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In electronics, a vacuum tube, electron tube (in North America), or thermionic 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  Lenovo S10-2 Battery 5200mAh .

Some special function vacuum tubes are filled with low-pressure gas: these are so-called soft tubes as distinct from the hard vacuum type which have the internal gas pressure reduced as far as possible. Almost all tubes depend on the thermionic emission of electrons  Lenovo Y730 Battery 5200mAh .

Vacuum tubes were critical to the development of electronic technology, which drove the expansion and commercialization of radio broadcasting, television, radar,sound reproduction, large telephone networks, analog and digital computers, and industrial process control  Lenovo Y710 Battery 5200mAh .

Some of these applications pre-dated electronics, but it was the vacuum tube that made them widespread and practical.

For most purposes, the vacuum tube has been replaced by solid-state devices such as transistors and solid-state diodes  Lenovo Y530 Battery 5200mAh .

Solid-state devices last much longer, are smaller, more efficient, more reliable, and cheaper than equivalent vacuum tube devices. However, tubes are still used in specialized applications: for engineering reasons, as replacements in older, non-solid-state, high-power radio frequency transmitters  Lenovo Y510 Battery 5200mAh ;

or for their aesthetic appeal and distinct sound signature, as in audio amplification. Cathode ray tubes until very recently were the primary display devices in television sets, video monitors, and oscilloscopes, although they are now being replaced by LCDs and other flat-panel displays  Lenovo S10-3 Battery 5200mAh .

A specialized form of the electron tube, the magnetron, is the source of microwave energy in microwave ovens and some radar systems. The klystron, a powerful but narrow-band radio-frequency amplifier, is commonly deployed by broadcasters as a high-power UHF televisiontransmitter  Lenovo G450 Battery 5200mAh .

Description

A vacuum tube consists of electrodes in a vacuum in an insulating heat-resistant envelope which is usually tubular. Many tubes have glass envelopes, though some types such as power tubes may have ceramic or metal envelopes Lenovo E30 Battery 5200mAh .

The electrodes are attached to leads which pass through the envelope via an airtight 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 an evacuated glass envelopeLenovo E40 Battery 5200mAh .

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 theanode) is positively charged relative to the filament (or cathode)  Lenovo E50 Battery 5200mAh .

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  Lenovo G460 Battery 5200mAh .

The vacuum tube diode conducts conventional current from plate (anode) to the filament (cathode); this is the opposite direction to the actual flow of electrons (called electron current).

Vacuum tubes require a large temperature difference between the hot cathode and the cold anode  Lenovo Y460 Battery 5200mAh .

Because of this, 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  Lenovo Y450 Battery 5200mAh .

Because the tube requires a vacuum to operate, convection cooling of the anode is not generally possible unless the anode forms a part of the vacuum envelope (in which case the cooling is by conduction through the anode material and then convection outside the vacuum envelope)  Lenovo N200 Battery 5200mAh .

Thus anode cooling occurs in most tubes through black-body radiation and conduction of heat to the outer glass envelope via the anode mounting frame. Cold cathode tubes do not rely on thermionic emission at the cathode and usually have some form of gas discharge as the operating principle  Lenovo T410 Battery 5200mAh ;

such tubes are used for lighting (neon lights and neon glow lamps) or as voltage regulators.

Sometimes another electrode, called a control grid, is added between the cathode and the anode. The vacuum tube is then known as a "triode  Lenovo T510 Battery 5200mAh ."

A triode is a voltage-controlled device, in that a voltage that is applied as an input to the grid can be used to modulate the rate of electron flow between anode and cathode. The relationship between this input voltage and the output current is determined by atransconductance function  Lenovo SL410 Battery 5200mAh .

Control grid current is practically negligible in most circuits. 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  Lenovo SL510 Battery 5200mAh .

History and development

The 19th century saw increasing research with evacuated tubes, such as the Geissler and Crookes tubes. Famous scientists who experimented with such tubes included Thomas Edison, Eugen Goldstein, Nikola Tesla, and Johann Wilhelm Hittorf among many others  Lenovo C100 Battery 5200mAh .

With the exception of early light bulbs (modernincandescent light bulbs are not evacuated but are filled with an inert gas), such tubes were only used in scientific research or as novelties. The groundwork laid by these scientists and inventors, however, was critical to the development of subsequent vacuum tube technology  Lenovo T400S Battery 5200mAh .

Although thermionic emission was originally reported in 1873 by Frederick Guthrie, it was Thomas Edison's 1884 investigation that spurned future research, the phenomenon thus becoming known as the "Edison Effect  Lenovo Y430 Battery 5200mAh ."

Edison patented what he found, but he did not understand the underlying physics, nor did he have an inkling of the potential value of the discovery. It wasn't until the early 20th century that the rectifying property of such a device was utilized, most notably by John Ambrose Fleming who used the diode tube to detect (demodulate) radio signals Ibm ThinkPad G40 Battery 5200mAh .

Lee De Forest's 1906 "audion" was also developed as a radio detector, and soon led to the developement of the triode tube. This was essentially the first electronic amplifier, leading to great improvements in telephony (such as the first coast-to-coast telephone line in the US) and revolutionizing the technology used in radio transmitters and receivers  Ibm ThinkPad X200T Battery 5200mAh .

The electronics revolution of the 20th century arguably began with the invention of the triode vacuum tube.

Diodes

The English physicist John Ambrose Fleming worked as an engineering consultant for firms including Edison Telephone and the Marconi Company  Apple A1280 battery 5200mAh .

In 1904, as a result of experiments conducted on Edison effect bulbs imported from the USA, he developed a device he called an "oscillation valve" (because it passes current in only one direction). The heated filament, or cathode, was capable of thermionic emission of electrons that would flow to the plate (or anode) when it was at a higher voltage Apple A1281 battery 5200mAh .

Electrons, however, could not pass in the reverse direction because the plate was not heated and thus not capable of thermionic emission of electrons.

Later known as the Fleming valve, it could be used as a rectifier of alternating current and as a radio wave detector Samsung R522 battery 5200mAh .

This greatly improved the crystal set which rectified the radio signal using an early solid-state diode based on a crystal and a so-called cat's whisker. Unlike modern semiconductors, such a diode required painstaking adjustment of the contact to the crystal in order for it to rectify  Samsung P467 battery 5200mAh .

The diode tube was a reliable alternative for rectifying radio signals. Higher power diode tubes or power rectifiers found their way into power supply applications until they were eventually replaced by silicon rectifiers in the 1960's  Hp Pavilion dm3 Battery 5400mAh .

Triodes

Originally, the only use for tubes in radio circuits was for rectification, not amplification. In 1906 Robert von Lieben filed for a patent   for a cathode ray tube which included magnetic deflection  Hp Pavilion dm4 Battery 5200mAh .

This could be used for amplifying audio signals and was intended for use in telephony equipment. He would later go on to help refine the triode vacuum tube Hp Pavilion cq62 Battery 5200mAh .

However it was Lee De Forest who in 1907 is credited with inventing the triode tube while continuing experiments to improve his original Audion tube, a crude forerunner of the triode  Hp Pavilion cq42 Battery 5200mAh .

By placing an additional electrode in between the filament (cathode) and plate, he discovered the ability of the resulting device to amplify signals of all frequencies. As the voltage applied to the so-called control grid(or simply "grid") was lowered from the cathode's voltage to somewhat more negative voltages Hp Pavilion cq72 Battery 5200mAh ,

the amount of current flowing from the filament to the plate would be reduced. The negative electrostatic field created by the grid in the vicinity of the cathode would inhibit thermionic emission and reduce the current to the plate Hp cq20 Battery 2600mAh .

Thus a few volts difference at the grid would make a large change in the plate current and could lead to a much larger voltage change at the plate, resulting in voltage and power amplification. In 1907, De Forest filed for a patent  for such a three-electrode version of his original Audion tube for use as an electronic amplifier in radio communications  Hp 2230 Battery 2600mAh .

This eventually became known as the triode.

De Forest's device was not strictly a vacuum tube, as he erroneously believed that it depended on the presence of residual gas remaining after evacuation  Dell Inspiron Mini 12 battery 7800mAh .

The De Forest company, in its Audion leaflets, even warned against operation which might lead to too high a vacuum! The Finnish inventor Eric Tigerstedt significantly improved on the original triode design in 1914, while working on hissound-on-film process in Berlin, Germany  Dell XPS M1730 battery 7800mAh .

The first true vacuum triodes in production 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  Dell XPS M2010 battery 7800mAh .

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; early diodes and triodes performed as such despite a rather high residual gas pressure. Techniques to produce and maintain better vacuums in tubes were then developed Dell Studio XPS 1340 battery 5200mAh/7800mAh.

Historically, vacuum levels in production vacuum tubes typically ranged from 10 µPa down to 10 nPa.

The non-linear operating characteristic of the triode caused early tube audio amplifiers to exhibit harmonic distortions at low volumes   Dell Inspiron 9100 battery 7800mAh .

This is not to be confused with the so-called overdrive distortion that tube amplifiers exhibit when driven beyond their linear region (known as the tube sound). To remedy the triode's nonlinear characteristics, engineers plotted curves of the applied grid voltage and resulting plate currents  Dell Inspiron N4010 battery 5200mAh ,

and discovered that there was a range of grid voltages allowing for relatively linear operation. In order to use this range, a negative voltage had to be applied to the grid to place the tube in the "middle" of the linear area with no signal applied. This was called the idle condition, and the plate current at this point the "idle current"  Dell Inspiron N3610 battery 5200mAh .

