By the end of the nineteenth century, electricity passed through evacuated glass tubes left a glowing patch on the far wall. What these 'cathode rays' were remained contested: most German physicists took them for waves in the ether, the British for streams of particles.

In 1897, at the Cavendish Laboratory in Cambridge, Thomson deflected the rays with both magnetic and electric fields, measured the deflection, and computed the particles' charge-to-mass ratio. The result was startling: about two thousand times larger than that of the lightest known ion. The same value came out regardless of the metal the cathode was made from — so this particle was a common constituent not of one element but of all matter. Thomson called it a 'corpuscle'; we call it the electron.

The consequences run two ways. Conceptually the atom stopped being the indivisible unit chemistry had inherited and became a system with an interior; Thomson's plum-pudding model gave way to Rutherford's nucleus in 1911 and to Bohr's orbits in 1913 — the chain of modern atomic physics starts here. Practically, understanding the electron as a controllable particle is the ground of everything from the cathode-ray tube to radio, television and ultimately all electronics. The charge-to-mass ratio Thomson measured was completed twelve years later when Millikan measured the charge itself.