J. J. Thomson, the Discovery of the Electron, and the Study of Atomic Structure

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J. J. Thomson, the Discovery of the Electron, and the Study of Atomic Structure

Overview

Late in the nineteenth century physicists were working hard to understand the properties of electricity and the nature of matter. Both subjects were transformed by the experiments of J. J. Thomson, who in 1897 showed the existence of the charged particles that came to be known as electrons. Along with the nearly contemporaneous discoveries of radioactivity and x rays, the discovery of the electron focused the attention of scientists on the problem of atomic structure, as well as on ways to put these invisible phenomena to use with inventions such as radio and television.

Background

Joseph John Thomson (1856-1940) spent his professional life at England's Cambridge University, where he passed in four years from prizewinning student (he was ranked second "wrangler" in the prestigious "mathematical tripos" examination in 1880) to head professor at the Cavendish Laboratory—a position previously held by James Clerk Maxwell (1831-1879) and Lord Rayleigh (1842-1919). Maxwell, who first put forth the theory of an electromagnetic field, set up the Cavendish Laboratory in 1874 as a place to pursue investigations in experimental physics and to provide electrical standards for industry. Although he died in 1879, his influence continued to be felt there among the physicists. Mathematical physics had long been established at Cambridge, and while Maxwell, Thomson, and most other Cambridge physicists continued to work successfully in this tradition, the Cavendish Laboratory helped make Cambridge an important center for experimental investigations as well. It remained the preeminent center for the study of subatomic physics into the early decades of the twentieth century.

Although Maxwell himself did not set an explicit agenda for his successors, in the period following his death many of Cambridge's physicists devoted themselves to developing, testing, and expanding Maxwell's theory of electricity and magnetism. An important part of this program was the effort to establish a microscopic basis for electromagnetic phenomena. This would enable physicists not only to describe the behavior of electricity, but also to understand how its phenomena were produced. In his own work, Maxwell had made use of two important research strategies: he derived mathematical relationships between measurable quantities, and he also created analogies or models between electrical effects and well-understood mechanical devices. Neither of these methods required an understanding of the nature of the phenomena themselves. Thomson and his colleagues began from Maxwell's framework, but the goals of their research came to include the understanding of fundamental atomic structure and processes.

Soon after taking over his position as head of the Cavendish Laboratory, Thomson began to study the discharge of electricity in gases. He sought to test various ideas about the dissociation of molecules in an electric field. The earliest experiments consisted of passing a discharge between two large parallel plates that functioned as electrodes, in a container filled with gas and connected to a vacuum pump that could vary the pressure within the container. These experiments, performed with many different gases, provided numerous observations but ultimately led to more puzzles around the relationships among electric current, gaseous discharge, and the chemical combination of atoms.

Thomson refined these investigations by attempting to study the same phenomenon, the discharge of electricity through a gas, with a different experimental arrangement. He filled a bulb with gas at a low pressure, and surrounded it with an electrical coil. The coil produced an electrical field within the bulb that was simpler to study than the one produced by an electrode, and Thomson was optimistic about its results. But by 1892, Thomson was frustrated with difficulties in attaining quantitative measurements in his experiments, and was once again looking for a new approach. He turned for awhile to the study of the electrolysis of steam, analyzing the appearance of hydrogen and oxygen under various experimental conditions. This gave him useful insights into the variable charge on atoms, and encouraged him that his experimental program was indeed elucidating the relationship between the processes of chemical combination and electromagnetism.

During 1895 and 1896, in a scientific atmosphere enriched by reports of the discovery of x rays and "Becquerel rays" or radioactivity, Thomson brought the study of cathode rays into his research on electricity and gases. He had studied cathode rays previously, trying to provide a theoretical framework to describe the rays. Now he entered the ongoing debate about whether these rays were themselves streams of charged particles, or some kind of disturbance in the underlying electrical "ether." Thomson sought to show that cathode rays could be deflected by a magnetic field—an observation that would confirm that the rays were charged particles. He did this by directing a stream of cathode rays through narrow slits into the field between two charged plates, and then measuring the stream's deflection. These experiments not only showed that the cathode rays were indeed charged particles, but also allowed Thomson to determine the ratio between the charge carried by the particles and their mass. The ratio of charge to mass turned out to be much higher than expected, and even more surprising, to be independent of the nature of the gas in the tube. Thus, these particles, or corpuscles as Thomson initially referred to them, must have a very small mass, and he also argued that since they seemed to be the same in all gases, they must actually be part of atoms themselves. Thomson had identified one of the primary building blocks of matter—what came to be called the electron.

Although Thomson's role in the discovery of the electron is indisputably central, it would be a mistake to believe it to be exclusively his accomplishment. In addition to the other researchers at the Cavendish Laboratory, important work on the discharge of electricity through gases and the nature of cathode rays was done by Arthur Schuster (1851-1934), Philipp Lenard (1862-1947), and Heinrich Hertz (1857-1894); Hendrik Lorentz (1853-1928) pursued Thomson's discovery, improved upon his measurements and endowed the corpuscles with the name "electron."

Impact

J. J. Thomson's identification of the electron in 1897 focused new attention on questions of atomic structure. Thomson conjectured that the electron was a fundamental building block of matter or atoms, and along with his colleagues at Cambridge attempted to build upon his discovery in order to model atomic structure with theoretical speculations and extensive experimental investigations, particularly scattering experiments. They struggled to explain many observations, such as the nature of positive charge, the relation between number of electrons and atomic weight, and the mechanical stability and chemical properties of atoms. While the Cambridge scientists and others working within the framework they had established came up with models of the atom that successfully accounted for many of these phenomena, the behavior of atoms came to be explained much more effectively as physicists adopted the ideas of quantum science beginning about 1912.

Other investigations also built upon Thomson's discovery. Further research by Thomson, as well as work by Henri Becquerel (1852-1908), Lenard, Ernst Rutherford (1871-1937), and others, helped to show that the electron identified by Thomson was the same as the negatively charged particles observed in phenomena such as radioactivity and the photoelectric effect. American scientist Robert Millikan (1868-1953) improved upon Thomson's measurement of the charge on the electron by observing the motion of charged oil drops. By the 1920s, scientists were studying electrons within the framework of quantum physics, and began to explore the theory that electrons behaved not only as particles but also as waves. Several Nobel Prizes were given for early research related to the discovery and study of the electron, including one to Thomson in 1906 and to Millikan in 1923. As testimony to Thomson's influence as a teacher, seven of his research assistants also went on to win Nobel Prizes for physical research.

The impact of the discovery of the electron extended far beyond science. Throughout the nineteenth century, research into electrical phenomena had been intertwined with efforts to advance practical uses of electricity such as the telegraph and electrical power. The investigations of Thomson's era helped bring about the rapid invention and development of "wireless telegraphy," or radio, and led to the invention of television and later the development of microwave technologies such as radar. Radio arose in part from investigations into the nature of the electromagnetic "ether" or atmosphere, a subject that Thomson also addressed in his research. The invention of television is more directly indebted to the discovery of the electron, as electronic television is based on cathode ray tubes in which a beam of electrons is aimed at a screen. While Thomson's experiments and theories did not result directly in any of these inventions, his contributions advanced understanding of the nature and behavior of electrical processes and atomic structure, making such technological developments easier and faster.

LOREN BUTLER FEFFER