Radio Broadcast, July, 1922, pages 199-205:
What Everyone Should Know About Radio History
By Prof. J. H. MORECROFT
AT A recent dinner attended by the writer, the principal speakers of the evening both took as their theme the complacence with which we Americans take for granted the many conveniences and comforts surrounding us, which the application of modern science has made possible. They were both foreign born, both had come to America when young, and both had achieved remarkable success scientifically and financially after adopting the United States as their new home. Both of them are endowed with keen intellects and sound judgment of men and events, which attributes no doubt contributed largely to their success, but both of them expressed the opinion later that they really saw and appreciated the advantages and opportunities of America so much more than the average American that, in the race for achievement, the native born was actually much handicapped because he took so much for granted, without inquiring how wonderful the things about him really were and how they came to be developed.
WHAT AN IMMIGRANT BOY SAW
Professor Pupin, one of our best known and most successful scientists, is fond of relating his early impressions of America; the first walk he took after landing at Castle Garden was through the lower part of New York where the streets were lined with poles carrying hundreds of telephone and telegraph wires. Having been told that signals and speech were being conveyed over these wires from city to city, scores of miles, he was filled with awe and amazement; what an opportunity there must be, he thought, in a land where such things were a part of the every day life of the people! To the native New Yorker these wire-laden poles meant nothing; he had seen them gradually installed around him, and they incited in him neither awe nor inspiration. But to young Pupin, fresh from a land of no scientific development, they spelled all kinds of possibility and opportunity; he didn't merely take them for granted, but inquired as to how and when and where and why these speech-carrying wires came about, how they operated, and later how their operation might be improved. The inspiration he received started him on that career which brought him fame and reward and made him finally the best known scientist in the field of telephone communication.
AFTER A CENTURY OF EFFORT
An art or science is of importance to mankind in direct proportion to the benefits men derive therefrom; the appreciation of radio, and to a certain extent the pleasure arising from it, will be greatly increased by a knowledge of its principles and development. The accomplishments of the early workers, marking out the trail which was to lead to the present state of the art, make interesting reading and serve well to lay the background for discussing the work of the later scientists and inventors whose contributions are directly incorporated in the radio receiving and transmitting equipments of to-day.
Every one is now becoming more or less familiar with radio communication, and it will soon be taken for granted as much as is the telephone; to the average person the radio entertainment every evening will soon cause no more wonder or interest than do the phonograph or movies. Actually, the simple receiving set of to-day, picking up music or speech from a transmitting station many miles distant, represents the result of nearly a century of effort and development by scores of scientists and inventors; before we become too complacent in the matter, and take the radio telephone in the same matter of fact way we do the rest of our applied science miracles, it is worth while to review their labors and progress, as a knowledge of their work will make the evening's radio concert the more pleasurable and appreciated. It is with this idea in mind that the following brief story of the wireless telegraph has been written.
The earlier name for communication between two stations without the use of connecting wires was the wireless telegraph, but for reasons to be shortly pointed out the term radio telegraph or radio communication is now generally used and preferred. There are three closely allied developments in the growth of the radio of to-day, all of which contributed their share toward our knowledge of the art. The first has to do with the early attempts to carry on ordinary telegraph communication without wires, the earth's surface forming the conducting medium between the two stations. A great deal of work was done in this field by many workers; the reward for a successful solution would have been great as it might have made unnecessary, to some extent, the very expensive cables being installed for transoceanic telegraphy. This scheme of using the earth for conductor found application during the war just past for communication from the front line trenches and is well known to those acquainted with the work of the Signal Corps, where it goes by the abbreviation of its French name, T.P.S. (Telegraphie Par Sol).
THEN THE IDEA OF INDUCTION WAS TRIED
A second line of work used no conducting medium whatsoever between the two stations; comparatively slow change of current in one coil was used to induce currents in another coil in the vicinity and these induced currents, by some prearranged code, were used to convey information. This work was begun in England and the United States at about the same time, by independent workers; it did not apparently promise much success at the time, but with our present knowledge of the art it seems that some of the experimenters missed the real solution of the problem by a very narrow margin. This scheme has recently received much public notice because of its application to the guiding of vessels into a harbor during the night or in a fog, when ordinary methods of navigation are not available. In this method of navigation, a cable laid in the channel is traversed by alternating current and coils placed on the sides of the vessel's hull receive induced currents from the cable and the navigator can maneuver his vessel by the relative strengths of the signals received on the two sides.
The third line of work involved the same general idea as the foregoing, but the changes of current were thousands of times as rapid as those formerly used; instead of using the ordinary phenomena of induction, as explained by Faraday and Henry, a new concept of radiated power was invoked and with this step taken, success was assured. As long as the communication between the two stations depended upon the induction ideas of Faraday and Henry the possible separation of the two stations was but a few times the dimensions of the coils used at the stations; when high frequency radiated power was utilized, the possible distance of communication was increased thousands of times and made feasible the transmission of signals between any two points located upon the surface of the earth.
THE FIRST EXPERIMENTS BY STEINHEIL
In 1837 Professor Steinheil, of Munich, while making some experiments with the telegraph apparatus ordinarily using two wires, one for the outgoing current and another for the return, found that it was possible to dispense with one of the two wires hitherto thought necessary, and use only one wire. This one wire was connected, at the transmitting end through battery and key, to large plates buried in the earth and at the receiving end it was similarly connected to ground through whatever type of receiving apparatus was used. He thus showed that the ordinary one wire telegraph system of to-day, using the earth as the return was possible. This experience evidently aroused Steinheil's imagination, as he suggested, in 1838, when discussing the results of his experiments, that it might be possible to carry on communication with no connecting wires at all between the two stations!
