J. J. Thomson

Sir Joseph John Thomson (1856–1940) was a distinguished British physicist. He was awarded the 1906 Nobel Prize in Physics for his significant theoretical and experimental contributions to understanding the conduction of electricity through gases. In 1897, Thomson demonstrated that cathode rays consisted of previously unidentified negatively charged particles, subsequently named electrons. His calculations indicated these particles possessed a mass considerably smaller than atoms and an exceptionally high charge-to-mass ratio. The discovery of the electron marked the identification of the first subatomic particle.

Sir Joseph John Thomson (18 December 1856 – 30 August 1940) was a British physicist. He received the 1906 Nobel Prize in Physics "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases." In 1897, he showed that cathode rays were composed of previously unknown negatively charged particles (now called electrons), which he calculated must have bodies much smaller than atoms and a very large charge-to-mass ratio. The electron was the first subatomic particle to be discovered.

Thomson is also credited with providing the initial evidence for isotopes of stable (non-radioactive) elements in 1912, a finding that emerged from his investigations into the composition of canal rays (positive ions). His collaborative experiments with Francis William Aston, aimed at characterizing positively charged particles, constituted the inaugural application of mass spectrometry and subsequently facilitated the invention of the mass spectrograph.

As an influential educator, Thomson mentored seven students who later became Nobel laureates: Ernest Rutherford (Chemistry, 1908), Lawrence Bragg (Physics, 1915), Charles Barkla (Physics, 1917), Francis Aston (Chemistry, 1922), Charles Thomson Rees Wilson (Physics, 1927), Owen Richardson (Physics, 1928), and Edward Appleton (Physics, 1947). Furthermore, his son, George Paget Thomson, jointly received the 1937 Nobel Prize in Physics with Clinton Davisson for their experimental demonstration of electron diffraction by crystals.

Biography

Joseph John Thomson was born on December 18, 1856, in Cheetham Hill, Manchester. His mother, Emma Swindells, originated from a prominent local textile family. His father, Joseph James Thomson, managed an antiquarian bookshop established by Thomson's great-grandfather. Joseph John had a younger brother, Frederick Vernon Thomson. Thomson maintained a reserved yet devout Anglican faith.

Education

Thomson's initial education took place in small private institutions, where he exhibited exceptional aptitude and a keen interest in scientific inquiry. In 1870, at the remarkably young age of 14, he gained admission to Owens College in Manchester (now the University of Manchester). There, he was significantly influenced by Balfour Stewart, Professor of Physics, who introduced him to the field of physical research. He commenced experiments on contact electrification, promptly publishing his inaugural scientific paper. His parents had intended for him to apprentice as an engineer at Sharp, Stewart & Co, a locomotive manufacturer; however, these plans were curtailed by his father's death in 1873.

In 1876, Thomson matriculated at Trinity College, Cambridge. He earned his B.A. in mathematics in 1880, achieving the distinction of Second Wrangler in the Tripos and 2nd Smith's Prizeman. The subsequent year, he successfully applied for and was appointed a Fellow of Trinity College. He completed his M.A. in 1883, also receiving the Adams Prize.

Career

On December 22, 1884, Thomson was appointed Cavendish Professor of Physics at the University of Cambridge. This appointment generated considerable surprise, as other candidates, including Osborne Reynolds and Richard Glazebrook, possessed greater age and laboratory experience. In contrast, Thomson was primarily recognized for his mathematical contributions and exceptional intellectual prowess.

Thomson received a knighthood in 1908 and was inducted into the Order of Merit in 1912. At Oxford, he delivered the 1914 Romanes Lecture, entitled The Atomic Theory. In 1918, he assumed the position of Master of Trinity College, Cambridge, which he occupied until his demise on August 30, 1940. His remains are interred in Westminster Abbey, alongside those of Isaac Newton and his former student, Ernest Rutherford.

