This tube made by J J Griffin & Sons is a modification of the original vacuum tube used by Sir William Crookes in his famous 'Maltese cros' experiment of 1887. When an electric discharge is pased through gas at very low presure, cathode rays are produced and make the glas of the tube containing the gas glow green. By placing a small metal cros in the path of the rays coming from the cathode and observing the shadow cast on the glas screen beyond it, Crookes demonstrated that cathode rays could cast shadows and travelled in straight lines. He theorised that cathode rays were made up of negatively charged particles, a theory confirmed when J J Thomson identified the particles as electrons in 1897.
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