.

The term _simple_ as applied in the headings of this and the next
chapter, is employed to designate switchboards adapted for so small a
number of lines that they may be served by a single or a very small
group of operators; each line is provided with but a single connection
terminal and all of them, without special provision, are placed
directly within the reach of the operator, or operators if there are
more than one. This distinction will be more apparent under the
discussion of transfer and multiple switchboards.

Mode of Operation. The cycle of operation of any simple manual
switchboard may be briefly outlined as follows: The subscriber desiring
a connection transmits a signal to the central office, the operator
seeing the signal makes connection with the calling line and places
herself in telephonic communication with the calling subscriber to
receive his orders; the operator then completes the connection with the
line of the called subscriber and sends ringing current out on that
line so as to ring the bell of that subscriber; the two subscribers
then converse over the connected lines and when the conversation is
finished either one or both of them may send a signal to the central
office for disconnection, this signal being called a clearing-out
signal; upon receipt of the clearing-out signal, the operator
disconnects the two lines and restores all of the central-office
apparatus involved in the connection to its normal position.

Component Parts. Before considering further the operation of manual
switchboards it will be well to refer briefly to the component pieces
of apparatus which go to make up a switchboard.

_Line Signal._ The line signal in magneto switchboards is practically
always in the form of an electromagnetic annunciator or drop. It
consists in an electromagnet adapted to be included in the line
circuit, its armature controlling a latch, which serves to hold the
drop or shutter or target in its raised position when the magnet is not
energized, and to release the drop or shutter or target so as to permit
the display of the signal when the magnet is energized. The symbolic
representation of such an electromagnetic drop is shown in Fig. 233.

[Illustration: Fig. 233. Drop Symbol]

_Jacks and Plugs._ Each line is also provided with a connection
terminal in the form of a switch socket. This assumes many forms, but
always consists in a cylindrical opening behind which are arranged one
or more spr