An Encyclopedia of the History of Technology part 74

PART FOUR: COMMUNICATION AND CALCULATION Figure 15.2: Three stages in the evolution of the telegraph: (a) Chappe’s 1794 semaphore; (b) von Soemmering’s 1809 electrolytic telegraph. A station agent telegraphed the page number with his first signal, and the number of the word on the page with his second. Phrase and place-name vocabularies, each again of 92 by 92 items, were developed; to distinguish them, an initial signal indicated the code; the second, the page; and the third, the item on the page. No public excitement over the semaphore system occurred until 1 September 1794, when Condé was recaptured. The Convention then ordered the extension of the line from Lille to Ostend, and a second line to be built to Strasbourg, which was completed in 1798 with 46 towers at a cost of 176,000 francs. Despite the practical success of these telegraph lines, they did not bring in revenue, but rather entailed great expense; maintenance and service in the eighth year of use cost 712 INFORMATION Figure 15.2: (c) Cooke and Wheatstone’s 1837 5-needle telegraph. 434,000 francs. Napoleon, losing enthusiasm for the venture, reduced the appropriation to 150,000 francs a year, and cancelled a planned Paris-Lyon line. Claude Chappe thought that his system might benefit business, as well as give an advantage in war. During the Restoration, interest in the telegraph increased and Chappe’s dream was at last realized—but he committed suicide in 1805, despondent over the slow progress and suffering from bladder trouble. However, his brothers continued to perfect the system; Abraham worked at overcoming the fog problem, anticipating British use of hydrogen fires during the Second World War. Semaphore systems remained in use well into the nineteenth century, even after the electric telegraph was developed (see p. 714). Figuier, writing in the 1860s, claimed that aerial telegraphers could send a dispatch in two minutes from Lille to Paris (a distance of 240km (150 miles), requiring 22 stations); in three minutes from Calais (270km (168 miles) and 33 stations); in eight minutes from Brest (600km (373 miles) and 54 stations); and in twenty minutes from Toulon (more than 1000km (620 miles) and 100 stations). The equivalent rates of speed would be, respectively, 7200, 5400, 4500 and 3000kph; even the worst case is three hundred times better than the then fastest system—a relay of horses and riders, such as the famed Pony Express in the USA. The electric telegraph The idea of the electric telegraph preceded its practical possibility by many decades. Chappe’s semaphore had demonstrated the need for rapid 713 PART FOUR: COMMUNICATION AND CALCULATION communication of messages, but until electricity was better understood, little progress was possible. There were attempts to do the job with static electricity, but the high voltage and very low current characteristic of this form of electricity did not allow reliable transmission beyond a few metres. The inventor of the earliest electric telegraph did not wait upon the epochmaking discoveries of Oersted, Faraday and their contemporaries (see Chapter 6). In Germany, S.T. von Soemmerring observed that electric current passed through an acid solution caused bubbles to appear (electrolytic decomposition of water into its elements, hydrogen and oxygen). He invented a telegraph system using this principle in 1809, which used 26 parallel wires to transmit letters of the alphabet a distance of up to two miles (Figure 15.2 (b)). He even designed an ingenious alarm to alert the receiving operator (this was really the first relay, although not electromechanical). However, the expense of so many conductors made the system economically impracticable. André-Marie Ampère, in 1820, invented the galvanometer which enabled electricity to be measured, and suggested using a galvanometer needle for telegraphy. W.F.Cooke and Charles Wheatstone invented a five-needle telegraph, which was patented in 1837 (Figure 15.3 (c)). Although still a parallel data-communication system like Soemmerring’s, this reduced the number of wires to only six, by representing alphabetic characters with a 2-out-of-5 code. By 1839, they had set up a 13-mile telegraph for the British railways using this system. It was not long before the cost of multiple conductors stimulated reduction of the number of needles to two, and finally to one. So the needle telegraph became the first serial data-communications system, with codes defined by sequences of needle deflections to make up each character. Samuel F.B.Morse, already a noted American painter, became interested in the possibilities of electrical communication in the 1830s. He built a telegraph system in 1835: the sender used notched metal strips to encode the alphabet, and the receiver was an electromagnetically-driven pendulum with a pencil attached which wrote the coded signals on a moving roll of paper. Morse’s knowledge of electricity was rudimentary, but by 1837 he had patented the telegraph, replaced the type-bar sender with what we now call a telegraph key, and simplified the receiver to a pen which put marks (dots and dashes) on paper tape. Early telegraphs used two wires, but it was soon found that one would do, the earth acting as the return path. With some government support, Morse started a telegraph service between Washington and Baltimore in 1844. When the line was extended to New Jersey, it attracted customers in the financial community who appreciated the commercial value of instantaneous communications. The visual receiver was replaced by a sounder, because operators could transcribe aurally transmitted codes to paper faster than the marks on paper tape. This early human-factor discovery shows the importance of inventor-user interaction in bringing an 714 INFORMATION idea to practical fulfilment. The so-called Morse code was