J. J. Thomson

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J. J. Thomson

Sir Joseph John Thomson, OM, FRS (18 December 185630 August 1940), often known as J. J. Thomson, was a British scientist. Thomson is credited with the discovery of the electron and isotopes, and the invention of the mass spectrometer.


listed in chronological order

  • The difficulties which would have to be overcome to make several of the preceding experiments conclusive are so great as to be almost insurmountable.
    • Warning about the non-conclusiveness for the experimental foundation of electrostatic theory, in a footnote of the third edition of: James Clerk Maxwell (1891). A Treatise on Electricity and Magnetism, Vol.1, 3rd Edition. Oxford University Press. p. 37. 
  • As the cathode rays carry a charge of negative electricity, are deflected by an electrostatic force as if they were negatively electrified, and are acted on by a magnetic force in just the way in which this force would act on a negatively electrified body moving along the path of these rays, I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter.
  • If, in the very intense electric field in the neighbourhood of the cathode, the molecules of the gas are dissociated and are split up, not into the ordinary chemical atoms, but into these primordial atoms, which we shall for brevity call corpuscles; and if these corpuscles are charged with electricity and projected from the cathode by the electric field, they would behave exactly like the cathode rays.
  • We see from Lenard's table that a cathode ray can travel through air at atmospheric pressure a distance of about half a centimetre before the brightness of the phosphorescence falls to about half its original value. Now the mean free path of the molecules of air at this pressure is about 10-5 cm., and if a molecule of air were projected it would lose half its momentum in a space comparable with the mean free path. Even if we suppose that it is not the same molecule that is carried, the effect of the obliquity of the collisions would reduce the momentum to half in a short multiple of that path. Thus, from Lenard's experiments on the absorption of the rays outside the tube, it follows on the hypothesis that the cathode rays are charged particles moving with high velocities, that the size of the carriers must be small compared with the dimensions of ordinary atoms or molecules. The assumption of a state of matter more finely subdivided than the atom of an element is a somewhat startling one; but a hypothesis that would involve somewhat similar consequences—viz. that the so-called elements are compounds of some primordial element—has been put forward from time to time by various chemists.
  • I have described at some length the application of Positive Rays to chemical analysis; one of the main reasons for writing this book was the hope that it might induce others, and especially chemists, to try this method of analysis. I feel sure that there are many problems in chemistry, which could be solved with far greater ease by this than any other method. The method is surprisingly sensitive — more so than even that of spectrum analysis, requires an infinitesimal amount of material, and does not require this to be specially purified; the technique is not difficult if appliances for producing high vacua are available.
    • Rays of Positive Electricity (1913).


  • This example illustrates the differences in the effects which may be produced by research in pure or applied science. A research on the lines of applied science would doubtless have led to improvement and development of the older methods—the research in pure science has given us an entirely new and much more powerful method. In fact, research in applied science leads to reforms, research in pure science leads to revolutions, and revolutions, whether political or industrial, are exceedingly profitable things if you are on the winning side.
    • Cited from Lord Rayleigh, The Life of Sir J. J. Thomson (1943), p. 199.
  • The electron: may it never be of any use to anybody!
    • A popular toast or slogan at J. J. Thompson's Cavendish Laboratory in the first years of the 1900s, as quoted in Proceedings of the Royal Institution of Great Britain, Volume 35 (1951), p. 251.

Quotes about J. J. Thomson[edit]

  • Cathode Rays... he adheres to the hypothesis that the rays are due to the violent projection of the negatively charged particles from the cathode. In another abstract from presumably the same lecture, he states that in the cathode discharge the matter is in something beyond the ordinary state and that the carriers of the discharge in a cathode ray are not atoms but something very much smaller; his conclusions are that the particles carrying the charge must be in a much more finely divided state than the ordinary molecule and possibly may be the primordial element; the numerical ration of the mass of the particle to the charge carried is about 1,100 times less than that deduced electrolytically for the hydrogen ion, showing that either the charge must be very great or the particle very small, and it is the latter which he thinks is the case.
    • The Electrical World, Vol. XXIX (Jan 2-Jun 26, 1897) citing J. J. Thompson. London Elec. Eng., May 7.
  • His reluctance to pay for elaborate or expensive equipment, perhaps the result of an impoverished childhood, had established the legendary "sealing wax-and-string" tradition of the Cavendish, where everyday materials were ingeniously used to make and patch up experimental equipment, with sealing wax proving particularly useful for vacuum seals.
    • Dianna Preston, Before the Fallout from Marie Curie to Hiroshima (2005).
  • J. J. Thompson was about to make the most significant find of the late nineteenth century... Thompson had been investigating the nature of cathode rays. He was convinced that they were some kind of electrified particles and, to prove his theory, began testing their behavior in electric or magnetic fields. By measuring both the extent to which such fields deflected them and their electric charge, he discovered that cathode rays consisted of very small negatively charged particles whose mass was about eighteen hundred times smaller than the lightest known substance—the hydrogen atom. ...He initially named these tiny carriers of electricity "corpuscles." Later they would become known as "electrons." The corpuscles were, in fact, the first subatomic particles to be found...
    • Dianna Preston, Before the Fallout from Marie Curie to Hiroshima (2005).
  • Thompson's work suggested an alternative version—the instability of matter—to that of the indivisible atom. It was revolutionary stuff.
    • Dianna Preston, Before the Fallout from Marie Curie to Hiroshima (2005).
  • Thompson and then his young men demolished a recurrent scientific myth—one that had surfaced again in the 1870's: that there was nothing left to be discovered, nothing new under the sun. Part of the immutable wisdom of the day, endorsed and believed long before the greatest of scientists, Isaac Newton, was a kind of billiard ball theory of the atom, which went back to the ancient Greeks. The word itself is from the Greek atomos, meaning "inidivisible."
    • Richard Reeves, A Force of Nature The Frontier Genius of Ernest Rutherford (2008).

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