Cold cathode

A cold cathode is a cathode that is not electrically heated by a filament. A cathode may be considered "cold" if it emits more electrons than can be supplied by thermionic emission alone. It is used in gas-discharge lamps, such as neon lamps, discharge tubes, and some types of vacuum tube. The other type of cathode is a hot cathode, which is heated by electric current passing through a filament. A cold cathode does not necessarily operate at a low temperature: it is often heated to its operating temperature by other methods, such as the current passing from the cathode into the gas.

A cold-cathode vacuum tube does not rely on external heating of an electrode to provide thermionic emission of electrons. Early cold-cathode devices included the Geissler tube and Plucker tube, and early cathode ray tubes. Study of the phenomena in these devices led to the discovery of the electron.

Neon lamps are used both to produce light as indicators and for special-purpose illumination, and also as circuit elements displaying negative resistance. Addition of a trigger electrode to a device allowed the glow discharge to be initiated by an external control circuit; Bell Laboratories developed a "trigger tube" cold cathode device in 1936.

Many types of cold-cathode switching tube were developed, including various types of thyratron, the krytron, cold cathode displays (Nixie tube) and others. Voltage regulator tubes rely on the relatively constant voltage of a glow discharge over a range of current, and were used to stabilize power supply voltages in tube-based instruments. A Dekatron is a cold-cathode tube with multiple electrodes that is used for counting. Each time a pulse is applied to a control electrode, a glow discharge moves to a step electrode; by providing ten electrodes in each tube and cascading the tubes, a counter system can be developed and the count observed by the position of the glow discharges. Counter tubes were used widely before development of integrated circuit counter devices.

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