Telephone Plant in the United States

For many years batteries were required at each subscriber's station to furnish the current for actuating the transmitter. An important forward step was the development of the common, or centralized battery, system of operation in which a storage battery at the central office overcame the necessity for local batteries at the individual substations. The common battery system also provided a way by which the subscriber could signal the operator merely by removing the receiver from its switch hook, thereby lighting a tiny electric lamp associated with his line at the switchboaid. Before this system was devised and per fected, the subscriber signaled the central office by turning a crank that caused a small electrical generator to send a current over his line which operated an electromechanical annunciator or "drop" at the switchboard. Improved forms of the latter sys tem, known as the "magneto," are still advantageous for use in small communities.

Buildings.—While, in small communities, telephone central of fice equipment is frequently installed in rented quarters, in the larger places the requirements of the service usually make desir able the erection of buildings specially adapted for telephone pur poses. The headquarters buildings, located in the larger cities, generally house not only the central administrative forces of the telephone company, but of ten several central office switchboards.

Telephone Lines.

Telephone lines connect (I) each tele phone to a central office; (2) each central office in a city to every other central office in that city; and (3) all cities, towns, villages and community centres, by means of a vast network of wires. These wire lines are of two forms—cable and open-wire. Cable consists of insulated copper wires, twisted in pairs and assembled into a core which is covered with an envelope of lead alloy. About 92 per cent. of the telephone wire mileage in the United States is in cable placed either underground or suspended aeri ally by means of steel rope attached to poles. Open-wire lines consist of bare wires, generally of copper, attached to glass in sulators supported on pins in crossarms attached to the tops of poles. The lines in a city area may be divided into two classes: (I) Subscribers' lines connecting the subscribers' stations to a central office and (2) trunk lines which connect the offices together. In a typical city installation, a rubber covered pair of wires, known as the "drop" wire, runs from the subscriber's premises to a cable terminal. In densely built blocks the con nection is frequently made by wires attached to the walls of buildings. From the cable terminal, the line is continued toward the central office in a small cable, often containing 25 or so pairs of wires and known as "block" cable, which connects with the larger "feeder" cables that run as directly as practicable to a cen tral office. In densely built areas the feeder cables are generally placed in underground ducts; elsewhere they are placed overhead.

As the result of the cable development work described in a later section, telephone cables containing many hundreds of pairs of wires are now available and are employed in the more con gested districts. Plate I., fig. 8 shows a cable, of the type used for main feeder routes, containing 1,818 pairs of 26-gauge wires. The installation of the subscribers' cable distribution plant, consisting of block cables, and main and subsidiary feeder cables is preceded by a careful engineering study of the present and probable future demand for telephone service in order that the plant may be so planned as to enable these demands to be adequately met.

Trunk cables usually contain larger wires than subscribers' cables. In large metropolitan districts, where there are many central offices, the number of trunk circuits required is very large; sometimes as high as one trunk circuit for each eight subscribers' lines. Trunk routes are usually "double-tracked," i.e., two groups of circuits (one for traffic in each direction) are ordinarily re quired between each pair of central offices, except where tandem or intermediate switching is employed. To provide adequate transmission efficiency in the case of the longer trunk circuits, it is frequently economical to "load" them, as described in a subsequent section. The loading coils are contained within large iron cases. The cable pairs used for subscribers' line distribution and inter-office trunking form the great bulk of the 87 million miles of wire used for telephone purposes in the United States. Toll and Long Distance Lines.—Until comparatively re cently, the toll and long distance plant was composed largely of open-wire lines, the largest size of hard drawn copper wire em ployed being 165 mils in diameter, weighing 435 pounds to the mile. Within recent years, radical improvements in the telephonic art have permitted the use of long intercity cables in increasing amounts until, at the present time, fully two-thirds of the toll and long distance wire mileage is in cables which cover important backbone routes. Figure No. 2 shows the present routes in north eastern United States. One of the first of these long inter-city cables was placed in service in 1906, between New York City and Philadelphia. In 1913 an underground cable was provided between Boston and Washington to care for the rapidly increasing traffic and to insure service under such weather conditions as might affect open-wire lines adversely. This cable was equipped with loading coils. It provided a storm-proof route along the northern Atlantic coast. Many of the more recent long cables shown in fig. 2 have been of the aerial type except where passing through urban areas. They have combined the use of loading coils and telephone repeaters and, for the longer distances, so-called "four wire" circuits have been used in which a separate talking path is utilized for transmission in each direction.