J. J. Thomson

How to use this course

J. J. Thomson trained as a mathematical physicist and became the head of the most productive experimental laboratory of his age. In 1897 he showed that cathode rays are streams of particles far lighter than any atom, and that the same particles come out of every kind of matter. He then tried to build atoms from them, measured their charge, and turned the deflection of positive ions into the first mass spectrometer. For 35 years he directed the Cavendish Laboratory in Cambridge, where a remarkable number of the next generation of physicists learned their trade.

This course follows his physics roughly in the order he did it, from the theoretical paper of 1881 to the isotopes of neon in 1913. Each result is derived with modern tools and set beside the evidence Thomson actually had.

Each technical lesson opens with a paragraph for the general reader, then gives precise statements, derivations and exercises with folded solutions. Lesson 05, the measurement of \(e/m\), is the centre of the course. Readers who have taken the companion course on Ernest Rutherford will meet the same ionization chamber in Lesson 06 and the end of the Thomson atom in Lesson 08.

Timeline

Manchester, 1856–1876

Joseph John Thomson was born on 18 December 1856 at Cheetham Hill, a suburb of Manchester. His father, also named Joseph James Thomson, ran an antiquarian bookshop that had been in the family for generations. His mother, Emma Swindells, came from a local textile family. He had one brother, Frederick Vernon, two years younger.

His parents intended him for engineering. The plan was an apprenticeship with Sharp, Stewart and Company, a firm of locomotive builders, and in the meantime, in 1870, he was sent to Owens College, the forerunner of the University of Manchester. He was 14, unusually young even for that time. His father’s death in 1873 put the apprenticeship premium out of reach, and he stayed at Owens on scholarships.

Owens College

Owens was small but strong in science. Balfour Stewart, the professor of physics, drew him into experimental research; his first paper, on contact electricity between insulators, came out of that laboratory. Osborne Reynolds, the professor of engineering, was another teacher he remembered. Above all Thomson acquired a solid training in mathematics, and his teachers urged him to try for Cambridge, where mathematical physics was then taught more thoroughly than anywhere in Britain.

He won a minor scholarship to Trinity College, and went up in 1876. He never left Cambridge for long again.

Trinity and the Tripos, 1876–1884

Cambridge mathematics in the 1870s meant the Mathematical Tripos, a punishing written examination that ranked every candidate in order. Success required a private coach, and the most successful coach of the century was Edward Routh, who prepared Thomson. In January 1880 Thomson was Second Wrangler, behind Joseph Larmor, and second Smith’s Prizeman. He was soon elected a Fellow of Trinity, and in 1883 became a lecturer in the college.

A theorist of fields and vortices

Thomson’s early research was theoretical and grew from the Treatise on Electricity and Magnetism of James Clerk Maxwell, published in 1873. In 1881 he asked what happens to the field of a charged sphere when it moves. The answer was that the moving charge carries magnetic energy with it, and so behaves as if its mass were increased. This is electromagnetic mass, the subject of Lesson 04, and it later made him receptive to the idea that matter might be electrical in nature.

His Adams Prize essay, judged in 1882 and published in 1883 as A Treatise on the Motion of Vortex Rings, took up William Thomson’s hypothesis that atoms are knotted vortex rings in an ideal fluid. Thomson (no relation to Kelvin) studied the stability of linked rings and suggested how chemical combination might arise from their interactions. The vortex atom failed, but two habits from this work stayed with him: picturing atoms as dynamical systems of interacting parts, and asking which arrangements of those parts are stable. Both return in the plum-pudding atom of 1904.

He also began experimental work in the Cavendish Laboratory, then directed by Lord Rayleigh. His Applications of Dynamics to Physics and Chemistry (1888) tried to derive physical and chemical laws from Lagrangian dynamics alone.

The Cavendish chair, 1884–1896

The Cavendish Laboratory opened in 1874 with Maxwell as its first professor. Rayleigh succeeded him in 1879 and resigned at the end of 1884. On 22 December 1884 the electors chose Thomson, aged 28. The choice surprised many. Older candidates, among them Osborne Reynolds and Richard Glazebrook, had far more experience of running laboratories, and Thomson was known chiefly as a mathematician. He had been elected a Fellow of the Royal Society the same year.

