Ernest Rutherford

Ernest Rutherford

The British physicist Ernest Rutherford, 1st Baron Rutherford of Nelson (1871-1937), discovered transmutation of the elements, the nuclear atom, and a host of other phenomena to become the most prominent experimental physicist of his time.

In searching for an experimental physicist to compare with Lord Rutherford, it is natural to think of Michael Faraday. Like Faraday, Rutherford instinctively knew what experiments would yield the most profound insights into the operations of nature; unlike Faraday, however, Rutherford established a school of followers by training a large number of research physicists. One of his colleagues observed that Rutherford always appeared to be on the "crest of the wave." Rutherford, with no sense of false modesty, replied, "Well! I made the wave, didn't I?" Then, after a moment's reflection, he added, "At least to some extent." Most physicists would agree that it was to a very large extent.

Ernest Rutherford was born on Aug. 30, 1871, in Spring Grove (Brightwater), near Nelson, New Zealand. His father, a Scot, was a wheelwright, farmer, timberman, and large-scale flax producer. Rutherford attended Nelson College, a secondary school (1886-1889), and then studied at Canterbury College in Christchurch, receiving his bachelor's degree in 1892. The following year he took his master's degree with honors in mathematics and physics.

First Research

Rutherford's interest in original research induced him to remain at Canterbury for an additional year. Using the rather primitive research facilities available to him, he proved that iron can be magnetized by the rapidly oscillating (and damped) electric field produced during the discharge of a Tesla coil. This indicated that electromagnetic (Maxwellian or Hertzian) waves might be detectable if they were allowed to demagnetize a magnetized wire, and by the end of 1894 he was sending and receiving these "wireless" signals in the laboratory.

In 1895 Rutherford arrived in Cambridge, where he became the first research student to work under J. J. Thomson at the Cavendish Laboratory. He improved his earlier instrumentation and was soon transmitting and receiving electromagnetic signals up to 2 miles' distance, a great achievement in those days. Thomson asked Rutherford to assist him in his own researches on the x-ray—induced conduction of electricity through gases. Within a year these studies led Thomson to his discovery of the electron.

Rutherford then explored still another recent find, A. H. Becquerel's 1896 discovery of radioactivity. Rutherford soon determined that the uranium rays were capable of ionizing gases. He also discovered something new, namely, that uranium emits two different types of radiation, a highly ionizing radiation of low penetrating power, which he termed alpha radiation, and a much lower ionizing radiation of high penetrating power, which he termed beta radiation.

Rutherford remained with Thomson at the Cavendish Laboratory until 1898; he was therefore extremely fortunate in being at precisely the right place at precisely the right time. His scientific horizons broadened enormously during these years; and his confidence increased greatly owing to Thomson's open recognition of his exceptional ability.

Radioactive Transformations

Rutherford's first professorship was the Macdonald professorship of physics at McGill University in Montreal. In 1900 he married Mary Newton; the following year their only child, Eileen, was born.

Concerning research, Rutherford knew precisely the area he wished to study: radioactivity. On his suggestion, R. B. Owens, a young colleague in electrical engineering, had prepared a sample of thorium oxide to study the ionizing power of thorium's radiations. Owens found, oddly enough, that the ionization they produced apparently depended upon the presence or absence of air currents passing over the thorium oxide. Nothing similar had ever been observed with uranium. It was this mystery that Owens, going on vacation, left for Rutherford to solve.

Rutherford designed a series of masterful experiments from which he concluded that thorium somehow produces a gas, which he called "thorium emanation." It was this gas that Owens's air currents had transported, thereby influencing the recorded ionization. Rutherford also found that any thorium emanation produced soon disappeared before his very eyes! By passing some thorium emanation through a long tube at a constant rate, Rutherford discovered that half the amount present at any given time disappeared ("decayed") roughly every minute—its "half-life." He also found that, if thorium emanation came into contact with a metal plate, the plate would acquire an "active deposit" which also decayed but which had a half-life of roughly 11 hours. Further studies revealed that pressure or other external conditions did not influence these half-lives. In addition, the "activities" of the substances as a function of time decayed exponentially, which Rutherford realized was possible only if the activity was directly proportional to the number of "ions" (atoms) present at any given time. In this way Rutherford discovered the first known radioactive gas, thorium emanation, and explored its behavior.

In 1900 Rutherford was joined by Frederick Soddy, a member of McGill's chemistry department. Together they resolved to isolate the sources of thorium's radioactivity by chemical separation techniques. By the end of 1901 their most important conclusions were, first, that thorium emanation is an inert gas like argon and, second, that thorium emanation is produced, not by thorium directly, but by some unknown, and apparently chemically different, element which they termed "thorium X." This was a key insight into the understanding of radioactivity, for it suggested that one element, thorium, can decay into a second element, thorium X, which in turn can decay into a third element, thorium emanation.

