The Fleming valve was the first practical application of
thermionic emission, discovered in 1873 by
Frederick Guthrie. While improving his
incandescent lamp in 1880,
Thomas Edison discovered that charged particles from a heated negative electrode moved through the vacuum and collected on a positive electrode, producing current. Later scientists called this phenomenon the
Edison effect and determined it was due to thermally-emitted electrons. Edison was granted a patent for this device as part of an electrical indicator in 1884, but did not find a practical use for it. Professor Fleming of
University College London consulted for the
Edison Electric Light Company from 1881 to 1891, and subsequently for the
Marconi Wireless Telegraph Company. In 1901 Fleming designed the transmitter used by
Guglielmo Marconi in the first transmission of radio waves across the Atlantic from
Poldhu,
England, to
Signal Hill, St. John's,
Newfoundland,
Canada. The distance between the two points was about . Although the contact, reported December 12, 1901, was widely heralded as a great scientific advance at the time, there is also some skepticism about the claim, because the received signal, the three dots of the
Morse code letter "S", was so weak the primitive receiver had difficulty distinguishing it from
atmospheric radio noise caused by static discharges, leading later critics to suggest it may have been random noise. Regardless, it was clear to Fleming that reliable transatlantic communication with the existing transmitter required a more sensitive receiving apparatus. valves derived from the Fleming valve, from the 1930s
(left) to the 1970s
(right) The receiver for the transatlantic demonstration employed a
coherer, which had poor sensitivity and degraded the tuning of the receiver. This led Fleming to look for a detector that was more sensitive and reliable while at the same time being better suited for use with tuned circuits. In 1904 Fleming tried an Edison effect bulb for this purpose and found that it worked well to rectify high-frequency oscillations and thus allow detection of the rectified signals by a
galvanometer. On November 16, 1904, he applied for a US patent for what he termed an oscillation valve. This patent was subsequently issued as number 803,684 and found immediate utility in detecting messages sent by Morse code. The Marconi company used the Fleming valve in its shipboard receivers until around 1916 when it was replaced by the
triode.
Oscillation valves The Fleming valve proved to be the start of a technological revolution. After reading Fleming's 1905 paper on his oscillation valve, American engineer
Lee de Forest in 1906 created a three-element vacuum tube, the
Audion, by adding a wire
grid between cathode and anode. It was the first electronic
amplifying device, allowing the creation of
amplifiers and continuous wave
oscillators. De Forest quickly refined his device into the
triode, which became the basis of long-distance
telephone and radio communications,
radars, and early digital computers for 50 years, until the advent of the
transistor in the 1950s. Fleming sued De Forest for infringing his valve patents, resulting in decades of expensive and disruptive litigation, which were not settled until 1943 when the
United States Supreme Court ruled Fleming's patent invalid.
Power applications Later, when
vacuum tube equipment began to be powered from
AC electrical outlets instead of DC batteries, the Fleming valve was developed into a
rectifier to produce the DC plate (anode) voltage required by other vacuum tubes. Around 1914
Irving Langmuir at
General Electric developed a high voltage version called the
Kenotron, which was used to power
x-ray tubes. As a rectifier, the tube was used for high voltage applications but its high internal
resistance made it inefficient in low voltage, high current applications. Until vacuum tube equipment was replaced by transistors in the 1970s, radios, and televisions usually had one or more diode tubes. == See also ==