RUTHERFORD, ERNEST

(b. between the settlements of Brightwater and Spring Grove, near Nelson, New Zealand, 30 August 1871: d. Cambridge, England, 19 October 1937)

physics, chemistry.

Both of Rutherford’s parents were taken as youngsters to New Zealand in the mid-nineteenth century. His father, James, from Perth, Scotland, acquired the skills of his wheelwright father and brought this technological inclination to his work: flax farming and processing, railroad-tie cutting, bridge construction, and small-scale farming. Although he was moderately successful in this range of endeavors, his family of a dozen children necessarily learned hard work and thrift. Rutherford’s mother, Martha Thompson, accompanied her widowed mother to New Zealand from Hornchurch, Essex, England, and a few years later took over her mother’s teaching post when she remarried. One need look no further than his parents for the source of Rutherford’s characteristic traits of simplicity, directness, economy, energy, enthusiasm, and reverence for education.

Success in the local schools brought Rutherford a scholarship to Nelson College, a nearby secondary school. Until this time he had tinkered with clocks, made models of the waterwheels his father used in his mills, and at the age of ten had a copy of Balfour Stewart’s science textbook; but he had not exhibited intellectual precocity or a predilection for a scientific career. At Nelson he excelled in nearly every subject, particularly mathematics, in which he was given a solid grounding by W. S. Littlejohn. Another scholarship took Rutherford in 1889 to Canterbury College, Christchurch, where he came under the influence of A. W. Bickerton, a man of contagious scientific enthusiasm whose cosmological theories were never taken seriously, and C. H. H. Cook, a rigorous and orthodox mathematician. At the conclusion of the three-year course Rutherford received his B.A. and a mathematical scholarship that enabled him to remain for another year. For his postgraduate work he obtained the M.A. in 1893, with double first-class honors in mathematics and mathematical physics and in physical science.

By this time Rutherford’s special talents must have been apparent, for he was encouraged to stay at Canterbury for yet another year, during which he began research on the magnetization of iron by high-frequency discharges, work that earned him the B.Sc. in 1894. His activities until mid-1895 are not known for certain: but he seems to have continued this line of research under Bickerton, taught briefly at a boys’ school, and fallen in love with his future wife, Mary Newton, the daughter of the woman in whose house he lodged.

In this first research Rutherford examined the magnetization of iron by a rapidly alternating electric current, such as the oscillatory discharge of a Leyden jar, and showed it to occur even with frequencies of over 10⁸ cycles per second. Heinrich Hertz, less than a decade before, had caused a sensation by detecting the radio waves predicted by Maxwell’s electromagnetic theory; and Rutherford, always interested in the latest scientific advances, probably was drawn to his own investigation involving high frequencies by Hertz’s work. More important than Rutherford’s initial observation—Joseph Henry had discovered the effect half a century earlier—was his finding that the alternating field diminished the magnetization of a needle that was already magnetized. This discovery enabled him to devise a detector of wireless signals before Marconi began his experiments, and during the next year or two Rutherford endeavored to increase the range and sensitivity of his device.

In 1895 Rutherford was awarded a scholarship established with the profits from the famous 1851 Exhibition in London. The terms of this award required attendance at another institution, and Rutherford chose Cambridge University’s Cavendish Laboratory, of which the director, J. J. Thomson, was the leading authority on electromagnetic phenomena . The university had just altered its rules to admit graduates of other schools, thereby enabling Rutherford to become the laboratory’s first research student. He brought with him to England his wireless wave detector and soon was able to receive signals from sources the discovery of X raysup to half a mile away. This work so impressed a number of Cambridge dons, J. J. Thomson included, that Rutherford quickly made a name for himself. Upon the discovery of X rays, Thomson asked Rutherford in early 1896 to join him in studying the effect of this radiation upon the discharge of electricity in gases. Although he might have hesitated, for Rutherford was anxious to earn enough to marry his fiancée in New Zealand and saw a limited use for his detector in lighthouse or lighthouse-to-shore communication, he could not refuse the honor of Thomson’s offer or the opportunity to investigate the most recently discovered physical phenomenon.

Out of this collaboration came a joint paper famous for its statement of a theory of ionization. The idea—that the X rays created an equal number of positive and negative carriers of electricity, or “ions,” s in the gas molecules—presumably was Thomson’s, while much of the experimentation that placed this formerly descriptive subject on a quantitative basis was Rutherford’s. The latter continued this work through 1897, measuring ionic velocities, rates of recombination, absorption of energy by gases, and the electrification of different gases while Thomson independently determined the existence of the particle later called the electron. Rutherford logically next examined the discharge of electricity by ultraviolet light, then conducted a similar study of the effects of uranium radiation. Again his inclination to pursue the most recent—and significant—problems led to a more detailed study of radioactivity. This was his field of endeavor for the next forty years; his work and that of his students was to make this the most significant area of physical science as radioactivity evolved into atomic physics and then into nuclear physics.

Radioactivity had been chanced upon in 1896 by Henri Becquerel, had enjoyed a brief period of moderate attention, and had then been abandoned even by its discoverer because it seemed relatively uninteresting among the numerous radiations being studied at the end of the nineteenth century. In early 1898 interest was somewhat revived when G. C. Schmidt and Marie Curie independently showed that not only uranium, but also thorium, exhibited this property. When Pierre and Marie Curie, with Gustave Bémont, announced later in 1898 the discovery of two new radioactive elements, polonium and radium, world scientific attention finally crystallized. Rutherford did not jump on this bandwagon, for his investigations had begun earlier, even before the discovery of thorium’s activity. It is likely, in fact, that his own work alone would have served the same purpose as radium in creating the science of radioactivity, if somewhat more slowly; for within a short time Rutherford, not Becquerel or the Curies, was the dominant figure in the field.

