TELEPHONE PLANT IN THE UNITED STATES The term "telephone plant" includes (I) the telephone appara tus and wiring at the subscribers' premises; (2) the central office switching equipment (with the buildings that contain it) for inter connecting subscribers' lines; and (3) the aerial and underground wires and cables with their pole lines and conduits, which connect the subscribers' stations with the central offices and the latter with each other, whether they be in the same city or in different cities. This plant makes it possible, at the present time, for any user of the telephone service to be connected promptly with any other station of the telephone system, and to converse easily, by electrical means, with the person called, after the connection is established, regardless of distance. The systems which enable this nation-wide service to be rendered are necessarily complex and intricate and they include a multitude of auxiliary devices and appurtenances. The following brief description deals only with the principal kinds of telephone plant.
Substation Sets.—There are three types of telephone sets in general use: (I) the "wall set" (Plate I., fig. 4) adapted for at tachment, at convenient height, on the wall of a room ; (2) the "desk set" consisting of a pedestal, rising from a substantial base, and supporting the transmitter, and the receiver hung on a switchhook; (3) the "hand set" (Plate I., fig. 5) in which a transmitter and a receiver, of suitable design, are attached at opposite ends of a handle. This unit normally rests on a cradle surmounting a base. Both the "desk" and the "hand" sets are provided with flexible connecting cords so that they are movable within the limits fixed by the length of the cord. With them
is used a bell box, usually mounted beneath a desk or in some other inconspicuous place, containing the call bell, induction coil and other auxiliary apparatus. For carrying on the telephonic conversation, two instruments are required, the transmitter and the receiver. The former converts the speech waves in the air into their electrical replicas on the wire ; the latter performs the reverse operation of converting electrical speech waves into sound waves in the air.
At first, a single instrument, placed alternately to the mouth and the ear, was used for talking and listening. Shortly, however, it was found desirable to provide each subscriber with two identical instruments, one (the receiver) to be used for listening, the other (the transmitter) to be used for talking. Bell's first instruments required a battery in the circuit, but the instruments that were first employed commercially had permanent magnets and the speech currents were generated by the motion of the diaphragm.
The Receiver—Although the basic scientific principle of the modern telephone receiver follows the original invention of Bell, the structure and design of the instrument have been changed in every essential feature and its efficiency has been greatly in creased as the result of continuous experimentation and develop ment work. An early commercial type of receiver was known, from its shape, as the "butter stamp." To the end of a perma nent magnet was attached a soft iron pole-piece on which was wound a coil of insulated, fine wire. A circular diaphragm of iron, supported by the case or shell of the instrument, was placed with its center close to the end of the pole-piece, but not in con tact with it. Speech currents, passing through the winding, varied the attraction of the permanent magnet for the diaphragm, caus ing the latter to vibrate and produce sound waves in the air corresponding to the speech currents.