Today this current would be called the quiescent or bias current. The controlling voltage was superimposed onto this fixed "bias" voltage, resulting in a linear variation of plate current in response to both positive and negative variation of the input voltage around that point  Dell Inspiron N5010 battery 5200mAh .

This concept is called grid bias. Many early radio sets had a third battery called the "C battery" (not to be confused with the modern C cell) whose positive terminal was connected to the cathode of the tubes (or "ground" in most circuits) and whose negative terminal supplied this bias voltage to the grids of the tubes  Dell Inspiron N7010 battery 5200mAh .

More modern circuits used cathode biasing in lieu of a separate negative power supply.

When triodes were first used in radio transmitters and receivers, it was found that tuned amplification stages had a tendency to oscillate unless their gain was very limited  Dell Inspiron 1464 battery 5200mAh .

This was due to the parasitic capacitance between the plate (the amplifier's output) and the control grid (the amplifier's input), known as the Miller capacitance. Eventually the technique of neutralization was developed whereby the RF transformer connected to the plate would include an additional winding in the opposite phase  Dell Inspiron 1564 battery 5200mAh .

This winding would be connected back to the grid through a small capacitor, and when properly adjusted would cancel the Miller capacitance. This technique was employed and led to the success of the Neutrodyne radio during the 1920's  Dell Inspiron 1764 battery 5200mAh .

However neutralization required careful adjustment and proved unsatisfactory when used over a wide ranges of frequencies.

Tetrodes and pentodes

In combatting stability problems and limited voltage gain due to the Miller effect, it was discovered that the addition of a second grid, located between the control grid and the plate  Dell Inspiron 1320 battery 5200mAh ,

known as the screen grid, could solve these problems. ("Screen" in this case refers to electrical "screening" or shielding, not physical construction: all "grid" electrodes in between the cathode and plate are "screens" of some sort rather than solid electrodes since they must allow for the passage of electrons directly from the cathode to the plate)  Dell Inspiron 1320n battery 5200mAh .

A positive voltage slightly lower than the plate voltage was applied to it, and was bypassed (for high frequencies) to ground with a capacitor. This arrangement decoupled the anode and the control grid, essentially eliminating the Miller capacitance and its associated problems  Dell Vostro 1710 battery 5200mAh .

Consequently higher voltage gains from a single tube became possible, reducing the number of tubes required in many circuits. This two-grid tube is called a tetrode, meaning four active electrodes, and was common by 1926.

However, the tetrode has one new problem   Dell Vostro 1720 battery 5200mAh .

In any tube, electrons strike the anode with sufficient energy to cause the emission of electrons from its surface. In a triode this so-called secondary emission of electrons is not important since they are simply re-captured by the more positive anode   Dell Studio XPS 1640 battery 5200mAh .

But in a tetrode they can be captured by the screen grid since it is at also at a high voltage, thus robbing them from the plate current and reducing the amplification of the device. Since secondary electrons can outnumber the primary electrons, in the worst case, particularly as the plate voltage dips below the screen voltage  Dell Vostro A840 battery 5200mAh ,

the plate current can actually go down with increasing plate voltage. This is termed negative resistanceand can itself cause instability. This is the so-called "tetrode kink" (see the reference for a plot of this effect in the RCA-235 tetrode)  Dell Vostro A860 battery 5200mAh .

Another consequence of secondary emission is that in extreme cases the current reaching the screen grid can cause it to overheat to the point of destroying the tube.

The solution was to add one more grid in between the screen grid and the plate, called the suppressor grid (since it supressed secondary emission current toward the screen grid)  Dell Studio 1535 battery 5200mAh/7800mAh .

This grid was held at the cathode (or "ground") voltage and its negative voltage (relative to the anode) electrostatically repelled secondary electrons so that they would be collected by the anode after all. This three-grid tube is called a pentode, meaning five electrodes   Dell Studio 1735 battery 5200mAh/7800mAh .

The pentode was invented in 1928 by Bernard D. H. Tellegen and became generally favoured over the simple tetrode.

A refinement of the tetrode or pentode for power applications is the beam tetrode or "beam power tube", discussed later in this article  Dell Vostro 1310 battery 5200mAh .

Improvements in construction and performance

The very earliest vacuum tubes strongly resembled incandescent light bulbs and were made by lamp manufacturers, who had the equipment for manufacture of glass envelopes and the powerful vacuum pumps required to evacuate the enclosures Dell Latitude E6400 battery,10400mAh/7800mAh .

After World War I, specialized manufacturers using more economical construction methods were set up to fill the growing demand for broadcast receivers. Bare tungsten filaments operated at a temperature of around 2200 °C  Dell Latitude E6400 Battery,10400mAh/7800mAh .

The development of oxide-coated filaments in the mid 1920s reduced filament operating temperature to a dull red heat (around 700 °C), which in turn reduced thermal distortion of the tube structure and allowed closer spacing of tube elements  Dell Latitude E6400 Battery,10400mAh/7800mAh .

This in turn improved tube gain, since the gain of a triode is inversely proportional to the spacing between grid and cathode. Development of the indirectly-heated cathode, with the filament inside a cylinder of oxide-coated nickel, further reduced distortion of the tube elements and also allowed the cathode heaters to be run from an AC supply Dell Latitude E6400 Battery,10400mAh/7800mAh .

Heat transfer and appearance of tubes

Many types of vacuum tubes can be recognized from their appearance. A considerable amount of heat is produced when tubes operate  Dell Latitude E6500 battery,10400mAh/7800mAh .

In most circuits the tube is about 30-60% efficient dependent on the class of operation (classes A, B, or C), which means that 40-70 % of input power to the stage is lost as heat. The requirements for heat removal significantly change the appearance of high-power vacuum tubes  Dell Latitude E6500 battery,10400mAh/7800mAh .

Most tubes contain two sources of heat when operating. The first one of these is the filament or heater. Some types contain a directly heated cathode. This is a filament similar to an incandescent electric lamp and some types glow brightly like a lamp, but most glow dimly   Dell Latitude E6500 battery,10400mAh/7800mAh .

(The "bright emitter" types possess atungsten filament alloyed with 1-3 % thorium which reduces the work function of the metal, giving it the ability to emit sufficient electrons at about 2000 degrees Celsius   Dell Latitude E6500 battery,10400mAh/7800mAh .

The "dull emitter" types also possess a tungsten filament but it is coated in a mixture of calcium, strontium and barium oxides, which emit electrons easily at much lower temperatures due to a monolayer of mixed alkali earth metals coating the tungsten when the cathode is heated to about 800-1000 degrees Celsius.)  HP Pavilion DV6-1210SA battery - 5200mAh/8800mAh

The second form of cathode is the indirectly heated form which usually consists of a nickel tube, coated on the outside with the same strontium, calcium, barium oxide mix used in the "dull emitter" directly heated types, and fitted with a tungsten filament inside the tube to heat it  HP Pavilion DV7 battery - 7200mAh .

This tungsten filament is usually uncoiled and coated in a layer of alumina, (aluminium oxide), to insulate it from the nickel tube of the actual cathode. This form of construction allows for a much greater electron emitting area and, because the heater is insulated from the cathode  Ibm THINKPAD X200 Battery 8800mAh, 11.1V Replacement ,

the cathode can be positioned in a circuit at up to 150 volts more positive than the heater or 50 volts more negative than the heater for most common types. It also allows all the heaters to be simply wired in series or parallel rather than some requiring special isolated power supplies such as specially insulated windings on power transformers or separate batteries Ibm THINKPAD X200 Battery 8800mAh, 11.1V Replacement .

For small-signal tubes such as used in radio receivers, heaters are rated from 50 mW to 5 watts, (directly heated), and about 500 mW to 8 watts for indirectly heated types. Once filament/heater power is included in total power consumption, small tubes have very poor efficiencies  Ibm THINKPAD X200 Battery 8800mAh, 11.1V Replacement .

A 6BM8/ECL82 audio stage consumes a total power of some 15 watts for 3.5 watts of useful audio power, giving an efficiency of around 23%. Some signal amplifiers, particularly high-frequency amplifiers such as the 6BA6, consume some 5.9 watts of power in normal operation and deliver only 1.1 watts of power at the plate  SONY VAIO VGN-FW21E Battery .

The second source of heat is generated at the anode, when electrons, accelerated by the voltage applied to the anode, strike the anode and impart a considerable fraction of their energy to it, raising its temperature. In tubes used in power amplifier or transmitting circuits, this source of heat will exceed the power dissipated in the cathode heater Asus M52N Battery

(The plates or anodes of 6L6 devices used in guitar amplifiers can sometimes be seen to reach red heat if the bias is set too high, they should not emit any visible radiation when driven at maximum ratings.) No tubes in domestic, music, or studio equipment should operate with glowing anodes  Dell Latitude E6400 ATG Battery .

This heat usually escapes the device by black body radiation from the anode/plate as infra red light. Some is conducted away through the connecting wires going to the base but none is convected in most types of tube because of the vacuum and the absence of any gas inside the bulb to convect  SONY VAIO VGN-FW21J Battery .

It is the way tubes get rid of heat which most affects their overall appearance, next to the type of unit (triode, pentode, etc.) they contain, or whether they contain more than one of these basic units. For devices required to radiate more than 500 mW or so, usually indirectly heated cathode types   SONY VAIO VGN-FW21L Battery ,

the anode or plate is often treated to make its surface less shiny, (see black body radiator), and to make it darker, either gray or black. This helps it radiate the generated heat and maintain the anode or plate at a temperature significantly lower than the cathode, a requirement for proper operation  SONY VAIO VGN-FW21M Battery .

Types 6BQ5/EL84 and 6BM8/ECL82 are examples of indirectly heated types with gray anodes.