He began by examining the Becquerel rays from uranium. Indeed, until about 1904 the emissions received far more attention than the emitters. Passage of the radiation through foils revealed one type that was easily absorbed and another with greater penetrating ability; these Rutherford named alpha and beta, “for simplicity.” While this work was in progress, Rutherford was seriously considering his future prospects. A lectureship or even better, a fellowship at Trinity College would allow him to marry. But either a Trinity regulation about length of residence or, as he felt, the prevailing Cambridge snobbery toward those who had been undergraduates elsewhere, especially in the colonies, prevented the offer of a fellowship. With little hope of success, for older men with far greater teaching experience had also applied, Rutherford entered the competition for the professorship of physics at McGill University. The Montreal authorities, however, were looking for someone to direct work in their well-equipped laboratory and were convinced by Thomson’s testimonial: “I have never had a student with more enthusiasm or ability for original research than Mr. Rutherford.”

Arriving at McGill in September 1898, Rutherford found a warm welcome: perhaps the best laboratory in the western hemisphere (it was financed by a tobacco millionaire who considered smoking a disgusting habit); widespread skepticism that he would measure up in research ability to his predecessor, H. L. Callendar: and a department chairman, John Cox, who soon voluntarily assumed some of Rutherford’s teaching duties when he recognized his colleague’s work in radioactivity had been solely with uranium minerals; in Montreal his first inclination was to examine thorium substances, since the activity of this element had been noticed only half a year earlier. When a colleague obtained erratic ionization measurements. Rutherford succeeded in tracing the irregularity to a gaseous radioactive product escaping from the thorium; and because he was uncertain of the nature of this product, in 1900 he gave it the deliberately vague name “emanation.” Within a short time the emanations from radium and actinium also were found, by Ernst Dorn and F. Giesel, respectively.

The number of known radioelements was increasing. Rutherford added several more to the list, the next being thorium active deposit, which in time was resolved into thorium A, B, C, and so on. The active deposit, or excited activity, which was laid down on surfaces touched by the decaying emanation, was found by Rutherford because of the apparent breakdown of good insulators and was described in Philosophical Magazine just one month after his announcement of the emanation. A curious feature he immediately noticed was that, unlike uranium, thorium, and radium, such materials as thorium emanation, radium emanation, their active deposits, and polonium lost their activities over periods of time. Moreover, the rate of this decrease was unique for each radioelement and thus an ideal identifying label. This meant that an exponential curve could be plotted for the half-life of each radioelement with a discernible decay period, and theory could thereby be compared with experiment.

Sir William Crookes, among others, doubted that uranium and thorium were intrinsically active; he believed, rather, that the active materials were only entrained with the atoms of these long-known elements. In 1900 he succeeded, through repeated dissolution and recrystallization of uranium nitrate, in preparing uranium that left no image on a photographic plate and in isolating the active constituent, called uranium X. But the confidence thereby generated in the stability of uranium was shaken little more than a year later, when Becquerel reexamined his materials, prepared by Crookes’s method, and found that his uranium X was inactive, while his uranium had regained its activity. By this time Rutherford had recognized the need for skilled chemical assistance in his radioactivity investigations and had secured the services of a young chemistry demonstrator at McGill, Frederick Soddy. Together they removed most of the activity from a thorium compound, calling the active matter thorium X; but they too found that the X product lost its activity and that the thorium recovered its original level in a few weeks. Had Becquerel’s similar finding for uranium not been immediately at hand, they might have searched for errors in their work. In early 1902, however, they began to plot the activities as a function of time, seeing evidence of a fundamental relationship in the equality of the time for thorium X to decay to half value and thorium to double in activity.

This work led directly to Rutherford’s greatest achievement at McGill, for with Soddy he advanced the still-accepted explanation of radioactivity. Becquerel for several years had considered the phenomenon a form of long-lived phosphorescence, although by the first years of the twentieth century he spoke vaguely of a “molecular transformation.” Crookes, in the British tradition of visualized mechanical models, had suggested a modified Maxwell demon sitting on each uranium atom and extracting the excess energy from faster-moving air molecules, this energy then appearing as uranium radiation. The Curies had considered several possibilities but inclined strongly toward the concept of an unknown ethereal radiation the existence of which is manifested only through its action on the heaviest elements, which then emit alpha, beta, and gamma rays as secondary radiations. Perhaps the most prescient idea was offered by Elster and Geitel—that the energy exhibited by radioactive substances comes not from external sources but from within the atoms themselves—but it was left to Rutherford and Soddy to add quantitative evidence to such speculation.

Their iconoclastic theory, variously called transformation, transmutation, and disintegration, first appeared in 1902 and was refined in the following year. Although alchemy had long been exorcised from scientific chemistry, they declared that “radioactivity is at once an atomic phenomenon and the accompaniment of a chemical change in which new kinds of matter are produced.” The radioactive atoms decay, they argued, each decay signifying the transmutation of a parent into a daughter element, and each type of atom undergoing its transformation in a characteristic period. This insight set the course for their next several years of research, for the task was then to order all the known radioelements into decay series and to search for additional members of these families.

The theory also explained the experimental decay and rise curves as a measure of a radioelement’s quantity and half-life. At equilibrium the same number of atoms of a parent transform as the number of atoms of its daughter and its granddaughter, and so on until a stable end product is obtained. But when a chemical process separates members of a series, the parent must regain its former activity as it produces additional daughters while its own numbers are maintained constant, unless it is the very first member of the family—whose numbers can only decrease. The daughter side of a chemical separation, however, is destined only to decay, for there is no means of replenishing its stock of transformed atoms.

Rutherford and Soddy saw that the apparently constant activities of uranium, thorium, and radium were due to half-lives that are long compared with human lives. This understanding overcame the puzzle at the core of all previous theories; for if the total radioactivity in the universe was growing smaller and tending to disappear, the law of conservation of energy would not be violated. They considered radioactivity a fundamental property of nature, fit to join the select group of electricity, magnetism, light, and gravity. Not the least remarkable thing about this theory which proclaimed that the atom was not indestructible was the uncontroversial way in which it was accepted. Aside from the elderly and unalterable Lord Kelvin and the constantly contentious Henry Armstrong, the transformation theory encountered little opposition, Chemists, especially, although it violated views about the unchangeability of atoms that they “absorbed with their mothers’ milk,” could not refute the evidence and at most could adopt a wait-and-see attitude.