He chose as his field the passage of electricity through gases, a subject full of striking phenomena and almost without theory. Glowing discharges, cathode rays and sparks seemed to obey no simple law. He held that understanding conduction in gases would reveal how electricity is attached to matter, and he worked on it for the next thirty years.

Family and books

In 1890 he married Rose Elisabeth Paget, daughter of Sir George Paget, Regius Professor of Physic at Cambridge. Their son George Paget Thomson was born in 1892, and a daughter, Joan, later. In 1893 Thomson published Notes on Recent Researches in Electricity and Magnetism, a sequel to Maxwell’s Treatise, and in 1895 Elements of the Mathematical Theory of Electricity and Magnetism.

Research students

In 1895 Cambridge began admitting graduates of other universities to work for a degree by research. The first arrivals at the Cavendish included Ernest Rutherford from New Zealand and John Townsend from Dublin. Their timing was lucky. Wilhelm Röntgen announced X-rays at the end of 1895, and X-rays made gases conduct in a controlled way. Thomson and Rutherford measured that conduction in 1896 (Lesson 06). In the autumn of 1896 Thomson lectured at Princeton; the lectures became The Discharge of Electricity through Gases (1897). The following spring he settled the nature of cathode rays.

Electromagnetic mass

For the general reader. A moving electric charge creates a magnetic field, and a magnetic field stores energy. So to set a charged ball moving you must supply energy to its field as well as to the ball itself. The ball behaves as if it were heavier than it is. Thomson worked this out in 1881. Twenty years later many physicists hoped that all mass might be of this electrical kind.

The calculation

Thomson’s 1881 paper, On the electric and magnetic effects produced by the motion of electrified bodies, considered a charged sphere moving through Maxwell’s ether. His numerical coefficient was in error; George FitzGerald and Oliver Heaviside corrected it, and Heaviside found the full field of a uniformly moving charge in 1888–89. The modern version for slow motion is short.

Take a charge \(e\) spread uniformly over the surface of a sphere of radius \(a\), moving with velocity \(v\ll c\). Outside the sphere the electric field is that of a point charge, and the magnetic field is given to first order in \(v\) by the Biot–Savart law,

where \(\theta\) is the angle between \(\mathbf v\) and \(\hat{\mathbf r}\). Inside the shell the field vanishes. The magnetic energy is

The force needed to accelerate the sphere includes \(m_{\mathrm{em}}\dot{\mathbf v}\): the field resists changes of velocity, which is what inertia means. The smaller the sphere, the larger the effect.

Velocity dependence and the 4/3 problem

In 1893 Thomson noted that the electromagnetic mass must depend on speed and grow without limit as \(v\to c\). George Searle computed the energy of a rapidly moving charged sphere exactly in 1897. Once the electron was found, the idea that its mass might be entirely electromagnetic became attractive. Walter Kaufmann measured the deflection of fast beta rays from 1901 onward to test it, and Max Abraham and Hendrik Lorentz built rival models of the electron.

The models had a flaw visible in the theorem above. If mass were \(U_E/c^2\), the coefficient would be 1, not \(\tfrac43\). The momentum and energy of the field do not transform together like those of a particle, because a charged shell is not stable: its parts repel. Henri Poincaré showed in 1905–06 that non-electromagnetic stresses holding the shell together restore consistency. Special relativity then made \(E=mc^2\) a property of all energy, and electromagnetic mass became a contribution to mass, not its origin. The classical electron radius \(r_e=e^2/(4\pi\varepsilon_0m_ec^2)\approx2.82\) fm is a remnant of this programme, and the divergence of the self-energy as \(a\to0\) reappears in quantum electrodynamics as the problem of mass renormalization.

Cathode rays and e/m

For the general reader. When electricity passes through a glass tube pumped nearly empty of air, a beam leaves the negative electrode and makes the glass glow where it strikes. For twenty years physicists argued whether these cathode rays were waves in the ether or a stream of particles. In 1897 Thomson bent the beam with both electric and magnetic forces and measured the ratio of mass to charge of the particles. It was more than a thousand times smaller than for the lightest charged atom, and it was the same whatever metal or gas he used. He concluded that the rays are made of a constituent common to all atoms, which we now call the electron.