Other internal elements of high-power tubes, such as control grids and screen grids, may also dissipate heat if carrying large currents  SONY VAIO VGN-FW21Z Battery .

Limits to grid dissipation are listed for such devices, to prevent distortion and failure of the grids.

Tubes used as power amplifier stages for radio transmitters may have additional heat exchangers, cooling fans, radiator fins, or other measures to improve heat transfer at the anode  SONY VAIO VGN-FW25T/B Battery .

Broadcast transmitters may use water-cooling or evaporative cooling for tube anodes. The water cooling system must withstand the high voltages present on the anode.

Low power rated tubes, such as the 1.4 volt filament  SONY VAIO VGN-FW26T/B Battery ,

directly heated tubes, designed for use in battery powered equipment, often retain shiny metal anodes as they produce so little heat. 1T4, 1R5 and 1A7 are examples of devices with shiny untreated anodes. Gas filled tubes, such as thyratrons, although they possess a greater plate dissipation than a "1 volt battery type"  SONY VAIO VGN-FW27/B Battery ,

still often possess a shiny metal anode finish as the gas filling conducts and convects the heat to the bulb wall. Types 884 and 2D21 are typical examples.

The outer electrode in most tubes is usually the anode   SONY VAIO VGN-FW27/W Battery .

Some small signal types, such as sharp and remote cut-off R.F. and A.F. pentodes and some pentagrid converters have a shield fitted around all the electrodes enclosing the anode. This shield is sometimes a solid metal sheet, treated to make it dull and gray like an anode or plate  SONY VAIO VGN-FW31ZJ Battery ,

and sometimes it is fabricated from expanded metal mesh, acting as a Faraday cage but allowing sufficient heat from the anode beneath to escape. Types 6BX6/EF80 and 6BK8/EF86 are typical examples of this shielded type commonly using expanded meshSONY VAIO VGN-FW27T/H Battery .

Types 6AU6/EF94 and 6BE6/EK90 are examples using a gray sheet metal cylindrical shield giving them a very similar overall appearance.

Indicators such as some "magic eye" tubes and the type 6977 fluorescent-anode type have glowing electrodes SONY VAIO VGN-FW29/B Battery .

Other variations

Frequency conversion can be accomplished by various methods in superheterodyne receivers. Tubes with 5 grids, called pentagrid converters, were generally used, although alternatives such as using a combination of a triode with a hexode were also used SONY VAIO VGN-FW290JRB Battery .

Even octodes have been used for frequency conversion. The additional grids are either control grids, with different signals applied to each one, or screen grids. In many designs a special grid acted as a second 'leaky' plate to provide a built-in oscillator, which then coupled this signal with the incoming radio signal  SONY VAIO VGN-FW290JTB Battery .

These signals create a single, combined effect (equivalent to a crude analog multiplier) on the plate current (and thus the signal output) of the tube circuit. The useful component of the output was the difference frequency between that of the incoming signal and that of the oscillator  SONY VAIO VGN-FW290JTH Battery .

The heptode, or pentagrid converter, was the most common of these. 6BE6 is an example of a heptode (note that the first number in the tube ID indicates the filament voltage). Octodes were rare in the US, the 7A8 was one example, but much more common in Europe particularly in battery operated radios where the lower power consumption was an advantage SONY VAIO VGN-FW33G/E1 Battery.

Toward the end of the tube era, precision control and screen grids, called frame grids, offered enhanced performance. Instead of the typically elliptical fine-gauge wire supported by two larger wires, a frame grid was a metal stamping with rectangular openings that surrounded the cathode  SONY VAIO VGN-FW33G/W Battery .

The grid wires were in a plane defined by the stamping, and the control grid was placed much closer to the cathode surface than traditional construction would permit  SONY VAIO VGN-FW33GW Battery .

To reduce the cost and complexity of radio equipment, by 1940 it was common practice to combine more than one function, or more than one set of elements in the bulb of a single tube. The only constraint was where patents, and other licencing considerations required the use of multiple tubes. See British Valve Association (Dell XPS M1210 Batteryhttp://www.hdd-shop.co.uk .

For example, the RCA Type 55 was a double diode triode used as a detector, automatic gain control rectifier and audio preamp in early AC powered radios. The same set of tubes often included the 53 Dual Triode Audio Output  Dell KM958 battery .

Another early type of multi-section tube, the 6SN7, is a "dual triode" which, for most purposes, can perform the functions of two triode tubes, while taking up half as much space and costing less  Dell Studio 1555 battery .

The 12AX7 is a dual high-gain triode widely used in guitar amplifiers, audio preamps, and instruments.

The invention of the 9-pin miniature tube base, besides allowing the 12AX7 family, also allowed many other multi section tubes, such as the 6GH8 triode pentode  Sony VGP-BPS13 battery .

Along with a host of similar tubes, the 6GH8 was quite popular in television receivers. Some color TV sets used exotic types like the 6JH8 which had two plates and beam deflection electrodes (it was known as the 'sheet beam' tube)  Sony VGP-BPS13/B battery .

Vacuum tubes used like this were designed for demodulation of synchronous signals, an example of which is color demodulation for television receivers.

The desire to include many functions in one envelope resulted in the General Electric Compactron Sony VGP-BPS13/S battery .

A typical unit, the 6AG11 Compactron tube contained two triodes and two diodes, but many in the series had triple triodes.

An early example of multiple devices in one envelope was the Loewe 3NF. This 1920s device had 3 triodes in a single glass envelope together with all the fixed capacitors and resistors required to make a complete radio receiver Sony VGP-BPS13A/B battery .

As the Loewe set had only one tube socket, it was able to substantially undercut the competition since, in Germany, state tax was levied by the number of sockets. However, reliability was compromised, and production costs for the tube were much greater. In a sense, these were akin to integrated circuits  Sony VGP-BPS13B/B battery .

In the US, Cleartron briefly produced the "Multivalve" triple triode for use in the Emerson Baby Grand receiver. This Emerson set also had a single tube socket, but because it used a four-pin base, the additional element connections were made on a "mezzanine" platform at the top of the tube base  Sony VGP-BPL9 battery .

Loewe were to also offer the 2NF (two tetrodes plus passive components) and the WG38 (two pentodes, a triode and the passive components).

The beam power tube is usually a tetrode with the addition of beam-forming electrodes, which take the place of the suppressor gridSony VGP-BPS13B/B battery .

These angled plates focus the electron stream onto certain spots on the anode which can withstand the heat generated by the impact of massive numbers of electrons, while also providing pentode behavior. The positioning of the elements in a beam power tube uses a design called "critical-distance geometry" Sony VGP-BPL15 battery ,

which minimizes the "tetrode kink", plate-grid capacitance, screen-grid current, and secondary emission effects from the anode, thus increasing power conversion efficiency. The control grid and screen grid are also wound with the same pitch, or number of wires per inch  Dell Inspiron E1505 battery .

Aligning the grid wires also helps to reduce screen current, which represents wasted energy. This design helps to overcome some of the practical barriers to designing high-power, high-efficiency power tubes. 6L6 was the first popular beam power tube, introduced by RCA in 1936  HP Pavilion dv6000 Battery .

Corresponding tubes in Europe were the KT66, KT77 and KT88 by GEC (the KT standing for "Kinkless Tetrode").

Variations of the 6L6 design are still widely used in guitar amplifiers, making it one of the longest lived electronic device families in history  HP Pavilion dv8000 battery .

Similar design strategies are used in the construction of large ceramic power tetrodes used in radio transmitters.

Special-purpose tubes

Some special-purpose tubes are constructed with particular gases in the envelope HP PAVILION DV6000 Battery,  8800mAh, 10.8V.

For instance, voltage regulator tubes contain various inert gases such as argon, helium or neon, and take advantage of the fact that these gases will ionize at predictable voltages. The thyratron is a special-purpose tube filled with low-pressure gas or mercury, some of which vaporizes   HP PAVILION DV8000 Battery,  8800mAh, 10.8V .

Like other tubes, it contains a hot cathode and an anode, but also a control electrode, which behaves somewhat like the grid of a triode. When the control electrode starts conduction, the gas ionizes, and the control electrode no longer can stop the current; the tube "latches" into conduction SONY VAIO VGN-FZ Battery .

Removing plate (anode) voltage lets the gas de-ionize, restoring its non-conductive state. Some thyratrons can carry large currents for

their physical size. One example is the miniature type 2D21, often seen in 1950s jukeboxes as control switches for relays HP Pavilion DV6000 battery - 8800mAh .

A cold-cathode version of the thyratron, which uses a pool of mercury for its cathode, is called an Ignitron (tm). It can switch thousands of amperes in its largest versions. Thyratrons containing hydrogen have a very consistent time delay between their turn-on pulse and full conduction, and have long been used in radar transmitters  HP Pavilion DV8000 battery - 7800mAh .

Thyratrons behave much like silicon-controlled rectifiers, or to be more chronologically precise, silicon controlled rectifiers mimic some of the behaviours of Thyratrons.

Tubes usually have glass envelopes, but metal, fused quartz (silica), and ceramic are possible choices  HP Pavilion DV6000 battery - 8800mAh .

The first version of the 6L6 used a metal envelope sealed with glass beads, while a glass disk fused to the metal was used in later versions. Metal and ceramic are used almost exclusively for power tubes above 2 kW dissipation. The nuvistor is a tiny tube made only of metal and ceramic  HP Pavilion DV8000 battery - 6600mAh .

In some power tubes, the metal envelope is also the anode. The 4CX1000A is an external anode tube of this sort. Air is blown through an array of fins attached to the anode, thus cooling it. Power tubes using this cooling scheme are available up to 150 kW dissipation  SONY VAIO VGN-FZ18 Battery .

