 50 to 200 kilowatts.

Fig. 11
    One could write an entire paper on the subject of the audion detectophone alone, if he was to even describe the applications of this most valuable instrument during the recent World War.
    Telephone and telegraph messages were in some instances intercepted one mile back of the lines by a sensitive microphone, and these signals then amplified by a six, eight, or ten-stage audion amplifier, until they were clearly audible and readable by our intelligence officers.
    Radio signals were in some cases picked up from far distant enemy stations, which altho in some cases of small size were of extreme strategic importance. See Fig. 11. Just to show how sensitive some of these measurements had become under the pressure of war, it may be mentioned that with a 10-stage audion amplifier in one of the college laboratories in New York City, radio signals were received with such powerful intensity, from the Nauen station in Germany, that they could be received on a loop-antenna six feet square, located in the basement and directional readings taken on this station, nearly 4,000 miles away, to within a fraction of a degree.
    The French engineers developed some very excellent radio amplifying sets of surprising sensitivity and at a later date we hope to present some of the features of these sets in a special paper.
    The English radio experts developed a 19-stage audion amplifier, with which it became possible to hear buzzer signals being transmitted between German warships over 100 miles away; and the beauty of this marvelous amplification work is that there is practically no distortion or attenuation of the current or signals, whether they be telegraphic or telephonic.
    A dot is a dot, and a dash is a dash, whether it is received and interpreted in the first stage or in the twelfth or nineteenth stage of the audion amplifier.

Fig. 12
    As an illustration of a newly devised "audion clock," consider the system of Fig. 12-A which shows how a three-electrode tube may be used to maintain undamped oscillations of a pendulum.* Two sets of magnet coils are respectively inserted in the grid and plate circuits of the tube as indicated and placed in front of a small iron armature which is integral with the pendulum. As the pendulum is swung out of position, it oscillates back and forth, moving one end of the armature alternately toward and away from the grid coil. This induces an alternating potential between the grid and filament of the tube, which in turn varies the current in the plate circuit and plate coil. There results a correspondingly varying attraction of the coil on the opposite end of the armature which, for suitable magnitude of the currents and proper polarity, of the connections has such a phase relation with respect to the oscillation cycle of the pendulum as to sustain its motion continuously. The energy expenditure from the plate battery is thus seen to compensate for the friction losses, which in the absence of the vacuum tube device would damp out the oscillations of the pendulum or bring it to rest.


    A somewhat different case is that of Fig. 12-B which, however, is merely a different application of the same fundamental principle. The grid and plate, coils are wound over iron cores having a gap in which is placed an iron disc which is free to rotate around the central axis. This disc is provided with a number of teeth. When set in motion, the teeth and slots of the disc alternately pass the iron core yoke of the grid coils, inducing an alternating grid E. M. F. which in turn synchronously varies the current in the plate coils. The attraction of the latter on the rotor teeth thus varies synchronously with the motion of the angular position of the two sets of coils around the disc and the proper polarity of the connections; these variations will occur at such times that the rotor is kept in continuous motion.

See "Radio Engineering Principles," by Lauer and Brown, 1920. Pages 293-295.