A Historical-Didactical Reconstruction of Planck’s Discovery of the Constant h

Notwithstanding the huge historical literature about the discovery of Planck's constant, superficial and often incorrect introductions prevail even in university textbooks. We retrace in a didactical perspective the main steps of Planck's research for a theoretical derivation of Kirchhoff's universal function, given by the black-body spectrum, from first principles as related to the theories of electromagnetism and thermodynamics, considering all his communications and papers from November 1899 to January 1901.