Editing J. J. Thomson (section)

==Career and research==
===Overview===
On 22 December 1884, Thomson was appointed [[Cavendish Professor of Physics]] at the [[University of Cambridge]].<ref name="Profile">{{cite web|title=Joseph John "J. J." Thomson|url=https://www.sciencehistory.org/historical-profile/joseph-john-j-j-thomson|publisher=[[Science History Institute]]|access-date=20 March 2018|date=June 2016}}</ref> The appointment caused considerable surprise, given that candidates such as [[Osborne Reynolds]] or [[Richard Glazebrook]] were older and more experienced in laboratory work.  Thomson was known for his work as a mathematician, where he was recognised as an exceptional talent.<ref name="Leadership">{{cite book|last1=Kim|first1=Dong-Won|title=Leadership and creativity : a history of the Cavendish Laboratory, 1871–1919|date=2002|publisher=Kluwer Acad. Publ.|location=Dordrecht|isbn=978-1402004759|url=https://books.google.com/books?id=iN13QvH8vnwC&pg=PA51|access-date=11 February 2015}}</ref>

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 [[Knight Bachelor|knighted]] in 1908 and appointed to the [[Order of Merit (Commonwealth)|Order of Merit]] in 1912. In 1914, he gave the [[Romanes Lecture]] in [[University of Oxford|Oxford]] on "The atomic theory". In 1918, he became Master of [[Trinity College, Cambridge|Trinity College]], [[University of Cambridge|Cambridge]], where he remained until his death. He died on 30 August 1940; his ashes rest in [[Westminster Abbey]],<ref>'The Abbey Scientists' Hall, A.R. p. 63: London; Roger & Robert Nicholson; 1966</ref> near the graves of Sir [[Isaac Newton]] and his former student [[Ernest Rutherford]].<ref name="sirJJrestingplace">{{cite web|last=Westminster Abbey| title= Sir Joseph John Thomson| url=http://www.westminster-abbey.org/our-history/people/sir-joseph-john-thomson}}</ref>

Rutherford succeeded him as [[Cavendish Professor of Physics]]. Six of Thomson's research assistants and junior colleagues ([[Charles Glover Barkla]],<ref>{{cite web |title=Charles Glover Barkla – Biographical |url=https://www.nobelprize.org/prizes/physics/1917/barkla/biographical/ |website=The Nobel Prize |publisher=Nobel Lectures, Physics 1901–1921, Elsevier Publishing Company |access-date=11 October 2022 |date=1967 |quote=he worked under J. J. Thomson at the Cavendish Laboratory in Cambridge.}}</ref> [[Niels Bohr]],<ref>{{cite web |title=Niels Bohr – Biographical |url=https://www.nobelprize.org/prizes/physics/1922/bohr/biographical/ |website=The Nobel Prize |publisher=Nobel Lectures, Physics 1922–1941, Elsevier Publishing Company, Amsterdam |access-date=18 October 2022 |date=1965 |quote=he made a stay at Cambridge, where he profited by following the experimental work going on in the Cavendish Laboratory under Sir J.J. Thomson’s guidance}}</ref> [[Max Born]],<ref>{{cite web |title=Max Born- Biographical |url=https://www.nobelprize.org/prizes/chemistry/1922/aston/biographical/ |website=The Nobel Prize |publisher=Nobel Lectures, Physics 1942–1962, Elsevier Publishing Company |access-date=11 October 2022 |date=1964 |quote=Born next went to Cambridge for a short time, to study under Larmor and J. J. Thomson.}}</ref> [[William Henry Bragg]], [[Owen Willans Richardson]]<ref>{{cite web |title=Sir Owen Willans Richardson, British physicist |url=https://www.britannica.com/biography/Owen-Willans-Richardson |work=Encyclopædia Britannica |access-date=18 October 2022 |quote=Richardson, a graduate (1900) of Trinity College, Cambridge, and a student of J. J. Thomson at the Cavendish Laboratory}}</ref> and [[Charles Thomson Rees Wilson]]<ref name="frs">{{Cite journal | last1 = Rayleigh | doi = 10.1098/rsbm.1941.0024 |title = Joseph John Thomson. 1856–1940 | journal = [[Obituary Notices of Fellows of the Royal Society]] | volume = 3 | issue = 10 | pages = 586–609 | year = 1941 | doi-access = free }}</ref>) won Nobel Prizes in physics, and two ([[Francis William Aston]]<ref>{{cite web |title=Francis W. Aston – Biographical |url=https://www.nobelprize.org/prizes/chemistry/1922/aston/biographical// |website=The Nobel Prize |publisher=Nobel Lectures, Physics 1922–1941, Elsevier Publishing Company |access-date=13 October 2022 |date=1966 |quote=At the end of 1909 he accepted the invitation of Sir J. J. Thomson to work as his assistant at the Cavendish Laboratory}}</ref> and Ernest Rutherford<ref name="nobelprize">{{cite web|title=Ernest Rutherford – Biography|url=https://www.nobelprize.org/prizes/chemistry/1908/rutherford/biographical/|publisher=NobelPrize.org|access-date=6 August 2013 |quote=as a research student at the Cavendish Laboratory under J.J. Thomson.}}</ref>) won Nobel prizes in chemistry. Thomson's son ([[George Paget Thomson]]) also won the 1937 Nobel Prize in physics for proving the wave-like properties of electrons.<ref>{{cite web |title=George Paget Thomson Biographical |url=https://www.nobelprize.org/prizes/physics/1937/thomson/biographical/ |website=The Nobel Prize |access-date=8 June 2022 |quote=he carried out experiments on the behaviour of electrons ... which showed that electrons behave as waves ...}}</ref>

