![]() Nearly a year later de Forest filed for a patent on the same invention, which he later sold with all audion rights to the American Telephone and Telegraph Company (AT & T).Īs radio began to boom, AT & T mounted a broad attack to overturn Armstrong's patent in favor of de Forest's. His basic feedback patent had been issued on October 6, 1914. As the 1920s wore on, Armstrong found himself enmeshed in a corporate war to control radio patents. Upon the success of early radio broadcasting, he became a millionaire, but he continued at Columbia as professor and eventual successor to Pupin.Īfter a celebratory trip to Paris, he returned to court Marion MacInnis, secretary to the president of RCA, David Sarnoff. Later he sold a lesser circuit to the newly organized Radio Corporation of America (RCA) for a large block of stock. In 1920, on a bid from Westinghouse Electric and Manufacturing Company, he sold rights to his two major circuits for $335, 000. By then, wireless was ready to erupt into radio broadcasting. Armstrong returned to Columbia with the rank of major and the ribbon of France's Legion of Honor. He called this the superheterodyne circuit, and although it detected no secret enemy transmissions, it is today the basic circuit used in 98 percent of all radio and television receivers. ![]() He was assigned to detect possibly inaudible shortwave enemy communications and thereby created his second major invention.Īdapting a technique called heterodyning found in early wireless, but little used, he designed a complex eight-tube receiver that in tests from the Eiffel Tower amplified weak signals to a degree previously unknown. Before his new circuit could gain wide use, however, awaiting improvements in the vacuum tube, the United States was plunged into World War I and Armstrong was commissioned as an officer in the U. ![]() Thus this single circuit yielded not only the first radio amplifier but also the key to the continuous-wave transmitter that is still at the heart of all radio operations.Īrmstrong received his engineering degree in 1913, filed for a patent, and returned to Columbia as an instructor and as assistant to the professor and inventor Michael Pupin. He later found that when feedback was pushed to a high level the tube produced rapid oscillations acting as a transmitter and putting out electromagnetic waves. Testing this concept in his turret room in Yonkers, he began getting distant stations so loudly that they could be heard without earphones. In the summer of 1912 Armstrong devised a new regenerative circuit in which part of the current at the plate was fed back to the grid to strengthen incoming signals. But in the usual receiver circuit the tube did no more than detect weak signals. In 1904 an English inventor, John Ambrose Fleming, had shown that this effect could be used as a wireless receiver two years later de Forest had added a vital element, a wire grid between filament and plate. The tube was based on Thomas Edison's 1883 discovery in his early lamp of a tiny anomalous electric current that flowed across a gap from the filament to a metal plate. Long analysis of the action within the audion tube suggested to him that it might be used to greater effect. ![]() While a junior at Columbia, Armstrong made his first major invention. But none of the instruments were able to amplify weak signals at the receiver, nor yet to provide stronger, more reliable power at the transmitter. He worked with every new device that came along, among them the so-called audion tube invented in 1906 by Lee de Forest. (age 63) Manhattan, New York, United StatesĪs a student at Yonkers High School, Armstrong built an antenna mast, 125 feet tall, on the family lawn to study wireless in all its aspects.
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