J. J. Thomson's plum-pudding atomic model: The making of a scientific myth

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 discoveries raised questions concerning 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. Although the historian Heilbron 2, 3 has argued persuasively that this expression misrepresents Thomson's theory, it is still mentioned in respectable professional texts (sometimes labeled “inappropriate”): see, e.g., 4-8. As evidence for its occurrence in popular accounts, we found that a Google search for “plum pudding atomic model” yields over 100,000 hits. In other words, the expression 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 Kragh1 has noted, between 1904 and about 1910 Thomson's model was generally accepted as the best available atomic theory.2 What distinguishes Thomson's theory is his assignment of a specific inner structure to the atom as well as a set of dynamical assumptions.

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, the atom is a spatial domain filled with numerous negative corpuscles whose total charge is neutralized somehow by the spatial positive electrification.3

In May 1903 Thomson gave a series of lectures at Yale University in which he discussed the consequences of recent advances in physics, including his own substantial contributions, remarking that “we must face the problem of the constitution of the atom, and see if we can imagine a model which has in it the potentiality of explaining the remarkable properties shown by radio-active substances” 11. Thomson then addressed the question, how do the corpuscles arrange themselves in a sphere? He began by considering one corpuscle and appealing to equilibrium as a guiding constraint 11 (pp. 111–117). The resulting theory was based on “the atoms of the chemical elements [which] are built up of large numbers of negatively electrified corpuscles revolving around the centre of a sphere filled with uniform positive electrification” 12. His conception of the atom now included corpuscles whose motions take place in orbits and, due to the forces exerted on the orbits, each orbit “may be constrained to occupy an invariable position with respect to the aggregate—as if, to take a rough analogy, the orbit was a tube bored through the aggregate, so that the orbit and the aggregate move like a rigid body…” 12 (p. 690). Although Thomson appeals to “a rough analogy”, what he presents is an atomic model. As far as we can determine, in his subsequent publications he invoked neither “analogy” nor “aggregation”—an indication that Thomson was groping for a way to address atomic phenomena. Both his methodology and terminology were in transition.

Thomson constructed his atomic theory so that it would be conducive to mathematical analysis where stability, as the title indicates, plays a critical role. The atom in equilibrium has n corpuscles and they are arranged at equal angular intervals on rings inside a positively electrified sphere, each corpuscle carrying a charge of negative electricity. The task was then to find a stable structure which comprises corpuscles bearing negative electric charges (i.e., electrons) immersed in a sphere of positive electrification to render the atom electrically neutral and yet capable of giving an account of its chemical properties. In Thomson's view his calculations showed that the proposed structure does indeed account for the chemical properties of the atom 1 p.258.

For Thomson the model served as a heuristic device exhibiting two principal features, namely, the stability and unity of atomic phenomena, both chemical and electrical.

Thomson's theory was part of the background for the various atomic theories that replaced his model, notably those of Ernest Rutherford (1871–1937) 14 and Niels Bohr (1885–1962) 15. In particular, Bohr was very much influenced by the work of his mentors, Thomson and Rutherford, whom he cited extensively throughout his Trilogy of 1913, in which he introduced the quantum theory of the atom.

Rutherford was well positioned to assess Thomson's contribution and he noted that, while Thomson's model was not consistent with newly found experimental results of deflections of particles, his theory “played a useful part” in pointing to fruitful lines of research. One such line was taken up by Bohr in his construction of the quantum theory of the atom in 1913.

Bohr described Thomson's theory and went on to claim that the configurations allowed in Thomson's atom-model “apparently do not exist for the second [i.e., Rutherford's] atom-model” 15 (p. 2). As is well known, in his quantum theory of the atom Bohr took as his point of departure the atomic structure which Rutherford had proposed and explicitly discarded that of Thomson, inaugurating a new era in physics exactly 100 years ago.4

The “plum pudding” in popularizations

Merck's Report was a practical journal of pharmacy as a profession and a business. In an essay published in December 1906, the question “What is matter?” was addressed, in which a report was given on a series of lectures delivered by Thomson earlier in 1906:

Now a plum pudding is a suet pudding with raisins, traditionally served at Christmas. Despite its name, this pudding contains raisins rather than plums. Clearly, the anonymous reporter knows his pudding: he describes the plum pudding correctly. But there is no correspondence between the embedded raisins and the revolving corpuscles for, according to Thomson's theory of 1906, the electrons revolve on rings about the center of the atom, and they are not distributed throughout the “pudding” as “raisins swimming about”. Merck's report tells us that the plum-pudding image was originally based on a misunderstanding of Thomson's theory.

The invocation of “plum pudding” (here without naming Thomson) is misleading in two respects, namely, the plums in fact are raisins and, more importantly, it is misleading, as we have seen, to liken Thomson's model to a plum pudding.6

The making of a myth

On all accounts plum pudding is a curious image for the atom, and one wonders what could have been the source for these anonymous reporters. One possibility is that they depended on a passage in the physics textbook, Properties of matter (1885) by P. G. Tait (1831–1901), which includes, inter alia, a survey of several hypotheses concerning the ultimate structure of matter.7 Against the limitations of two conceptions, the discrete hard atom on the one hand, and continuous mathematical points on the other, Tait presents what he considers a more plausible theory: “Matter is continuous (i.e. not made up of particles situated at a distance from one another) and compressible, but intensely heterogeneous; like a plum-pudding, for instance, or a mass of brick-work” 23. Tait's plum-pudding image was offered as an attempt to reconcile the continuous with the discrete; this was a problem of accommodation within the classical domain. The imagery is designed to depict matter in general; the atom did not yet have a structure.

Given these historical data, we notice a curious fact, namely, a misleading popular image entered scientific and scholarly discussions. As we have seen, physicists like Rutherford and Bohr who responded professionally to Thomson's model atom conceived it in his own terms and did not present any pictures or introduce any analogies to facilitate comprehension. Indeed, there was no need for such imagery. However, contemporary popularizers, who sought to make the emerging modern physics accessible to the general public, wished to avoid technical aspects of the theory, offering instead a vivid image. It is unfortunately a frequent occurrence that popularizations misrepresent scientific theories; however, what is surprising in the “plum-pudding” case, is the fact that the scholarly literature adopted this misleading characterization of Thomson's model atom. At the outset Thomson considered the atom a structural unity within which numerous negatively charged corpuscles transverse a positively charged space. The model changed over the years, eventually becoming a positively electrified space within which relatively few electrons set on rings circulate about a mathematical center. The scene is energetic, clearly dynamical, and nowhere are raisins/plums stuck in a pudding.