J. J. Thomson
Joseph John Thomson, commonly known as J. J. Thomson, was a prominent British physicist whose work fundamentally altered the understanding of atomic structure. Born in 1856, he began his studies at Owens College before transferring to Trinity College, Cambridge, where he excelled in mathematics and later became a fellow. Thomson's most significant scientific achievement was the discovery of the electron in 1897, which he identified through experiments with cathode rays. This groundbreaking finding provided vital evidence that atoms are not indivisible, as previously believed, but consist of smaller particles.
Thomson also proposed a model of the atom, often visualized as a sphere of positive charge with electrons embedded within it. Although this model was eventually supplanted by Ernest Rutherford's nuclear atom model, it represented a critical step in atomic theory. Throughout his career, Thomson was deeply involved in research and education, leading the Cavendish Laboratory and mentoring many students, some of whom became notable physicists themselves. His legacy extends beyond his discoveries; he contributed to the development of the field of subatomic physics and authored influential texts that shaped the discipline. Thomson's work laid the foundation for modern physics and chemistry, impacting the study of atomic and subatomic particles for generations to come.
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J. J. Thomson
English physicist
- Born: December 18, 1856; Cheetham Hill, England
- Died: August 30, 1940; Cambridge, England
J. J. Thomson discovered the electron, for which he received the Nobel Prize in Physics in 1906, and subsequently proposed a model for the interior of the atom. His discovery rendered false the belief in the atom as the indivisible building block of matter.
Primary fields: Physics; mathematics
Specialty: Atomic and molecular physics
Early Life
Joseph John Thomson was the oldest son of Joseph James Thomson, a bookseller, and Emma Swindells. The young Thomson was to be apprenticed to an engineering firm, but while awaiting an opening, he entered Owens College in Manchester at the age of fourteen. Two years later, Thomson’s father died, and his mother could not afford the apprenticeship fee. With scholarships, Thomson was able to remain at Owens, from which he received a certificate and prize in engineering in 1876.
Later that year, Thomson entered Trinity College at Cambridge University and spent the rest of his life there. Studying mathematics, he obtained the second-highest score in the 1880 Tripos final examination and thus earned the title second wrangler, a distinction he shared with other such illustrious physicists as James Clerk Maxwell and Lord Kelvin. In 1880, he was elected a fellow of Trinity, where his mathematical research extended some of Maxwell’s earlier results in electrodynamics.
Thomson became a lecturer in mathematics at Trinity in 1882 and a university lecturer in 1883. The following year, at the age of only twenty-eight, Thomson succeeded Lord Rayleigh as Cavendish Professor of Experimental Physics and director of the Cavendish Laboratory. In 1890, he married Rose Paget, one of the first women admitted to advanced study at Cambridge. The Thomsons had two children; their son George Paget Thomson became a famous physicist as well.
Life’s Work
Thomson was elected a fellow of Trinity on the basis of his thesis on energy transformation, a topic that first interested him at Owens College and that remained an important theme in his research throughout his life. From his work on electrodynamics, he published a theoretical proof that a moving electrified sphere undergoes an increase in mass because of its charge, one of the earliest indications of the relationship between mass and energy. While this was an important result itself, it was even more important in stimulating other advances from Maxwell’s work, by not only Thomson but also other physicists.
In 1882, Thomson won the Adams Prize for an essay on the interaction of two closed vortices in an incompressible fluid. In A Treatise on the Motion of Vortex Rings (1883), he extended his results to the “vortex atom” proposed by Kelvin in 1867. After determining the arrangements necessary for the stability of two or more vortex rings in a frictionless fluid, Thomson used this model to explain the combining power of various elements in some simple molecules.
At Cavendish, Thomson chose the topic of gas discharge for experimental investigation. It had been known since the 1850s that the passage of an electric current through an evacuated tube causes the small amount of gas remaining in the tube to glow. This topic regained interest in the early 1880s as a result of new studies on cathode rays, the emanation from the cathode (negative electrode) in evacuated tubes. Some of Thomson’s earlier theoretical work, in fact, was aimed at an improved understanding of cathode rays. English physicists generally believed the rays were streams of fast-moving charged particles, while German physicists generally regarded them as electromagnetic phenomena. Sir William Crookes had shown that a magnetic field deflects the path of cathode rays, and while this evidence suggested charged particles, it was not conclusive.