PROFESSOR MORSE'S WIRELESS
In 1842, Professor Morse in America, actually did establish telegraphic communication between two stations on the opposite banks of a river, there being no wires at all crossing the river. Along one bank of the river he laid a wire in which were contained his sending battery and key; this wire terminated in two metal plates placed in the river itself. These plates were separated from each other by a distance greater than the width of the river. A similar wire and set of plates was used on the opposite side of the river, the plates on one bank being opposite those on the other. The receiving galvanometer was inserted in series with this second wire. When the sending switch was closed it sent current through the river water from one plate on the sending bank to the other. The current spread throughout the river and some of it strayed to the opposite bank, flowing through the opposite plates and wire and thus through the receiving instrument. Although but a small part of the current reached the opposite bank it was sufficient to actuate the galvanometer used for receiving, and thus wireless telegraphy was an accomplished fact. It may be noted that quite long wires were necessary on the two banks of the stream so it could not logically be called wireless communication, but it must be remembered that such is always the fact with our present radio stations. In a modern radio trans-Atlantic station the sending antenna may contain 50 miles of wire in the overhead network and perhaps even more buried underground. The essential point in wireless communication is that there must be no wires connecting one station with the other.
BRITISH SCIENTISTS WHO CONTRIBUTED
In 1859, in Dundee, Lindsay was working along the same lines that Morse had followed, apparently unacquainted with Morse's experiments. He made many tests and endeavored to find the laws of transmission distance in terms of the size of plates used, length of land wires, size of galvanometer coil, etc. He came to the conclusion that if two plates were immersed in the ocean, one off the most northerly part of Scotland and the other off the southern coast of England, if a powerful set of batteries was used for sending, and if a galvanometer coil weighing two hundred pounds were used at the receiving station, it would be possible to send messages from England to America through the ocean water. We know now that the laws he deduced were not quite correct and that such a scheme is not feasible. The idea of a receiving coil weighing two hundred pounds is interesting when we consider that the coil of the galvanometer actually used to-day weighs less than an ounce.
In 1845 Wilkins, in England, suggested that Morse's scheme be used in establishing wireless communication with France, across the English Channel, the same feat that was to make Marconi famous fifty years later, using a different and more effective form of transmission.
Many more experimenters than the few mentioned here worked in this field, endeavoring to eliminate the connecting wire between the two stations, among them Professor Trowbridge, of Harvard. He reached the conclusion that trans-Atlantic communication by Morse's scheme might be possible if the two plates to be submerged in the ocean were as far apart as are Nova Scotia and Florida. The wire thus required to connect the two plates would be as long as the distance to be traversed, a statement which gives the approximate range for this type of wireless transmission. The laws of the spreading of current were better known to Trowbridge than they were to Lindsay when he first put out his project, and furthermore the telephone receiver had been invented in the mean time which gave to the scheme a receiver much more sensitive than anticipated by Lindsay.
Trowbridge also put forth the quite feasible scheme of fitting a ship with submerged plates in bow and stern (or bow plate and a trailing insulated wire astern, carrying the second plate at its end) and sending out into the ocean an interrupted current which would spread out all around the ship; another ship similarly equipped with plates and a telephone receiver for listening, would be able to detect the presence of the first ship, thus rendering collision in case of fog much less likely. If the present scheme of radio communication had not come into the field, it seems likely that Trowbridge's scheme would have been universally adopted. If the trailing wire should be one quarter of a mile long, a second ship would be able to detect the presence of the first at a distance of about one half a mile and this would evidently give sufficient warning to prevent collision.
ALEXANDER GRAHAM BELL'S EXPERIMENTS
In 1882 Alexander Graham Bell tried out the scheme of using two charged metal plates immersed in water for communication. Using boats with a submerged plate at the bow and the second plate at the end of a trailing wire one hundred feet long, using interrupted current in one boat and the telephone receiver for the detector in the other, he was able to get signals when the boats were separated about one half a mile. This possible distance will be much less when the boats are in salt water than when in the fresh water of a river, however.
In the T. P. S. scheme of the army, two iron stakes are driven into the earth at a separation as great as feasible; a powerful buzzer, with battery and key, is placed in series with the wire which connects these two stakes. If two other stakes are driven into the ground some distance behind the front line trench where the first pair of stakes is driven, and this second pair of stakes is connected by a wire in series with which is a sensitive telephone receiver, the system forms a possible communication link from a position where other types of communication are impossible.
HOW MODERN RADIO DIFFERS
It is to be noticed that in the schemes of communication so far described the sending and receiving stations each connect two points on the earth's surface and the transmitting and receiving apparatus are connected between these two points; low frequency currents are caused to traverse the earth's surface and a small part of the transmitted current reaches the surface where the receiving points are located. This is true wireless telegraphy, as much so as the type used to-day for radio broadcasting, and the two methods have many points in common. The line connecting the two contact points at the receiving station should be essentially parallel to the similar line at the transmitting station; the transmitted power is sent in all directions in both schemes so that but a very small fraction of the transmitted power is actually received. In the modern radio scheme each station uses two points in a similar manner, but one of them is on the earth's surface and the other is up in the air. The transmitting and receiving antennae should both be vertical, that is, parallel to each other as in the foregoing schemes. The essential difference of the two schemes lies in the frequency of current used in the transmitting antenna, and the factor of height of the two stations.
THE IDEA OF MUTUAL INDUCTION
A second possible method of wireless communication was opened up when the laws of electro-magnetic induction, discovered independently by Faraday in England and Henry in America, were made known. When a current flows through a coil, a magnetic field is set up in the space surrounding the coil. When the current in the coil is varied, the magnetic field will correspondingly vary, and if another coil is placed in proximity to the first, and so situated in the magnetic field, the changing magnetic field will set up a voltage in the second coil and if this is connected to some detecting device (such as a telephone or galvanometer) any change of current in the first will be recorded in the second. In this method real wireless communication is possible, there being no connection to the earth at either station. The amount of current which can be set up in the second coil by the changing current in the first decreases very rapidly with increasing distance between the two coils, so much so that the scheme is useful over only small distances. Thus if we have two coils say ten feet in diameter, the possible distance of communication would be probably less than two hundred feet.
