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History of Automation: The Triode Vacuum Tube the Evolution into Automation, Industrial, and Machine Control

In 1904, Sir John Ambrose Fleming invented the two-electrode vacuum-tube rectifier, which he called an “oscillation valve”. He received a U.S. patent on November 16, 1904. The device also had other names: Thermionic Valve, based on the principle of the thermionic electron emission from the hot cathode; the Vacuum Diode, based on the fact that the electrodes were suspended and operated in a partial vacuum, and the operation of the device only allowed current to flow in one direction (diode operation); the kenotron, from “kenosis” (emptying) & “electron”; a Thermionic Tube (American word for British “valve”); and the Fleming Valve or Fleming Oscillation Valve. At the time of discovery, it was not understood that the valve could be useful for rectification, and Fleming’s intent was simply to create an oscillator. Early radio reception was accomplished by the use of “crystal sets” or “cat whisker radios” and this valve pioneered the movement into more sensitive and effective instruments.

The Supreme Court of the United States later invalidated Fleming’s patent because of an improper disclaimer and, additionally, maintained that the technology in the patent was known art when filed. There were many disputes regarding the patent and subsequent inventions based on the principles of this device. This invention (the diode) is often considered to have been the beginning of electronics, for this was the first vacuum tube ever developed. Fleming’s diode was used in every kind of electronic device for many decades afterward, until it was superseded by solid state technology more than 50 years later.


In 1906, U.S. researcher and engineer Lee De Forest added a control “grid” to Fleming’s valve to allow creation of a vacuum tube RF detector. He called the tube the Audion, and his work led Fleming to accuse him of copying his ideas. De Forest’s device however was shortly refined by him…


…and Edwin H. Armstrong into the first electronic amplifier, renaming the new tube into the triode. The triode was vital in the creation of long-distance telephone and radio communications, television, RADAR, and early electronic analog and digital computers.


The Audion became the design of the triode tube, in which the grid element was located between the filament or cathode and plate, and was patented January 29, 1907. The Audion however was not completely evacuated and contained some gas at low pressure, thought to be necessary by De Forest, and caused erratic operation and shortened the life of the filament.  Invented primarily as a radio receiver detector, the Audion did not see much use until its ability to amplify was recognized around 1912 by several researchers, who used it to build the first successful amplifying radio receivers and electronic oscillators. The many uses for amplification and oscillation motivated its rapid development. By 1913 improved versions with higher vacuum were developed by Harold Arnold at American Telephone and Telegraph Company which had purchased the rights to the Audion from De Forest, and Irving Langmuir at General Electric, who named his tube the “Pliotron“.



These were the first vacuum tube triodes. The name triode appeared later, when it became necessary to distinguish it from other kinds of vacuum tubes with more or fewer elements (e.g. diodes, tetrodes, pentodes, etc.). There were lengthy lawsuits between De Forest and von Lieben, and De Forest and the Marconi Company, representing John Ambrose Fleming who invented the diode.

The discovery of the triode’s amplifying ability in 1912 revolutionized electrical technology, creating the new field of electronics, the technology of active (amplifying) electrical devices. The triode was immediately applied to many areas of communication. Triode based radio transmitters replaced the cumbersome inefficient spark gap transmitters, allowing the transmission of sound by amplitude modulation (AM). Amplifying triode radio receivers, which now had the power to drive loudspeakers, replaced weak crystal “cat whisker” radios, which had to be listened to with very sensitive headphones. This resulted in the evolution of radio to be the first instantaneous mass communication medium, with the beginning of radio broadcasting occurring around 1920. Triodes made transcontinental telephone service possible. Vacuum tube triode repeaters, invented at Bell Telephone after its purchase of the Audion rights, allowed telephone calls to travel beyond the unamplified limit of about 800 miles. The opening by Bell of the first transcontinental telephone line was celebrated 3 years later, on January 25, 1915. Other inventions made possible by the triode were television, public address systems, electric phonographs, and talking motion pictures, and even were used in analog and digital computers long before solid-state semiconductors became widely known.

The triode also evolved into the thyratron which is basically a very large triode tube that is filled with mercury vapor or another conducting ionizing gas that acts like a switch when a small current is applied to the grid. The thyratron made possible many industrial and electrical inventions requiring control of massive currents in welding machines or electrical motor control such as in locomotives, that would otherwise require massive electromechanical relays or could not otherwise exist. Thyratrons and various offshoots of these tubes (valves) were used for many years, even into 2004 when a 12-pulse 3 phase semiconductor thyrister array replaced the older massive mercury-vapor based thyratron 6-pulse static valve bridge in the Sylmar Converter Station that is currently used to invert 1000 kV 3100 Amp DC line voltage supplied in Washington State to 3-phase AC grid power for Los Angeles, California.

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