Dutch physicist, professor in Utrecht.
He was the only son of the merchandiser Nathan Leonard Ornstein (20 April 1835, Nijmegen – 2 October 1916, Den Haag) and Sophia Maria Manson (14 February 1846, Den Haag – 11 April 1922, Utrecht), who married 17 July 1872 in Den Haag.
He studied theoretical physics at Leiden University under Hendrik Antoon Lorentz from 1898 to 1908. On March 26, 1908 (16.00 PM) he became a Ph.D in Leiden with a study of the applications of the statistical mechanics of Josiah Willard Gibbs (1839) to molecular problems. He dedicated it to his parents.
After assisting A.H. Lorentz in Leiden, he became a lecturer at Groningen University. On April 29, 1909 he held his first public lecture here on the role of mathematics in mathematical physics.
In 1914 he was appointed professor of physics at the University of Utrecht (UU) at age 34. He succeeded the physical chemist Peter Debye. He accepted the appointment on January 25, 1915 with an oration with the title: Problemen der kinetische theorie van de stof (problems of the kinetic theory of matter).
In 1922 he became director of the Fysisch Laboratorium in the Bijlhouwerstraat 6 in Utrecht. He extended his research interests to experimental physics. But he was a clumsy person and was feared by his Ph.D. students for accidentally messing up their experimental setups.
With enlightened absolutism he ruled the lab with remarkable results: He was the promotor of 88 dissertations according the website of the UU. He also stimulated the funding of science via the industry. He was then a pioneer in this. With Gilles Holst, director of Philips Research Laboratories, he was a driving force behind establishing the Dutch Physical Society in 1921. He was its chairman from 1939 to 1940.
Ornstein was of Jewish heritage. When the Netherlands were occupied by Nazi Germany during World War II, the astronomer and musician Peter van de Kamp (26 December 1901, Kampen – 18 May 18 1995, Amsterdam) offered him to bring him and his family to America. But he refused as he felt he was needed in his Laboratory.
After Nazi laws ordered Berufsverbote for academic Jews, Ornstein was dismissed from his Laboratory in September 1940. In November 1940 he was forced to leave the Utrecht University. On November 29, 1940, Ornstein himself withdrew his membership of the Dutch Physical Society. He also withdrew himself more and more from public life events to the point that he did not want to receive any guests at home.
He died on May 20, 1941 in Utrecht.
Personal
He married Jeanette Hoofiën on June 29, 1911. He got two daughters from this marriage. He divorced Hoofiën on March 24, 1927 and remarried Estella Meijer on March 8, 1929. He got two sons from this marriage. His son L.Th.M. Ornstein became a professor in plasma research. .
From 1918 tot 1922 he was the chairman of the Dutch Zionist Society. He was interested in the Montessori education and he was always a great stimulant to his students.

Link to Wikipedia