Audion was an electronic detecting or amplifying vacuum tube
invented by American electrical engineer
Lee de Forest
Lee de Forest in
1906. It was the first triode, consisting of
an evacuated glass tube containing three electrodes: a heated
filament, a grid, and a plate. It is important in the history of
technology because it was the first widely used electronic device
which could amplify; a small electrical signal applied to the grid
could control a larger current flowing from the filament to
The original triode
Audion had more residual gas in the tube than
later versions and vacuum tubes; the extra residual gas limited the
dynamic range and gave the
Audion nonlinear characteristics and
erratic performance. Originally developed as a radio receiver
detector by adding a grid electrode to the Fleming valve, it found
little use until its amplifying ability was recognized around 1912 by
several researchers, who used it to build the first amplifying
radio receivers and electronic oscillators. The many practical
applications for amplification motivated its rapid development, and
Audion was superseded within a few years by improved
versions with higher vacuum.
1.1 Patents and disputes
1.2 Kenotron and Pliotron
2 Applications and use
4 Further reading
5 External links
One of the earliest
Audion radio receivers, constructed by De Forest
Audion tubes were mounted upside down, with the delicate
filament hanging down, to prevent it from sagging and touching the
grid. This was a detector (rectifier) and two stage audio amplifier
unit; the radio signal came from a separate "tuner" unit.
It had been known since the middle of the 19th century that gas flames
were electrically conductive, and early wireless experimenters had
noticed that this conductivity was affected by the presence of radio
waves. De Forest found that gas in a partial vacuum heated by a
conventional lamp filament behaved much the same way, and that if a
wire were wrapped around the glass housing, the device could serve as
a detector of radio signals. In his original design, a small metal
plate was sealed into the lamp housing, and this was connected to the
positive terminal of a 22 volt battery via a pair of headphones, the
negative terminal being connected to one side of the lamp filament.
When wireless signals were applied to the wire wrapped around the
outside of the glass, they caused disturbances in the current which
produced sounds in the headphones.
This was a significant development as existing commercial wireless
systems were heavily protected by patents; a new type of detector
would allow De Forest to market his own system. He eventually
discovered that connecting the antenna circuit to a third electrode
placed directly in the current path greatly improved the sensitivity;
in his earliest versions, this was simply a piece of wire bent into
the shape of a grid-iron (hence "grid").
Audion provided power gain; with other detectors, all of the power
to operate the headphones had to come from the antenna circuit itself.
Consequently, weak transmitters could be heard at greater distances.
Patents and disputes
De Forest and everybody else at the time greatly underestimated the
potential of his original device, imagining it to be limited to mostly
military applications. It is significant that he apparently never saw
its potential as a telephone repeater amplifier, even though crude
electromechanical note magnifiers had been the bane of the telephone
industry for at least two decades. (Ironically, in the years of patent
disputes leading up to World War I, it was only this "loophole" that
allowed vacuum triodes to be manufactured at all, since none of De
Forest's patents specifically mentioned this application).
(left) The first prototype
Audion with the grid (zigzag wires) between
the filament and plate. (right) Later design of an audion tube.
The grid and plate are in two parts on either side of the central
filament. In both these tubes the filament is burned out.
De Forest was granted a patent for his early two-electrode version of
Audion on November 13, 1906 (U.S.
Patent 841,386), but the
"triode" (three electrode) version was patented in 1908 (U.S. Patent
879,532). De Forest continued to claim that he developed the Audion
independently from John Ambrose Fleming's earlier research on the
thermionic valve (for which Fleming received Great Britain patent
24850 and the American
Fleming valve patent U.S.
Patent 803,684), and
De Forest became embroiled in many radio-related patent disputes. De
Forest was famous for saying that he "didn't know why it worked, it
just did".
He always referred to the vacuum triodes developed by other
researchers as "Oscillaudions", although there is no evidence that he
had any significant input to their development. It is true that after
the invention of the true vacuum triode in 1913 (see below), De Forest
continued to manufacture various types of radio transmitting and
receiving apparatus, (examples of which are illustrated on this page).
