n one general plan of action and that is to so arrange the
circuits that whenever a line is switched at one section, such an
electrical condition will be established on the forward contacts of all
of its multiple jacks that any operator at any other section in
attempting to make a connection with that line will be notified of the
fact that it is already switched by an audible signal, which she will
receive in her head receiver. On the other hand the arrangement is such
that when a line is not busy, _i. e._, it is not switched at any of the
positions of the switchboard, the operator on attempting to make a
connection with such a line will receive no such guarding signal and
will, therefore, proceed with the connection.

We may liken a line in a multiple switchboard to a lane having a number
of gates giving access to it. One of these gates--the answering jack--is
for the exclusive use of the proprietor of that lane. All of the other
gates to the lane--the multiple jacks--are for affording means for the
public to enter. But whenever any person enters one of these gates, a
signal is automatically put up at all of the other gates forbidding any
other person to enter the lane as long as the first person is still
within.

[Illustration: Fig. 336. Principle of Multiple Switchboard]

=Diagram Showing Multiple Board Principle.= For those to whom the
foregoing description of the multiple board is not altogether clear, the
diagram of Fig. 336 may offer some assistance. Five subscribers' lines
are shown running through four sections of a switchboard. Each of these
lines is provided with a multiple jack on each section of the board.
Each line is also provided with an answering jack and a line signal on
one of the sections of the board. Thus the answering jacks and the line
signals of lines _1_ and _2_ are shown in Section I, that of line _4_ is
shown in Section II, that of line _3_ in Section III, and that of line
_5_ in Section IV. At Section I, line _1_ is shown in the condition of
having made a call and having had this call answered by the operator
inserting one of her plugs into its answering jack. In response to the
instructions given by the subscriber, the operator has inserted the
other plug of this same pair in the multiple jack of line _2_, thus
connecting these two lines for conversation. At Section III, line _3_ is
shown as having made a call, and the operator as having answered by
inserting one of her plugs into the ans