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electron tube, device consisting of a sealed enclosure in which electrons flow between electrodes separated either by a vacuum (in a vacuum tube) or by an ionized gas at low pressure (in a gas tube). The two principal electrodes of an electron tube are the cathode and the anode or plate. The simplest vacuum tube, the diode, has only those two electrodes. When the cathode is heated, it emits a cloud of electrons, which are attracted by the positive electric polarity of the anode and constitute the current through the tube. If the cathode is charged positively with respect to the anode, the electrons are drawn back to the cathode. However, the anode is not capable of emitting electrons, so no current can exist; thus the diode acts as a rectifier, i.e., it allows current to flow in only one direction. In the vacuum triode a third electrode, the grid, usually made of a fine wire mesh or similar material, is placed between the cathode and anode. Small voltage fluctuations, or signals, applied to the grid can result in large fluctuations in the current between the cathode and the anode. Thus the triode can act as a signal amplifier, producing output signals some 20 times greater than input. For even greater amplification, additional grids can be added. Tetrodes, with 2 grids, produce output signals about 600 times greater than input, and pentodes, with 3 grids, 1,500 times. X-ray tubes maintain a high voltage between a cathode and an anode. This enables electrons from the cathode to strike the anode at velocities high enough to produce X rays. A cathode-ray tube can produce electron beams that strike a screen to produce pictures, as in some oscilloscopes and older television displays. Gas tubes behave similarly to vacuum tubes but are designed to handle larger currents or to produce luminous discharges. In some gas tubes the cathode is not designed as an electron emitter; conduction occurs when a voltage sufficient to ionize the gas exists between the anode and the cathode. In these cases the ions and electrons formed from the gas molecules constitute the current. Electron tubes have been replaced by solid-state devices, such as transistors, for most applications. However they are still used in high-power transmitters, specialty audio equipment, and some oscilloscopes. A klystron is a special kind of vacuum tube that is a powerful microwave amplifier; it is used to generate signals for radar and television stations.
CRT(1) (CRunTime) See runtime library.
(2) (Cathode Ray Tube) A vacuum tube used as a display screen in a computer monitor or TV. The viewing end of the tube is coated with phosphors, which emit light when struck by electrons.
In the past, CRT was a popular term for a computer display terminal. Today, "monitor" is the correct term as computer displays have long since migrated from CRTs to LCD panels (see flat panel display). Likewise, TV sets no longer use CRTs (see flat panel TV).
Electrons and Phosphors
The CRT works by heating a cathode which causes electrons to flow. Accelerating and focusing anodes turn the electrons into a fine beam that is directed to the phosphors by magnetic fields that are generated by steering coils. The viewing end of a color CRT tube is coated with red, green and blue phosphor dots, and separate "electron guns" bombard their respective colors a line at a time in a prescribed sequence (see raster scan).
The resulting color displayed on screen is derived by the intensity of the electron beams as they strike the red, green and blue phosphors and cause them to glow at each pixel location. See cathode and vacuum tube.
Back to the 1800s
The first oscilloscope tube was developed in 1897 by German scientist Ferdinand Braun. Using a fluorescent screen and still known as a "Braun tube" in Germany, his "cathode-ray oscilloscope" was used to display the patterns of electronic signals. Although better known for inventing the CRT, Braun shared the Nobel Prize in 1909 with Guglielmo Marconi for wireless telegraphy.
|The Braun Tube - 1897|
|Using a bellows, it took a strong man to evacuate the air from this tube. The successor to Sir William Crookes' vacuum tubes some 20 years earlier, these tubes used unheated "cold" cathodes that required a huge voltage. (Image courtesy of O'Neill's Electronic Museum)|
|Bulky But Magic in the 1950s|
|Although clunky by comparison to today's color screens, millions of people were thrilled when they first watched CRT-based monochrome TV. (Image courtesy of Vintage Vibe, www.thevintagevibestore.com)|
|CRT vs. Flat Panel|
|The CRT gave way to LCD panels in the late 1990s, taking less space, less power and emitting less radiation. This high-quality EIZO LCD monitor was state-of-the-art in 1999. (Image courtesy of EIZO Nanao Technologies Inc.)|
|CRT Front Projection|
|The first data and TV projectors used CRTs, and although mostly abandoned, they continue to provide the highest quality. In 2006, this home theater was built by a serious video enthusiast. See front-projection TV. (Images courtesy of Kal of CurtPalme.com)|
|CRT Rear Projection|
|Although big and bulky, the Pioneer Elite Pro-107 was perhaps the best CRT-based rear-projection TV ever made. Still working fine after 17 years, this unit was sold for a pittance in 2010. See rear-projection TV.|
vacuum tubeAn electronic device that controls the flow of electrons in a vacuum. It is used as a switch, amplifier or display screen (CRT). Used as on/off switches, vacuum tubes allowed the first computers to perform digital computations. Although tubes made a comeback in high-end stereo components, they have long since been abandoned for TVs and computer monitors. See vacuum tube types, audiophile, tube amplifier and Vintage Radio Museum.
|Early Vacuum Tube|
|Early vacuum tubes were used to amplify signals for radio and other audio devices. This one was made in 1915. Tubes were not used as switches in calculating machines until 1939. (Image courtesy of AT&T.)|
|Tubes in the 21st Century|
|Many audiophiles claim vacuum tubes amplify music better than transistors. These high-end Model One amplifiers (collectively weighing 212 pounds) were designed by legendary audio engineer Mark Levinson.|
|Vacuum tubes have come in myriad shapes and sizes over the years, and the Vintage Radio and Communications Museum has one of the finest collections. (Images courtesy of Vintage Radio and Communications Museum of Connecticut, www.vrcmct.org)|
|Amazing - A Tube Could Be This Small|
|This image is from an article in the October 1947 issue of Mechanix Illustrated that highlighted the huge reduction in vacuum tube size. The article's author could not have imagined that in the future several trillion transistors could fit inside the small vacuum tube. (Image courtesy of Mechanix Illustrated.)|