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the latter out of engagement with a ratchet wheel

w when the switch hook is pulled down. The ratchet wheel w and the contact arms being mounted on the same shaft with a spiral spring, the action just noted permits the spring to return

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FIG. 112.-Details of the Automatic Attachment in the Wall Set Shown in Fig. III

the contact arm to its normal position. After the pawl p has been raised to a certain point, the dog slips out of the notch on it, permitting the pawl again to come in contact with the ratchet wheel w and be ready for the next use of the telephone. To prevent the pawl from engaging with the ratchet wheel before the contact arm s has reached its final position, the second dog g is provided; this is pressed by a spring so as to project under the pin. k, carried on the pawl p, and holds p away from the ratchet wheel until s has traveled to its normal position. Then a cam on the ratchet wheel forces the dog g away from the pin k and allows the pawl to drop into position.

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

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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

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FIG. 114.-Telephone Switchboard for a Small Exchange

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

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