Editing Cathode ray (section)

== Properties of cathode rays and the experiments that revealed them ==
During the last quarter of the 19th century dozens of historic experiments were conducted to try to find out what cathode rays were.<ref name="Brona">{{cite web | last=Brona | first=Grzegorz | title=The Cathode Rays | work=Atom - The Incredible World | url=http://library.thinkquest.org/19662/high/eng/cathoderays.html | access-date=2008-09-27 | display-authors=etal | url-status=dead | archive-url=https://web.archive.org/web/20090211185645/http://library.thinkquest.org/19662/high/eng/cathoderays.html | archive-date=2009-02-11 }}</ref> There were two theories: British scientists [[William Crookes|Crookes]] and [[C. F. Varley|Cromwell Varley]] believed they were particles of 'radiant matter', that is, electrically charged [[atoms]]. German researchers E. Wiedemann, [[Heinrich Hertz]], and [[Eugen Goldstein]] believed they were '[[Luminiferous aether|aether]] vibrations', some new form of [[electromagnetic wave]]s, and were separate from what carried the current through the tube.<ref name="Pais" />{{rp|79-81}}<ref name="Thomson" >{{cite book
  | last = Thomson
  | first = J. J.
  | title = The Discharge of Electricity through Gasses
  | publisher = Charles Scribner's Sons
  | date = 1903
  | location = New York
  | language = 
  | url = https://archive.org/details/bub_gb_Ryw4AAAAMAAJ/page/n5/mode/2up
  | archive-url= 
  | archive-date=
  | doi = 
  | id = 
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  | jfm =}}</ref>{{rp|189-190}}<ref name="Brona" /> The debate continued until [[J. J. Thomson]] measured cathode ray’s mass, proving they were a previously unknown negatively charged particle in an atom, the first [[subatomic particle]], which he called a 'corpuscle' but was later renamed the 'electron'.

===Straight line motion===
[[Julius Plücker]] in 1869 built a tube with an anode shaped like a [[Maltese Cross]] facing the cathode. It was hinged, so it could fold down against the floor of the tube. When the tube was turned on, the cathode rays cast a sharp cross-shaped shadow on the fluorescence on the back face of the tube, showing that the rays moved in straight lines.<ref name="Pais">{{cite book | last = Pais | first = Abraham | author-link = Abraham Pais | title = Inward Bound: Of Matter and Forces in the Physical World | publisher = Oxford Univ. Press | year = 1986 | location = UK | url = https://books.google.com/books?id=mREnwpAqz-YC&pg=PA81 | isbn = 978-0-19-851997-3 }}</ref>{{rp|79}} This fluorescence was used as an argument that cathode rays were electromagnetic waves, since the only thing known to cause fluorescence at the time was [[ultraviolet]] light. After a while the fluorescence would get 'tired' and the glow would decrease.<ref name="Thomson" />{{rp|143}} If the cross was folded down out of the path of the rays, it no longer cast a shadow, and the previously shadowed area would fluoresce more strongly than the area around it.

