A vacuum tube is a tube (usually made of glass) with most of the air pumped out, and usually containing two electrodes—two pieces of metal with a potential difference (a voltage difference) applied between them. Electrons, being negatively charged, are repelled out of the negative electrode and fly through the vacuum toward the positive electrode, to which they are simultaneously being attracted. The positive electrode is called the anode; the negative electrode is called the cathode.
Common examples of vacuum tubes in which electrons flow from a cathode to an anode are cathode ray tubes, such as television tubes and computer monitors, and x-ray tubes. In an x-ray tube, the kind of metal that the anode is made of determines the kind of x rays (i.e., the x-ray energy) that the tube emits.
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