Cathode Rays
A glowing ray bends in a field — and turns out to be a particle far lighter than any atom.
A faint glow in an empty glass tube bent sideways in a magnet's grip — and Thomson realised he was watching a piece of matter smaller than the atom itself.
The big idea
In the late 1800s, physicists could make a strange greenish glow stream across an evacuated glass tube from its negative terminal. They called it a cathode ray, and argued for decades about what it was: a ripple in the invisible "aether" that supposedly filled space, or a hail of tiny charged particles?
Thomson settled it with a measurement. He pushed the ray through electric and magnetic fields, watched how far it bent, and worked backwards to the ratio between each particle's electric charge and its mass. The answer was astonishing: the particles were about a thousand times lighter than the lightest atom, hydrogen. The atom — long thought to be the smallest, indivisible scrap of matter — had something even smaller inside it. We now call that something the electron.
How it came about
Cathode-ray tubes were a hot puzzle of the 1890s. German physicists, led by experiments that made the rays look wave-like, were sure they were a disturbance in the aether. The British leaned toward particles. A sharp piece of evidence cut against the British view: Heinrich Hertz had tried to deflect the rays with an electric field and seen nothing — surely particles carrying charge would have swerved?
Thomson, running the Cavendish Laboratory at Cambridge, found the flaw. Hertz's tube still held too much gas, which carried away the charge and cancelled the field. Pump the tube to a far better vacuum, Thomson found, and the ray bends crisply toward the positive plate — exactly as a stream of negative particles must. Then he measured how much it bent in both electric and magnetic fields, and the small mass fell straight out of the numbers.
Why it mattered
For more than two thousand years “atom” had meant “uncuttable” — the final, smallest piece of stuff. Thomson found a piece smaller still, and the same one no matter which gas or metal he used. That meant every atom of every element shares a common ingredient: the electron. It was the first crack in the solid atom, and it opened the road to the nucleus, to quantum physics, and to all of modern electronics. Thomson was careful, though: he had measured a ratio, not the particle's mass alone — that exact figure would take two more years and other hands to pin down.
A way to picture it
Imagine rolling a ball past a strong fan and a downhill slope at once. How far it drifts tells you something about the ball — a heavy bowling ball barely budges, a light ping-pong ball veers wildly. Thomson couldn't see his particles, but he could see how far the beam “veered” in electric and magnetic fields. The huge swerve told him these particles were feather-light — far too light to be whole atoms.
Where it sits
Thomson's electron sits at the hinge between classical and modern physics. Behind it lie Faraday and Maxwell, who tied electricity and magnetism together; ahead of it lies a cascade — Rutherford (his own student) finding the nucleus, Bohr quantising the atom, and the whole quantum revolution. The Library's threads on electromagnetism and on the quantum world meet here, in a glass tube where the first piece of the atom was finally weighed.
The question — two rival views
The electric deflexion Hertz had missed
Negatively charged particles of matter
As the cathode rays carry a charge of negative electricity, are deflected by an electrostatic force as if they were negatively electrified, and are acted on by a magnetic force in just the way in which this force would act on a negatively electrified body moving along the path of these rays, I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter.
Matter in a new state
Thus on this view we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state: a state in which all matter—that is, matter derived from different sources such as hydrogen, oxygen, &c.—is of one and the same kind; this matter being the substance from which all the chemical elements are built up.