Editing Electron (section)

=== Discovery of free electrons outside matter ===
{{see also|Cathode ray|J. J. Thomson#Discovery of the electron}}

[[File:Cyclotron motion wider view.jpg|right|thumb|alt=A round glass vacuum tube with a glowing circular beam inside|A beam of electrons deflected by a magnetic field into a circle<ref>
{{cite book
 | last1 = Born
 | first1 = M.
 | last2 = Blin-Stoyle
 | first2 = R.J.
 | last3 = Radcliffe
 | first3 = J.M.
 | year = 1989
 | title = Atomic Physics
 | url = https://books.google.com/books?id=NmM-KujxMtoC&pg=PA26
 | page = 26
 | publisher = [[Courier Dover]]
 | isbn = 978-0-486-65984-8
 | access-date = 2020-08-25
 | archive-date = 2021-01-26
 | archive-url = https://web.archive.org/web/20210126003322/https://books.google.com/books?id=NmM-KujxMtoC&pg=PA26
 | url-status = live
}}</ref>]]

While studying electrical conductivity in [[rarefied]] gases in 1859, the German physicist [[Julius Plücker]] observed the radiation emitted from the cathode caused phosphorescent light to appear on the tube wall near the cathode; and the region of the phosphorescent light could be moved by application of a magnetic field.<ref>{{Cite journal|last=Plücker|first=M.|date=1858-12-01|title=XLVI. Observations on the electrical discharge through rarefied gases|url=https://doi.org/10.1080/14786445808642591|journal=The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science|volume=16|issue=109|pages=408–418|doi=10.1080/14786445808642591|issn=1941-5982}}</ref>  In 1869, Plücker's student [[Johann Wilhelm Hittorf]] found that a solid body placed in between the cathode and the phosphorescence would cast a shadow upon the phosphorescent region of the tube. Hittorf inferred that there are straight rays emitted from the cathode and that the phosphorescence was caused by the rays striking the tube walls. Furthermore, he also discovered that these rays are deflected by magnets just like lines of current.<ref>{{Cite book |last=Darrigol |first=Olivier |url=https://books.google.com/books?id=ysMf2pAid94C&pg=PA277 |title=Electrodynamics from Ampère to Einstein |date=2003 |publisher=OUP Oxford |isbn=978-0-19-850593-8 |language=en}}</ref>

In 1876, the German physicist [[Eugen Goldstein]] showed that the rays were emitted perpendicular to the cathode surface, which distinguished between the rays that were emitted from the cathode and the incandescent light.  Goldstein dubbed the rays [[cathode ray]]s.<ref name="leicester" /><ref name="Whittaker">
{{cite book
 |last=Whittaker
 |first=E.T.
 |author-link=E. T. Whittaker
 |title=[[A History of the Theories of Aether and Electricity]]
 |volume=1
 |publisher=Nelson |place=London
 |year=1951
}}</ref>{{rp|393}} Decades of experimental and theoretical research involving cathode rays were important in [[J. J. Thomson]]'s eventual discovery of electrons.<ref name="arabatzis" /> Goldstein also experimented with double cathodes and hypothesized that one ray may repulse another, although he didn't believe that any particles might be involved.<ref name=":0">{{Cite journal |last=Thomson |first=George |date=1970 |title=An Unfortunate Experiment: Hertz and the Nature of Cathode Rays |url=https://www.jstor.org/stable/530878 |journal=Notes and Records of the Royal Society of London |volume=25 |issue=2 |pages=237–242 |doi=10.1098/rsnr.1970.0032 |jstor=530878 |issn=0035-9149}}</ref>