The debate

William Crookes in England held that cathode rays were charged molecules. Most German physicists thought they were a disturbance in the ether. Heinrich Hertz had found in 1883 that the rays were not deflected by an electric field between plates inside the tube, and later that they pass through thin metal foil. Philipp Lenard built a tube with a thin aluminium window and followed the rays several centimetres into the open air, which seemed impossible for molecules. On the other side, Jean Perrin showed in 1895 that the rays carry negative charge into a collector, and Arthur Schuster had estimated their charge-to-mass ratio from magnetic bending. In January 1897 Emil Wiechert reported that the mass-to-charge ratio was more than a thousand times smaller than for a charged atom.

Thomson’s three experiments

Charge follows the rays. Thomson repeated Perrin’s experiment with the collector placed off the axis of the tube. It charged negatively only when a magnet bent the rays into it. The negative charge and the rays could not be separated.

Electric deflection. With a much better vacuum, the rays bent towards the positive plate. Thomson explained Hertz’s failure: at higher pressure the rays ionize the residual gas, which then conducts and screens the plates.

Measurement of \(m/e\). He used two methods, described below, with tubes containing air, hydrogen and carbon dioxide and with cathodes of different metals. The ratio did not depend on any of them.

Method 1: charge, heat and bending

Let \(N\) particles of mass \(m\), charge \(e\) and speed \(v\) strike a collector in some interval. A thermocouple measures the heat they deliver, \(W=\tfrac12Nmv^2\), and an electrometer the charge, \(Q=Ne\). In a magnetic field \(B\) the beam bends with radius \(\rho\), where \(mv=eB\rho\). Eliminating \(N\) and \(v\):

Method 2: crossed fields

The beam passes between parallel plates of length \(L\) carrying a transverse electric field \(E\), and then travels a distance \(D\) to the glass. A magnetic field \(B\) perpendicular to both the beam and \(E\) can be applied over the same region.

Derivation. Balance requires \(eE=evB\). With \(B\) off, the transit time through the plates is \(L/v\) and the transverse acceleration \(eE/m\), so the displacement at the exit is \(\tfrac12(eE/m)(L/v)^2\) and the exit slope is \(eEL/(mv^2)\). Over the field-free distance \(D\) the beam moves a further \(D\,eEL/(mv^2)\). Adding the two gives the formula. Thomson neglected the fringing fields at the ends of the plates, which affects \(L\), and the magnetic field of his coils extended beyond the plates. His values from the two methods lay within a factor of about three of the modern one, \(e/m_e=1.759\times10^{11}\) C kg\(^{-1}\), and both far above the electrolytic value for hydrogen, about \(9.6\times10^{7}\) C kg\(^{-1}\).

The interpretation

A small \(m/e\) could mean a large charge or a small mass. Lenard’s range measurements favoured a small mass: the particles travel far through gases that stop molecules quickly. Thomson proposed that cathode rays are matter in a new state, subdivided far beyond ordinary atoms, and that this matter is a constituent of all the chemical elements. He called the particles corpuscles and used that word for years; the name electron, coined by George Johnstone Stoney in 1891 for the unit of charge, won out, and Thomson adopted it by 1914. He announced the result at the Royal Institution on 30 April 1897 and published it in the Philosophical Magazine in October. Walter Kaufmann in Berlin obtained similar values in 1897 but did not draw the same conclusion. Proof that the particles are light required an independent measurement of \(e\) (Lesson 07).

Conduction of electricity in gases

For the general reader. Air is normally an excellent insulator. Shine X-rays through it, though, and it conducts. Thomson and his students showed that the radiation splits gas molecules into positive and negative ions, which drift in an electric field and recombine when left alone. This simple picture explained the strange shape of the current–voltage curve and gave physics its first reliable radiation detector. The Nobel Prize of 1906 was awarded for this body of work.

Ions in a gas

In 1896 Thomson and Rutherford exposed air between two parallel plates to X-rays and measured the current as the voltage was raised. At low voltage the current was proportional to the voltage. At higher voltage it levelled off at a saturation value that no further increase could raise. The air kept its conductivity for a short time after the X-rays were switched off, and lost it if the gas was blown through glass wool or a strong field was applied.

Their model: radiation produces ion pairs at a rate \(q\) per unit volume per second. Positive and negative ions recombine at a rate proportional to the product of their concentrations, \(\alpha n_+n_-\). In an electric field they drift with velocities \(k_\pm E\), where \(k_\pm\) are the mobilities.