Item after item now fell into place. Soddy, turning from thorium to uranium, found that it decayed into a new radioactive element, "uranium X." Next, Rutherford came to understand the crucial fact that each radioactive transformation is accompanied by the instantaneous emission of a single alpha or beta particle. Rutherford also proved by a simple calculation that in radioactive transformations enormous quantities of energy are released, which, he argued could be derived only from an internal atomic source.

Although some links were still missing, Rutherford's revolutionary theory of radioactive transformations was essentially complete by early 1904. He summarized the results of all of his own researches, as well as those of the Curies and other physicists, in his Bakerian lecture, "The Succession of Changes in Radioactive Bodies," of May 19, 1904, which he delivered before the Royal Society of London. In this lecture, one of the classics in the literature of physics, he presented the complete mathematical formulation of his theory, identified the four radioactive series— uranium, thorium, actinium, and radium (neptunium)—and established the principle, albeit tacitly, that any radioactive element can be uniquely identified by its half-life.

Rutherford also delivered a lecture at the Royal Institution in which he dwelled at some length on an important consequence of his theory—its implications for the age of the earth. He realized that lead, a stable element, is the end product of each radioactive series. This meant that, by determining the relative amounts of, say, uranium and lead in a sample of rock, its age can be calculated—which is the basis of the radioactive dating method.

Rutherford's researches attracted a number of scientists to McGill. His activities there—teaching, experimenting, writing his famous book Radioactivity —were prodigious. Recognition came to Rutherford early: he was elected a Fellow of the Royal Society in 1902, was awarded the society's Rumford Medal in 1905, and delivered the Yale University Silliman Lectures and received his first honorary degree in 1906. In 1908 he received the Nobel Prize—in chemistry! Rutherford later remarked that he had in his day observed many transformations of varying periods of time, but the fastest he had ever observed was his own from physicist to chemist. He refused to disappoint the Nobel Committee, however, and titled his Nobel lecture "The Chemical Nature of the Alpha-Particles from Radioactive Substances."

Nuclear Atom and Artificial Transmutations

In 1907 Rutherford arrived at the University of Manchester to succeed Sir Arthur Schuster as Langworthy professor of physics. Rutherford seems to have enjoyed teaching at Manchester more than at McGill. As he later wrote to his friend B.B. Boltwood of Yale University: "I find the students here regard a full professor as little short of Lord God Almighty…. It is quite refreshing after the critical attitude of Canadian students."

By early 1908 Rutherford was ready to test some new ideas. One of the first questions he wanted to settle was the nature of alpha particles. He devised a very simple scheme for capturing alpha particles, from purified radium emanation, in a glass enclosure. There the alpha particles acquired free electrons and formed a gas which spectroscopic analysis proved to be helium. This work took on much broader significance as a result of another observation, namely, that alpha particles can be scattered by various substances. His coworkers, H. Geiger and E. Marsden, allowed alpha particles to strike various metal foils (for example, gold and platinum) and counted that between 3 and 67 alpha particles per minute—or about 1/8000 of those present in the incident beam—were scattered backward, that is, through more than a right angle.

Two years elapsed before Rutherford achieved the insights necessary for a satisfactory explanation of Geiger and Marsden's experiments. He had to realize that the alpha particle is not of atomic dimensions but that it can be considered to be a point charge in scattering theoretical calculations and that the number of electrons per atom is relatively small—on the same order of magnitude, numerically speaking, as the atom's atomic weight. He also had to realize the extreme improbability of obtaining Geiger and Marsden's results if the alpha particle was multiply scattered by presumably widely separated electrons in the atom, as a 1904 atomic model, as well as a 1910 scattering theory, of Thomson's suggested. In early 1911 Rutherford became convinced, through rather extensive calculations, that Geiger and Marsden's alpha particles were being scattered in hyperbolic orbits by the intense electric field surrounding a dense concentration of electric charge in the center of the atom—the nucleus. The nuclear atom had been born.

No one, however, noticed the new arrival. It was apparently not even mentioned, for example, at the famous 1911 Solvay Conference in Brussels, which Rutherford, Albert Einstein, Max Planck, and many other prominent physicists attended. Whatever novelty contemporary physicists attached to Rutherford's paper seems to have been to his scattering theory rather than to his model of the atom— which was only one of many models present in the literature. Only after Niels Bohr exploited the nucleus in developing his famous 1913 quantum theory of the hydrogen atom, and only after H.G.J. Moseley attached to the nucleus a unique atomic number through his well-known 1913-1914 x-ray experiments, was the full significance of Rutherford's nuclear model generally appreciated. Only then, for example, did the concept of isotopes become generally and clearly recognized.

