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Vacuum Tubes

Vacuum Tubes

A vacuum tube is an electronic device used for the processing of electrical signals. It consists of two or more electrodes inside a metal or glass tube which has been evacuated, hence the name.

In the mid-1800s Sir William Crookes (18321919) performed early experiments with passing electric current through an evacuated glass tube. In 1883 Thomas Edison (18471931) noticed that current would flow between two electrodes inside a light bulb if the negative electrode was heated. John Ambrose Fleming (18491945) constructed the first practical diode tube , containing two electrodes. When the heated electrode, called the cathode, was at a negative voltage compared to the other electrode, called the anode or plate, electrons flowed from the cathode to the anode. When the voltages were reversed, electron flow was prevented.

This type of action is called rectification ; it is used to change alternating current into direct current. This is a basic operation needed in radio receivers to demodulate a radio frequency signal into audio.

Later, Lee De Forest (18731961) developed a tube with three electrodes, called a triode. The third electrode was called a grid. It was a fine mesh placed between the cathode and the anode. De Forest discovered that a small change in voltage on the grid produced a large change in current flow between the cathode and anode. A positive voltage attracts the electrons from the cathode toward the anode and produces a larger current. A negative voltage repels the electrons and produces less current. Thus, the current flow is proportional to the voltage of the grid. Called amplification, this discovery was central to the growth of the electronics industry.

Further developments in vacuum tube technology led to the development of the tetrode, which contained four electrodes; the pentode with five electrodes; and others. The additional electrodes are used to enhance the amplification action of the basic triode: extending power, availability, frequency, efficiency, or fidelity.

The cathode of all vacuum tubes must be heated. The heat is supplied by passing a high direct current through the cathode or, more commonly, by providing another element, called a filament, near the cathode and passing high current through it. The filament is not considered an electrode since it is electrically isolated from the other elements and its sole purpose is to heat the cathode. It is the filament that produces the characteristic glow of the vacuum tube.

The filament represents many of the disadvantages of the vacuum tube. It requires a lot of power, which is essentially wasted energy since it does not add to the output power of the device. In a large device, a cooling system is needed to remove the heat generated by this process. Also, filament burn-out is the most common failure mechanism of most vacuum tubes.

Vacuum Tubes in Computers

The first practical electronic digital computer was the Electronic Numerical Integrator and Computer (ENIAC), built in 1946 at the University of Pennsylvania's Moore School of Electrical Engineering. It weighed 33 metric tons (60,000 pounds), contained 18,000 vacuum tubes, and consumed 150,000 watts of electricity. It was originally designed to be able to compute artillery ballistics tables for the U.S. Ballistics Research Lab. However, it was a general purpose computer that could be programmed by connecting the machine's modules with cables.

Prior to ENIAC, mathematicians computed ballistics tables with mechanical adding machines, a process that took up to twenty hours for each one. Because ENIAC was electronicrather than mechanicaland programmable, it was able to perform about 5,000 integer additions per second, reducing the time required to generate a ballistics table to about thirty seconds. (By comparison, a modern supercomputer can perform more than one billion floating point operations per second.)

Of course, the reliability of ENIAC was a real concern. As noted earlier, a vacuum tube filament is susceptible to burn out. Some felt that a device with so many tubes would never work long enough to produce useful results. By de-rating the tubes (running them with less than full rated voltage and current), the ENIAC team managed to keep the system running for several days without failure. This was a significant accomplishment.

Still, reliability was a continual problem for ENIAC and other vacuum tube-based computers. Another significant cause of failure with these early computers was large insects that crawled between vacuum tube electrodes and caused short circuits. The process of finding and fixing these short circuits was called debugging. The term has endured; it is used today to refer to the process of finding and fixing errors in the computer's program, or software.

Vacuum Tubes Today

Although they have been replaced with transistors and other integrated circuits in many low-power applications, vacuum tubes are still used in many high-power applications, including specialized sensors and television and computer display devices.

The only type of vacuum tube used in modern computer systems is the cathode ray tube (CRT) , which is the main component in a computer display monitor. The cathode assembly in a cathode ray tube is called an electron gun. Located in the narrow tube neck, it generates a very narrow beam of electrons that are accelerated at high speed toward the anode. The anode is a large rectangular screen coated with phosphors that glow when struck by the beam.