During the 1870s, the English chemist and physicist Sir [[William Crookes]] developed the first cathode-ray tube to have a [[vacuum|high vacuum]] inside.<ref name="dekosky">
{{cite journal
 | last = DeKosky | first = R.K.
 | year = 1983
 | title = William Crookes and the quest for absolute vacuum in the 1870s
 | journal = [[Annals of Science]]
 | volume = 40 | issue = 1 | pages = 1–18
 | doi =10.1080/00033798300200101
}}</ref> He then showed in 1874 that the cathode rays can turn a small paddle wheel when placed in their path.  Therefore, he concluded that the rays carried momentum. Furthermore, by applying a magnetic field, he was able to deflect the rays, thereby demonstrating that the beam behaved as though it were negatively charged.<ref name="leicester">
{{cite book
 | last = Leicester
 | first = H.M.
 | year = 1971
 | title = The Historical Background of Chemistry
 | url = https://books.google.com/books?id=aJZVQnqcwv4C&pg=PA221
 | pages = 221–222
 | publisher = [[Courier Dover]]
 | isbn = 978-0-486-61053-5
 | access-date = 2020-08-25
 | archive-date = 2022-02-04
 | archive-url = https://web.archive.org/web/20220204082418/https://books.google.com/books?id=aJZVQnqcwv4C&pg=PA221
 | url-status = live
}}</ref> In 1879, he proposed that these properties could be explained by regarding cathode rays as composed of negatively charged gaseous [[molecule]]s in a fourth [[state of matter]], in which the mean free path of the particles is so long that collisions may be ignored.<ref name=Whittaker />{{rp|394–395}}

In 1883, not yet well-known German physicist [[Heinrich Hertz]] tried to prove that cathode rays are electrically neutral and got what he interpreted as a confident absence of deflection in electrostatic, as opposed to magnetic, field. However, as [[J. J. Thomson]] explained in 1897, Hertz placed the deflecting electrodes in a highly-conductive area of the tube, resulting in a strong screening effect close to their surface.<ref name=":0" /> 

The German-born British physicist [[Arthur Schuster]] expanded upon Crookes's experiments by placing metal plates parallel to the cathode rays and applying an [[electric potential]] between the plates.<ref name="schu1890">{{Cite journal|last=Schuster|first=Arthur|date=1890|title=The discharge of electricity through gases|journal=Proceedings of the Royal Society of London|volume=47|pages=526–559|doi=10.1098/rspl.1889.0111|s2cid=96197979|doi-access=free}}</ref> The field deflected the rays toward the positively charged plate, providing further evidence that the rays carried negative charge. By measuring the amount of deflection for a given [[electric field|electric]] and [[magnetic field]], in 1890 Schuster was able to estimate the [[Mass-to-charge ratio|charge-to-mass ratio]]{{efn|Older sources list charge-to-mass rather than the modern convention of mass-to-charge ratio.}} of the ray components. However, this produced a value that was more than a thousand times greater than what was expected, so little credence was given to his calculations at the time.<ref name="leicester" /> This is because it was assumed that the charge carriers were much heavier [[hydrogen]] or [[nitrogen]] atoms.<ref name="schu1890" /> Schuster's estimates would subsequently turn out to be largely correct.

In 1892 [[Hendrik Lorentz]] suggested that the mass of these particles (electrons) could be a consequence of their electric charge.<ref>{{cite magazine |first=Frank |last=Wilczek |url=https://www.scientificamerican.com/article.cfm?id=happy-birthday-electron |title=Happy birthday, electron |magazine=Scientific American |date=June 2012 |access-date=2022-02-24 |archive-date=2013-11-01 |archive-url=https://web.archive.org/web/20131101121817/http://www.scientificamerican.com/article.cfm?id=happy-birthday-electron |url-status=live }}</ref>

