Bohr's first term as a Copenhagen professor began auspiciously. A 21-year-old Dutch physicist named Hendrik Kramers, equipped with a strong background, sought Bohr out with the hope of becoming his assistant. Kramers had chosen Denmark mainly because of its neutrality, but his desire to work with Bohr was keen. Bohr could not have foreseen the contributions that this young man would add to his work, as he served as a faithful assistant for ten years, leaving only for a professorship at Utrecht. Kramers remained a friend even beyond that period. The two worked together again after World War II to urge the limitation of nuclear weapons.Kramers's help contributed greatly to one particularly significant paper: "On the Quantum Theory of Line Spectra." This paper elaborated on the correspondence principle, a concept first introduced by Bohr in 1913. In a general sense, the
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