To a large degree Rutherford spent the next years mining this rich vein of interpretation. Working with Soddy and using the new liquid-air machine given to McGill by its wealthy benefactor, he condensed emanation at low temperatures, proving that it is a gas. Other tests convinced them that emanation belonged to the family of inert gases found not long before by Sir William Ramsay. Soddy then left Montreal in 1903 for London, where he and Ramsay proved spectroscopically that helium is produced during transformations from radium emanation. Such work was highly important, for there were numerous radioelements of which the chemical identity and place in the decay series were uncertain.

Helium, while not a radioelement, was of particular interest because of Rutherford’s certainty that, as a positive ion, it was identical with the alpha particle. And the alpha particle, being of ponderable mass, he saw as the key in the change from an element of one atomic weight to an element of another. It fascinated Rutherford also because he could appreciate the enormous speed and energy with which it is ejected from a decaying atom. In 1903 he was able to deflect it in electric and magnetic fields, thereby showing its positive charge, but his charge-to-mass ratio measurement lacked the precision required to distinguish between a helium atom with two charges and a hydrogen atom with one charge. The proof of the particle’s identity awaited Rutherford’s transfer to Manchester, although he determined many useful facts about the alpha particle, such as the number emitted per second from one gram of radium, a constant that is the basis for several other important quantities, including the half-life of radium, and in 1906 made another assault upon the e/m ratio.

Halfway between Soddy’s departure in 1903 and Otto Hahn’s arrival at McGill in 1905, Rutherford found another chemist of comparable skill upon whom he could rely. This was Bertram Boltwood, who had proved circumstantially that uranium and radium are related, thus linking two previously separate decay series, and who in 1907 discovered ionium, the immediate parent of radium, which went far in proving the uranium-radium connection directly. Since Boltwood remained in New Haven, Connecticut, his collaboration with Rutherford was conducted through the mails. This work extended from determination of the quantity of radium present per gram of uranium in minerals to Rutherford’s suggestion that, if quantity and rate of formation of a series’ end product were known, it would be possible to calculate the age of the mineral. R. J. Strutt in England followed up this idea, using the helium found in radioactive substances; but the variable amount of this gas that escaped permitted only minimum age determinations. Bolt-wood showed the universal occurrence of lead with uranium minerals; considered this the series’ final product; and, using Rutherford’s value for radium’s half-life and their figure for the amount of radium in a gram of uranium, was able to calculate the rate of formation for lead. The ages of some of his rock samples were over a billion years, furnishing for the first time quantitative proof of the antiquity of the earth.

Many other problems in radioactivity were pursued by Rutherford, sometimes alone, sometimes with one of the research students in the strong school he established. Among the projects in his laboratory were measurements of radiation energy, studies of beta- and gamma-ray properties, attempts to change rates of decay by extreme conditions of temperature, efforts to place actinium in a decay series, and investigations of the radioactivity of the earth and atmosphere. Few advances in this science throughout the world failed to be reflected in the work at McGill. Nor, single-minded though he was, did Rutherford entirely abandon other areas of science: radio, the conduction of electricity in gases, and N rays received some attention.

Rutherford’s nine years at McGill, filled with the great work that brought him the Nobel Prize for chemistry in 1908, were no less replete with other professional activities. He was in great demand as a speaker and traveled frequently to distant parts of the United States and to England, to give a lecture, a series of talks, or a summer-session course. While he could not be expected to refuse the honor of speaking at the Royal Institution, the Bakerian lecture to the Royal Society (1904), or the Silliman lectures at Yale University (1905), some well-wishers urged him to limit his outside engagements. His time was also consumed in writing Radio-Activity, the first textbook on the subject and recognized as a classic at its publication in 1904. So fast did the science progress, however, that Rutherford prepared a second edition the following year that was 50 percent larger. No sooner was this done than he faced the task of fashioning the Silliman lectures into a book. Small wonder that he confined his writing to journals for the next several years.

A veritable fallout of honors began to descend upon him, continuing for the rest of his life. Rutherford thoroughly enjoyed this recognition, for, while not vain, he was fully aware of his own worth. The Royal Society offered him fellowship in 1903 and the Rumford Medal the next year, while various universities presented both honorary degrees and job offers. While he was happy at McGill, Rutherford desired to return to England, where he would be closer to the world’s leading scientific centers. Thus when Arthur Schuster offered to resign from his chair at the University of Manchester on the condition that Rutherford succeed him, the post and the laboratory were sufficiently attractive for Rutherford to make the move in 1907.

If the Cavendish, under Thomson, was the premier physics laboratory in England, Manchester, under Rutherford, was easily the second, Schuster had built a fine structure less than a decade earlier and bequeathed to his successor a strong research department, his assistant, Hans Geiger, and a personally endowed readership in mathematical physics, filled in turn by Harry Bateman, C. G. Darwin, and Niels Bohr. Rutherford’s great and growing fame attracted to Manchester (and later to Cambridge) an extraordinarily talented group of research students who made profound contributions to physics and chemistry.

On his return to England, Rutherford had only a few milligrams of radioactive materials, a quantity insufficient for even his own research. In a generous gesture the Austrian Academy of Sciences sent, from the Joachimsthal uranium mines under its control, about 350 milligrams of radium chloride, as a joint loan to Rutherford and Ramsay. Unfortunately, Ramsay wished to retain possession indefinitely, while both saw the wisdom of leaving the supply undivided: so until the Vienna authorities sent another comparable radium supply for Rutherford’s exclusive use, he was limited to work with the “draw” of emanation that Ramsay sent periodically from London. To a degree this determined most of Rutherford’s initial investigations at Manchester, an extensive study of radium emanation: but he always found emanation and its active deposit decay products more convenient sources than radium itself.

