What Everyone Should Know About Radio History Part II

Radio Broadcast Magazine ^ | August 1922 | J. H. Morecroft

[Steely Tom]

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 Armstrone was on 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 to 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 therin 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 led 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 conculusion 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 the grid circuit are suitably connected (by magnetic induction or otherwise) the reactions occuring 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 all are 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 monoply 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 frequncy 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 a 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 this simple coupling idea of Armostrong 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, NJ. 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 of 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 to see 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 be drummy 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 wherein the present amplifying power of his circuit is greatly increased.

[ABN 505]

“Empire of the Air” a great book on the radio wars between brillant men and early technology.

[Steely Tom]

Without the Fleming Valve, the Audion would never have existed. And without the Edison incandescent bulb, the Fleming Valve would never have existed.

Yes, in a SF short story I read as a kid, the author identifies Edison as the inventor of the "monode."

And I forgot about the Fleming Valve, thanks for mentioning it.