Remarkable as was the discovery of electromagnetic induction it contributed but little directly to the problem of wireless transmission of signals over appreciable distances; it is of course used throughout the transmitting and receiving sets wherever two circuits are coupled together magnetically, but in so far as the actual transmission of the power is concerned it gave but little promise. In 1891, however, Trowbridge suggested an interesting use of this principle, which, had it come about, would have much resembled a modern radio installation. His idea involved the installation of large coils in the rigging of a ship, these coils to be as large as could be carried from the ship's spars. If the current in the coil of one ship should be interrupted many times a second, a telephone receiver connected to the coil of a neighboring ship would receive a signal and so permit the transmission of messages. Trowbridge further pointed out that such coils would permit the determination of the relative direction of the two ships from each other, a rôle filled to-day by the radio compass.
DOLBEAR, EDISON, AND STEVENSON
In 1883 Dolbear described his scheme for wireless signaling in which he used at each station an elevated wire, grounded on only one end; he was able to get communication over a distance of half a mile and some of his notes on the working of his scheme indicate that he was very close to a real solution of the problem.
In 1885 Edison and his associates devised a scheme for signaling to moving trains by induction from the telegraph wires running parallel to the railroad tracks. The currents induced in the train receiving apparatus were received with the train at high speed and the system had the advantage that the same wires could be used simultaneously for regular telegraph traffic. In Edison's apparatus the currents had to "jump" from the telegraph wires to the train, a distance of thirty to forty feet; it was evidently to this extent a system of wireless telegraphy.
The most remarkable achievement using the principle of magnetic induction was accomplished by Stevenson in England in 1892; he was able to establish reliable communication from the mainland to an island half a mile distant, using at his two stations large horizontal coils two hundred yards in diameter. In the transmitting coil the current from a few cells was interrupted by scratching a contact on a file and in the receiving coil a telephone receiver was used for detecting the induced currents.
WHY "WIRELESS" CHANGED TO "RADIO"
We have now come to the point in the development of wireless communication where the really important work begins; it is worth while to review what had been done in the rather more than half century which had elapsed since Steinheil had used the earth for one of the conductors of his telegraph system and had then put forth the proposition to do away completely with any wire connecting the two stations communicating with each other. A host of experimenters had worked on Steinheil's idea of using the earth or water as the only connection between the two stations, with some success, the most promising being the work of Bell; the feasible distance of communication by this scheme, however, seemed to be sharply limited to a few miles at most. Electrostatic as well as electromagnetic induction had both had their adherents, and considerable success had rewarded their efforts as evidenced by Edison's telegraphy with moving trains and Stevenson's transmission from mainland to island. The promise of much greater distance was rather slight with all of these schemes, however, and the time was ripe for the introduction of some new and radical step in the problem.
This new step was rapidly forthcoming; the energy radiated by very high frequency alternating currents and some simple scheme for detecting the high frequency currents, were the new concepts which were to give the development the wonderful progress which it so soon showed. Incidentally, the new idea of using radiated energy, as contrasted to the previous schemes, gives us the reason for the change of name from wireless telegraphy, up to now a proper name for the art, to that of radio communication, indicating that the power used in carrying the message was not due to conduction through the earth's surface, or to magnetic induction, but to energy which was actually shaken free from the transmitting station antenna, and left to travel freely in all directions.
MAXWELL'S THEORY OF RADIATED POWER
The theoretical work of Clerk Maxwell carried out during the period from 1860 to 1870 and published in complete form in 1873 showed that energy may be radiated from an electric circuit and that this energy shaken free from the circuit follows the same laws as does ordinary light. In fact, Maxwell made light and radiated electric energy exactly the same kind of a disturbance in the universal ether, Maxwell had, of course, no idea of the usefulness of this startling concept; he was a scientist, of the pure kind as contrasted to the applied, and his work was done in the spirit of pure science. It was the truth regarding certain natural phenomena as he saw it, and it is in the pursuit of the truth about Nature's activities that men like Maxwell pass their lives. Their material reward is generally nil, but that matters to them not at all; the joy of finding out the secrets of nature is the only reward required to keep them stimulated for further work. We shall point out later the work of another pure scientist who predicted theoretically that the modern vacuum tube was possible; others made the tubes and reaped the financial reward. To those buying the tubes to-day it undoubtedly seems that they are still reaping their reward.
Maxwell's theory of radiated power was the subject of much scientific argument and discussion; for many years this theory lacked any experimental evidence, either for or against it. The English scientists in general adopted the theory, but those of the continent were against it as being more complex and difficult to understand than the older theories of light and electricity. At the suggestion of von Helmholtz, probably the best known of German physicists, Heinrich Hertz was pursuaded to take up the problem of connecting experimentally the behavior of light and electromagnetic waves. Hertz had almost given up the idea of carrying out this experiment when he noticed a peculiar event taking place in another experiment he was working on. He was discharging a condenser through a spiral inductance coil, when he noticed that another coil in the vicinity produced small sparks every time the discharge took place in the first circuit. This phenomenon is the same as takes place every time a spark transmitter is operated to-day; the current in the antenna of a spark set is excited by the oscillatory discharge in the so-called local circuit.
AN ACCIDENT STARTED HERTZ
The sparks in the second coil took place with such regularity that Hertz decided to investigate their action. It will be noticed that this beginning of Hertz's remarkable work was the result of accident; if the second coil had not been in the neighborhood of the first when the discharges were taking place, no spark would have been noticed in the second and probably nothing further on the problem would have been done by Hertz and some one else might have carried out his epoch-making work; in fact, Professor Oliver Lodge, in England, would have been almost sure to have carried out the work if Hertz had not started when he did.
