Thomson, Joseph John
THOMSON, JOSEPH JOHN
(b. Cheetham Hill, near Manchester. England, 18 December 1856; d. Cambridge, England, 30 August 1940)
physics.
Thomson came to physics for want of money to enter engineering. His father, a bookseller, sent him to Owens College to mark time until a leading engineer, to whom he was to be apprenticed, had an opening; but the father died before the vacancy occurred, and the family then could not afford the premium. With the help of small scholarships Thomson continued to an engineering degree at Owens College, which had an excellent scientific faculty including Osborne Reynolds, Henry Roscoe, Balfour Stewart (under whom Thomson did his first experimental work [1]), and Thomas Barker, the professor of mathematics, a former senior wrangler. On Barker’s advice Thomson remained at Owens in order to work for an entrance scholarship in mathematics offered by Barker’s old college, Trinity (Cambridge). He won a minor scholarship and in 1876 went up to the university where he would spend the rest of his life [58:13–32].
He read for the mathematical tripos, which at that time covered a wide range of pure mathematics as well as applications to many branches of physics. To “wrangle” successfully, that is, to place high on the tripos list, one needed great facility at computation and an ability to cope with the sort of models, or “physical analogies,” prized by the school of Kelvin, Stokes, and Maxwell. But one required neither knowledge nor experience of experimental physics; and so Thomson, who prepared himself by following diligently the advice of his coach, E. J. Routh, did no more than put his foot into the Cavendish Laboratory, and never met Maxwell [58:95, 129], whose work was to inspire his own. He emerged second from the tripos of 1880, after Joseph Larmor, who, like himself, became a Cambridge professor.
Thomson stayed on at Trinity, which awarded him a fellowship in 1881. He followed three lines of mathematical work, apparently diverse in content and style, but forming a coherent group for disciples of Maxwell and continuing research interests for himself. He seldom abandoned an idea he had once developed.
Fellow of Trinity . In Maxwell’s practice there is a play, and sometimes a tension, between advancing theory by developing special mechanical models or analogies and deducing basic equations from the most general dynamical relationships. In the first mood, for example, Maxwell reached the equations of electromagnetism via an elaborate picture of a hydrodynamical contrivance supposed responsible for the phenomena; in the second, he obtained them directly from a Lagrangian constructed from known relationships between measurable quantities. The advantage of the second procedure, as Maxwell emphasized, is that one need not know (as one did not know) anything about the “mechanism” at work. The advantage of the first procedure is that, as Thomson would say, it fixes ideas, aids the memory, and, above all, suggests unexpected new directions of experimentation [15:1].
Thomson’s earliest effort at designing models was stimulated by the subject set for the Adams Prize of 1882, “a general investigation of the action upon each other of two closed vortices in a perfect incompressible fluid.” In his winning Treatise [5] Thomson carried the matter further than required, to an application which, for him, gave it “the greater part of the interest it possesse[d]” [5:2], namely Kelvin’s theory of the vortex atom (1867). Here the atoms of a gas are represented as reentrant vortices in a frictionless fluid, rather like smoke rings in air; but the vortices, unlike the rings, are eternal, and therefore could reproduce the permanence of the Victorian atom.
The theory appealed to Thomson’s romantic strain, to his recurrent wish for a quantitative, mechanistic, and “ultimate” [39:1]–by which he meant not “unique” but parsimonious [5:1]–account of the physical world. In this sense the theory of the vortex atom is perhaps the most fundamental ever started, for it hoped to make do with nothing besides the several perfections of its primitive fluid and pure mathematical analysis. “The difficulties of this method are enormous,” Maxwell had written, “but the glory of surmounting them would be unique.”1 Thomson’s Treatise is perhaps the most glorious episode in this hopeless struggle.
In adapting the pertinent hydrodynamics to the theory of the vortex atom, Thomson was guided, and perhaps even inspired, by the experiments of A. M. Mayer as interpreted by Kelvin [5:107]. Mayer, whose striking results would remain fresh in Thomson’s mind [e.g., 26:313–314], had investigated the equilibrium configurations of n vertical magnetized needles floating on water and exposed to the attraction of a large fixed magnet. It appeared that if n≤5 the magnets would arrange themselves in a single circle, while for large n several concentric rings were required. On the basis of his kinetic representation of magnetism, Kelvin had inferred that to every stable arrangement found by Mayer there must be a counterpart formed by straight columnar vortices. Thomson therefore examined the stability of m vortex rings so coupled that their nearest portions always ran parallel, like threads wrapped symmetrically about a toroid, without crossing one another. Stability required that the vortices be of equal strength, and that m≤6.
To apply these results to the problem of chemical constitution (to which Thomson, in striking contrast to most physicists, gave continuing consideration “e.g., 39:120–141; 55:28–112”]), observe that each of the linked threads in Thomson’s arrangement can itself be a combination of n(≤6) vortices of equal strength. Let the strengths of all vortex atoms be multiples of that of hydrogen, taken as one. The oxygen atom clearly has strength two. Nitrogen gives trouble, apparently requiring a vortex of strength two in NO and of strength one in NH3. Carbon likewise has its ambiguities (CO, CH4); and in general the table of valences with which Thomson concluded his Treatise was useless for the chemist. But it was quite characteristic of him–at least in regard to “ultimate” theories– to be satisfied with gross qualitative agreement between experiment and the quantitative results he extracted at great labor from simple mechanical representations. Much of his important work on atomic structure, and the theories of chemical action [55:12–26] and of the nature of light [57] which he developed late in life show the same curious procedure: precise calculation and ingenious analogy applied with great virtuosity to secure only a rough fit with a few data. No doubt this method–or rather mood, for it by no means characterizes all of Thomson’s work–helped him to those “happy intuitions” and “inspired generalizations,” to that “abundance of ideas” and “endless fertility in invention” which led and impressed his contemporaries.2 But it was also a method that became sterile in proportion to its success; for its qualitative conquests prepared the way for an exact physics that had no need for it.
A second line in Thomson’s early researches descends from Maxwell’s phenomenological strain. As part of the dissertation written for his fellowship [58:21], he elaborated an idea that had occurred to him at Owens College, and to which he would return [e.g., 53]–that the potential energy of a given system might be replaced by the kinetic energy of imaginary masses connected to it in an appropriate way. This notion, which anticipated the better-known scheme of Heinrich Hertz, could be made analytic by employing a form of Lagrange’s equations worked out by Routh [12:12–15]. From the inspection of a Lagrangian, therefore, one not only cannot determine the underlying mechanism, but in general one cannot tell whether one confronts an ordinary system or one with Thomsonian masses.
Like Maxwell, Thomson was prepared to exploit this result in two ways. First, the fact that one can replace potential energy (“[which] cannot be said, in the strict sense of the term, to explain anything” [12:15]) by kinetic energy supported the hope for a theory based solely on the properties of matter in motion. “When we have done this we have got a complete explanation of any phenomenon and any further explanation must be rather metaphysical than physical” “[ibid].”. Thomson had in mind a theory like the vortex-ring atom. But, second, the fact that a given Lagrangian is compatible with any number of models is a strong recommendation for avoiding them all, especially since a primary goal of physics–the discovery of new phenomena–can be reached merely by manipulating an appropriate Lagrangian in a prescribed manner.
