In 1927, Kronig returned to Europe for good and worked in different prominent centres of research: Copenhagen, London, Zürich (where for a year he was Pauli's assistant).
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Around 1930 he settled in the Netherlands: first in Utrecht, then in Groningen, first as Dirk Coster's assistant, and from 1931 as an associate professor, and since 1939 as a full professor at the Delft University of Technology where he stayed until his retirement in 1969.
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During 1925 Ralph Kronig, who presented the idea of electron spin some months before George Uhlenbeck and Samuel Goudsmit, was working as an assistant to Landé.
A solution to this problem was suggested in early 1925 by George Uhlenbeck and Samuel Goudsmit, students of Paul Ehrenfest (who rejected the idea), and independently by Ralph Kronig, one of Landé's assistants.