The emanation could easily be purified in liquid air, and Rutherford soon determined the volume of this gas in equilibrium with one gram of radium. This corrected earlier results by Ramsay and A. T. Cameron, and, by confirming his calculated amount, removed some doubt cast on the accuracy of radioactive data and theory. With the spectros-copist Thomas Royds, Rutherford next photographed the spectrum of emanation, not examined since Ramsay and Collie’s visual observations in 1904. Such work involved him in scientific controversy, which he usually sought to avoid; but after Soddy left Ramsay’s laboratory, the latter’s contributions to radioactivity were noted for their almost uniform incorrectness. Although an expert at handling minute quantities of rare gases, Ramsay never took the trouble to learn well the techniques of radioactivity. His imprecise work, coupled with a strong desire to gain priority, led him to publish quickly numerous results that Rutherford and others in this field felt compelled to correct. Further contributions to emanation studies included Rutherford’s examination in 1909 of its vapor pressure at different low temperatures, and, with Harold Robinson in 1913, measurement of its heating effect.

Never one to limit the scope of his investigations—he preferred to advance across radioactivity in a wide path—Rutherford pursued “his” alpha particles in 1908. These were his favorites: the beta particles were too small and, being electrons, too common. The alphas, however, were massive, of atomic dimension: and he could clearly visualize them hurtling out of their parent atoms with enormous speed and energy. Certainly these would be the key to the physicist’s classic goal: an understanding of the nature of matter. Until that time nothing had changed Rutherford’s early conviction that the alpha particle was a doubly charged helium atom, but he had not succeeded in proving that belief. In 1908 he and Geiger were able to fire alpha particles into an evacuated tube containing a central, charged wire and to record single events. Ionization by collision, a process studied by Rutherford’s former colleague at Cambridge, J. S. E. Townsend, caused a magnification of the single particle’s charge sufficient to give the electrometer a measurable “kick.” By this means they were able to count, for the first time accurately and directly, the number of alpha particles emitted per second from a gram of radium.

This experiment enabled Rutherford and Geiger to confirm that every alpha particle causes a faint but discrete flash when it strikes a luminescent zinc sulfide screen, and thus led directly to the widespread method of scintillation counting. It was also the origin of the electrical and electronic methods of particle counting in which Geiger later pioneered. But at this time the scintillation technique, now proved reliable, was more convenient. This counting work also led Rutherford and Geiger to the most accurate value of the fundamental electric charge e before Millikan performed his oil-drop experiment. They measured the total charge from a radium source and divided it by the number of alphas counted to obtain the charge per particle. Since this figure was about twice the previous values of e. they concluded that the alpha was indeed helium with a double charge. But Rutherford still desired decisive, direct proof; and here his skilled glassblower came to his aid. Otto Baumbach in 1908 was able to construct glass tubes thin enough to be transparent to the rapidly moving alpha particles yet capable of containing a gas. Such a tube was filled with emanation and was placed within a larger tube made of thicker glass. In time, alpha particles from the decaying emanation penetrated into and were trapped in the space between inner and outer tubes: and when Royds sparked the material in this space, they saw the spectrum of helium.

As in Montreal, Rutherford found chemical help in Manchester of the highest quality. Boltwood spent a year with him, during which time they redetermined more accurately the rate of production of helium by radium. By combining these results with those from the counting experiments mentioned above, they obtained Avogadro’s number more directly than ever before. There were new researchers too—Alexander Russell, Kasimir Fajans, and Georg von Hevesy—fitting radioelements into the periodic table, generating information and ideas on which displacement laws and concept of isotopy would be based, and working on branching of the decay series, periods of the short-lived elements, and other radiochemical problems.

Rutherford’s greatest discovery at Manchester—in fact, of his career—was of the nuclear structure of the atom. In retrospect, its origins can be seen in the slight evidences of alpha particle scattering in thin metal foils or sheets of mica, which he noticed while at McGill, and in similar scattering by air molecules in his later electrical counting experiments with Geiger. With a view to learning more about this scattering, both because it introduced experimental difficulties leading to less precise results and because it bore upon the perplexing question of the nature of alpha and beta absorption in matter, Geiger made a quantitative study of the phenomenon. Counting the scintillations produced by scattered alphas, he found that they increased with the atomic weight of the target foil and, until the particles could no longer penetrate the foil, with its thickness. Only very small angular deflections from the beam were measured and, as expected, fewer particles were bent through the larger angles.

In 1909 Rutherford and Geiger decided that Ernest Marsden, who had not yet taken his bachelor’s degree, was ready for a real research problem, Much has been said of Rutherford’s great insight in suggesting that Marsden look for large-angle alpha particle scattering; but, inspired though it was, it came logically from knowledge of the “diffuse” scattering still interfering with Geiger’s measurement of small-angle scattering. On the other hand, Rutherford was aware that the alpha particle, being very fast and massive, was not likely to be scattered backward by the accumulated effect of a number of small deflections. His urging the experiment upon Marsden may well have been an example of his characteristic willingness to try “any damn fool experiment” on the chance that it might work. This one worked magnificently. Geiger then joined Marsden, and the two measured the exceedingly small number of particles that were deflected not only through ninety degrees, but more. Ruther-ford’s reaction on learning of this—rather embellished over the years—has become a classic: “It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you.”

Rutherford pondered long over the implications of this experiment, for it was early 1911 before he announced that he knew what the atom looked like. The small deflections investigated by Geiger could be reasonably explained by the theory of multiple scattering then current. This was based on the “plum pudding” model of the atom—a sphere of positive electrification in which electrons (plums) were regularly positioned—proposed by Lord Kelvin and highly refined by Thomson. The alpha particle was believed to suffer numerous collisions with the atoms of the target foil, each collision resulting in a small deflection; and a probability distribution for each angle could be calculated to compare with experiment. But for large angles the comparison failed; multiple scattering theory predicted virtually no deflections, while Geiger and Marsden found a measurable few.

Thomson’s multiple-scattering theory, moreover, was challenged regarding beta particle encounters, its area of special competence: John Madsen, in Australia, obtained data on beta deflections that suggested that this type of scattering was done in a single collision. Other experiments, conducted at Manchester by William Wilson, showed that beta particles suffered inelastic collisions in their passage through matter; like the alpha particles, therefore, they gradually lost their energy, and it was possible to think that both particles experienced the same type of encounters.