Derivation. The decay law follows by separating variables: \(dn/n^2=-\alpha\,dt\). At small voltages the ions are swept out much more slowly than they recombine, so the concentration stays near \(n_\infty\) and the current density is \(n_\infty e(k_++k_-)E\), proportional to \(V\). At large voltages each ion reaches a plate before it can meet a partner, so the current equals the charge of one sign produced per second in the whole volume \(Ad\). The crossover occurs when the drift time \(d/(kE)\) becomes comparable to the recombination time \(\tau=1/(\alpha n_\infty)=1/\sqrt{q\alpha}\).

The saturation current is independent of the field and directly proportional to the ionization rate, which made the ionization chamber a quantitative instrument. Rutherford took it to McGill and used it to sort the radiations of uranium. Paul Langevin, who worked at the Cavendish in 1897–98, went on to develop the theory of ion recombination and mobility. Thomson collected the field in Conduction of Electricity through Gases (1903; second edition 1906), and he and his son George revised it in two volumes in 1928–33.

Beyond the weak field

At still higher fields the current rises again, because the ions, especially the electrons, gain enough energy between collisions to ionize molecules themselves. John Townsend worked out this regime from 1900 onward (Lesson 10). It explains the spark, the glow discharge and, later, the Geiger counter.

The charge of the electron

For the general reader. Knowing only the ratio of charge to mass, Thomson could not yet say that the corpuscle was light. He needed its charge. He found it by a clever indirect route: in damp air each ion becomes the seed of a tiny water droplet, so counting droplets counts ions. The charge came out close to that of a hydrogen ion in electrolysis. The mass of the corpuscle therefore had to be about a thousandth of a hydrogen atom.

Wilson’s clouds

C. T. R. Wilson had been studying cloud formation at the Cavendish since 1895. He found that when moist, dust-free air is expanded suddenly, it cools and becomes supersaturated, and droplets then form on ions if the expansion exceeds a certain ratio. John Townsend in 1897 estimated the ionic charge from clouds formed on the charged gas released in electrolysis. In 1898 Thomson applied Wilson’s expansion method to air ionized by X-rays, and in 1899 to the negative carriers released from a metal plate by ultraviolet light.

Derivation. The Stokes drag on a sphere at low Reynolds number is \(6\pi\eta ru\). Setting it equal to the weight \(\tfrac43\pi r^3\rho g\) gives the result.

The method

1. Ionize a known volume and measure, with an electrometer, the current in a known weak field. With the known mobility this gives the charge per unit volume, \(ne\).
2. Expand the gas to form a cloud. From the expansion ratio and the thermodynamics of water vapour, compute the mass of water condensed per unit volume.
3. Time the fall of the top of the cloud. Stokes’ law gives the droplet radius and hence the mass per droplet.
4. Divide: the number of droplets per unit volume is \(n\). Then \(e=(ne)/n\).

Thomson obtained \(6.5\times10^{-10}\) electrostatic units for X-ray ions in 1898, and \(6.8\times10^{-10}\) esu in 1899 for the carriers released by ultraviolet light. In the 1899 paper, On the masses of the ions in gases at low pressures, he also measured \(m/e\) for these carriers by magnetic and electric deflection, and for the carriers emitted by a hot carbon filament. The values matched those for cathode rays. Since the charge matched that of the hydrogen ion in electrolysis, the mass was about \(1.4\times10^{-3}\) of a hydrogen atom.

Thomson’s values were about 40% too high, since \(1\) esu \(=3.336\times10^{-10}\) C makes \(6.8\times10^{-10}\) esu equal to \(2.3\times10^{-19}\) C. The cloud method assumed that every ion caught a drop and that every drop held one ion. Harold A. Wilson, another Cavendish student, improved the method in 1903 by watching the cloud fall with and without an electric field. Robert Millikan, starting from Wilson’s version, replaced the cloud with single oil drops and from 1909 to 1913 obtained \(e\) to a fraction of a percent.

The plum-pudding atom

For the general reader. If every atom contains electrons, and atoms are electrically neutral, something must carry the balancing positive charge. In 1904 Thomson proposed that the positive charge fills the whole atom like a jelly, with the electrons embedded in it, arranged in rings and moving in orbits. A journalist later compared this to raisins in a pudding, and the name stuck, though Thomson never used it. The model failed, but it was the first serious attempt to explain chemistry and the periodic table by the arrangement of electrons.