However, although he routinely described these devices as using
"Audions", they actually used high-vacuum triodes, using circuitry
very similar to that developed by other experimenters.
Columbia University student
Edwin Howard Armstrong
Edwin Howard Armstrong worked
with professor John Harold Morecroft to document the electrical
principles of the Audion. Armstrong published his explanation of the
Audion in Electrical World in December 1914, complete with circuit
diagrams and oscilloscope graphs. In March and April 1915,
Armstrong spoke to the
Institute of Radio Engineers in New York and
Boston, respectively, presenting his paper "Some Recent Developments
Audion Receiver", which was published in September. A
combination of the two papers was reprinted in other journals such as
the Annals of the New York Academy of Sciences. When Armstrong and
De Forest later faced each other in a dispute over the regeneration
patent, Armstrong was able to demonstrate conclusively that De Forest
still had no idea how it worked.
The problem was that (possibly to distance his invention from the
Fleming valve) De Forest's original patents specified that
low-pressure gas inside the
Audion was essential to its operation
Audion being a contraction of "Audio-Ion"), and in fact early Audions
had severe reliability problems due to this gas being adsorbed by the
metal electrodes. The Audions sometimes worked extremely well; at
other times they would barely work at all.
As well as De Forest himself, numerous researchers had tried to find
ways to improve the reliability of the device by stabilizing the
partial vacuum. Much of the research that led to the development of
true vacuum tubes was carried out by
Irving Langmuir in the General
Electric (GE) research laboratories.
Kenotron and Pliotron
Audions and early triodes developed from them, 1918.
Bottom row (D): De Forest Audions
Third row (C): Pliotrons, developed at
General Electric by Langmuir
Second row (B): triodes developed at
Western Electric which bought the
rights from De Forest in 1913. These were used in telephone repeaters
which made possible the first transcontinental telephone line in 1915.
Top row (A): French triodes. The French government gained the right to
manufacture Audions in 1912 when De Forest failed to renew his French
patents for lack of $125.
General Electric Company Pliotron
Langmuir had long suspected that certain assumed limitations on the
performance of various low-pressure and vacuum electrical devices,
might not be fundamental physical limitations at all, but simply due
to contamination and impurities in the manufacturing process.
His first success was in demonstrating that, contrary to what Edison
and others had long asserted, incandescent lamps could function more
efficiently and with longer life if the glass envelope was filled with
low-pressure inert gas rather than a complete vacuum. However, this
only worked if the gas used was meticulously 'scrubbed" of all traces
of oxygen and water vapor. He then applied the same approach to
producing a rectifier for the newly developed "Coolidge" X-ray tubes.
Again contrary to what had been widely believed to be possible, by
virtue of meticulous cleanliness and attention to detail, he was able
to produce versions of the Fleming
Diode that could rectify hundreds
of thousands of volts. His rectifiers were called "Kenotrons" from the
Greek keno (empty, contains nothing, as in a vacuum) and tron (device,
He then turned his attention to the
Audion tube, again suspecting that
its notoriously unpredictable behaviour might be tamed with more care
in the manufacturing process.
However he took a somewhat unorthodox approach. Instead of trying to
stabilize the partial vacuum, he wondered if it was possible to make
Audion function with the total vacuum of a Kenotron, since that
was somewhat easier to stabilize.
He soon realized that his "vacuum"
Audion had markedly different
characteristics from the De Forest version, and was really a quite
different device, capable of linear amplification and at much higher
frequencies. To distinguish his device from the
Audion he named it the
"Pliotron", from the Greek plio (more or extra, in this sense meaning
gain, more signal coming out than went in).
Essentially, he referred to all his vacuum tube designs as Kenotrons,
the Pliotron basically being a specialized type of Kenotron. However,
because Pliotron and Kenotron were registered trademarks, technical
writers tended to use the more generic term "vacuum tube". By the
mid-1920s, the term "Kenotron" had come to exclusively refer to vacuum
tube rectifiers, while the term "Pliotron" had fallen into disuse.
Ironically, in popular usage, the sound-alike brands "Radiotron" and
"Ken-Rad" outlasted the original names.
