well Rocky, A vacuum tube is a glass tube surrounding a vacuum What makes it interesting is that when electrical contacts are put on the ends,it is basically a tube with anode and cathode on the either ends of the tube which is vacuum when a low signal is passed from one end on the other end an amplified is taken as output
simslarly what a transistor does
here very high voltage is induced inbetween anode and cathode and thus that high voltage is observed by low voltage ac current and thus it is amplifed. BUt it was bulky and low efficientand thus transistor replaced them in 1945.

Answered by Uttam , an ibibo Master, at 2:25 PM on May 01, 2008

A vacuum tube, electron tube ,thermionic valve, or just valve , is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space.

Answered by Sharad Singh , an ibibo Master, at 1:44 PM on April 25, 2008

a vacuum tube, electron tube (in North America), thermionic valve, or just valve (elsewhere, especially in Britain), is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space, often tubular in form. Many devices called vacuum tubes are filled with low-pressure gas: these are so-called soft valves (or tubes); as distinct from the hard vacuum type, which have the internal gas pressure reduced as far as possible. Almost all depend on the thermal emission of electrons, hence thermionic.

Vacuum tubes were critical to the development of electronics technology, which drove the expansion and commercialization of radio broadcasting, television, radar, high fidelity sound reproduction, large telephone networks, modern types of digital computer, and industrial process control. Some of these applications pre-dated electronics, but it was electronics that made them widespread and practical; electronics has driven mechanical computers such as slide-rules to the point of obsolescence.

Answered by Smarty , an ibibo Master, at 4:53 PM on April 14, 2008

The conduction of electricity in thin gases in vacuum tubes was the key to the discovery of the electron in 1897. A modern version of the vacuum tube is the television monitor.

Answered by bhanu pratap , an ibibo Citizen, at 8:06 PM on April 13, 2008

The English physicist John Ambrose Fleming worked as an engineering consultant for many technology firms of his day, including Edison Telephone; in 1904, as a result of experiments conducted on Edison Effect bulbs imported from the USA and while working as scientific adviser to the Marconi company, he developed a device he called an "oscillation valve" (because it passes current in only one direction) or kenotron, which can also be used as part of a radio wave detector. Later known as the Fleming valve and then the diode, it allowed electrical current to flow in only one direction, enabling the rectification of alternating current. Its operation is described in greater detail in the previous section.
In 1906 Robert von Lieben filed[2] for a three electrode amplifying vacuum tube. His invention included also a beam focusing electromagnet.
In 1907 Lee De Forest placed a bent wire serving as a screen, later known as the "grid" electrode, between the filament and plate electrode. As the voltage applied to the grid was varied from negative to positive, the number of electrons flowing from the filament to the plate would vary accordingly. Thus the grid was said to electrostatically "control" the plate current. The resulting three-electrode device was therefore an excellent and very sensitive amplifier of voltages. DeForest called his invention the "Audion". In 1907, DeForest filed[3] for a three-electrode version of the Audion for use in radio communications. The device is now known as the triode. De Forest's device was not strictly a vacuum tube, but clearly depended for its action on ionisation of the relatively high levels of gas remaining after evacuation. The De Forest company, in its Audion leaflets, warned against operation which might cause the vacuum to become too hard. The Finnish inventor Eric Tigerstedt significantly improved on the original triode design in 1914, while working on his sound-on-film process in Berlin, Germany. The first true vacuum triodes were the Pliotrons developed by Irving Langmuir at the General Electric research laboratory (Schenectady, New York) in 1915. Langmuir was one of the first scientists to realize that a harder vacuum would improve the amplifying behaviour of the triode. Pliotrons were closely followed by the French 'R' Type which was in widespread use by the allied military by 1916. These two types were the first true vacuum tubes. Historically, vacuum levels in production vacuum tubes typically ranged between 10 µPa to 10 nPa.

