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Main article: "Semaphore line
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A replica of one of "Chappe's "semaphore towers (18th century).

A "'semaphore telegraph', also called a 'semaphore line', 'optical telegraph', 'shutter telegraph chain', 'Chappe telegraph', or 'Napoleonic semaphore', is a system used for conveying information by means of visual signals, using towers with pivoting arms or shutters, also known as blades or paddles. Information is encoded by the position of the mechanical elements; it is read when the shutter is in a fixed position.[2][6]

Semaphore lines were a precursor of the "electrical telegraph. They were far faster than "post riders for conveying a message over long distances, but far more expensive and less private than the electrical telegraph lines which would later replace them. The maximum distance that a pair of semaphore telegraph stations can bridge is limited by geography, weather and the availability of light; thus, in practical use, most optical telegraphs used lines of relay stations to bridge longer distances. Each relay station would also require its complement of skilled operator-observers to convey messages back and forth across the line.

The modern design of semaphores was first foreseen by the British "polymath "Robert Hooke, who first gave a vivid and comprehensive outline of visual telegraphy in an 1684 submission to the "Royal Society. His proposal (which was motivated by military concerns following the "Battle of Vienna the preceding year) was not put into practice during his lifetime.[7][8]

The first operational optical semaphore line arrived in 1792, created by the French engineer "Claude Chappe and his brothers, who succeeded in covering "France with a network of 556 stations stretching a total distance of 4,800 kilometres (3,000 mi). It was used for military and national communications until the 1850s.

Many national services adopted signaling systems different from the Chappe system. For example, "Britain and "Sweden adopted systems of shuttered panels (in contradiction to the Chappe brothers' contention that angled rods are more visible). In "Spain, the engineer "Agustín de Betancourt developed his own system which was adopted by that state. This system was considered by many experts in Europe better than Chappe's, even in France.

These systems were popular in the late 18th to early 19th century but could not compete with the "electrical telegraph, and went completely out of service by 1880.[1]

Semaphore signal flags[edit]

Flag semaphore
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A naval signaler transmitting a message by flag semaphore (2002).

"Semaphore Flags is the system for conveying information at a distance by means of visual signals with hand-held flags, rods, disks, paddles, or occasionally bare or gloved hands. Information is encoded by the position of the flags, objects or arms; it is read when they are in a fixed position.

Semaphores were adopted and widely used (with hand-held "flags replacing the mechanical arms of "shutter semaphores) in the maritime world in the 19th century. They are still used during "underway replenishment at sea and are acceptable for emergency communication in daylight or, using lighted wands instead of flags, at night.

The newer flag semaphore system uses two short poles with square flags, which a signaler holds in different positions to convey letters of the alphabet and numbers. The transmitter holds one pole in each hand, and extends each arm in one of eight possible directions. Except for in the rest position, the flags cannot overlap. The flags are colored differently based on whether the signals are sent by sea or by land. At sea, the flags are colored red and yellow (the "Oscar flags), while on land, they are white and blue (the "Papa flags). Flags are not required, they just make the characters more obvious.

Optical fiber[edit]

Fiber-optic communication

"Optical fiber is the most common type of channel for optical communications. The transmitters in optical fiber links are generally "light-emitting diodes (LEDs) or "laser diodes. "Infrared light, rather than "visible light is used more commonly, because optical fibers transmit infrared wavelengths with less "attenuation and "dispersion. The signal encoding is typically simple "intensity modulation, although historically optical phase and "frequency modulation have been demonstrated in the lab. The need for periodic "signal regeneration was largely superseded by the introduction of the "erbium-doped fiber amplifier, which extended link distances at significantly lower cost.

Signal lamps[edit]

Signal lamp and "Aviation light signals
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An "air traffic controller holding a signal light gun that can be used to direct aircraft experiencing a radio failure (2007).

"Signal lamps (such as Aldis lamps), are visual signaling devices for optical communication (typically using Morse code). Modern signal lamps are a focused lamp which can produce a pulse of light. In large versions this pulse is achieved by opening and closing shutters mounted in front of the lamp, either via a manually operated pressure switch or, in later versions, automatically.

With hand held lamps, a "concave mirror is tilted by a trigger to focus the light into pulses. The lamps are usually equipped with some form of optical sight, and are most commonly deployed on naval vessels and also used in airport control towers with coded "aviation light signals.

"Aviation light signals are used in the case of a "radio failure, an "aircraft not equipped with a radio, or in the case of a hearing-impaired pilot. "Air traffic controlers have long used signal light guns to direct such aircraft. The light gun's lamp has a focused bright beam capable of emitting three different colors: red, white and green. These colors may be flashing or steady, and provide different instructions to aircraft in flight or on the ground (for example, "cleared to land" or "cleared for takeoff"). Pilots can acknowledge the instructions by wiggling their plane's wings, moving their "ailerons if they are on the ground, or by flashing their "landing or "navigation lights during night time. Only 12 simple standardized instructions are directed at aircraft using signal light guns as the system is not utilized with "Morse code.

