There’s a lot to say about Christian Christiansen (1843-1917) as a person and as a physicist. But the main point here is just that he was Niels Bohr’s physics teacher at the university level, and his PhD advisor.
But B. knew Christiansen for many years in advance of his university studies. For Christiansen was one of the group of four scientists (including Christian Bohr, Harald Høffding, and Vilhelm Thomsen) who met regularly in the Bohr home for discussion.
Høffding reminisces about Christiansen:
Ogsaa ham kom jeg, især i senere Aar, til at staa nær, og ogsaa her kom mig en ejendommelig Natur imøde, en ung Kraft, der tidligt fandt sit bestemte Virkeomraade, men tillige et Sind, som led af Tynge og trængte til et absolut Tilhold i den Tro, han var voxet op i, dog med aaben Sans opsøgt det Menneskelige under alle Former og til alle Tider.
Christiansen wrote textbooks and articles in physics. He also wrote several works of popular science.