Above that level, water or water-vapor cooling are used. The highest-power tube currently available is the Eimac 4CM2500KG, a forced water-cooled power tetrode capable of dissipating 2.5 megawatts. (By comparison, the largest power transistor can only dissipate about 1 kilowatt.)   SONY VAIO VGN-FZ21E Battery

Another very high power tube is the Eimac 8974, a 1.25 megawatt tetrode used in military and commercial radio-frequency installations.

An extremely specialized tube is the Krytron, which is used for extremely precise, rapid high-voltage switching SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh, 11.10V .

Due to their intended purpose, the initiation of the precise sequence of detonations used to set off a nuclear weapon, they are heavily controlled at an international level.

Medical imaging equipment, such as radiographic and nuclear imaging, use special vacuum tubes  SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh, 11.10V .

Radiographic, fluoroscopic, and CT X-ray imaging equipment use a specially designed vacuum tube diode, which has a rotating anode to dissipate the large amounts of heat developed during operation, and a focused cathode. They are housed in an aluminum housing which is filled with a dielectric oil  SONY VAIO VGN-FW21E Battery .

Nuclear imaging equipment uses photomultiplier tube arrays to detect radiation.

Miniature tubes

The miniature vacuum tube made tubes smaller by eliminating the Bakelite base. It was invented in 1938 SONY VAIO VGN-NR11S/S Battery .

Instead of the separate base, the pins are fused in the glass base of the envelope. This forces the sealing tip to the top of the envelope. Making tubes smaller reduced the voltage that they could work at, and also the power of the filament, so the older style continued to be used for high power rectifiers, valve amplifier output stages and certain transmitting tubes SONY VAIO VGN-NR11M/S Battery .

Miniature tubes with a size roughly that of half a cigarette were used in hearing-aid amplifiers.

Development continued, and led to the sub miniature tubes and the "acorn" valve (named due to its shape) SONY VAIO VGN-NR11Z/S Battery .

Batteries

Batteries provided the voltages required by tubes in early radio sets. As many as three different voltages were required, using three different batteries. The "A" batteries or LT (low-tension) battery provided the filament voltage  SONY VAIO VGN-NR11Z/T Battery .

Tube heaters were designed for single, double or triple-cell lead-acid batteries, giving nominal heater voltages of 2 V, 4 V or 6 V. In portable radios, dry batteries were sometimes used with 1.5 or 1 V heaters. Reducing filament consumption improved the life span of batteries   Sony VAIO VGN-FZ21E Battery .

By 1955, receiving tubes using 50 mA down to as little as 10 mA for the heaters had been developed, but they were swept aside by development of the transistor.

The plate voltage was provided by "B" batteries or the HT (high-tension) supply or battery  Sony VAIO VGN-FW21E Battery .

These were generally of dry cell construction, containing many small 1.5 volt cells in series. They typically came in ratings of 22.5, 45, 67.5, 90 or 135 volts  Sony VAIO VGN-NR11S/S Battery .

Some sets used a grid bias battery or "C" batteries, although many circuits used grid leak resistors, voltage dividers or cathode bias to provide proper tube bias. These batteries had very low drain  Sony VAIO VGN-NR11Z/S Battery .

AC power

Replacement of batteries was a major cost of operation for early radio receiver users. The development of the battery eliminator, and, in 1925, batteryless receivers operated by household power, reduced operating costs and contributed to the growing popularity of radio  Sony VAIO VGN-NR11M/S Battery .

A power supply using a transformer with several windings, one or more rectifiers (which may themselves be vacuum tubes), and large filter capacitors provided the required direct current voltages from the alternating current source  Sony VAIO VGN-NR11Z/T Battery .

As a cost reduction measure, especially in high-volume consumer receivers, all the tube heaters could be connected in series across the AC supply, and the plate voltage derived from a half-wave rectifier directly connected to the AC input, eliminating the need for a heavy power transformer  SONY VAIO VGN-FZ180E Battery .

As an additional feature, these radios could be operated on AC or DC mains. While this arrangement limited the plate voltage (and so, indirectly, the output power) that could be obtained, the resulting supply was adequate for many purposes. A filament tap on the rectifier tube provided the 6 volt, low current supply needed for a dial light  SONY VAIO VGN-FZ220E Battery .

The so-called series string approach did have one safety defect: the chassis of the receiver was connected to one side of the power supply, presenting a shock hazard. Engineers reduced this hazard by enclosing the chassis in a plastic case, making the back out of particle board  SONY VAIO VGN-FZ340E Battery ,

and riveting the power cord chassis plug to the back so that consumers would not be able to power the radio while the chassis was accessible. (Technicians and tinkerers routinely bypassed this by using a separate cord, known colloquially as a "cheater cord" or "widowmaker.")  SONY VAIO VGN-FZ430E Battery

Many consumer AM radio manufacturers of the era used a virtually identical circuit with the tube complement of 12BA6, 12BE6, 12AV6, 35W4, and 50C5, giving these radios the nickname All American Five or simply "Five Tube Radio." Although millions of such receivers were produced, they have now become collector's items SONY VAIO VGN-FZ460E Battery .

Direct and indirect heating

It became common to use the filament to heat a separate electrode called the cathode, and to use this cathode as the source of electron flow in the tube rather than the filament itself  SONY VAIO VGN-FZ480E Battery .

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. Development of vacuum tubes that could use alternating current for the heater supply allowed elimination of one rectifier element  SONY VAIO VGN-FZ4000 Battery .

Reliability

One reliability problem of tubes with oxide cathodes is the possibility that the cathode may slowly become "poisoned" by gas molecules from other elements in the tube, which reduce its ability to emit electrons  SONY VAIO VGN-FZ31E Battery .

Trapped gases or slow gas leaks can also damage the cathode or cause plate-current run away due toionization of free gas molecules. Vacuum hardness and proper selection of construction materials are the major influences on tube lifetime  SONY VAIO VGN-FZ31B Battery .

Depending on the material, temperature and construction, the surface material of the cathode may also diffuse onto other elements. The resistive heaters that heat the cathodes may break in a manner similar to incandescent lamp filaments, but rarely do, since they operate at much lower temperatures than lamps  SONY VAIO VGN-FZ31J Battery .

The heater's failure mode is typically a stress-related fracture of the tungsten wire or at a weld point and generally occurs after accruing many thermal (power on-off) cycles. Tungsten wire has a very low resistance when at room temperature  SONY VAIO VGN-FZ31M Battery .

A negative temperature coefficient device, such as a thermistor, may be incorporated in the equipment's heater supply or a ramp-up circuit may be employed to allow the heater or filaments to reach operating temperature more gradually than if powered-up in a step-function  SONY VAIO VGN-FZ31Z Battery .

Low-cost radios had tubes with heaters connected in series, with a total voltage equal to that of the line (mains). Following World War II, tubes intended to be used in series heater strings were redesigned to all have the same ("controlled") warm-up time. Earlier designs had quite-different thermal time constants  SONY VAIO VGN-FZ38M Battery .

The audio output stage, for instance, had a larger cathode, and warmed up more slowly than lower-powered tubes. The result was that heaters that warmed up faster also temporarily had higher resistance, because of their positive temperature coefficient  SONY VGP-BPS8 Battery .

This disproportionate resistance caused them to temporarily operate with heater voltages well above their ratings, and shortened their life.

Another important reliability problem is caused by air leakage into the tube  SONY VGP-BPS13/S Battery .

Usually oxygen in the air reacts chemically with the hot filament or cathode, quickly ruining it. Designers developed tube designs that sealed reliably. This was why most tubes were constructed of glass. Metal alloys (such as Cunife and Fernico) and glasses had been developed for light bulbs that expanded and contracted in similar amounts, as temperature changed SONY VGP-BPS13A/B Battery .

These made it easy to construct an insulating envelope of glass, while passing connection wires through the glass to the electrodes.

When a vacuum tube is overloaded or operated past its design dissipation, its anode (plate) may glow red  SONY VGP-BPS13B/B Battery .

In consumer equipment, a glowing plate is universally a sign of an overloaded tube. However, some large transmitting tubes are designed to operate with their anodes at red, orange, or in rare cases, white heat.

Vacuum

The vacuum inside the envelope must be as perfect, or "hard", as possible  SONY VGP-BPS13A/S Battery .

Any gas atoms remaining might be ionized at operating voltages, and will conduct electricity between the elements in an uncontrolled manner. This can lead to erratic operation or even catastrophic destruction of the tube and associated circuitry SONY VGP-BPS13AS Battery .

Unabsorbed free air sometimes ionizes and becomes visible as a pink-purple glow discharge between the tube elements.

To prevent any remaining gases from remaining in a free state in the tube, modern tubes are constructed with "getters", which are usually small, circular troughs filled with metals that oxidize quickly, with barium being the most common Dell Inspiron 1320n Battery .

While the tube envelope is being evacuated, the internal parts except the getter are heated by RF induction heating to extract any remaining gases from the metal. The tube is then sealed and the getter is heated to a high temperature, again by radio frequency induction heating  Dell Studio 1450 Battery .

This causes the material to evaporate, absorbing/reacting with any residual gases and usually leaving a silver-colored metallic deposit on the inside of the envelope of the tube. The getter continues to absorb any gas molecules that leak into the tube during its working life   Dell Studio 1457 Battery .

If a tube develops a crack in the envelope, this deposit turns a white color when it reacts with atmospheric oxygen. Large transmitting and specialized tubes often use more exotic getter materials, such as zirconium  Dell Latitude D610 Battery .

Early gettered tubes used phosphorus based getters and these tubes are easily identifiable, as the phosphorus leaves a characteristic orange or rainbow deposit on the glass. The use of phosphorus was short-lived and was quickly replaced by the superior barium getters   Toshiba NB100 Battery .