By the end of 1910, Rutherford began tying these several factors into a new atomic model and theory of scattering. The alpha projectile, he said, changed course in a single encounter with a target atom. But for this to occur, the forces of electrical repulsion (or attraction—it made no difference for the mathematics) had to be concentrated in a region of 10^-13 centimeters, whereas the atom was known to measure 10^-8 centimeters. This meant that the atom consisted largely of empty space, with a very tiny and very dense charged nucleus at the center and opposite charges somehow placed in the surrounding void. Rutherford next calculated the probability of such single scattering at a given angle and found his predictions confirmed experimentally by Geiger and Marsden. The scientific community, however, was not impressed; this novel theory of the atom was not opposed, but largely ignored.

There were some, though, whose scientific orientation made them more likely than Rutherford to see the implications of a nuclear atom. One was Niels Bohr, who first met Rutherford in 1911 and later spent extended periods at Manchester. To Bohr it was apparent that radioactivity must be a phenomenon of the nucleus, while an element’s chemical and physical properties were influenced by the electrons about this core. He brilliantly fitted chemical, radioactive, and spectroscopic data into the nuclear atom. His success in 1913 in applying quantum considerations to the orbital electron of hydrogen, thereby explaining its optical spectrum, eventually drew deserved attention to Rutherford’s model of the atom. Bohr also treated heavier elements, and his attention to their electron arrangements brought the nuclear atom into chemistry.

H. G. J. Moseley was another of Rutherford’s students whose work showed the fertility of the nuclear concept. In 1913, immediately after Max von Laue proved the wave nature of X rays and Rutherford’s good friend at Leeds, W. H. Bragg, and his son, W. L. Bragg, who succeeded Rutherford at both Manchester and Cambridge, showed how to measure X-ray wavelengths by reflecting them from crystals, Moseley determined the wavelength of a particular line in the X-ray spectra of a large number of elements. When he organized his data according to each element’s place in the periodic table, the wavelength (or frequency) of each line varied in regular steps. Only one thing, Moseley said, could change by such a constant amount: the positive charge on the atom’s nucleus. Previously the organization of elements by atomic weights into the periodic table was seen by some as nothing more than fortuitous and by others as signifying a profound law of nature. It was Moseley’s contribution to show that the profundity lay in the ordering of elements not by their weights but by their atomic numbers or nuclear charges, for it was precisely these charges that determined the number of orbital electrons and, hence, the chemical nature of the atom.

More was yet to come from Rutherford’s school concerning the nucleus. Fajans and Soddy, both “alumni,” and Russell, still at Manchester, each proposed a scheme to place the numerous radioelements into the periodic table. Russell’s suggestion, a few months before Moseley’s work mentioned above, and the more accurate versions that followed from the other two, stated simply that the daughter of an alpha-emitting element was two places to the left of the parent in the table, while the daughter of a beta-emitter was one place to the right. Moseley’s work, showing that each place in the periodic table corresponds to a change of one nuclear charge, allowed further insight to these displacement laws, for the alpha particle bore a charge of +2 and the beta particle a charge of ?1. That an alpha decay followed by two beta decays would lead back to the same place in the periodic table but, with a loss of about four atomic weight units, soon indicated the concept of isotopy.

Other important work was accomplished at Manchester, such as the Geiger-Nuttall rule connecting the range of an alpha particle with the average lifetime of the parent atom, beta- and gamma-ray spectroscopy, and the measurement by Rutherford and E. N. da C. Andrade in 1914 of gamma-ray wavelengths by the crystal technique. In all of these investigations Rutherford either played a direct part or kept closely abreast of developments during his daily rounds of the laboratory. At Manchester these rounds were possible, for Rutherford was largely spared time-consuming administrative duties and other chores. But he was increasingly busy, and service on the Council of the Royal Society, the presidency of Section A of the British Association for the Advancement of Science in 1909, attendance at several overseas conferences, and his numerous lectures took him more and more away from the Midlands.

The outbreak of World War I caught Rutherford in Australia at a meeting of the British Association for the Advancement of Science. On his return to England, he found his laboratory virtually empty, for before governments found scientists useful in wartime and before conscription was introduced, many young scientists felt it was their duty to enlist for action. Rutherford himself was called upon to serve as a civilian member of the Admiralty’s Board of Invention and Research committee dealing with submarine problems.

As the war progressed and hydrophone research became centralized at a naval base, Rutherford found time to return to his more customary line of investigation. A few years before, Marsden had noticed scintillations on a screen placed far beyond the range of alpha particles when these particles were allowed to bombard hydrogen. Rutherford repeated the experiment and showed that the scintillations were caused by hydrogen nuclei or protons. This was easily understood, but when he substituted nitrogen for the hydrogen, he saw the same proton flashes. The explanation he gave in 1919 stands beside the transformation theory of radioactivity and the nuclear atom as one of Rutherford’s most important discoveries. This, he said, was a case of artificial disintegration of an element. Unstable, or radioactive, atoms disintegrated spontaneously; but here a stable nucleus was disrupted by the alpha particle, and a proton was one of the pieces broken off.

This line of work was to be the major theme for the remainder of Rutherford’s career, which he spent at Cambridge from 1919. In that year Thomson was appointed master of Trinity College and decided to resign as director of the Cavendish Laboratory. The postwar period saw great activity in the game of professorial “musical chairs,” but to no one’s surprise Rutherford was elected as Thomson’s successor. With him came James Chadwick, a former research student at Manchester who had spent the war years interned in Germany; he was to become Rutherford’s closest collaborator. During the 1920’s they determined that a number of light elements could be disintegrated by bombardment with swift alpha particles; as a corollary, they measured the distance of closest approach between projectile and target to ascertain both the size of the nucleus and that the inverse-square force law applied at this small distance.

There was no doubt that the alpha particle caused such elements as nitrogen, boron, fluorine, sodium, aluminum, and phosphorus to disintegrate. But did the alpha merely bounce off the target nucleus, which then emitted a proton, or did it combine with this nucleus? While these two reactions would form different elements, the number of atoms was too small for chemical tests. C. T. R. Wilson, who still worked independently in the Cavendish Laboratory, had perfected a cloud chamber before the war that was an ideal instrument to resolve this problem. If the alpha bounced off its target, there would be three tracks diverging from the collision point; the alpha, the proton, and the recoil nucleus. If, on the other hand, the alpha and the target formed a compound nucleus, there would be only two trails: the proton and the compound nucleus. From the photographs of some 400,000 alpha-particle tracks., P. M. S. Blackett in 1925 showed that it was the latter process which occurred, for he found eight doubly branched tracks from nuclear collisions. Later work at the Cavendish by Blackett and G. P. S. Occhialini, with cosmic rays triggering coincidence counters and thus photographing themselves in a cloud chamber, confirmed the discovery of the positron, made shortly before by Carl D. Anderson in California.

