Editing Cold cathode (section)

==Details==
The cathode is the negative electrode. Any gas-discharge lamp has a positive (anode) and a negative electrode. Both electrodes alternate between acting as an anode and a cathode when these devices run with [[alternating current]].

[[File:Cold Cathodes.jpg|thumb|A standard computer case fitted with blue and green cold-cathode tubes]]
[[File:Cold Cathode Fluorescent Lamp.JPG|thumb|Cold-cathode fluorescent lamp backlight]]

A ''cold cathode'' is distinguished from a [[hot cathode]] that is heated to induce [[thermionic emission]] of [[electron]]s. Discharge tubes with hot cathodes have an envelope filled with low-pressure gas and containing two electrodes. Hot cathode devices include common [[vacuum tube]]s, [[fluorescent lamp]]s, high-pressure discharge lamps and [[vacuum fluorescent display]]s.

The surface of cold cathodes can emit [[secondary electrons]] at a ratio greater than unity (breakdown).  An electron that leaves the cathode will collide with neutral gas molecules.  The collision may just excite the molecule, but sometimes it will knock an electron free to create a positive ion.  The original electron and the freed electron continue toward the anode and may create more positive ions (see [[Townsend avalanche]]). The result is for each electron that leaves the cathode, several positive ions are generated that eventually crash onto the cathode. Some crashing positive ions may generate a secondary electron. The discharge is self-sustaining when for each electron that leaves the cathode, enough positive ions hit the cathode to free, on average, another electron. External circuitry limits the discharge current.  Cold-cathode discharge lamps use higher voltages than hot-cathode ones. The resulting strong electric field near the cathode accelerates ions to a sufficient velocity to create free electrons from the cathode material.

Another mechanism to generate free electrons from a cold metallic surface is [[field electron emission]]. It is used in some [[x-ray tube]]s, the [[field-emission microscopy|field-electron microscope]] (FEM), and [[field-emission display]]s (FEDs).

Cold cathodes sometimes have a [[rare-earth]] coating to enhance electron emission. Some types contain a source of [[beta radiation]] to start [[ionization]] of the gas that fills the tube.<ref>{{US Patent|1860149}}, Discharge tube.</ref> In some tubes, [[glow discharge]] around the cathode is usually minimized; instead there is a so-called positive column, filling the tube.<ref>{{US Patent|2103033}}, Electron emissive electrode.</ref><ref>{{US Patent|1316967}}, Gaseous-conduction lamp.</ref><ref group="note">Positive column is part of a glow discharge, such as in the [[Moore lamp]].</ref> Examples are the [[neon lamp]] and [[nixie tube]]s. Nixie tubes too are cold-cathode neon displays that are in-line, but not in-plane, display devices.

Cold-cathode devices typically use a complex [[high-voltage power supply]] with some mechanism for limiting current. Although creating the initial space charge and the first arc of current through the tube may require a very high voltage, once the tube begins to heat up, the electrical resistance drops, thus increasing the [[electric current]] through the lamp. To offset this effect and maintain normal operation, the supply voltage is gradually lowered. In the case of tubes with an ionizing gas, the gas can become a very hot [[plasma (physics)|plasma]], and electrical resistance is greatly reduced. If operated from a simple power supply without current limiting, this reduction in resistance would lead to damage to the power supply and overheating of the tube electrodes.