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Sir Joseph John “J. J.” Thomson, OM, FRS (18 December 1856 – 30 August 1940) was a Britishmarker physicist and Nobel laureate, credited for the discovery of the electron and of isotopes, and the invention of the mass spectrometer. He was awarded the 1906 Nobel Prize in Physics for the discovery of the electron and his work on the conduction of electricity in gases.


Joseph John Thomson was born in 1856 in Cheetham Hillmarker, Manchestermarker in England, of Scottishmarker parentage. His father died when he was 16 years old. In 1870 he studied engineering at University of Manchestermarker known as Owens College at that time, and moved on to Trinity College, Cambridgemarker in 1876. In 1880, he obtained his BA in mathematics (Second Wrangler and 2nd Smith's prize) and MA (with Adams Prize) in 1883. In 1884 he became Cavendish Professor of Physics. One of his students was Ernest Rutherford, who would later succeed him in the post. In 1890 he married Rose Elisabeth Paget, daughter of Sir George Edward Paget, KCB, a physician and then Regius Professor of Physic at Cambridge. He fathered one son, George Paget Thomson, and one daughter, Joan Paget Thomson, with her. One of Thomson's greatest contributions to modern science was in his role as a highly gifted teacher, as seven of his research assistants and his aforementioned son won Nobel Prizes in physics. His son won the Nobel Prize in 1937 for proving the wavelike properties of electrons.

He was awarded a Nobel Prize in 1906, "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases." He was knighted in 1908 and appointed to the Order of Merit in 1912. In 1914 he gave the Romanes Lecture in Oxfordmarker on "The atomic theory". In 1918 he became Master of Trinity Collegemarker, Cambridgemarker, where he remained until his death. He died on 30 August 1940 and was buried in Westminster Abbeymarker, close to Sir Isaac Newton.

Thomson was elected a Fellow of the Royal Society on 12 June 1884 and was subsequently President of the Royal Society from 1915 to 1920.

Sir Joseph John Thomson.


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.

First experiment

In his first experiment, Thomson investigated the idea that the negative charge in the cathode-ray tube could be separated by using magnetism. In previous work, Jean Baptiste Perrin had demonstrated that negative electric charge is given off by the cathode by setting up a cathode ray tube with a metal cup connected to an electrometer located opposite the cathode. With this apparatus, Perrin detected that a negative electrical charge was built up on the electrometer when cathode rays were passed along the tube. When the cathode rays were deflected away from the metal cup very little charge was detected by the electrometer.

Perrin's experiment had demonstrated that negative electrical charge is emitted from the cathode and that this charge is deflected by magnetic fields. However, objections were raised by scientists whoattributed cathode rays to a process in the aether, to the effect that the transport of negative charge had not been proven to have anything to do with the cathode rays.

Thomson constructed the cathode ray tube shown here, with the metal cup of the electrometer set to one side, away from the undeflected path of the cathode rays. When the tube was activated the electrometer showed no charge unless the beam was magnetically deflected towards the metal cup. Thomson was able to conclude that the negative charge always followed exactly the same path as the cathode rays and was therefore a property of the cathode rays.

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.

Third experiment

Thomson's third experiment.
In his third experiment, Thomson measured the mass-to-charge 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 mass to charge ratio was over a thousand times lower than that of a hydrogen ion (H+), suggesting either that the particles were very light and/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. He conducted this experiment in 1897.

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 of the atom.

Nobel Prize

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. He notes that prior to his work:
  1. the (negatively charged) cathode was known to be the source of the cathode rays;
  2. the cathode rays were known to have the particle-like property of charge;
  3. were deflected by a magnetic field like a negatively charged particle;
  4. had the wave-like property of being able to penetrate thin metal foils;
  5. had not yet been subject to deflection by an electric field.

Thomson succeeded in causing electric deflection because his cathode ray tubes were sufficiently evacuated that they developed only a low density of ions (produced by collisions of the cathode rays with the gas remaining in the tube). Their ion densities were low enough that the gas was a poor conductor, unlike the tubes of previous workers, where the ion density was high enough that the ions could screen out the electric field. He found that the cathode rays (which he called corpuscles) were deflected by an electric field in the same direction as negatively charged particles would deflect. With the electrons moving along, say, the x-direction, the electric field E pointing along the y-direction, and the magnetic field B pointing along the z-direction, by adjusting the ratio of the magnetic field B to the electric field E he found that the cathode rays moved in a nearly straight line, an indication of a nearly uniform velocity v=E/B for the cathode rays emitted by the cathode. He then removed the magnetic field and measured the deflection of the cathode rays, and from this determined the charge-to-mass ratio e/m for the cathode rays. He writes: "however the cathode rays are produced, we always get the same value of e/m for all the particles in the rays. We may...produce great changes in the velocity of the particles, but unless the velocity of the particles becomes so great that they are moving nearly as fast as light, when other considerations have to be taken into account, the value of e/m is constant. The value of e/m is not merely independent of the is independent of the kind of electrodes we use and also of the kind of gas in the tube."

Thomson notes that "corpuscles" are emitted by hot metals and "Corpuscles are also given out by metals and other bodies, but especially by the alkali metals, when these are exposed to light. They are being continually given out in large quantities and with very great velocities by radioactive substances such as uranium and radium; they are produced in large quantities when salts are put into flames, and there is good reason to suppose that corpuscles reach us from the sun." Thomson also describes water drop experiments that enabled him to obtain a value for e that is about twice the modern value, and close to the then current value for the charge on a hydrogen ion in an electrolyte.

Isotopes and mass spectrometry

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 neon is composed of atoms of two different atomic masses (neon-20 and neon-22), that is to say of two isotopes. This was the first evidence for isotopes of a stable element; Frederick Soddy had previously proposed the existence of isotopes to explain the decay of certain radioactive elements.

Thomson's 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.

Other work

In 1905 Thomson discovered the natural radioactivity of potassium.

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

Awards and Recognition

In 1991 the thomson (symbol: Th) was proposed as a unit to measure mass-to-charge ratio in mass spectrometry.


  • 1883. A Treatise on the Motion of Vortex Rings: An essay to which the Adams Prize was adjudged in 1882, in the University of Cambridge. London: Macmillan and Co., pp. 146. Recent reprint: ISBN 0-5439-5696-2.
  • 1888. Applications of Dynamics to Physics and Chemistry. London: Macmillan and Co., pp.326. Recent reprint: ISBN 1-4021-8397-6.
  • 1893. Notes on recent researches in electricity and magnetism: intended as a sequel to Professor Clerk-Maxwell's 'Treatise on Electricity and Magnetism'. Oxford Univ. Press, pp.xvi and 578. 1991, Cornell University Monograph: ISBN 1-4297-4053-1.
  • 1921 (1895). Elements Of The Mathematical Theory Of Electricity And Magnetism. London: Macmillan and Co. Scan of 1895 edition.
  • (with J.H. Poynting). A Text book of Physics in Five Volumes: Properties of Matter, Sound, Heat, Light, and Magnetism & Electricity.
  • Navarro, Jaume, 2005, " Thomson on the Nature of Matter: Corpuscles and the Continuum," Centaurus 47(4): 259-82.


  3. Thomson's 1906 Nobel Prize lecture


  • 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
  • J.J. Thomson, " "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," Philosophical Magazine Series 6, Volume 7, Number 39, pp 237-265. This paper presents the classical "plum pudding model" from which the Thomson Problem is posed.
  • The Master of Trinity at Trinity College, Cambridgemarker
  • JJ Thomson, The Electron in Chemistry: Being Five Lectures Delivered at the Franklin Institute, Philadelphia (1923).

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