Heroes of the Telegraph/Chapter 4 > 자유게시판

The advantage of doing this is that messages can be sent at a steady, fast rate making maximum use of the available telegraph lines. The economic advantage of doing this is greatest on long, busy routes where the cost of the extra step of preparing the tape is outweighed by the cost of providing more telegraph lines. A feature of the Baudot code, and subsequent telegraph codes, was that, unlike Morse code, every character has a code of the same length making it more machine friendly. Early teleprinters used the Baudot code, a five-bit sequential binary code. The Baudot code was used on the earliest ticker tape machines (Calahan, 1867), a system for mass distributing information on current price of publicly listed companies. The first widely used system (Wheatstone, 1858) was first put into service with the British General Post Office in 1867. A novel feature of the Wheatstone system was the use of bipolar encoding. Use of the heliograph declined from 1915 onwards, but remained in service in Britain and British Commonwealth countries for some time. A fused 13 A version of this continued to be available after the introduction of BS 1363, illustrating that BS 546 was not used exclusively at any time.

An improved version (Begbie, 1870) was used by British military in many colonial wars, including the Anglo-Zulu War (1879). At some point, a morse key was added to the apparatus to give the operator the same degree of control as in the electric telegraph. The ink may, therefore, be electrified to an exalted state, or high potential as it is called, while the body of the instrument, including the paper and metal writing-tablet, are in connection with the earth, and at low potential, or none at all, for the potential of the earth is in general taken as zero. The minor features of the recorder have been simplified by other inventors of late; for example, magnets of steel have been substituted for the electro-magnets which influence the swinging coil; and the ink, instead of being electrified by the mouse-mill, is shed on the paper by a rapid vibration of the siphon point. The cost of one wire by itself is vastly larger than where many are run together, the cost of the pipe and for laying being not much greater for fifty wires than for one, and the cost of single wire cables being greater per mile of wire than multiple wire cables, so that the expense of putting such a system as one of our telephone exchanges entirely under-ground would place the cost of the instruments entirely out of reach of the subscribers.

Multiple messages can be sequentially recorded on the same run of tape. A heliograph is a telegraph that transmits messages by flashing sunlight with a mirror, usually using Morse code. A teleprinter is a telegraph machine that can send messages from a typewriter-like keyboard and print incoming messages in readable text with no need for the operators to be trained in the telegraph code used on the line. The first true printing telegraph (that is printing in plain text) used a spinning wheel of types in the manner of a daisy wheel printer (House, 1846, improved by Hughes, 1855). The system was adopted by Western Union. This was a telegraph code developed for use on the French telegraph using a five-key keyboard (Baudot, 1874). Teleprinters generated the same code from a full alphanumeric keyboard. Wigwag achieved this by using a large flag-a single flag can be held with both hands unlike flag semaphore which has a flag in each hand-and using motions rather than positions as its symbols since motions are more easily seen. In a punched-tape system, the message is first typed onto punched tape using the code of the telegraph system-Morse code for instance. The Colomb shutter (Bolton and Colomb, 1862) was originally invented to enable the transmission of morse code by signal lamp between Royal Navy ships at sea.

Current flowing through transmission lines induces a magnetic field that surrounds the lines of each phase and affects the inductance of the surrounding conductors of other phases. An LCDI cord has a fine wire mesh around the conductors, and circuitry to detect current leaking from the conductors to the mesh, which would happen if the cord were damaged or frayed. 2. Anti-short Bushings are cheap ways to add a layer of protection against accidental cuts to the conductors where the cable joins the connector. White sheathing indicates NM cable with 14-gauge conductors. Thus, it was not compatible with black and white TV sets in use across America. The heliograph was ideal for use in the American Southwest due to its clear air and mountainous terrain on which stations could be located. It was found necessary to lengthen the morse dash (which is much shorter in American Morse code than in the modern International Morse code) to aid differentiating from the morse dot. Wigwag was used extensively during the American Civil War where it filled a gap left by the electrical telegraph. Miles' enemies used smoke signals and flashes of sunlight from metal, but lacked a sophisticated telegraph code.

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