Switchboards.—If its use had been re stricted to isolated lines only, the telephone would undoubtedly have remained scarcely more than an interesting scientific curiosity. With the development of the instruments themselves, the necessity for connecting one telephone sta tion with another, by means of some switching device, at once became apparent.
The first interconnection of telephone lines by means of a switchboard took place at Bos ton in 1877. A few burglar-alarm lines were equipped at the station ends with box telephones. At the burglar-alarm office, each line was con nected to a small metal block. These blocks were so arranged, in relation to other metal being mounted on a wooden board. Each line was connected through the pivot, with one of the rotatable metal strips. Every other line was connected with, a metal button, these buttons being so placed, around the circumference of a circle, that, by rotating the strip, connection could be made with any desired button and line.
Shortly following this, many different types of switchboards were made employing either plugs or button switches in various arrange ments for making the connections. Fig. 37 shows one of these types.
An important step was taken when the flexi ble connecting cord, terminating at each end in a plug, adapted to connect with any line, was adopted as a substitute for the hand switch described above. Fig. 38 shows an early type of board employing connecting cords. In this style of board each subscriber's line was pro blocks placed closely beside them, that metal plugs could he employed to connect the blocks together in various ways, thus permitting the lines to be interconnected as desired. (Fig. 35).
The first commercial switchboard was in vided with an electromagnetic annunciator the shutter or index of which moved, when a sub scriber operated the magneto-generator at his station, thus attracting the operator's attention to the fact that a connection was desired. Each subscriber's line was also connected at the switchboard to a device known as a °spring jack?' The insertion of one plug of a con necting cord into the spring-jack associated with the line of the calling subscriber and the plug at the other end of the same cord into the jack of the line of the called subscriber, placed these subscribers in connection so that they could talk with each other. The cords were provided with listening and ringing keys. The former enabled the operator to communicate with the calling subscriber to ascertain the line with which connection was desired. The latter per mitted the operator to send alternating current over the called line for the purpose of ringing the called subscriber's bell. An electromagnetic
disconnect annunciator in each cord circuit enabled either subscriber, by operating his mag neto-generator, to signal to the operator that the conversation had ended.
as the °transfer° board. As the number of lines grew, it gave less satisfactory service and it was seen that, unless something better was developed, the switchboard would be a serious limitation on the growth of the business. The solution was found by multiplying the points of access to each subscriber's line so that a means of access (spring-jack) to each sub scriber's line in the office appeared in the face of this switchboard within reac.h of every operator, thus obviating the necessity for trunking calls between operators in the same office. The switchboard in which this °mul tiple° principle was first used was known as the °series multiple° board. The invention of the multiple was made by L. B. Firman. A simplified diagram of the wiring of this board is shown in Fig. 39, which also illustrates the construction of the spring-jack. In this ar Switchboards of this general type were used until the number of lines terminating in a telephone central office had grown so large as to exceed the ability of one operator to com plete all of the connections. This situation was met by placing two or more switchboards side by side, the operators completing the connec tions by reaching across, with their flexible cords, from one board to another. As the number of sections of switchboard placed side by side increased still further, the distance to which an operator could reach was exceeded. The next step was to join the sections of switchboard by metal strips or wires. An operator could then connect a calling sub scriber with . a certain strip and request another operator, more conveniently situated, to .connect the called subscriber to the same strip. These strips constituted the first use of the so-called °trunk line° between switch boards. The practice led to much confusion, due to the operators calling out the desired numbers, and resulted in the development of means enabling the operators to communicate along the switchboard by telephoning one another. This type of switchboard was known rangement each subscriber's line was carried through the board with branching taps to the ring (outer contact) of the jack associated with the line at each operator's position. The other side of the line was carried through the board in series with the cut-off spring contact of each jack associated with the line. The ob ject of the spring contact was to disconnect the line signal from the line while talking was going on.