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The Private Branch Exchange.—This is a small · central office differing from the main telephone exchange only in being smaller. It is intended to relieve the latter from calls not extending outside the building in which it is located, and to effect a saving in the amount of wire required for an installation. Instead of each station in a building being connected with the main telephone exchange, it would in the present case be onnected with the private branch switchboard, and those calls not extending outside the building would be handled on this board entirely irrespective of the main exchange. By means of a few trunk lines running from the branch switchboard to the main switchboard, calls to parties outside the building can be put through by operators stationed at these boards. The use of a few trunk lines and short connecting wires to the different telephone sets, instead of individual wires extending the entire distance between the telephones and the main exchange, effects a considerable saving in copper. This kind of installation is especially adapted and widely used in office buildings, hotels, and factories. Owing to the similarity between a private branch exchange and a small central exchange such as will next be considered, no separate description and discussion of the former will be necessary.
A Small Central Exchange.-By a small central exchange is here meant one whose capacity does not exceed 200 stations. The line conductors leading into such an exchange may be either open
wires or cables, and as a rule complete metallic circuits must be provided for.
Protection to the exchange apparatus is usually similar to that afforded the telephone set in the user's premises. This, it will be remembered, comprises a fuse, a heat coil, and a carbon lightning arrester for each side of the line. A terminal head like that shown in Fig. 95, but fitted with fuses for each line wire serves well for protection from abnormal currents if utilized on the last pole of the line, while on the main distributing board in the exchange may be mounted the carbon lightning arresters and the heat coils for intercepting sneak currents. The main distributing board's chief function, however, is to provide fixed terminals for the line conductors and fixed terminals for the wires running to the exchange switchboard, so that these two sections of wiring will be entirely independent of each other and either one can be changed without necessitating a change in the other. It is then possible to shift any outside circuit to any switchboard circuit by simply changing the “jumper” wires connecting them together.
To facilitate this work, the terminals are numbered as in the distributing board shown in Fig. 113. In the case of open wire lines the terminals are numbered to correspond to the insulator pins carrying the conductors connected to the terminals. The pins in a complete metallic installation are numbered as follows: standing with the back to the exchange and facing the direction of the
pole line, the two points at the extreme left of the top cross-arm are each No. 1, the next two pins No. 2, and so on toward the right, continuing with the left-hand pins on the second cross-arm in the same manner. The distribution board, Fig. 113, is of hard wood, fitted with fuses s and carbon lightning arresters a, but carries no heat coils. It is, however, fitted at c for plug tests whereby grounds, short circuits, crossed wires and open lines may be determined. A testing wire is readily plugged in on any circuit as shown at n. The board is mounted on the wall or any flat surface near the switchboard, and is connected with the switchboard
Fig. 113.—Distributing Board Fitted with Protective
Apparatus and Test Plugs
apparatus by No. 22 B. & S. gage copper wires, tinned and formed into cables of 26 pairs each. The wires are cach covered with a layer of silk and a layer of cotton thread placed one above the other. The cotton thread on one conductor
of a pair is white and on its mate is colored, so as to distinguish the one from the other. After being paraffined, the insulated conductors are bound
together with cotton braid saturated in powdered soapstone and painted to exclude moisture.
The Exchange Switchboard for 100-line circuits or stations is shown in Fig. 114. It consists of
the following parts: the line drops a a, i for each line circuit, by means of which the switchboard operator is signaled; the clearing-out drops cc, 1 for each of the 1o operator's circuits provided, by means of which the operator is notified when a conversation between two parties connected through the board is finished; the spring jacks e e, which are numbered to correspond to the line drops to which they are wired and are connected one to each line circuit; the plugs s s, which, being the terminals of the operator's 10 circuits, are therefore 20 in number and which when inserted in the jacks connect together the line circuits wired to them; the ringing and listening keys k k, 1 for each of the operator's 1o circuits, and which when depressed forward connect the magneto generator at g with the station plugged, for signaling the party there, and when depressed backward connect the operator's receiver p in circuit for enabling her to determine from the signaling party the station desired and to listen in on the conversation and learn if the proper parties are connected; and the operator's iransmitter t, which is adjustable as to height, for enabling the operator to speak with the parties connected, when necessary. Telephone switchboards up to 200 stations capacity are built along the same lines as the one in Fig. 114.
Each Line Circuit terminates in the switchboard as shown in Fig. 115, t'and t being the line wires, d the distributing board, S the spring jack, and A the line drop. One side of the line circuit is