actually the work of Morse’s assistant, Alfred Vail. The telegraph was one invention which did not have be marketed; the public was eager and ready to pay for good communications. By 1852, more than 18,000 miles of wire covered the eastern third of the USA, with service provided by many small companies. In 1856, Western Union was created, and extended the telegraph to the west coast in 1861 with government support; a message cost senders $1 per word. There had been several attempts to develop submarine cables for telegraphy, but the first, under the English Channel, ruptured in 1850. Improvements in cable-making, and the development of specially equipped cable-laying ships, enabled construction of a reliable link from Dover to Calais in 1851. A much more audacious undertaking, a transatlantic cable, was proposed by an American, Cyrus W.Field, in 1856, and was successfully laid by HMS Agamemnon between 1857 and 1858. However, a few months afterwards, operator error put 2000 volts across the cable which rendered it unusable. It was not until after the American Civil War, in 1865, that Field attempted another link; this time, the cable was spun out from a single ship, Brunel’s Great Eastern. This sail-and-steam leviathan carried almost 5000km (3,100 miles) of cable weighing over 5000 tonnes. However, after laying 2000km (1250 miles) between Ireland and Newfoundland, a defective length snapped; after ten days’ unsuccessful grappling, the spot was marked and the attempt abandoned. Another cable was manufactured by the Telegraph Construction and Maintenance Company of the UK, and laid by the Great Eastern in 1866: the commercial communications link so long sought between Europe and the United States was at last established. The first telecommunications network—organizational rather than physicalwas that of the Associated Press, begun in the 1840s by a group of New York newspapers to share telegraph expenses. Many technical improvements were made in both the United States and Europe, including the paper-tape perforator of Wheatstone (1855), and the multiplex telegraph of Emile Baudot (1874). However, the most radical departure, which led the way to the modern era of telegraphy, was the invention of the teletypewriter (or teleprinter) by E.E. Kleinschmidt in the US in 1928. This allowed operators to compose messages using a typewriter-like keyboard to punch paper tape, which was then torn off and fed into a tape reader for transmission. At the receiving end, the message was printed out on paper strips (later, directly on a roll of paper). Thus, the days of the telegraph operator, sending with a lightning touch on the key, and listening to the clicks of the sounder were ended. However, Morse is still used in radiotelegraphy, where it can get a message through static and difficult transmission conditions when electromechanical and electronic alternatives are unworkable. 715 PART FOUR: COMMUNICATION AND CALCULATION Telex The sending and delivery of a telegram has always been a hybrid type of service, part electronic and therefore very rapid, part manual and therefore slow and labour-intensive. The sender had to go to a telegraph office and print out his message in capital letters on a special form; the clerk had to count the words and compute the amount due from a tariff schedule; then the telegraph operator had to key in the message. At the other end, the reverse took place; the message would be printed out on a paper strip, cut into segments and pasted on a form, and then had to be delivered by hand to the recipient. By the time most customers had their own telephones (the 1930s in the US and Canada; later elsewhere), the US telegraph carrier Western Union would accept outgoing telegrams over the phone, and read incoming telegrams to recipients who were phone subscribers. However, the unique feature of the telegram among public telecommunications services has always been the delivery of a written message, providing legal proof to sender and receiver. Most customers would insist on physical delivery even though the message had been read to them over the telephone; in such cases, telegraph officers would resort to the postal system. For business users, the whole system seemed archaic. In the US they turned increasingly to the telephone whenever written proof was not essential. In Europe, with its many languages and complex cross-border tariffs, the telephone did not have the same ease of use or economy. Therefore, when the first telex network was put into service in Germany in 1933, a great unmet demand was released. From its start, with only nineteen subscribers in Berlin and Hamburg, telex service had an explosive growth. In Germany subscriptions grew at more than 100 per cent annually up to the Second World War; similar growth, but at lower rates, was experienced in other European countries. By the early 1980s, the number of telex subscribers world-wide exceeded 1.5 million, more than half of them in Europe. Today, telex is a world-wide, switched public teleprinter service. Therefore it is like the telephone, in that subscribers are loaned terminal equipment, can dial up other subscribers themselves, can receive messages, and are charged on the basis of time and distance. However, unlike the telephone, messages must be keyed in and received on teleprinters. Up to the 1970s, the usual practice was to keypunch a series of messages on paper tape, dial up the recipient’s teleprinter, and put the strip of tape in the sender’s teleprinter; this store-and-forward process has been greatly enhanced by the use of computers in telex officers, and by microprocessors and electronic memory capability in terminal equipment. Also, in store-and-forward operation, the same message may be sent to many recipients (a type of message broadcasting), and re-dialling done automatically when receiving terminals are busy. 716 ... - tailieumienphi.vn