Rutherford subsequently succeeded him as Cavendish Professor. Notably, six of Thomson's research assistants and junior colleagues—Charles Glover Barkla, Niels Bohr, Max Born, William Henry Bragg, Owen Willans Richardson, and Charles Thomson Rees Wilson—were awarded the Nobel Prize in Physics, while two others, Francis William Aston and Ernest Rutherford, received the Nobel Prize in Chemistry. His son, George Paget Thomson, was also a recipient of the 1937 Nobel Prize in Physics for experimentally demonstrating the wave-like properties of electrons.

Research

Early Research

Thomson's award-winning master's thesis, entitled Treatise on the motion of vortex rings, reflects his nascent interest in atomic structure. Within this work, Thomson provided a mathematical description of the dynamics inherent in Lord Kelvin's vortex theory of the atom.

Thomson authored numerous publications that explored both the theoretical and empirical aspects of electromagnetism. His research encompassed an analysis of James Clerk Maxwell's electromagnetic theory of light, the introduction of the concept of electromagnetic mass for charged particles, and a demonstration that a charged body in motion would exhibit an apparent increase in mass.

A significant portion of Thomson's contributions to the mathematical modeling of chemical processes is recognized as foundational to early computational chemistry. In a subsequent publication, the book titled Applications of dynamics to physics and chemistry (1888), Thomson theoretically and mathematically investigated energy transformation, positing that all energy could potentially be kinetic. His subsequent volume, Notes on recent researches in electricity and magnetism (1893), expanded upon Maxwell's seminal work, Treatise upon electricity and magnetism, and was occasionally dubbed "the third volume of Maxwell." This book underscored physical methodologies and experimental approaches, featuring numerous illustrations and diagrams of experimental equipment, particularly those related to the conduction of electricity through gases. His third book, Elements of the mathematical theory of electricity and magnetism (1895), served as an accessible introduction to diverse topics and gained considerable acclaim as an academic textbook.

In 1896, Thomson delivered a series of four lectures during a Additionally, he presented a six-lecture series at Yale University in 1904.

The Electron's Discovery

Prior to Thomson's work, scientists like William Prout and Norman Lockyer had theorized that atoms comprised a more fundamental constituent, which they believed to be comparable in size to the smallest atom, hydrogen. However, in 1897, Thomson became the first to propose that a fundamental atomic unit was over 1,000 times smaller than an atom, thereby introducing the concept of the subatomic particle now identified as the electron. This groundbreaking insight emerged from his investigations into the characteristics of cathode rays. On April 30, 1897, Thomson advanced his hypothesis after observing that cathode rays (then referred to as Lenard rays) traversed air significantly further than anticipated for particles of atomic dimensions. He determined the mass of cathode rays by quantifying the heat produced upon their impact with a thermal junction and correlating this measurement with the magnetic deflection of the rays. His experimental findings indicated that cathode rays were not only more than 1,000 times less massive than a hydrogen atom but also possessed a consistent mass irrespective of their atomic origin. Consequently, he deduced that these rays consisted of extremely light, negatively charged particles, which served as universal atomic building blocks. Thomson initially termed these particles "corpuscles," but the scientific community subsequently adopted the designation "electron," a term proposed by George Johnstone Stoney in 1891, predating Thomson's discovery.

By April 1897, Thomson had only preliminary evidence suggesting the electrical deflectability of cathode rays, a phenomenon previously doubted by researchers such as Heinrich Hertz. One month following his announcement of the corpuscle, Thomson successfully demonstrated that cathode rays could be consistently deflected by an electric field, provided the discharge tube was evacuated to an exceptionally low pressure. Through a comparative analysis of cathode ray deflection by both electric and magnetic fields, he acquired more precise measurements of the mass-to-charge ratio, which corroborated his earlier estimations. This methodology subsequently became the standard technique for determining the electron's charge-to-mass ratio. In 1899, he further quantified the electron's charge as approximately 6.8×10⁻¹⁰ esu.