With its charge of +2, the alpha particle was too strongly repelled by the large numbers of positive charges on the nuclei of the heavier elements to cause disintegrations. To overcome this potential barrier, Rutherford recognized that projectiles might be accelerated but disregarded the alpha and proton because electrical engineers in the 1920’s could not furnish the voltage required even to match the energy of alphas from natural radioactive sources. Instead, he inclined toward the idea of electron acceleration, with the thought that this projectile, once past the orbital electrons of similar charge, would be attracted to the nucleus. In the last few years of the 1920’s T. E. Allibone, one of a growing number at the Cavendish with engineering training—a new trend in physics—attempted disintegrations with accelerated electrons, but without success. George Gamow, on one of his several visits to Cambridge, then pointed out that the new wave mechanics predicted that a small number of particles of relatively low energy could tunnel through the potential barrier around a nucleus instead of climbing over it. This put the matter of effecting disintegrations by accelerated particles of positive charge (and with more mass than the electron) back into the range of laboratory possibilities.

John Cockcroft and E. T. S. Walton built an apparatus capable of accelerating protons through several hundred thousand volts, with which, in 1932, they succeeded in bombarding lithium and producing alpha particles. By measuring the energy of these products, they further offered experimental proof of Einstein’s famous relationship E =mc². The mass values were furnished with great accuracy by another long-time Cavendish member, F. W. Aston, who, like C. T. R. Wilson, worked to a large degree independently. In 1919 Rutherford had produced artificial disintegrations by natural means, that is, by alphas from naturally decaying radioactive materials. The Cockcroft-Walton work of 1932, artificial disintegrations by artificial means—that is, by accelerated particles—made Rutherford a believer in the quantum mechanics on which it was based. Although he was not generally interested in highly mathematical physical theories, especially ones difficult to visualize, these new ideas of the late 1920’s worked—and that was Rutherford’s criterion.

An instance of his willingness to use, if not fully understand, quantum mechanics was Rutherford’s construction with Marcus Oliphant of a special discharge tube that generated a far more copious supply of protons than the Cockcroft-Walton apparatus and at lower voltages. His faith in the ability of these protons to tunnel through the potential barrier at these lower energies was rewarded with a number of disintegrations. But the heavier elements still resisted such bombardments; and it was clear that, for them, projectiles of greater energy were required. Ernest Lawrence, in California, had built the cyclotron a few years before and generously shared his plans with Rutherford. A new high-voltage laboratory was also planned for the Cavendish, to house a two-million-volt, commercially built apparatus; but neither of these heralds of the new age of “big science and big money” was in significant use by the time of Rutherford’s death in 1937.

There were other important activities at the Cavendish, some with direct connections to Rutherford’s main interest in disintegrations and others with no connection at all. Among the latter were the work of G. I. Taylor on problems in classical physics, E. V. Appleton on radio waves, and Peter Kapitza on phenomena in intense magnetic fields and at low temperatures. More in the mainstream of the laboratory’s orientation was the long series of investigations by C. D. Ellis on beta- and gamma-ray spectra. Since these rays, as well as the alpha, come from the decaying nucleus, they offered a view of the energy levels in this nucleus and, hence, an insight to nuclear structure.

Not long after the discovery of heavy water in the United States, Rutherford obtained a small quantity of this precious fluid and in 1934, with Oliphant and Paul Harteck, bombarded deuterium with deuterons. This reaction was notable for the first achievement of what is now called fusion, as well as for the production of tritium. Another major advance at the Cavendish, the significance of which is not sufficiently appreciated, was the application by C. E. Wynn-Williams of Heinrich Greinacher’s ideas for electronic amplification of ionization. With the Geiger-Müller tube, which was based on a different principle, and especially with Wynn-Williams’ tubes and associated electronics, research workers in the laboratory were able to count particles at much higher rates than with scintillations and with other benefits.

Rutherford recognized the value to his experimentalists of contact with theoretical physicists and encouraged their presence in the laboratory. Some, such as Gamow, came as visitors for a period. The Cambridge theoreticians, however, were by administrative fiat in the mathematics department and were somewhat isolated from the Cavendish. A notable exception was Ralph Fowler, Rutherford’s son-in-law, whose advice was eagerly sought during the nearly two decades that Rutherford directed the laboratory. Along with the Cockcroft-Walton experiment, the most important discovery during this period by one of Rutherford’s colleagues was Chadwick’s proof, in 1932, of the existence of the neutron. Rutherford had long considered the neutron a possibility and in his 1920 Bakerian lecture to the Royal Society had predicted its likely properties. Chadwick made several attempts to detect the neutral particle, but none was successful until he learned of experiments by the Joliot-Curies in Paris, in which, they said, extremely penetrating gamma rays were emitted. As he suspected, Chadwick found the rays were not gammas but neutrons: and not long afterward Norman Feather, also at the Cavendish, showed that neutrons were capable of causing nuclear disintegrations.

Even by the beginning of Rutherford’s second period at Cambridge, he was a public figure. Increasingly beset with outside calls upon his time, he had less and less opportunity for his own research and for keeping abreast of his students’ work. Yet, with the tradition of enthusiasm for research that he had established earlier, his still frequent rounds to “ginger up” his “boys”, and Chadwick’s invaluable assistance, the laboratory’s output remained far more than respectable.

