In Quantum Generations, the Danish historian Helge Kragh writes:
As we have seen, when J. J. Thomson made his celebrated experiments in 1897, the electron was a well-known, if hypothetical, entity. However, this does not mean that Thomson discovered experimentally what Lorentz and others had predicted theoretically. Thomson's particle was at first thought to be different from earlier versions of the electron, and it took several years until the different pictures merged into a single and unified conception of the electron.So Thomson distinguished himself by claiming that he had discovered something other than what Lorentz and others had predicted, a claim that turned out to be entirely wrong. Thomson used his electrons to construct a plum puddling model of the atom, which was interesting but quickly rejected. His rivals did experiments that were much more accurate.
The question of the nature of cathode rays -- were they corpuscular or etherial processes? -- was not much discussed in England until 1896, when Roentgen's discovery forced cathode rays into prominence ...
Thomson named his primordial particles "corpuscles." Since the name "electron" was already in use and electron theory was on its way to become a fashionable branch of theoretical physics, why didn't he call the particles by that name? Briefly put, Thomson did not conceive his particle to be iden-tical with the Lorentz-Larmor particle and he stressed the difference by choosing another name. According to Thomson, the cathode ray corpuscles were not ethereal-charges without matter, as the electron theoreticians would have it-but charged material particles, proto-atoms of a chemical nature. In his October 1897 paper, Thomson briefly considered the possi-bility of subjecting the corpuscular matter to "direct chemical investigation," but rejected the idea because the amount of the corpuscular substance pro-
duced in a cathode ray tube was much too small. The identification of cor-puscles with free electrons was first suggested by George FitzGerald im-mediately after Thomson had announced his discovery. ...
Thomson is celebrated as the discoverer of the electron because he sug-gested corpuscles to be subatomic constituents of matter, elementary parti-cles; because he provided this suggestion with some experimental evidence; and because his contemporaries and later physicists accepted and substanti-ated the claim. He did not discover the electron simply by measuring the elm value of cathode rays. Such measurements, more accurate than Thomson's, were being made at the same time by Emil Wiechert and Walter Kaufmann in Germany. Wiechert's first result was e/m = 2 X 107 emu/g and Kauf-mann initially obtained about 10^7 emu/g which, later the same year, he im-proved to 1.77 X 10^7 emu/g. Thomson's mean value was 0.77 X 10^7 emu/g, to be compared with the modem value of 1.76 X 10^7 emu/g. Although Kaufmann, like Thomson, varied the material of the cathode and the gas in the tube, he did not suggest from his data that the cathode rays were cor-puscular. Wiechert did, but he did not make the same sweeping generaliza-tion as his colleague in Cambridge and thus missed one of the most impor-tant discoveries in the history of physics.
His son received the 1937 Nobel Prize for proving that the electron was a wave, and thereby disproving that it was a corpuscle.