In a series of papers [7, 9], lectures, and a book, Applications of Dynamics [12], Thomson illustrated how to guess at a term in the Lagrangian from a consideration of known phenomena and how, from the term once admitted, to deduce the existence and magnitudes of other effects. He also showed that a time-average of the Lagrangian could play the part of the entropy in certain problems usually handled by the second law of thermodynamics. One of his most important contributions in this line was the development of the notion, perhaps original with him, that electricity flows in much the same way in metals as in electrolytes [e.g., 12: 289–304]. He was to return to this idea in founding the electron theory of metals, first supposing the electrons free [30], as in Arrheniu’s picture of a dilute solution, and then, in the hope of accounting for a difficulty in the theory of specific heats, allowing them only intermittent liberty [39:86–102], as in Grotthu’s theory of electrolytic conduction [62:419–420,425].
Three characteristics of these Applications deserve notice. For one, Thomson shows himself a master of the literature, not excluding the pertinent papers of German experimentalists. He was to keep fully abreast of the journals (from which he sometimes took ideas whose origin he later forgot) until World War I [59:150–151, 219]. Second, the moderate phenomenology of the Applications, a work which eschews the specification of dynamical processes, recurs in much of Thomson’s later work. His pioneering theory of the conduction of electricity in gases [24, 28], for example, merely assumes the existence of ions, and describes their behavior not in terms of the electrodynamics of their interactions, but via parameters–especially measures of mobility and recombination–to be fixed by experiment. Only later [e.g., 45] did he sketch a theory of the process of ionization.
Third, Thomson, in common with most of the Cambridge school of mathematical physicists, took it for granted that an appropriate Lagrangian could always be found, or, in other words, that in principle all physical phenomena could be explained mechanically. Further, he thought that one or another of the possible dynamical explanations of a given phenomenon, whose existence is guaranteed by the Lagrangian, ought to be made explicit whenever possible. In this mood, as contrasted to that of the theorist of vortex atoms, Thomson did not require that models of diverse phenomena be consistent among themselves, nor that they avoid action at a distance; but that they admit only those sorts of forces and interactions with which physicists had become familiar since the time of Newton. He never totally abandoned this point of view, which caused him and contemporaries like Lodge and Schuster to deprecate the quantum theory as a screen of ignorance, a cowardly substitute for “a knowledge of the structure of the atom” [50a:27]. Sometimes, as in his theory of the speckled wave-front [34:63–65: 42], designed to explain the selective ionizing power of X and γ rays, his efforts to cope with the quantum could advance the subject. But after 1910 his schemes for avoiding novelties like Einstein’s approach to the photo-effect [49] and Bohr’s deduction of the Balmer formula [52], would become increasingly farfetched and fruitless.
The last of Thomson’s early research lines was the mathematical development of Maxwell’s electrodynamics. His first important results included the discovery of the so-called electromagnetic mass, or extra inertia, possessed by electrified bodies in virtue of their charge [2], and the calculation (in error by a factor of two) of the force–now known as the Lorentz force–exerted by a magnetic field on a moving electrified sphere “ibid.”. These results were not only important in themselves: they also marked or sparked the beginning of the rapid harvest of Maxwellian fruits by Fitzgerald, Heaviside, Lamb, Poynting, and Thomson himself “eg., 6”.
One is struck by the literalness with which Thomson at first cultivated Maxwell’s theory. Not that he clung slavishly to his model, for his thorough report on electrical theories to the British Association for the Advancement of Science [8] points to obscurities in Maxwell’s formulation and discusses competing systems sympathetically. But he tried to remain true to what he considered the peculiar mark of Maxwell’s theory: the dielectric “displacement” D, whose divergence represented what other electricians called the electric fluid and whose time rate of change, even in the absence of matter, gave magnetic effects like those of an ordinary current. By obscuring the concept of charge, displacement caused much of the malaise felt by Continental readers of Maxwell, and it could lead even English ones astray. In his important paper of 1881 [2], Thomson reached incorrect results by ascribing the magnetic field of a moving charged sphere solely to D outside it, thereby ignoring the most important factor, the convection of the charge [cf. 62:306–307;60:24, 55].
In subsequent work Thomson replaced “displacement” (an “unfortunate” term [8:125]) by “polarization,” which he represented in terms of electrostatic “tubes of force” supposed to begin and end on “atoms,” each tube conferring the electrolytic unit of electricity, or “electron,” on its termini [15:1–52]. From this representation, which in its ingenious details shows the hand of the essayist on vortex motion [cf. 58:94], Thomson recovered all the usual formulae of Maxwellian electrodynamics. He also thereby [15:13] stressed the notion of electromagnetic momentum stored in the medium (as a consequence of the translation of the tubes, the cause of magnetism), a notion used by himself [e.g., 39:24] and others to save the equality of action and reaction in electrodynamics [62:366], and to demonstrate the existence of “some invisible “material” universe, which we may call the ether” [36a:235].
It need scarcely be said that, although Thomson believed strongly that students should form some mental picture of the mechanism of the electromagnetic field, he did not urge his own as unique or even as particularly meritorious. “Which particular method the student should adopt is for many purposes of secondary importance, provided that he does adopt one” [15:vii]. “A theory of matter is a policy rather than a creed” [39:1]. He himself used models different in scope and degree of reduction, and, not seldom, conflicting in character; and after the discovery of the electron he freely admitted anti-Maxwellian bugaboos, like electric charge, into his partial pictures of metallic conduction [e.g., 30], atomic structure [e.g., 35], and chemical combination [39:120–139]. Such laxity of course could not be permitted in an “ultimate” theory of electricity [53, 56, 57].
Cavendish Professor . In 1884 Lord Rayleigh, who had succeeded Maxwell, resigned the Cavendish Professorship of Experimental Physics. Thomson had by then completed a few imperfect bits of laboratory work [cf. 44:80; 58:97], including a determination, at Rayleigh’s suggestion, of the ratio of the electrostatic to the electromagnetic unit of electricity “[4, corrected in 13]”. Rayleigh had intended to collaborate in this work which, apart from its imperfection, was typical of the Cavendish during his era; but Thomson, unaware of many of the pitfalls, ran away with the project, published hastily, and gave his colleagues, including the Professor, to doubt that he had any future in experimental physics [59:18–20]. With these credits and his mathematics, he competed for the chair; much to his surprise [58:98], and to the great annoyance of some of his competitors, who included Fitzgerald, Glazebrook, Larmor, Reynolds, and Schuster, he was elected. It says much for the wisdom of the electors, among whom the ancient wranglers Stokes, William Thomson, W. D. Niven, and George Darwin, one of the judges of Thomson’s Adams Prize essay, were probably most influential.