From 1921, when he succeeded Thomson, until his death, Rutherford was professor of natural philosophy at the Royal Institution in London, a post that entailed several lectures each year. There were numerous other public lectures to which great honor was attached, such as his presidential address to the British Association for the Advancement of Science in 1923. Between 1925 and 1930 he was president of the Royal Society, and following this he became chairman of the advisory council to the British government’s Department of Scientific and Industrial Research. Both posts involved many public appearances, such as opening conferences, and new laboratories, in addition to administrative and policy-making chores. Although liberal-minded, Rutherford customarily side-stepped political issues. Yet he felt he could not remain idle when Nazi Germany expelled hundreds of Jewish scholars; and from 1933 he was president of the Academic Assistance Council which sought to obtain positions and financial aid for these refugees.

In work that may be characterized as radioactivity at McGill, atomic physics at Manchester, and nuclear physics at Cambridge, Rutherford, more than any other, formed the views now held concerning the nature of matter. It is to be expected that numerous honors would come to such a man, called the greatest experimental physicist of his day and often compared with Faraday. In 1922 he received the Copley Medal, the highest award given by the Royal Society. Dozens of universities and scientific societies awarded him honorary degrees and memberships, For the fame Rutherford brought to the British Empire—not for his relatively minor services to his government—he was made a knight in 1914 and a peer (Baron Rutherford of Nelson) in 1931. King George V personally honored him in 1925 by conferring on him the Order of Merit, which is limited to a handful of the most distinguished living Englishmen.

BIBLIOGRAPHY

I. Original Works. Approximately two dozen boxes of Rutherford’s correspondence and miscellaneous papers are preserved in the Cambridge University Library. Most of his published papers have been reprinted, under the scientific direction of Sir James Chadwick, in Collected Papers of Lord Rutherford of Nelson, 3 vols. (London, 1962–1965). Rutherford’s books are Radio-Activity (Cambridge, 1904; 2nd ed., 1905); Radioactive Transformations (London, 1906); Radioactive Substances and Their Radiations (Cambridge, 1913); Radiations From Radioactive Substances (Cambridge, 1930), written with J. Chadwick and C. D. Ellis; and The Newer Alchemy (Cambridge, 1937). A portion of his correspondence is reproduced in Lawrence Badash, ed., Rutherford and Boltwood, Letters on Radioactivity (New Haven, 1969). Badash has also compiled the Rutherford Correspondence catalog (New York, 1974).

II. Secondary Literature. There are three biographies written by Rutherford’s former students: A. S. Eve, Rutherford (Cambridge, 1939); Norman Feather, Lord Rutherford (London, 1940); and E. N. da C. Andrade, Rutherford and the Nature of the Atom (London, 1964). Other biographies and partial biographies include Ivor Evans, Man of Power, the Life Story of Baron Rutherford of Nelson, O. M., F. R. S. (London, 1939); John Rowland, Ernest Rutherford, Atom Pioneer (London, 1955); Robin McKown, Giant of the Atom, Ernest Rutherford (New York, 1962); John Rowland, Ernest Rutherford, Master of the Atom (London, 1964); D. Danin, Rutherford (Moscow, 1966), in Russian: O. A. Staroselskaya-Nikitina, Ernest Rutherford, 1871–1937 (Moscow, 1967), in Russian; E. S. Shire,Rutherford and the Nuclear Atom (London, 1972); and, especially valuable for personal information, Mark Oliphant, Rutherford, Recollections of the Cambridge Days (Amsterdam, 1972).

Collections, of articles about Rutherford include Rutherford by Those Who Knew Him, which is the first five Rutherford lectures of the Physical Society, by H. R. Robinson, J. D. Cockcroft, M. L. Oliphant, E. Marsden, and A. S. Russell reprinted from Proceedings of the Physical Society, 1943–1951; a series of Rutherford memorial lectures, by J. D. Cockcroft, J. Chadwick, E. Marsden, C. Darwin, E. N. da C. Andrade, P. M. S. Blackett, T. E. Allibone, and G. P. Thomson, in Proceedings of the Royal Society, from 1953 on; J. B. Birks, ed.,Rutherford at Manchester (London, 1962); Albert Parry, ed.,Peter Kapitsa on Life and Science (New York, 1968); Notes and Records of the Royal Society27 (Aug. 1972), an issue devoted to Rutherford, with articles by M. L. Oliphant, H. Massey, N. Feather, P. M. S. Blackett, W. B. Lewis, N. Mott, P. P.O’Shea, and J. B. Adams: P. L. Kapitza, ed., Rutherford—Scholar and Teacher, On the Hundredth Anniversary of his Birth (Moscow, 1973), which is in Russian.

Among the wide range of articles written about Rutherford during his lifetime. obituary notices, recollections, and historical studies are the following: A. S. Eve, “Some Scientific Centres. VIII. The Macdonald Physics Building. McGill University, Montreal”, in Nature74 (1906), 272–275; J. A. Harker, “Some Scientific Centres, XI. The Physical Laboratories of Manchester University”, in Nature, 76 (1907), 640–642: “The Extension of the Physical and Electrotechnical Laboratories of the University of manchester”, in Nature89 (1912), 46; N. Bohr, “Sir Ernest Rutherford, O. M., P.R.S”., in Nature Supplement, 118 (1926), 51–52; O. Hahn and L. Meitner, “Lord Rutherford zum Sechzigsten Geburtstag”, in Die Naturwissenschaften, 19 (1931), 729: M. de Broglie, “Scientific Worthies; The Right Hon. Lord Rutherford of Nelson, O.M., F.R.S.,” in Nature, 129 (1932), 665–669; and J. G. Crowther, “Lord Rutherford, O. M., F. R. S”., in Great Contemporaries (London, 1935), pp. 359–370.

Obituary notices are in the Times (London) 20, 21, 22, and 26 Oct. 1937; New York Times, 20 Oct. 1937 and 21 Jan. 1938; Nature, 140 (1937), 717, 746–755, 1047–1054; and 141 (1938), 841–842.

