Invention History: The Father of FM

Positive Feedback

Armstrong's first important contribution to radio was regeneration, which opened the door for commercialization of the medium. Before regeneration, the world of radio consisted mostly of wireless telegraphers, amateur enthusiasts, and laboratory scientists. Like most advances, it was a cumulative effort. While regeneration is directly attributable and unique to Armstrong, his work was built upon a foundation laid by others.

The breakthrough occurred in 1912, while on summer break just prior to starting his senior year. As usual, Armstrong was holed up in his room cum laboratory at his parents' home in Yonkers, hard at work on his radio experiments. His receivers‘ circuits now included audion tubes, state-of-the-art electronics for the day.

Lee DeForest patented the audion, or triode amplifier, in 1906. Although DeForest (1873-1961) is considered an electronics industry pioneer, he wasn't particularly successful. A three-time loser in marriage, DeForest founded a couple dozen companies that he subsequently ran into the ground. In 1913, U.S. Attorney Daniel W. Baker sued him for defrauding shareholders with "absurd" promises; although he was acquitted.

Still, the audion boosted the strength of weak electrical signals, and was a relatively fast switching device for those pre-solid-state times.

DeForest adapted his tube from the world's first vacuum tube, which was patented in November 1904 by British Electrical Engineer Sir John Ambrose Fleming.

Fleming's vacuum tube - also known as the Fleming, or Thermionic valve - was the first practical application of the "Edison Effect", or thermionic emission. Thomas Edison discovered the effect in 1883 in conjunction with the creation of the incandescent light bulb. Edison observed a flow of electrons from the heated filament to a nearby metal plate, but failed to pursue the matter.

Fleming's valve was a diode that consisted of two metal plates encased in a vacuum and connected to separate circuits. The tube made possible the detection of high frequency wireless signals by early radio receivers.

Fleming's device is the foundation on which the whole of the modern electronics industry is based. The diode was a basic component in the manufacture of every piece of electronic gear until the advent of the solid-state era brought about by the transistor.

DeForest added a grid between the two plates, which boosted the strength of the signal flowing through the circuit. It was a major advance, revolutionary even. As revolutionary as it may have been, however, DeForest's tube was a poor amplifier.

Armstrong employed positive feedback to improve the audion's amplification capabilities. Basically, he redirected a portion of the output back through the circuit.

Feedback dramatically increased the amplification of the radio signal by a factor of thousands. The receiver's normally weak output could be clearly heard without earphones from across the room in Yonkers by Armstrong and his sister, Edith.

Back at Columbia, Armstrong worked to fine tune his discovery. Results were impressive. Greater amounts of feedback caused the circuit to oscillate, making it capable of transmitting its own signal. Lesser amounts resulted in a more sensitive receiver.

The patent for regeneration was granted in 1914, and shortly thereafter, radios incorporating the new technology began coming into the marketplace.


Armstrong gave a number of public demonstrations of his regeneration marvel. He wowed the audience members with broadcast signals originating from San Francisco, Hawaii, Ireland, and other far away locales captured by a huge antenna hung between Philosophy, Havemeyer, and Schermerhorn halls on the Columbia campus.

David Sarnoff, the future head of the Radio Corporation of America, or RCA, became a believer in the potential of radio from one such demonstration. He befriended Armstrong, and became an early benefactor. Later on he would become a dangerous adversary.

Lee DeForest was reportedly in the audience at a demonstration sponsored by the Institute of Radio Engineers (IRE). The IRE is now known as the Institute of Electrical and Electronics Engineers, or IEEE, following a merger with the rival American Institute of Electrical Engineers in 1963.

The demonstration provided the first opportunity for DeForest to hear the potential of his audion tube fully realized.

On the other hand, DeForest may not have attended one of Armstrong's demonstrations. However it occurred, DeForest learned about Armstrong's work, and its reliance of his precious audion.

DeForest was appalled by the upstart. He found it especially galling that one so young should be so much more knowledgeable about the nature of his invention. The force behind his determination to maintain control of the audion was probably fueled by the fact that it represented DeForest's one legitimate success in an otherwise lackluster professional career.

Armed with a new understanding the tube's capabilities, thanks to Armstrong, DeForest entered the laboratory on a mission to unlock the secrets of regeneration. He had the financial backing of AT&T, which hoped to gain a market advantage by partnering with the inventor of the tube around which everything revolved.

Considering that Armstrong had already proven its existence, it was a foregone conclusion that DeForest would develop his own regenerative circuitry, and he did. He based the precedence of his patent claim over Armstrong's entirely on a notation about a particular type of howling of a circuit he observed in the laboratory in 1912. The howling was caused by feedback that was causing the circuit to oscillate.

Of course, Deforest neglected to mention anything about his efforts to suppress the noise. Still, his claim of precedence was enough to interfere with Armstrong's established patent, and begin a legal battle that would end 14-years later in the Supreme Court.

Major Armstrong in UniformMajor Armstrong in UniformWhile DeForest labored to usurp credit for Armstrong's discovery, Armstrong published a series of technical papers, which explained for the first time how the audion worked, something that even its inventor didn't understand. Learning about the mechanics that caused his tube to do the things it did only increased DeForest's determination to gain exclusive control of it.

The IRE, in 1917, made Armstrong the very first recipient of its Medal of Honor Award for his discovery of regeneration. That same year the U.S. entered the war in Europe. Armstrong joined the U.S. Army Signal Corps within which he would rise to the rank of major by the end of the conflict.

Apr 15, 2007
by Michelle
Michelle's picture

Mike, Is FM used for


Is FM used for anything else but radio? Is the inventor of FM inventor of FM radio?

Apr 16, 2007
by Mike (not verified)

Yes, EH Armstrong is

Yes, EH Armstrong is considered the inventor of FM. It's used a lot outside of radio. TV  sound is FM. Military, NASA. Cellphones sound technology is based on it. 


Jul 6, 2008
by Anonymous