J. J. Thomson

• 1. Introduction

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  J. J. Thomson, born on the 18th of December 1856 in Cheetham Hill, Manchester, grew up in a household shaped by antiquarian books and a mother rooted in the local textile trade. He never became the locomotive engineer his parents once intended. His father's death in 1873 cut short those apprenticeship plans, and Thomson instead went to Cambridge, where he would eventually upend humanity's understanding of matter itself.

  In 1897, he announced that cathode rays were made of particles more than a thousand times lighter than the smallest atom. That finding cracked open the atom, revealing the first subatomic particle ever identified. The discovery raised an unsettling question: if atoms were divisible, what were they made of, and what held them together?

  Thomson's answer, his so-called plum pudding model, turned out to be wrong. But the path from that wrong model to the correct one ran directly through Thomson's laboratory, and through the careers of the students he trained there.

• 2. An Unlikely Appointment

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  On the 22nd of December 1884, Thomson was appointed Cavendish Professor of Physics at the University of Cambridge, and the appointment caused considerable surprise. Candidates such as Osborne Reynolds and Richard Glazebrook were older and far more experienced in practical laboratory work. Thomson was known primarily as a mathematician, recognized as an exceptional talent, but hardly the obvious choice to lead one of Britain's premier experimental physics posts.

  He had arrived at Trinity College, Cambridge in 1876 and taken his B.A. in mathematics in 1880, finishing as Second Wrangler in the Tripos and 2nd Smith's Prizeman. A fellowship followed the next year, and his M.A. came in 1883 with the Adams Prize. His prize-winning master's work, a treatise on the motion of vortex rings, was already probing atomic structure through mathematics.

  By the time he reached the Cavendish Chair, Thomson had published work examining James Clerk Maxwell's electromagnetic theory of light, had introduced the concept of electromagnetic mass of a charged particle, and had shown mathematically that a moving charged body would apparently increase in mass. His 1888 book on the transformation of energy went so far as to suggest that all energy might be kinetic. His 1893 book built directly on Maxwell's foundational treatise and was sometimes called the third volume of Maxwell. These were not the publications of a man who stayed comfortably in pure mathematics.

• 3. Cracking the Cathode Ray

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  Before Thomson's experiments, physicists disagreed sharply about what cathode rays actually were. Some held that they were immaterial, like light, a process in the aether. Others argued they were wholly material, the paths of particles charged with negative electricity. Thomson found the particle hypothesis specific enough to test, and he set about doing exactly that.

  His first round of experiments investigated magnetic deflection. He produced cathode rays in a side tube and passed them through an anode into a larger chamber, where a magnet bent their path. A fluorescent screen let him track where the rays went. Whatever the material of the anode or the gas in the tube, the deflection remained the same, pointing toward a universal origin for the rays.

  For the charge question, Thomson built a Crookes tube fitted with an electrometer placed to the side, out of the direct path of the rays. He found that the electrometer registered a charge only when he used a magnet to bend the cathode ray toward it. The negative charge and the rays, he concluded, were the same thing.

  Electrical deflection proved trickier. Earlier investigators, including Heinrich Hertz, had failed to observe it and concluded electric fields had no effect on cathode rays. Thomson suspected their tubes held too much residual gas. A denser gas meant more ions from electron collisions, and those ions electrically screened out the applied field. His solution was a much better vacuum. With residual gas sparse enough that space charge could not screen the field, he observed clear deflection when plates connected to a battery bent the beam toward the positive plate and away from the negative one. The experiments ran in May-June 1897.

  Thomson's classic measurement then combined electric and magnetic deflection to extract the mass-to-charge ratio of the rays. He placed his discharge tube between the poles of a large electromagnet and varied the magnetic field until the magnetic and electric deflections matched. The resulting calculation showed the mass-to-charge ratio was over a thousand times lower than that of a hydrogen ion. Critically, every cathode material gave the same result. He announced his conclusion on the 30th of April 1897: these particles were a universal building block of atoms, each more than a thousand times lighter than the hydrogen atom. He called them corpuscles.

