In 1917, after his military service during
World War I, Fokker returned to
Leiden University as an assistant to
Hendrik Lorentz and
Paul Ehrenfest. In 1921, he became a physics teacher at the Gymnasium of Delft, and in 1923 was appointed Professor of Applied Physics at Technische Hoogeschool Delft. In 1928, he succeeded Lorentz as Curator of the Physical Cabinet at
Teylers Museum in
Haarlem, a position he held until his retirement in 1955. Fokker made several contributions to
special relativity, and some less well-known contributions to
general relativity, particularly in the area of
geodetic precession, the phenomena of precession of a freely falling gyroscope in a
gravitational field. Also
absorber theory of electrodynamics. Fokker began to study
music theory during the
Second World War, when Leiden University was closed; partly this was due to a desire to convince the
Nazis he would be of no use to the war effort, and partly it was a response to reading the work of
Christiaan Huygens on the
31 equal temperament. In 1938, Fokker – along with
Dirk Coster and
Otto Hahn – helped Austrian Jewish physicist
Lise Meitner escape from
Nazi-occupied Austria to the Netherlands. Historian
Ruth Lewin Sime writes :"Fokker and Coster both knew that university positions were virtually unavailable for foreigners. Laboratory space was not a problem, however, either in Groningen or Haarlem. 'Perhaps we can tap colleagues for regular contributions,' Coster suggested. Fokker set a goal of f.20,000, enough to support Meitner for five years, and immediately began contacting colleagues for advice and donations." They were unsuccessful in obtaining funding, but Fokker succeeded in getting official permission for Meitner to leave, although he was unable to telegraph that to her due to secrecy. She escaped barely in time to evade arrest. The year 1942 consequently marked a turning point in his life; after then he wrote many pieces in 31-equal, which are notable for using the 7th
harmonic as a consonant interval (31-equal has a much better approximation of the 7th harmonic than the ubiquitous 12-equal). He also made notable contributions to music theory, such as the
Fokker periodicity block. In 1949, Fokker became a Member of the
Royal Netherlands Academy of Arts and Sciences. Fokker died on 24 September 1972 in
Beekbergen at the age of 85.
Musical instruments File:Fokker organ keyboard design.png|thumb|200px|Schematic representation of Fokker's microtonal keyboard for a 31EDO pipe organ (Fokker organ) with two manuals and pedal, built by him. Fokker designed and had built a number of keyboard instruments capable of playing microtonal scales via a
generalized keyboard. The best-known of these is his 31-tone equal-tempered organ, which was installed in Teyler's Museum in Haarlem in 1951. It is commonly called the Fokker organ. The Fokker organ is currently property of the
Huygens-Fokker Foundation and it moved to the Bamzaal in
Muziekgebouw aan 't IJ. Regularly concerts take place on this instrument in the Bamzaal. ==See also==