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The cathode is one of the two electrodes that are present in any system in which electricity is entering and leaving a region; the other electrode is called the anode. The electric current enters through one of the electrodes and leaves through the other.

Two general kinds of systems employ electrodes: vacuum tubes (also called gas discharge tubes) and electrochemical cells.

In a vacuum tube, the cathode is the negative electrode-the electrode that carries a negative potential with respect to the other one. The cathode is often heated to drive out electrons, which then fly through the vacuum toward the positive electrode, the anode. These streams of electrons are referred to as cathode rays. Cathode ray tubes are vacuum tubes that are widely used as oscilloscopes, television tubes, and computer monitors.

Electrochemical cells are of two types: voltaic cells (also called galvanic cells) and electrolytic cells. In a galvanic cell, such as an automobile battery, an electric current is produced by a chemical oxidation-reduction reaction. In an electrolytic cell, such as a cell designed for the electrolysis of water, the chemical oxidation-reduction reaction is produced by an externally-supplied electric current. In either case, the cathode is defined as the electrode at which the chemical reduction process is taking place in the cell—that is, the electrode at which electrons are being taken up by atoms, molecules, or ions. The anode, on the other hand, is the electrode at which the oxidation process is taking place—that is, the electrode at which electrons are being given off by atoms, molecules, or ions.

See also Cathode ray tube.

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