Giora Hon, University of Haifa, Israel, and Bernard R. Goldstein, University of Pittsburgh, U.S.A.
Constitution, Structure, System, and Model: Imagining the Atom ‒ An Early Twentieth-Century Story

We focus on Niels Bohr’s use of the term “model” in 1913. The title of his trilogy, “On the Constitution of Atoms and Molecules”, contrasts with that of Ernest Rutherford’s experimental paper of 1911, “The Scattering of alpha and beta Particles by Matter and the Structure of the Atom”, and that of Joseph J. Thomson’s theoretical paper of 1904, “On the Structure of the Atom.” Yet, in the very opening paragraph of his trilogy, Bohr identified what he called Rutherford’s theory with an “atom-model”. We ask, how does this usage differ from previous usages of model and other similar terms such as those that were invoked by Joseph Larmor (“model”, 1900), Jean Perrin (“constitution”, and “hypothèse”, 1901), Ludwig Boltzmann (essay on “model”, 1902), Lord Kelvin (“model”, 1904), Hantaro Nagaoka (“system”, and “mechanical analogy”, 1904)? We compare these usages with those of leading physicists in the late nineteenth century. The historical evidence indicates that Bohr’s usage in 1913 was unusual at the time (and unexplained), but his invocation of model had far-reaching consequences for the introduction of modeling into science in general and physics in particular. We examine the different usages of “constitution”, “structure”, “system”, and “model” in describing the atom, and consider their philosophical presuppositions.