J.J. Thomson

A bookseller’s son who never quite left the lab

Joseph John Thomson was born on December 18, 1856 in Cheetham Hill, a suburb of Manchester, the elder son of a small bookseller of Scottish descent. His father had hoped to apprentice him to a local engineering firm, but the apprenticeship fee proved a shilling too many, and at fourteen the boy was sent instead to Owens College, the predecessor of the University of Manchester. It was a piece of household economy that quietly changed the shape of twentieth-century physics. Engineering, he later admitted, was never in his hands. Apparatus was. He could imagine a glass tube, a coil, and a magnet, but a lathe and a milling machine left him cold.

At Owens he fell under the influence of Balfour Stewart, an experimentalist with a taste for problems no one else would touch, and Thomas Barker, a mathematician who pressed his students to read Maxwell’s Treatise on Electricity and Magnetism the year it came out. A scholarship carried Thomson on to Trinity College, Cambridge, where he came second in the Mathematical Tripos of 1880. He stayed at Trinity, took the Adams Prize in 1882 for an essay on vortex rings, and in 1884, on the resignation of Lord Rayleigh, was elected Cavendish Professor of Experimental Physics at the age of twenty-seven. The senior men of the lab were quietly horrified. He made his own glassware so badly that his assistants developed the habit of finishing his pieces for him out of pure mercy. But he had the one quality the chair required: the willingness to see the world through Maxwell’s equations even when the world declined to cooperate.

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…

1897 and a question that would not leave him alone

For most of the 1880s and early 1890s Thomson worked on the discharge of electricity through gases, a fashionable subject that nobody quite understood. Sealed glass tubes pumped down to low pressure, with metal electrodes at either end and a voltage across them, glowed with strange colors and produced a faint green fluorescence on the far wall. The German school, following Heinrich Hertz, held that the cathode rays were a vibration in the aether, akin to light. The British school, drawing on William Crookes, insisted they were streams of charged particles. By the mid-1890s the argument had been raging for twenty years without resolution.

Thomson did not invent the equipment that decided the matter. The cathode-ray tube, the deflection plates, even the basic geometry of the experiment, were all in the literature. What he did was insist that the rays be subjected to both a measured electric field and a measured magnetic field at the same time, with the apparatus calibrated carefully enough that the answer would be a number rather than a hand-waving argument. In a long string of experiments in early 1897 he showed that the rays carried negative charge, that they were deflected by an electric field (which Hertz had famously failed to demonstrate, owing to residual gas in his tube), and that the ratio of charge to mass, e/m, came out roughly a thousand times larger than for any known ion. The carriers, he announced at the Royal Institution on April 30, 1897, were “corpuscles” much smaller than atoms, and they emerged from any cathode regardless of the metal used.

The corpuscle was the first subatomic particle. The name electron had been coined by George Johnstone Stoney in 1891 for the unit of electric charge; it slid sideways onto Thomson’s corpuscles and stayed there. In 1899 he closed the loop by measuring the charge alone, using a cloud chamber adapted from C.T.R. Wilson’s condensation experiments, and the mass came out at roughly one two-thousandth of a hydrogen atom. For the first time in history, something inside the atom had been weighed. The Nobel Prize in Physics followed in 1906.

Plum pudding and a model that almost worked

In 1904 Thomson published the model of the atom that bears his name in popular memory. The atom, he proposed, was a roughly uniform sphere of positive electrification, with the negative electrons embedded in it like raisins in a pudding, held in place by electrostatic forces and arranged in rotating rings whose stability he worked out in painstaking detail. The image of plum pudding came from his own analogy and the cartoonists of the day seized on it gleefully.

The model is now mostly remembered as the thing Rutherford refuted in 1911, when the gold-foil scattering experiment forced the positive charge into a tiny dense nucleus. But the plum pudding was not a foolish guess. It was the first serious attempt to give the atom an internal structure on which actual calculations could be done, and Thomson’s ring-stability analysis foreshadowed the shell language of later atomic theory. When the model fell, it fell because a young colonial in Manchester had built a better experiment, not because Thomson had been careless.

The Cavendish factory

The other reason Thomson’s name still echoes through twentieth-century physics has less to do with his own experiments than with the laboratory he ran. From 1884 until 1919 he directed the Cavendish at a time when Cambridge had no real graduate program and no formal notion of an experimental research school. He built one. He took in young physicists from across the British Empire, gave them an apparatus, a corner of bench space, and a problem of their own, and trusted them to figure out the rest. Seven of his students or assistants went on to win Nobel Prizes, among them Ernest Rutherford, William Henry Bragg, Owen Richardson, Charles Glover Barkla, Francis Aston, and C.T.R. Wilson. He could be vague at the bench, quick to wander off into his own thoughts mid-meeting. But he was generous with credit and intuitive about who needed pushing and who needed leaving alone.

He married Rose Paget, one of the first women allowed to do research in the Cavendish, in 1890. Their son George Paget Thomson, born in 1892, grew up in the laboratory. In 1937 the Nobel Committee gave the prize in Physics jointly to G.P. Thomson and Clinton Davisson for showing, by electron diffraction through thin films, that the same electrons J.J. had discovered as particles in 1897 also behaved as waves. Father and son had won Nobel Prizes for proving, in essence, opposite descriptions of the same object. There is no closer experimental embodiment of the wave-particle duality at the heart of quantum mechanics, and Thomson, who lived to see his son’s prize awarded, took quiet delight in it.

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…

Master of Trinity, and the long slow exit

In 1918 Thomson stepped down as Cavendish Professor and was elected Master of Trinity College, a position he held for the rest of his life. He was knighted in 1908, given the Order of Merit in 1912, and elected President of the Royal Society in 1915. He continued to attend the Cavendish on most days, sitting in on tea-time discussions and offering the occasional sardonic observation about the new quantum mechanics, which he found ugly and difficult to love. He died at the Master’s Lodge on August 30, 1940, in the second summer of the war, aged eighty-three. His ashes were buried in Westminster Abbey, near Newton and Kelvin, not far from where Rutherford had been laid three years before.

What he gave the quantum story is hard to overstate. The electron, first weighed in 1897, is the carrier of every chemical bond, the particle whose orbits Bohr quantized in 1913, whose statistics Pauli described in 1925, whose wavefunction Schrodinger solved in 1926, and whose spin Dirac built into relativistic quantum mechanics in 1928. None of those chapters would have a subject without Thomson’s patient cathode-ray tubes. He was the man who showed, against an entire century of chemical assumption, that the atom was not the floor of matter; that something smaller and more universal lived inside it; and that the place to look for it was a glowing glass tube in a basement laboratory in Cambridge.