Thomson posited that these corpuscles originated from the atoms of the residual gas within his cathode-ray tubes. This led him to conclude that atoms were not indivisible but rather composed of these fundamental corpuscles. In 1904, Thomson proposed an atomic model, theorizing that the atom consisted of a sphere of positive matter where electrostatic forces governed the arrangement of the corpuscles. To account for the atom's overall electrical neutrality, he suggested that the corpuscles were dispersed within a homogeneous expanse of positive charge. In this "plum pudding model," electrons were conceptualized as being embedded within the positive charge, akin to raisins in a plum pudding, though in Thomson's formulation, they were not static but in rapid orbital motion.

Thomson's discovery coincided with Walter Kaufmann and Emil Wiechert's determination of the accurate mass-to-charge ratio for these cathode rays, later identified as electrons.

The scientific community adopted the designation electron for these particles, largely influenced by the advocacy of George Francis FitzGerald, Joseph Larmor, and Hendrik Lorentz. George Johnstone Stoney initially coined this term in 1891 as a provisional name for the fundamental unit of electrical charge, which remained undiscovered at that time. For several years, Thomson opposed the use of "electron" due to his disagreement with physicists who referred to a "positive electron" as the elementary unit of positive charge, mirroring the "negative electron" as the elementary unit of negative charge. Thomson consistently favored "corpuscle," which he rigorously defined as negatively charged. By 1914, he eventually conceded, incorporating the term "electron" into his publication, The Atomic Theory. In 1920, Rutherford and his colleagues collectively decided to name the nucleus of the hydrogen ion "proton," thereby establishing a distinct nomenclature for the smallest known independently existing positively-charged particle of matter.

Isotopes and Mass Spectrometry

In 1912, during an investigation into the composition of positively charged particles, then referred to as canal rays, Thomson and his research assistant, F. W. Aston, directed a stream of neon ions through both magnetic and electric fields. They subsequently measured its deflection by positioning a photographic plate in its trajectory. The observation of two distinct light patches on the photographic plate indicated two different parabolic deflections, leading to the conclusion that neon comprises atoms of two varying atomic masses (neon-20 and neon-22), thus representing two isotopes. This groundbreaking finding constituted the initial empirical evidence for isotopes of a stable element; Frederick Soddy had earlier theorized the existence of isotopes to elucidate the decay mechanisms of specific radioactive elements.

Thomson's successful separation of neon isotopes based on their mass represented the inaugural application of mass spectrometry. This technique was later refined and expanded into a comprehensive methodology by F. W. Aston and A. J. Dempster.

Experiments Involving Cathode Rays

Previously, physicists engaged in discourse regarding the nature of cathode rays, questioning whether they were immaterial, akin to light (described as "some process in the aether"), or, as Thomson posited, "in fact wholly material, and ... mark the paths of particles of matter charged with negative electricity." While the aetherial hypothesis lacked specificity, the particle hypothesis offered sufficient clarity for Thomson to subject it to empirical investigation.

Magnetic Deflection

Thomson initiated his research by examining the magnetic deflection of cathode rays. These rays were generated within a side tube positioned on the left of the experimental apparatus, subsequently traversing the anode and entering the primary bell jar, where a magnet caused their deflection. Thomson traced the trajectory of these rays by observing the fluorescence produced on a gridded screen inside the jar. His findings indicated that the deflection of the rays remained constant, irrespective of the anode material or the gas present in the jar, thereby implying a consistent form for the rays regardless of their source.

Electrical Charge

Adherents of the aetherial theory acknowledged the potential for negatively charged particles to be generated within Crookes tubes; however, they contended that these particles were merely incidental by-products and that the cathode rays themselves possessed an immaterial nature. Thomson undertook an inquiry to ascertain the feasibility of isolating the electrical charge from the rays.

Thomson engineered a Crookes tube incorporating an electrometer positioned laterally, outside the direct trajectory of the cathode rays. He was able to delineate the ray's path by observing the phosphorescent luminescence it generated upon striking the tube's surface. Thomson noted that the electrometer recorded an electrical charge exclusively when he magnetically diverted the cathode ray towards it. This observation led him to conclude that the negative charge and the rays were intrinsically linked.

