Martin Hans Christian Knudsen (February 15, 1871 in Hasmark on

Funen Funen ( da, Fyn, ), with an area of , is the third-largest island of Denmark, after Zealand and Vendsyssel-Thy. It is the 165th-largest island in the world. It is located in the central part of the country and has a population of 469,947 as o ...

– May 27, 1949 in Copenhagen) was a Danish physicist who taught and conducted research at the

Technical University of Denmark The Technical University of Denmark ( da, Danmarks Tekniske Universitet), often simply referred to as DTU, is a polytechnic university and school of engineering. It was founded in 1829 at the initiative of Hans Christian Ørsted as Denmark's fir ...

. He is primarily known for his study of molecular gas flow and the development of the Knudsen cell, which is a primary component of molecular beam epitaxy systems. Knudsen received the University's gold medal in 1895 and earned his master's degree in physics the following year. He became lecturer in physics at the University in 1901 and professor in 1912, when Christian Christiansen (1843–1917) retired. He held this post until his own retirement in 1941. Knudsen was renowned for his work on kinetic-molecular theory and low-pressure phenomena in gases. His name is associated with the Knudsen flow, Knudsen diffusion,

Knudsen number The Knudsen number (Kn) is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale. This length scale could be, for example, the radius of a body in a fluid. The number is na ...

, Knudsen layer and Knudsen gases. Also there is the Knudsen equation; two instruments, the Knudsen absolute manometer and Knudsen gauge; and one gas pump that operates without moving parts, the

Knudsen pump The Knudsen pump also referred to as "thermal transpiration pump" or "Knudsen compressor" is a gas pump that utilizes no moving parts. Instead it uses thermal transpiration, the phenomenon that gas molecules drift from the hot end to the cold end o ...

. His book, ''The Kinetic Theory of Gases'' (London, 1934), contains the main results of his research. Knudsen was also very active in physical oceanography, developing methods of defining properties of

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appr ...

. He participated as hydrographer on the Ingolf expedition in the

North Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe and ...

in 1895-96. By means of his for the purpose constructed precision thermometer, capable of measuring water temperature in the deep sea with a precision of 1/100°C, it was demonstrated that the water masses at the sea floor north of the Wyville Thompson Ridge were consistently a few degrees colder than south of the ridge and likely explained the differences in the deep sea fauna on either sides of the ridge. He was editor of ''Hydrological Tables'' (Copenhagen–London, 1901). In 1927, he was one of the participants of the fifth

Solvay Conference The Solvay Conferences (french: Conseils Solvay) have been devoted to outstanding preeminent open problems in both physics and chemistry. They began with the historic invitation-only 1911 Solvay Conference on Physics, considered a turning point ...

on Physics that took place at the International Solvay Institute for Physics in Belgium. He was awarded the Alexander Agassiz Medal of the U.S.

National Academy of Sciences The National Academy of Sciences (NAS) is a United States nonprofit, non-governmental organization. NAS is part of the National Academies of Sciences, Engineering, and Medicine, along with the National Academy of Engineering (NAE) and the Nat ...

in 1936. He was made a Commander First Class of the

Order of the Dannebrog The Order of the Dannebrog ( da, Dannebrogordenen) is a Danish order of chivalry instituted in 1671 by Christian V. Until 1808, membership in the order was limited to fifty members of noble or royal rank, who formed a single class known ...

References

Literature

# Matematiken i Danmark (Mathematics in Denmark) by Niels Nielsen, Gyldendalske Boghandel, Nordisk Forlag, København. Vol. I: 1801-1908, published 1910; Vol. II: 1528-1800, published 1912. (In Danish). Note: This important work is a compilation of Danish mathematical writings. In addition to mathematicians, the short biographical notes include many physicists, astronomers, and early natural philosophers, including medical practitioners. # Kemien i Danmark (Chemistry in Denmark) by Stig E. Veibel. Vol. I: Kemiens historie i Danmark (The History of Chemistry in Denmark), 1939. Vol. II: Dansk Kemisk Bibliografi, 1800-1935 (Danish Chemistry Bibliography), 1943. Vol III: Danske Kemikere (Danish Chemists), edited by Bodil Jerslev, 1968 (Three of six chapters by Stig Veibel). All published by Nyt Nordisk Forlag, Copenhagen. (In Danish). Note: Vol. II is a compilation of Danish publications in chemistry and includes works by physicists, geologists, biologists, and physicians, along with biographical information. # Dictionary of Scientific Biography, Charles Coulston Gillispie, editor, Scribner's & Sons, New York, 1980 (16 Volumes). Supplement of 1990 (Volumes 17 and 18). {{DEFAULTSORT:Knudsen, Martin 1871 births 1949 deaths Danish physicists Members of the Prussian Academy of Sciences Fluid dynamicists University of Copenhagen faculty Rectors of the University of Copenhagen Members of the German Academy of Sciences at Berlin Commanders First Class of the Order of the Dannebrog People from Nordfyn Municipality