, and that in
dotted lines the space that will be required by the expected future
equipment.

In Fig. 430 is shown a group of five line-switch units, representing a
total of five hundred lines. The length of such a unit is practically
fourteen feet and the breadth over all about twenty-two inches.

Fig. 431 shows a general view of this Lansing office, taken from a point
of view indicated at _A_ on the floor plan of Fig. 429. Fig. 432 shows
the main distributing frame, which is of ordinary type; Fig. 433 shows a
closer view of some of the primary line switches; Fig. 434 is a view of
the secondary line switches and first selectors, the latter being on the
right; Fig. 435 is a view of the frequency selectors and second
selectors, the former being used in connection with party-line work; and
Fig. 436 is a view of the toll distributing frame and harmonic
converters for party-line ringing.

A general view of the main switching room in the Grant Avenue office of
the Home Telephone Company of San Francisco is given in Fig. 437, this
being taken before the work of installation had been fully completed.
The present capacity of the equipment is 6,000 and the ultimate 12,000
lines. This office is one of a number of similar ones recently installed
for the Home Telephone Company in San Francisco, the combination of
which forms by far the largest automatic exchange yet installed. The
scope of the plans is such as to enable 125,000 subscribers to be served
without any change in the fundamental design, and by means merely of
addition in equipment and lines as demanded by the future subscriptions
for telephone service.

[Illustration: Fig. 437. Grant Avenue Office--San Francisco]

CHAPTER XXXIV

PRIVATE BRANCH EXCHANGES

=Definitions.= A telephone exchange devoted to the purely local uses of
a private establishment such as a store, factory, or business office, is
a private exchange. If, in addition to being used for such local
communication, it serves also for communication with the subscribers of
a city exchange, it becomes in effect a branch of the city exchange and,
therefore, a private branch exchange. The term "P. B. X." has become a
part of the telephone man's vocabulary as an abbreviation for private
branch exchange.

Private exchanges for purely local use require no separate treatment as
any of the types of switching equipments for interconnecting the lines
for communication, that have been or that will be descr