Electron, Discovery of

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ELECTRON, DISCOVERY OF

By 1890 many chemists and physicists believed that atoms must have some sort of structure, a belief stemming from a conviction that nature was essentially simple, and that atoms of the many different elements might represent arrangements of a more fundamental unit. This was often taken to be the hydrogen atom, an idea first suggested by William Prout in 1816. Chemists, however, based their atomic ideas on evidence from spectroscopy, while physicists were trying to understand kinetic theory.

In 1860 Gustov Kirchoff and Robert Bunsen showed that each element emitted its own characteristic line spectrum if heated in a flame. Spectral analysis led to a spate of discoveries of new elements, making the variety even more bewildering. The observation that hydrogen and helium were the only two elements present in the Sun prompted both Norman Lockyer and William Crookes to suggest that atoms somehow evolved from these two elements.

Meanwhile physicists were following up the success of the kinetic theory of gases which proposed that macroscopic effects such as heat could be explained by the motion of atoms and molecules. Some idea of atomic structure was needed to establish the types of vibration possible. However, models devised for the kinetic theory disagreed with the results of spectral analysis.

James Clerk Maxwell's electromagnetic theory, published in 1862–1873, and Michael Faraday's work on electrolysis pointed toward a possible solution. Maxwell had suggested that the vibrations of light were not mechanical, as previously thought, but electromagnetic, while Faraday's laws of electrolysis implied that electricity existed in discrete units with a charge equal to that on the hydrogen ion. Several physicists, notably F. Richarz and H. Ebert, suggested that if the atom contained discrete charge units, then their oscillations might explain the emission of line spectra. In 1891 George Johnstone Stoney named these units of charge electrons and attempted to find out how big they were by reconciling the spectroscopic and kinetic data.

Simultaneously, the theorists Henrik Anton Lorentz and Joseph Larmor were independently trying to accommodate discrete charges within Maxwell's theory that were expressed in terms of a continuous and all-pervading electromagnetic medium known as the ether. They suggested that charges could be modeled by vortices or strain centers in the ether, and Larmor adopted the term electron to describe his charge. In 1896, Pieter Zeeman's discovery of magnetic splitting of spectral lines upheld these ideas for it could be predicted from Lorentz's theory and allowed, for the first time, the size of the vibrating spectroscopic charges to be calculated: they proved to have a mass to charge ratio about 2,000 times smaller than the hydrogen atom, suggesting that the vibration was that of the electron itself. However, there was still no way of manipulating the electrons or measuring them directly.

For Lorentz and Larmor the electron was embedded within the atom but played no role in determining its chemical nature. This view was to change in the years 1895–1905 following the discovery of X rays, radioactivity, and Joseph John Thomson's investigations of cathode rays.

Cathode rays were discovered by Julius Plücker in 1858. They are found when an electric potential is applied across a gas at low pressure and are detected by a fluorescent glow where they hit the glass at the end of the discharge tube. They were known to travel in straight lines and to be deflected by a magnetic field, but by about 1880 the initial interest had died down and most physicists did not consider cathode rays very important. They seemed peripheral to major theoretical concerns and were difficult to experiment on, requiring at least half a day of hand pumping to evacuate the tube, which then frequently broke due to the poor composition of glass available.

However, in 1895, Wilhelm Roentgen discovered X rays, which are emitted when cathode rays hit a target. The discovery caused a furor and the understanding of gaseous discharge and of the behavior of discharge tubes advanced rapidly; it also revived interest in the nature of the cathode rays that caused X rays. Speedy recognition that cathode rays were negatively charged particles about 2,000 times smaller than atoms depended primarily on the work of four men: Philip Lenard, who had followed up Heinrich Hertz's discovery of 1892 that cathode rays could pass out of the discharge tube through a thin foil of metal and showed that cathode rays traveled much further than expected through gases and that their absorption depended on the molecular weight of the gas; Emil Wiechert; Walter Kaufmann; and J. J. Thomson, who measured the charge to mass ratio (e /m ) of the rays by various means. While Wiechert's experiments predated Thomson's by a few months and Kaufmann's were often regarded as more reliable, Thomson went the furthest theoretically.

