ach trunk is chosen and connected with, conditions are
established, by means not unlike the busy test in multiple manual
switchboards, which will guard that trunk and its associated apparatus
against appropriation by any other line or apparatus as long as it is
held in use. Likewise, as soon as any subscriber's line is put into use,
either by virtue of a call being originated on it, or by virtue of its
being connected with as a called line, conditions are automatically
established which guard it against being connected with any other line
as long as it is busy. These guarding conditions of both trunks and
lines, as in the manual board, are established by making certain
contacts, associated with the trunks or lines, assume a certain
electrical condition when busy that is different from their electrical
condition when idle; but unlike the manual switchboard this different
electrical condition does not act to cause a click in any one's ear, but
rather to energize or de-energize certain electromagnets which will
establish or fail to establish the connection according to whether it is
proper or improper to do so.

_Local and Inter-Office Trunks._ The groups of trunks that are used in
building up connections between subscribers' lines may be local to the
central office, or they may extend between different offices. The action
of the two kinds of trunks, local or inter-office, is broadly the same.

CHAPTER XXIX

THE AUTOMATIC ELECTRIC COMPANY'S SYSTEM

Almost wherever automatic telephony is to be found--and its use is
extensive and rapidly growing--the so-called Strowger system is
employed. It is so named because it is the outgrowth of the work of
Almon B. Strowger, an early inventor in the automatic telephone art.
That the system should bear the name of Strowger, however, gives too
great prominence to his work and too little to that of the engineers of
the Automatic Electric Company under the leadership of Alexander E.
Keith.

=Principles of Selecting Switch.= The underlying features of this
automatic system have already been referred to in the abstract. A better
grasp of its principles may, however, be had by considering a concrete
example of its most important piece of apparatus--the selecting switch.
The bare skeleton of such a switch, sufficient only to illustrate the
salient point in its mode of operation, is shown in Fig. 380. The
essential elements of this are a vertical shaft capable of both
longitudinal