Cathode

The cathode is one of 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. Both vacuum tubes (also called gas discharge tubes) and electrochemical cells use electrodes.

In a vacuum tube, the cathode is the negative electrode, which carries a negative potential with respect to the other. The cathode is often heated to drive out electrons, which then fly through the vacuum toward the positive electrode, or 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.

There are two types of electrochemical cells: voltaic or galvanic cells, and electrolytic cells. In a galvanic cell, such as an automobilebattery, an electric current is produced by a chemical oxidation-reduction reaction. In an electrolytic cell, such as one designed for the electrolysis of water, the oxidation-reduction reaction is produced by an externally supplied electric current. In either case, the cathode is the electrode at which chemical reduction is taking place—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.