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Professional and personal files including Armstrong's correspondence with professional associations, other engineers, and friends, his research notes, circuit diagrams, lectures, articles, legal papers, and other related materials. Of his many inventions and developments, the most important are: 1) the regenerative or feedback circuit, 1912, the first amplified radio reception, 2) the superheterodyne circuit, 1918, the basis of modern radio and radar, 3) superregeneration, 1922, a very simple, high-power receiver now used in emergency mobile service, and 4) frequency modulation - FM, 1933, static-free radio reception of high fidelity. More than half the files concern his many lawsuits, primarily with Radio Corporation of America, over infringement of the Armstrong patents. Litigation continued until 1967. Other files deal with his work in the Marcellus Hartley Research Laboratory at Columbia University, 1913-1935, and with the American Expeditionary Forces in France during World War I, his Air Force contracts for communications development, Army research during World War II, the Radio Club of America, the Institute of Radio Engineers, FM development at his radio station at Alpine, N.J., the use of FM in television, his involvement in Federal Communications Commission hearings and legislation, and his work with the Zenith Radio Corporation. Also, letters to H.J. Round

Personal and professional correspondence, including 25 long letters from Professor Henry F. Herbig; manuscripts (mainly speeches); specifications for patents in electrical fields; technical and personal photographs; and memorabilia. Included is a copy of the famous "shot in hand" x-ray photograph, ca. 1896, one of the first ever to be taken. This collection also contains the correspondence, manuscripts, documents, and memorabilia of Professor Pupin's daughter, Varvara Smith, and his son-in-law, Louis Graham Smith. His daughter's letters and documents deal with her financial difficulties, her administration of Pupin's estate and her claims against Columbia University. Louis G. Smith's letters deal with his anti-Communist sentiments and his manuscripts are mainly ideas for popular songs and plays. There are three letters (photostatic copies) to Smith from Dwight D. Eisenhower.

This collection, representing Hildburgh's student days at Columbia University, contains student records, class memorabilia, notebooks of electrical engineering experiments, projects, and research for his bachelor's and master's theses.