J.J. Thomson at
home in his
study
in 1899.


"At first there were very few who believed in the existence of these bodies smaller than atoms."

Thomson presented three hypotheses about cathode rays based on his 1897 experiments:

  1. Cathode rays are charged particles (which he called "corpuscles").
  2. These corpuscles are constituents of the atom.
  3. These corpuscles are the only constituents of the atom.

HEAR J.J. THOMSON talk about the size of the electron.


Thomson's speculations met with some skepticism. The second and third hypotheses were especially controversial (the third hypothesis indeed turned out to be false). Years later he recalled, "At first there were very few who believed in the existence of these bodies smaller than atoms. I was even told long afterwards by a distinguished physicist who had been present at my lecture at the Royal Institution that he thought I had been 'pulling their legs.'"



Joseph Larmor

The word "electron," coined by G. Johnstone Stoney in 1891, had been used to denote the unit of charge found in experiments that passed electric current through chemicals. In this sense the term was used by Joseph Larmor, J.J. Thomson's Cambridge classmate. Larmor devised a theory of the electron that described it as a structure in the ether (the invisible elastic fluid that was proposed as a substrate for light and other electrical phenomena). But Larmor's theory did not describe the electron as a part of the atom. When the Irish physicist George Francis FitzGerald suggested in 1897 that Thomson's corpuscles were really "free electrons," he was actually disagreeing with Thomson's hypotheses. FitzGerald had in mind the kind of "electron" described by Larmor's theory.

Gradually scientists accepted Thomson's first and second hypotheses, although with some subtle changes in their meaning. Experiments by Thomson, Lenard, and others through the crucial year of 1897 were not enough to settle the uncertainties. Real understanding required many more experiments over later years.

Ernest Rutherford
Theories about the atom proliferated in the wake of Thomson's 1897 work. If Thomson had found the single building block of all atoms, how could atoms be built up out of these corpuscles? Thomson proposed a model, sometimes called the "plum pudding" or "raisin cake" model, in which thousands of tiny, negatively charged corpuscles swarm inside a sort of cloud of massless positive charge. This theory was struck down by Thomson's own former student, Ernest Rutherford. Using a different kind of particle beam, Rutherford found evidence that the atom has a small core, a nucleus. Rutherford suggested that the atom might resemble a tiny solar system, with a massive, positively charged center circled by only a few electrons. Later this nucleus was found to be built of new kinds of particles (protons and neutrons), much heavier than electrons.


Table of Contents:

Exhibit Home
J.J. Thomson
Mysterious Rays
1897 Experiments
Corpuscles to Electrons
Legacy for Today
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Legacy for Today

1997- American Institute of Physics