Electrical Deflection

During May and June of 1897, Thomson conducted experiments to determine if cathode rays could be deflected by an electric field. Although prior researchers had been unsuccessful in observing such deflection, Thomson attributed their failures to experimental deficiencies, specifically the excessive gas pressure within their vacuum tubes.

Thomson engineered a Crookes tube with a superior vacuum. The tube's initial section housed a cathode, which projected the rays. These rays were collimated into a focused beam by two metallic slits; the first slit also served as the anode, while the second was grounded. The beam then proceeded between two parallel aluminum plates, which, when connected to a battery, established an electric field. The tube concluded with a large spherical section where the beam's impact on the glass produced a glowing patch. Thomson affixed a scale to this sphere's surface to measure the beam's deflection. Previous experiments encountered an issue where electron beams colliding with residual gas atoms within a Crookes tube would ionize them, creating a space charge of electrons and ions that electrically screened externally applied electric fields. In contrast, Thomson's Crookes tube featured such a low density of residual atoms that the space charge generated was insufficient to screen the external electric field, thus allowing him to successfully observe electrical deflection.

Connecting the upper plate to the battery's negative terminal and the lower plate to its positive terminal resulted in a downward displacement of the luminous patch. Conversely, reversing the polarity caused the patch to shift upwards.

Determination of the Mass-to-Charge Ratio

In his seminal experiment, Thomson determined the mass-to-charge ratio of cathode rays by quantifying their deflection in a magnetic field and comparing this with their electric deflection. He employed the identical apparatus from his previous experiment, but positioned the discharge tube between the poles of a large electromagnet. His results revealed that the mass-to-charge ratio was over a thousand times lower than that of a hydrogen ion (H⁺), suggesting that the particles were either exceptionally light, highly charged, or both. Significantly, cathode rays originating from every cathode consistently yielded the same mass-to-charge ratio. This finding stands in contrast to anode rays, now recognized as positive ions emitted from the anode, whose mass-to-charge ratio varies depending on the anode material. Thomson himself remained circumspect about the implications of his work, referring to these entities as 'corpuscles' rather than 'electrons' in his Nobel Prize acceptance speech.

Thomson's calculations are summarized below, employing his original notation where F represents the electric field and H denotes the magnetic field:

The electric deflection is quantified by the following expression: ${\displaystyle \Theta =Fel/mv^{2}}$, where Θ signifies the angular electric deflection, F is the applied electric field intensity, e represents the charge of the cathode ray particles, l is the length of the electric plates, m denotes the mass of the cathode ray particles, and v is their velocity. The magnetic deflection is given by: ${\displaystyle \phi =Hel/mv}$, where φ is the angular magnetic deflection and H is the applied magnetic field intensity.

The magnetic field was adjusted until the magnetic and electric deflections achieved equivalence, at which point the relationship ${\displaystyle \Theta =\phi ,Fel/mv^{2}=Hel/mv}$ was established. This equation can be simplified to yield ${\displaystyle m/e=H^{2}l/F\Theta }$. Since the electric deflection (Θ) and magnetic field strength (H) were independently measured, and the electric force (F) and length (l) were known constants, the mass-to-charge ratio (m/e) could be precisely calculated.

Conclusions

Given that cathode rays possess a negative electrical charge, exhibit deflection by an electrostatic force consistent with negative electrification, and respond to a magnetic force precisely as a negatively charged entity moving along their trajectory would, the inescapable conclusion is that these rays represent negative electrical charges conveyed by material particles.

Regarding the origin of these particles, Thomson theorized that they emanated from gas molecules situated near the cathode.

He posited that if, within the exceptionally strong electric field adjacent to the cathode, gas molecules undergo dissociation and fragment not into conventional chemical atoms but into these fundamental "primordial atoms"—termed corpuscles for conciseness—and if these corpuscles are electrically charged and propelled from the cathode by the electric field, their behavior would precisely mirror that of cathode rays.

Thomson conceptualized the atom as comprising these corpuscles orbiting within a diffuse sphere of positive charge, a model famously known as the plum pudding model. This hypothesis was subsequently disproven when his student, Ernest Rutherford, demonstrated that the atom's positive charge is, in fact, concentrated within a central nucleus.

