Discussion of "A History of Some Foundations of Modern Radio-Electronics Technology" Page 1257

tober,” that de Forest was in Gloucester, for he had gone first to the Bell people and was with them the last two days of October.²³

Fig. 8 of the paper shows a concentric form of vacuum tube structure (with axial filament surrounded by cylindrical grid and plate), a design attributed to G. W. Pierce. As if it were new, Hammond referred it to the General Electric Company as “the proper triode design.” But such form of tube was not new; it was to be seen, along with measured E-I characteristics, in a German Scientific paper.²⁴

MODERN INTERMEDIATE FREQUENCY CIRCUITRY (SECTION IV)

This section of the paper recounts some interesting early inventing by Hammond, but by not fully revealing other contemporary developments of intermediate frequency circuitry, gives the impression of that technique having originated largely in Hammond. Actually, it started before him and developed without knowledge of his activity, secret as it was.

In a patent interference Hammond was awarded a claim which he interprets as giving him “the broad subject matter” of intermediate-frequency circuitry.²⁵ The claim is quoted as reading:

A carrier wave transmission system comprising means for receiving and detecting the energy of a modulated wave, means for selecting a component of said detected energy, and means for detecting said selected component.

Thus, the claim is for tandem detection-selection-detection. The paper asserts, “The entire principle of IF selectivity is expressed by the words ‘selecting a component’ regardless of whether the unselected components were to be utilized otherwise as in multiplex reception, or were to be discarded as in simplex telephonic reception.”

What is disclosed in Hammond Patent No. 1,491,772 (1912–1924), of which the above is claim 46? The modulated “carrier transmission wave” is simply that of an intermittent spark discharge of definite group frequency. That group frequency is recovered in the output of a detector, is selected and then detected again, down to the signal frequency. The receiving selectivity is enhanced by tuning to the spark frequency as well as to the radio frequency.

Now this addition of selecting the spark frequency after detection and then rectifying it to obtain the signal frequency, was not new as of 1912. In 1909–1911, the Telefunken singing-spark, quenched-gap, system of wireless telegraphy which contained it was well known. The receiver comprised: a radio-frequency tuner, a detector, an amplifier sharply tuned to the spark frequency of about 500 cycles, a second detector, and a telegraph signal recorder. (The tone-frequency signals, instead of being rectified, could be read in headphones or on a loudspeaker.) The amplifier element was electromechanical, a stretched steel wire tuned to the spark frequency drove a microphone, and three such elements in tandem made up the amplifier, highly selective as it was. The system was described fully in the German technical press of 1910–1911 and in The Electrician of London, page 249, November 24, 1911. Since Hammond was familiar with German developments of the time, he must have known about it. A transmitting and receiving set was imported into the U. S. by the Telefunken Company of America and exhibited and offered for sale at their quarters in the little tower of the Trinity Building at 111 Broadway, New York. The present writer was employed by this American subsidiary of the Telefunken Company in 1909–1910 and helped set up and operate the apparatus. Among the U. S. military people to whom it was shown was the then Lieutenant G. C. Sweet, U.S.N. What became of this particular set of apparatus is not known, but other quenched spark sets were sold to the Army and the Navy, and altogether the system became well-known at the time. Hence in allowing to Hammond, as of 1924 on the basis of a 1912 application, this claim which reads on the Telefunken system, the Patent Office must have overlooked prior art. In view of it Hammond cannot be credited with originating the double-detection technique, starting, as it did, with audio-frequency IF.

Of Alexanderson’s 1912 tuned-radio-frequency receiving circuit shown in Fig. 12, it is said that if the tubes of the first stage were all detectors (they being in parallel), “then the system would be of the intermediate frequency type . . . .”²⁶ But there was then no basis for calling them detectors; the “if” is a pure supposition of the authors, made in the light of later developments. Hence the claim that Hammond’s October, 1912, conference with Alexanderson had “disclosed the ultimate selective receiver with double detection . . .” (meaning superheterodyne) impresses one as unjustified.

