What was the subject of Niels Bohr's master's degree work?

Niels Bohr's master's degree work concerned the electron theory of metals.

Bohr received his master's degree from the University of Copenhagen in 1909 after studying how electron theory could explain the physical properties of metals. The subject drew on contemporary ideas associated with the Drude and Lorentz models, which treated metals as containing mobile electrons whose behavior could help account for conduction and other properties.

The topic was not yet Bohr's famous atomic model. His master's work came before his 1913 theory of quantized atomic energy levels and focused on applying electron theory to bulk matter. It also led naturally into his doctoral research: his 1911 dissertation, commonly titled “Studies on the Electron Theory of Metals,” expanded the earlier investigation substantially.

A common mix-up is to identify quantum radiation theory or atomic nuclear structure as the master's subject. Those belong to Bohr's later intellectual development. His early metal research exposed him to the strengths and limits of classical electron models, experience that helped prepare him for the quantum ideas he would later apply to atoms.