Additional Research

In 1905, Thomson identified the inherent radioactivity of potassium.

By 1906, Thomson had experimentally established that each hydrogen atom possesses only one electron, a finding that contradicted earlier theoretical frameworks proposing variable electron counts.

Between 1916 and 1918, Thomson presided over the "Committee appointed by the Prime Minister to enquire into the Position of Natural Science in the Educational System of Great Britain." The committee's findings, released in 1918, became widely recognized as the Thomson Report.

Personal Life

In 1890, Thomson wed Rose Elisabeth Paget at St Mary the Less church. Rose, the daughter of Sir George Edward Paget, a distinguished physician and later Regius Professor of Physic at Cambridge, harbored an interest in physics. From 1882 onwards, women were permitted to attend demonstrations and lectures at the University of Cambridge. Rose's attendance at these sessions, including those delivered by Thomson, ultimately fostered their relationship.

The couple had two children: George Paget Thomson, who subsequently received a Nobel Prize for his research on the electron's wave properties, and Joan Paget Thomson (later Charnock), who pursued a career as an author, producing children's literature, non-fiction works, and biographies.

Honors and Distinctions

Memberships

Awards

Commemorations

In November 1927, Thomson inaugurated the Thomson building at The Leys School, Cambridge, which was named in his honor.

In 1991, the thomson (symbol: Th) was proposed as a unit for quantifying the mass-to-charge ratio in mass spectrometry, acknowledging his contributions.

J. J. Thomson Avenue, located within the University of Cambridge's West Cambridge campus, bears his name.

The Thomson Medal Award, supported by the International Mass Spectrometry Foundation, is named in honor of Thomson.

The Institute of Physics Joseph Thomson Medal and Prize also commemorates Thomson.

Thomson Crescent in Deep River, Ontario, intersects with Rutherford Avenue.

• History of physics

References

In 1883, A Treatise on the Motion of Vortex Rings: An essay to which the Adams Prize was adjudged in 1882, in the University of Cambridge was published by Macmillan and Co. in London, spanning 146 pages. A recent reprint is available with ISBN 0-543-95696-2.

