Joseph John Thomson’s discovery of the electron enabled scientists to explain the nature of cathode rays, provided other explanations for problems with currents in gases, and paved the way for advances in understanding atomic structure.

In his celebrated work Treatise on Electricity and Magnetism (1873), James Clerk Maxwell stressed the need to study the complex processes involved in electric discharge in gases in order to understand the nature of the charge and the medium. In 1879, an English chemist, Sir William Crookes Crookes, William —who had invented the “Crookes tube” Crookes tubes and was the first to observe radiations emitted from a cathode in an evacuated glass tube through which electric discharges occurred— Cathode-ray tubes published an extensive list of attributes of these rays. Crookes noted that, among other properties, cathode rays cast shadows and were bent by a magnetic field: He concluded that they were made up of particles. At the suggestion of Hermann von Helmholtz, Eugen Goldstein Goldstein, Eugen of Berlin studied cathode rays exhaustively and published an impressive paper in the English Philosophical Magazine in 1880, firmly convinced that these rays were a form of waves. Thus, in 1880, the divergence of opinion regarding the nature of cathode rays became a central problem. Crookes and the leading English physicists believed that cathode rays consisted of electrified particles, whereas the German physicists, led by Heinrich Hertz, were certain that the rays were waves. Electrons;discovery
Nobel Prize recipients;Joseph John Thomson[Thomson]
[kw]Thomson Wins the Nobel Prize for Discovering the Electron (Dec. 10, 1906)
[kw]Nobel Prize for Discovering the Electron, Thomson Wins the (Dec. 10, 1906)
[kw]Electron, Thomson Wins the Nobel Prize for Discovering the (Dec. 10, 1906)
Electrons;discovery
Nobel Prize recipients;Joseph John Thomson[Thomson]
[g]Sweden;Dec. 10, 1906: Thomson Wins the Nobel Prize for Discovering the Electron[01740]
[c]Science and technology;Dec. 10, 1906: Thomson Wins the Nobel Prize for Discovering the Electron[01740]
[c]Physics;Dec. 10, 1906: Thomson Wins the Nobel Prize for Discovering the Electron[01740]
Thomson, Joseph John
Wilson, Charles Thomson Rees
Röntgen, Wilhelm Conrad
Lenard, Philipp

In 1883, Hertz found that one could bend cathode rays by applying a magnetic field outside a discharge tube. His attempt to measure the magnetic field caused by the discharge inside the tube (between two parallel glass plates, which enabled him to determine the current distribution therein) gave no significant correlation to the direction of the cathode rays. Further, Hertz applied static electric fields both inside and outside the tube through parallel conducting plates connected to batteries up to 240 volts. This would presumably produce a force perpendicular to the direction of the rays, deflecting them if they were composed of charged particles. In both situations, he obtained a null result. His long series of experiments seemed to confirm the basic premise with which he had started—namely, that cathode rays were waves. His pupil, Philipp Lenard, continued the study of cathode rays, concentrating on their properties outside the tube, which made them easier to handle. He showed that once out of the tube, the rays rendered the air a conducting medium and blackened the photographic plates; moreover, the distance they traveled depended on the weight-per-unit area of matter, not on its chemical properties, and the magnetic deflection was independent of the gas inside the tube. Like Hertz, Lenard believed that he was dealing with a wave phenomenon.

In 1895, Jean-Baptiste Perrin, Perrin, Jean-Baptiste repeating Crookes’s experiment with improved equipment, succeeded in collecting from cathode rays negatively charged particles in an insulated metal cup. This appeared to cast doubt about their wave aspects and consequently, by late 1895, two divergent views prevailed among the leading physicists as to the nature of electric charges. One group thought of them as portions of fluids consisting of large numbers of “molecules of electricity,” or electrons. The other group regarded the “charge” as a result of an unknown form of stress in ether, attached to matter being rendered visible. Therefore, the nature of the cathode rays still remained to be resolved.