Luckily the personnel of the laboratory, including one who had expected to be its chief, remained on; and so the introductory courses set up in Rayleigh’s time, and especially those for the many candidates for part I of the Natural Science Tripos, continued to function smoothly while the new Professor found his way. The same staff later (1888) introduced courses for intending physicians, whose fees quickly became an important part of the Cavendish’s finances [44:84–89; 61:250–280; 59:19–21].
Thomson chose the phenomena of the gas discharge, whose study Maxwell had recommended, for experimental investigation. The subject had attracted attention in the early 1880’s owing largely to the work of Crookes and Goldstein on the cathode rays [62:350–353]. Indeed, the ostensible motivation of Thomson’s Maxwellian computations of 1881 was to provide a theoretical guide to the further study of the rays, which he, like Crookes, took to be “particles of matter highly charged with electricity and moving with great velocities” [2:229; 58:91–93]. Two years later Thomson had again turned his attention to the discharge, guided this time by the theory of the vortex atom [3].
The vortical mechanism for chemical bonding, he observed, works only when combining vortex rings have approximately the same size and velocity. Any disturbance in the medium, like the approach of another vortex atom, may alter the critical parameters and prevent linkage or disrupt unions. Now an electric field may be represented by a distribution of velocity in the medium; and the chemical decomposition it stimulates would be the immediate cause of the discharge. In this odd form Thomson introduced an idea of the utmost importance for future work: that the gas discharge proceeds in analogy to electrolysis, by the disruption of chemical bonds. Initially, as was only natural, he regarded the particles into which the molecules separated under the influence of the field as “atoms.” Later researches (and, one presumes, a relaxation of his Maxwellian literalness) helped him to see the “atoms” first as “ions,” that is, charge carriers of atomic dimensions, and then as mixtures of ions and “bodies much smaller than atoms.”
The electrolytic analogy suggested that important clues to the mechanics of gas discharge might come from studies of dielectric breakdown in poorly conducting liquids, or from decomposition of polyatomic gases by sparks. Thomson and his students worked on the one [e.g., 11] and the other [10, the Bakerian lecture of 1887, continued in 16 and 18], and acquired many data without much advance in understanding. By the early 1890’s he had concluded that a study of the striated positive light was the most promising avenue to the understanding of the discharge. As for the cathode rays, which had seemed significant in the early 1880’s, they now appeared to him but a “local” and “secondary” matter [15:114–115].
Controversy returned the neglected cathode rays to the center of attention. Most English physicists, including Thomson, had taken them to be streams of charged particles, primarily because their paths curved in a magnetic field; while most German physicists, arguing chiefly from their ability to cause glass to fluoresce, had considered them an “aether disturbance” akin to ultraviolet light [62: 351–354]. In the early 1890’s the English were put on the defensive by Philipp Lenard, who aggressively pursued the discovery of his master, Heinrich Hertz, that the rays could be passed through thin metal foil impermeable to particles of gas-theoretical dimensions. (Another objection, based on Hertz’s inability to deflect the rays in an electrostatic field, was regarded less seriously; what weight it had was largely reduced by Perrin’s direct detection of the charge carried by the rays, and vanished altogether when Thomson obtained the deflection in a better vacuum than Hertz had commanded [26:296].) Thomson tried in turn to undermine the position of the etherists by showing that the cathode rays moved at less than the velocity of light [17], but his results–off by two orders of magnitude–did not convince his opponents. At this point Röntgen prepared to enter the fray, and in the process discovered X rays.
Thomson, who had all the apparatus to hand, immediately found that the new rays turned gases they traversed into conductors of electricity [19, 20], and so offered a means much more convenient than disruptive discharge for producing gaseous ions [58:326]. Under his guidance the advanced students at the Cavendish rushed to exploit the new tool, and to make the accurate measurements of ionic parameters on which the Professor built his theory of gas discharge. The first edition of the famous Conduction of Electricity Through Gases [33] is a monument to these coordinated researches, in which McClelland [33], Rutherford [24], Townsend, and Zeleny played principal parts [59:74, 125; 60:38–41]. The presence at this time of these “research students”–graduates of other institutions allowed, by a reform introduced in 1895, to work for a research degree without first obtaining a Cambridge B.A.–was a great stroke of luck, as Thomson fully recognized [44:93; 58:325]. It provided talented and highly motivated men who not only developed the Professor’s ideas in work of the highest quality, but also helped raise the enthusiasm of younger recruits to the laboratory [61:269–271].
Thomson also saw in the X rays a possible explanation for the “startling” [37:3] transparency of metal foils to cathode rays: might it not be that in fact no penetration occurs, that cathode rays striking one surface of the metal produce X rays there which in turn create new “ions,” alias “cathode rays,” on the far side? [22] This ingenious subterfuge did not long survive the attacks of Lenard who, at Thomson’s invitation [59:55], brought his campaign to the British Association in 1896. Thomson allowed himself to be persuaded of the importance of Lenard’s work, and particularly of his discoveries that (a) the magnetic “deflectibility” of rays passed outside the tube depends only on the conditions within it, (b) that these external “Lenard rays” lost their power of causing fluorescence, that is, were absorbed, in proportion to the density, and independent of the chemical character, of the environment, and (c) that the mean free path of the rays outside the tube far exceeded the value to be expected if they consisted of gaseous ions [cf. 25: 430–431]. One suspects that it was in the process of digesting Lenard’s results that Thomson first entertained the idea that the cathode rays consisted of bodies smaller than atoms.
To explore the matter further he employed Schuster’s old technique of magnetic bending; for from the measured radius of curvature R of a beam of cathode rays deflected by a magnet of strength H one can infer a value for e/m, the ratio of the charge to the mass of the hypothetical cathode-ray particle (e/m = v/HR). Since the values for similar ratios, E/M, were known for ions produced in electrolysis, a comparison of e/m to E/M might provide a clue to the nature of the particle.3
To obtain e/m by Schuster’s method the velocity v of the rays is required. If one takes for v either Thomson’s faulty measurement [17] or, as Schuster had done, the mean speed of a gas molecule (3kT/M)1/2, e/m ∼E/M, that is, one confirms the standard English theory which assimilated the rays to streams of charged particles of atomic dimensions. Lenard’s intervention pushed Thomson to devise a more direct way of obtaining v. He found two. In the first [25:432; 26:302–306], the heat T = nmv2/2 delivered by a stream of n “corpuscles” (as Thomson was to call the cathode-ray particle) to a Faraday cup was compared to the total charge, Q = ne, simultaneously conveyed, whence v2 = (e/m) · (2T/Q). In the second [26:307–309], which exploited Thomson’s discovery of the electrostatic deflection of the rays, v came from balancing an electric force eF against a magnetic one (e/c)vH’ to give no net deviation of the beam, whence v = cF/H’. The rough result of these measurements was that e/m exceeded by a factor of 1,000 the E/M for the ion of hydrogen, which has the largest charge-to-mass ratio of the chemical elements. The same anomalous value characterized all the cathode rays Thomson tried, irrespective of the material of the electrodes or of the nature of the gas in the discharge tube in which they were produced [26:306, 309].