===Early work===
Thomson's prize-winning master's work, ''Treatise on the motion of vortex rings'', shows his early interest in atomic structure.<ref name="Nobel1906" /> In it, Thomson mathematically described the motions of [[William Thomson, 1st Baron Kelvin|William Thomson]]'s vortex theory of atoms.<ref name="Leadership" />
 
Thomson published a number of papers addressing both mathematical and experimental issues of electromagnetism.  He examined the [[electromagnetic theory of light]] of [[James Clerk Maxwell]], introduced the concept of [[Electromagnetic mass|electromagnetic mass of a charged particle]], and demonstrated that a moving charged body would apparently increase in mass.<ref name="Leadership" />

Much of his work in mathematical modelling of chemical processes can be thought of as early [[computational chemistry]].<ref name="Profile" /> In further work, published in book form as ''Applications of dynamics to physics and chemistry'' (1888), Thomson addressed the transformation of energy in mathematical and theoretical terms, suggesting that all energy might be kinetic.<ref name="Leadership" /> His next book, ''Notes on recent researches in electricity and magnetism'' (1893), built upon Maxwell's ''Treatise upon electricity and magnetism'', and was sometimes referred to as "the third volume of Maxwell".<ref name="Nobel1906" /> In it, Thomson emphasized physical methods and experimentation and included extensive figures and diagrams of apparatus, including a number for the passage of electricity through gases.<ref name="Leadership" /> His third book, [http://catalog.hathitrust.org/Record/001985977 ''Elements of the mathematical theory of electricity and magnetism''] (1895)<ref>{{cite journal|author=Mackenzie, A. Stanley|author-link=Arthur Stanley Mackenzie|title=Review: ''Elements of the Mathematical Theory of Electricity and Magnetism'' by J. J. Thomson|journal=Bull. Amer. Math. Soc.|year=1896|volume=2|issue=10|pages=329–333|url=https://www.ams.org/journals/bull/1896-02-10/S0002-9904-1896-00357-8/S0002-9904-1896-00357-8.pdf|doi=10.1090/s0002-9904-1896-00357-8|doi-access=free}}</ref> was a readable introduction to a wide variety of subjects, and achieved considerable popularity as a textbook.<ref name="Leadership" />
[[File:Thomson-13.jpg|alt=First page to Notes on Recent Researches in Electricity and Magnetism (1893)|thumb|200x200px|First page to ''Notes on Recent Researches in Electricity and Magnetism'' (1893)]]
A series of four lectures, given by Thomson on a visit to [[Princeton University]] in 1896, were subsequently published as ''Discharge of electricity through gases'' (1897).  Thomson also presented a series of six lectures at [[Yale University]] in 1904.<ref name="Nobel1906" />