The electron beam is guided by a strong magnetic field induced by deflection coils around the tube neck. The electron beam traces a raster scan pattern that covers the entire surface of the screen at a speed higher than the human eye can detect. The intensity of the beam determines the brightness of the spot.

Colors seen on the computer screen are produced by different phosphors that glow red, green, or blue. A color tube has three electron guns, one for each color. Each gun can only "see" spots on the tube corresponding to its color. Other colors are made by combinations of varying intensities of the three primary colors. The cathode ray tube has become so common in computer display devices that the acronym CRT has come to be synonymous with the entire display unit.

see also Early Computers; Generations, Computers; Integrated Circuits; Transistors.

Donald M. McIver

Bibliography

Bell, David. Electronic Devices and Circuits. Englewood Cliffs, NJ: Prentice Hall, 1986.

Benedict, R. Ralph. Electronics for Scientists and Engineers, 2nd ed. Englewood Cliffs, NJ: Prentice Hall, 1976.

Internet Resources

"ENIAC-on-a-Chip." Moore School of Electrical Engineering, University of Pennsylvania. <http://www.ee.upenn.edu/~jan/eniacproj.html>

"John W. Mauchly and the Development of the ENIAC Computer." Van Pelt Library, University of Pennsylvania. <http://www.library.upenn.edu/special/gallery/mauchly/jwmintro.html>

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Vacuum Tube

Vacuum tube

A vacuum tube is a hollow glass cylinder from which as much air as possible has been removed. The cylinder also contains two metal electrodes: the cathode, or negative electrode, and the anode, or positive electrode. Current flows within a vacuum tube from the cathode, which has an excess of electrons, to the anode, which has a deficiency of electrons.

Vacuum tubes were a subject of great interest among both scientists and inventors at the end of the nineteenth century. Among scientists, vacuum tubes were used to study the basic nature of matter. Among inventors, vacuum tubes were used as a means of controlling the flow of electric current within an electrical system.

One of the first practical vacuum tubes was invented by English electrical engineer John Ambrose Fleming (18491945). Fleming's device permitted the flow of electric current in one direction (from cathode to anode) but not in the other (from anode to cathode). It was, therefore, one of the first devices that could be used to control the direction of flow of electric current. Because it consisted of two parts, Fleming's invention is called a diode. Fleming himself referred to the device as a thermionic valve because, like a water valve, it controlled the flow of electricity.

In 1906, American inventor Lee de Forest (18731961) discovered a way to improve the efficiency with which vacuum tubes operate. He installed a third element in the diode: a metal screen between the anode and cathode. This modification of the diode was given the name triode because it consists of three parts rather than two.

Applications

For more than half a century, the vacuum tube had an enormous number of applications in research and communications. They were usedin radio receivers as well as in early digital computers. Incorporated intophoto tubes, they were used in sound equipment, making it possible torecord and retrieve audio from motion picture film. In the form of cathode-ray tubes, they were used to focus an electron beam, leading to theinvention of oscilloscopes (which measure changes in voltage over time),televisions, and cameras. As microwave tubes, they were used in radar, early space communication, and microwave ovens. When modified asstorage tubes, they could be used to store and retrieve data and, thus, wereessential in the advancement of computers.

Despite their many advantages, vacuum tubes had many drawbacks.They are extremely fragile, have a limited life, are fairly large, and require a lot of power to operate. The successor to the vacuum tube, thetransistor, invented by Walter Brattain, John Bardeen, and William Shockley in 1948, overcame these drawbacks. After 1960, small, lightweight, low-voltage transistors became commercially available and replaced vacuum tubes in most applications. With the creation of microscopic vacuumtubes (microtubes) in the 1990s, however, vacuum tubes are again beingused in electronic devices.

Words to Know

Anode: Also known as target electrode; the positively charged electrode in an X-ray tube.

Cathode: The negatively charged electrode in an X-ray tube.

Cathode-ray tube (CRT): A form of vacuum tube in which a beam of electrons is projected onto a screen covered with a fluorescent material in order to produce a visible picture.

Electrode: A material that will conduct an electrical current, usually a metal, used to carry electrons into or out of an electrochemical cell.

Transistor: A device capable of amplifying and switching electrical signals.

[See also Cathode-ray tube; Superconductor; Transistor ]

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vacuum tube

vac·u·um tube • n. an electron tube containing a near-vacuum that allows the free passage of electric current.

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vacuum tube

vacuum tube: see electron tube.

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