A vacuum tube, also called a ‘valve’ in British English, is a device that controls electric current flow in a high vacuum between electrodes (conductors that emit or receive electrons). Tubes were used in many radios, television sets, and amplifiers until they were supplanted by lower cost transistors in the 1960s that performed the same function but used less electricity and were more durable.
In a vacuum tube, a cathode (an electrode that emits electrons) is heated, as in a light bulb, so it will emit electrons. This is called ‘thermionic emission.’ The electrons are accelerated from the cathode to the anode (an electrode that receives electrons) by the electric field in the tube. Vacuum tubes must be hot to work. Most are made of glass, thus are fragile and can break. Vacuum tubes were used in the first computers like the ENIAC, which were large and need much work to continue operating.
Although the vacuum tube was invented by John Ambrose Fleming in 1904, it was Thomas Edison who later discovered the ‘Edison effect.’ The Edison effect states that electricity does not necessarily need a solid material to move through; it can move through gas or vacuums as well. Without this realization, vacuum tubes would never have been invented. After Ambrose Fleming’s initial invention, Lee De Forest invented the ‘audion’ in 1906, a device that amplified vacuum tubes (which was improved by others as the ‘triode’ in 1908) and used in the first telephone amplifiers. Many other kinds were invented for various purposes.
In the 1940s, the invention of semiconductor devices made it possible to produce solid-state devices, which are smaller, more efficient, reliable, durable, safer, and more economical than thermionic tubes. Beginning in the mid-1960s, thermionic tubes were being replaced by the transistor, a semiconductor device used to amplify or switch electronic signals and electrical power. However, the cathode-ray tube (CRT) remained the basis for television monitors and oscilloscopes until the early 21st century.
By the late 1960s, most radios, television sets, and amplifiers began using transistors instead of tubes because they were much smaller, worked on lower voltages, and used less power. In addition, unlike vacuum tubes, they were much less likely to be damaged by being dropped and had extremely long life. Eventually, they were also much cheaper than glass vacuum tubes. High powered electronics such as broadcasting transmitters were transistorized more slowly. Television receivers continued using the cathode ray tube until the mid-2000s.
In the 21st century, vacuum tubes are rarely used in common electronic equipment. Many devices today rely on the transistor over the vacuum tube. Some devices that still use the vacuum tube, however, include systems which need high frequency operation, high-power output or very high amplification, such as television transmission, X-ray machines, radar, and microwave ovens. Additionally, audiophiles who enjoy listening to music on high-quality home stereo systems sometimes buy amplifiers which use vacuum tubes. All amplifiers distort to some degree; some audiophiles prefer the sound produced by tube amplifiers. The reasons for the difference in sound between tube and solid state electronic devices is a continuing debate among audiophiles.
The Daily Omnivore 
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