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Semaphore (communication)

Semaphore was originally a flag signalling system devised by the Chappe brothers in France.

Claude Chappe began development when he and his four brothers lost their means of support because of the French Revolution. They determined by experiment that it was easier to see the angle of a rod than determine the presence of a panel. Their system was composed of black movable wooden arms, the position of which indicated alphabetic letters. The Chappe system was controlled by only two handles, and was mechanically simple, and reasonably rugged. Night operation with lamps on the arms was unsuccessful.

The arms showed seven positions each, and the connecting cross bar had four different angles, for a total of 196 codes.

A crucial innovation was to use a group of trained, dedicated men to pass the signals.

The first Chappe semaphore line was established between Paris and Lille in 1792. It was used to carry dispatches for a war between France and Austria. In 1794, it brought news of a French capture of Condé-sur-l'Escaut from the Austrians less than an hour after it occurred. Other lines were built, including a line from Paris to Toulon. The system was widely copied by other European states, and was used by Napoleon to coordinate his empire and army.

The first symbol of a message to Lille would pass 120 miles through 15 stations in only nine minutes. The speed of the line varied with the weather, but the line to Lille typically transferred 36 codes, a complete message, in about 32 minutes.

By 1824, the Chappe brothers were promoting the semaphore lines for commercial use, especially to transmit the costs of commodities.

Many European governments instituted a semaphore service as part of their postal union. Many national services adopted signaling systems different from the Chappe system. For example, Britain and Sweden adopted systems of flapping panels (in contradiction to the Chappe brothers' discovery that angled rods are more visible). Britain developed a series of semaphore towers[?] which allowed rapid communications between London and the naval dockyards at Portsmouth.

Semaphore lines had several crucial advantages over post roads (roads with stations to change horses). First, a semaphore message could easily travel at several thousand miles per hour. Second, with large signals and a telescope, the distance between stations could be as much as 30Km (20mi), over mountain ranges and bad terrain, reducing investment and the number of stations over other forms of communication. Finally, techniques were developed to permit a semaphore relay line to serve a region, not just a single town, permitting a service to amortize the line's expense over several towns, and reach the headquarters of a bivouaced army.

The semaphores' crucial disadvantages were that they were affected by weather, especially fog and rain, and they could be read by anyone with the training.

The first code book was developed for use with semaphore lines. The directors of the Chappes' corporation used a secret code that took 92 of the basic codes two at a time to yield 8,464 words and phrases.

Napoleon Bonaparte saw the military advantage in being able to transmit information between locations, and carried a portable semaphore with his headquarters. This allowed him to coordinate forces and logistics over longer distances than any other army of his time.

Semaphores were adopted and widely used (with hand-held flags replacing the mechanical arms) in the maritime world in the early 1800s. Semaphore signals were used, for example, at the Battle of Trafalgar.

The semaphores were successful enough that Samuel Morse was unsuccessful when he attempted to sell the electric telegraph[?] to the French government. However, France finally committed to replace semaphores with electric telegraphs in 1846. Note that electric telegraphs are both more private and unaffected by weather. Many contemporaries predicted the failure of electric telegraphs because "they are so easy to cut."

The last stationary semaphore link in regular service was in Sweden, connecting an island with a mainland telegraph line. It went out of service in 1880.

Relative Costs

The semaphore system was cleverly designed, and provided a strategic advantage for France in a difficult time. However, it was almost 30 times more expensive per message than the electric telegraph. Here's a brief breakdown using $US:

Semaphore line, 120 miles (Paris to Lille)

  • 15 towers ($1,500,000)

  • At least 15 full-time operators ($450,000/year).

  • Operates at most ten hours a day.

  • Sends roughly 2 words per minute (1 symbol per minute, at 2 symbols per phrase, using the efficient directors' codebook).

  • Cost to send one word one mile, at 10% interest: $0.0114

Electric Telegraph line, 120 miles

  • At least six full-time operators ($180,000/year)

  • Poles, right-of-way, wires, installation: $15,000/mile, ($1,800,000)

  • Operates 24 hours a day.

  • Sends 15 words per minute (includes breaks for the operators).

  • Cost to send one word one mile, at 10% interest: $0.000380

External Links

  • Chappe's semaphore (http://people.deas.harvard.edu/~jones/cscie129/images/history/chappe)

See also optical telegraph.

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