===Discovery of the electron===
Several scientists, such as [[William Prout]] and [[Norman Lockyer]], had suggested that atoms were built up from a more fundamental unit, but they envisioned this unit to be the size of the smallest atom, hydrogen. Thomson in 1897 was the first to suggest that one of the fundamental units of the atom was more than 1,000 times smaller than an atom, suggesting the subatomic particle now known as the electron. Thomson discovered this through his explorations on the properties of cathode rays. Thomson made his suggestion on 30 April 1897 following his discovery that cathode rays (at the time known as [[Philipp Lenard|Lenard rays]]) could travel much further through air than expected for an atom-sized particle.<ref name="referenceB">{{cite journal |last=Thomson |first=J.J. |year=1897 |url=https://books.google.com/books?id=vBZbAAAAYAAJ&pg=PA104 |title=Cathode Rays |journal=The Electrician |volume=39 |page=104}}</ref> He estimated the mass of cathode rays by measuring the heat generated when the rays hit a thermal junction and comparing this with the magnetic deflection of the rays. His experiments suggested not only that cathode rays were over 1,000 times lighter than the hydrogen atom, but also that their mass was the same in whichever type of atom they came from. He concluded that the rays were composed of very light, negatively charged particles which were a universal building block of atoms. He called the particles "corpuscles", but later scientists preferred the name [[electron]] which had been suggested by [[George Johnstone Stoney]] in 1891, prior to Thomson's actual discovery.<ref>{{cite book |last=Falconer |first=Isobel  |year=2001 |chapter=Corpuscles to electrons |chapter-url=https://isobelf.files.wordpress.com/2013/08/falconer_corpusclestoelectrons_preprint.pdf |editor1-last=Buchwald |editor1-first=J. Z.|editor2-last=Warwick |editor2-first=A. |title=Histories of the Electron |publisher=MIT Press |pages=77–100 |isbn=978-0262024945}}</ref>

In April 1897, Thomson had only early indications that the cathode rays could be deflected electrically (previous investigators such as [[Heinrich Hertz]] had thought they could not be). A month after Thomson's announcement of the corpuscle, he found that he could reliably deflect the rays by an electric field if he evacuated the discharge tube to a very low pressure. By comparing the deflection of a beam of cathode rays by electric and magnetic fields he obtained more robust measurements of the mass-to-charge ratio that confirmed his previous estimates.<ref name="PhilMag">{{cite journal|last1=Thomson|first1=J. J.|title=Cathode Rays|journal=Philosophical Magazine|date=7 August 1897|volume=44|issue=269|page=293|url=https://zenodo.org/records/1431235/files/article.pdf|access-date=4 August 2014|series=5|doi=10.1080/14786449708621070}}</ref> This became the classic means of measuring the charge-to-mass ratio of the electron. Later in 1899 he measured the charge of the electron to be of {{val|6.8|e=−10|u=esu}}.<ref>{{cite journal |last1=Thomson |first1=J.J. |title=On the masses of the ions in gases at low pressures |journal=The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science |date=1899 |volume=48 |issue=295 |pages=547–567 |doi=10.1080/14786449908621447 |url=https://www.tandfonline.com/doi/abs/10.1080/14786449908621447 |access-date=28 December 2024}}</ref>

