A vacuum tube is a hollow glass cylinder containing a positive electrode and a negative electrode between which is conducted in a full or partial vacuum. A grid between these electrodes controls the flow of electricity.
The hollow cylinder of a vacuum tube contains a filament, typically tungsten coated with another metal. When the filament is sufficiently heated by an electric current, it emits electrons. This filament, or electrode, which emits electrons is known as a cathode and has a negative charge. Because it has a negative charge, it attracts electrons, thus nullifying the process. Therefore, free electrons must be supplied to the cathode. This is usually done by connecting the cathode to the negative terminal of a generator or battery. The other electrode, known as an anode, has a positive charge. The electrons move from the cathode to the anode, resulting in a oneway current within the tube.
In 1884, Thomas Edison, while working on his incandescent light bulb, inserted a metal plate between glowing filaments. He observed that electricity would flow from the positive side of the filament to the plate, but not from the negative. He did not understand why this was so and treated this effect (now known as the Edison effect) as a curiosity. Unwittingly, he had created the first diode.
Later, John Ambrose Fleming of England, one of Edison's former assistants, became involved in designing a radio transmitter for Guglielmo Marconi. In 1904 Fleming realized that the diode had the ability to convert alternating current (AC) into direct current (DC), and incorporated it into his very efficient radio wave detector. Fleming called his device the thermionic valve because it used heat to control the flow of electricity just as a valve controls the flow of water. In the United States the invention became known as a vacuum tube.
In Germany, Arthur Wehnelt, who also worked with thermionic emission, had applied for a patent in January 1904 for a tube that converted AC into DC. However, he neglected to mention the use of the device in radio wave detection and was unable to sell his invention for that purpose after Fleming applied for his own patent.
Lee de Forest (1873-1961) improved on Fleming's valve by adding a third element in 1906, thus inventing the triode. This made an even better radio wave detector but, like Edison, he did not realize the full potential of his invention; his device, called an audion, created an electrical current that could be amplified considerably.
In 1912 Edwin Howard Armstrong realized what de Fest had wrought. He used the triode to invent a regenerative circuit that not only received radio signals, it amplified them to such a degree they could be sent to a loudspeaker and heard without the use of headphones.
Diodes were usually made of two concentric cylinders, one inside the other. The cathode emitted electrons and the anode collected them. Fleming's thermionic valve operated at a temperature of 4,532°F (2,500°C), generating a considerable amount of heat. Deforest placed a grid between the cathode and anode. The electrons passed through the triode's grid, inducing a larger current to flow.
These early vacuum tubes were called soft valves. The vacuum was not the best and some air remained within the tube, shortening its lifespan. Langmuir devised a more efficient vacuum pump in 1915; with a better vacuum, the tubes lasted longer and were more stable. The improved tubes were called hard valves and their operating temperature dropped to 3,632°F (2,000°C). In 1922 the temperature was reduced yet again, to 1,832°F (1,000°C), with the introduction of new elements. Indirect heating improved tube efficiency.
Triodes were limited to low frequencies of less than one megahertz. In 1927 American physicist Albert Wallace Hull (1880-1966) invented the tetrode to eliminate high-frequency oscillations and improve the frequency range. A year later the pentode, which improved performance at low voltage, was developed and became the most commonly used valve.
Over the course of years, a variety of vacuum tubes came into use. Low-voltage/low-power tubes were used in radio receivers as well as early digital computers. Photo tubes were used in sound equipment, making it possible to record and retrieve audio from motion picture film. The cathode-ray tube focused an electron beam, leading to the invention of oscilloscopes, televisions, and cameras. Microwave tubes were used in radar, early space communication, and microwave ovens. Storage tubes, which could store and retrieve data, were essential in the advancement of computers.
Despite its numerous advantages, the vacuum tube had many drawbacks. It was extremely fragile, had a limited life, was fairly large, and required a lot of power to operate its heating element. The successor to the vacuum tube, the transistor, invented by Walter Houser Brattain, John Bardeen, and William Shockley in 1948, had none of these drawbacks. After 1960 the small, lightweight, low-voltage transistors became commercially available and replaced vacuum tubes in most applications, but with the creation of microscopic vacuum tubes (microtubes) in the 1990s, vacuum tubes are again being used in electronic devices.
See also Cathode ray tube.
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Goodman, Billy. "Return of the Vacuum Tube." Discover 11 (March 1990): 55-57.