J.J. Thomson discovers the electron at the Cavendish

A picture of nature is rarely overturned in a single evening, and almost never on a public stage. More often the working physicists in the back rows of a Royal Institution Friday lecture sit through the discourse politely, take notes, and go home to consider whether the speaker has overstated. On the evening of the thirtieth of April 1897, in the lecture theatre at Albemarle Street, the working physicists in the back rows sat through the hour and left the room knowing that the conception of the atom they had been taught at school, the indivisible elemental unit of matter that John Dalton had set down in 1808 and that had governed every chemistry-and-physics textbook of the nineteenth century, had not survived the last forty minutes.

THE CAVENDISH MAN

Joseph John Thomson is forty years old in the spring of 1897. He was born at Cheetham Hill in northern Manchester on the eighteenth of December 1856, eldest son of Joseph James Thomson, a Manchester antiquarian-bookseller of long Lowland Scots descent on the Thomson side, and Emma Swindells. He was schooled at Owens College Manchester (the modern University of Manchester) from fourteen, took the open scholarship to Trinity College, Cambridge, in 1876 in his twentieth year, was second wrangler in the Mathematical Tripos in 1880, was elected Fellow of Trinity in 1881, and on the death of Lord Rayleigh's predecessor was elected Cavendish Professor of Experimental Physics at the University of Cambridge on the twenty-second of December 1884 in his twenty-ninth year, the youngest holder of the chair in the institution's history.

He had inherited the Cavendish Laboratory from James Clerk Maxwell (founder, 1874) and Lord Rayleigh (Cavendish Professor 1879 to 1884). He took it across the next decade into the leading single experimental-physics research school in the world. By 1897 the Cavendish was running fifteen separate research programmes under his direction, employed twenty-three research students and three senior demonstrators, and had become the institution to which every aspiring European-and-North-American experimental physicist applied for the postdoctoral year. He had married Rose Paget, one of the first women admitted to research at the Cavendish, in 1890.

THE CATHODE RAYS

Cathode rays had been observed in evacuated glass tubes since the 1860s. When a high-voltage electric current was passed between two electrodes in a glass tube from which the air had been substantially pumped out, the negative electrode (the cathode) gave off a faint glowing emanation that travelled across the tube in straight lines, cast sharp shadows of any object in their path, and made phosphorescent screens glow on impact. By the 1880s the question of what the rays actually were had divided the physics-and-chemistry establishment into two schools. The German school, led by Heinrich Hertz at Bonn, held that the rays were electromagnetic waves analogous to light. The British school, led by William Crookes in London, held that the rays were streams of negatively-charged particles. Neither side had decisive experimental evidence. The question had been open for thirty years.

Thomson took up the question in the autumn of 1895 in his thirty-ninth year. He had three new experimental techniques available that had not been available to the earlier investigators: a better Geissler-and-Sprengel mercury vacuum pump that could evacuate his tubes more thoroughly than the previous generation's apparatus, the new high-voltage Ruhmkorff induction coil that could drive higher cathode-ray currents, and the careful electrostatic-and-electromagnetic deflection apparatus he had designed himself with his Cavendish research student Ernest Everett. He spent the autumn of 1895 and the whole of 1896 working through the systematic series of measurements: the magnetic deflection of the rays in known field strengths, the electrostatic deflection in known electric fields, the measured ratio of the two deflections, and the calculation of the charge-to-mass ratio of whatever the rays were composed of.

THE NUMBER

The result he reached by February 1897 was unambiguous and unprecedented. The cathode rays were particles. They carried a negative electric charge. Their charge-to-mass ratio (the quantity Thomson designated e/m) was approximately seventeen hundred times larger than the charge-to-mass ratio of hydrogen ions in electrolysis, the lightest then-known charged particle. The cathode-ray particles were therefore either far more highly charged than hydrogen ions or far less massive, or both. By a further set of measurements through the spring of 1897 (the cloud-chamber drop-method that his Cavendish student Charles Wilson was simultaneously developing for the measurement of single-particle charges), he was able to show that the charge was approximately the same as the hydrogen-ion charge, and that the mass was therefore approximately one-seventeen-hundredth of the hydrogen atom, the lightest known atom.

He had measured a particle of matter substantially smaller than any atom. The conclusion was that the cathode-ray particle was a sub-atomic constituent of all matter, common to every chemical element, knocked out of the cathode atoms by the high-voltage electric field. The atom was therefore not the indivisible elemental particle Dalton had described, but a composite object containing this still-smaller negatively-charged constituent. Thomson called the particle a corpuscle. The Stoney name electron, coined in Dublin in 1894 by the Trinity College physicist George Johnstone Stoney for the natural unit of electric charge, was applied to the Thomson particle within ten years and has been the universal name since.

THE FRIDAY EVENING DISCOURSE

He reported the result formally at the Royal Institution Friday Evening Discourse on the thirtieth of April 1897. The Friday Evening Discourses were the central single public-scientific lecture series in late-Victorian Britain; the lectures were delivered at the Royal Institution at Albemarle Street on Friday evenings through the academic year, ran to about an hour each, and were attended by an audience of approximately five hundred Fellows of the Royal Society, members of the scientific press and the educated London public, in evening dress. The platform was the original Faraday demonstration bench that Michael Faraday had used to demonstrate electromagnetic induction in 1831 in the same room.

Thomson took the platform at eight o'clock and spoke for about fifty-five minutes on the experimental work at the Cavendish. He had brought with him three of the original demonstration tubes from the Cavendish (the deflection tubes that had been used in the spring measurements) and demonstrated the magnetic and electrostatic deflection of the cathode-ray beam at the platform with a small portable Ruhmkorff coil. He set out the e/m measurement, the calculation of the corpuscle mass, the conclusion that the corpuscle was a sub-atomic constituent common to every element. The audience response (recorded in the Royal Institution Proceedings of the next morning) was reserved-but-attentive; the implications were too large to be absorbed across a single Friday lecture.

He published the full mathematical results in the Philosophical Magazine in October 1897 in the paper Cathode Rays. The paper is on every modern syllabus of the foundational works of twentieth-century physics. He was elected President of the Royal Society in 1915, was awarded the Nobel Prize in Physics in 1906, was knighted in 1908, was admitted to the Order of Merit in 1912, and held the Cavendish Professorship for thirty-five years until his retirement in 1919 in succession by his Cavendish student Ernest Rutherford. He died at Cambridge on the thirtieth of August 1940 and was buried in Westminster Abbey beside Newton and Darwin. The Thomson name in modern physics carries the weight of the Friday evening at Albemarle Street in April 1897.