At least two other physicists–Emil Wiechert and Walther Kaufmann, both then beginning distinguished careers–had independently obtained the same sort of data about the rays, and had inferred the correct magnitude of e/m by deducing v from the energy which would be acquired by a particle falling through the full potential V of the tube (mv2/2=eV). The equation could scarcely be justified theoretically, as Thomson liked to observe [e.g., 58:339]; but it gave the right order, and for this reason had been rejected by Schuster before the advent of Lenard. Despite their possessing most or all of the relevant data, neither Wiechert nor Kaufmann discovered the electron. Wiechert came closest: guided by the older Continental ideas about electricity [62:198 ff.], then recently revived by Lorentz, he identified the cathode-ray particle as a disembodied atom of electricity, a fundamental entity distinct from common matter. Kaufmann found nothing at all but an argument against “the hypothesis that assumes the cathode rays to be charged particles shot from the cathode.”4
When Thomson, following his method, sought a representation of his striking data, he did not forget his old concerns: the vortex atom and Mayer’s magnets, the problem of chemical combination and the nature of electricity [cf. 58:94]. It was doubtless these which pushed him to “discover” the electron, that is, to claim far more for the “corpuscle” than the data authorized. For from its large e/m he inferred its small mass (by assuming that its charge was of the order of the electrolytic unit); from its small mass he inferred (what scarcely follows) its small size; from its small size, its penetrability, and an answer to Lenard; from its size again and from the apparent independence of its e/m from the circumstances of its production, that it is a constituent of all chemical atoms. Or, rather, the constituent: for, as Thomson pointed out [26:311–314], if the chemical atoms are built up of corpuscles, arranged in rings in the style of Mayer’s magnets, one immediately glimpses an electrodynamic explanation of the periodic properties of the elements and saves Prout’s vexed hypothesis–that the elements are built up of multiples of a single basic unit, or “protyle”–from the old objections based on deviations of atomic weights from integers. And there is more. Although Thomson called his particle “corpuscle” in order not to prejudge the value of its charge, which he initially believed to be larger than the “electron” [26:312; 60:55], he came quickly to believe that the corpuscle carried the elementary unit of electricity [27:544–545]. Apparently the protyles of matter and electricity were inseparable.
The initial evidence for Thomson’s claims consisted primarily of the values of e/m for cathode rays of different provenance and of Lenard’s law of absorption, which would follow if atoms contained corpuscles (and nothing but corpuscles capable of slowing cathode rays) in proportion to their weights. (To this might be added the similar law for the absorption of X rays found by McClelland, which Thomson had earlier tied to Prout’s hypothesis [21], and which no doubt aided his digestion of Lenard.) Few physicists in 1897 were prepared to believe on this basis that the world was made of corpuscles.
Two years later the claim seemed more than plausible. In the interim, Thomson, following up old experiments of Elster and Geitel [59:108–112], had managed to find other particles–those liberated from metals by ultraviolet light and from carbon filaments by heat–which possessed approximately the same value of e/m as the corpuscle [29]. Moreover, he had succeeded in measuring e alone by exploiting, in a way suggested by Town-send and with an apparatus designed by C. T. R. Wilson, his earlier study of the ability of charged particles to promote condensation of water vapor [14; cf. 58:342–343; 61:195–205; 59:101–105]. The measurement employed X rays or ultraviolet light to create ions in a saturated gas, ions which, as Wilson had painstakingly proved, did indeed serve as condensation nuclei. The gas was then expanded and n droplets formed; from the mass and rate of fall of the fog one can compute n, whence e = Q/n, Q being the total charge carried down by the droplets. The result, which agreed to order of magnitude with estimates of the electrolytic “electron” [27:544; 29:562–563], turned out to be 30 percent too high. A second try [31] erred equally by defect. Thomson liked to leave the second decimal place to someone else [60:169]; in this case he left the first as well.
All this evidence, however imposing, left a large logical gap in Thomson’s theory; for, strictly speaking, it did not bear on his claim that the normal atom consisted of corpuscles. By great good luck this gap was filled even as Thomson prepared his first lecture on the cathode rays [25]. For just then Zeeman established that the particle that, on the theory of Lorentz, gave rise to the spectral lines split by his magnet, possessed an e/m 1,000 times that of electrolytic hydrogen. The corpuscle not only belonged in normal atoms, but was responsible for their line spectra. The weight of evidence tipped in Thomson’s favor. When he outlined it to the British Association in 1889 [29] it immediately “carried conviction” [58:341]: “The scientific world seemed suddenly to awake to the fact that their fundamental conceptions had been revolutionized.”5
Atomic Structure . After consolidating his evidence about the electron (as physicists renamed the corpuscle when they came to believe in it) Thomson returned to the problem he had raised when introducing it as Prout’s protyle: what causes the electrons of an atom to arrange themselves in the periodic manner implied by the table of the elements? He aimed at a theory that would postulate nothing but a few properties of the universal corpuscle; even the positive electrification apparently needed to retain atomic electrons and to neutralize matter would, he hoped, be reduced to an electronic property [59:140–141]. There were many good reasons, some of which had been identified before 1900 by Fitzgerald, Larmor, and Rayleigh, for believing that such a theory (even admitting two sorts of charges) could not explain important atomic characteristics like the frequencies of spectral lines. Fortunately, Thomson disregarded this counsel of despair, which the failure or barrenness of models proposed by others had made the more compelling when he took up the work in 1903 [34:90–139].
He then represented the positive electrification, which he had not been able to eliminate, in a manner Kelvin had used for a primitive model of a radium atom: a diffuse sphere of constant charge density through which electrons move subject solely to electrostatic forces. Thomson always regarded this model as a pis aller, an unsatisfactory incarnation of that “something which causes the space through which the corpuscles “in an atom” are spread to act as if it had a “compensating” charge of positive electricity” [29:565]. But it was easily visualized, and yielded much of what he wanted: for assuming the electrons constrained to circulate in a single plane through the atom’s center (another pis aller, to ease calculations) Thomson showed that to insure mechanical stability the electrons, under the influence of electrostatic forces alone, must distribute themselves into rings in the manner of Mayer’s magnets [35].
He drew several important qualitative conclusions. First, since the electrons, unlike the magnets, move in accelerated paths, they must radiate, and consequently no arrangement of them can be permanent. This apparent menace proved a great advantage. The radiation from a ring of p electrons decreases very quickly as p increases, the radius and angular velocity of the particles remaining the same [37]; hence if the rings of an atom are well populated its internal motions might decay very slowly until a critical velocity is reached and the whole explodes. We call such explosions radioactivity. (The obvious inference, that all elements must be radioactive, kept several Cavendish men busy for years [cf. 61:235–237].) The relative stability of matter depends, in this theory, on n, the number of electrons in an atom. Since, in 1904. Thomson still took n to be on the order of 1,000 times the atomic weight A, he did not fear imminent radiation collapse.