While on this matter of Hammond’s 1912 approach to the double-detection, IF, technique, it is to be observed that the twice-tandem-interrupted kind of spark transmitter of Hammond Patents No. 1,491,773 and No. 1,491,774, is shown in the little book by Miessner where one reads: “Fig. 82 illustrates a type of transmitter-receiver unit suggested by the writer in 1911.”²⁷

Another claim²⁸ for a “first” must be corrected:

The intermediate-frequency principle was first applied outside the laboratory to the solution of a World War I communication problem of high military importance.

The apparatus is said to have been

. . . constructed by the Hammond Laboratories in 1917–18.

and

Delivered to the U. S. Army at Tours, France, . . . on October 10, 1918.

Actually, the IF principle had been applied “outside the laboratory” a year earlier here in the U. S. This was the short wave multiplex radio telephone system developed by the Western Electric Company, more directly by R. A. Heising, in 1916 and installed on two U. S. battleships. It is described briefly in the Craft-Colpitts paper on “Radio Telephony” given before the AIEE on February 21, 1919. Not only was this ahead of Hammond’s military application, but the technique was superior, enabling a plurality of carrier channels of intermediate frequency taken from the wire carrier art, to be conveyed over a radio carrier.

Fig. 14 of the paper shows the double-detection circuit of the 1917–1918 Hammond Chaffee system, which was covered by two patents.²⁹ Only one of these patents shows feedback on the first stage, namely the Chaffee one, and it specifically says it is to increase sensitivity and that the feedback should not be allowed to oscillate. Yet the authors in afterthought say it was “usable” for heterodyne reception, and assert: “Structurally, therefore, the receiver was of the most general ‘superheterodyne’ variety, since both detectors could be, and during adjustment often were, of an oscillatory nature.” Of course both detectors were “of an oscillatory nature,” detection of oscillations being their function, and they would even tend to self-oscillation. But the first detector was not used in the self-oscillating condition; the Chaffee patent enjoined against it. Thus, one sees that the authors use “weasel words” to give the impression they had a true superheterodyne in 1918, whereas they did not.

The first to arrive at the true superheterodyne and apply for a patent on it, was one Lucian Lévy of Paris. He invented it as a highly selective antistatic receiver in 1917. Soon thereafter E. H. Armstrong learned of Lévy’s receiver while in Paris with the AEF, the present writer has learned from Lévy. Armstrong’s acquaintance with the principle may have started in New York with the Western Electric Company before he went abroad, but his full appreciation of it probably stemmed from Lévy more than from Hammond as the present authors suggest, although he probably knew also of the Hammond-Chaffee system. Upon returning to the U. S. after the war, Armstrong sought to exploit the superheterodyne as a means of amplifying and receiving the higher frequencies, and managed to be allowed a patent. But he had nothing new in principle and finally lost to others, mostly to Lévy, all the patent claims. In 1919 Lévy had offered his American rights for sale to the French house of the Western Electric Company, namely La Material Telephonique. In this way knowledge of his work came to the attention of Bell System engineers. They themselves had evolved the principle of frequency step-up-and-down, but so gradually and generally that they had not appreciated all its inventive features. But upon learning what Lévy was patenting they did appreciate it as a high-gain, highly-selective, stable receiver, and purchased his American patent application. Prosecution of it in the Patent Office resulted in Lévy securing the definitive patent in the U. S. to the superheterodyne as a means of amplifying and selecting, prevailing over several contestants, including Armstrong and Hammond. (Patent No. 1,734,038 of 1918–1929.) The writer in recent years asked Lévy about his invention

²³ Footnote 1, p. 1198.
²⁴ O. von Baeyer, “Ueber langsame Kathodenstrahlen,” (Concerning low-velocity cathode rays), Phys. Z., vol. 10, pp. 168–176; March, 1909.
²⁵ Footnote 1, p. 1203.
²⁶ Footnote 1, p. 1201.
²⁷ B. F. Miessner, “Radio Dynamics,” D. Van Nostrand Co., Inc., New York, N. Y.; 1916. See ch. 17, “A means of obtaining selectivity,” p. 145.
²⁸ Footnote 1, p. 1201.
²⁹ Footnote 1, p. 1201, reference 50.