• 4. A Model and Its Limits

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  Thomson concluded that atoms were divisible, and in 1904 he proposed what he thought they looked like from the inside. His model pictured the atom as a sphere of positive matter, with corpuscles distributed inside it in a uniform sea of positive charge. The corpuscles were not stationary; they orbited rapidly. The arrangement came to be called the plum pudding model, with the electrons imagined as raisins embedded in the positive matter, though Thomson's own version had them moving, not sitting still.

  In 1906, a direct consequence of this experimental program was his demonstration that hydrogen held only a single electron per atom. Previous theories had left open the possibility of several electrons per hydrogen atom.

  Thomson resisted calling his particles electrons for years. The name had been coined by George Johnstone Stoney in 1891 as a tentative label for the basic unit of electrical charge, before any particle had actually been found. After Thomson's discovery, George Francis FitzGerald, Joseph Larmor, and Hendrik Lorentz pushed for the scientific community to adopt the term. Thomson balked because some physicists were already speaking of a positive electron as the elementary unit of positive charge, and he wanted to reserve his term, corpuscle, strictly for the negatively charged particle. He finally relented by 1914, using the word electron in his book The Atomic Theory.

  The plum pudding model itself did not survive long. Thomson's own former student Ernest Rutherford showed that the positive charge of an atom is concentrated in a tiny nucleus, not spread uniformly throughout the sphere. Rutherford succeeded Thomson as Cavendish Professor, a handoff that says something about the intellectual lineage Thomson built.

• 5. Isotopes and Mass Spectrometry

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  In 1912, Thomson and his research assistant F. W. Aston turned their attention to canal rays, the streams of positively charged particles traveling in the direction opposite to cathode rays. They channelled a stream of neon ions through both a magnetic and an electric field, then measured the deflection by placing a photographic plate in the beam's path.

  Two distinct patches of light appeared on the plate. Two different parabolas of deflection meant two different atomic masses in the same neon gas: neon-20 and neon-22. This was the first evidence for isotopes in a stable, non-radioactive element. Frederick Soddy had earlier proposed that isotopes exist to explain the decay of radioactive elements, but Thomson's neon result was something new: a chemically pure, stable substance turning out to be a mixture of atoms with different masses.

  Thomson's separation of those neon isotopes by mass was the first example of mass spectrometry. Aston and A. J. Dempster subsequently improved and extended the technique into a general analytical method. In 1991, a proposed unit for measuring mass-to-charge ratio in mass spectrometry was named the thomson, symbol Th, in his honour. The Thomson Medal Award, sponsored by the International Mass Spectrometry Foundation, also carries his name.

  In 1905, separately from the isotope work, Thomson had also identified the natural radioactivity of potassium.

• 6. Teacher of Nobel Laureates

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  Seven of Thomson's students went on to win Nobel Prizes. Ernest Rutherford took the Chemistry prize in 1908. Lawrence Bragg won Physics in 1915. Charles Barkla took Physics in 1917. Francis Aston, the same Aston who worked beside Thomson on the neon isotope experiments, took Chemistry in 1922. Charles Thomson Rees Wilson won Physics in 1927. Owen Richardson won Physics in 1928. Edward Appleton won Physics in 1947, seven years after Thomson's death.

  The reach of that influence extended into Thomson's own family. His son George Paget Thomson shared the 1937 Nobel Prize in Physics with Clinton Davisson for the experimental discovery of the diffraction of electrons by crystals. The father had shown that electrons were particles; the son shared a prize for showing they had wave-like properties. Thomson married Rose Elisabeth Paget in 1890 at the church of St Mary the Less. Rose was the daughter of Sir George Edward Paget, a physician and Regius Professor of Physic at Cambridge, and her own interest in physics predated her relationship with Thomson. She had begun attending lectures and demonstrations at Cambridge from 1882 onward, when women were first permitted to do so, and she attended Thomson's.

  Thomson gave a series of four lectures at Princeton University in 1896, and six more at Yale University in 1904. He was knighted in 1908 and appointed to the Order of Merit in 1912. In 1918, he became Master of Trinity College, Cambridge, a post he held until his death on the 30th of August 1940. His ashes rest in Westminster Abbey, near the graves of Isaac Newton and Ernest Rutherford.