X-ray tubes. These are devices in which electrical energy is transformed into X-rays. The characteristics of such tubes vary according to the use for which they are designed. They consist essentially of a cathode from which the electrons are emitted, and a target (anti-cathode or anode) on which the electrons impinge, thus causing it to emit X-rays. In some cases, the tubes also have a number of intermediate electrodes for accelerating the stream of electrons. The electrodes are mounted in a tube or container, usually of glass, with the appropriate electrical contacts. The tube is often mounted in an electrically insulated metal container filled with oil. Sometimes the tube is gas filled, but more usually it is maintained at a high degree of vacuum.
glass envelopes for X-ray tubes (heading 70.11):