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Vacuum tubes, or '''tubes''' or '''thermionic valves''' are arrangements of electrodes surrounded by [[vacuum]] within an insulating, temperature-resistant envelope. Although the envelope was classically glass, power tubes often use ceramics, and military tubes often use glass-lined metal.
<div style="float:right; width:232px; margin-left:1em; text-align:center">[[Image:diode.jpg|Diagram of Vacuum-Tube Diode]]<br>
'''Diode'''<br><br>
'''Triode'''</div>
Vacuum tubes resemble incandescent [[light bulb]]s, in that they have a filament sealed in a glass envelope, which has been evacuated of all air. Tubes have a ''filament'' heated by an electric current. When hot, the filament releases [[electron]]s into the vacuum, a process called '''[[thermionic|thermionic emission]]'''. The resulting negatively-charged cloud of electrons is called a [[space charge]]. These electrons will be electrostatically drawn to a positively charged outer metal plate called the ''[[anode]]'', or more commonly just the ''plate''. This results in a measurable electrical current "flowing" throught he vacuum of the tube from the filament to the plate.
Electrons do not flow from the plate back toward the filament, even if the charge on the plate is made negative in relation to the filament, because the plate is not heated, and therefore does not emit enough electrons to develop a space charge. This arrangement of a filament and plate is called a ''[[diode]]'' and was invented in [[1904]] by [[John Ambrose Fleming]], scientific adviser to the [[Marconi]] company, based on an [[Edison Effect|observation]] by [[Thomas Edison]].
The next innovation, due to [[Lee DeForest]] in [[1907]], was to place another electrode, the ''grid'', between the filament and plate. The grid is a bent wire or screen. De Forest discovered that the current flow from filament to plate depended on the [[voltage]] applied to the grid, and that the current drawn by the grid was very low. As the applied voltage of the grid varied from negative to positive, the current of electrons flowing from the filament to the plate would vary correspondingly. Thus the grid was said to "control" the plate current. The resulting three-electrode device was therefore an excellent amplifier. DeForest called his invention the '''audion''', but it is better known as a '''triode'''. The valve equivalent of a [[transistor]], triodes were used in early [[valve amplifier]]s.
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Many further innovations followed. It became common to use the filament to heat a separate electrode called the ''cathode'', and to use the cathode as the source of electron flow in the tube rather than the filament itself. This minimized the introduction of "hum" when the filament was energized with [[alternating current]]. In such tubes, the filament is called a heater to distinguish it as an inactive element.
<div style="float:left
[[image:valve.radio.250pix.jpg]]<br>
<small>''A two-valve home-made radio from 1958. The valves are the two glass columns with the dark tops. The leads at the bottom connect to the low-voltage filament supply and to the high-voltage anode supply.''
<br>[[media:valve.radio.600pix.jpg|Larger version]]</small>▼
▲[[media:valve.radio.600pix.jpg|Larger version]]
</div>
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