Secondly, the electronic distributions calculated by Thomson supported analogies to the behavior of the chemical elements and, in particular, the conclusion that the electronic populations of the atoms of contiguous elements in the periodic table differ by a single unit. Had he made this a principle, it would probably have modified his thinking about the order of n; but, as was his practice with such models, he did not take the results of his calculations literally, never assigned the value of n for any given element, and, very probably, did not anticipate that exact assignments could soon be made.
In fact the first substantial advances in atomic theory arose from efforts to obtain n as a function of A. Here Thomson once again led the way by showing how to estimate n from measurements of the scattering of light (dispersion), X and β rays [36]. All the data, including some collected at the Cavendish, were interpreted via formulae computed by Thomson under the guidance of his model atom; and the formulae for the scattering of X [33: 268] and of β rays [36:773] were the first of their kind. The upshot was that the population of the atom had been grossly overestimated, and that n appeared to lie between two-tenths and twice the atomic weight.
A great many experimental studies on the scattering of X, β, and γ rays were then put in train at the Cavendish [61:237]. The multiplication of data prompted Thomson to improve upon his theory of β scattering [43], which, however, rested on an unjustified assumption: that, regardless of the thickness of the scatterer, a β particle acquires a measurable deflection only as a result of encounters with a great many atomic electrons. It was this theory that served Rutherford first as pattern and then as counterfoil during his classical analysis of the scattering of α particles. Although the results of this analysis–the single-scattering theory, the new approximation n = A/2, and the nuclear atom–forced the rejection of Thomson’s model, they should be viewed not as evidence of the failure, but as proof of the value, of his methods.
Thomson’s discovery of the order of n did much more than recommend the cultivation of scattering theory. For one, it undermined the radiative stability of the atom and, by reducing the number of spectral emitters, made what Rayleigh called the “bog of spectroscopy” more mushy than ever. For another, it demonstrated that the chief part of the atomic mass must belong to the positive charge. Thomson tended to ignore the first set of problems, although he once troubled to suggest how a single electron might, during its capture by an ionized atom, emit most or all of the line spectrum [39: 157–162]. It was different with the new-found substantiality of the plus charge, which no longer could be referred persuasively to a “property of the corpuscle.”
Thomson’s last important experimental work, which extended over many years, was devoted to determining the nature of positive electricity. He concentrated on the “canal” or “positive” rays which can be constructed from the ions in a discharge tube by passing them through a perforated cathode. In earlier studies, by W. Wien in particular, the E/M of the rays had been found by deflecting them in superposed electric and magnetic fields, and catching them on a photographic plate; from the position of the traces on the film it appeared that they consisted of gaseous ions, and especially of hydrogen, which occurred irrespective of the gas filling the tube. Wien inclined to attribute the ubiquitous hydrogen to release of impurities absorbed by the walls of the tube. But this conclusion, as well as the general interpretation of Wien’s results, was made problematic by the width of the traces, which Thomson ascribed to the neutralization of ions in the rays by collisions with molecules of the residual gas. His first effort, therefore, was to remove the molecules by realizing the highest vacuum obtainable [cf. 58:350–357]. Wien’s bands then broke up, as theory required, into parabolic traces, each deposited by ions possessing different velocities and a common E/M [38].
Many ionic species disclosed themselves, and always H+, which Thomson accordingly took to be the positive protyle for which he was looking [38:575; 39:19, 23; 40:12–13]; but after a long exchange with Wien he conceded that the hydrogen was not protyle but impurity [46:248]. During the exchange Thomson introduced many ingenious improvements in experimental technique and in the analysis of the traces. By 1913 his instrument had become sensitive enough to distinguish ionic species of atomic weights 20 and 22 in a neon discharge. At first [48:593] he thought the heavier species a new element, or perhaps a molecular peculiarity, NeH2+; but eventually he came around to the new view of Rutherford’s school and recognized that he had been the first to isolate isotopes of stable elements [54:88]. In this work Thomson had the help not only of his long-time assistant, E. Everett, but also of Francis Aston, who returned to it after the war and perfected the mass spectroscope, which brought him the Nobel Prize.
Teacher and Administrator . Aston was one of seven Nobel Prizemen, twenty-seven Fellows of the Royal Society, and dozens of professors of physics trained at the Cavendish during Thomson’s tenure [58:435–438]. Thomson was an excellent teacher and, when in good form, an unsurpassable lecturer [61:257; 59:42–43], clever, challenging, presuming neither too much nor too little, enthusiastic, and imperturbable. He took pedagogy seriously, on all levels. He interested himself in the improvement of science education in the secondary schools [22, 47] as well as in the universities, for which he and his close friend J. H. Poynting prepared several excellent texts. He kept his own lectures up to date both at the Royal Institution, where he became Professor of Natural Philosophy in 1905 (in addition to his Cambridge post), and at the Cavendish [cf. 61:273–278]. For the benefit of the advanced students in the laboratory he established in 1893 the Cavendish Physical Society, a fortnightly seminar in the German manner in which recent work–including his own–was reviewed and criticized [61:226, 271; 59:41].
Thomson was not himself a good experimentalist, being clumsy with his hands [60:73; 58:118], but he had a genius for designing apparatus and diagnosing its ills [59:175]. This trait, together with his wide and up-to-date interests, his enthusiasm, imagination, and resourcefulness, made him an excellent director of research throughout his tenure of the Cavendish chair. He resigned the professorship in 1919, in favor of Rutherford [59:215–218], before his lack of sympathy for Bohr’s new physics could do any damage.
Thomson made every effort to place his best students, and gave generously of his time to keep those who took professorships in the colonies alive professionally. He would see their papers through the press, select demonstrators for them, advise on job openings and laboratory construction, and report recent progress in physics. As administrator of the Cavendish he gave his demonstrators great freedom and interfered as little as possible with laboratory routine [61:226]. He extended the buildings twice, once with accumulated laboratory fees [59:46], and again with Lord Rayleigh’s Nobel Prize money, generously given the University for the purpose [59:155–156]. For a time, particularly in the 1890’s, the need to save for expansion left little for research [61:270; 59:47–48], and it may be that Thomson could then have done more to improve the finances of the laboratory. He had a good eye for investment himself, and died a moderately wealthy man [59:262].
Thomson received a great many honors, including the Nobel Prize (1906), a knighthood (1908), the Order of Merit (1912), and the Presidency of the Royal Society, which he assumed in 1915. He therefore bore the burden of directing the Society’s efforts to assist in the war [cf. 51] and of restraining some of its superpatriots from trying to oust Fellows of German descent like Schuster [59: 181–195]. The tact and energy with which he accomplished these tasks were widely recognized. In 1918 Thomson became Master of his old college, Trinity. He guided its affairs with his wonted geniality and good sense until a few months before his death.
NOTES
1. “Atom.” in Encyclopaedia Britannica (9th ed., 1875).
2. The first two phrases are A. Righi’s. “Sir J. J. Thomson,” in Nature, 91 {1918), 4–5; the second pair come from N. Bohr. ibid., 118 (1926), 879. and 59:150, respectively.
