Discovery of the Electron (1897): J.J. Thomson's Cathode Rays

The discovery of the electron in 1897 by J. J. Thomson at the Cavendish Laboratory in Cambridge showed that atoms are not indivisible. Thomson announced on 30 April 1897 that cathode rays, the mysterious glow inside evacuated glass tubes, were streams of negatively charged particles far smaller than the lightest atom. It was the first subatomic particle ever identified and the starting point of modern atomic physics.

Background: the cathode ray puzzle

From the late 1850s physicists had passed electric currents through glass tubes from which most of the air had been pumped. Julius Plücker in Bonn and his student Johann Hittorf noticed a glow on the glass opposite the negative electrode, the cathode, and found that objects in the tube cast shadows. In 1876 Eugen Goldstein named the effect cathode rays.

What the rays were became a long dispute. Many German physicists, including Heinrich Hertz and Philipp Lenard, thought they were waves in the ether, because they passed through thin metal foils. In 1883 Hertz tried to bend them with an electric field and failed, which seemed to rule out charged particles. In Britain, William Crookes argued in the 1870s that they were streams of charged molecules, which he called “radiant matter”. In 1895 Jean Perrin in Paris collected negative charge from the rays, strong evidence for particles, though not decisive.

The question mattered because it touched the nature of matter itself. If cathode rays were waves, they were a new form of radiation, like light. If they were particles, they had to be carried by something material, and their behaviour might reveal what atoms were made of. By the mid-1890s the rival camps were roughly divided along national lines, and the debate had become one of the most closely watched in physics.

Thomson and the Cavendish Laboratory

Joseph John Thomson (1856–1940), born near Manchester, became Cavendish Professor of Experimental Physics at Cambridge in 1884, aged only 28. He was a gifted theorist who also directed a busy laboratory. From the early 1890s he worked on how gases conduct electricity, and the discovery of X-rays in late 1895 (see Röntgen’s discovery of X-rays) gave new tools for that research.

Thomson set out to settle the nature of cathode rays. He presented his first conclusions at a Friday evening discourse at the Royal Institution in London on 30 April 1897 and published the full account, titled “Cathode Rays”, in the Philosophical Magazine in October 1897.

The experiments

Thomson’s work rested on three linked experiments.

Charge and rays cannot be separated. He repeated Perrin’s test with an improved tube. When a magnet bent the rays into a side chamber connected to an electrometer, negative charge was recorded; when the rays were steered away, almost none arrived. The charge travelled with the rays.

Electric deflection. Thomson suspected Hertz had failed because the leftover gas in his tube became conducting and screened the electric field. With better vacuum pumps he obtained a clear deflection between two metal plates inside the tube, towards the positive plate, as expected for negative particles.

The tubes themselves were made by the laboratory’s glassblower, Ebenezer Everett, who worked closely with Thomson for decades. Rays from the cathode passed through slits in metal plugs to form a narrow beam, which travelled between the deflecting plates and struck the far end of the tube, where a glowing patch on the glass, marked with a scale, showed how far the beam had been bent.

Mass-to-charge ratio. By balancing the electric deflection against a magnetic one, and in a separate method by measuring the heat delivered by the rays, he calculated the speed of the particles and the ratio of their mass to their charge. The ratio was roughly a thousand times smaller than for a hydrogen ion, the lightest charged particle then known from electrolysis. It was the same whatever gas filled the tube and whatever metal made the cathode.

Thomson concluded that the rays were made of tiny particles, which he called corpuscles, present in all matter. In 1899 he measured their charge separately, using clouds of droplets formed around charged particles, a technique developed with his student C. T. R. Wilson and with John Townsend. The charge was close to that of a hydrogen ion, so the mass had to be extremely small. Modern values give the electron about 1/1836 of the mass of a hydrogen atom.

Was Thomson alone?

Other physicists reached similar results in 1897. Walter Kaufmann in Berlin measured the mass-to-charge ratio of cathode rays very accurately, and Emil Wiechert in Königsberg estimated it too. Pieter Zeeman’s 1896 discovery that magnetic fields split spectral lines had already been explained by Hendrik Lorentz as the work of small charged particles inside atoms. Thomson is credited because he combined the measurements with the bold claim that these particles were a universal building block of atoms.

The word electron had been coined by the Irish physicist George Johnstone Stoney in 1891 for a natural unit of electric charge. Others soon applied it to Thomson’s corpuscles, and the name stuck, though Thomson himself long preferred his own term.

Thomson’s claim was not accepted at once. Some colleagues found the idea of particles smaller than atoms hard to believe, and he later recalled that a distinguished physicist who heard his Royal Institution lecture thought he had been pulling the audience’s leg. Within a few years, however, the electron was found in many other settings: in the beta rays of radioactive substances (see Becquerel’s discovery of radioactivity), in the photoelectric effect and in the hot filaments of lamps.

The plum pudding model

If atoms contained light negative electrons but were neutral overall, there had to be positive charge somewhere. In 1904 Thomson proposed that the electrons sat in rings inside a sphere of evenly spread positive charge. Popular accounts compared it to plums in a pudding; the nickname was not Thomson’s own. The model explained some patterns in chemistry and was widely discussed.

It did not last. Experiments by Hans Geiger and Ernest Marsden under Ernest Rutherford, Thomson’s former student, showed in 1909 that a few alpha particles bounced back from thin gold foil. In 1911 Rutherford explained this with a tiny, dense, positive nucleus (see Rutherford’s discovery of the atomic nucleus).

Legacy

Thomson received the 1906 Nobel Prize in Physics, was knighted in 1908 and became Master of Trinity College, Cambridge, in 1918. Several of his students and assistants, including Rutherford, Wilson and Francis Aston, won Nobel Prizes. He died on 30 August 1940 and is buried in Westminster Abbey near Isaac Newton.

In a well-known twist, his son George Paget Thomson shared the 1937 Nobel Prize in Physics for showing that electrons can be diffracted like waves. The father showed the electron to be a particle; the son showed it also behaved as a wave, a central idea of quantum mechanics. The electron itself became the basis of electronics, from radio valves and television tubes to transistors and modern computers.

Timeline

• 1858–1869: Plücker and Hittorf study the glow and shadows in evacuated tubes.
• 1876: Eugen Goldstein names the effect cathode rays.
• 1883: Hertz fails to deflect cathode rays with an electric field.
• 1895: Jean Perrin shows that cathode rays carry negative charge.
• 1897, 30 April: Thomson announces his conclusions at the Royal Institution.
• 1897, October: His paper “Cathode Rays” appears in the Philosophical Magazine.
• 1899: Thomson measures the charge of the particles.
• 1904: Thomson publishes his model of the atom.
• 1906: Thomson receives the Nobel Prize in Physics.

Why it matters

The discovery of the electron proved that atoms have internal parts, opening the path to the nuclear atom, quantum theory and chemistry based on electron structure. Nearly every electrical and electronic technology of the 20th and 21st centuries depends on controlling the particle Thomson identified.