During World War One, beginning in April, 1917, the United States government assumed control of most of the U. S. radio industry, in addition to the telegraph and telephone lines. It also gained access to all radio patents, which meant the military could use the best equipment that had been developed by the various competing companies. The two year period of military control also saw numerous advances in radio engineering, the most important being the perfection of vacuum-tube based radio equipment, particularly transmitters. Also, extensive improvements were made on the production line, so that thousands of identical vacuum tubes could now be mass produced, without having to test and grade each and every tube, as the DeForest Company had needed to do.
Annual Reports of the War Department for the fiscal year ended June 30, 1919
Report of George O. Squier, Major General, Chief Signal Officer of the Army, October 15, 1919
The application to the radio intercommunication of the vacuum tube--perhaps more properly called the thermionic tube or bulb--is one of the most interesting developments in the whole field of applied science. For not only has it made possible what has been justly heralded as one of the most spectacular achievements of the whole war--the airplane radiophone--but the confidence growing out of the extensive experience with the vacuum tube in warfare, coupled with its extreme adaptability, have resulted in a rapidly increasing amount of radio development involving its use.
Prewar history.--The vacuum tube was known in various forms before the war. Following extensive experiments with the so-called Edison effect, Fleming some years ago produced the well-known Fleming valve--a current rectifying device, capable therefore of being used as a detector of radio signals. This device contains two elements, an incandescent filament emitting electrons and a plate upon which an alternating voltage is impressed, both placed within an evacuated bulb. Later Dr. Lee De Forest introduced an important modification by placing a wire mesh or "grid" between the filament and the plate. A small voltage variation on this grid produces the same current change through the tube as would a much larger voltage variation on the plate, thus adding amplifying properties to the detector characteristics of the Fleming valve. De Forest called his device the "audion." Later, with superior facilities for evacuation available and with a more intimate knowledge of the laws of thermionic emission from hot bodies, improvements and modifications were made in the audion or vacuum tube by both the General Electric Co. and the Western Electric Co., the latter designating their product as "vacuum tube," and the former the "pliotron."
In addition to acting as detectors and amplifiers, as mentioned above, vacuum tubes can function in two other important ways:
1. As oscillators. In properly designed circuits containing inductance and capacity they will act as radio frequency generators for use in transmitting or receiving radio signals.
2. As modulators. By suitable connection to an oscillator circuit or antenna they can be made to vary the power radiated so that the envelopes of the waves transmitted shall have any desired wave form, as, for example, the speech waves from an ordinary telephone transmitter.
The most striking use made of vacuum tubes prior to the time we entered the war was the transmission of speech by radio from Washington to Paris and Honolulu during the experiments carried out by the American Telephone & Telegraph Co. and the Navy Department. Vacuum tubes were used as the radio frequency generator for transmitting and for detector and amplifier in receiving.
When the United States entered the war, vacuum tubes already were in use by the allied forces for various signaling purposes. The French particularly had been quick to recognize the military value of vacuum tubes and had, previous to June, 1917, developed very creditable tubes and apparatus. In America tubes were in limited use as "repeaters" on telephone lines, and as detectors and amplifiers in laboratories and radio stations. The total production, however, in this country did not exceed 300 or 400 a week.
Early in our participation in the war it became evident that vacuum tubes would be required in very large quantities in order to meet the growing demands for radio communication and signaling. It was equally evident that service conditions not hitherto anticipated would require great mechanical strength, freedom from disturbance under extreme vibration, and uniformity of product sufficient to make possible absolute interchangeability of the tubes in sets, without the necessity of readjusting when changing tubes. To these conditions must be added that of minimum size consistent with dependable operation.
To make such a device, with its complicated yet accurately constructed metallic system within a practically perfect vacuum, is no small problem even when made in the laboratory on the individual unit basis by a skilled operator who appreciates the delicacy of the job. To turn out tubes by the thousands by factory methods involves almost infinitely greater difficulties. How well certain companies, in collaboration with the Signal Corps, have succeeded in solving these difficulties is indicated by the fact that recently the total rate of production in the United States of high-quality standardized tubes was considerably in excess of 1,000,000 a year. This rate of production could be made many times greater on short notice.
As an example of the difficulties which this quantity production has involved may be mentioned that of evacuation. The degree of vacuum required is such that unusual methods of exhaust are necessary. The heating of the tubes in electric ovens is supplemented by heating of the elements of the tube by excessive filament and plate electrical power input. Molecular pumps are employed, necessitating an extremely large number of pumps to handle quantity production. Special treatment of metal arts prior to assembly is employed to reduce the gas given off by them during the exhaust process.
Another problem is that of making the complicated metallic structure of all tubes exactly alike in order to insure identical electrical properties. As an indication of progress in this direction it may be stated that one company is prepared to manufacture in quantity a certain tube in which the clearance between filament and grid is only three-hundredths of an inch, the allowable variation being of course only a small percentage of this.
Manufacturing in quantity involves careful inspection. The problem of specifying definitely the required performance of tubes the development of adequate testing specifications, the placing of standardized testing and inspection methods, personnel, and equipment in the various factories so that tubes manufactured at different times and places would after passing inspection be uniform and interchangeable--these questions were entirely new and have been solved almost entirely by the Signal Corps engineers.
Tubes developed by the Signal Corps may be divided into two general classes--the tungsten-filament types as developed and manufactured by the General Electric Co. and the De Forest Radio Telephone & Telegraph Co., and the coated-filament or Wehnelt cathode types as developed and manufactured by the Western Electric Co. The coated-filament tubes so far have proven superior to the tungsten-filament tubes for Signal Corps use. Both classes have been standardized as regards base, exterior dimensions, filament current and voltage, and, in addition, plate voltage and output for transmitting tubes; and amplifying power and detecting power for receiving tubes. Except in certain special cases, the Signal Corps uses two types of tubes, one for transmitting and another for receiving. The French and the British have been using one type for both transmitting and receiving, but present tendencies of the British are toward different tubes for different duties.
Vacuum tubes are now employed for electric-wave detection, radio-frequency, and audio-frequency amplification, radiotelephony, particularly in the airplane radiophone, continuous-wave radiotelegraphy, voltage and current regulators on generators, and for other miscellaneous purposes. However, varied as are the applications at present, the uses, actual and potential, growing out of war-development work have proved that the art of vacuum-tube engineering and the application of its products to radio engineering, telephone and telegraph engineering, and particularly to electrical engineering in general, are still in their early infancy. That vacuum tubes in various forms and sizes will, within a few years, become widely used in every field of electrical development and application is not to be denied.
The engineering advancement accomplished in less than two years represents at least a decade under the normal conditions of peace, and our profession will, it is hoped, profit by this particular salvage of war, which offers perhaps the most striking example extant of a minimum "time-lag" between the advanced "firing line" of so-called pure physics and applied engineering.
The Chief Signal Officer considers that the work of standardization and quality production of vacuum tubes, accomplished during the last 18 months under the pressure of military necessity, represents an advance in the art of electrical engineering which will prove of inestimable industrial and scientific value to this country and to the engineering world at large.
The vacuum-tube development, in common with other radio-development activities of the Signal Corps, has been controlled and directed entirely by the radio-development organization. Vacuum-tube work was carried on by the Research Section and its successor, the technical department of the radio laboratories, both in charge of Capt. Ralph Bown. Other personnel engaged in this work is as follows: Lieut. Brunson, Mr. Keller, Lieut. R. E. Bitner, Lieut. S. S. Mackeown, Lieut. Weeks, Lieut. Stacey, Capt. Webb, Lieut. Batsel, Capt. Gray, and Dr. Richtmyer.