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 correspondence principle stated that any new scientific theory must be able to explain all the phenomena which a preceding theory explained. Coming at a time when atomic phenomena were puzzling scientists everywhere, the correspondence principle served as a guideline for new theories: they had to account not only for activity at the atomic level but be applicable to conventional phenomena as well, since classical physics had been successful in doing so. Using this principle, Bohr and Kramers successfully investigated such details as the fine structure of spectra and went on to show how atomic radiation could be reconciled with classical laws regarding the motion of particles.
As a professor, Bohr taught standard subjects such as mechanics and thermodynamics. He also taught atomic theory, which was more closely related to his research interests. He held colloquia that encouraged students to follow the most recent developments. He was elected to the Royal Danish Academy of Sciences and Letters in 1917 and later served as its president for many years. But while he enjoyed prestige, he also lacked resources, working mainly in a single room that gave him no room for experiments. With support from Rutherford and others, Bohr applied to the government as well as private foundations for help in setting up a laboratory. Thus began the University Institute for Theoretical Physics.
Rutherford wrote to Bohr with a proposal that would be hard to refuse. Manchester was also planning to establish a research center for modern physics. Rutherford indicated to Bohr the current state of his research, whose groundbreaking implications Bohr instantly recognized. Rutherford also expressed his sincere desire that Bohr be with him to interpret his new and startling results. Bohr struggled with his decision, recognizing the superior resources that England had to offer and the satisfaction he gained from working with Rutherford. Nevertheless, he felt a duty to his country and university, which had made his opportunities possible and had allowed him to realize his dream of starting his own institute. Rutherford was asked to replace Thomson as Cavendish professor at Cambridge.
While struggling with the pressures surrounding the new institute, Bohr received an invitation from Planck to lecture on spectral theory in Berlin. Accepting eagerly, Bohr arrived to meet Planck and, for the first time, Albert Einstein. Long and intense discussions of physics ensued. The members of the group admired each other. Just as Bohr had made some of his first contributions based on extensions of theories by Planck and Einstein, Einstein had recently expanded upon rules governing Bohr's concept of stationary states.
One fundamental difference between Einstein and Bohr soon became evident. In his lecture on spectral theory, Bohr made the argument that certain determinations could not be measured exactly. This belief irked Einstein, whose lifelong resistance to the element of chance would set him in disagreement with many physicists whose work in quantum mechanics could not be reconciled to Einstein's convictions. Bohr attributed Einstein's stubbornness to a reluctance to renounce certain ideals that Bohr himself had to give up. Regardless, both scientists enjoyed this meeting and looked forward to future encounters, challenging themselves to work out arguments in the interim.