A plate, usually called anode in Britain, is a type of

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...

that forms part of a

vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as ...

. It is usually made of sheet metal, connected to a wire which passes through the glass envelope of the tube to a terminal in the base of the tube, where it is connected to the external circuit. The plate is given a positive

potential Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social sciences to indicate things that are in a state where they are able to change in ways ranging from the simple r ...

, and its function is to attract and capture the electrons emitted by the

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...

. Although it is sometimes a flat plate, it is more often in the shape of a cylinder or flat open-ended box surrounding the other electrodes.

Construction

The plate must dissipate heat created when the electrons hit it with a high velocity after being accelerated by the voltage between the plate and cathode. Most of the waste power used in a vacuum tube is dissipated as heat by the plate. In low power tubes it is usually given a black coating, and often has "fins" to help it radiate heat. In power vacuum tubes used in radio transmitters, it is often made of a refractory metal like

molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lead ...

. and is part of a large

heat sink A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, ...

that projects through the glass or ceramic tube envelope and is cooled by

radiation cooling In the study of heat transfer, radiative cooling is the process by which a body loses heat by thermal radiation. As Planck's law describes, every physical body spontaneously and continuously emits electromagnetic radiation. Radiative cooling has ...

, forced air or water.Robert B. Tomer, ''Getting the most out of vacuum tubes'', Howard W. Sams, Indianapolis, USA 1960, Library of Congress card no. 60-13843, available on the Internet Archive. Chapter 1

Secondary emission

A problem in early vacuum tubes was ''

secondary emission In particle physics, secondary emission is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the em ...

''; electrons striking the plate could knock other electrons out of the metal surface. In some tubes such as

tetrode A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). ...

s these ''secondary electrons'' could be absorbed by other electrodes such as grids in the tube, resulting in a current out of the plate. This current could cause the plate circuit to have negative resistance, which could cause unwanted parasitic oscillations. To prevent this most plates in modern tubes are given a chemical coating which reduces secondary emission.

External links

*https://web.archive.org/web/20101007201649/http://pentalabs.com/tubeworks.html – The history of vacuum tubes
The Thermionic Detector – HJ van der Bijl (October 1919)How vacuum tubes really work
– Thermionic emission and vacuum tube theory, using introductory college-level mathematics.

– FAQ from rec.audio

Fleming discovers the thermionic (or oscillation) valve, or 'diode'.

References

*Shiers, George, "The First Electron Tube", Scientific American, March 1969, p. 104. *Tyne, Gerald, ''Saga of the Vacuum Tube'', Ziff Publishing, 1943, (reprint 1994 Prompt Publications), p. 30–83. *RCA '' Radiotron Designer's Handbook'', 1953 (4th Edition). Contains chapters on the design and application of receiving tubes. Vacuum tubes Electrodes {{Electronics-stub

Construction

Secondary emission

See also

External links

References