On 14 December 1900 Max Planck gave his seminal lecture to the German Physical Society, introducing the quantum concept for the first time. To join in the celebration of this crucial episode in the history of modern physics, we have invited two prominent historians of science who have studied the event, its origins and its implications. They will discuss, respectively, the related questions of Planck's original understanding of his idea and his fellow physicists' understanding of it once it had been introduced.

Planck's famous communication of December 1900 cannot be understood without taking into account an essential difference between his and Boltzmann's conceptions of the relation between the micro-world and the macro-world. For Boltzmann, macroscopic laws are in principle completely determined by the underlying microscopic dynamics. For Planck, the microscopic dynamics is partly undetermined and must be supplemented by additional assumptions in order to reach macroscopic laws. Thanks to this leeway, Planck could introduce finite energy elements and derive his famous black-body law without giving up the continuity and determinism of former dynamical theories. The purpose of this paper is to explain the origins, peculiarities and evolution of Planck's approach; to show how it ultimately yielded the correct black-body law; and to examine the status of the energy elements which Planck thereby introduced.

In 1914 O.W. Richardson published an advanced textbook: The Electron Theory of Matter, based on his lectures to graduate students at Princeton where he was professor of physics from 1906 to 1914. The book got a very favourable review (signed N.B.) in Nature and must have sold well, for a second, revised, edition containing a much extended account of Niels Bohr's new atomic theory appeared in 1916. Based on this book and on some of Richardson's research papers from 1901 to 1916 I shall try to describe his view of the quantum, a view which I claim to be typical of the period.