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Cathode rays are streams of electrons observed in vacuum tubes. If an evacuated glass tube having gas pressure less than 0.02 mm is equipped with two electrodes and a high voltage (10000 to 12000 V) is applied, the glass opposite of the negative electrode is observed to glow with a greenish fluorescence, due to electrons emitted from and travelling perpendicular to the cathode (the electrode connected to the negative terminal of the voltage supply). Electrons were first discovered as the constituents of cathode rays, in 1897 by British physicist J. J. Thomson. Cathode ray tubes (CRTs) using a focused beam of electrons deflected by electric or magnetic fields, create the image in a classic television set.

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