A telegraph code is one of the character encodings used to transmit information by telegraphy. Morse code is the best-known such code. Telegraphy usually refers to the electrical telegraph, but telegraph systems using the optical telegraph were in use before that. A code consists of a number of code points, each corresponding to a letter of the alphabet, a numeral, or some other character. In codes intended for machines rather than humans, code points for control characters, such as carriage return, are required to control the operation of the mechanism. Each code point is made up of a number of elements arranged in a unique way for that character. There are usually two types of element (a binary code), but more element types were employed in some codes not intended for machines. For instance, American Morse code had about five elements, rather than the two (dot and dash) of International Morse Code. Codes meant for human interpretation were designed so that the characters that occurred most often had the fewest elements in the corresponding code point. For instance, Morse code for E, the most common letter in English, is a single dot ( ▄ ), whereas Q is ▄▄▄ ▄▄▄ ▄ ▄▄▄ . These arrangements meant the message could be sent more quickly and it would take longer for the operator to become fatigued. Telegraphs were always operated by humans until late in the 19th century. When automated telegraph messages came in, codes with variable-length code points were inconvenient for machine design of the period. Instead, codes with a fixed length were used. The first of these was the Baudot code, a five-bit code. Baudot has only enough code points to print in upper case. Later codes had more bits (ASCII has seven) so that both upper and lower case could be printed. Beyond the telegraph age, modern computers require a very large number of code points (Unicode has 21 bits) so that multiple languages and alphabets (character sets) can be handled without having to change the character encoding. Modern computers can easily handle variable-length codes such as UTF-8 and UTF-16 which have now become ubiquitous.
Manual telegraph codes
Optical telegraph codes
Prior to the electrical telegraph, a widely used method of building national telegraph networks was the optical telegraph consisting of a chain of towers from which signals could be sent by semaphore or shutters from tower to tower. This was particularly highly developed in France and had its beginnings during the French Revolution. The code used in France was the Chappe code, named after Claude Chappe the inventor. The British Admiralty also used the semaphore telegraph, but with their own code. The British code was necessarily different from that used in France because the British optical telegraph worked in a different way. The Chappe system had moveable arms, as if it were waving flags as in flag semaphore. The British system used an array of shutters that could be opened or closed.
Chappe code The Chappe system consisted of a large pivoted beam (the regulator) with an arm at each end (the indicators) which pivoted around the regulator on one extremity. The angles these components were allowed to take was limited to multiples of 45° to aid readability. This gave a code space of 8×4×8 code points, but the indicator position inline with the regulator was never used because it was hard to distinguish from the indicator being folded back on top of the regulator, leaving a code space of 7×4×7 = 196. Symbols were always formed with the regulator on either the left- or right-leaning diagonal (oblique) and only accepted as valid when the regulator moved to either the vertical or horizontal position. The left oblique was always used for messages, with the right oblique being used for control of the system. This further reduced the code space to 98, of which either four or six code points (depending on version) were control characters, leaving a code space for text of 94 or 92 respectively. The Chappe system mostly transmitted messages using a code book with a large number of set words and phrases. It was first used on an experimental chain of towers in 1793 and put into service from Paris to Lille in 1794. The code book used this early is not known for certain, but an unidentified code book in the Paris Postal Museum may have been for the Chappe system. The arrangement of this code in columns of 88 entries led Holzmann & Pehrson to suggest that 88 code points might have been used. However, the proposal in 1793 was for ten code points representing the numerals 0–9, and Bouchet says this system was still in use as late as 1800 (Holzmann & Pehrson put the change at 1795). The code book was revised and simplified in 1795 to speed up transmission. The code was in two divisions, the first division was 94 alphabetic and numeric characters plus some commonly used letter combinations. The second division was a code book of 94 pages with 94 entries on each page. A code point was assigned for each number up to 94. Thus, only two symbols needed to be sent to transmit an entire sentence – the page and line numbers of the code book, compared to four symbols using the ten-symbol code. In 1799, three additional divisions were added. These had additional words and phrases, geographical places, and names of people. These three divisions required extra symbols to be added in front of the code symbol to identify the correct book. The code was revised again in 1809 and remained stable thereafter. In 1837 a horizontal only coding system was introduced by Gabriel Flocon which did not require the heavy regulator to be moved. Instead, an additional indicator was provided in the centre of the regulator to transmit that element of the code.
Edelcrantz code
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