Which academy sponsored Bohr's 1905 gold-medal competition?

The Royal Danish Academy of Sciences and Letters sponsored Bohr's 1905 gold-medal competition.

In 1905, the academy announced a prize problem concerning the surface tension of liquids. It invited a quantitative investigation of a method based on vibrations in a liquid jet, building on Lord Rayleigh's earlier theory. Bohr entered while still a student at the University of Copenhagen and developed an experimental and theoretical study of water's surface tension.

The competition helped launch Bohr's scientific career. Copenhagen University lacked a dedicated physics laboratory, so he worked in his father Christian Bohr's physiology laboratory and carried out experiments under difficult conditions. He refined the underlying theory by accounting for viscosity, surrounding air, and larger oscillation amplitudes.

Bohr submitted the work in 1906, although the competition had been announced the previous year. The academy informed him on 23 February 1907 that he had won its gold medal. The result is sometimes loosely described as a 1905 prize, but 1905 was the announcement year; the medal followed the completed submission and judging.