Applications and use
Audion AM radio transmitter, built by Lee De Forest and
announced April, 1914
Some of the earliest
Audion AM radio transmitters, built by De Forest
around 1916. The invention of the
Audion oscillator in 1912 made
inexpensive sound radio transmission possible, and was responsible for
the advent of radio broadcasting around 1920.
Audion advertisement, Electrical Experimenter magazine, 1916
De Forest continued to manufacture and supply Audions to the US Navy
up until the early 1920s, for maintenance of existing equipment, but
elsewhere they were regarded as well and truly obsolete by then. It
was the vacuum triode that made practical radio broadcasts a reality.
Prior to the introduction of the Audion, radio receivers had used a
variety of detectors including coherers, barretters, and crystal
detectors. The most popular crystal detector consisted of a small
piece of galena crystal probed by a fine wire commonly referred to as
a "cat's-whisker detector". They were very unreliable, requiring
frequent adjustment of the cat's whisker and offered no amplification.
Such systems usually required the user to listen to the signal through
headphones, sometimes at very low volume, as the only energy available
to operate the headphones was that picked up by the antenna. For long
distance communication huge antennas were normally required, and
enormous amounts of electrical power had to be fed into the
Audion was a considerable improvement on this, but the original
devices could not provide any subsequent amplification to what was
produced in the signal detection process. The later vacuum triodes
allowed the signal to be amplified to any desired level, typically by
feeding the amplified output of one triode into the grid of the next,
eventually providing more than enough power to drive a full-sized
speaker. Apart from this, they were able to amplify the incoming radio
signals prior to the detection process, making it work much more
Vacuum tubes could also be used to make superior radio transmitters.
The combination of much more efficient transmitters and much more
sensitive receivers revolutionized radio communication during World
By the late 1920s such "tube radios" began to become a fixture of most
Western world households, and remained so until the introduction of
transistor radios in the mid-1950s.
In modern electronics, the vacuum tube has been largely superseded by
solid state devices such as the transistor, invented in 1947 and
implemented in integrated circuits in 1959, although vacuum tubes
remain to this day in such applications as high-powered transmitters,
guitar amplifiers and some high fidelity audio equipment.
^ a b c Okamura, Sōgo (1994). History of Electron Tubes. IOS Press.
pp. 17–22. ISBN 9051991452.
^ De Forest patented a number of variations of his detector tubes
starting in 1906. The patent that most clearly covers the
Patent 879,532, Space Telegraphy, filed January 29, 1907, issued
February 18, 1908
^ a b De Forest, Lee (January 1906). "The Audion; A New Receiver for
Wireless Telegraphy". Trans. of the AIEE. American Institute of
Electrical and Electronic Engineers. 25: 735–763.
doi:10.1109/t-aiee.1906.4764762. Retrieved January 7, 2013. The
link is to a reprint of the paper in the Scientific American
Supplement, No. 1665, November 30, 1907, p.348-350, copied on Thomas
H. White's United States Early Radio History website
^ a b c Godfrey, Donald G. (1998). "Audion". Historical Dictionary of
American Radio. Greenwood Publishing Group. p. 28. Retrieved
January 7, 2013.
^ a b Amos, S. W. (2002). "Triode". Newnes Dictionary of Electronics,
4th Ed. Newnes. p. 331. Retrieved January 7, 2013.
^ Hijiya, James A. (1992). Lee de Forest. Lehigh University Press.
p. 77. ISBN 0934223238.
^ a b c d e Lee, Thomas H. (2004). Planar
Microwave Engineering: A
Practical Guide to Theory, Measurement, and Circuits. Cambridge
University Press. pp. 13–14. ISBN 0521835267.
^ a b Hempstead, Colin; Worthington, William E. (2005). Encyclopedia
of 20th-Century Technology, Vol. 2. Taylor & Francis. p. 643.
^ a b Nebeker, Frederik (2009). Dawn of the Electronic Age: Electrical
Technologies in the Shaping of the Modern World, 1914 to 1945. John
Wiley & Sons. pp. 14–15. ISBN 0470409746.
