Controversy about electron spin
Leiden University in the Netherlands was a prominent centre for theoretical physics. Hendrik Antoon Lorentz (1853–1929) had worked there at the end of the nineteenth and the beginning of the twentieth century. On the recommendation of Albert Einstein, his successor was the Austrian Paul Ehrenfest (external link), whom Wolfgang Pauli had already met in 1922 during the "Bohr Festival" in Göttingen.
In 1925 Ehrenfest's assistants Georg Eugen Uhlenbeck (1900–1988) and Samuel Abraham Goudsmit (1902–1978) published their idea of electron spin, i.e. a clearly imaginable rotation of the electron on its axis. This hypothesis had already been put forward by Ralph Kronig (1904–1995), who was later Pauli’s assistant in Zurich.