Knudsen, Martin Hans Christian

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Knudsen, Martin Hans Christian

(b. Hansmark, Denmark, 15 February 1871; d. Copenhagen, Denmark, 27 May 1949)

physics, hydrography.

Knudsen’s parents owned a small estate, and he led the healthy and simple existence of a country boy. His outstanding abilities soon became evident; and upon entering the University of Copenhagen in 1890, he began to study physics, mathematics, astronomy, and chemistry. In 1896 he was granted the M.S. with physics as his main subject. In 1895 Knudsen had answered one of the prize questions posed by the University of Copenhagen on electrical sparks and had received the university’s gold medal. In 1901 the position of docent was established so that he could teach physics to medical students. When C. Christiansen retired in 1912, Knudsen succeeded him as professor of physics, a position he held until 1941, when he retired. He also taught at the Technical University.

Knudsen’s scientific work was centered mainly on the properties of gases at low pressure; and using simple methods he obtained important results for further development as well as for the technology of the vacuum. In one of his earliest projects he examined the escape of gases through a small hole and obtained a confirmation of the correctness of the predictions of the kinetic molecular theory; in particular he was responsible for the first indirect experimental confirmation of Maxwell’s law of the distribution of velocity, the experimental determination of the flow of gases through the small hole being in accordance with the formula calculated on the basis of Maxwell’s law. At the same time Knudsen conducted a study of the flow of gases through narrow tubes and thus arrived at the laws of molecular diffusion. He used an elegant application of diffusion to describe the vapor pressure of mercury at low temperature. Continuing this research, he next examined the behavior of a gas at low pressure in a container in which there is a temperature gradient. This led to a quantitative theory of the “radiometer forces” at low pressure and the discovery of the absolute manometer, now commonly called the Knudsen manometer. Knudsen was also responsible for an extensive series of basic investigations into the behavior of gases at very low pressures and found that it is not the mean free path of the molecules following collisions, but the dimension of the container, which is decisive.

Knudsen also occupies an important place in hydrography. He developed methods to define the various properties of seawater and was very active as an administrator. It was mainly through his initiative that the Central Committee for Oceanic Research of the International Council for Exploration of the Sea was based in Copenhagen. From 1902 to 1947 he was the Danish delegate to the council, the last fourteen years serving as vice-president, and he edited the Bulletin hydrografique from 1908 to 1948.

Besides his work as a researcher and administrator Knudsen carried a heavy load as teacher of physics at the University of Copenhagen and the Polytechnical Institute. Well liked by all his colleagues, he received many honors, both Danish and foreign, for his research and administrative achievements.


I. Original Works. A complete bibliography is given in Fysisk Tidsskrift, 47 (1949), 159–164. The main results of Knudsen’s experimental research in the field of kinetic molecular theory of gases are summarized in his small book The Kinetic Theory of Gases (London, 1934). He was editor of Hydrological Tables (Hydrographische Tabellen) (Copenhagen–London, 1901). A short autobiography is in Innbjudning til filosofie doktorpromotion vid Lunds universitets 250 årsfest (1918), p. xliii.

II. Secondary Literature. A biography of Knudsen by H. M. Hansen is in Dansk Biografisk Leksikon, XII (Copenhagen, 1937), 615–618. Obituaries by Niels Bohr and E. R. H. Rasmussen are in Fysisk Tidsskrift, 47 (1949), 145–159. See also Mogens Pihl, Betydningsfulde danske bidrag til den klassiske fysik (Copenhagen, 1972).

Mogens Pihl