Thomson set out to demonstrate that an electric field could also deflect the path of cathode rays, a goal that others had not achieved, and he finally succeeded in 1897 with highly evacuated tubes. This result finally offered conclusive evidence that cathode rays are indeed particles. He also developed several methods for experimentally determining the charge-to-mass (e/m) ratio of these particles. Regardless of the source of the cathode rays, this ratio was always the same, and its value implied a very large charge or a very small mass for each particle. Although other physicists had determined similar e/m values, only Thomson assumed the charge to be equal to the smallest-known charge of an ion and thus concluded that the mass of the particle was more than one thousand times smaller than that of a hydrogen atom (the ratio is now known to be about 1/1836). Thus, Thomson discovered the electron and received the Nobel Prize in Physics in 1906.
With the idea of the electron as the fundamental unit of electricity and a universal constituent of the atom, Thomson proposed a model for the internal structure of the atom. His model consisted of a sphere of diffuse positive charge in which were distributed negatively charged electrons that exactly balanced the positive charge. Despite drawbacks to the model, it had the advantage of being easily visualized, though it shortly gave way to the more successful nuclear atom proposed by Thomson’s student, Ernest Rutherford, in 1911.
After 1906, Thomson turned his attention to “canal rays,” streams of positively charged ions that travel in the opposite direction to cathode rays in discharge tubes. His investigation of their properties continued for many years and helped pave the way for Rutherford’s 1914 discovery of the proton, which has an equal and opposite charge to that of the electron. As Thomson’s experimental techniques grew more sophisticated, he was able to separate gaseous products differing very little in mass. In 1913, for example, the two separated species from a neon discharge proved to be the two most abundant isotopes of the element known as neon. Although different isotopes of the same element were known to exist in radioactive transformations, Thomson produced the first evidence for the existence of isotopes of a stable element.
Thomson resigned his positions at the Cavendish in 1919 and was succeeded by Rutherford. As an honorary professor, he worked daily at the Cavendish and remained master of Trinity College from 1918 until shortly before his death in 1940. Throughout his career, Thomson received numerous honors, prizes, and honorary degrees. He served as president of the Cambridge Philosophical Society, the British Association for the Advancement of Science, and the Royal Society of London.
Impact
Thomson’s most notable and lasting scientific achievement was his discovery of the electron. Once scientists accepted the electron as a constituent of all atoms, the century-old belief in the atom as the indivisible building block of matter was rendered false. Atoms have internal structure, and much of the research in both physics and chemistry since 1897 has been to elucidate that structure. For physicists, the electron remains of interest as one of the fundamental subatomic particles, but for chemists, the behavior of electrons, especially the atom’s outermost valence electrons, is crucial in determining the properties and chemical behavior of each element.
Thomson’s importance, however, extends far beyond his scientific achievements. He wrote several treatises on research topics and coauthored a widely used four-volume textbook of physics. At Cambridge, Thomson developed the Cavendish Laboratory into a preeminent research institution for subatomic physics in the early twentieth century. An outstanding teacher and leader, he trained numerous physicists, many of whom became professors themselves and seven of whom won the Nobel Prize. In introductory physics and chemistry textbooks, Thomson is often relegated to a historical footnote, but his contributions, both direct and indirect, have a direct bearing on more of the topics in those books than many realize.
Bibliography
Davis, E. A., and I. J. Falconer. J. J. Thomson and the Discovery of the Electron. London: Taylor, 1997. Print. A survey of Thomson’s discovery of the electron within the context of his life and other achievements.
Kim, Dong-Won. Leadership and Creativity: A History of the Cavendish Laboratory, 1871–1919. Boston: Kluwer, 2002. Print. Traces the success of Cavendish through its leaders and scientists, with a strong focus on Thomson.
Thomson, J. J. Recollections and Reflections. 1936. Cambridge: Cambridge UP, 2011. Print. Thomson’s retrospective of his own life and career.