[[File:J.J Thomson.jpg|thumb|upright|[[J. J. Thomson]]]]
While studying naturally [[Fluorescence|fluorescing]] minerals in 1896, the French physicist [[Henri Becquerel]] discovered that they emitted radiation without any exposure to an external energy source. These [[Radioactive decay|radioactive]] materials became the subject of much interest by scientists, including the New Zealand physicist [[Ernest Rutherford]] who discovered they emitted particles. He designated these particles [[alpha particle|alpha]] and [[beta particle|beta]], on the basis of their ability to penetrate matter.<ref>
{{cite journal
 | last = Trenn | first = T.J.
 | year = 1976
 | title = Rutherford on the Alpha-Beta-Gamma Classification of Radioactive Rays
 | journal = [[Isis (journal)|Isis]]
 | volume = 67 | issue = 1 | pages = 61–75
 | jstor = 231134
 | doi = 10.1086/351545
 | s2cid = 145281124
}}</ref> In 1900, Becquerel showed that the beta rays emitted by [[radium]] could be deflected by an electric field, and that their mass-to-charge ratio was the same as for cathode rays.<ref>
{{cite journal
 | last = Becquerel | first = H.
 | year = 1900
 | title = Déviation du Rayonnement du Radium dans un Champ Électrique
 | journal = [[Comptes rendus de l'Académie des sciences]]
 | volume = 130
 | pages = 809–815
 |language=fr}}
</ref> This evidence strengthened the view that electrons existed as components of atoms.<ref name="BaW9091">[[#refBaW2001|Buchwald and Warwick (2001:90–91).]]</ref><ref>
{{cite journal
 | last = Myers
 | first = W.G.
 | year = 1976
 | title = Becquerel's Discovery of Radioactivity in 1896
 | url = https://jnm.snmjournals.org/cgi/content/abstract/17/7/579
 | journal = [[Journal of Nuclear Medicine]]
 | volume = 17
 | issue = 7
 | pages = 579–582
 | pmid = 775027
 | access-date = 2022-02-24
 | archive-date = 2008-12-22
 | archive-url = https://web.archive.org/web/20081222023947/http://jnm.snmjournals.org/cgi/content/abstract/17/7/579
 | url-status = live
}}</ref>

In 1897, the British physicist [[J. J. Thomson]], with his colleagues [[John Sealy Townsend|John S. Townsend]] and [[Harold A. Wilson (physicist)|H. A. Wilson]], performed experiments indicating that cathode rays really were unique particles, rather than waves, atoms or molecules as was believed earlier.<ref name="thomson" /> By 1899 he showed that their charge-to-mass ratio, ''e''/''m'', was independent of cathode material.  He further showed that the negatively charged particles produced by radioactive materials, by heated materials and by illuminated materials were universal.<ref name="thomson" /><ref>
{{cite web
 |last         = Thomson
 |first        = J.J.
 |year         = 1906
 |title        = Nobel Lecture: Carriers of Negative Electricity
 |url          = https://nobelprize.org/nobel_prizes/physics/laureates/1906/thomson-lecture.pdf
 |publisher    = [[Nobel Foundation|The Nobel Foundation]]
 |access-date  = 2008-08-25 |df=dmy-all
 |archive-url  = https://web.archive.org/web/20081010100408/https://nobelprize.org/nobel_prizes/physics/laureates/1906/thomson-lecture.pdf
 |archive-date = 2008-10-10
 |url-status   = dead
}}</ref> Thomson measured ''m''/''e'' for cathode ray "corpuscles", and made good estimates of  the charge ''e'', leading to value for the mass ''m'', finding a value 1400 times less massive than the least massive ion known: hydrogen.<ref name=Whittaker/>{{rp|364}}<ref name="thomson" /> In the same year [[Emil Wiechert]] and [[Walter Kaufmann (physicist)|Walter Kaufmann]] also calculated the ''e''/''m'' ratio but did not take the step of interpreting their results as showing a new particle, while J. J. Thomson would subsequently in 1899 give estimates for the electron charge and mass as well: ''e''&nbsp;≈&nbsp;{{val|6.8|e=-10|u=[[Statcoulomb|esu]]}} and ''m''&nbsp;≈&nbsp;{{val|3|e=-26|u=g}}<ref>{{cite journal|last=[[Abraham Pais]]|date=1997|title=The discovery of the electron – 100 years of elementary particles|url=https://www.slac.stanford.edu/pubs/beamline/pdf/97i.pdf|journal=Beam Line|volume=1|pages=4–16|access-date=2021-09-04|archive-date=2021-09-14|archive-url=https://web.archive.org/web/20210914142755/https://www.slac.stanford.edu/pubs/beamline/pdf/97i.pdf|url-status=live}}</ref><ref>{{cite journal |last=Kaufmann |first=W. |date=1897 |title=Die magnetische Ablenkbarkeit der Kathodenstrahlen und ihre Abhängigkeit vom Entladungspotential |url=https://dx.doi.org/10.1002/andp.18972970709 |journal=Annalen der Physik und Chemie |volume=297 |issue=7 |pages=544–552 |doi=10.1002/andp.18972970709 |bibcode=1897AnP...297..544K |issn=0003-3804 |access-date=2022-02-24 |archive-date=2022-02-24 |archive-url=https://web.archive.org/web/20220224105619/https://onlinelibrary.wiley.com/doi/10.1002/andp.18972970709 |url-status=live }}</ref>
[[File:Millikan.jpg|thumb|upright|[[Robert Andrews Millikan|Robert Millikan]]]]