Hertz's own report of his brilliant and important experiments is available, as the original papers of Hertz have been translated into English and published under the title of "Electric Waves"; for the most part the book is non-mathematical and makes very interesting reading. As Hertz felt his way in this new field his reports had all the fascination of those of the explorer of unknown lands. His various papers followed one another so rapidly that in the space of only two years, 1887-1889, he had covered practically the whole field and had established firmly the laws of electric wave propagation as we know them to-day. He showed that the waves sent off from an electric circuit carrying high frequency current traveled with the same velocity as does light, that these waves could be reflected by mirrors and refracted by prisms and lenses just the same as light. He measured the length of the waves with which he was experimenting, and found that his detecting circuit must be of the same natural frequency as the transmitter if the response was to be appreciable. As one reads the account of these experiments he feels that Hertz's laboratory was really the birthplace of the radio art and cannot help feeling regret that this keen experimenter could not live long enough to see the wonderful practical benefits which mankind was to receive as the direct result of his work, carried out in the interest of pure science. It is because of the results following from the work of such men as Hertz that our most highly developed industries are to-day spending millions of dollars annually in the support of purely scientific research; the directors of these immense laboratories know too well that no real scientific truth can be discovered without bringing with it some application which will benefit the industry itself.
Very shortly after the death of Hertz in 1894, the world began to hear of the modest successes of Marconi, whose optimism and aggressiveness, combined with the wonderful foundation of knowledge which Hertz had given, soon showed that the possible reliable distance of radio communication was probably limited only by the extent of the earth's surface. In our next number will be taken up the work of the later and better known inventors and scientists, Marconi, Fleming, De Forest, Fessenden, Armstrong and others, who, building on the work of those earlier experimenters we have mentioned in this number, have given us the modern radio telephone.
Radio Broadcast, August, 1922, pages 294-302:
What Everyone Should Know About Radio History
By Prof. J. H. MORECROFT
A FEW years after the publication of Hertz's work in 1888 the scientific world heard rumors of the experiments of Guglielmo Marconi, then about 20 years old. He had been a student of Physics under Professor Rosa, at the Leghorn Technical School, and had especially made himself acquainted with the work of Professor Righi, who had been making experiments similar to those of Hertz, extending Hertz's work into the region of very short electric waves, about one centimeter long.
MANY contributions to scientific development have been the result of accident; something strange and unexpected has happened in the course of an experiment and has thus started a keen mind in search of its significance. But not so with Marconi; it is evident in reading of his early experiments and progress that he had set out, intentionally and with premeditation, to develop the laboratory work of Hertz into a successful scheme of communication. And once having started on the problem he stuck to it with a persistence seldom seen in a scientific worker. His progress was methodical, and followed the line suggested by his experimentation; there are no wonderful jumps in the methods of attacking the problem or in the results achieved. The development brought out by Marconi from 1895 to 1902 is an excellent example of scientific attack and accomplishment; with keen insight as to what was happening, Marconi took the logical steps to increase the distance over which he could carry on signalling and also the certainty of the communication.
His enthusiasm and ability steered him clear of the thorny and tedious path which must be trod by many inventors; the British Post Office Department and many prominent scientists gave him assistance and encouragement in carrying out his tests. It was in England that Marconi found the conditions best suited to the development of his new scheme of telegraphy; the British Empire has always been foremost in the development of communications as it is evidently of utmost importance for the close cooperation of its component parts. Until the United States entered the field of worldwide radio the British cables practically controlled the field of international communication. This of course gave to her traders a great advantage over others and enabled them nearly to control world trading. It is no wonder therefore that Marconi was so ably assisted in his development work in England. Its success would give the British Dominions still better control over the world's trade routes.
As everyone at all acquainted with radio knows, it involves the generation and radiation of high frequency waves at the transmitting station and some means of detecting them at the receiving station. Marconi started by using at his transmitting station radiators similar to Hertz's, but used at his receiving station a more sensitive indicator than was used by Hertz--a device known as the Branly coherer. The coherer, in the form first used by Marconi, was a small piece of glass tubing with metal terminals in each end, the space between these ends being filled with metallic filings, loosely in contact. It possessed a remarkable property by virtue of which if high frequency voltages were impressed on its terminals the contacts between its particles of metal dust became much more intimate so that the electrical resistance of the device became much less. This effect could be taken advantage of in the scheme of Marconi very well; a battery connected through the coherer could ordinarily force but little current through it hecause of its high resistance, but when it was affected by the high frequency waves sent out by the transmitting station its resistance fell to a low value and thus the battery could send much more current through it and so ring a bell or operate a printing telegraph, etc. This coherer of Branly, which was considerably improved by Marconi, was probably the most important single factor in Marconi's early work, it so far exceeded in sensitiveness Hertz's spark-gap receiver that it increased the possible distance of communication hundreds of times.
EFFECT OF HEIGHT AND SIZE OF ANTENNA
EARLY in his work Marconi got the idea of using at his transmitter and receiver a vertical wire, to the upper end of which was connected some large metallic body (such as a tin-covered cubical frame) and the lower end of which was connected to metal plates laid on the ground. He found that with his vertical wires six feet high he could communicate one hundred feet and with them twelve feet high he could get the same amount of signal at a distance of three hundred feet, and when they were twenty-four feet high he got the same signal strength at twelve hundred feet. Furthermore with his twenty-four foot wires--, if he increased the size of the metal bodies connected to the upper end, the possible distance was very much increased; thus with metal cubes about three feet on a side his transmission distance was three times as much as when they were only one foot on a side. These experiments, which were carried out in 1895, it will be noted, gave to Marconi ideas regarding the effeciency of an antenna as a radiator or receiver which we accept as correct to-day after much more refined measurements of the quantities involved. If an antenna is to send out much power, it must be high, and, further, it must have a large spread of wires at the top and be suitably connected to good earth plates, or ground, as we call it.
In 1896 Marconi went to England with his apparatus and there took out his first patent on wireless telegraphy in that year. His work interested Sir William Preece, of the British Government telegraph service; this eminent engineer at once realized the wonderful advance Marconi had made over previous attempts along this line, and gave to the young inventor his hearty support and approval. Although Marconi made no startling new invention he had availed himself of the known possibilities of Hertzian waves and had improved the Branly coherer and had made a combination which worked. When the validity of Marconi's claim to an invention was questioned Sir William Preece made the following comment:
"He has not discovered any new rays; his receiver is based on Branly's coherer. Columbus did not invent the egg but he showed how to make it stand on end. and Marconi has produced from known means a new electric eye more delicate than any known electrical instrument and a new system of telegraphy that will reach places hitherto inaccessible. Enough has been done to prove and show that for shipping and lighthouse purposes it will be a great and valuable acquisition."