Unlike the barium getters, the phosphorus did not absorb any further gases once it had fired.

Transmitting tubes

Large trans

mitting tubes have carbonized tungsten filaments containing a small trace (1% to 2%) of thorium  Toshiba Satellite M65 battery .

An extremely thin (molecular) layer of thorium atoms forms on the outside of the wire's carbonized layer and, when heated, serve as an efficient source of electrons. The thorium slowly evaporates from the wire surface, while new thorium atoms diffuse to the surface to replace them  Toshiba Satellite M60 battery .

Such thoriated tungsten cathodes usually deliver lifetimes in the tens of thousands of hours. The end-of-life scenario for a thoriated-tungsten filament is when the carbonized layer has mostly been converted back into another form of tungsten carbide and emission begins to drop off rapidly  Dell Latitude D830 Battery ;

a complete loss of Thorium has never been found to be a factor in the end-of-life in a tube with this type of emitter. The highest reported tube life is held by an Eimac power tetrode used in a Los Angeles radio station's transmitter, which was removed from service after 80,000 hours (~9 years) of operation Dell Latitude D620 Battery .

It has been said[who?] that transmitters with vacuum tubes are better able to survive lightning strikes than transistor transmitters do. While it was commonly believed that at rf power levels above approx. 20 kilowatts, vacuum tubes were more efficient than solid state circuits  Dell Inspiron Mini 10 Battery ,

this is no longer the case especially in medium wave (AM broadcast) service where solid state transmitters at nearly all power levels have measurably higher efficiency. FM broadcast transmitters with solid state power amplifiers up to approx. 15 kW also show better overall mains-power efficiency than tube-based power amplifiers  Sony VGN-FW11S Battery .

Receiving tubes

Cathodes in small "receiving" tubes are coated with a mixture of barium oxide and strontium oxide, sometimes with addition of calcium oxide or aluminium oxide   Sony VGN-FW11M Battery .

An electric heater is inserted into the cathode sleeve, and insulated from it electrically by a coating of aluminium oxide. This complex construction causes barium and strontium atoms to diffuse to the surface of the cathode when heated to about 780 degrees Celsius, thus emitting electrons  Sony VGN-FW139E/H battery .

Failure modes

A catastrophic failure is one which suddenly makes the vacuum tube unusable. A crack in the glass envelope will allow air into the tube and destroy it  Dell Latitude E5400 Battery .

Cracks may result from stress in the glass, bent pins or impacts; tube sockets must allow for thermal expansion, to prevent stress in the glass at the pins. Stress may accumulate if a metal shield or other object presses on the tube envelope and causes differential heating of the glass. Glass may also be damaged by high-voltage arcing Dell Latitude E4200 Battery .

Tube heaters may also fail without warning, especially if exposed to over voltage or as a result of manufacturing defects. Tube heaters do not normally fail by evaporation like lamp filaments, since they operate at much lower temperature  Dell Inspiron 300M Battery .

The surge of inrush current when the heater is first energized causes stress in the heater, and can be avoided by slowly warming the heaters, gradually increasing current. Some tubes intended for series string operation of the heaters across the supply will have a definite controlled warm-up time to avoid excess voltage on some heaters as others warm up Dell Studio 1737 battery .

Directly-heated filament-type cathodes as used in battery-operated tubes or some rectifiers may fail if the filament sags, causing internal arcing. Excess heater-to-cathode voltage in indirectly heated cathodes can break down the insulation between elements and destroy the heater  Dell Inspiron E1505 battery .

Arcing between tube elements can destroy the tube. An arc can be caused by applying plate potential before the cathode has come up to operating temperature, or by drawing excess current through a rectifier which damages the emission coating. Arcs can also be initiated by any loose material inside the tube, or by excess screen voltage  Dell RM791 battery .

An arc inside the tube allows gas to evolve from the tube materials, and may deposit conductive material on internal insulating spacers.

Degenerative failures

Degenerative failures cause the performance of the tube to slowly deteriorate with time  Toshiba Satellite P10 Battery .

Overheating of internal parts, such as control grids or mica spacer insulators, can result in trapped gas escaping into the tube; this can reduce performance. A getter is used to absorb gases evolved during tube operation, but has only a limited ability to combine with gas   Acer Aspire One battery 10400mAh .

Control of the envelope temperature prevents some types of gassing. A tube with very bad internal gas may have a visible blue glow when plate voltage is applied.

Gas and ions within the tube contribute to grid current which can disturb operation of a vacuum tube circuit SONY VGN-FZ210CE Battery .

Another effect of overheating is the slow deposit of metallic vapors on internal spacers, resulting in inter-element leakage.

Tubes on standby for long periods, with heater voltage applied, may develop high cathode interface resistance and display poor emission characteristics  Dell Precision M70 Battery .

This effect occurred especially in pulse anddigital circuits, where tubes had no plate current flowing for extended times.

Cathode depletion describes the loss of emission after thousands of hours of normal use  Toshiba Satellite L305 Battery .

Sometimes emission can be restored for a time by raising heater voltage either for a short time or a permanent increase of a few percent. Cathode depletion was uncommon in signal tubes but was a frequent cause of failures of monochrome television cathode-ray tubes  Toshiba Satellite T4900 Battery .

Other failures

Vacuum tubes may have or develop defects in operation that makes an individual tube useless in one device, but which may not prevent its satisfactory operation in another system Toshiba PA3399U-2BRS battery .

Microphonics refers to internal vibration of tube elements, which modulates the signal from the tube in an undesirable way; sound or vibration pick-up may affect the signals, or even cause uncontrolled howling if a feedback path develops between a microphonic tube and, for example, a loudspeaker  Toshiba Satellite A200 Battery .

Leakage current between AC heaters and the cathode may couple into the circuit, or electrons emitted directly from the ends of the heater may also inject hum into the signal. Leakage current due to internal contamination may also inject noise  Toshiba Satellite 1200 Battery .

Colossus

Colossus (and its successor Colossus Mk2) was built by the British during World War II to substantially speed up the task of breaking the German high levelLorenz encryption. Based on 1500 vacuum tubes, Colossus replaced an earlier machine based on relay and switch logic (the Heath Robinson)  Toshiba Satellite M300 Battery .

Colossus was able to break in a matter of hours messages that had previously taken several weeks. Colossus Mk2 used a total of around 2000 vacuum tubes. Colossus was the first ever use of vacuum tubes on such a large scale for a single machine. The largest project previously had used just 150 tubes and had proven to be extremely unreliable  SONY VGP-BPS13A/Q Battery .

The main design problem at Colossus's inception was how to make vacuum tube based equipment reliable when the tubes were used in large numbers.

The Colossus computer's designer, Dr. Tommy Flowers, had a theory that most of the unreliability was caused during power down and (mainly) power up  SONY VGP-BPS13A/Q Battery .

Once Colossus was built and installed, it was switched on and left switched on running from dual redundant diesel generators (the wartime mains supply being considered too unreliable). The only time it was switched off was for conversion to the Colossus Mk2 and the addition of another 500 or so tubes  SONY VGP-BPS13B/Q Battery .

Another 9 Colossus Mk2s were built, and all 10 machines ran with a surprising degree of reliability. The 10 Colossi consumed 15 kilowatts of power each, 24 hours a day, 365 days a year—nearly all of it for the tube heaters  SONY VGP-BPS13B/Q Battery .

Whirlwind

To meet the reliability requirements of the early digital computer Whirlwind, it was necessary to build special "computer vacuum tubes" with extended cathode life  SONY VGP-BPS13/Q Battery .

The problem of short lifetime was traced to evaporation of silicon, used in the tungsten alloy to make the heater wire easier to draw. Elimination of the silicon from the heater wire alloy (and paying extra for more frequent replacement of the wire drawing dies) allowed production of tubes that were reliable enough for the Whirlwind project  SONY VGP-BPS13/Q Battery .

The tubes developed for Whirlwind later found their way into the giant SAGE air-defense computer system. High-purity nickel tubing and cathode coatings free of materials that can poison emission (such as silicates and aluminium) also contribute to long cathode life. The first such "computer tube" was Sylvania's 7AK7 of 1948  SONY VGP-BPS21A/B Battery .

By the late 1950s it was routine for special-quality small-signal tubes to last for hundreds of thousands of hours, if operated conservatively. This reliability made mid-cable amplifiers in submarine cables possible  SONY VGP-BPS21/S Battery .

World War II

Near the end of World War II, to make radios more rugged, some aircraft and army radios began to integrate the tube envelopes into the radio's castaluminium or zinc chassis  SONY VGP-BPS21B Battery .

The radio became just a printed circuit with non-tube components, soldered to the chassis that contained all the tubes. During WWII in 1942, rugged metal vacuum tubes were mounted in anti-aircraft shells. These proximity fuzes made anti-aircraft shells 6 times more effective  SONY VGP-BPS21A Battery .

In the fall of 1944, artillery shells with proximity fuses were used. The tiny tubes were later known as "subminiature" types. They were widely used in 1950s military and aviation electronics  SONY VGP-BPS21 Battery .

Applications

Vacuum tubes were critical to the development of electronic technology, which drove the expansion and commercialization of radio broadcasting, television,radar, sound reproduction, large telephone networks, analog and digital computers, and industrial process control  Sony VGP-BPS21A/B Battery .

Some of these applications pre-dated electronics, but it was the vacuum tube that made them widespread and practical.

Tubes were heavily used in the early generations of electronic devices, such as radios, televisions, and early computers such as the Colossus which used 2000 tubes, the ENIAC which used nearly 18,000 tubes, and the IBM 700 series Sony VGP-BPS21/S Battery.