The name "electron" was adopted for these particles by the scientific community, mainly due to the advocation by [[George Francis FitzGerald|G. F. FitzGerald]], [[Joseph Larmor|J. Larmor]], and [[Hendrik Lorentz|H. A. 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 |last=Stoney |first=George Johnstone |author-link=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 name="GJStoney" />

The electron's charge was more carefully measured by the American physicists [[Robert Andrews Millikan|Robert Millikan]] and [[Harvey Fletcher]] in their [[Oil drop experiment|oil-drop experiment]] of 1909, the results of which were published in 1911. This experiment used an electric field to prevent a charged droplet of oil from falling as a result of gravity. This device could measure the electric charge from as few as 1–150 ions with an error margin of less than 0.3%. Comparable experiments had been done earlier by Thomson's team,<ref name="thomson" /> using clouds of charged water droplets generated by electrolysis, and in 1911 by [[Abram Ioffe]], who independently obtained the same result as Millikan using charged microparticles of metals, then published his results in 1913.<ref>
{{cite journal
 | last1 = Kikoin | first1 = I.K.
 | last2 = Sominskiĭ | first2 = I.S.
 | year = 1961
 | title = Abram Fedorovich Ioffe (on his eightieth birthday)
 | journal = [[Uspekhi Fizicheskikh Nauk|Soviet Physics Uspekhi]]
 | volume = 3 | pages = 798–809
 | doi = 10.1070/PU1961v003n05ABEH005812
 | bibcode = 1961SvPhU...3..798K
 | issue = 5
}} Original publication in Russian: 
{{cite journal
 | last1 = Кикоин | first1 = И.К.
 | last2 = Соминский | first2 = М.С.
 | year = 1960
 | title = Академик А.Ф. Иоффе
 | journal = [[Uspekhi Fizicheskikh Nauk|Успехи Физических Наук]]
 | volume = 72 | issue = 10 | pages = 303–321
 | doi = 10.3367/UFNr.0072.196010e.0307
 | doi-access = free
}}</ref> However, oil drops were more stable than water drops because of their slower evaporation rate, and thus more suited to precise experimentation over longer periods of time.<ref>
{{cite journal
 | last = Millikan
 | first = R.A.
 | year = 1911
 | title = The Isolation of an Ion, a Precision Measurement of its Charge, and the Correction of Stokes's Law
 | journal = [[Physical Review]]
 | volume = 32
 | issue = 2
 | pages = 349–397
 | doi = 10.1103/PhysRevSeriesI.32.349
 | bibcode = 1911PhRvI..32..349M
 | url = https://authors.library.caltech.edu/6437/1/MILpr11b.pdf
 | access-date = 2019-06-21
 | archive-date = 2020-03-17
 | archive-url = https://web.archive.org/web/20200317204458/https://authors.library.caltech.edu/6437/1/MILpr11b.pdf
 | url-status = live
}}</ref>

Around the beginning of the twentieth century, it was found that under certain conditions a fast-moving charged particle caused a condensation of [[supersaturation|supersaturated]] water vapor along its path. In 1911, [[Charles Thomson Rees Wilson|Charles Wilson]] used this principle to devise his [[cloud chamber]] so he could photograph the tracks of charged particles, such as fast-moving electrons.<ref>
{{cite journal
 | last1 = Das Gupta | first1 = N.N.
 | last2 = Ghosh | first2 = S.K.
 | year = 1999
 | title = A Report on the Wilson Cloud Chamber and Its Applications in Physics
 | journal = [[Reviews of Modern Physics]]
 | volume = 18 | pages = 225–290
 | doi = 10.1103/RevModPhys.18.225
 | bibcode=1946RvMP...18..225G
 | issue = 2
}}</ref>