Plate I., fig. 2, shows a cross-section of a type of receiver now widely used in the United States. Among the improvements em bodied in it are a bipolar magnet with two pole-pieces and wind ings, welded construction, dust-proof case, special kinds of steel, accurate spacing of diaphragm and pole-pieces and, throughout, the most effective proportioning of its parts. These features collectively have resulted in increasing its efficiency and the naturalness of its reproduction of the voice.
The Transmitter—The transmitters first used were of the magneto type, similar to receivers, but Bell himself early pointed the way to improvements by his liquid transmitter which con tained the principle of the variation of the electrical resistance of the transmitting element under the control of the vibrations of the voice. Important improvements in the transmitter came from the use of the principle of the microphone, employing vari able resistance contacts in a battery circuit. Following the Ber liner and Edison transmitters of 1877 based on this principle came the Blake transmitter, an improved type, introduced into the Bell System in 1878. These early instruments were super seded by the granular carbon type, growing out of an invention by Hunnings and employing a variable resistance element consisting of a small chamber filled with granules of carbon. The "solid back" type, an improved form of granular carbon transmitter and a prototype of modern instruments, was the invention of Anthony C. White. Scientific research has been continuously, and still is, directed toward the further improvement of transmitters. Plate I., fig. 1, shows a cross-section of a modern telephone transmitter.
Substation Wiring Plans.—In the case of business offices and residences, additional telephone facilities in great variety, be yond the scope of a single set, are frequently employed to meet the ever-increasing requirements of modern life. "Extension" telephone sets, connected with the substation line, enable calls to be originated or received from different points in the sub scriber's residence or place of business. By pressing a button, an extension telephone may be cut off when desired and, where several stations, and more than one line are provided, any one of the stations may be connected to any one of the lines. In some
cases, means are provided whereby a secretary may answer calls on either of two lines and, by suitable switch keys, perform vari ous operations such as holding a call on one line while the prin cipal is talking on another. Many combinations of master sta tions, control stations and extension bells in combination with one or more lines are available for the telephone user.
Private Branch Exchanges.—A "private branch exchange" is a switchboard, generally of small size, located at the subscriber's premises. To it are connected the substation sets of the sub scriber's establishment and it is connected with the central office switchboard by a sufficient number of trunk lines to handle the traffic. Plate I., fig. 3, shows a private branch exchange switchboard for a small number of stations and trunk lines. This is capable of being placed on a desk and operated by a clerk who may also perform other duties. With this equipment all of the operations of connecting and disconnecting the local stations with each other and with the central office are performed by means of switch keys. Larger private branch exchanges resemble small central office switchboards, and require the attendance of one or more operators. Private branch exchanges are also available which operate in a manner similar to the dial central offices to be described later.