The model

Thomson’s paper On the structure of the atom appeared in the Philosophical Magazine in March 1904. He took the positive charge as a uniform sphere, an idea Lord Kelvin had also proposed, and asked how \(n\) electrons inside it would arrange themselves. He restricted the calculation to electrons moving in one plane, arranged in concentric rings, and found that a single ring becomes unstable if it holds too many electrons, so that larger numbers must form several rings. He illustrated the arrangements with Alfred Mayer’s experiment of magnetized needles floating in water under a large magnet: five needles form a pentagon, but six push one into the centre. The number of electrons in successive rings suggested a periodicity reminiscent of the periodic table.

Proof. By Gauss’s law only the charge inside radius \(r\) acts, namely \(Ze\,r^3/R^3\), and it acts as if concentrated at the centre. The force is \(-e\cdot Ze\,r^3/R^3/(4\pi\varepsilon_0r^2)\) along \(\hat{\mathbf r}\), which is the stated linear restoring force.

A model with a definite natural frequency was attractive because atoms emit spectral lines at definite frequencies. The difficulty was that real spectra contain many lines, related by formulas such as Balmer’s, which no arrangement of oscillating electrons reproduced.

Counting electrons

Thomson’s first estimates assumed that all the atom’s mass was electromagnetic mass of electrons, which required about a thousand or more electrons in a hydrogen atom. In 1906 he used three kinds of data, the scattering of X-rays, the absorption of beta rays and the optical dispersion of gases, to argue that the number of electrons is of the same order as the atomic weight, and that hydrogen has a single electron. The positive sphere then had to carry most of the mass. The X-ray argument rests on a formula now called Thomson scattering.

Derivation. In a wave with field amplitude \(E_0\), the electron’s acceleration is \(a=(eE_0/m_e)\cos\omega t\). The Larmor formula gives the radiated power \(P=e^2a^2/(6\pi\varepsilon_0c^3)\), with time average \(e^4E_0^2/(12\pi\varepsilon_0m_e^2c^3)\). The incident intensity is \(I=\tfrac12c\varepsilon_0E_0^2\). Then \(\sigma_T=\langle P\rangle/I=e^4/(6\pi\varepsilon_0^2m_e^2c^4)\), which equals \(\tfrac{8\pi}3r_e^2\). Each electron scatters independently, so a layer containing \(N\) electrons per unit area scatters a fraction \(N\sigma_T\). Charles Barkla’s measurements of X-ray scattering by light elements, together with this formula, gave about half the atomic weight in electrons per atom.

The end of the model

A uniform sphere of positive charge can deflect a fast alpha particle only slightly in one encounter. Thomson developed in 1910 a theory of compound scattering by many small deflections. The large-angle scattering observed by Hans Geiger and Ernest Marsden in 1909 could not be produced this way, and in 1911 Rutherford replaced the sphere by a nucleus (see the Rutherford course, Lesson 07). The question Thomson had posed, how electrons arrange themselves to produce the periodic table, was taken up by Niels Bohr in 1913. The mathematical problem of the minimum-energy arrangement of \(n\) equal charges on a sphere, which grew from the 1904 paper, is still called the Thomson problem.

Positive rays and isotopes

For the general reader. If a discharge tube makes negative particles fly one way, it makes positive ions fly the other. Thomson let these positive rays through a narrow hole and bent them with electric and magnetic fields at once. Each kind of ion landed on a photographic plate along its own parabola, so the plate showed at a glance which ions were present. When he put neon in the tube he saw two parabolas, at masses 20 and 22. Neon is a mixture of atoms of the same element but different mass: isotopes. The instrument was the first mass spectrometer.

Canal rays

Eugen Goldstein found in 1886 that rays stream backwards through holes in a perforated cathode; he called them canal rays. Wilhelm Wien showed in 1898–99 that they are positively charged and measured their charge-to-mass ratio with crossed fields. Their \(e/m\) values were of the order of electrolytic ions, so they were atoms or molecules that had lost electrons. From 1907 Thomson studied them at much lower pressures, where ions travel without collisions, and developed what he called the method of positive rays.

