The monocord switchboard may be used with either a magneto telephone, camp telephone, field telephone Model 1917, buzzerphone or service buzzer. The operator’s telephone set is not furnished as a part of the switchboard and a separate telephone set of one type previously mentioned must be supplied for this purpose.
Switchboard Frame.—The switchboard frame is made of hard wood, varnished in order to make it moisture proof. Its function is to hold the various units together and to protect them from dust and mechanical injury. In the back of the frame there are three horizontal brass bars extending the width of the board. In addition to providing a mechanical support for the various units, the top bar serves as a common ground connection and the middle and bottom bars as common night bell and battery connections for all units. Three Fahnestock clips on American made boards and binding posts on French made boards are installed at the top of the frame and four at the bottom. On the French made boards the upper three binding posts are marked S1, S2 and T, and are used for connecting by independent wires, respectively, the night bell and ground. The two posts at the bottom of the frame marked ZS and CS are used for connecting the two poles of the night bell battery. To the other two marked P1 and P2 are connected the operators telephone and the operators plug. The terminals at the top of the American made boards are marked A, A1 and G, corresponding with the French S1, S2 and T, and those at the bottom are marked B, B1, L1 and L2, corresponding to ZS, CS, P1 and P2, respectively.
If two or more multi-unit boards are connected in parallel for operation at one exchange, the interconnections between boards in order to use one ground, one night bell and one battery for the whole exchange are made. The corresponding binding posts at the top and bottom of the frames are simply connected to each other, as S1 and S2 of No. 1 board to S1 and S2, respectively, of No. 2 board.
Unit Panel.—Each unit comprises all the necessary apparatus for the exchange terminals of one line (two wires). The various parts are listed below according to their position on the panel from top to bottom:
- Two connecting blocks for the line wires.
- Lightning arrester between these blocks.
- Two fuses.
- Line number plate.
- Line drop signal and night bell contact.
- Jack.
- Cord and line plug.
Each unit is held in the frame by means of two machine screws, one at the top and one at the bottom of the unit, which engages the brass bars in back of the board and hold the unit firmly in place. Rigidity of construction is essential, as constant use and transferring from one place to another tend to loosen the units from the frame and disturb the sensitiveness of the adjustment of the line drop.
Lightning Arrester.—The lightning arrester is a simple toothed washer held against the panel by the supporting screw. This screw is grounded through the upper brass bar. If lightning comes in on the line wires, the main portion of it jumps the small air gap from the binding post to the grounded washer and thence passes to the ground, so that only a small portion of the high frequency current flows through the switchboard apparatus. If the latter portion is at all heavy, the fuses burn out, opening the circuit through the switchboard with a much wider gap, and hence higher resistance than that in the circuit to ground across the arrester.
Line Fuses.—Two glass enclosed removable fuses, which fit into spring connections, are provided to protect each side of the line from excessive currents. In order that burned out fuses may be readily seen, the panel is painted white behind them. The glass is usually smoked up more or less when a fuse burns out and this against the white background is easily noticed.
Line Number Plate.—The line number plate is a small white celluloid strip on which the line number is written. This number may easily be erased if it is necessary to change it.
Line Drop Signal.—The line drop signal consists of a shutter held normally in a vertical position by a brass trip latch. This trip latch is attached to the armature of a small electromagnet which is normally connected across the line through the anvil and the jack tip contact spring of the switchboard jack. When the coils of the magnet are energized by a current sent over the wire from a calling station, the armature and trip latch are lifted, thereby releasing the shutter, which falls by gravity to a horizontal position and attracts the attention of the operator. The electromagnet is adjusted to operate on a very small current. For locking the shutter in the vertical position and protecting it from mechanical injury during transport, a flat spring lever is provided which may be turned up from a pivot at one end to press against the shutter.