The researches that Rutherford fostered at Manchester—partly for which he was knighted in 1914—were not confined to alpha scattering and atomic structure. For example, he and his coworkers studied the chemistry and modes of decay of the radioactive elements; the scattering, the wavelengths, and the spectra of gamma rays; and the relationship between the range of alpha particles and the lifetime of the elements from which they are emitted.

Most of this immense activity was brought to a halt at the outbreak of World War I. Rutherford became associated with the Admiralty Board of Invention and Research early in the war, and he carried out experiments relating to the detection of submarines, devising a variety of microphones, diaphragms, and underwater senders and receivers to study underwater sound propagation. He supplied American scientists with a vast amount of information when the United States entered the war in 1917.

In 1919 Rutherford and William Kay found, as the culmination of a long series of investigations, that when alpha particles strike hydrogen—or, in a more famous experiment, nitrogen—recoil "protons" (Rutherford's term) are produced. Rutherford realized at once that he had achieved the first artificial nuclear transmutation (alpha particle + nitrogen to proton + oxygen) known to man. He gave a full account of his and Kay's work in 1920 in his second Bakerian lecture, "Nuclear Constitution of Atoms." One surprising prediction he made in this lecture was that of a "kind of neutral doublet," perhaps a faint premonition of the neutron. Rutherford's discovery of artificial transmutation was, in general, a fitting capstone to his brilliant career at Manchester.

Cambridge and Honors

In 1919 Rutherford became Cavendish Professor of Physics and Director of the laboratory and, a bit later, Fellow of Trinity College, Cambridge. As the occupant of the most prestigious chair of physics in England, and, concurrently, as the holder of a Professorship of Natural Philosophy at the Royal Institution (1921), Rutherford was more and more called upon to deliver public lectures and serve in various professional offices. In 1923 he was elected President of the British Association for the Advancement of Science; in 1925, the same year in which he gained admittance into the coveted Order of Merit, he became President of the Royal Society for the customary 5-year term. In 1933 he accepted the presidency of the Academic Assistance Council, formed to aid Nazi-persecuted Jewish scholars. He died on Oct. 19, 1937, in Cambridge.

Portrait of the Man

C. P. Snow has provided the following portrait of Rutherford in mature life: "He was a big, rather clumsy man, with a substantial bay window that started in the middle of the chest. I should guess that he was less muscular than at first sight he looked. He had large staring blue eyes and a damp and pendulous lower lip. He didn't look in the least like an intellectual. Creative people of his abundant kind never do, of course, but all the talk of Rutherford looking like a farmer was unperceptive nonsense. His was really the kind of face and physique that often goes with great weight of character and gifts. It could easily have been the soma of a great writer. As he talked to his companions in the streets, his voice was three times as loud as any of theirs, and his accent was bizarre…. It was part of his nature that, stupendous as his work was, he should consider it 10 per cent more so. It was also part of his nature that, quite without acting, he should behave constantly as though he were 10 per cent larger than life. Worldly success? He loved every minute of it: flattery, titles, the company of the high official world."

Further Reading

Rutherford's scientific papers, together with introductory notes by James Chadwick and other physicists, were assembled in The Collected Papers of Lord Rutherford of Nelson (3 vols., 1962-1965). Selections from his papers are in J. B. Birks, ed., Rutherford at Manchester (1962), and Alfred Romer, ed., The Discovery of Radioactivity and Transmutation (1964). Lawrence Badash edited Rutherford and Boltwood: Letters on Radioactivity (1969).

Arthur S. Eve, Rutherford: Being the Life and Letters of the Rt. Hon. Lord Rutherford (1939), is a full-length biography; Eve and James Chadwick wrote the obituary notice of Rutherford in the Royal Society of London, Obituary Notices of Fellows of the Royal Society, vol. 3 (1936-1938). Three other full-length biographies are Ivor B. N. Evans, Man of Power: The Life Story of Baron Rutherford of Nelson (1939); John Rowland, Ernest Rutherford: Atom Pioneer (1955); and Edward N. da C. Andrade, Rutherford and the Nature of the Atom (1964). A brief biography is C. M. Focken, Lord Rutherford of Nelson (1938). Extremely interesting recollections by H.R. Robinson, J. D. Cockcroft, M.L. Oliphant, E. Marsden, and A.S. Russell were published between 1943 and 1951 and separately reprinted in 1954 by the Physical Society of London under the title Rutherford: By Those Who Knew Him (1954). For help with questions on physics see W.E. Burcham, Nuclear Physics: An Introduction (1963). □