r may move the
wiper arm of his selecting switch into connection with the line of any
other subscriber.

_The "Up-and-Around" Movement._ The elemental idea to be grasped by the
discussion so far, is the so-called "up-and-around" method of action of
the selecting switches employed in this system. This preliminary
discussion may be carried a step further by saying that the arrangement
is such that when a subscriber presses both his keys and grounds both of
the limbs of his line, such a condition is brought about as will cause
all holding pawls to be withdrawn from the shaft, and thus allow it to
return to its normal position with respect to both its vertical and
rotary movements. No attempt has been made in Fig. 380 to show how this
is accomplished.

=Function of Line Switch.= Such a system as has been briefly outlined in
the foregoing would require a separate selecting switch for each
subscriber's line and would be limited to use in exchanges having not
more than one hundred lines. In the modern system of the Automatic
Electric Company, the requirement that each subscriber shall have a
selective switch, individual to his own line, has been eliminated by
introducing what is called an _individual line switch_ by means of which
any one of a group of subscribers' lines, making a call, automatically
appropriates one of a smaller group of selecting switches and makes it
its own only while the connection exists.

=Subdivision of Subscribers' Lines.= The limitation as to the size of
the exchange has been overcome, without increasing the number of bank
contacts in any selecting switch, by dividing the subscribers' lines
into groups of one hundred and causing selecting switches automatically
to extend the calling subscriber's line first into a group of groups
corresponding, for instance, to the thousand containing the called
subscriber's line, and then into the particular group containing the
line, and lastly, to connect with the individual line in that group.

=Underlying Feature of Trunking System.= It will be remembered that in
the chapter on fundamental principles of automatic systems, it was
stated that the subscriber, by means of the signal transmitter at his
station, was made to govern the action of the central-office apparatus
in the selection of a proper group of trunks; and the group being
selected, the central-office apparatus was made to act automatically to
pick out and connect with the first idle trunk of such gr