Sir William's belief in the usefulness of the young inventor's scheme has been amply justified, as we now know; in fact, his estimate of the value of Marconi's work was all too small.
From 1896 on Marconi gave many demonstrations, gradually increasing the size of his apparatus and correspondingly the distance over which he could communicate. In 1898 a set was in actual operation connecting the Goodwin Sands lightship with the shore; with the success thus far reached it was evidently only a matter of perseverance and material resources to accomplish transoceanic communication, the goal towards which many of the earlier experimenters, dealing with currents in the ocean water, had striven with no success. In 1899 he had in operation two stations bridging the English Channel, and during the month of December, 1901, the first transatlantic signals were received by him in Newfoundland. For these first transoceanic tests his receiving aerial in Newfoundland was a wire supported by a kite, and the transmitting aerial, in Cornwall, on the west coast of England, was two hundred feet long and one hundred and sixty feet high. The spark transmitter used in Cornwall had an electrical capacity of only about 10 kilowatts and its efficiency must have been extremely low. In judging the ability required of Marconi in getting these first messages it must be remembered that to-day, with much more efficient transmitting sets, and receiving circuits thousands of times as sensitive as was Marconi's coherer, we use hundreds of kilowatts of power to get reliable transoceanic communication. The success of Marconi's first transoceanic tests speaks volumes therefore for his experimental ability. Bold indeed would be the experimenter to-day who would attempt transoceanic signalling with an inefficient spark coil transmitter, and a coherer for a receiver!
TUNING OF TRANSMITTER AND RECEIVER
THE waves used in the early experiments were not much more than "splashes" in the ether (whatever that mysterious substance may be); the receiving apparatus could not be tuned, and the great gain in loudness of signals which tuning makes possible could not be obtained. Pupin, in America, had pointed out the possibility of using tuned circuits at low frequency, and from 1899 to 1901 Marconi did much experimental work in trying to use tuning (or syntonizing, as it was then called) and took out several British patents on the application of tuned circuits to wireless communication.
He accomplished a great deal, using two tuned circuits, loosely coupled at the transmitting station and two similarly arranged circuits at the receiving station. The arrangement of circuits he used in 1900 was as good as that we use to-day in a modern spark transmitting set; of course there are certain features which have been improved since then, such as the quenched spark gap, and the crystal detector or vacuum tube detector, at the receiving station. We must remember that in all this important development work of Marconi's he was using as detector the coherer, which, although Preece regarded it as the most sensitive electrical eye possible, was but a very crude and insensitive piece of apparatus compared to that we use at present. It had to be continually tapped by a buzzer attachment to maintain it in a sensitive condition; after a signal had been received through the coherer the dust particles were cohered, and had to be shaken apart by the taps of the buzzer before they could again function to receive another signal.
PRACTICAL TRANS-ATLANTIC COMMUNICATION
IN 1907 the company developing Marconi's work opened the well known stations at Clifden, in Ireland, and Glace Bay, in Nova Scotia. Regular commercial business was carried on at the rate of ten cents a word. This Clifden-Glace Bay service was the pioneer radio link between America and Europe and much very valuable data was gained during the first few years of its operation. In spite of its novelty and isolation in the technical field (for years no other radio transmission rivalled it), the service was surprisingly uniform and reliable. It was in the study of the operation of this service that Marconi first found out that it was very difficult for a radio signal to cross the sunrise or sunset line; when the sun had risen in Ireland but had not yet come up in Glace Bay the sunrise line was somewhere in the Atlantic and the signals had to cross this line. A remarkable fading effect in the strength of signals was noted; in fact transmission across the line was practically impossible. We still have to contend with strange fading phenomena in radio transmission, but this special sunrise seems to occur to an appreciable extent only for the comparatively short waves used in the early days. Now, with waves 20,000 meters long, the effect is of much less importance. It was in the operation of this Clifden-Glace Bay service that the tremendous difference between transmission in the summer and that in the winter was forcibly brought out. The interference by atmospheric disturbances is thousands of times as troublesome during the summer as in the winter months, so that the amount of power required for summer traffic must be many times as great to get the same reliability in transmission as in the winter months.
Many of the amateurs will recall the consistent transmission of the low pitched musical note of Glace Bay; it used to be the station by which we could test the condition of our sets, regularly sending its dots and dashes across the ocean. The Clifden station was not as easy to "get," of course, as was Glace Bay but many of the good amateur experimenters used to do it, nevertheless.
OTHER EARLY WORKERS IN THE FIELD
OF COURSE, work as remarkable as was that of young Marconi excited the interest of every scientific and technical man of the day and many of them contributed valuable ideas to the rapid development of the new art. In England Fleming was closely associated with Marconi, and was undoubtedly of great assistance in the early experiments helping to design properly the circuits and apparatus. Later he contributed the Fleming valve about which more is said later on. Lodge and Muirhead made important contributions and were granted various patents, particularly with regard to the coherer, which was unquestionably the weakest point in Marconi's whole scheme. This coherer, which occupied the same position in the receiving circuit as does the crystal or tube detector of to-day, with its buzzer for de-cohering after every dot, was rather complicated and unsatisfactory in its performance, and many of the workers endeavored to modify it so as to improve its performance. With the simple crystal detector of to-day, or the vacuum tube, the work of the early experimenters would have progressed much faster and farther.