See also A. S. Russell, “More About Lord Rutherford”, in The Listener, 18 (1937), 966; A. N. Shaw, “Rutherford at McGill”, in the McGill News (Winter 1937), no pagination; C. M. Focken, “Lord Rutherford of Nelson. a Tribute to New Zealand’s Greatest Scientist”, a 19-page brochure (privately printed in New Zealand, n.d., but ca. 1938); the obituary notices by R. A. Millikan, in Yearook of the American Philosophical Society for 1938, 386–388; F. R. Terroux, “The Rutherford Collection of Apparatus at McGill University”, in Transactions of the Royal Society of Canada, 32 (1938), 9–16; the obituary notice by E. F. Burton, in University of Toronto Quarterly, 7 (1938), 329–338; A. George, “Lord Rutherford ou I’Alchimiste”, in La Revue de France, (1938), 525–533; the obituary notice by G. Guében, in Revue des Questions Scientifiques, 113 (1938), 5–19; the obituary notice by A. S. Eve and J. Chadwick, in Obituary Notices of the Royal Society of London. 2 (1938), 395–423; H. Geiger, “Memories of Rutherford in Manchester”, in Nature, 141 (1938), 244; H. Geiger, “Das Lebenswerk von Lord Rutherford of Nelson”, in Die Naturwissenschaften, 26 (1938), 161–164; and obituary notices in Proceedings of the Physical Society, 50 (1938), 441–466.

Other articles are an obituary notice by E. Marsden, in Transactions and Proceedings of the Royal Society of New Zealand, 68 (1938), 4–16, to which is appended a partial bibliography compiled by C. M. Focken, pp. 17–25; “51 Years as Laboratory Steward”, an interview with W. Kay, in the Munchester Guardian, 27 Dec. 1945; H. Tizard, “The Rutherford Memorial Lecture”, in Journal of the Chemical Society (1946), 980–986; “Rutherford Commemoration, paris, 7 and 8 November 1947”, in Notes and Records of the Royal Society, 6 (1948), 67–68; H. Dale, “Some Personal Memories of Lord Rutherford of Nelson”, in Cawthrron Lecture Series, no. 25 (1950); P. M. S. Balckett, “Rutherford and After”, in the Manchester Guardian Weekly, 63 (14 Dec. 1950), 13; E. N. da C. Andrade, “The Birth of the Nuclear Atom”, in Scientific American, 195 (1956), 93–104; C. P. Snow, “The Age of Rutherford”, in Atlantic Monthly, 202 (Nov. 1958), 76–81; C. D. Ellis, “Rutherford; One Aspect of a Complex Character”, in Trinity Review (Lent 1960), 13–15; J. E. Geake, “Rutherford in Manchester”, in Contemporary Physics, 3 (1961), 155–158; “The jubilee of the Nuclear Atom”, in Endeavour. 21 (1962), 3–4; N. Feather. an essay-review of volume one of Rutherford’s collected papers, in Contemporary Physics, 4 (1962), 73–76; W. A. Kay, “Recollections of Rutherford. Being the Personal Reminiscences of Lord Rutherford’s Laboratory Assistant. Here Published for the First Time. Recorded and Annotated by Samuel Devons”, in The Natural Philosopher, 1 (1963), 127–155; W. E. Burcham, “Rutherford at Manchester, 1907–1919”, in Contemporary Physics, 5 (1964), 304–308; T. H. Osgood and H. S. Hirst, “Rutherford and 32 (1964). 681–686; P. L. Kapitza, “Recollections of Lord Rutherford”, in Proceedings of the Royal Society. A294 (1966). 123–137; M. L. Oliphant, “The Two Ernests” [Rutherford and lawrence], in Physics Today, 19 (Sept. 1966). 35–49. (Oct. 1966). 41–51; L. Badash, “How the ‘Newer Alchemy’ Was Received”, in Scientific American, 215 (1966), 88–95; L. Badash, “Rutherford Boltwood, and the Age of the Earth; The Origin of Radioactive Dating Techniques”, in proceedings of the American philosophical Society. 112 (1968), 157–169; J. L. Heilbron. “The Scattering of ? and ? Particles and Rutherford’s Atom”, in Archive for History of Exact Sciences, 4 (1968), 247–307; L. Badash, “The Importance of Being Ernest Rutherford”, in Science, 173 (1971), 873; T. Trenn, “Rutherford and Soddy; From a Search for Radioactive Constituents to the Disintegration Theory of Radioactivity”, in RETE Strukturgeschichte der Naturwissenschaften1 (1917), 51–70; and T. Trenn. “The Geiger-Marsden Scattering Results and Rutherford’s Atom. July 1912 to July 1913: The shifting Significance of Scientific Evidence”, in Isis,65 (1974), 74–82.

For information about some of Rutherford’s colleagues. see the various articles in he DSB and Albert Parry. ed., Peter Kapitsa on Life and Science (New York, 1968); Sir Ernest Marsden. 80th Birthday Book (Wellington, New Zealand. 1969); and Robert Reid. Marie Curie (New York, 1974).

Various aspects of the history of radioactivity may be found in the following selections: T. W. Chalmers, A Short History of Radio-Activity (London, 1951); Alfred Romer. The Restless Atom (Garden City. New York, 1960): A. Romer. ed., The discovery of Radioactivity and Transmutation (New York, 1964); L. Badash, The Early Developments in radioactivity, With Emphasis on Contributions From the United States (Ph.D. Diss. Yale University, 1964): L. Badash, “Radioactivity Before the Curies”, in American Journal of Physics, 33 (1965), 128–135: L. Badash, “Chance Favors the Prepared Mind: Henri Becquerel and the Discovery of Radioactivity”, in Archives Internationales d’Histoire des Sciences, 18 (1965), 55–66: L. Badash, “The discovery of Thorium’s Radioactivity”, in Journal of Chemical Education, 43 (1966), 219–220: L. Badash, “Becquerel’s ‘unexposed’ Photographic Plates”, in Isis, 57 (1966), 267–269: L. Badash, “An Elster and Geitel Failure: Magnetic Deflection of Beta Rays”, in Centaurus.11 (1966), 236–240; A. Romer, ed.,Radiochemistry and the Discovery of Isotopes (New York, 1970): Marjorie Malley, “The Discovery of the Beta particle”, in American Journal of Physics, 39 (1971), 1454–1460; and Thaddeus J. Trenn. The Rise and Early Development of the Disintegration Theory of Radioacativity (Ph.D. Diss.. University of Wisconsin, 1971).

Lawrence Badash