Editing Optical telegraph (section)

====Chappe system technical operation====
The Chappe brothers determined by experiment that it was easier to see the angle of a rod than to see the presence or absence of a panel. Their semaphore was composed of two black movable wooden arms, connected by a cross bar; the positions of all three of these components together indicated an alphabetic letter. With counterweights (named ''forks'') on the arms, the Chappe system was controlled by only two handles and was mechanically simple and reasonably robust. Each of the two 2-metre-long arms could display seven positions, and the 4.6-metre-long cross bar connecting the two arms could display four different angles, for a total of 196 symbols (7×7×4). Night operation with lamps on the arms was unsuccessful.<ref>Holzmann & Pehrson, p. 213</ref> To speed up transmission and to provide some semblance of security, a [[code]] book was developed for use with semaphore lines. The Chappes' corporation used a code that took 92 of the basic symbols two at a time to yield 8,464 coded words and phrases.

The revised Chappe system of 1795 provided not only a set of codes but also an operational protocol intended to maximize line [[throughput]]. Symbols were transmitted in cycles of "2 steps and 3 movements."

*'''Step 1, movement 1 (setup)''': The operator turned the indicator arms to align with the cross bar, forming a non-symbol, and then turned the cross bar into position for the next symbol.
*'''Step 1, movement 2 (transmission)''': The operator positioned the indicator arms for current symbol and waited for the downline station to copy it.
*'''Step 2, movement 3 (completion)''': The operator turned the cross bar to a vertical or horizontal position, indicating the end of a cycle.

In this manner, each symbol could propagate down the line as quickly as operators could successfully copy it, with [[Acknowledgement (data networks)|acknowledgement]] and [[Flow control (data)|flow control]] built into the protocol. A symbol sent from Paris took 2 minutes to reach Lille through 22 stations and 9 minutes to reach Lyon through 50 stations. A rate of 2–3 symbols per minute was typical, with the higher figure being prone to errors. This corresponds to only 0.4–0.6 [[wpm]], but with messages limited to those contained in the code book, this could be dramatically increased.<ref>Holzmann & Pehrson</ref><ref>{{cite web |url=http://chappe.ec-lyon.fr/message.html |title=Les dépêches |website=chappe.ec-lyon.fr |access-date=11 January 2022 |archive-url=https://web.archive.org/web/20140202183712/http://chappe.ec-lyon.fr/message.html |archive-date=2 February 2014 |url-status=dead}}</ref> An additional benefit is that, if the code is kept secret, the content of transmitted messages can be concealed from both onlookers and system operators, even if they are aware that a message is being transmitted. This has remained an important feature of encrypted communications even as the technology for transmitting data has evolved.<ref>{{Cite news |date=2013-06-16 |title=How Napoleon's semaphore telegraph changed the world |url=https://www.bbc.com/news/magazine-22909590 |access-date=2024-11-08 |work=BBC News |language=en-GB}}</ref>