The parabola method

Ions pass through a fine tube in the cathode and enter a region of length \(L\) where an electric field \(E\) and a magnetic field \(B\) are both directed along the \(y\)-axis, perpendicular to the beam. A photographic plate stands a distance \(D\) beyond. The electric force deflects ions along \(y\), the magnetic force along \(x\). For small deflections, write \(K=L(L/2+D)\).

Derivation. Both deflections follow the pattern of Lesson 05: an impulse acting for time \(L/v\), followed by drift. The electric impulse is \(qEL/v\), giving \(y=(qEL/(mv^2))(L/2+D)\). The magnetic force \(qvB\) is perpendicular to both \(\mathbf v\) and \(\mathbf B\), so along \(x\), giving \(x=(qBL/(mv))(L/2+D)\). Then \(x^2=q^2B^2K^2/(m^2v^2)\) and \(y/x^2=mE/(qB^2K)\).

Ions in the discharge are produced at various points and gain various energies, so each species traces a segment of its parabola. Ions accelerated through the full cathode fall have the highest speed and the smallest deflections, which sets a sharp inner end to each segment. Thomson reversed the magnetic field halfway through an exposure to produce mirror-image parabolas, which allowed him to measure them from their common axis without locating the undeflected spot. Calibrating with known ions such as \(\mathrm H^+\), \(\mathrm O^+\) or \(\mathrm{CO}^+\), he could read off the mass of any other trace.

Neon

In 1912 the plates for neon showed, besides the strong parabola at 20, a faint one at 22. Thomson and his assistant Francis Aston considered and excluded the obvious alternatives, and Thomson announced in 1913 that atmospheric neon is a mixture of two gases. The same year Frederick Soddy, working on radioactive decay chains, coined the word isotope for chemically identical atoms of different mass. Neon was the first case among the stable elements. Thomson also identified \(\mathrm H_3^+\) in the discharge, which he called \(X_3\), a molecular ion later important in astrochemistry.

After the war Aston built a mass spectrograph, reported in 1919, which focused ions of the same \(m/q\) but different speeds onto a single point. With it he found isotopes in many elements and formulated the whole number rule: on the scale \({}^{16}\mathrm O=16\), isotopic masses are very nearly whole numbers, and non-integer chemical atomic weights are averages over isotopes. He received the Nobel Prize in Chemistry in 1922. The small departures from whole numbers, Aston’s packing fraction, measure nuclear binding energy. Every modern mass spectrometer, from the isotope-ratio instruments of geochemistry to the proteomics machines of biology, descends from Thomson’s parabola plates.

The Cavendish school

For the general reader. Thomson was clumsy with his hands but extraordinary at suggesting experiments and at seeing what results meant. Under him the Cavendish became a nursery of physicists. The instruments and ideas of his students, the cloud chamber, the theory of gas discharges, thermionic emission and X-ray diffraction, became the working tools of twentieth-century physics.

The laboratory

Thomson directed the Cavendish from 1884 to 1919. After the research-student reform of 1895 it attracted graduates from across the British Empire, continental Europe and the United States. Thomson met each researcher daily, proposed problems, and often sat beside the apparatus to watch the results. His own technical skill was limited, and his assistants, notably Ebenezer Everett, built most of his apparatus. Among those who worked under him were Rutherford, Townsend, C. T. R. Wilson, Charles Barkla, Owen Richardson, Paul Langevin, Robert Strutt, Harold A. Wilson, G. I. Taylor, Lawrence Bragg, Francis Aston and Edward Appleton. Rutherford (chemistry, 1908), Bragg (1915), Barkla (1917), Aston (chemistry, 1922), C. T. R. Wilson (1927), Richardson (1928) and Appleton (1947) all received Nobel Prizes.

The cloud chamber

Wilson’s discovery that ions nucleate droplets rests on the effect of curvature on vapour pressure. A very small drop evaporates unless the vapour is strongly supersaturated, and the charge of an ion lowers the supersaturation needed.

A drop of molecular size would need an enormous supersaturation to grow. Wilson found that in clean air droplets formed on negative ions at an expansion ratio of about 1.25 and on positive ions at about 1.31, while uncharged nuclei needed much more. By 1911 he could photograph the track of a single alpha or beta particle as a line of droplets. The cloud chamber became the main visual detector of nuclear and particle physics until the 1950s; it revealed the positron in 1932.