^ a b Armstrong, E. H. (September 1915). "Some Recent Developments in
Audion Receiver". Proceedings of the IRE. 3 (9): 215–247.
doi:10.1109/jrproc.1915.216677. . Republished as Armstrong, E. H.
(April 1997). "Some Recent Developments in the
Audion Receiver" (PDF).
Proceedings of the IEEE. 85 (4): 685–697.
^ De Forest, Lee (May 1930). "Evolution of the
Vacuum Tube" (PDF).
Radio News. Experimenter Publications. 9 (11): 990. Retrieved August
^ a b Armstrong, E. H. (December 12, 1914). "Operating Features of the
Audion". Electrical World. 64 (24): 1149–1152.
^ McNicol, Donald Monroe (1946). Radio's Conquest of Space the
Experimental Rise in Radio Communication. Taylor & Francis.
1973 postage stamp honoring De Forest's audion
Radio Corp. v. Radio Engineering Laboratories, 293 U.S. 1 (United
States Supreme Court 1934).
Hong, Sungook (2001), Wireless: From Marconi's Black-box to the
Audion, MIT Press, ISBN 9780262082983
Where Good Ideas Come From, Chapter V, Steven Johnson, Riverhead
Wikimedia Commons has media related to Audion.
Wikisource has original text related to this article:
Practical Pointers on the Audion
1906 photograph of the original
Audion tube, from New York Public
Langmuir, Irving (September 1997) , "The Pure Electron Discharge
and Its Applications in Radio Telegraphy and Telephony" (PDF),
Proceedings of the IEEE, 85 (9): 1496–1508,
doi:10.1109/jproc.1997.628726 . Reprint of Langmuir, Irving
(September 1915), "The Pure Electron Discharge and Its Applications in
Radio Telegraphy and Telephony", Proceedings of the IRE, 3 (3):
261–293, doi:10.1109/jrproc.1915.216680 . (Includes comments
from De Forest.)
The Audion: A new Receiver for Wireless Telegraphy, Lee de Forest,
Scientific American Supplement No. 1665, November 30, 1907, pages
348-350, Scientific American Supplement No. 1666, December 7, 1907,
Lee De Forest's
Audion Piano on '120 years Of Electronic Music'
https://books.google.com/books?id=YEASAAAAIAAJ&pg=PA166 De Forest
and Armstong debate
Cole, A. B. (March 1916). "Practical Pointers on the Audion". QST:
Amplifier Bulb is entirely different from the
Audion Detector Bulb in construction and vacuum. [page 43]
Also page 43 stating,
Audion Detector Bulbs are not adapted for the reception of
continuous waves, because the vacuum is not correct for the purpose
and because the filaments must be operated at such a high intensity
that they give very short service, making them unnecessarily
Also page 44 stating,
BLUE DISCHARGE OF GLOW
This appears in some
Audion Bulbs and not in others. If allowed to
persist, the vacuum automatically increases. For this reason the glow
should not be allowed to appear and certainly not to continue, as the
vacuum may rise to a very high value, requiring very high voltage in
the “B” battery.
Bipolar junction transistor
Bipolar junction transistor (BJT)
Field-effect transistor (FET)
Constant-current diode (CLD, CRD)
Heterostructure barrier varactor
Insulated-gate bipolar transistor
Insulated-gate bipolar transistor (IGBT)
Integrated circuit (IC)
Light-emitting diode (LED)
Silicon controlled rectifier
Silicon controlled rectifier (SCR)
Unijunction transistor (UJT)
Pentagrid (Hexode, Heptode, Octode)
Vacuum tubes (RF)
Backward-wave oscillator (BWO)
Crossed-field amplifier (CFA)
Inductive output tube
Inductive output tube (IOT)
Traveling-wave tube (TWT)
Cathode ray tubes
Beam deflection tube
Magic eye tube
Video camera tube
audio and video
Pentagrid (Hexode, Heptode, Octode)
Cathode ray tube
Beam deflection tube
Inductive output tube
Video camera tube
List of vacuum tubes