Thomson believed that the corpuscles emerged from the atoms of the trace gas inside his [[cathode-ray tube]]s. He thus concluded that atoms were divisible, and that the corpuscles were their building blocks. In 1904, Thomson suggested a model of the atom, hypothesizing that it was a sphere of positive matter within which electrostatic forces determined the positioning of the corpuscles.<ref name="Profile" /> To explain the overall neutral charge of the atom, he proposed that the corpuscles were distributed in a uniform sea of positive charge.  In this "[[plum pudding model]]", the electrons were seen as embedded in the positive charge like raisins in a plum pudding (although in Thomson's model they were not stationary, but orbiting rapidly).<ref>{{citation|title=Modern Inorganic Chemistry|first=Joseph William|last=Mellor|publisher=Longmans, Green and Company|year=1917|page=868|url=https://books.google.com/books?id=1iQ7AQAAMAAJ&pg=PA868|quote=According to J. J. Thomson's hypothesis, atoms are built of systems of rotating rings of electrons.}}</ref><ref>{{harvtxt|Dahl|1997}}, p. 324: "[https://books.google.com/books?id=xUzaWGocMdMC&pg=PA324 Thomson's model, then, consisted of a uniformly charged sphere of positive electricity (the pudding), with discrete corpuscles (the plums) rotating about the center in circular orbits, whose total charge was equal and opposite to the positive charge.]"</ref>

Thomson made the discovery around the same time that [[Walter Kaufmann (physicist)|Walter Kaufmann]] and [[Emil Wiechert]] discovered the correct mass to charge ratio of these cathode rays (electrons).<ref>{{cite journal |last1=Chown |first1=Marcus |title=Forum: Just who did discover the electron? |journal=New Scientist |date=29 March 1997 |issue=2075 |url=https://www.newscientist.com/article/mg15320756-400-forum-just-who-did-discover-the-electron-marcus-chown-says-the-truth-is-not-quite-as-the-history-books-suggest/ |access-date=17 October 2020 |quote=Marcus Chown says the truth is not quite as the history books suggest.}}</ref>