Photophone[edit]

Photophone

The "photophone (originally given an alternate name, "radiophone) is a communication device which allowed for the "transmission of speech on a beam of "light. It was invented jointly by "Alexander Graham Bell and his assistant "Charles Sumner Tainter on February 19, 1880, at Bell's 1325 'L' Street laboratory in Washington, D.C.[9][10] Both were later to become full associates in the "Volta Laboratory Association, created and financed by Bell.

On June 21, 1880, Bell's assistant transmitted a wireless voice telephone message of considerable distance, from the roof of the "Franklin School to the window of Bell's laboratory, some 213 meters (about 700 ft.) away.[11][12][13][14]

Bell believed the photophone was his most important "invention. Of the 18 "patents granted in Bell's name alone, and the 12 he shared with his collaborators, four were for the photophone, which Bell referred to as his 'greatest achievement', telling a reporter shortly before his death that the photophone was "the greatest invention [I have] ever made, greater than the telephone".[15]

The photophone was a precursor to the "fiber-optic communication systems which achieved popular worldwide usage starting in the 1980s.[16][17][18] The master patent for the photophone (U.S. Patent 235,199 Apparatus for Signalling and Communicating, called Photophone), was issued in December 1880,[13] many decades before its principles came to have practical applications.

Free-space optical communication[edit]

Free-space optical communication and "Optical wireless communications

Free-space optics (FSO) systems are generally employed for '"last mile' "telecommunications and can function over distances of several kilometers as long as there is a clear "line of sight between the source and the destination, and the optical receiver can reliably decode the transmitted information.[19] Other free-space systems can provide high-data-rate, long-range links using small, low-mass, low-power-consumption subsystems.[20]

More generally, transmission of unguided optical signals is known as "optical wireless communications (OWC). Examples include medium-range "visible light communication and short-distance "IrDA, using infrared LEDs.

Heliograph[edit]

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"Heliograph: Australians using a heliograph in North Africa (1940).
Heliograph

A "heliograph ("Greek: Ἥλιος "helios, meaning "sun", and γραφειν graphein, meaning "write") is a wireless solar "telegraph that signals by flashes of "sunlight (generally using "Morse code) reflected by a "mirror. The flashes are produced by momentarily pivoting the mirror, or by interrupting the beam with a shutter.

The heliograph was a simple but effective instrument for instantaneous optical communication over long distances during the late 19th and early 20th century. Its main uses were in military, surveys and forest protection work. They were standard issue in the British and Australian armies until the 1960s, and were used by the Pakistani army as late as 1975.[5]

See also[edit]

References[edit]

Citations[edit]

  1. ^ a b Chapter 2: Semaphore Signalling "ISBN 978-0-86341-327-8 Communications: an international history of the formative years R. W. Burns, 2004
  2. ^ a b Telegraph Vol 10, Encyclopædia Britannica, 6th Edition, 1824 pp. 645-651
  3. ^ "Nation Park Service Fire History Timeline". 
  4. ^ "Lewis and Clark Journals, July 20, 1805". 
  5. ^ a b Harris, J.D. Wire At War - Signals communication in the South African War 1899–1902. Retrieved on 1 June 2008. Note a discussion on the heliograph use during the Boer War.
  6. ^ Telegraph, Volume 17 of The Edinburgh encyclopaedia, pp. 664-667, 1832 David Brewster, ed.
  7. ^ Calvert, J.B. The Origin of the Railway Semaphore, "Boston University, 15 April 2000, Revised 4 May 2007.
  8. ^ McVeigh, Daniel P. An Early History of the Telephone: 1664-1865, Part 2, "Columbia University in The City of New York, Institute For Learning Technologies, 2000.
  9. ^ Bruce 1990, pg. 336
  10. ^ Jones, Newell. First 'Radio' Built by San Diego Resident Partner of Inventor of Telephone: Keeps Notebook of Experiences With Bell, San Diego Evening Tribune, July 31, 1937. Retrieved from the University of San Diego History Department website, November 26, 2009.
  11. ^ Bruce 1990, pg. 338
  12. ^ Carson 2007, pg. 76-78
  13. ^ a b Groth, Mike. Photophones Revisted, 'Amateur Radio' magazine, "Wireless Institute of Australia, Melbourne, April 1987 pp. 12–17 and May 1987 pp. 13–17.
  14. ^ Mims 1982, p. 11.
  15. ^ Mims 1982, p. 14.
  16. ^ Morgan, Tim J. "The Fiber Optic Backbone", "University of North Texas, 2011.
  17. ^ Miller, Stewart E. "Lightwaves and Telecommunication", "American Scientist, Sigma Xi, The Scientific Research Society, January–February 1984, Vol. 72, No. 1, pp. 66-71, Issue Stable URL.
  18. ^ Gallardo, Arturo; "Mims III, Forrest M.. Fiber-optic Communication Began 130 Years Ago, "San Antonio Express-News, June 21, 2010. Accessed January 1, 2013.
  19. ^ Clint Turner (October 3, 2007). "A 173-mile 2-way all-electronic optical contact". Modulated light web site. Retrieved June 28, 2011. 
  20. ^ Wilson, K. "Recent Development in High-Data Rate Optical Communications at JPL". Jet Propulsion Laboratory. NASA Technical Reports Server. Retrieved 4 October 2011. 

Bibliography[edit]

Further reading[edit]

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