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 of ...

– May 27, 1949 in Copenhagen) was a

Danish Danish may refer to: * Something of, from, or related to the country of Denmark People * A national or citizen of Denmark, also called a "Dane," see Demographics of Denmark * Culture of Denmark * Danish people or Danes, people with a Danish a ...

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 fi ...

. He is primarily known for his study of molecular gas flow and the development of the

Knudsen cell In crystal growth, a Knudsen cell is an effusion evaporator source for relatively low partial pressure elementary sources (e.g. Ga, Al, Hg, As). Because it is easy to control the temperature of the evaporating material in Knudsen cells, they are c ...

, which is a primary component of

molecular beam epitaxy Molecular-beam epitaxy (MBE) is an epitaxy method for thin-film deposition of single crystals. MBE is widely used in the manufacture of semiconductor devices, including transistors, and it is considered one of the fundamental tools for the devel ...

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 Free molecular flow describes the fluid dynamics of gas where the mean free path of the molecules is larger than the size of the chamber or of the object under test. For tubes/objects of the size of several cm, this means pressures well below 10− ...

Knudsen diffusion In physics, Knudsen diffusion, named after Martin Knudsen, is a means of diffusion that occurs when the scale length of a system is comparable to or smaller than the mean free path of the particles involved. An example of this is in a long pore wi ...

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 named ...

Knudsen layer The Knudsen layer, also known as evaporation layer, is the thin layer of vapor near a liquid or solid. It is named after Danish physicist Martin Knudsen (1871–1949). Definition At the interface of a vapor and a liquid/solid, the gas interaction w ...

and

Knudsen gas A Knudsen gas is a gas in a state of such low density that the average distance travelled by the gas molecules between collisions (mean free path) is greater than the diameter of the receptacle that contains it. If the mean free path is much greater ...

es. Also there is the

Knudsen equation In fluid dynamics, the Knudsen equation is used to describe how gas flows through a tube in free molecular flow. When the mean free path of the molecules in the gas is larger than or equal to the diameter of the tube, the molecules will intera ...

; two instruments, the Knudsen absolute manometer and

Knudsen gauge Knudsen may refer to: People Knudsen is a surname of Scandinavian origin, derived from the personal name Knud (Canute) and literally meaning "Knud's son." Notable people with the surname include: * Anthony Carl Knudsen (1874–1931), American b ...

; and one gas pump that operates without moving parts, the Knudsen pump. His book, ''The Kinetic Theory of Gases'' (London, 1934), contains the main results of his research. Knudsen was also very active in physical

oceanography Oceanography (), also known as oceanology and ocean science, is the scientific study of the oceans. It is an Earth science, which covers a wide range of topics, including ecosystem dynamics; ocean currents, waves, and geophysical fluid dynamic ...

, 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 appro ...

. He participated as hydrographer on the

Ingolf expedition HDMS ''Ingolf'' was a Danish schooner-rigged steam gunboat build in iron and launched in 1876. ''Ingolf'' marks a transition between the traditional gunships with muzzle-loading cannons placed along the sides of the ship and modern breechloading ...

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 The Wyville Thomson Ridge is a bathymetric feature of the North Atlantic Ocean floor ca. 200 km in length, located between the Faroe Islands and Scotland. The ridge separates the Faroe–Shetland Channel to the north from the Rockall Trough ...

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 i ...

on Physics that took place at the International Solvay Institute for Physics in Belgium. He was awarded the

Alexander Agassiz Medal The Alexander Agassiz Medal is awarded every three years by the U.S. National Academy of Sciences for an original contribution in the science of oceanography. It was established in 1911 by Sir John Murray in honor of his friend, the scientist Ale ...

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 Nati ...

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 a ...

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