switch, a saving of about ninety per cent of the first selectors
was effected, since the number of first selectors was thereby reduced
from a number equal to the number of lines in a group to a number equal
to the number of simultaneous connections resulting from calls
originating in that group. In other words, by the line switch, the
number of first selectors is determined by the traffic rather than by
the number of lines.

=Scheme of Trunking.= With this understanding as to the names and
broader functions of the things involved, Fig. 381 may now be
understood. The line switch of the single line, as indicated here, has
only the power of selection among three trunks, but it is to be
understood that in actual practice, it would have access to a greater
number, usually ten. So, also, throughout this diagram we have shown the
apparatus and trunks arranged in groups of three instead of in groups of
ten, only the first three thousands groups being indicated and the first
three hundreds groups in each thousand. Again only three levels instead
of ten are indicated for each selecting switch, it being understood that
in the diagram the various levels are represented by concentric arcs of
circles, and the trunk contacts by dots on these arcs.

_Line-Switch Action._ When the subscriber, whose line is shown at the
bottom of the figure, begins to make a call, the line switch acts to
connect his line with one of the first selector trunks available to it.
This selection is entirely preliminary and, except to start it, is in no
way under the control of the calling subscriber. The calling line now
has under its control a first selector which, for the time being,
becomes individual to it. Let it be assumed that the line switch found
the first of the first selector trunks already appropriated by some
other switch, but that the second one of these trunks was found idle.
This trunk being appropriated by the line switch places the center one
of the first selectors shown under the control of the subscriber's line.
This first selector then acts in response to the first set of selective
impulses sent out by his signal transmitter.

[Illustration: DEAN HARMONIC CONVERTER Dry Cell Type for Magneto
Exchange. _The Dean Electric Co._]

[Illustration: Fig. 381. Scheme of Trunking]

_First Selector Action._ We will assume that the calling subscriber
desires to connect with No. 3213. The first movement of the subscriber's
signal transmitter will