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 . 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
The Edelcrantz system was used in Sweden and was the second largest network built after that of France. The telegraph consisted of a set of ten shutters. Nine of these were arranged in a 3×3 matrix. Each column of shutters represented a binary-coded octal digit with a closed shutter representing "1" and the most significant digit at the bottom. Each symbol of telegraph transmission was thus a three-digit octal number. The tenth shutter was an extra-large one at the top. Its meaning was that the codepoint should be preceded by "A".
One use of the "A" shutter was that a numeral codepoint preceded by "A" meant add a zero (multiply by ten) to the digit. Larger numbers could be indicated by following the numeral with the code for hundreds (236), thousands (631) or a combination of these. This required fewer symbols to be transmitted than sending all the zero digits individually. However, the main purpose of the "A" codepoints was for a codebook of predetermined messages, much like the Chappe codebook.
The symbols without "A" were a large set of numerals, letters, common syllables and words to aid code compaction. Around 1809, Edelcrantz introduced a new codebook with 5,120 codepoints, each requiring a two-symbol transmission to identify.
+ Edelcrantz alphabetABCDEFGHIJKLMNOPQRST003026055112125162210254274325362422450462500530610UVWXYZÅÄÖ12345678900000640650710711712713723737001002004010020040100200400236631
There were many codepoints for error correction (272, error), flow control, and supervisory messages. Usually, messages were expected to be passed all the way down the line, but there were circumstances when individual stations needed to communicate directly, usually for managerial purposes. The most common, and simplest situation was communication between adjacent stations. Codepoints 722 and 227 were used for this purpose, to get the attention of the next station towards, or away from, the sun, respectively. For more remote stations codepoints 557 and 755 respectively were used, followed by the identification of the requesting and target stations.
Wig-wag Flag signalling was widely used for point-to-point signalling prior to the optical telegraph, but it was difficult to construct a nationwide network with hand-held flags. The much larger mechanical apparatus of the semaphore telegraph towers was needed so that a greater distance between links could be achieved. However, an extensive network with hand-held flags was constructed during the American Civil War. This was the wig-wag system which used the code invented by Albert J. Myer. Some of the towers used were enormous, up to 130 feet, to get a good range. Myer's code required only one flag using a ternary code. That is, each code element consisted of one of three distinct flag positions. However, the alphabetical codepoints required only two positions, the third position only being used in control characters. Using a ternary code in the alphabet would have resulted in shorter messages because fewer elements are required in each codepoint, but a binary system is easier to read at long distance since fewer flag positions need to be distinguished. Myer's manual also describes a ternary-coded alphabet with a fixed length of three elements for each codepoint.
Electrical telegraph codes Cooke and Wheatstone and other early codes
Many different codes were invented during the early development of the electrical telegraph. Virtually every inventor produced a different code to suit their particular apparatus. The earliest code used commercially on an electrical telegraph was the Cooke and Wheatstone telegraph five needle code (C&W5). This was first used on the Great Western Railway in 1838. C&W5 had the major advantage that the code did not need to be learned by the operator; the letters could be read directly off the display board. However, it had the disadvantage that it required too many wires. A one needle code, C&W1, was developed that required only one wire. C&W1 was widely used in the UK and the British Empire.
Some other countries used C&W1, but it never became an international standard and generally each country developed their own code. In the US, American Morse code was used, whose elements consisted of dots and dashes distinguished from each other by the length of the pulse of current on the telegraph line. This code was used on the telegraph invented by Samuel Morse and Alfred Vail and was first used commercially in 1844. Morse initially had code points only for numerals. He planned that numbers sent over the telegraph would be used as an index to a dictionary with a limited set of words. Vail invented an extended code that included code points for all the letters so that any desired word could be sent. It was Vail's code that became American Morse. In France, the telegraph used the Foy-Breguet telegraph, a two-needle telegraph that displayed the needles in Chappe code, the same code as the French optical telegraph, which was still more widely used than the electrical telegraph in France. To the French, this had the great advantage that they did not need to retrain their operators in a new code.
Standardisation—Morse code
In Germany in 1848, Friedrich Clemens Gerke developed a heavily modified version of American Morse for use on German railways. American Morse had three different lengths of dashes and two different lengths of space between the dots and dashes in a code point. The Gerke code had only one length of dash and all inter-element spaces within a code point were equal. Gerke also created code points for the German umlaut letters, which do not exist in English. Many central European countries belonged to the German-Austrian Telegraph Union. In 1851, the Union decided to adopt a common code across all its countries so that messages could be sent between them without the need for operators to recode them at borders. The Gerke code was adopted for this purpose.