3. This agrees with the account in 37:3, with the order of ideas in 25:430–432, and with the order of events in 1896–1897. Lenard’s role has become less prominent in Thomson’s definitive announcement of his discovery [26], and has altogether disappeared from the retrospective account in 44:95, which ascribes the awakening of “doubts” about the ionic interpretation of the rays solely to the results of the bending experiments. Whittaker [62:361], Rayleigh [59:80], and G. P. Thomson [60:44–45] all follow this version, which Thomson enlarged in 58:333–335.
4. W. Kaufmann, “Die magnetische Ablenkbarkeit der Kathodenstrahlen und ihre Abhängigkeit vom Entladungspotential,” in Annalen der Physik, 61 (1897), 544–552,
5. A. Schuster, The Progress of Physics 1875–1908 (Cambridge, 1908). 70–71.
BIBLIOGRAPHY
I. Most of Thomson’s important papers were published in the Philosophical Magazine, which he took to be, and helped to make, the leading English journal for physics. His results would often be reprinted, more or less reworked, in books, two of which became fundamental texts in their fields [33, 50]. No full bibliography of his works exists; the best, that in the obituary notice by the 4th Baron Rayleigh (Obituary Notices of Fellows of the Royal Society, 3 [1941], 587–609), contains some 250 items and yet is quite incomplete. It omits letters to Nature, at least one of which [19] was important; and misses contributions to cooperative works like the Encyclopaedia Britannica, Watt’s Chemical Dictionary, and the Recueil des travaux offerts . . . à H. A. Lorentz (The Hague, 1900). Other important omissions include Thomson’s Nobel Prize speech [37], his Rede lecture [21], and his contribution to James Clerk Maxwell. A Memorial Volume (New York, 1931). Additional items are supplied by Poggendorff and by Rayleigh [59:292]. A useful but incomplete list of Thomson’s publications from 1880 to 1909 may be gathered from 61:285–323.
There follows a list in order of publication of the works of Thomson mentioned in the text (PM = Philosophical Magazine; PCPS = Proceedings of the Cambridge Philosophical Society; PRI = Proceedings of the Royal Institution; PRS = Proceedings of the Royal Society; PT = Philosophical Transactions of the Royal Society; RBA = Reports of the British Association for the Advancement of Science).
[1] “Experiments on Contact Electricity Between Non-Conductors,” in PRS, 25 (1877), 369–372.
[2] “On the Electric and Magnetic Effects Produced by the Motion of Electrified Bodies,” in PM. 11 (1881), 229–249.
[3] “On a Theory of Electric Discharge in Gases,” in PM, 15 (1883), 427–434.
[4] “On the Determination of the Number of Electrostatic Units in the Electromagnetic Unit of Electricity,” in PRS, 35 (1883), 346–347.
[5] Treatise on the Motion of Vortex Rings (London, 1883).
[6] “On Electrical Oscillations . . .,” in Proceedings of the London Mathematical Society, 15 (1884), 197–218.
[7] “On Some Applications of Dynamical Principles to Physical Phenomena,” in PT, 176 pt. 2 (1885), 307–342.
[8] “Report on Electrical Theories,” in RBA (1885), 97–155.
[9] “Some Applications of Dynamical Principles to Physical Phenomena, PT, 178A (1887), 471–526.
[10] “On the Dissociation of Some Gases by the Electric Discharge,” in PRS, 42 (1887), 343–344.
[11] “On the Rate at Which Electricity Leaks Through Liquids Which Are Bad Conductors of Electricity,” in PRS, 42 (1887), 410–429, written with H. F. Newall.
[12] Applications of Dynamics to Physics and Chemistry (London, 1888).
[13] “On Determination of ‘v,’ the Ratio of the Electromagnetic Unit of Electricity to the Electrostatic Unit,” in PT, 181 (1889), 583–621, written with G. F. C. Searle.
[14] “The Electrolysis of Steam,” in PRS, 53 (1893), 90–110.
[15] Notes on Recent Researches in Electricity and Magnetism (Oxford, 1893).
[16] “On the Effect of Electrification and Chemical Action on a Steam Jet. . .,” in PM, 36 (1893), 313–327.
[17] “On the Velocity of the Cathode-Rays,” in PM, 38 (1894), 358–365
[18] “On the Electrolysis of Gases,” in PRS, 58 (1895), 244–257.
[19] “The Röntgen Rays,” in Nature, 53 (1896), 391–392.
[20] “On the Discharge of Electricity Produced by the Röntgen Rays”, in PRS, 59 (1896), 274–276.
[21] “The Röntgen Rays,” Rede lecture, in Nature, 54 (1896), 302–306.
[22] “Presidential Address,” section A, in RBA (1896), 699–706.
[23] “On the Leakage of Electricity Through Dielectrics Traversed by Röntgen Rays,” in PCPS, 9 (1896), 126–140, written with J. A. McClelland.
[24] “On the Passage of Electricity Through Gases Exposed to Röntgen Rays,” in PM, 42 (1896), 392–407, written with E. Rutherford.
[25] “Cathode Rays,” in PRI, 15 (1897), 419–432.
[26] “Cathode Rays, in PM, 44 (1897), 293–316.
[27] “On the Charge of Electricity Carried by the Ions Produced by Röntgen-Rays,” in PM. 46 (1898), 528–545.
[28] “On the Theory of the Conduction of Electricity Through Gases by Charged Ions,” in PM, 41 (1899), 253–268.
[29] “On the Masses of the Ions in Gases at Low Pressures,” in PM, 48 (1899), 547–567.
[30] “Indications relatives á la constitution de la matiére, in Rapports du congrés international de physique (Paris, 1900), III , 138–151.
[31] “On the Charge of Electricity Carried by Gaseous Ions,” in PM, 5 (1903). 346–355.
[32] “The Magnetic Properties of Systems of Corpuscles Describing Circular Orbits,” in PM, 6 (1903), 673–693.
[33] “Conduction of Electricity Through Gases (Cambridge, 1903).
[34] “Electricity and Matter (New Haven, 1904).
[35] “On the Structure of the Atom . . .,” in PM, 7 (1904), 237–265.
[36] “On the Number of Corpuscles in an Atom,” in PM, 11 (1906), 769–781.
[36a] “On the Light Shown by Recent Investigations of Electricity on the Relation Between Matter and Ether, Adamson lecture (Manchester, 1907); reprinted in Annual Report of the Smithsonian Institution (1908), 233–244.
[37] “Carriers of Negative Electricity,” in Les prix Nobel en 1906 (Stockholm, 1908).
[38] “On Rays of Positive Electricity,” in PM, 13 (1907), 561–575.
[39] The Corpuscular Theory of Matter (London, 1907).
[40] “Presidential Address,” in RBA (1909), 3–24.
[41] “Positive Electricity,” in PM, 18 (1909), 821 –845.
[42] “On a Theory of the Structure of the Electric Field and Its Application to Röntgen Radiation and to Light,“PM, 19 (1910), 301–313.
