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The Metallic Telegraph SystemThere is a growing tendency among communication companies to place important circuits in cable whenever it is practicable to do so. The metallic telegraph system was designed especially to provide telegraph service over such cable circuits. These telegraph channels are often provided by superimposing the telegraph messages on the cable pairs already carrying telephone speech currents. The composite arrangement is common. The cable conductors are very close together, are loaded, and are provided with repeaters (page 399). Interference from the superposed telegraph would be excessive if the currents and voltages used for telegraphing were not kept far below the values used for ordinary telegraphy.43 Line currents of 4 or 5 milliamperes are used in typical installations, these being of the same order of magnitude as the telephone currents, but of lower frequency.
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Last Update: 2011-05-30