(7) The vertical portion of the switchshelf carries the operator's telephone jack and the hand-generator crank. The
operator's headset (transmitter and receiver) is corded to a plug. When this plug is inserted in the telephone jack, the
headset is connected electrically into the switchboard circuits. Almost every switchboard is equipped with a hand
generator. This generator is used for ringing the bells at the stations when the ringing power supply fails or when no other
ringing power is available. The hand generator is operated by rotating the hand-generator crank in a clockwise direction.
Operating a ringing switch while turning the hand generator crank connects ringing power to the associated cord.
Access to parts of the switchboard apparatus and wiring is gained by removing a panel from the back
of the board. Generally, this panel is removed by lifting upward and outward on the panel. Access to the switches shown
on the switchshelf, operator's telephone jack, hand generator, and associated wiring is obtained by unlocking and raising
the switchshelf. A special switchboard key is necessary to unlock the switchshelf.
Operation of Common-Battery Switchboard.
To make clear the main function of the switchboard in relation to the telephone lines and stations connected to
it, a brief over-all picture of the operation of a common-battery switchboard is given in this paragraph. A number of
important details, omitted here for simplicity, are described fully in later paragraphs. The present discussion covers a cord
circuit and the telephone stations which it interconnects through the switchboard.
a. Cord circuit.
(1) Figure 40 is a simplified diagram of a cord circuit in a common-battery switchboard. This circuit consists of
two cords, an answer cord and a call cord, each of which contains three conductors: tip (T), ring (R), and sleeve (S). The
tip conductors of the two cords are connected, and their junction is connected to the positive (grounded) battery bus
through a filter network (par. 46 and fig. 38).