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The Electron

The Electron. What is it and what’s it doing in my pudding?. Excuse me… how can you discover a particle so small that nobody has ever seen one? J.J. Thomson 1897.

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The Electron

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  1. The Electron What is it and what’s it doing in my pudding?

  2. Excuse me… how can you discover a particle so small that nobody has ever seen one? J.J. Thomson 1897

  3. Glowing tubes full of gas proliferated – and gained scientific importance – in 1855, when glassblower Heinrich Geissler developed an improved vacuum pump. It was one of Geissler's tubes that Julius Plücker used when he first observed cathode rays in 1859.

  4. Plucker's student, Johann Wilhelm Hittorf, put solid objects inside the tube between the cathode and the glow. The objects cast shadows. Hittorf concluded that the cathode was emitting something that traveled in straight lines, like light rays.

  5. Crookes’ Tube • Invented by William Crookes 1875 • Maltese Cross Tube (on left)

  6. A later manifestation of Crookes’ Tube • German physicist Eugen Goldstein called them “cathode rays” • Since they… well… were emitted from the cathode • Goldstein discovered positive rays which were emitted by the anode (“Canal” rays)

  7. J.J. Thomson • Using devices similar to Crookes’ tube, studied cathode rays • Identified cathode rays as “electrons” • Term coined by G. Johnstone Stoney (1891) • Determined with basic physics • e/m = 1.8 x 10-8 C/g • Either they’re really small or highly charged • Problem solved w/ Millikan’s “oil-drop” exp. in 1908

  8. J. J. Thomson in his office at the Cavendish Laboratory. A colleague of Thomson's, Lord Rayleigh, said that "J.J. had something to say on nearly any subject that might turn up. He as a good raconteur, but also a good listener, and knew how to draw out even shy members of the company.... J.J., while talking, paced the room vigorously in a manner rather suggestive of a caged lion."

  9. Plum-Pudding Model

  10. Ernest Rutherford (1871-1937) …came to the Cavendish Laboratory as a young man from New Zealand, and was a research student under Thomson. Using a-rays emitted by radioactive elements to probe into atoms, around 1913 he showed that Thomson's “plum pudding” model of the atom was untenable.

  11. Gold-Foil Experiment Other names to remember: Hans Geiger, Ernest Marsden

  12. Modern View of the Atom

  13. So what do you do with a newly characterized electron beam? • Shoot it at things, of course

  14. Introducing… Wilhelm Röntgen (1895) • Using a Crookes tube, accidentally discovered x-rays • Discovery ranks among shortest scientific “gestation” periods • 1st Nobel Prize

  15. Crooke’s-Hittorf X-ray Tube

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