MARCONI'S WORK TAKEN UP IN GERMANY
WORD of Marconi's work having reached Germany, Professor A. Slaby came to England in 1897 to see the experiments. He himself had been trying to use the ideas of his illustrious countryman, Hertz, to obtain communication over appreciable distances, and had met with but meager success. Slaby was quick to recognize the superiority of Marconi's work over his own and gave him generous praise after seeing but a few of his experiments. In analyzing Marconi's work, Slaby (who afterwards became one of the foremost wireless inventors of Germany) replied to some of the criticism which had been raised against the novelty of Marconi's work in the following words: "It was urged that the production of Hertz rays, their radiation through space, the construction of the electrical eye*--all this was known before. True; all this had been known to me also, and yet I was never able to exceed one hundred meters.
"In the first place Marconi has worked out a clever arrangement for his apparatus which by the use of the simplest means produces a sure technical result. Then he has shown that such telegraphy (writing from afar) was to be made possible only through, on the one hand, earth connection between the apparatus, and, on the other hand, the use of long extended upright wires. By this simple, but extraordinarily effective, method he raised the power of radiation in the electric forces a hundred fold."
After witnessing Marconi's experiments and returning to Germany, Slaby began active development of wireless along the lines already taken by Marconi, and, associated with Count von Arco, developed the well known Slaby-Arco wireless apparatus. Professors Braun and Zenneck made valuable contributions also to the German wireless development. In 1903 Slaby and von Arco and Braun joined interests with the Allgemeine Electricitäts Gesellschaft and Siemens and Halske to found the Gesellschaft für Drahtlose Telegraphie, which firm put out the excellent wireless apparatus used in the "Telefunken" system.
To the scientific and theoretical side of radio Drude, Abraham, Wien, and Seibt, in Germany contributed; in France Poincaré, Branly, and Ferrie; in Italy, besides Righi there were Bellini and Tosi (who did the pioneer work in the radio compass); in America, Pupin, Trowbridge, Pierce, Fessenden, and Stone helped in the early developments.
WORK OF FLEMING
ALTHOUGH there were many engineers and scientists of valuable assistance to Marconi in his early work, of these J. A Fleming was by far the most important, judging by the contributions he made. Fleming assisted in making the generating apparatus at the transmitting stations more powerful and reliable, using an alternating current generator and transformer in place of the spark coil used earlier by Marconi. His great contribution to the art was not along these lines, however, but in furnishing a more reliable and sensitive detector of the high frequency radio currents set up in the receiving aerial. The coherer, and later the magnetic detector, had been used by Marconi; the magnetic detector was more reliable than the coherer but even this was far less useful than the Fleming valve, the forerunner of the wonderful vacuum tube used to-day in all good sets.
In 1883 Thomas A. Edison had noticed a peculiar action taking place in some of the special incandescent lamps with which he was working at the time. Experiments carried out with a bulb in which there had been sealed a metal plate close by the filament but insulated therefrom, showed that if the metal plate was made electrically positive with respect to the filament, current could pass through the vacuous space between the filament and plate, but if the plate was made negative with respect to the filament no current could flow. Here there was evidently a kind of electrical gate, or one-way valve, and the idea was patented by Edison in 1884. The phenomenon was given the name "Edison effect."
Fleming had used some of these bulbs having the extra electrode inside and when working with Marconi he got the idea of using this effect to permit the detection of the high frequency currents in a receiving aerial. Using a coil for transmitter and another for receiver, the same as Hertz had done, he utilized one of these bulbs with a direct current galvanometer in series to see if the direct current instrument would indicate. His first tests were successful and indicated that such a type of rectifier would probably be much more useful than the coherer.
Fleming took out a patent in Great Britain in 1904 and in America in 1905, the patent covering the idea of using the Edison effect in detecting high frequency signals. It has been frequently stated that Fleming did not invent this device, the well-known Fleming valve,--that his accomplishment was merely the application of an old idea in a new field. This is undoubtedly true, but such application has repeatedly been rated as invention and it has been judicially confirmed that Fleming's work did constitute an invention. The life of this patent is, of course, now expired, so that the construction of a two electrode tube for detecting radio signals is now permissible for anyone.
Marconi used some of the early Fleming oscillation valves (as Fleming called them), and found them much more satisfactory than the coherer then used. The ordinary crystal detector had not yet been discovered, so the production of the oscillation valve by Fleming constituted a real advance in the art. In sensitiveness those valves which have been tested by the writer are about equal to an ordinary crystal, but they have the advantage, of course, compared to the crystal that it is not necessary to hunt for a "good point." As long as the batteries are not run down, and the filament is hot, a valve will always function properly, whereas it cannot similarly be known for a crystal that it is operative or not, but recourse must be had to testing with locally produced signals.
Fleming apparently made a quite thorough investigation of his valves, and it is worth while noting some of his remarks regarding their behavior. He had noticed that the current flowing across the vacuous space in the valve was strongly affected by the action of either an electric or a magnetic field. The control of the electron stream by the magnetic field is the basis of the action of the "magnetron," a device developed in the General Electric Laboratory during the last year or two. The action of an electric field on the electron flow to the plate of course foreshadows the control of the plate current by an electric field applied either internally (the De Forest audion) or externally, as done in the Marconi audion, with external grid, a type of tube known to but few radio experimenters. It seems strange that Fleming did not at once jump to the idea of the audion, but the history of science is full of just such occurrences--a worker on the point of making an important discovery, yet missing it by the merest chance.
In recent years developments have been carried out and patents have been granted for Fleming valves which have been more thoroughly evacuated than were Fleming's valves. It scarcely seems that these devices (styled "kenotrons") are inventions; they employ no action unknown to Fleming, even though they do permit the use of much higher plate voltages than was possible with Fleming's original tubes. It seems, however, that Fleming appreciated the significance of a good vacuum for the proper operation of his valves. In one of the specifications of his 1904 patent he states: "As a very high vacuum should be obtained in the bulb, and a considerable quantity of air is contained in the conductors, these should be heated when the bulb is being exhausted. The filament can be conveniently heated by passing a current through it while the cylinder can be heated by surrounding the bulb with a resistance coil through which a current is passed, the whole being enclosed in a box lined with asbestos, or the like.** When the cylinder is replaced by any form of conductor which can be heated by passing a current through it, this method is usually more convenient than that just described."
