Martin Hans Christian Knudsen (15.02.1871— 27.05.1949) was a Danish physicist and oceanographer, a member of the Royal Danish Academy of Sciences and Letters (1909) and its secretary in 1917—46. He was born in Hansmark (the island of Funen). He graduated from the University of Copenhagen (1896), where he worked (professor in 1912-41).
His research concerned the kinetic theory of gases, molecular physics, heat, hydrography and oceanology. He studied the behaviour and properties of gases at low pressures and obtained results important for the development of vacuum technology. He developed the theory of heat conduction at low pressures (1910 — 11). He constructed (1910) a manometer (the Knudsen manometer). He gave the first indirect proof of the Maxwell distribution of molecular velocities. Studying the flow of gases through narrow tubes, he established the law of molecular diffusion.
In 1923 —34 he was vice-president of the International Union of Pure and Applied Physics.
The H. Ørsted Medal (1916).