For most purposes, the vacuum tube has been replaced by solid-state devices such as transistors and solid-state diodes. Solid-state devices last much longer, are smaller, more efficient, more reliable, and cheaper than equivalent vacuum tube devices. However, tubes are still used in specialized applications  Sony VGP-BPS21A Battery :

for engineering reasons, as in high-power radio frequency transmitters; or for their aesthetic appeal and distinct sound signature, as in audio amplification. Cathode ray tubes are still used as display devices in television sets, video monitors, and oscilloscopes, although they are being replaced by LCDs and other flat-panel displays  Sony VGP-BPS21 Battery .

A specialized form of the electron tube, the magnetron, is the source of microwave energy in microwave ovens and some radar systems. The klystron, a powerful but narrow-band radio-frequency amplifier, is commonly deployed by broadcasters as a high-power UHF television transmitter  Sony Vaio PCG-5G2L Battery .

Vacuum tubes are less susceptible than corresponding solid-state components to the electromagnetic pulse effect of nuclear explosions. This property kept them in use for certain military applications long after transistors had replaced them elsewhere  Sony Vaio PCG-5G3L Battery .

Vacuum tubes are still used for very high-powered applications such as industrial radio-frequency heating, generating large amounts of RF energy forparticle accelerators, and power amplification for broadcasting. In microwave ovens, cost-engineered magnetrons efficiently generate microwave power on the order of hundreds of watts  Sony Vaio PCG-5J1L Battery .

Many audiophiles, professional audio engineers, and musicians prefer the tube sound of audio equipment based on vacuum tubes over electronics based ontransistors. There are companies which still make specialized audio hardware featuring tube technology  Sony Vaio PCG-5K2L Battery .

A common usage is in the high-end microphonepreamplifiers preferred by professional music recording studios, and in electric guitar amplification. The sound produced by a tube based amplifier with the tubes overloaded (overdriven) has defined the texture of some genres of music such as classic rock and blues  Sony Vaio PCG-5J2L Battery .

Guitarists often prefer tube amplifiers for the warmth of their tone and the natural compression effect they can apply to an input signal.

Cathode-ray tubes (CRTs) are a highly-evolved type of vacuum tube, described elsewhere  Sony Vaio PCG-5K1L Battery .

In 2002, computer motherboard maker AOpen brought back the vacuum tube for modern computer use by releasing the AX4GE Tube-G motherboard. This motherboard uses a Sovtek 6922 vacuum tube (a version of the 6DJ8) as part of AOpen’s TubeSound Technology. AOpen claims that the vacuum tube brings superior sound  Sony Vaio PCG-6S2L Battery .

Cooling

Like any electronic device, vacuum tubes produce heat while operating. This waste heat is one of the principal factors that affect tube life  . The majority of this waste heat originates in the anode though some grids may also require cooling to remove excess heat  Sony Vaio PCG-6S3L Battery .

For example, the cooling of the screen grid in an EL34 is facilitated by the addition of two small radiators or "wings," located near the top of the tube. The heater (filament) also contributes to the total waste heat. A tube'sdata sheet will normally identify the maximum amount of heat each element may dissipate  Sony Vaio PCG-6V1L Battery .

The method of anode cooling is dependent on the construction of the tube itself. For tubes with internal anodes such as the 12AX7 or EL34, the cooling occurs by radiating the heat by black body radiation from the anode to the glass envelope   Sony Vaio PCG-6W1L Battery .

Natural air circulation, convection, then removes the heat from the envelope. Tube shields that aided heat dispersal could be retrofitted on certain select types of tubes. These shields act by improving heat conduction from the surface of the tube to the shield itself by means of tens of copper tongues in contact with the glass tube  Sony Vaio PCG-6W2L Battery ,

and have an opaque, black outside finish for improved heat radiation. The ability to remove heat may be further increased by implementing forced air cooling, adding fins to the anode, and operating the anode at red hot temperatures. All of these measures are implemented in the 4-1000A transmitting tube  Sony Vaio PCG-6W3L Battery .

The amount of heat that may be removed from a tube with an internal anode is limited  . Tubes with external anodes may be cooled using forced air, water, vapor, and multiphase. The 3CX10,000A7 is an example of a tube with an external anode cooled by forced air  Sony Vaio PCG-7111L Battery .

The water, vapor, and multiphase cooling techniques all depend on the high specific heat andlatent heat of water. The 8974 is an example of a water cooled tube and is among the largest commercial tube available today  Sony Vaio PCG-7112L Battery .

In a water cooled tube, the anode voltage appears directly on the cooling water surface, thus requiring the water to be an electrical insulator. Otherwise the high voltage can be conducted through the cooling water to the radiator system  Sony Vaio PCG-7113L Battery ;

hence the need for deionized water. Such systems usually have a built-in water-conductance monitor which will shut down the high-tension supply (often tens of kilovolts) if the conductance becomes too high.

Other vacuum tube devices

Many devices were built during the 1920–1960 period using vacuum-tube techniques  Sony Vaio PCG-7133L Battery .

Most such tubes were rendered obsolete by semiconductors; some techniques for integrating multiple devices in a single module, sharing the same glass envelope have been discussed above, such as the Loewe 3NF. Vacuum-tube electronic devices still in common use include the magnetron, klystron,photomultiplier, x-ray tube, traveling-wave tube and cathode ray tube Sony Vaio PCG-7Z2L Battery .

The magnetron is the type of tube used in all microwave ovens. In spite of the advancing state of the art in power semiconductor technology, the vacuum tube still has reliability and cost advantages for high-frequency RF power generation. Photomultipliers are still the most sensitive detectors of light  Sony Vaio PCG-8Y1L Battery .

Many televisions,oscilloscopes and computer monitors still use cathode ray tubes, though flat panel displays are becoming more popular as prices drop.

Many of the better tube radios had so-called "tuning eye" indicator tubes behind their front panels  Sony Vaio PCG-8Y2L Battery ,

with just the top of the tube showing. These tubes were used as a visual indication of received signal strength, and an aid to properly tuning in a station.

Secondary emission is the term for what happens when electrons in a vacuum strike certain materials, and the impacts cause electrons to be emitted  Sony Vaio PCG-8Z1L Battery .

For some materials, more electrons are emitted than originally hit the surface. Such devices, called electron multipliers, amplify the current represented by the incoming electrons. Several stages (as many as 15 or so) can be cascaded for high gain, and are essential parts of very sensitive phototubes, usually called photomultipliers or multiplier photoubes  Sony Vaio PCG-8Z2L Battery .

The image orthicon TV studio camera tubes also used multistage photomultipliers.

For decades, electron-tube designers tried to use secondary emission to obtain more amplification in vacuum tubes with hot cathodes  SONY VAIO VGN-FW31J Battery ,

but they suffered from short life because the material used for the secondary-emission electrode (called a dynode) "poisoned" the tube's hot cathode. (For instance, the interesting RCA 1630 secondary-emission tube was marketed, but did not last.)   SONY VAIO VGN-FW31E Battery

However, eventually, Philips of The Netherlands developed the EFP60 tube that had a satisfactory lifetime, and was used in at least one product, a laboratory pulse generator. However, transistors were rapidly improving, and eclipsed tubes in general  SONY VAIO VGN-FW31B Battery .

A variant, called a channel electron multiplier, is a curved tube, such as a helix, coated on the inside with material with good secondary emission. One type had a little funnel to capture incoming electrons. The tube was resistive, and its ends were connected to enough voltage to create repeated cascades of electrons SONY VAIO VGN-FW31M Battery .

Tektronix made a high-performance wideband oscilloscope CRT with a channel electron multiplier plate behind the phosphor layer. This plate was a bundled array of a huge number of short individual c.e.m. tubes that accepted a low-current beam and intensified it to provide a display of practical brightness  SONY VAIO VGN-FW31Z Battery .

(The electron optics of the wideband electron gun could not provide enough current to directly excite the phosphor.)

The fluorescent displays commonly used on videocassette recorders, some microwave oven control panels, and automotive dashboards are vacuum tubes  SONY VAIO VGN-FW38M Battery .

using phosphor-coated anodes to form the display characters, and a heated filamentary cathode as an electron source. These are referred to as "VFDs", or vacuum fluorescent displays. Because the filaments are in view, they must be operated at temperatures where the filament does not glow visibly  SONY VAIO VGN-AW41JF Battery .

Often found in automotive applications, their high brightness allows reading the display in daylight. VFD tubes are flat and rectangular, as well as relatively thin. Typical VFD phosphors emit a broad-spectrum greenish-white light, permitting use of color filters    SONY VAIO VGN-AW41JF/H Battery .

This type of phosphor provides a bright glow with only modest operating voltage, low tens of volts.

Some tubes, such as magnetrons, traveling-wave tubes, carcinotrons, and klystrons, combine magnetic and electrostatic effects  SONY VAIO VGN-AW41MF Battery .

These are efficient (usually narrow-band) RF producers and still find use in radar, microwave ovens and industrial heating. Traveling-wave tubes (TWTs) are very good amplifiers; they are used in some communications satellites. High-powered klystron amplifier tubes can provide hundreds of kilowatts in the UHF range  SONY VAIO VGN-AW41MF/H Battery .

Gyrotrons or vacuum masers, used to generate high-power millimetre band waves, are magnetic vacuum tubes in which a small relativistic effect, due to the high voltage, is used for bunching the electrons. Gyrotrons can generate very high powers (hundreds of kilowatts)  SONY VAIO VGN-FW30B Battery .