Townsend’s avalanche

John Townsend, one of the first research students of 1895, became Wykeham Professor of Physics at Oxford in 1900. He explained the rise of current above saturation (Lesson 06): an electron accelerated by the field makes \(\alpha\) new ion pairs per unit length.

The first form follows from \(dn/dx=\alpha n\). For the second, each cathode electron gives \(e^{\alpha d}-1\) ion pairs, whose ions return \(\gamma(e^{\alpha d}-1)\) new cathode electrons; summing the geometric series gives the denominator. The breakdown condition is the origin of Paschen’s law for sparking voltages.

Thermionic emission and X-ray diffraction

Owen Richardson showed from 1901 that the current of electrons emitted by a hot metal rises steeply with temperature, and in its modern form \(j=A\,T^2e^{-W/k_BT}\), where \(W\) is the work function. It underlies every vacuum tube. In 1912, as a young research student at Cambridge, Lawrence Bragg interpreted Max von Laue’s X-ray diffraction patterns as reflections from planes of atoms, giving the Bragg condition \(n\lambda=2d\sin\theta\). With his father William Henry Bragg he founded X-ray crystallography.

Master of Trinity, 1918–1940

The Nobel Prize in Physics for 1906 was awarded to Thomson in recognition of his theoretical and experimental investigations on the conduction of electricity by gases. He was knighted in 1908, presided over the British Association in 1909, and received the Copley Medal in 1914. His later books, The Corpuscular Theory of Matter (1907) and Rays of Positive Electricity and Their Application to Chemical Analysis (1913), summarized his programme.

War and public service

The First World War emptied the Cavendish of most of its researchers. Thomson served as President of the Royal Society from 1915 to 1920 and was involved in organizing scientific advice for the war effort. From 1916 to 1918 he chaired the government committee on the position of natural science in the British educational system. Its 1918 report, known as the Thomson Report, argued for more and better science teaching in schools and universities.

Trinity

In 1918 the Crown appointed him Master of Trinity College. In 1919 he resigned the Cavendish chair, and Rutherford, his former student, succeeded him. Thomson kept a room and a small research group in the laboratory and continued to publish on positive rays and on the arrangement of electrons in atoms and molecules. His Franklin Institute lectures, The Electron in Chemistry (1923), treated chemical bonds as shared electrons, alongside the ideas of Gilbert Lewis and Irving Langmuir. He was a sociable and popular Master, fond of sport and of gardening, and held the post until his death.

In 1937 his son George Paget Thomson shared the Nobel Prize in Physics with Clinton Davisson for showing that electrons are diffracted by crystals. The father had shown that the electron is a particle; the son showed that it also behaves as a wave. Both were right, and quantum mechanics explained how.

Honours, death and legacy

Besides the Nobel Prize, his honours included the Royal Medal (1894), the Hughes Medal (1902), the Copley Medal (1914), the Albert Medal (1915), the Franklin Medal (1922), the Faraday Medal (1925) and the Order of Merit. His memoir, Recollections and Reflections, appeared in 1936.

He died in Cambridge on 30 August 1940, aged 83. His ashes were buried in the nave of Westminster Abbey, near the graves of Isaac Newton and of Rutherford, who had died three years earlier.

Legacy

• The electron: the first subatomic particle, identified as a universal constituent of matter. Electronics, from the vacuum tube to the transistor, is the technology of Thomson’s corpuscle.
• Atomic structure: the 1904 model set the research programme of explaining chemical properties by the number and arrangement of electrons, completed by Bohr, Pauli and quantum mechanics.
• Mass spectrometry and isotopes: the parabola method and its successors measure isotopic composition in geochronology, nuclear physics, medicine and biology.
• Electromagnetic mass: the idea that field energy contributes to inertia fed into relativity and, later, into the renormalization of mass in quantum field theory.
• The Cavendish: he turned a small university laboratory into the model of the modern research institute, and trained much of the next generation of British physics.