In 1895, at the University of Würzburg, while studying discharges produced by an induction coil in an evacuated Crookes tube, Wilhelm Conrad Röntgen accidentally discovered X rays produced by cathode rays as they impacted a platinum target. Among other properties, such as penetrability through matter, X rays were found to ionize gaseous media, making it conduct, which accelerated the study of conductivity in gases. Sir Joseph John Thomson’s use of the property of X rays X rays proved to be the pivotal point in guiding him toward the discovery of the electron.

In the hope of resolving the controversy on the nature of cathode rays, Thomson repeated Perrin’s experiment with minor modifications in the collection and measurement of the charges. Using a magnetic field to bend the rays, he collected them in a metal cup placed away from the direct line. He found that the charge in the cup reached a steady state after attaining a maximum value, which he correctly explained as caused by leakage into surrounding space. Hertz had failed to observe electric deflection of the cathode rays. Thomson, using two conducting plates between the cathode rays within the tube, applying an electrostatic field between the plates, and utilizing a better vacuum technique compared to that available to Hertz, was able to observe deflection of the rays, showing that they were composed of negatively charged particles. He correctly explained that Hertz’s failure to observe electric deflection of the cathode rays was caused by their ionizing property in excess amount of gas in the tube, thus shielding them from the very field meant to deflect them. By the simultaneous application of the electric and magnetic fields to the cathode rays, Thomson obtained the velocity v of the cathode-ray particles. On the assumption that the particles carried a charge e and had mass m, Thomson succeeded in obtaining the crucial ratio of charge to mass, that is, e/m, showing that it was seventeen hundred times the corresponding value for hydrogen atoms. He further showed that the constant e/m was independent of velocity v, the kind of electrodes used, and the type of gas inside the cathode-ray tube.

Using Charles Thomson Rees Wilson’s newly developed “cloud chamber,” Thomson was able to obtain the value of the charge e; from the ratio e/m, it was simple to compute the numerical value of m. Hence the smallness of the mass, combined with the relatively large velocity of the cathode-ray particles, also explained Hertz’s observation, namely, that the rays penetrated thin sheets of metals. Obviously, massive particles could not do so. From Lenard’s result of constancy of magnetic deflection of the rays and independence of chemical properties, Thomson soon realized that he had discovered a universal module of atoms found in radioactive substances, alkali metals bombarded by ultraviolet light, and in a variety of gaseous discharge phenomena. Thomson’s discovery that the cathode-ray particle is universal and fundamental to the understanding of the structure of all matter unraveled the puzzling aspect of conductivity of gases, the nature of electricity, and the wave-particle controversy.

Thomson’s discovery of the electron—as well as the recognition of the fact that it carried a natural unit of charge and was a universal component of all atoms—marked the beginning of a new and exciting period in atomic research. Further confirmation concerning the particle nature of cathode rays came from other quarters. For example, Pieter Zeeman’s observation of the widening D lines in the spectrum of sodium, explained by Hendrik Antoon Lorentz’s theory, caused by changed electron configurations of the atoms in the presence of a magnetic field, gave a value of e/m, comparable to those obtained by Thomson.

Based on his discovery and a study of mechanical stability of the electrons under the influence of the electrostatic force, Thomson showed that the electron must circulate about the atom’s center, constrained to move in concentric circles. Calling the cathode-ray particles “corpuscles” and speculating that their number increased proportionally to the atomic weights, Thomson attempted to explain the structure of the chemical elements and their properties. From this early model, he drew several important conclusions. First, because the electrons must accelerate as they move in circles around the atomic center, they will radiate. Therefore, such an arrangement of electrons cannot be stable, because n, the number of electrons, was assumed to be of the order of one thousand times the atomic weight A. Because experimental work on alpha, beta, and gamma scattering, performed under the supervision of Thomson at the Cavendish Laboratory Cavendish Laboratory to verify his theory and obtain n of chemical elements, had led to negative conclusions, this served as a basis for Ernest Rutherford’s Rutherford, Ernest model of the nuclear atom. Rutherford model of the atom

