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Cathode rays

Thomson conducted a series of experiments with cathode rays and cathode ray tubes leading him to the discovery of electrons and subatomic particles. Thomson used the cathode ray tube in three different experiments.

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First experiment

In his first experiment, he investigated whether or not the negative charge could be separated from the cathode rays by means of magnetism. He constructed a cathode ray tube ending in a pair of cylinders with slits in them. These slits were in turn connected to an electrometer. Thomson found that if the rays were magnetically bent such that they could not enter the slit, the electrometer registered little charge. Thomson concluded that the negative charge was inseparable from the rays.

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Second experiment

Thomson's second experiment.

In his second experiment, he investigated whether or not the rays could be deflected by an electric field (something that is characteristic of charged particles). Previous experimenters had failed to observe this, but Thomson believed their experiments were flawed because they contained trace amounts of gas. Thomson constructed a cathode ray tube with a practically perfect vacuum, and coated one end with phosphorescent paint. Thomson found that the rays did indeed bend under the influence of an electric field, in a direction indicating a negative charge.

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Third experiment

Thomson's third experiment.

In his third experiment, Thomson measured the charge-to-mass ratio of the cathode rays by measuring how much they were deflected by a magnetic field and how much energy they carried. He found that the charge to mass ratio was over a thousand times higher than that of a hydrogen ion (H⁺), suggesting either that the particles were very light or very highly charged.

Thomson's conclusions were bold: cathode rays were indeed made of particles which he called "corpuscles", and these corpuscles came from within the atoms of the electrodes themselves, meaning that atoms are in fact divisible. The "corpuscles" discovered by Thomson are identified with the electrons which had been proposed by G. Johnstone Stoney.

Thomson imagined the atom as being made up of these corpuscles swarming in a sea of positive charge; this was his plum pudding model. This model was later proved incorrect when Ernest Rutherford showed that the positive charge is concentrated in the nucleus.

Thomson's discovery was made known in 1897, and caused a sensation in scientific circles, eventually resulting in him being awarded a Nobel Prize in Physics in 1906.

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Isotopes and mass spectrometry

In the bottom right corner of this photographic plate are markings for the two isotopes of neon: neon-20 and neon-22.

In 1913, as part of his exploration into the composition of canal rays, Thomson channelled a stream of ionized neon through a magnetic and an electric field and measured its deflection by placing a photographic plate in its path. Thomson observed two patches of light on the photographic plate (see image on right), which suggested two different parabolas of deflection. Thomson concluded that the neon gas was composed of atoms of two different atomic masses (neon-20 and neon-22).

This separation of neon isotopes by their mass was the first example of mass spectrometry, which was subsequently improved and developed into a general method by Thomson's student F. W. Aston and by A. J. Dempster.

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Other work

In 1906 Thomson demonstrated that hydrogen had only a single electron per atom. Previous theories allowed various numbers of electrons.^[1]

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Awards

• Royal Medal (1894)
• Hughes Medal (1902)
• Nobel Prize for Physics (1906)
• Copley Medal (1914)

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Bibliography

• A Treatise on the Motion of Vortex Rings: An essay to which the Adams Prize was adjudged in 1882, in the University of Cambridge (1883), Macmillan and Co., London, pp.146, ISBN 0-5439-5696-2 (recent reprint)
• Applications of Dynamics to Physics and Chemistry (1888), Macmillan and Co., London, pp.326, ISBN 1-4021-8397-6 (recent reprint)
• Notes on recent researches in electricity and magnetism: intended as a sequel to Professor Clerk-Maxwell's 'Treatise on Electricity and Magnetism' (1893), Oxford at the Clarendon Press, pp.xvi and 578, ISBN 1-4297-4053-1 (1991 Cornell University Monograph)
• Elements Of The Mathematical Theory Of Electricity And Magnetism (1895 1st Ed; 1921 5th Ed), Macmillan and Co., London, an on-line scan of the 1895 Ed.
• A Text book of Physics in Five Volumes: Properties of Matter, Sound, Heat, Light, and Magnetism & Electricity by Thomson and J.H. Poynting

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References

• Dahl, Per F., "Flash of the Cathode Rays: A History of J.J. Thomson's Electron". Institute of Physics Publishing. June, 1997. ISBN 0-7503-0453-7
• JJ Thomson (1897), Cathode rays, Philosophical Magazine, 44, 293 — Discovery of the electron
• JJ Thomson (1913), Rays of positive electricity, Proceedings of the Royal Society, A 89, 1-20 — Discovery of neon isotopes
• "On the Structure of the Atom": an Investigation of the Stability and Periods of Oscillation of a number of Corpuscles arranged at equal intervals around the Circumference of a Circle; with Application of the Results to the Theory of Atomic Structure" — J.J. Thomson's 1904 paper proposing the plum pudding model.
• The Master of Trinity at Trinity College, Cambridge

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External links

Wikimedia Commons has media related to:

Joseph John Thomson

• The Discovery of the Electron
• The Nobel Prize in Physics 1906
• Annotated bibliography for Joseph J. Thomson from the Alsos Digital Library
• Essay on Thomson life and religious views
• The Cathode Ray Tube site
• Notes on recent researches in electricity and magnetism Cornell University Library Historical Monographs Collection. {Reprinted by} Cornell University Library Digital Collections