Editing Cathode (section)

====Hot cathode====
{{Main|Hot cathode}}
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| image1   = Dubulttriode darbiibaa.jpg
| caption1 = Two indirectly-heated cathodes (orange heater strip) in ECC83 dual triode tube
| width1   = 137
| image2   = Triode-english-text.svg
| caption2 = Cutaway view of a [[triode]] vacuum tube with an indirectly-heated cathode ''(orange tube)'', showing the heater element inside
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[[File:Triode schematic labeled.svg|thumb|[[Schematic symbol]] used in [[circuit diagram]]s for vacuum tube, showing cathode]]

A hot cathode is a cathode that is heated by a [[electrical filament|filament]] to produce electrons by [[thermionic emission]].<ref name="Avadhanulu" /><ref name="Whitaker">Ferris, Clifford "Electron tube fundamentals" in {{cite book
 |last        = Whitaker
 |first       = Jerry C.
 |title       = The Electronics Handbook, 2nd Ed.
 |publisher   = CRC Press
 |date        = 2013
 |pages       = 354–356
 |url         = https://books.google.com/books?id=FdSQSAC3_EwC&pg=PA355
 |isbn        = 978-1420036664
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20140102024350/http://books.google.com/books?id=FdSQSAC3_EwC&pg=PA355
 |archive-date = 2 January 2014
}}</ref> The filament is a thin wire of a [[refractory metal]] like [[tungsten]] heated red-hot by an electric current passing through it. Before the advent of transistors in the 1960s, virtually all electronic equipment used hot-cathode [[vacuum tube]]s. Today hot cathodes are used in vacuum tubes in radio transmitters and microwave ovens, to produce the electron beams in older [[cathode-ray tube]] (CRT) type televisions and computer monitors, in [[x-ray generator]]s, [[electron microscope]]s, and [[fluorescent tube]]s.

There are two types of hot cathodes:<ref name="Avadhanulu" />
* '''Directly heated cathode''': In this type, the filament itself is the cathode and emits the electrons directly. Directly heated cathodes were used in the first vacuum tubes, but today they are only used in [[fluorescent tube]]s, some large transmitting vacuum tubes, and all X-ray tubes.
* '''Indirectly heated cathode''': In this type, the filament is not the cathode but rather heats the cathode which then emits electrons. Indirectly heated cathodes are used in most devices today. For example, in most vacuum tubes the cathode is a nickel tube with the filament inside it, and the heat from the filament causes the outside surface of the tube to emit electrons.<ref name="Whitaker" /> The filament of an indirectly heated cathode is usually called the ''heater''. The main reason for using an indirectly heated cathode is to isolate the rest of the vacuum tube from the electric potential across the filament. Many vacuum tubes use [[alternating current]] to heat the filament. In a tube in which the filament itself was the cathode, the alternating [[electric field]] from the filament surface would affect the movement of the electrons and introduce hum into the tube output. It also allows the filaments in all the tubes in an electronic device to be tied together and supplied from the same current source, even though the cathodes they heat may be at different potentials.

In order to improve electron emission, cathodes are treated with chemicals, usually compounds of metals with a low [[work function]]. Treated cathodes require less surface area, lower temperatures and less power to supply the same cathode current. The untreated tungsten filaments used in early tubes (called "bright emitters") had to be heated to {{convert|1400|°C|°F|abbr=on}}, white-hot, to produce sufficient thermionic emission for use, while modern coated cathodes produce far more electrons at a given temperature so they only have to be heated to {{convert|425|–|600|C|F}}<ref name="Avadhanulu" /><ref name="IanPoole">{{cite web
 |last        = Poole
 |first       = Ian
 |title       = Vacuum tube electrodes
 |work        = Vacuum Tube Theory Basics Tutorial
 |publisher   = Radio-Electronics.com, Adrio Communications
 |date        = 2012
 |url         = http://www.radio-electronics.com/info/data/thermionic-valves/vacuum-tube-theory/tube-electrodes.php
 |access-date  = 3 October 2013
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20131104104630/http://www.radio-electronics.com/info/data/thermionic-valves/vacuum-tube-theory/tube-electrodes.php
 |archive-date = 4 November 2013
}}</ref><ref name="Jones">{{cite book
 |last        = Jones
 |first       = Martin Hartley
 |title       = A Practical Introduction to Electronic Circuits
 |publisher   = Cambridge Univ. Press
 |date        = 1995
 |location    = UK
 |pages       = 49
 |url         = https://books.google.com/books?id=EEcemABAU44C&pg=PA49
 |isbn        = 978-0521478793
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20140102024515/http://books.google.com/books?id=EEcemABAU44C&pg=PA49
 |archive-date = 2 January 2014
}}</ref> There are two main types of treated cathodes:<ref name="Avadhanulu" /><ref name="Whitaker" />

[[Image:Neon lamp on DC.JPG|thumb|upright=0.5|Cold cathode ''(lefthand electrode)'' in [[neon lamp]] ]]
* Coated cathode – In these the cathode is covered with a coating of [[alkali metal]] oxides, often [[barium]] and [[strontium]] oxide. These are used in low-power tubes.
* Thoriated tungsten – In high-power tubes, [[ion]] bombardment can destroy the coating on a coated cathode.   In these tubes a directly heated cathode consisting of a filament made of tungsten incorporating a small amount of [[thorium]] is used. The layer of thorium on the surface which reduces the work function of the cathode is continually replenished as it is lost by diffusion of thorium from the interior of the metal.<ref name="Sisodia">{{cite book
 |last        = Sisodia
 |first       = M. L.
 |title       = Microwave Active Devices Vacuum and Solid State
 |publisher   = New Age International
 |date        = 2006
 |pages       = 2.5
 |url         = https://books.google.com/books?id=mKs53pET-bkC&pg=SA2-PA4
 |isbn        = 978-8122414479
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20140102024156/http://books.google.com/books?id=mKs53pET-bkC&pg=SA2-PA4
 |archive-date = 2 January 2014
}}</ref>