Free electron lasers, used to generate high-power coherent light and perhaps even X rays, are highly relativistic vacuum tubes driven by high-energy particle accelerators.

Particle accelerators can be considered vacuum tubes that work backward, the electric fields driving the electrons, or other charged particles  SONY VAIO VGN-FW50B Battery .

In this respect, a cathode ray tube is a particle accelerator.

A tube in which electrons move through a vacuum (or gaseous medium) within a gas-tight envelope is generically called an electron tube SONY VAIO VGN-FW51B/W Battery .

Some condenser microphone designs use built-in vacuum tube preamplifiers.

Vacuum tubes using field electron emitters

In the early years of the 21st century there has been renewed interest in vacuum tubes, this time with the electron emitter formed on a flat silicon substrate, as in integrated circuit technology  SONY VAIO VGN-FW51MF Battery .

This subject is now called vacuum nanoelectronics. The most common design uses a cold cathode in the form of a large-area field electron source (for example a field emitter array). With these devices, electrons are field-emitted from a large number of closely spaced individual emission sites  SONY VAIO VGN-FW51MF/H Battery .

Their claimed advantages include greatly enhanced robustness combined with the ability to provide high power outputs at low power consumptions. Operating on the same principles as traditional tubes, prototype device cathodes have been fabricated in several different ways  SONY VAIO VGN-FW51ZF Battery .

Although a common approach is to use a field emitter array, one interesting idea is to etch electrodes to form hinged flaps – similar to the technology used to create the microscopic mirrors used in Digital Light Processing) that are stood upright by an electrostatic charge  SONY VAIO VGN-FW51ZF/H Battery .

Such integrated microtubes may find application in microwave devices including mobile phones, for Bluetooth and Wi-Fi transmission, in radar and for satellite communication. Presently they are being studied for possible applications in field emission display technology, but significant production problems seem to exist  SONY VAIO VGN-FW52JB Battery .

A Crookes tube is an early experimental electrical discharge tube, invented by British physicist William Crookes  and others around 1869-1875,  in which cathode rays, that is electrons, were discovered  SONY VAIO VGN-FW54FB Battery .

A development of the Geissler tube, it consists of a partially evacuated glass cylinder of various shapes, with two metal electrodes, one at either end. When a high voltage is applied between the electrodes, electrons travel in straight lines from the cathode to the anode  SONY VAIO VGN-FW70DB Battery .

It was used by Crookes, Johann Hittorf, Juliusz Plücker, Eugen Goldstein, Heinrich Hertz, Philipp Lenard and others to discover the properties of cathode rays, culminating in J. J. Thomson's 1897 identification of cathode rays as negatively-charged particles, which were later named electrons SONY VAIO VGN-FW71DB/W Battery .

Crookes tubes are now used only for demonstrating cathode rays.

Wilhelm Röntgen discovered x-rays with the Crookes tube in 1895. The term is also used for the first generation, cold cathode x-ray tubes, which evolved from the experimental Crookes tubes and were used until about 1920  SONY VAIO VGN-FW72JGB Battery .

Crookes tubes were cold cathode tubes, meaning they didn't have a heated filament in them to release electrons like later electronic vacuum tubes. Instead, electrons were generated by ionization of the residual air by a high DC voltage (from a few kilovolts to 100 kV) applied between the electrodes, usually by aninduction coil (Ruhmkorff coil)  SONY VAIO VGN-FW73JGB Battery .

They require a small amount of air in them to function, from 10?6 to 5×10?8 atmosphere (7×10?4 - 4×10?5 torr or 0.1 - 0.005pascal).

When high voltage is applied to the tube, the electric field accelerates the small number of electrically charged ions always present in the gas, created by natural processes like radioactivity  SONY VAIO VGN-FW74FB Battery .

These collide with other gas molecules, knocking electrons off them and creating more positive ions in a chain reaction. All the positive ions are attracted to the cathode or negative electrode. When they strike it, they knock large numbers of electrons out of the surface of the metal, which in turn are repelled by the cathode and attracted to the anode or positive electrode SONY VAIO VGN-FW81HS Battery .

These are the cathode rays.

Enough of the air has been removed from the tube that most of the electrons can travel the length of the tube without striking a gas molecule  SONY VAIO VGN-FW81NS Battery .

The high voltage accelerates these low-mass particles to a high velocity (about 37,000 miles per second, or 59,000 km/s, 20% of the speed of light, for a typical tube voltage of 10 kV ). When they get to the anode end of the tube, they have so much momentum that, although they are attracted to the anode, many fly past it and strike the end wall of the tube SONY VAIO VGN-FW81S Battery .

When they strike atoms in the glass, they knock their orbital electrons into a higher energy level. When the electrons fall back to their original energy level, they emit light. This process, called fluorescence, causes the glass to glow, usually yellow-green. The electrons themselves are invisible, but the glow reveals where the beam of electrons strikes the glass  SONY VAIO VGN-FW82DS Battery .

Later researchers painted the back wall of the tube inside with a phosphor, a fluorescent chemical such as zinc sulfide, to make the glow more visible. After striking the wall, the electrons eventually make their way to the anode, flow through the anode wire, the power supply, and back to the cathode  SONY VAIO VGN-FW82JS Battery .

The above only describes the motion of the electrons. The full details of the action in a Crookes tube are complicated, because it contains a nonequilibrium plasma of positively charged ions, electrons, and neutral atoms which are constantly interacting  SONY VAIO VGN-FW82XS Battery .

At higher gas pressures, above 10?6 atm (0.1 Pa), this creates different colored glowing regions in the gas, depending on the pressure in the tube (see diagram). The details were not fully understood until the development of plasma physics in the early 20th centurySONY VAIO VGN-FW83DS Battery .

Crookes tubes evolved from the earlier Geissler tubes, experimental tubes which are similar to modern neon tube lights. Geissler tubes had only a low vacuum, around 10?3 atm (100 Pa), and the electrons in them could only travel a short distance before hitting a gas molecule   SONY VAIO VGN-FW83JS Battery .

So the current of electrons moved in a slow diffusion process, constantly colliding with gas molecules, never gaining much energy. These tubes didn't create beams of cathode rays, only a pretty glow discharge that filled the tube as the electrons struck the gas molecules and excited them, producing light  SONY VAIO VGN-FW83XS Battery .

Crookes was able to evacuate his tubes to a lower pressure, 10?6 to 5x10?8 atm, using an improved Sprengel mercury vacuum pump made by his coworker Charles A. Gimingham. He found that as he pumped more air out of his tubes, a dark area in the glowing gas formed next to the cathode  SONY VAIO VGN-FW90NS Battery .

As the pressure got lower, the dark area, called the Crookes dark space, spread down the tube, until the inside of the tube was totally dark. However, the glass envelope of the tube began to glow at the anode end  SONY VAIO VGN-FW90S Battery .

What was happening was that as more air was pumped out of the tube, there were fewer gas molecules to obstruct the motion of the electrons, so they could travel a longer distance, on average, before they struck one. By the time the inside of the tube became dark, they were able to travel in straight lines from the cathode to the anode, without a collision   SONY VAIO VGN-FW91NS Battery .

They were accelerated to a high velocity by the electric field between the electrodes, both because they didn't lose energy to collisions, and also because Crookes tubes required a higher voltage. By the time they reached the anode end of the tube, they were going so fast that many flew past the anode and hit the glass wall   SONY VAIO VGN-FW91S Battery .

The electrons themselves were invisible, but when they hit the glass walls of the tube they excited the atoms in the glass, making them give off light or fluoresce, usually yellow-green. Later experimenters painted the back wall of Crookes tubes with fluorescent paint, to make the beams more visible  SONY VAIO VGN-FW92DS Battery .

This accidental fluorescence allowed researchers to notice that objects in the tube, such as the anode, cast a sharp-edged shadow on the tube wall. Johann Hittorf was first to recognise in 1869 that something must be travelling in straight lines from the cathode to cast the shadow  SONY VAIO VGN-FW92JS Battery .

In 1876, Eugen Goldsteinproved that they came from the cathode, and named them cathode rays (Kathodenstrahlen).

At the time, atoms were the smallest particles known, the electron was unknown, and what carried electric currents was a mysterySONY VAIO VGN-FW93DS Battery .

Many ingenious types of Crookes tubes were built to determine the properties of cathode rays (see below). The high energy beams of pure electrons in the tubes revealed their properties much better than electrons flowing in wires. The colorful glowing tubes were also popular in public lectures to demonstrate the mysteries of the new science of electricity  SONY VAIO VGN-FW93JS Battery .

Decorative tubes were made with fluorescent minerals, or butterfly figures painted with fluorescent paint, sealed inside. When power was applied, the fluorescent materials lit up with many glowing colors.

In 1895, Wilhelm Röntgen discovered x-rays emanating from Crookes tubes  SONY VAIO VGN-FW139E/H Battery .

The many uses for x-rays were immediately apparent, the first practical application for Crookes tubes.

Crookes tubes were unreliable and temperamental. Both the energy and the quantity of cathode rays produced depended on the pressure of residual gas in the tube  SONY VAIO VGN-FW145E/W Battery .

Over time the gas was absorbed by the walls of the tube, reducing the pressure. This reduced the amount of cathode rays produced and caused the voltage across the tube to increase, creating 'harder' more energetic cathode rays. Soon the pressure got so low the tube stopped working entirely  SONY VAIO VGN-FW190EBH Battery .

The electronic vacuum tubes invented later around 1906 superseded the Crookes tube. These operate at a still lower pressure, around 10?9 atm (10?4 Pa), at which there are so few gas molecules that they don't conduct by ionization   SONY VAIO VGN-FW190EDH Battery .

