J J Thomson and the Discovery of the Electron

Abstract

Davis and Falconer's book will help dispel the impression that Thomson discovered the electron, collected his Nobel Prize and disappeared from the history books. This interesting book locates Thomson's ideas in their historical and philosophical context, revealing a consistent world view that runs through all of his work. In fact this world view could be regarded as the key to why we remember Thomson as the discoverer of the electron rather than any of the many others who were carrying out similar experiments at the same time. It is well known that the name `electron' had been coined by Stoney six years before Thomson's 1897 discovery, but for Stoney an electron was the basic unit of charge and not a subatomic particle. Thomson's interest in gaseous discharge was linked to his desire to explain the underlying structure of matter and link it to Maxwell's electromagnetism. For Thomson cathode rays revealed more than a basic unit of charge, they revealed a new, subatomic particle that happened to carry this charge. This is why Thomson declined to call his `corpuscles' electrons for many years - the discovery was more a step along the way toward a theory of atomic structure than an end in itself. It could be regarded as the start of particle physics. It demonstrated that there was a particle common to all atoms and that a unifying theory was possible. Thomson continued to develop his atomic models and managed to account for ionization, radiation, chemical combination and radioactivity in a consistent way before his model was superseded by the Rutherford planetary model. But even Thomson's attempts to unify atomic physics by constructing all atoms from similar corpuscles were themselves a consequence of his underlying belief that the ultimate ground of physical reality was the ether. This idea, which ties him to the late nineteenth century, had developed from his early work in electromagnetism and analytical dynamics in which corpuscles emerged as structures in the ether (originally he thought they were some kind of vortex ring) interacting via Faraday tubes (localized connections in the ether). The usual cursory mention of the `plum-pudding model' in which Thomson's negatively charged corpuscles are embedded in a vague positively charged fluid does little justice to the subtlety and adaptability of Thomson's atomic theories. In later life he accepted the predictive and explanatory powers of relativity and quantum theory but saw them as mathematical constructs and not directly representational. He still identified space with the ether and thought that deeper levels of explanation were required. The structure of the book is interesting. There is a short but significant foreword by David Thomson, JJ's grandson. This affectionate reminiscence is an important point of contact, since most of the book deals with his ideas and influence. The rest of the book treats his life more or less in chronological order but the chapters are separated by a number of papers written or presented by JJT (mainly drawn from Proceedings of the Royal Institution or Philosophical Magazine). These account for almost half the book and are fascinating to read in parallel with the text. They have been reproduced as facsimiles, and this makes them slightly more difficult to read since the quality of the originals was not wonderful, but it does give them an authentic flavour (although I would have occasionally liked a little more information about some of the line drawings). The other important aspect of the book is its emphasis on Thomson's influence at the Cavendish, where he succeeded Rayleigh as Professor of Experimental Physics in 1884. He is described as `a committed college man, with a reputation for attacking fundamental problems, seeking unification within an ether-based, mechanical physics; a man with an enormous fertility of theoretical invention and a fairly cavalier approach to experiment'. He encouraged and supported a growing number of students, many of whom went on to carry out first rate work, and the Cavendish entered the twentieth century as the most important centre for fundamental atomic research in the world. Thomson's contributions to fundamental science were rewarded with the Nobel Prize; he also had a pivotal advisory role to government, was President of the Royal Society and Master of Trinity College. For many years he was the most important scientist in Britain, and yet few physicists know that much about him. This book gives us an opportunity to put that right, and I would recommend it to anyone who wants to understand the transition from classical to modern physics and learn about a modest but imposing man of unusual vision and compassion.