===Perpendicular emission===
[[file:Crookes tube for heating 1879.png|thumb|upright=0.4|Crookes tube with concave cathode]]
[[Eugen Goldstein]] in 1876 found that cathode rays were always emitted perpendicular to the cathode's surface.<ref name="Thomson" />{{rp|138}}<ref>Goldstein E. (1876). ''Monat der Berl. Akad''., p. 284.</ref> If the cathode was a flat plate, the rays were shot out in straight lines perpendicular to the plane of the plate. This was evidence that they were particles, because a luminous object, like a red hot metal plate, emits light in all directions, while a charged particle will be repelled by the cathode in a perpendicular direction.  Cathode rays heat matter which they strike.<ref name="Thomson" />{{rp|145}}  If the electrode was made in the form of a concave spherical dish, the cathode rays would be focused to a spot in front of the dish.<ref name="Brona" />  This could be used to heat samples to a high temperature.
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===Electrostatic deflection===
Cathode rays path can be deflected by an [[electric field]].  [[Heinrich Hertz]] built a tube with a second pair of metal plates to either side of the cathode ray beam, a crude [[Cathode-ray tube|CRT]]. If the cathode rays were [[charged particle]]s, their path should be bent by the [[electric field]] created when a [[voltage]] was applied to the plates, causing the spot of light where the rays hit to move sideways. He did not find any bending, but it was later determined that his tube was insufficiently evacuated, causing accumulations of [[surface charge]] which masked the electric field. Later Arthur Schuster repeated the experiment with a higher vacuum. He found that the rays were attracted toward a positively charged plate and repelled by a negative one, bending the beam. This was evidence they were negatively charged, and therefore not electromagnetic waves.
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===Magnetic deflection===
[[file:Crookes magnetic deflection tube.png|thumb|Crookes magnetic deflection tube]]
The rays path can be deflected by a [[magnetic field]].  [[William Crookes|Crookes]] put a [[magnet]] across the neck of the tube, so that the North pole was on one side of the beam and the South pole was on the other, and the beam travelled through the [[magnetic field]] between them. The beam was bent down, perpendicular to the magnetic field.<ref name="Thomson" />{{rp|150-151}} To reveal the path of the beam, [[William Crookes|Crookes]] invented a tube ''(see pictures)'' with a cardboard screen with a [[phosphor]] coating down the length of the tube, at a slight angle so the electrons would strike the phosphor along its length, making a glowing line on the screen. The line could be seen to bend up or down in a transverse magnetic field. This effect (now called the [[Lorentz force]]) was similar to the behavior of electric currents in an [[electric motor]] and showed that the cathode rays obeyed [[Faraday's law of induction]] like currents in wires. Both electric and magnetic deflection were evidence for the particle theory, because electric and magnetic fields have no effect on a beam of light waves in vacuum.

===Paddlewheel===
[[file:Crookes paddlewheel tube.png|thumb|Crookes's paddlewheel tube, from his 1879 paper ''On Radiant Matter'']]
[[William Crookes|Crookes]] put a tiny vaned [[turbine]] or [[paddlewheel]] in the path of the cathode rays, and found that it rotated when the rays hit it.<ref name="Thomson" />{{rp|146-149}} The paddlewheel turned in a direction away from the cathode side of the tube, suggesting that the force of the cathode rays striking the paddles was causing the rotation. [[William Crookes|Crookes]] concluded at the time that this showed that cathode rays had [[momentum]], so the rays were likely [[matter]] particles. However, later it was concluded that the paddle wheel turned not due to the momentum of the particles (or electrons) hitting the paddle wheel but due to the [[Crookes radiometer|radiometric effect]].<ref name="Brona" />  When the rays hit the paddle surface they heated it, and the heat caused the gas next to it to expand, pushing the paddle. This was proven in 1903 by [[J. J. Thomson]] who calculated that the momentum of the electrons hitting the paddle wheel would only be sufficient to turn the wheel one revolution per minute.<ref name="Brona" /> All this experiment really showed was that cathode rays were able to heat surfaces.

===Negative electric charge===
[[Jean-Baptiste Perrin]] wanted to determine whether the cathode rays actually carried negative [[Electric charge|charge]], or whether they just accompanied the charge carriers, as the Germans thought. In 1895 he constructed a tube with a 'catcher', a closed aluminum cylinder with a small hole in the end facing the cathode, to collect the cathode rays.<ref name="Thomson" />{{rp|161-165}} The catcher was attached to an [[electroscope]] to measure its charge. The electroscope showed a negative charge, proving that cathode rays really carry negative electricity.

===Anode rays===
[[file:Anode Ray Tube.jpg|thumb|upright=0.75|Special tube with perforated cathode, producing anode rays ''(top, pink)'']]
Goldstein found in 1886 that if the cathode is made with small holes in it, streams of a faint luminous glow will be seen issuing from the holes on the back side of the cathode, facing away from the anode.<ref>Goldstein E. (1886) Berliner Sitzungsberichte, 39, p.391</ref><ref name="Thomson" />{{rp|158-159}} It was found that in an electric field these [[anode ray]]s bend in the opposite direction from cathode rays, toward a negatively charged plate, indicating that they carry a positive charge. These were the positive [[ion]]s which were attracted to the cathode, and created the cathode rays. They were named ''canal rays'' (''Kanalstrahlen'') by Goldstein.<ref>{{cite web | title = Concept review Ch.41 Electric Current through Gasses | work = Learning Physics for IIT JEE | year = 2008 | url = http://iit-jee-physics.blogspot.com/2008/03/concept-review-ch41-electric-current.html | access-date = 2008-11-11}}</ref>