Instead, they use a more reliable and controllable source of electrons, a heated filament or hot cathode which releases electrons by thermionic emission. The ionization method of creating cathode rays used in Crookes tubes is today only used in a few specialized gas discharge tubes such as krytron  SONY VAIO VGN-FW21E Battery .

The technology of manipulating electron beams pioneered in Crookes tubes was applied practically in the design of vacuum tubes, and particularly in the invention of the cathode ray tube by Ferdinand Braun in 1897  SONY VAIO VGN-FW21J Battery .

Discovery of x-rays

When the voltage applied to a Crookes tube is high enough, around 5,000 volts or greater, it can accelerate the electrons to a fast enough velocity to create x-rays when they hit the anode or the glass wall of the tube  SONY VAIO VGN-FW21L Battery .

The fast electrons emit x-rays when their path is bent sharply as they pass near the high electric charge of an atom's nucleus, a process called bremsstrahlung, or they knock an atom's inner electrons into a higher energy level, and these in turn emit x-rays as they return to their former energy level, a process called x-ray fluorescence  SONY VAIO VGN-FW21M Battery .

Many early Crookes tubes undoubtedly generated x-rays, because early researchers such as Ivan Pulyui had noticed that they could make foggy marks on nearby unexposed photographic plates. On November 8, 1895, Wilhelm Röntgen was operating a Crookes tube covered with black cardboard when he noticed a nearby fluorescent screen faintly glowing  SONY VAIO VGN-FW21Z Battery .

He realized that some unknown invisible rays from the tube were able to pass through the cardboard and make the screen fluoresce. He found that they could pass through books and papers on his desk. Röntgen began to investigate the rays full time, and on December 28, 1895 published the first paper on x-rays SONY VAIO VGN-FW25T/B Battery .

He received the first Nobel Prize in physics for his discovery.

The medical applications of x-rays created the first practical use for Crookes tubes, and workshops began manufacturing specialized Crookes tubes to generate x-rays, the first x-ray tubes  SONY VAIO VGN-FW26T/B Battery .

The anode was made of a heavy metal, usually platinum, which generated more x-rays, and was tilted at an angle to the cathode, so the x-rays would radiate through the side of the tube. The cathode had a concave spherical surface which focused the electrons into a small spot around 1 mm in diameter on the anode  SONY VAIO VGN-FW27/B Battery ,

in order to approximate a point source of x-rays, which gave the sharpest radiographs. These cold cathode type x-ray tubes were used until about 1920, when they were superseded by the hot cathode Coolidge x-ray tube.

Experiments with Crookes tubes

Crookes tubes were used in dozens of experiments to try to find out what cathode rays were SONY VAIO VGN-FW27/W Battery .

There were two theories: Crookes and Cromwell Varley believed they were 'corpuscles' or 'radiant matter', that is, electrically charged atoms. German researchers E. Wiedemann, Heinrich Hertz, and Eugen Goldstein believed they were 'aether vibrations', some new form of electromagnetic waves, and were separate from what carried the current through the tube SONY VAIO VGN-FW27T/H Battery .

The debate continued until J. J. Thomson measured their mass, proving they were a previously unknown negatively charged particle, which he called a 'corpuscle' but was later named electron.

Maltese cross

Juliusz Plücker in 1869 built an anode shaped like a Maltese Cross in the tube  SONY VAIO VGN-FW29/B Battery .

It was hinged, so it could fold down against the floor of the tube. When the tube was turned on, it cast a sharp cross-shaped shadow on the fluorescence on the back face of the tube, showing that the rays moved in straight lines. After a while the fluorescence would get 'tired' and decrease  SONY VAIO VGN-FW290JRB Battery .

If the cross was folded down out of the path of the rays, it no longer cast a shadow, and the previously shadowed area would fluoresce stronger than the area around it.

Perpendicular emission

Eugen Goldstein in 1876 found  that cathode rays were always emitted perpendicular to the cathode's surface SONY VAIO VGN-FW290JTB Battery .

If the cathode was a flat plate, the rays were shot out in straight lines perpendicular to the plane of the plate. This was evidence that they were particles, because a luminous object, like a red hot metal plate, emits light in all directions, while a charged particle will be repelled by the cathode in a perpendicular direction  SONY VAIO VGN-FW290JTH Battery .

If the electrode was made in the form of a concave spherical dish, the cathode rays would be focused to a spot in front of the dish. This could be used to heat samples to a high heat.

Deflection by electric fields

Heinrich Hertz built a tube with a second pair of metal plates to either side of the cathode ray beam, a crude CRT SONY VAIO VGN-FW52JB Battery .

If the cathode rays were charged particles, their path should be bent by the electric field created when a voltage was applied to the plates, causing the spot of light where the rays hit to move sideways. He didn't find any bending, but it was later determined that his tube was insufficiently evacuated, causing accumulations of surface charge which masked the electric field SONY VAIO VGN-FW51B/W Battery .

Later Artur Shuster repeated the experiment with a higher vacuum. He found that the rays were attracted toward a positively charged plate and repelled by a negative one, bending the beam. This was evidence they were negatively charged, and therefore not electromagnetic waves  SONY VAIO VGN-FW50B Battery .

Deflection by magnetic fields

Crookes put a magnet across the neck of the tube, so that the North pole was on one side of the beam and the South pole was on the other, and the beam travelled through the magnetic field between them SONY VAIO VGN-FW47GYH Battery .

The beam was bent down, perpendicular to the magnetic field. This was similar to the behavior of electric currents in an electric generator and showed that the cathode rays obeyed Faraday's law like currents in wires.

Paddlewheel

Crookes put a tiny vaned turbine or paddlewheel in the path of the cathode rays, and found that it rotated when the rays hit it SONY VAIO VGN-FW465GJB Battery .

The paddlewheel turned in a direction away from the cathode side of the tube, suggesting that the rays were coming from the cathode. Crookes concluded at the time that this showed that cathode rays had momentum, so the rays were likely matter particlesSONY VAIO VGN-FW465GJ/BE1 Battery .

However later it was proposed that the paddle wheel turned not due to the momentum of the particles (or electrons) hitting the paddle wheel but due to the radiometric effect. When the rays hit the paddle surface they heated it, and the heat caused the gas next to it to expand, pushing the paddle  SONY VAIO VGN-FW465J/B Battery .

This was proven in 1903 by J. J. Thompson who calculated that the momentum of the electrons hitting the paddle wheel would only be sufficient to turn the wheel one revolution per minute. So all this experiment really demonstrated was the ability of cathode rays to heat objects  SONY VAIO VGN-FW45TJB Battery .

Charge

Jean-Baptiste Perrin wanted to determine whether the cathode rays actually carried negative charge, or whether they just accompanied the charge carriers, as the Germans thought. In 1895 he constructed a tube with a 'catcher', a closed aluminum cylinder with a small hole in the end facing the cathode, to collect the cathode rays  SONY VAIO VGN-FW45GJB Battery .

The catcher was attached to an electroscope to measure its charge. The electroscope showed a negative charge, proving that cathode rays really carry negative electricity.

Canal rays

Goldstein found in 1886 that if the cathode is made with small holes in it, streams of a faint luminous glow will be seen issuing from the holes on the back side of the cathode, facing away from the anode   SONY VAIO VGN-FWZJ/H Battery .

It was found that anode rays bend in the opposite direction from cathode rays, toward a negatively charged plate. These were the positive ions which were attracted to the cathode, and created the cathode rays. They were named canal rays (Kanalstrahlen) by Goldstein   SONY VAIO VGN-FW41M/H Battery .

Doppler shift

Eugen Goldstein thought he had figured out a method of measuring the speed of cathode rays. If the glow discharge seen in the gas of Crookes tubes was produced by the moving cathode rays, the light radiated from them in the direction they were moving, down the tube, would be shifted in frequency due to the Doppler effect  SONY VAIO VGN-FW41J/H Battery .

This could be detected with a spectroscope because the emission linespectrum would be shifted. He built a tube shaped like an 'L', with a spectroscope pointed through the glass of the elbow down one of the arms. He measured the spectrum of the glow when the spectroscope was pointed toward the cathode end  SONY VAIO VGN-FW41E/H Battery ,

then switched the power supply connections so the cathode became the anode and the electrons were moving in the other direction, and again observed the spectrum looking for a shift. He didn't find one, which he calculated meant that the rays were travelling very slowly  SONY VAIO VGN-FW37J Battery .

It is now recognized that the glow in Crookes tubes is emitted from gas atoms hit by the electrons, not the electrons themselves. Since the atoms are thousands of times more massive than the electrons, they move much slower, accounting for the lack of Doppler shift  SONY VAIO VGN-FW378J/B Battery .

Lenard window

Philipp Lenard wanted to see if cathode rays could pass out of the Crookes tube into the air. He built a tube with a 'window' in the glass envelope made of aluminum foil just thick enough to hold the atmospheric pressure out (later called a Lenard window) facing the cathode so the cathode rays would hit it  SONY VAIO VGN-FW378DH Battery .

He found that something did come through. Holding a fluorescent screen up to the window caused it to fluoresce, even though no light reached it. A photographic plate held up to it would be darkened, even though it wasn't exposed to light. The effect had a very short range of about 2 inches   SONY VAIO VGN-FW378J/B Battery .

He measured the ability of cathode rays to penetrate sheets of material, and found they could penetrate much farther than moving atoms could. Since atoms were the smallest particles known at the time, this was first taken as evidence that cathode rays were waves  SONY VAIO VGN-FW37J Battery .

Later it was realized that electrons were much smaller than atoms, accounting for their greater penetration ability. Lenard received the 1905 Nobel Prize in physics for this work  SONY VAIO VGN-FW37J/B Battery .

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