Connections

People linked to J. J. Thomson across this collection. A name without a link has no page here yet. See J. J. Thomson in the network of all 337 lives →

Teachers and advisors

• John William Strutt, Lord Rayleigh 1880-1884
• Osborne Reynolds 1870-1876
• Balfour Stewart 1870-1876
• Edward Routh

Students

• C. T. R. Wilson
• Ernest Rutherford 1895-1898
• G. I. Taylor
• Lawrence Bragg
• Paul Langevin 1897-1898
• Charles Barkla
• Edward Appleton
• Harold Albert Wilson
• John Sealy Townsend 1895-1900
• Owen Richardson
• Robert John Strutt, 4th Baron Rayleigh

Influenced by

• James Clerk Maxwell
• Jean Perrin
• Wilhelm Wien
• William Crookes
• William Thomson, Lord Kelvin

Influenced

• Henri Becquerel 1899-1900

Colleagues and collaborators

• Ferdinand Braun 1897
• Frederick Soddy
• Jean Perrin
• Niels Bohr 1911-1912
• Oliver Heaviside
• Wilhelm Wien
• Francis Aston 1910-1919
• Joseph Larmor 1880
• Richard Glazebrook

Rivals

• Philipp Lenard

Family

• George Paget Thomson

Also mentioned in this life

• Amedeo Avogadro
• Jean-Baptiste Biot
• Henry Cavendish
• Michael Faraday
• Benjamin Franklin
• Carl Friedrich Gauss
• Hans Geiger
• Heinrich Hertz
• Hendrik Antoon Lorentz
• Robert Millikan
• Isaac Newton
• Wolfgang Pauli
• Henri Poincaré
• George Gabriel Stokes
• Max von Laue

Fields

• atomic physics 7 lives
• electromagnetism 12 lives

On CoursesHub

• Atomic & Optical Physics
• Classical Electrodynamics

Sources and further reading

Biography

1. J. J. Thomson, Wikipedia (with extensive references).
2. J. J. Thomson, biographical, NobelPrize.org.
3. The Nobel Prize in Physics 1906, NobelPrize.org.
4. American Institute of Physics, The Discovery of the Electron, online exhibit.
5. J. J. Thomson, Recollections and Reflections, G. Bell and Sons, London, 1936.
6. Lord Rayleigh (R. J. Strutt), The Life of Sir J. J. Thomson, O.M., Cambridge University Press, 1942.
7. G. P. Thomson, J. J. Thomson and the Cavendish Laboratory in His Day, Nelson, 1964.
8. E. A. Davis and I. J. Falconer, J. J. Thomson and the Discovery of the Electron, Taylor and Francis, 1997.

Physics

1. J. J. Thomson, On the electric and magnetic effects produced by the motion of electrified bodies, Phil. Mag. 11 (1881), 229–249.
2. J. J. Thomson, A Treatise on the Motion of Vortex Rings, Macmillan, 1883.
3. J. J. Thomson, E. Rutherford, On the passage of electricity through gases exposed to Röntgen rays, Phil. Mag. 42 (1896).
4. J. J. Thomson, Cathode rays, Phil. Mag. 44 (1897), 293–316. Annotated text (Classic Chemistry, Le Moyne College).
5. J. J. Thomson, On the masses of the ions in gases at low pressures, Phil. Mag. 48 (1899), 547–567.
6. J. J. Thomson, On the structure of the atom, Phil. Mag. 7 (1904), 237–265.
7. J. J. Thomson, On the number of corpuscles in an atom, Phil. Mag. 11 (1906), 769–781.
8. J. J. Thomson, Rays of positive electricity, Proc. R. Soc. A 89 (1913), 1–20; and the book Rays of Positive Electricity and Their Application to Chemical Analysis, Longmans, 1913.
9. J. J. Thomson, Conduction of Electricity through Gases, Cambridge University Press, 1903; 3rd ed. with G. P. Thomson, 1928–33.
10. F. W. Aston, Isotopes, Edward Arnold, 1922.
11. Electromagnetic mass, Wikipedia (history from Thomson to Poincaré).
12. Plum pudding model, Wikipedia (the 1904–1910 models and the origin of the name).
13. R. P. Feynman, R. B. Leighton, M. Sands, The Feynman Lectures on Physics, Vol. II, ch. 28 (electromagnetic mass).
14. J. D. Jackson, Classical Electrodynamics, 3rd ed., Wiley, 1999, §14.8 (Thomson scattering) and ch. 16 (self-energy).