Thomson was unique among British physicists in his concern to explain the chemical properties of atoms. In 1882 he had shown how the then-popular theory that atoms consisted of vortex rings in the ether could account for the periodic table. He spent thirteen years experimenting on gaseous discharge (but not specifically on cathode rays) guided by his own concept of a discrete electric charge modeled by the end of a vortex tube in the ether. In 1896 he established his theory of discharge by ionization on a firm mathematical footing. He then demonstrated that the magnetic deflection of the cathode rays was the same, regardless both of the cathode material and of the gas in the discharge tube, and refined Jean Perrin's experiment of 1895 that showed that the rays carried with them a negative electric charge. Then, guided by the rays' uniform magnetic deflection and by Lenard's absorption data, which he could explain on the assumption that the cathode rays were interacting with the individual components of atoms, and prompted by his previous speculations about discrete charges and structured atoms, Thomson proposed that cathode rays were subatomic, negatively charged particles from which all atoms were built up. He announced his ideas at the Royal Institution on April 30, 1897.

Thomson's proposals suggested something radically new. His particles were not just charges embedded within atoms. They provided the essential mass of the atom and its chemical constitution. To mark the distinction from electrons he called the particles corpuscles. He supported his hypothesis by measuring the charge to mass ratio of the corpuscles. Initially, he did this by comparing the magnetic deflection of the rays with their heating effect when they hit a target. Later in 1897, after he had succeeded in deflecting the rays electrically, he devised his classic e /m experiment that compared the electric and magnetic deflections of the rays. Both sets of results suggested that the nature of the corpuscles was independent of the material in the discharge tube and that they were about 1,000 times smaller than the hydrogen atom. Thomson confirmed the small size of corpuscles in 1899 when he succeeded in measuring their charge independently of their mass using the particles released in the photoelectric effect.

Thomson's suggestion was difficult to accept: to his contemporaries it sounded like alchemy because an atom emitting corpuscles seemed as though it should change its chemical nature. They preferred George Fitzgerald's alternative proposal that cathode rays were free Larmor-type electrons. Thus the name electron became firmly attached to the particles several years before the realization that the particles were indeed essential constituents of the chemical atom. This realization did not come until work on radioactivity demonstrated the identity of beta and cathode ray particles and showed that atoms could and did split up and change their chemical nature.

Fitzgerald's suggestion ensured the early importance of Thomson's cathode ray particles by tying them into the attempt by Lorentz, Henri Poincaré, Kaufmann, and others to formulate an entirely electromagnetic theory of matter; an attempt which fostered, but eventually seemed incompatible with, Einstein's relativity theory.

Meanwhile, Thomson was incorporating corpuscles into his widely applicable discharge theory and his atomic theory. The mass of Thomson's atom was entirely due to the corpuscles (hence there must be thousands of them). He arranged them in rings within a uniform sphere of positive electrification and set them spinning to ensure stability. Radioactive decay became an inevitable consequence, for rotating corpuscles emit energy and slow down; the rate of emission is less if there are lots of corpuscles in the ring, and Thomson's model demanded thousands; yet, the time still comes when the rings have slowed down, become unstable, and the atom flies apart. Prior to decay, the arrangement of corpuscles in rings provided an explanation for the periodic table, valence, and ionic bonding.

Thomson's atom theory proved untenable in 1906 when he showed how to use scattering data to calculate the number of corpuscles in the atom and found that there could be only hundreds, not the thousands, necessary to ensure reasonable stability. But, it was his concept, as interpreted by Ernest Rutherford and Niels Bohr, that ensured that the electron became fundamental to new theories of atoms, chemical bonding, and materials and, together with relativity and quantum mechanics, to ideas of the nature of matter.