[43] “On the Scattering of Rapidly Moving Electrified Particles,” in PM, 23 (1912), 449–457.
[44] “Survey of the Last Twenty-five Years,” in A History of the Cavendish Laboratory, 1871–1910 (London, 1910), 75–101.
[45] “Ionization by Moving Electrified Particles,” in PM, 23 (1912), 449–457.
[46] “Further Experiments on Positive Rays,” in PM, 24 (1912), 209–253.
[47] “The Functions of Lectures and Textbooks in Science Teaching,” in Nature, 88 (1912), 399–400.
[48] “Some Further Applications of the Method of Positive Rays,” in PRI, 20 (1913), 591–600.
[49] “On the Structure of the Atom,” in PM, 26 (1913), 792–799.
[50] “Rays of Positive Electricity and Their Application to Chemical Analysis (London, 1913).
[50a] The Atomic Theory, Romanes lecture (Oxford. 1914).
[51] “Presidential Address,” in PRS, 93A (1916), 90–98; PRS, 94A (1917), 182–90; PRS, 95A (1918), 250–257.
[52] “On the Origin of Spectra and Planck’s Law,” in PM, 37 (1919), 419–446.
[53] “Mass, Energy and Radiation,” in PM, 39 (1920), 679–689.
[54] “Opening of the Discussion on Isotopes,” in PRS, 99A (1921), 87–94.
[55] “The Electron in Chemistry (Philadelphia, 1923).
[56] “On the Analogy Between the Electromagnetic Field and a Fluid Containing a Large Number of Vortex Filaments,” in PM, 12 (1931), 1057-1063.
[57] “On Models of the Electric Field and of the Photon,” in PM, 16 (1933), 809–845.
[58] “Recollections and Reflections (London, 1936).
II. Thomson’s notebooks have been deposited at the Cambridge University Library (Add. 7654/NB), which also has three boxes of his correspondence, primarily incoming (Add. 7654 [ii]) and, in the Rutherford Papers (Add. 7653), some forty letters from him, bits of which were published by A. S. Eve, Rutherford (New York, 1939). The Royal Society Library also has a few Thomson autographs, primarily twenty-six letters to Schuster (Sch. 331–356). Indications of other holdings may be found in R. M. MacCleod, Archives of British Men ofScience (London, 1972), and in T. S. Kuhn et al., Sources for History of Quantum Physics (Philadelphia, 1967).
III. The chief biographies of Thomson are the following:
[59] “Lord Rayleigh, The Life of Sir J, J. Thomson, O.M. (Cambridge, 1943).
[60] “G. P. Thomson, J. J. Thomson and the Cavendish Laboratory in His Day (New York, 1965).
For assessments of Thomson’s work, see the following:
[61] H. F. Newall, E. Rutherford, C.T.R. Wilson, N. R. Campbell, L. R. Wilberforce et al., A History of the Cavendish Laboratory (London, 1910).
[62] E. T. Whittaker, A History of Theories of Aether and Electricity. I . The Classical Theories, 2nd ed. (New York, 1951).
[63] R.McCormmach, “J.J. Thomson and the Structure of Light,” in British Journal for the History of Science, 3 (1967), 362–387.
[64] V. M. Dukov, Elektron: istoria otkritia i izuchenia svoistov (Moscow, 1966), 108–154.
[65] J. L. Heilbron, “The Scattering of α and β Particles and Rutherford’s Atom,” in Archive for History of Exact Science. 4 (1968), 247–307.
[66] D. Topper, “Commitment to Mechanism: J. J. Thomson, The Early Years,” ibid., 7 (1971), 393–410.
J. L. Heilbron
Thomson, Joseph John
Thomson, Joseph John
ENGLISH PHYSICIST 1856–1940
Joseph John Thomson, always known as "J. J.," was born in Manchester, England, on December 18, 1856. His fame derives primarily from his discovery of the electron in 1897. He studied physics and mathematics, first in Manchester, and in 1876 went to Trinity College, Cambridge University, and never left. He graduated in 1880 and in 1884 succeeded Lord Rayleigh as professor of physics and director of the Cavendish Laboratory. (When he retired from Cavendish in 1919 he passed the baton to Ernest Rutherford). Thomson made Cambridge a world center for atomic physics. He won the Nobel Prize for physics in 1906 for his work on the electron, and seven of his research associates went on to win Nobel Prizes. The electron could almost be said to have been a family heirloom, as his son, George Paget Thomson, won the Nobel Prize for physics (in 1937) for showing the wave nature of the electron.
His early work in electromagnetism led him to say, in 1893: "There is no other branch of physics which affords us so promising an opportunity of penetrating the secret of electricity." He turned his attention to cathode rays, and his subsequent investigations of these rays led him to the idea that they consisted of bodies smaller than atoms. Thomson's main contribution to science was the clear identification of the electron and its characterization as an elementary, subatomic particle in 1897. He showed that cathode rays were deflected by both magnetic and electric fields, and he was able to measure a cathode ray's charge/mass (e/m ) ratio. Figure 1 is a schematic of diagram of his apparatus, showing how a beam of electrons can be subjected to opposing electric and magnetic fields, which can be adjusted until their effects balance. This enabled him to estimate the mass of the electron as 1/1,837 of a hydrogen atom. The electron was the first subatomic particle to be discovered, and he made the inspired guess that it was a universal constituent of matter. He said: "… [W]e have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much
further than in the ordinary gaseous state: a state in which all matter … is of one and the same kind; this matter being the substance from which all the chemical elements are built up." He announced his discovery in the course of a public lecture at the Royal Institution in London, on April 30, 1897, in which he said: "Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen?" Thomson referred to electrons as "corpuscles" (even in his 1906 Nobel lecture).
Thomson devised the famous plum pudding model of the atom, in which electrons were compared to negative plums embedded in a positively charged pudding. The idea was wrong, and his successor at Cambridge, Ernest Rutherford, was soon to develop the nuclear model of the atom.
Thomson investigated positive rays, which consist of ionized atoms, beginning in 1906. He was able to use a combination of electric and magnetic fields to separate different charged atoms of elements on the basis of their charge/mass ratios. He was the first to show that neon contained two atoms of slightly different masses, in a paper published in 1913. As part of the conclusion of the paper he wrote: "There can, therefore, I think, be little doubt that what has been called neon is not a simple gas but a mixture of two gases, one of which has an atomic weight about 20 and the other about 22. The parabola due to the heavier gas is always much fainter than that due to the lighter, so that probably the heavier gas forms only a small percentage of the mixture." The two forms of neon were called isotopes by Frederick Soddy. One of Thomson's students, Frederick Aston, developed Thomson's idea of multiple species of an element, and in 1919 Aston produced the first mass spectrograph (an instrument that determined isotopic ratios), ancestor of today's mass spectrometer.
