Fig. 291. Cord Circuit with Double Lamp Signals
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In Fig. 291 is shown a cord circuit, adaptable to magneto switchboards, provided with double lamp signals instead of clearing-out drops. Two high-wound locking relays are bridged across the line, the cord strands being divided by 1-microfarad condensers. When the high-wound coil of either relay is energized by the magneto current from the subscriber's station, the relay pulls up and closes a locking circuit including a battery and a coil 2, the contact 3 of the locking relay, and also the contact 4 of a restoring key. This circuit may be traced from the ground through battery, coil 2, contact 3 controlled by the relay, and contact 4 controlled by the restoring key, and back to ground. In multiple with the locking coil 2 is the lamp, which is illuminated, therefore, whenever the locking circuit is closed. Pressure on the restoring key breaks the locking circuit of either of the lamps, thereby putting out the lamp and at the same time restoring the locking relay to its normal position.

Lamps vs. Drops in Cord Circuits. So much has been said and written about the advantages of incandescent lamps as signals in switchboards and about the merits of the common-battery method of supplying current to the subscribers, that there has been a tendency for people in charge of the operation of small exchanges to substitute the lamp for the drop in a magneto switchboard in order to give the general appearance of common-battery operations. There has also been a tendency to employ the common-battery system of operation in many places where magneto service should have been used, a mistake which has now been realized and corrected. In places where the simple magneto switchboard is the thing to use, the simpler it is the better, and the employment of locking relays and lamp signals and the complications which they carry with them, is not warranted.

Switchboard Assembly. The assembly of all the parts of a simple magneto switchboard into a complete whole deserves final consideration. The structure in which the various parts are mounted, referred to as the cabinet, is usually of wood.

Functions of Cabinet. The purpose of the cabinet is not only to form a support for the various pieces of apparatus but also to protect them from dust and mechanical injury, and to hold those parts that must be manipulated by the operator in such relation that they may be most convenient for use, and thus best adapted for carrying out their various functions. Other points to be provided for in the design of the cabinet and the arrangement of the various parts within are: that all the apparatus that is in any way liable to get out of order may be readily accessible for inspection and repairs; and that provision shall be made whereby the wiring of these various pieces of apparatus may be done in a systematic and simple way so as to minimize the danger of crossed, grounded, or open circuits, and so as to provide for ready repair in case any of these injuries do occur.

Wall-Type Switchboards. The simplest form of switchboard is that for serving small communities in rural districts. Ordinarily the telephone industry in such a community begins by a group of farmers along a certain road building a line connecting the houses of several of them and installing their own instruments. This line is liable to be extended to some store at the village or settlement, thus affording communication between these farmers and the center of their community. Later on those residing on other roads do the same thing and connect their lines to the same store or central point. Then it is that some form of switchboard is established, and perhaps the storekeeper's daughter or wife is paid a small fee for attendance.

Fig. 292. Wall Switchboard with Telephone
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