Editing J. J. Thomson (section)

===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"/>
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====Electrical deflection====
{{more citations needed|section|date=August 2017}}<!--only first paragraph has a citation-->
{{multiple image
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 | image1 = JJ Thomson Cathode Ray 2.png
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 | 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.
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 | caption2 = The cathode ray (blue line) was deflected by the electric field (yellow).
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[[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.
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====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.
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====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.