J.J. Thomson - the Electron

Cathode rays

In 1855 Heinrich Geissler, a glass blower from Bonn, invented the mercury vapor vacuum pump to remove air from a space. Soon thereafter, scientists began probing the properties of evacuated space. Of particular interest was the discovery that evacuated space in a Geissler tube could conduct electricity. Over the next forty years, physicists in Europe compiled a list of interesting phenomena. Some of this work is described here.
See a brief history at http://www.aip.org/history/electron/jjrays.htm#geissler


 Here is an illustrated piece about :Sir William Crookes

The Electron Centennial Page addresses these topics:
Smaller than the Atom, A Mathematician Turns to Experiment
Bullets or Waves? A Surprising Result
Decade of Discovery, Thomson's Atom

J.J. Thomson and the Discovery of the Electron

Are cathode rays charged particles or are they part of the electromagnetic spectrum? By 1895, these were the properties that cathode rays exhibited:

1) cathode rays cast shadows (read: they travel in straight lines)

2) cathode rays cause certain chemical reactions to occur (they caused photo emulsions to decompose.)

3) Any metal cathode could produce cathode rays. (likely to be a universal property of matter)

4) Cathode rays are deflected by an electric field (repelled by the negative plate)

5) Cathode rays are deflected by a magnetic field

 If cathode rays were charged particles, what was the size of the charge and, for that matter, what was the mass of the particle? The keystone experiment in this long line of research was done by (Sir Joseph John) J.J. Thomson in England in 1897. Try as he would, he could not determine either the charge or the mass , one independent of the other. The best he could produce was e/m, the charge to mass ratio for cathode rays particles. He determined e/m to be 1.76 x 10^11 Coulombs/kilogram. By itself, this number has little meaning. Thompson needed something by which he could compare e/m. he measured q/m, the charge (e stands for the charge on the electron, q , the charge on everything else) to mass ratio for hydrogen ions, heretofore the smallest particle on the planet

. He found

 e/m = 1846 q/m

Thomson pursued this line of reasoning regarding this equation: If e = q,then

 m (H+ ion) = 1846 m (CR particles)

The results of his experiments, coupled with some bold conjecture, lead to the conclusion that atoms have constituent parts


 Henry Moseley and the discovery of the proton

From the time of Mendeleev, the order of the elements had been determined mostly by increasing atomic mass. It was the work of Harry (Henry Gwyn-Jefferies) Moseley in the analysis of the lines in the X-ray spectra of the elements that came the suggestion that atomic number, the place of an element on the periodic table, was more important than atomic weight. One does not know much about Moseley. Born in 1887, he died at Gallipoli in 1915, shot by a sniper



Two Excellent Sites


 The Atomic Number Puzzle
The Philosopher's Stone.
A Peculiar Prediction
James Chadwick and the Beryllium Rays
The Modern Atom

Particles from the Sky
The Big Machines
The Particle Zoo
The Eightfold Way
Probing the Proton
The Plum Pudding Proton

Last edited 12/26/05

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