The name "electron" was adopted for these particles by the scientific community, mainly due to the advocation by [[George Francis FitzGerald]], [[Joseph Larmor]], and [[Hendrik Lorentz]].<ref name=OHara1975>
{{cite journal
 | last =O'Hara
 | first =J. G.
 | title =George Johnstone Stoney, F.R.S., and the Concept of the Electron
 | journal =Notes and Records of the Royal Society of London
 | volume =29
 | issue =2
 | pages =265–276
 | publisher =Royal Society
 | date =March 1975
 | jstor =531468
 | doi =10.1098/rsnr.1975.0018
 | s2cid =145353314
}}</ref>{{rp|273}} The term was originally coined by [[George Johnstone Stoney]] in 1891 as a tentative name for the basic unit of electrical charge (which had then yet to be discovered).<ref>{{cite journal |author=George Johnstone Stoney |year=1891 |title=On the Cause of Double Lines and of Equidistant Satellites in the Spectra of Gases |journal=The Scientific Transactions of the Royal Dublin Society |volume=4 |pages=583–608 |url=https://digitalarchive.rds.ie/files/show/4769}}</ref><ref>{{cite journal |journal=Philosophical Magazine |author=George Johnstone Stoney |date=1894 |title=Of the "Electron", or Atom of Electricity |series=Series 5 |volume=38 |issue=233 |pages=418–420 |url=https://archive.org/details/londonedinburgh5381894lon/page/418/mode/2up}}</ref> For some years Thomson resisted using the word "electron" because he didn't like how some physicists talked of a "positive electron" that was supposed to be the elementary unit of positive charge just as the "negative electron" is the elementary unit of negative charge. Thomson preferred to stick with the word "corpuscle" which he strictly defined as negatively charged.<ref>{{cite journal |year=1907 |author=J. J. Thomson |title=The Modern Theory of Electrical Conductivity of Metals |journal=Journal of the Institution of Electrical Engineers |volume=38 |issue=183 |pages=455–468|doi=10.1049/jiee-1.1907.0026 |url=https://books.google.com/books?id=Ni9HAQAAMAAJ&pg=PA467}}: "Perhaps I can best show my appreciation by trying to answer the questions which Professor Silvanus Thompson addressed to me. I think his first question was a question rather of notation, as to the difference between the electron and the corpuscle. I prefer the corpuscle for two reasons: first of all, it is my own child, and I have a kind of parental affection for it; and, secondly, I think it has one merit which the term electron has not. We talk about positive and negative electrons, and I think when you use the same term for the two the suggestion is that there is an equality, so to speak, in the properties. From my point of view the difference between the negative and the positive is essential, and much greater than I think would be suggested by the term positive electron and negative electron. Therefore I prefer to use a special term for the negative units and call it a corpuscle. A corpuscle is just a negative electron."</ref> He relented by 1914, using the word "electron" in his book ''The Atomic Theory''.<ref>{{cite book |author=J. J. Thomson |year=1914 |title=The Atomic Theory |publisher=Oxford Clarendon Press |url=https://archive.org/details/atomictheorythom00thomrich/page/n3/mode/2up}}</ref> In 1920, Rutherford and his fellows agreed to call the nucleus of the hydrogen ion "proton", establishing a distinct name for the smallest known positively-charged particle of matter (that can exist independently anyway).<ref>{{cite journal |author=Orme Masson |date=1921 |title=The Constitution of Atoms |journal=The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science |volume=41 |issue=242 |pages=281–285 |doi=10.1080/14786442108636219 |url=https://zenodo.org/records/1430963/files/article.pdf%3Fdownload%3D1&ved=2ahUKEwjN-YzeqIiGAxVyUqQEHaM_COwQFnoECBwQAQ&usg=AOvVaw1G76aUFXByKGSDekUwv2sa}}<br/>Footnote by Ernest Rutherford: 'At the time of writing this paper in Australia, Professor Orme Masson was not aware that the name "proton" had already been suggested as a suitable name for the unit of mass nearly 1, in terms of oxygen 16, that appears to enter into the nuclear structure of atoms. The question of a suitable name for this unit was discussed at an informal meeting of a number of members of Section A of the British Association [for the Advancement of Science] at Cardiff this year. The name "baron" suggested by Professor Masson was mentioned, but was considered unsuitable on account of the existing variety of meanings. Finally the name " proton" met with general approval, particularly as it suggests the original term "protyle " given by Prout in his well-known hypothesis that all atoms are built up of hydrogen. The need of a special name for the nuclear unit of mass 1 was drawn attention to by Sir Oliver Lodge at the Sectional meeting, and the writer then suggested the name "proton."'</ref>

===Isotopes and mass spectrometry===
[[File:Discovery of neon isotopes.JPG|thumb|In the bottom right corner of this photographic plate are markings for the two isotopes of neon: neon-20 and neon-22.]]
In 1912, as part of his exploration into the composition of the streams of positively charged particles then known as [[canal rays]], Thomson and his research assistant [[Francis William Aston|F. W. Aston]] channelled a stream of neon ions through a magnetic and an electric field and measured its deflection by placing a photographic plate in its path.<ref name="ReferenceA" /> They observed two patches of light on the photographic plate (see image on right), which suggested two different parabolas of deflection, and concluded that neon is composed of atoms of two different atomic masses (neon-20 and neon-22), that is to say of two [[isotope]]s.<ref>J.J. Thomson (1912) "Further experiments on positive rays," ''Philosophical Magazine'', series 6, '''24''' (140):  209–253.</ref><ref>J. J. Thomson (1913) "Rays of positive electricity", ''Proceedings of the Royal Society'' A, '''89''': 1–20.</ref> 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 [[Francis William Aston|F. W. Aston]] and by [[A. J. Dempster]].<ref name="Profile" /><ref name="Jones">{{cite web |author-first=Mark |author-last=Jones|title=Gas Chromatography-Mass Spectrometry  |url=https://www.acs.org/content/acs/en/education/whatischemistry/landmarks/gas-chromatography-mass-spectrometry.html |publisher=American Chemical Society |access-date=19 November 2019}}</ref>

