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The Hittorf  tube that was being used by Röntgen when he made the observations that led to the discover of X-rays was a simple affair. It could have been made by any competent laboratory glass-blower. It consisted of a glass bulb, into which were sealed two electrodes. A partial vacuum had been created in the tube, and its operation was dependant upon the presence of this residual gas.
When a high voltage was applied across the tube, the gas became ionized, and current flowed between the electrodes. If the electrode at the narrow end of the rube was attached to the negative pole of the supply, and the electrode nearest the wide end of the tube attached to the positive pole, the tube was filled with a faint glow, but an area on the broad end of the tube would glow brightly.