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Harry Nyquist (pron. [nʏ:kvɪst], not [naɪkwɪst] as often pronounced), (February 7, 1889 – April 4, 1976) was an important contributor to information theory.

He was born in Nilsby, Sweden. He emigrated to the USA in 1907 and entered the University of North Dakota in 1912. He received a Ph.D. in physics at Yale University in 1917. He worked at AT&T's Department of Development and Research from 1917 to 1934, and continued when it became Bell Telephone Laboratories in that year, until his retirement in 1954.

As an engineer at Bell Laboratories, he did important work on thermal noise ("Johnson–Nyquist noise"), the stability of feedback amplifiers, telegraphy, facsimile, television, and other important communications problems. With Herbert E. Ives, he helped to develop AT&T's first facsimile machines that were made public in 1924. In 1932, he published a classical paper on stability of feedback amplifiers (H. Nyquist, "Regeneration theory", Bell System Technical Journal, vol. 11, pp. 126-147, 1932). Nyquist stability criterion can now be found in all textbooks on feedback control theory.

His early theoretical work on determining the bandwidth requirements for transmitting information, as published in "Certain factors affecting telegraph speed" (Bell System Technical Journal, 3, 324–346, 1924), laid the foundations for later advances by Claude Shannon, which led to the development of information theory.

In 1927 Nyquist determined that the number of independent pulses that could be put through a telegraph channel per unit time is limited to twice the bandwidth of the channel. Nyquist published his results in the paper Certain topics in Telegraph Transmission Theory (1928). This rule is essentially a dual of what is now known as the Nyquist–Shannon sampling theorem.

Nyquist received the IRE Medal of Honor in 1960 for "fundamental contributions to a quantitative understanding of thermal noise, data transmission and negative feedback." In October 1960 he was awarded the Stuart Ballantine Medal of the Franklin Institute "for his theoretical analyses and practical inventions in the field of communications systems during the past forty years including, particularly, his original work in the theories of telegraph transmission, thermal noise in electric conductors, and in the history of feedback systems." In 1969 he was awarded the National Academy of Engineering's fourth Founder's Medal "in recognition of his many fundamental contributions to engineering."

Nyquist lived in Pharr, Texas after his retirement, and died in Harlingen, Texas on April 4, 1976.

• Nyquist plot
• Nyquist–Shannon sampling theorem
• Nyquist frequency
• Nyquist stability criterion
• Nyquist ISI criterion
• Johnson–Nyquist noise

• IEEE page about Nyquist
• Nyquist criterion page with photo of Nyquist with John R. Pierce and Rudy Kompfner