• 1883. A Treatise on the Motion of Vortex Rings: An essay to which the Adams Prize was adjudged in 1882, in the University of Cambridge. London: Macmillan and Co., pp. 146. Recent reprint: ISBN 0-543-95696-2.
• In 1888, Applications of Dynamics to Physics and Chemistry was released by Macmillan and Co. in London, comprising 326 pages. A recent reprint bears ISBN 1-4021-8397-6.
• In 1893, Notes on recent researches in electricity and magnetism: intended as a sequel to Professor Clerk-Maxwell's 'Treatise on Electricity and Magnetism' was published by Oxford University Press, totaling xvi and 578 pages. A 1991 Cornell University Monograph edition is available with ISBN 1-4297-4053-1.
• Thomson, Joseph John (1893) authored Notes on recent researches in electricity and magnetism, published in Oxford by Clarendon Press.
• Thomson, Joseph John (1900) published the German edition of Discharge of electricity through gases in Leipzig, through Johann Ambrosius Barth.
• Thomson, Joseph John (1904) authored the English edition of Electricity and matter, published in Oxford by Clarendon Press.
• Thomson, Joseph John (1905) released the Italian edition of Electricity and matter, published in Milano by Hoepli.
• Thomson, Joseph John (1908) published the German edition of Corpuscular theory of matter in Braunschweig, through Vieweg und Sohn.
• In 1921, a reprint of the 1895 edition of Elements of the Mathematical Theory of Electricity And Magnetism was published by Macmillan and Co. in London, based on a scan of the original.
• A Text book of Physics in Five Volumes, co-authored with J.H. Poynting, includes: (1) Properties of Matter, (2) Sound, (3) Heat, (4) Light, and (5) Electricity and Magnetism. This series was initially dated 1901 and subsequently released in revised editions.
• Dahl, Per F. (1997) authored Flash of the Cathode Rays: A History of J J Thomson's Electron, published in Bristol and Philadelphia by Institute of Physics Publishing, with ISBN 0-7503-0453-7.
• J.J. Thomson (1897) published "Cathode Rays" in The Electrician (volume 39, page 104), and it was also featured in Proceedings of the Royal Institution on 30 April 1897, pages 1–14. This publication marked the initial announcement of the "corpuscle," preceding the definitive mass and charge experiment.
• J.J. Thomson (1897) published "Cathode rays" in Philosophical Magazine (volume 44, page 293), detailing the seminal measurement of the electron's mass and charge.
• J.J. Thomson (1904) authored "On the Structure of the Atom: an Investigation of the Stability and Periods of Oscillation of a number of Corpuscles arranged at equal intervals around the Circumference of a Circle; with Application of the Results to the Theory of Atomic Structure," published in Philosophical Magazine, Series 6, Volume 7, Number 39, pages 237–265. This seminal paper introduced the classical "plum pudding model," which subsequently led to the formulation of the Thomson Problem.
• J. J. Thomson (1906) published "On the Number of Corpuscles in an Atom" (PDF) in Philosophical Magazine, Series 6, Volume 11 (issue 66), pages 769–781. The article's DOI is 10.1080/14786440609463496.
• Joseph John Thomson (1908) published On the Light Thrown by Recent Investigations on Electricity on the Relation Between Matter and Ether: The Adamson Lecture Delivered at the University on November 4, 1907 through University Press.
• J.J. Thomson (1912) published "Further experiments on positive rays" in Philosophical Magazine (volume 24, pages 209–253), which contained the initial announcement of the two neon parabolae.
• J.J. Thomson (1913) authored "Rays of positive electricity," published in Proceedings of the Royal Society, Series A, volume 89, pages 1–20, detailing the discovery of neon isotopes.
• J.J. Thomson (1923) published The Electron in Chemistry: Being Five Lectures Delivered at the Franklin Institute, in Philadelphia.
• Thomson, Sir J. J. (1936) published Recollections and Reflections in London with G. Bell & Sons, Ltd. This work was subsequently republished as a digital edition by Cambridge University Press in 2011, as part of the Cambridge Library Collection series.
• Thomson, George Paget (1964) authored J.J. Thomson: Discoverer of the Electron, published in Great Britain by Thomas Nelson & Sons, Ltd.
• Davis, Edward Arthur and Falconer, Isobel (1997) co-authored J.J. Thomson and the Discovery of the Electron, with ISBN 978-0-7484-0696-8.
• Falconer, Isobel (1988) published "J.J. Thomson's Work on Positive Rays, 1906–1914" in Historical Studies in the Physical and Biological Sciences, volume 18, issue 2, pages 265–310.
• Falconer, Isobel (2001) contributed "Corpuscles to Electrons" to Histories of the Electron, edited by J. Buchwald and A. Warwick, published by MIT Press in Cambridge, Massachusetts, on pages 77–100.
• Navarro, Jaume (2005). "J. J. Thomson on the Nature of Matter: Corpuscles and the Continuum." Centaurus, vol. 47, no. 4, pp. 259–282. Bibcode:2005Cent...47..259N. doi:10.1111/j.1600-0498.2005.00028.x.
• Downard, Kevin M. (2009). "J. J. Thomson Goes to America." Journal of the American Society for Mass Spectrometry, vol. 20, no. 11, pp. 1964–1973. Bibcode:2009JASMS..20.1964D. doi:10.1016/j.jasms.2009.07.008. PMID 19734055. S2CID 34371775.

• The Discovery of the Electron Archived 16 March 2008 at the Wayback Machine
• Annotated bibliography for Joseph J. Thomson from the Alsos Digital Library for Nuclear Issues
• The Cathode Ray Tube site
• Photos of some of Thomson's remaining apparatus at the Cavendish Laboratory Museum
• Works by J. J. Thomson at Project Gutenberg
• A history of the electron: JJ and GP Thomson published by the University of the Basque Country (2013)