{{external media | width = 180px | float = right | headerimage= [[File:Title page On the Chemical Combination of Gases by Joseph John Thomson 1856-1940.jpg|180px]] | video1 = [https://www.youtube.com/watch?v=WH-U_qCEzT0 ''The Early Life of J. J. Thomson: Computational Chemistry and Gas Discharge Experiments'']}}

===Experiments with cathode rays===
Earlier, physicists debated whether cathode rays were immaterial like light ("some process in the [[luminiferous aether|aether]]") or were "in fact wholly material, and ... mark the paths of particles of matter charged with negative electricity", quoting Thomson.<ref name="PhilMag" /> The aetherial hypothesis was vague,<ref name="PhilMag" /> but the particle hypothesis was definite enough for Thomson to test.

====Magnetic deflection====
Thomson first investigated the [[magnetic deflection]] of cathode rays. Cathode rays were produced in the side tube on the left of the apparatus and passed through the anode into the main [[bell jar]], where they were deflected by a magnet. Thomson detected their path by the [[fluorescence]] on a squared screen in the jar. He found that whatever the material of the anode and the gas in the jar, the deflection of the rays was the same, suggesting that the rays were of the same form whatever their origin.<ref>{{cite journal |last=Thomson |first=J. J. |date=8 February 1897 |title=On the cathode rays |journal=Proceedings of the Cambridge Philosophical Society |volume=9 |page=243}}</ref>

====Electrical charge ====
[[File:JJ Thomson Cathode Ray Tube 1.png|left|thumb|The cathode-ray tube by which J.&nbsp;J. Thomson demonstrated that cathode rays could be deflected by a magnetic field, and that their negative charge was not a separate phenomenon]]
While supporters of the aetherial theory accepted the possibility that negatively charged particles are produced in [[Crookes tube]]s,{{Citation needed|date=June 2012}} they believed that they are a mere by-product and that the cathode rays themselves are immaterial.{{Citation needed|date=June 2012}} Thomson set out to investigate whether or not he could actually separate the charge from the rays.

Thomson constructed a Crookes tube with an [[electrometer]] set to one side, out of the direct path of the cathode rays. Thomson could trace the path of the ray by observing the phosphorescent patch it created where it hit the surface of the tube. Thomson observed that the electrometer registered a charge only when he deflected the cathode ray to it with a magnet. He concluded that the negative charge and the rays were one and the same.<ref name="referenceB"/>
{{Clear}}

====Electrical deflection====
{{more citations needed|section|date=August 2017}}<!--only first paragraph has a citation-->
{{multiple image
 | align = right 
 | direction = vertical
 | width = 452
 | footer =
 | image1 = JJ Thomson Cathode Ray 2.png
 | alt1 =
 | caption1 = Thomson's illustration of the Crookes tube by which he observed the deflection of cathode rays by an electric field (and later measured their mass-to-charge ratio). Cathode rays were emitted from the cathode C, passed through slits A (the anode) and B ([[Ground (electricity)|grounded]]), then through the electric field generated between plates D and E, finally impacting the surface at the far end.
 | image2 = Thomson cathode ray exp.gif
 | alt2 =
 | caption2 = The cathode ray (blue line) was deflected by the electric field (yellow).
}}
[[File:JJThomsonGasDischargeTubeElectronCavendishLab2013-08-29-17-11-41.jpg|left|thumb|Cathode-ray tube with electrical deflection]]

In May–June 1897, Thomson investigated whether or not the rays could be deflected by an electric field.<ref name="ReferenceA"/> Previous experimenters had failed to observe this, but Thomson believed their experiments were flawed because their tubes contained too much gas.

Thomson constructed a [[Crookes tube]] with a better vacuum. At the start of the tube was the cathode from which the rays projected. The rays were sharpened to a beam by two metal slits – the first of these slits doubled as the anode, the second was connected to the earth. The beam then passed between two parallel aluminium plates, which produced an electric field between them when they were connected to a battery. The end of the tube was a large sphere where the beam would impact on the glass, created a glowing patch. Thomson pasted a scale to the surface of this sphere to measure the deflection of the beam. Any electron beam would collide with some residual gas atoms within the Crookes tube, thereby ionizing them and producing electrons and ions in the tube ([[space charge]]); in previous experiments this space charge electrically screened the externally applied electric field. However, in Thomson's Crookes tube the density of residual atoms was so low that the space charge from the electrons and ions was insufficient to electrically screen the externally applied electric field, which permitted Thomson to successfully observe electrical deflection.

