April 30, 1897

J. J. Thomson

J. J. Thomson publicly proposes subatomic cathode-ray "corpuscles"

The particle now called the electron entered this lecture under a different name: 'corpuscle.'

30 April 1897: a Friday evening at the Royal Institution

J. J. Thomson delivered a lecture titled 'Cathode Rays' at the Royal Institution's Friday evening meeting in London on 30 April 1897, with Sir Frederick Bramwell in the chair.

His argument was that cathode rays followed the paths of fast, negatively charged particles rather than being transverse vibrations in the ether.

Atoms as aggregations of something smaller

Thomson proposed that atoms were aggregations of much smaller, mutually similar particles — his 'corpuscles' — and suggested these formed cathode rays when torn from gas molecules. He asked the audience to picture ordinary atoms as mostly holes.

Using a thermal junction, an electrometer and magnetic deflection, he estimated the corpuscles' mass-to-charge ratio at m/e = 1.6 × 10⁻⁷ in his stated units, against 10⁻⁴ for a hydrogen atom carrying an ionic charge — and concluded the carriers were smaller than hydrogen atoms.

What the lecture did not say

'Discovery of the electron' is retrospective language. Thomson used 'corpuscle'; the term electron had been coined by George Johnstone Stoney for a unit of charge. He also did not announce a particle 1,800 times lighter than a proton — the proton and the nuclear atom were not yet established, and his comparison was of mass-to-charge ratios.

A printed account followed in The Electrician in May, and a fuller 'Cathode Rays' paper appeared later in 1897; immediate universal acceptance did not follow.

Sources

Researched 23 Aug 2026 4 sources 3 audit passes

How this was checked

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What the sources leave uncertain

The date and lecture are secure, but “discovery of the electron” is retrospective language. Thomson called the entities corpuscles, and historians emphasize that Emil Wiechert and Walter Kaufmann were conducting closely related work. Thomson’s distinctive contribution was the subatomic, universal-constituent interpretation.