On j
On j.j. thomsonJ.J. Thomson
Science lecturers who traveled from town to town in the middle nineteenth century delighted audiences by showing them the ancestor of the neon sign. They took a glass tube with wires embedded in opposite ends . . . put a high voltage across . . . pumped out most of the air . . . and the interior of the tube would glow in lovely patterns. In 1859 a German physicist sucked out still more air with an improved pump and saw that where this light from the cathode reached the glass it produced a fluorescent glow. Evidently the cathode emitted some kind of ray that was illuminating the glass.
What could these rays be? One possibility was that they were waves traveling in a hypothetical invisible fluid called the ether (similar to the quintessence of Aristotle). At that time, many physicists thought that this ether was needed to carry light waves through apparently empty space. Maybe cathode rays were similar to light waves? Another possibility was that cathode rays were some kind of material particle. Yet many physicists, including J.J. Thomson, thought that all material particles themselves might be some kind of structure built out of ether, so these views were not so far apart.
Experiments were needed to resolve the uncertainties. When physicists moved a magnet near the glass, they found they could push the rays about. Nevertheless, when the German physicist Heinrich Hertz passed the rays through an electric field created by metal plates inside a cathode ray tube, the rays were not deflected in the way that would be expected of electrically charged particles. Hertz and his student Philipp Lenard also placed a thin metal foil in the path of the rays and saw that the glass still glowed, as though the rays slipped through the foil. Did that not prove that cathode rays were some kind of waves?
Other experiments cast doubt on the idea that these were ordinary particles of matter, for example gas molecules as some suggested. In France, Jean Perrin had found that cathode rays carried a negative charge. In Germany, in January 1897 Emil Wiechert made a puzzling measurement indicating that the ratio of their mass to their charge was over a thousand times smaller than the ratio for the smallest charged atom. When Lenard passed cathode rays through a metal foil and measured how far they traveled through various gases, he concluded that if these were particles, they had to be very small.
Drawing on work by his colleagues, J.J. Thomson refined some previous experiments, designed some new ones, carefully gathered data, and then . . . made a bold speculative leap. Cathode rays are not only material particles, he suggested, but in fact the building blocks of the atom: they are the long-sought basic unit of all matter in the universe.
One hundred years ago, amidst glowing glass...
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