Additionally, Thomson’s discovery of the order of n fostered the development of the scattering theory, which was to play an important role in the evolving research work in atomic and nuclear physics. The instability of his model atom was instrumental in the formulation of the quantum hypothesis and the discovery of the atomic quantum levels. Finally, Thomson’s electron distribution in atoms provided analogies to the behavior of the chemical elements and in particular the population of electrons in the atoms of contiguous elements in the periodic table that differed by unity. Electrons;discovery
Nobel Prize recipients;Joseph John Thomson[Thomson]

• Buchwald, Jed Z., and Andrew Warwick, eds. Histories of the Electron: The Birth of Microphysics. Cambridge, Mass.: MIT Press, 2001. Collection of essays on Thomson and on the early physics of the electron, produced by the Dibner Institute. Bibliographic references and index.
• Crowther, J. G. The Cavendish Laboratory, 1874-1974. New York: Science History, 1974. Leads readers from the origin of the Cavendish Laboratory through its period of rapid growth to its present-day organization, discussing the crucial role of the personnel of this unique research laboratory. This volume, in addition to providing a complete account of the researchers at Cavendish and their achievements, contains an exhaustive list of references.
• Kim, Dong-Won. Leadership and Creativity: A History of the Cavendish Laboratory, 1871-1919. Boston: Kluwer, 2002. A condensed version of the author’s dissertation, this volume details Thomson’s research at the Cavendish Laboratory, as well as his role as a leader and inspiration of other physicists.
• Nobelstiftelsen, ed. Physics, 1901-1921. New York: Elsevier, 1967. Includes Sir Joseph John Thomson’s Nobel lecture of 1906, detailing his discovery of the electron, which is relatively short, easy to follow, and by far the best source of information on the subject. His biographical sketch, although brief, contains a complete list of honors bestowed on him.
• Strutt, Robert John, Fourth Baron Rayleigh. The Life of Sir J. J. Thomson. Cambridge, England: Cambridge University Press, 1942. At Cavendish in 1899, Strutt began his research work on ionization produced by radiations from various radioactive substances and was close to Thomson’s own research activities. This biography of Thomson is the authoritative text that is often quoted, for good reason. One begins to appreciate the contribution of Thomson and the people he inspired in the field of science after reading this work.
• Thomson, George Paget. J. J. Thomson and the Cavendish Laboratory in His Day. London: Thomas Nelson, 1964. George Paget Thomson, the son of Sir Joseph John Thomson, an eminent physicist, who received the Nobel Prize for his work on the diffraction of electrons in crystals in 1937, narrates the life and achievements of his famous father and those who worked with him at the University of Cambridge. This volume contains not only the story of Thomson’s life and work but also a lucid account of the history of the development of physics at the beginning of the twentieth century in England and Europe.
• Thomson, J. J. Cathode Rays. Stanford, Calif.: Academic Reprints, 1954. This classic scientific paper, originally published in the Philosophical Magazine (1897), in which Thomson methodically details his findings concerning the cathode rays and his discovery of the electron, provides a proper historical perspective and an understanding of the working of a powerful mind. Although the subject matter is technical, the reader should find it highly rewarding.
• _______. Recollections and Reflections. London: G. Bell & Sons, 1936. An autobiography is necessarily “recollections and reflections” in the true sense of the words. Thomson was an accomplished writer, scholar, and teacher accustomed to scrupulously accurate recording of his thoughts and observations. In this last work, Thomson gives a candid account of his many achievements, preserving a balanced historical perspective. This volume spans the history of physics at its most exciting period and is a pleasure to read.

First Practical Photoelectric Cell Is Developed

Einstein Describes the Photoelectric Effect

Thomson Confirms the Possibility of Isotopes

Aston Builds the First Mass Spectrograph and Discovers Isotopes

Discovery of the Compton Effect

Cockcroft and Walton Split the Atom

Discovery of the Cherenkov Effect

Yukawa Proposes the Existence of Mesons

Hahn Splits the Uranium Atom