The local circuit, or locking circuit, which included the lamp was carried through a pair of contacts in the corresponding jacks so arranged that when the plug was inserted in answer to the call, this locking lamp circuit would be open, thereby extinguishing the lamp and also unlocking the relay.  There seems to be absolutely no good reason why lamp signals should be substituted for mechanical drops in magneto switchboards.  There is no need for the economy in space which the lamp signal affords, and the complications brought in by the locking relays, and the requirements for maintaining a local battery suitable for energizing the lamps are not warranted for ordinary cases.

[Illustration:  Fig. 291.  Cord Circuit with Double Lamp Signals]

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.