It is evident from this, to anyone, that Fleming did appreciate the importance of high vacua in electron tubes and it therefore seems that later tubes, which are much more completely exhausted than were Fleming's, do not constitute an invention, but are merely the embodiment of Fleming's ideas, carried out to a higher degree than was possible for him with the then rather imperfect and difficult methods of evacuation available to him.
In 1911 Willows and Hill developed a Fleming valve in which the hot filament was a lime-coated strip of platinum, a so-called Wehnelt cathode, because Wehnelt was the first to point out the advantage of using such a cathode. If the hot filament is covered with certain oxides the electrons are emitted at a much lower temperature than if a pure metal is used, and thus less filament current is required to get a certain number of electrons. This type of oxide coated filament has been developed by the Western Electric Company for its long distance telephone repeaters.
In 1904 Dr. Lee De Forest was working in America on the use of the flame as a rectifier for high frequency radio currents. He took out a patent in 1905 on a bulb having two hot filaments connected in a peculiar manner, the intended functioning of which is not at all apparent to one comprehending the radio art. The wording of the patent claims is not that of a scientist but that of a shrewd patent attorney trying to hide some mysterious secrets in a superfluity of high sounding terms, instead of giving the exposition of a fact or operation discovered by the inventor. This is the trouble with too many of our patents, from the standpoint of one trying to understand what the inventor has done; the patent attorney tries to put the wording in as indefinite a form as possible so that, no matter what may happen in the future, the wording of the patent, it may be argued, anticipates just that development.
De Forest's claims were gradually changed until they finally described a device identical with Fleming's valve, which he styled the "audion". In the published accounts of the action of the audion De Forest seems to have thought the action very mysterious although Fleming had explained the action apparently quite satisfactorily some time before. During his early work it seems as though De Forest were deliberately trying to avoid giving Fleming credit for the work done in developing the oscillating valve. Had De Forest studied Fleming's writings at all, he would not have thought the action of the valve so mysterious.
In 1907 De Forest made his real contribution to the radio art; he somehow conceived the idea of interposing a metallic mesh, or grid, between the two elements of a Fleming valve and thus gave us that wonderful piece of apparatus, the three electrode tube. Much litigation ensued because of De Forest's claims that his device did not embody the principles of the Fleming valve. De Forest's claims and explanations were extremely dubious; it was about this time that the writer asked De Forest, at a public scientific meeting, a simple question as to how the audion functioned and to which answer was made that "his patent attorneys had told him to say nothing as to how the audion functioned." What patent attorneys can do for a scientist! Had it not been for experimenters like Armstrong of Columbia University, Langmuir of the General Electric Company, and van der Bijl of the Western Electric Company, we should know but little to-day of the action of the three electrode tube.
It is to be pointed out, however, that little as De Forest contributed to an explanation of his device, the thing which he actually did, namely the insertion of the third electrode into a Fleming valve, was a most wonderful contribution to the radio art. As a matter of fact, in the opinion of the writer, this was the most important single step taken in the whole development of radio communication. Let us give De Forest credit for this wonderful achievement, even though he was so reluctant to give credit to the other workers in the field, principally Fleming, on whose work the possibility of the audion depended.
R. A. FESSENDEN
FROM 1900 to 1907 Professor R. A. Fessenden was extremely active in the development of various phases of radio. The files of the Patent Office at Washington pay tribute to his activities during this period--his patents are counted by the dozen. Besides several patents on rectifiers, to compete with the coherer and the magnetic detector, he devoted himself among other things to radio-telephony; as early as 1901 he had laid down the essential principles of the art. The high frequency alternator which was later taken up by the General Electric Company, and further developed by their staff of engineers, and given the name Alexanderson alternator, was first conceived, patented, and built by Fessenden. The compressed air condenser, a very efficient form of condenser for large transmitting stations, is an invention of Fessenden; many of these are used to-day in the U. S. Government station at Arlington from which the standard time signals are sent out.
The most important of Fessenden's contributions to present day radio, however, is probably that by which he showed possible the reception of continuous wave telegraph signals by the so-called "beat method," or heterodyne reception. He first mentioned this scheme in 1902, but did not apparently use it much until 1907 when he described the method and gave several schemes for using it, among others the electrostatic and electrodynamic telephones. The heterodyne method of reception is not only a very ingenious scheme for overcoming a difficulty (absence of wave train frequency in continuous wave transmission), but it is an extremely sensitive method and it made feasible most of the early long distance radio transmission. The importance of the heterodyne scheme was increased tremendously when Armstrong discovered that the vacuum tube detector itself could be used for generating the required local high frequency currents.
WORK OF ARMSTRONG
DURING 1911 and 1912 E. H. Armstrong was studying for the degree of Electrical Engineer at Columbia University; he was not an especially brilliant student, in fact in many of his courses he did rather poorly. The writer knows because Armstrong was one of his students. The characteristics of alternating current machinery in general, did not prove very enticing to the young student, not because he was lazy or indifferent but because he had a hobby--and a vision. He was experimenting at his home with wireless apparatus and trying to find out how the three electrode audion of De Forest worked. If De Forest confessed in public that the action was too mysterious for him to explain, then Armstrong would explain it for him! Which he promised to do, and did very shortly.
After graduating, Armstrong continued at Columbia as assistant to the writer in the radio laboratory; later he worked with Prof. M. I. Pupin, continuing his study of the three electrode tube. As the writer looks back to those days it seems undoubtedly true that Armstrong understood the action of the audion better than anyone else in the world. Day and night he thought and talked of nothing but the audion; his devotion to this study, and perseverance therein finally brought rich reward--he was granted a patent, the validity of which was recently confirmed, which gives to him credit for being the first really to understand the action of the three electrode tube.