When the upper plate was connected to the negative pole of the battery and the lower plate to the positive pole, the glowing patch moved downwards, and when the polarity was reversed, the patch moved upwards.
{{Clear}}

====Measurement of mass-to-charge ratio====

[[File:JJ Thomson exp3.gif|thumb]]

In his classic experiment, Thomson measured the [[mass-to-charge ratio]] of the cathode rays by measuring how much they were deflected by a magnetic field and comparing this with the electric deflection. He used the same apparatus as in his previous experiment, but placed the discharge tube between the poles of a large electromagnet. He found that the mass-to-charge ratio was over a thousand times ''lower'' than that of a hydrogen ion (H<sup>+</sup>), suggesting either that the particles were very light and/or very highly charged.<ref name="PhilMag"/> Significantly, the rays from every cathode yielded the same mass-to-charge ratio. This is in contrast to [[anode rays]] (now known to arise from positive ions emitted by the anode), where the mass-to-charge ratio varies from anode-to-anode.  Thomson himself remained critical of what his work established, in his Nobel Prize acceptance speech referring to "corpuscles" rather than "electrons".

Thomson's calculations can be summarised as follows (in his original notation, using F instead of E for the electric field and H instead of B for the magnetic field):

The electric deflection is given by <math>\Theta = Fel / mv^2</math>, where Θ is the angular electric deflection, F is applied electric intensity, e is the charge of the cathode ray particles, l is the length of the electric plates, m is the mass of the cathode ray particles and v is the velocity of the cathode ray particles. The magnetic deflection is given by <math>\phi = Hel / mv</math>, where φ is the angular magnetic deflection and H is the applied magnetic field intensity.

The magnetic field was varied until the magnetic and electric deflections were the same, when  <math>\Theta = \phi, Fel / mv^2 = Hel / mv</math>. This can be simplified to give <math>m/e = H^2 l/F\Theta</math>. The electric deflection was measured separately to give Θ and H, F and l were known, so m/e could be calculated.
{{Clear}}

====Conclusions====
{{blockquote|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.|J. J. Thomson<ref name="PhilMag" />}}

As to the source of these particles, Thomson believed they emerged from the molecules of gas in the vicinity of the cathode.

{{blockquote|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.|J. J. Thomson<ref name="Philosophical Magazine 1897">{{cite journal |last=Thomson |first=J. J.|url=http://web.lemoyne.edu/~GIUNTA/thomson1897.html |title=Cathode rays |journal=Philosophical Magazine |volume=44 |page=293 |year=1897}}</ref>}}

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

===Other work===
In 1905, Thomson discovered the natural [[radioactivity]] of [[potassium]].<ref name='Phil Mag 1905'>{{cite journal|doi=10.1080/14786440509463405|title=On the emission of negative corpuscles by the alkali metals|journal=Philosophical Magazine |series=Series 6|year=1905|first=J. J. |last=Thomson|volume=10|issue=59|pages=584–590|url=https://zenodo.org/record/1430786}}</ref>

In 1906, Thomson demonstrated that [[hydrogen]] had only a single [[electron]] per atom. Previous theories allowed various numbers of electrons.<ref>{{The Timetables of Science|pages=411}}</ref><ref name='Phil Mag 1906'>{{cite journal|title=On the Number of Corpuscles in an Atom |journal=Philosophical Magazine |date=June 1906 |first=J. J. |last=Thomson |volume=11 |issue= 66|pages=769–781 |url=https://zenodo.org/record/1430808 |access-date=4 October 2008 |doi=10.1080/14786440609463496 |url-status= }}</ref>