Lee De Forest with Audion Tube


Thomas Edison placed a metal plate between glittering filaments in 1884 while building on his incandescent light bulb. He noticed that electricity would travel to the scale from the positive side of the filament but not the negative. He could not understand why and considered it a curiosity (now known as the Edison effect). He developed the first diode unwittingly.


Later, a radio transmitter for Guglielmo Marconi was designed by John Ambrose Fleming of England, one of Edison's former assistants. In 1904 Fleming discovered that the diode could convert AC into direct current (DC) and integrated it into its highly effective detector. Fleming named his invention the thermionic valve because heat regulates the electrical flow, much as a valve controls water flow. The device was known in the United States as a vacuum tube.

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Arthur Wehnelt, who also experimented with thermionic emissions in Germany, filed in January 1904 for a patent regarding a tube that transformed AC into DC. However, he failed to specify the application of the apparatus to detect radio waves, and when Fleming filed for his own patent, he was unable to market his invention.


The valve of Fleming was enhanced by adding a third element in 1906, which led Lee de Forest (1873-1961) to develop the triode. This made a radio wave detector much better, but, like Edison, he did not know all of the possibilities of his invention; his gadget, dubbed the auditory system, produced an electric current that could be significantly increased.


Edwin Howard Armstrong understood in 1912 what de Fest had done. He utilized the triode to create a regenerative system that received radio signals and amplified them so that they could be sent to a loudspeaker and heard without using a headphone.


Diodes typically consisted of two concentrated cylinders, one within the other. The cathode emitter and collected electrons. The thermionic valve of Fleming was operated at a temperature of 4,532°F (2,500°C) and produced significant heat. A grid was put between the cathode and the anode. The electrons flowed across the grid of the triode generating a greater flow.


These early vacuum tubes were referred to as soft valves. The vacuum was not the greatest, and some air was in the tube, and its lifetime was shortened. In 1915 Langmuir designed a more effective vacuum pump; the tubes lasted longer and were more stable with a better vacuum. Complex valves were termed the upgraded tubes, and the working temperature fell to 2,000 °C (3,632 °F). In 1922 the temperature was further lowered by introducing additional elements to 1,832°F (1,000°C). The efficiency of the tube increased by indirect heating.


Triodes were restricted to less than one megahertz low frequencies. In 1927, Albert Wallace Hull (1880-1966), an American scientist, developed a tetrode to remove high-frequency oscillations and increase frequency range. A year later, the pentode was created and the most frequently used valve, improving performance at low voltage.


Several vacuum tubes have been used throughout the years. In radio receivers and early digital computers, low-voltage/low-power tubes were employed. Photo tubes were employed in sound equipment, which enabled the film to record and recover sounds. The cathode-ray tube concentrated on an electron beam which led to oscilloscopes, TVs and cameras being invented. In radar, early space communication and microwave ovens, microwave tubes were employed. To develop computers, storage tubes that could store and retrieve data were needed.


Despite its many benefits, the vacuum tube had several inconveniences. It was very delicate, with a relatively long life and much electricity needed to run its heating element. None of these inconveniences replaced the vacuum tube, the transistor, developed by Walter Houser Brattain, John Bardeen, and William Shockley in 1948. After 1960, commercially accessible, tiny, lightweight, low voltage transistors were replaced by vacuum tubing in most applications; however, vacuum tubing was reused in electronic devices by developing microscopic vacuum tubes (microtubes) in the 1990s.