choosing and with a novel technique which greatly simplified the experiments. The day after he sent in his doctoral thesis, he married Helen Newmark, who remained his lifelong companion and became the mother of his two daughters, Nancy Lictenstein and Margaret Beels.

The Rabis soon went on a traveling fellowship to Europe, where he worked intermittently with Sommerfeld, Heisenberg, Bohr, and Pauli. The Stern-Gerlach experiment demonstrating the reality of space quantizations had earlier sparked Rabi's keen interest in quantum mechanics and so, while working in Hamburg with Pauli, Rabi became a frequent visitor to Stern's molecular beam laboratory. During one of these visits Rabi suggested a new form of deflecting magnetic field; Stern in characteristic fashion invited Rabi to work on it in his laboratory, and Rabi in an equally characteristic fashion accepted. Rabi's work in Stern's laboratory was decisive in turning his interest toward molecular beam research.

RESEARCH AND TEACHING AT COLUMBIA

Rabi returned from Europe to join the faculty at Columbia University and to begin atomic beam research in his own laboratory. In 1931 he and Gregory Breit developed the important Breit-Rabi formula, which showed how the magnetic energy of an atom and its effective magnetic moment vary with the strength of the external magnetic field. These changes occur because the atomic configuration varies from the electron angular momentum being primarily coupled to the nucleus at a low external field to being principally coupled to the external magnetic field at a high field.

Utilizing the Breit-Rabi formula and an atomic beam apparatus which deflected the atomic magnetic moments with inhomogeneous magnetic fields, Rabi, V. Cohen, and others3 were able to determine the strengths of the electron-



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