Central office switching equipment may be of the manually operated or of the dial type. To this equipment runs one line (or dinarily one pair of wires) from each telephone individually served. The manual switchboard must provide means, such as tiny electric lamps, one associated with each line, whereby the calling party can signal the operator when he removes his telephone from its switchhook to originate a call ; means such as a cord and plug and listening key whereby an operator can connect her telephone set momentarily to the calling party's line to receive the call; means such as the multiple and out-going trunk jacks to enable the operator to connect the calling party's line with the line of the person with whom he wishes to talk, whether that line be connected to the same switchboard as is his own, or to some other switchboard in the same city, or whether it must be reached over a trunk line to some suburban point or over a "toll" or "Long Distance" line extending to some remote place; means for determining whether the line called for is already in use, or "busy"; means for ringing the bell at the called station and, finally, when signals indicate that the conversation is ended, means for restoring all circuits to readiness for another call.
The general functions of the dial system are similar to those of the manually operated system, the electro-mechanisms of the former, however, being so contrived that most operations, in stead of being performed by human operators, are completed by means of switches set in operation and controlled by electrical impulses which are sent over the line from substation dial.
There are certain liniits to the number of telephones which can be given service advantageously on one switchboard. One of these is the economic limit which indicates that, in order to seep down the length and cost of the lines connecting the telephones with the central office, the subscribers' stations in certain con veniently sized areas should be grouped to one central office in each area and the several central offices interconnected by trunk lines. The locations for the central offices, the boundaries of their areas, the type of switchboard to employ and the character and number of the trunk lines are all matters requiring careful study and intricate planning. To reach the smaller central offices, in outlying sections of a large city, from central offices in a distant portion of the city, it is sometimes desirable to use two or more trunk lines connected in "tandem" by the various operators at the points between the two offices.
Historical.—The "first telephone switchboard was put into serv ice in New Haven, Connecticut, on January 28, 1878, and pro vided primitive means for accomplishing the fundamental opera tions of interconnecting substation lines. It served 21 stations on eight grounded lines. In contrast, Plate I., fig. 6 shows a large
manually operated switchboard in use in 1928. Such switchboards as this are capable of serving perhaps 20,000 telephones on ro,000 lines and may require as many as 120 operators on duty at the busiest period of the day. One of these large switchboards takes over a year to build and install. It will frequently contain as much as 4,00o miles of internal wiring and as many as two mil lion soldered connections. Plate II., fig. 3 is a rear view of a section of this kind of switchboard and Plate II., fig. 5 shows the "main" frame by means of which the subscribers' lines are connected with the switchboard.
The electrical circuit of the manually operated common battery switchboard is shown diagrammatically in fig. 1, its operation being explained in the text below the figure. Switchboards employ ing electrical mechanism, controlled by dials at the subscribers' stations, to replace human operators as far as practicable, have come into wide use both in the United States and elsewhere. This "dial" apparatus is the subject of a subsequent section. The "multiple" principle was an important step in the evolution of the modern type of switchboard. It is employed in dial as well as in • manually operated switchboards. As the telephone business grew, the number of subscribers which a single operator could serve was soon exceeded. For a time the situation was met by providing trunk circuits from one operator to another, but this reduced the speed of service. The invention of the multiple principle relieved this condition by permitting any one of the operators at a switchboard to establish a connection directly be tween any subscriber whose line she served and any other sub scriber connected with the switchboard.
In the multiple system, a "jack" or switch for every line is placed within the reach of each operator at the switchboard. In practice this means one multiple jack per line for every three adjacent operators. With the provision of this increased facility for interconnecting subscribers' lines, it became necessary for the operator to know whether or not a line, with which connection was desired, was already in use through a connection made at some remote part of the switchboard. The method and means for determining the condition of the line is known as the "busy test." Its invention was a necessary accompaniment to the use of the multiple principle. To accomplish the busy test the operator touches the "sleeve" of the jack with the "tip" of a connecting plug. If the line is busy, a click is heard in the operator's receiver. Plate II., fig. 3, shows the rear of a multiple switchboard ; the upper part of the cabling is that used to carry the "multiple" of subscribers' lines through the switchboard.