Physics Forum Ann. Phys. (Berlin) 525, No. 8–9, A129–A133 (2013) / DOI 10.1002/andp.201300732 THEN & NOW J. J. Thomson’s plum-pudding atomic model: The making of a scientiic myth Giora Hon and Bernard R. Goldstein Figure 1 Joseph J. Thomson giving a lecture demonstration in the 1890s (Photo: courtesy of Niels Bohr Archive, Copenhagen). Why “plum pudding”? Joseph J. Thomson (1856–1940) was one of the leading physicists at the turn of the last century. In 1906 he was awarded the Nobel Prize in physics for discovering the electron and for his work on the conduction of electricity in gases. Thomson’s dis- coveries raised questions concern- ing the nature of the atom. He demonstrated that the atom is not the simplest unit of matter; rather, it has a structure. Thomson’s atomic theory has informally been called the “plum-pudding” model, but the early history of this expression has not been elucidated. The expression is not in Thomson [1], the seminal paper in which Thomson developed his atomic theory, nor is it in any of his subsequent publications. Al- though the historian Heilbron [2, 3] has argued persuasively that this ex- pression misrepresents Thomson’s theory, it is still mentioned in re- spectable professional texts (some- times labeled “inappropriate”): see, e.g., [4–8]. As evidence for its oc- currence in popular accounts, we found that a Google search for “plum pudding atomic model” yields over 100,000 hits. In other words, the ex- pression is a commonplace and, as we show, it is not simply an artifact of recent popularization. So, what were the circumstances that led to calling Thomson’s theory the “plum- pudding” model? Thomson’s atom model and its impact As Kragh 1 has noted, between 1904 and about 1910 Thomson’s model 1 See [4], p. 20. was generally accepted as the best available atomic theory. 2 What dis- tinguishes Thomson’s theory is his assignment of a specific inner struc- ture to the atom as well as a set of dy- namical assumptions. In 1899, in a paper addressed to the British Association for the Ad- vancement of Science, Thomson in- troduced the essential ingredients of his model without any mathematical formulas: I regard the atom as containing a large number of smaller bod- ies which I will call corpuscles; .... In the normal atom, this assem- blage of corpuscles forms a sys- tem which is electrically neutral. Though the individual corpuscles behave like negative ions, yet when they are assembled in a neutral atom the negative effect is bal- anced by something which causes the space through which the cor- puscles are spread to act as if it had a charge of positive electricity equal in amount to the sum of the negative charges on the corpuscles [10], p. 565. These are preliminary remarks in which Thomson was exploring possibilities: he posited something which behaves as if it had a positive charge. But Thomson was certain about the overall system, namely, 2 For a brief account of contemporary alter- native atomic theories with references to the original papers, see [9]. C  2013 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim A129