|J. J. Thomson|
Sir Joseph John Thomson (1856-1940). Portrait by Arthur Hacker.
|Born||18 December 1856
Cheetham Hill, Manchester, UK
|Died||30 August 1940 (aged 83)
|Alma mater||University of Manchester
University of Cambridge
|Academic advisors||John Strutt (Rayleigh)
Edward John Routh
|Notable students||Charles Glover Barkla
Charles T. R. Wilson
Francis William Aston
J. Robert Oppenheimer
William Henry Bragg
H. Stanley Allen
Daniel Frost Comstock
T. H. Laby
Balthasar van der Pol
Geoffrey Ingram Taylor
|Known for||Plum pudding model
Discovery of electron
Discovery of isotopes
Mass spectrometer invention
First m/e measurement
Proposed first waveguide
Coining term 'delta ray'
Coining term 'epsilon radiation'
|Notable awards||Nobel Prize for Physics (1906)|
Thomson is the father of Nobel laureate George Paget Thomson.
Sir Joseph John “J. J.” Thomson, OM, FRS (18 December 1856 – 30 August 1940) was a British 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 Hill, Manchester, England. His mother came from a local textile family. His father ran an antiquarian bookshop founded by a great-grandfather from Scotland (hence the Scottish spelling of his surname). He died when Thomson was 16 years old. In 1870 he studied engineering at University of Manchester known as Owens College at that time, and moved on to Trinity College, Cambridge 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 Oxford on "The atomic theory". In 1918 he became Master of Trinity College, Cambridge, where he remained until his death. He died on 30 August 1940 and was buried in Westminster Abbey, 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.
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.
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.
|"This experiment shows that however we twist and deflect the cathode rays by magnetic forces, the negative electrification follows the same path as the rays, and that this negative electrification is indissolubly connected with the cathode rays"|
|—J. J. Thomson|
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 who attributed 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.
When Thomson studied cathode ray tubes in 1895, he realized that the resulting rays were little more than streams of negatively charged particles. By bending the rays with a magnetic source and then measuring energy of the rays he could deduce the average mass per charge ratio of the particles in the tube. The ratio he uncovered was over a thousand times smaller than the known ratio of a hydrogen ion (aka proton) meaning it must be orders of magnitude smaller. This particle turns out to be what is now known as the electron.
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.
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.
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:
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 velocity...it 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.
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.
Henry Montagu Butler
|Master of Trinity College, Cambridge
George Macaulay Trevelyan
Sir Joseph John Thomson, OM, FRS (18 December 1856 – 30 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.
|Born||18 December 1856|
Cheetham Hill, Manchester, UK
|Died||30 August 1940 (aged 83)|
|Alma mater||University of ManchesterUniversity of Cambridge|
|Known for||Plum pudding modelDiscovery of electronDiscovery of isotopesMass spectrometer inventionFirst m/e measurementProposed first waveguideThomson scatteringThomson problemCoining term 'delta ray'Coining term 'epsilon radiation'Thomson (unit)|
|Thomson is the father of Nobel laureate George Paget Thomson.|
Sir Joseph John "J.J." Thomson, OM, FRS (18 December 1856 – 30 August 1940) was a British physicist and Nobel laureate. He discovered the electron and isotopes, and invented the mass spectrometer. he was awarded the Nobel Prize in Physics in 1906 for his discovery of the electron and his work on the conduction of electricity in gases. John Joseph Thomson in1893 said : "There is no other branch of physics which affords us so promising an opportunity of penetrating the secret of electricity."