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Cathode ray
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===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. {{breakafterimages}}
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