Thomson was a great advocate of pure research, in contrast to applied research, declaring: "[R]esearch in applied science leads to reforms, research in pure science leads to revolutions, and revolutions, whether political or industrial, are exceedingly profitable things if you are on the winning side." Thomson was knighted in 1908 and received many awards and honors. He died during the early part of World War II, on August 30, 1940, and is buried in Westminster Abbey near Sir Isaac Newton, in recognition of his great contributions to science.
see also Magnetism; Spectroscopy.
Peter E. Childs
Bibliography
Dahl, Per F. (1997). Flash of the Cathode Rays: A History of J. J. Thomson's Electron. Philadelphia: Institute of Physics Pub.
Falconer, Isobel (1997). "J. J. Thomson and the Discovery of the Electron." Physics Education 32(4): 226–231.
Gerward, Leif (1997). "The Discovery of the Electron." Physics Education 32(4): 219–225.
Thomson, J. J. (1936). Recollections and Reflections. London: G. Bell and Sons.
Internet Resources
Articles about Thomson. Cavendish Laboratory. Available from <http://www.phy.cam.ac.uk/cavendish/history/>.
"The Discovery of the Electron." American Institute of Physics. Available from <http://www.aip.org/history/electron/>.
"Information on the Electron." Science Museum of London. Available from <http://www.iop.org/Physics/Electron/>.
Thomson, J. J. Articles. Available from <http://webserver.lemoyne.edu/faculty/giunta>.
Thomson, J. J. "Carriers of Negative Electricity." Nobel Lecture. Nobel e-Museum. Available from <http://www.nobel.se/physics/laureates/>.
Sir Joseph John Thomson
Sir Joseph John Thomson
1856-1940
English Physicist
As the director of the Cavendish Laboratory at Cambridge University, J. J. Thomson was instrumental in many important experiments and advances that marked the transition from classical to modern physics at the turn of the twentieth century. He discovered the electron in 1897 and was awarded a Nobel Prize for Physics in 1906. With his student Ernest Rutherford (1871-1937), also a Nobel laureate, Thomson made many fundamental discoveries concerning the properties of ionizing radiation.
Thomson was born in 1856 in the north of England. He studied physics at Cambridge and, at only 28 years old, was greatly surprised to be simultaneously named director of the Cavendish Laboratory at Cambridge and professor of experimental physics. By his own description, the laboratory he took over used "string and sealing wax" as its equipment, presenting him with a considerable challenge. Under his supervision Cavendish became one of the world's preeminent nuclear physics laboratories.
Just three years later, in 1897, Thomson successfully demonstrated that cathode rays are streams of electrons. radiation. Further research in this area allowed Thomson to determine the mass of the electron to be less than one two-thousandth that of the hydrogen atom, until then the lightest bit of matter known to exist. This led to the realization that electrons are subatomic—units of matter smaller than the atom itself. These same experiments also showed that electrons were negatively charged.
Previous research using magnetic fields had shown that cathode rays (electrons) could be deflected by magnetic fields. This is the principle underlying the operation of televisions, computer monitors, and other CRTs (cathode ray tubes). However, Thomson was the first to place cathode rays in an electrical field by bringing oppositely-charged electric plates next to a beam of cathode rays. When he applied electric current to the plates, the beam deflected towards the positively-charged plate, indicating that the rays had a negative charge. Further experiments in both electric and magnetic fields showed the ratio of mass between hydrogen atoms (protons) and the cathode rays (electrons). For this work, Thomson was awarded the Nobel Prize in 1906.
Additional research by Thomson showed that the interaction between electrons and matter could produce x-rays and that, conversely, xrays interacting with matter could produce electrons. Thomson also developed the first modern atomic model, albeit one that did not stand the test of time. In his "plum pudding" model, Thomson envisaged a sphere of positive changes with an equal number of negative charges (electrons) embedded within. This model was later supplanted by a number of others, leading eventually to the current model in which a cloud of electrons forms a shell surrounding a nucleus comprised of both protons and neutrons.
It may be argued that Thomson's mentoring of Rutherford was as important as his own discoveries in physics. Rutherford, who also went on to win the Nobel Prize for chemistry, was a pivotal figure in the formation of contemporary physics. Among his major discoveries, he proposed that radioactivity results from the disintegration of radioactive atoms, facilitated the development of today's model of atomic structure, and conducted other groundbreaking research into the nature of matter.
Thomson married and his son, George, also went into physics. George Thomson (1892-1975) was awarded the Nobel Prize in 1937 for research in electron diffraction by crystals. It is interesting to note that the work conducted by J.J. Thomson was based on the material properties of the electron while his son's work depended on the wave-like properties of electrons. The elder Thomson was knighted in 1908, and died in 1940 at the age of 84. He was buried near Isaac Newton (1642-1727) in Westminster Abbey in London.
P. ANDREW KARAM
Sir Joseph John Thomson
Sir Joseph John Thomson
The English physicist Sir Joseph John Thomson (1856-1940) is credited with the discovery of the electron.
On Dec. 18, 1856, J. J. Thomson was born at Cheetham Hill near Manchester. His father, a bookseller and publisher, planned a career in engineering for Joseph, but since no apprenticeship could be found for him in any engineering firm, he was sent "temporarily" to college in Manchester at the age of 14. As a result of his ability and determination, he won a scholarship in 1876 and entered Trinity College, Cambridge; he remained there for the rest of his life.
After graduation Thomson began working in the Cavendish Laboratory, which was under the direction of Lord Rayleigh. Thomson's brilliance brought him membership in the Royal Society at 27 and his appointment as Rayleigh's successor at 28. He proved to be inspiring and effective both as a teacher and as a research director, and as time passed, students came to him from all over the world. He sometimes had as many as 40 to advise at once, and for the first quarter of the 20th century the Cavendish Laboratory, where Thomson insisted that theory should be considered "a policy, not a creed, " was the world center for particle research.
Thomson began his studies of the properties of "cathode rays" in 1894 and proved in 1895 that they carried a negative charge. In 1897 he passed the rays through a vacuum and showed that they are deflected in both magnetic and electric fields. He was thus able to determine the ratio of the charge to the mass of the supposed particles and showed that its mass was about 2, 000 times larger than the mass of the hydrogen atom. The identification of the electron necessitated a revision of the atomic concept: Thomson visualized it as a mass of positively charged matter in which electrons were distributed like raisins in a cake.
About 1906 Thomson turned his attention to "positive rays"—positively charged ions. By 1912, using his deflection techniques and measuring the charge to mass ratio, he had shown that neon was a mixture of at least two kinds of atoms, with differing deflectibilities. Thomson had thus opened the door to the world of isotopes and had provided a beginning for the method of analysis now known as mass spectrography.
During his career Thomson published 13 books and over 200 papers. He was awarded the Nobel Prize in physics in 1906 and was knighted in 1908. In 1918 he abandoned research to become master of Trinity College, where he died on Aug. 30, 1940.
Further Reading
Thomson's Recollections and Reflections (1936) is one of the notable scientific autobiographies. A full-length biography is Robert J. S. Rayleigh, The Life of J. J. Thomson (1942). The sketch in James G. Crowther, British Scientists of the Twentieth Century (1952), is excellent. □