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All the telegraph wires which enter an office are grouped in a switchboard which also accommodates conduc tors to all the telegraph apparatus in the office. This switch board (Plate I. fig. 2) is usually a collection of jacks arranged in groups or singly. Each jack or group of jacks is connected to one line wire or one operating set and the two are joined by plugs and cords. Line wires and wires to operating sets do not termi nate directly on the switchboard, but instead go to a distributing frame (Plate I. fig. I) placed immediately behind the switch board. The distributing frame has connecting blocks in vertical rows on one side and in horizontal rows on the other. On the vertical side are all wires from the outside and from the operating sets. The horizontal side has permanent connection with all the jacks of the switchboard. The two sides, vertical and horizontal, are joined by cross-connection wires which may be rapidly changed if required. Normally, each particular wire is assigned to some particular set. If a temporary change is necessary owing to the failure of a set or a wire it is made by means of the plugs and jacks on the switchboard. If the change is to be permanent it is made on the distributing frame by moving the cross-connect ing wire. On the front of the switchboard is a shelf which is equipped with the necessary apparatus for testing wires and locat ing faults, consisting of a Morse set and plugs and a volt-milam meter for the former and a Wheatstone bridge set for the latter. Records are kept at the switchboard of all the outside lines con nected to it, their length, the route they follow and the material of which they are made. Periodical measurements are made of the resistance of each line and recorded on a special form. If a fault develops in any circuit the attendant can determine at once if it lies inside the office, at the switchboard or in the set, or outside on the line. If the latter, he can discover by means of the volt-milammeter the nature of the fault (open, cross or ground, etc.) and with the Wheatstone bridge the location of the fault.
Telegraph operation is almost always carried on by means of an earth return, i.e., one wire only runs between the two points and the electrical current returns by way of the earth. This feature makes it imperative that each telegraph office be provided with a good earth or ground. The requirements are that the resistance shall be low—for the larger offices not• more than i ohm—and the connection as direct as possible. A public water-supply system, with its large quantity of metal piping laid in the soil, offers the best ground and is always used where available. Gas pipes are nearly as good but their use is attended with the risk of a spark igniting an escape of gas. Where neither water nor gas are attainable made grounds must be constructed by sinking iron pipes or metal in the earth. Made grounds are never as satisfactory as water or gas pipe grounds owing to the smaller areas of contact. Connection is made to the ground by means of large size rubber-covered wire. run without splices, kinks, coils or sharp bends. These last con siderations are important from the point of view of the protec tion of the office from lightning and other electrical currents that may come in contact with the wires and cables entering the building. Before being carried into the switchboard each wire
is provided with an arrester and fuse. The arrester consists of two small carbon blocks separated from each other by only a few thousandths of an inch of air, one side being connected to the line and the other to earth. Fuses are placed in the line to prevent a foreign current, which may have been picked up by the outside wire or cable, from damaging the instruments in the office.
The arrester limits the voltage that can reach the wiring or appa ratus by diverting to earth potentials that would injure the apparatus or wiring or the employees.
In any central office means must be provided for carrying messages between different parts of the room. A message received on a wire in one corner may be sent out again on a wire in another corner and a rapid method of transference is necessary. The messages are conveyed mechani cally from the operating tables to a distributing centre, where they are sorted and sent out on other conveyors to different sec tions of the room. Many types of conveyor are in general use, moving belts being the most common. The main considerations are speed and accessibility, with all features eliminated that would tend to lose, mutilate or delay a message. Belts are run at speeds of from 200 to 250 ft. per minute. Other types of conveyors have been developed with speeds as high as soo ft. per minute. The distributing centre is the focal point of the operating room and is placed in a central position. Pneumatic tubes (q.v.) are also widely used both for carrying business within the confines of an office and for connection with other offices near by.
The conductors necessary to establish connections between various telegraph offices consist either of bare wires supported overhead on poles, or of groups of insulated wires held together by a common covering or sheath, to form cables. Cables may be carried overhead on poles or they may be laid underground. Overhead lines constitute the larger portion of the plant used to connect offices in one town with those in another town, although the use of cable is increasing rapidly. Connections between offices in the same town, and between offices and patrons, generally require the use of under ground cable, since space is usually not available in towns for overhead wires or cables.
In general, bare wires consist of hard-drawn copper, although galvanized iron wire has been used quite extensively, and bronze wire is now being tried out with considerable success. The advan tage of copper for line conductors is its high conductivity. Iron wire, while cheaper than copper, corrodes more rapidly except in dry climates and is not suitable for long or high speed circuits, because of its high resistance. Bronze wire is stronger than cop per and less subject to mechanical injury, but its resistance is somewhat greater and it is slightly more expensive than copper. The copper wire ordinarily used for overhead telegraph lines has a diameter of 0.114 in. and weighs 210 lb. to the mile. Other sizes of copper wire are employed to a limited extent, the largest having a diameter of 0.165 inch. The most common size of iron wire used to-day has a diameter of 0.165 in. and weighs 38o lb. to the mile.