In using the audion as a detector of wireless signals certain coils were required, and Armstrong accidentally placed two of these coils much nearer to each other than they should normally be and lo--a strange noise was heard in the telephones. This strange noise started Armstrong to work on his wonderful discoveries.
It was noted in the first part of this history that the more or less accidental occurrence of a small spark started Hertz on his epoch making discoveries, and certainly it was as much an accident that led to Armstrong's work. But by those who may, at this point, think that an accident may some day make them also famous, let it be remembered that after the accidental noting of something unusual it was a long and difficult road which lead to the complete explanation and utilization of the phenomenon involved.
The noise which Armstrong heard was the beat note between the oscillation being set up by the De Forest audion he was using and a signal being sent out from some continuous wave station. He found that the pitch of the note varied with the adjustment of his circuit, and by keen intuition he came to the conclusion that the tube he was using was oscillating at a high frequency. He pursued the study of the action until it became very clear to him and he made patent application for his idea--which is fundamentally this: If the plate circuit of a three electrode tube and grid circuit are suitably connected (by magnetic induction or otherwise) the reactions occurring between the two circuits tend to set up alternating current in that circuit which has a condenser and coil connected together, the value of inductance and capacity determining the frequency of the alternating current generated.
He found out that even if the adjustment was not sufficiently carried out to make the tube oscillate, still the interconnection of the plate and grid circuits might cause a tremendous increase in signal strength. This is the "feed-back" or regenerative idea for which Armstrong's work is known.
Since Armstrong's first work appeared, innumerable circuits, with fancy names sometimes attached, have been published, the "inventor" probably thinking many times that the idea was entirely new. They are all embraced by Armstrong's patent, however, if they function by the interaction of the plate and grid circuits of the tube which can be brought about by the use of various connections of condensers and coils. In general there must be made provision for the energy which is resident in the plate circuit battery to get into the grid circuit if oscillations are to be maintained; if this provision involves the electrical or magnetic interconnection of the plate and grid circuits by use of condensers and coils suitably arranged, the idea comes under Armstrong's feedback claims. It is of course possible, that some other action may be found by which case the present monopoly on the use of regeneration would be temporarily broken.
WHAT ARMSTRONG'S CIRCUIT MAKES POSSIBLE
IT SEEMS a simple thing to couple together the plate and grid circuits of a vacuum tube and one would scarcely believe the importance of such an evident possibility. The results of the coupling are however very important. When a continuous wave signal is received the ordinary crystal detector or vacuum tube detector does not yield a signal because there is no variation in the amplitude of the high frequency current, a variation with a frequency in the audible range. If, however, the local circuit is continually excited by a high frequency current, when the high frequency signal is received the two high frequencies will act together and produce "beats", and the frequency of these beats is the same as the difference in frequency of the two different currents. This method, as mentioned previously, is the result of Fessenden's work.
Armstrong's idea evidently enables the vacuum tube which is being used as detector to act also as a generator of the high frequency currents which serve to produce the beats when the continuous wave signals arrive. Not only does the simple coupling idea of Armstrong thus permit the audion to act as a receiver of continuous wave signals, but it also makes it an extremely sensitive receiver at the same time, if the adjustments are carefully carried out. The writer well remembers one night, before Armstrong had published his explanation of the action of the oscillating tube, spent at the Marconi's then new station at Belmar, N. J. Mr. Weagant, the chief engineer of the American Marconi Company, and Mr. Sarnoff, at present manager of the Radio Corporation, were also witnesses of those early tests when Armstrong showed us how his circuits could "pick up" the continuous wave stations on the Pacific coast-- stations with only a few kilowatts of power. To hear the note of the station changed at will, by a turn of a handle on one of the boxes, was a severe puzzle for the Marconi engineer, especially as Armstrong, like a proper inventor, had everything completely hidden in boxes, with the lids securely screwed down. And nary a chance did the chief engineer have to peep inside! He would surely have been surprised had he seen how simple the whole thing was.
As another illustration of the remarkable advance in sensitiveness made possible by Armstrong's invention, the writer recalls hearing in his laboratory at Columbia, on several occasions, a station on our west coast in communication with one at Honolulu, and the two stations were continually calling for "repeats". They were only 2,000 miles apart, over the ocean, and the laboratory was 3,000 miles over land from the nearer one and 5,000 miles from the farther. Both stations were received at the laboratory clearly by using Armstrong's apparatus, yet they could not understand each other, using the receiving apparatus then in general use.
Besides the wonderful amplification of signal obtainable by the feed-back principle, the selectivity of a circuit is greatly improved so that stations sending on nearly the same wave length cause no interference. This idea is of more value in telegraphy than in telephony; in the latter the receiving circuit must not be too selective or else the speech will not be clear but will bedrummy in quality, and indistinct.
Armstrong has also given to us a valuable idea in his special short wave amplifier, and has just startled the radio world with what he has named his "super-regenerative" scheme whereby the present amplifying power of his circuit is greatly increased.
OTHER WORKERS IN THE FIELD
IN THIS brief history of the art many names have necessarily been omitted. Pupin, in his early work on tuning alternating current circuits, did much to show how to make radio signals more free from interference; Lowenstein showed the importance of properly adjusting the potential of the grid of the three electrode tube if it is to operate efficiently as an amplifier; Pickard and others discovered the utility of the various crystal detectors used in the cheaper radio sets of to-day. Round, in England, and Meissner, in Germany, both were on the track of the regenerative action of the tube when Armstrong found it, and they were not far behind Armstrong. Poulsen and Pederson, in Denmark have been responsible for the development of the tremendous arc generators used in long distance transmitting stations, such as that erected at Lyons, France, by the American engineers during the war. Alexanderson, in America, and Goldschmidt, in Germany, have perfected wonderful high frequency generators. General Squiers has shown possible the transmission of radio frequency currents over ordinary telephone wires resulting in our present "wired wireless". And the great research laboratories of the General Electric and Western Electric companies, with such men as Hull, White, Heising, and others, have contributed tremendously to bringing the art of radio to its present high state of development.
*By which was meant the coherer.