===Spectral shift===
[[Eugen Goldstein]] thought he had figured out a method of measuring the speed of cathode rays. If the [[glow discharge]] seen in the gas of Crookes tubes was produced by the moving cathode rays, the light radiated from them in the direction they were moving, down the tube, would be shifted in [[frequency]] due to the [[Doppler effect]].<ref name="Brona" /> This could be detected with a [[spectroscope]] because the [[emission line]] [[spectrum]] would be shifted. He built a tube shaped like an "L", with a spectroscope pointed through the glass of the elbow down one of the arms. He measured the spectrum of the glow when the spectroscope was pointed toward the cathode end, then switched the power supply connections so the cathode became the anode and the electrons were moving in the other direction, and again observed the spectrum looking for a shift. He did not find one, which he calculated meant that the rays were traveling very slowly. It was later recognized that the glow in Crookes tubes is emitted from gas atoms hit by the electrons, not the electrons themselves. Since the atoms are thousands of times more massive than the electrons, they move much slower, accounting for the lack of Doppler shift.

=== Lenard window ===
[[file:Lenard window tube labeled.svg|thumb|Lenard window tube ]]
[[Philipp Lenard]] wanted to see if cathode rays could pass out of the Crookes tube into the air. See diagram. He built a tube with a "window" ''(W)'' in the glass envelope made of [[aluminum foil]] just thick enough to hold the atmospheric pressure out (later called a "Lenard window") facing the cathode ''(C)'' so the cathode rays would hit it.<ref name="Thomson" />{{rp|182-188}}  He found that something did come through. Holding a fluorescent screen up to the window caused it to fluoresce, even though no light reached it. A [[photographic plate]] held up to it would be darkened, even though it was not exposed to light. The effect had a very short range of about {{convert|2.5|cm|in|2}}. He measured the ability of cathode rays to penetrate sheets of material, and found they could penetrate much farther than moving atoms could. Since atoms were the smallest particles known at the time, this was first taken as evidence that cathode rays were waves. Later it was realized that electrons were much smaller than atoms, accounting for their greater penetration ability. Lenard was awarded the [[Nobel Prize in Physics]] in 1905 for his work.

===Wave-particle duality===
[[Louis de Broglie]] later (1924) suggested in his doctoral dissertation that electrons are like photons and can act as [[Matter wave|waves]]. The wave-like behaviour of cathode rays was later directly demonstrated using reflection from a nickel surface by [[Davisson–Germer experiment|Davisson and Germer]],<ref>{{Cite journal |last1=Davisson |first1=C. |last2=Germer |first2=L. H. |date=1927 |title=Diffraction of Electrons by a Crystal of Nickel |journal=Physical Review |volume=30 |issue=6 |pages=705–740 |doi=10.1103/PhysRev.30.705|doi-access=free |bibcode=1927PhRv...30..705D }}</ref> and transmission through celluloid thin films and later metal films by [[George Paget Thomson]] and Alexander Reid<ref>{{Cite journal |last1=Thomson |first1=G. P. |last2=Reid |first2=A. |date=1927 |title=Diffraction of Cathode Rays by a Thin Film |journal=Nature |language=en |volume=119 |issue=3007 |pages=890 |doi=10.1038/119890a0 |issn=1476-4687|doi-access=free |bibcode=1927Natur.119Q.890T }}</ref> in 1927. (Alexander Reid, who was Thomson's graduate student, performed the first experiments but he died soon after in a motorcycle accident<ref>{{Cite journal |last=Navarro |first=Jaume |date=2010 |title=Electron diffraction chez Thomson: early responses to quantum physics in Britain |url=https://www.cambridge.org/core/product/identifier/S0007087410000026/type/journal_article |journal=The British Journal for the History of Science |language=en |volume=43 |issue=2 |pages=245–275 |doi=10.1017/S0007087410000026 |issn=0007-0874|url-access=subscription }}</ref> and is rarely mentioned.)