Editing J. J. Thomson (section)

====Electrical deflection====
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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.
 | image2 = Thomson cathode ray exp.gif
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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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