Answered by yogesh , an ibibo Citizen, at 8:01 PM on April 13, 2008

It is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space, often tubular in form.
Vacuum tubes operate primarily on the function of the heat gradient difference between the hot cathode and the cold anode. Because of this operating requirement, vacuum tubes are inherently power-inefficient; enclosing the tube within a heat-retaining envelope of insulation would allow the entire tube to reach the same temperature, resulting in electron emission from the anode that would counter the normal one-way current flow. Because the tube requires a vacuum to operate, convection cooling of the anode is typically not possible. Instead anode cooling occurs primarily through black-body radiation and conduction of heat to the outer glass envelope via the anode mounting frame. Cold cathode tubes do exist but are used primarily in lighting systems, where unidirectional power regulation is not the functional purpose of the tube.[citations needed]
The vacuum tube is a voltage-controlled device, with the relationship between the input and output circuits determined by a transconductance function. The solid-state device most closely analogous to the vacuum tube is the JFET, although the vacuum tube typically operates at far higher voltage (and power) levels than the JFET.

Answered by anil kumar , an ibibo Citizen, at 7:52 PM on April 13, 2008

In electronics, a vacuum tube, electron tube (in North America), thermionic valve, or just valve (elsewhere, especially in Britain), is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space, often tubular in form. Many devices called vacuum tubes are filled with low-pressure gas: these are so-called soft valves (or tubes); as distinct from the hard vacuum type, which have the internal gas pressure reduced as far as possible. Almost all depend on the thermal emission of electrons, hence thermionic.
Vacuum tubes were critical to the development of electronics technology, which drove the expansion and commercialization of radio broadcasting, television, radar, high fidelity sound reproduction, large telephone networks, modern types of digital computer, and industrial process control. Some of these applications pre-dated electronics, but it was electronics that made them widespread and practical; electronics has driven mechanical computers such as slide-rules to the point of obsolescence.
For most purposes, the vacuum tube has been replaced by solid-state semiconductor devices such as transistors and solid-state diodes: for most applications, they are smaller, more efficient, more reliable, and cheaper—either as discrete devices or as integrated circuits. However, tubes are still used in specialized applications: for engineering reasons, as in high power radio frequency transmitters; or for their aesthetic appeal, as in modern audio amplification. Cathode ray tubes are still used as display devices in television sets, video monitors, and oscilloscopes, although they are being replaced at various rates by LCDs and other flat-panel displays. A specialized form of the electron tube, the magnetron, is the source of microwave energy in microwave ovens and some radar systems.

Answered by amit , an ibibo Citizen, at 7:42 PM on April 13, 2008

A vacuum tube consists of arrangements of electrodes in a vacuum within an insulating, temperature-resistant envelope. Although the envelope is classically glass, power tubes often use ceramic and metal. The electrodes are attached to leads which pass through the envelope via an air tight seal. On most tubes, the leads are designed to plug into a tube socket for easy replacement.
The simplest vacuum tubes resemble incandescent light bulbs in that they have a filament sealed in a glass envelope which has been evacuated of all air. When hot, the filament releases electrons into the vacuum: a process called thermionic emission. The resulting negatively-charged cloud of electrons is called a space charge. These electrons will be drawn to a metal plate inside the envelope, if the plate (also called the anode) is positively charged relative to the filament (or cathode). The result is a flow of electrons from filament to plate. This cannot work in the reverse direction because the plate is not heated and does not emit electrons. This very simple example described can thus be seen to operate as a diode: a device that conducts current only in one direction. The vacuum tube diode conducts conventional current from plate (anode) to the filament (cathode); this is the opposite direction to the flow of electrons (called electron current).

Answered by savesh singh , an ibibo Citizen, at 7:34 PM on April 13, 2008

Vacuum Tubes: In electronics, a vacuum tube (or thermionic valve outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space.

Answered by khushvinder , an ibibo Citizen, at 7:02 PM on April 13, 2008

a vacuum tube, electron tube (in North America), thermionic valve, or just valve (elsewhere, especially in Britain), is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space, often tubular in form. Many devices called vacuum tubes are filled with low-pressure gas: these are so-called soft valves (or tubes); as distinct from the hard vacuum type, which have the internal gas pressure reduced as far as possible. Almost all depend on the thermal emission of electrons, hence thermionic. This temperature dependence distinguishes them from solid-state devices, which require no initial warm-up period.

Answered by Anil , an ibibo Advisor, at 1:22 PM on April 13, 2008