fairs would only be found in cases where several small communities
were grouped closely enough together to make them operate as a single
exchange district, and that is rather unusual.

The subject of inter-office trunking so far as manual switchboards are
concerned is, therefore, confined mainly to trunking between a number of
offices each equipped with a manual multiple switchboard.

=Necessity for Multi-Office Exchanges.= Before taking up the details of
the methods and circuits employed in trunking in multi-office systems,
it may be well to discuss briefly why the multi-office exchange is a
necessity, and why it would not be just as well to serve all of the
subscribers in a large city from a single huge switchboard in which all
of the subscribers' lines would terminate. It cannot be denied, when
other things are equal, that it is better to have only one operator
involved in any connection which means less labor and less liability of
error.

The reasons, however, why this is not feasible in really large
exchanges are several. The main one is that of the larger investment
required. Considering the investment first from the standpoint of the
subscriber's line, it is quite clear that the average length of
subscriber's line will be very much greater in a given community if all
of the lines are run to a single office, than will be the case if the
exchange district is divided into smaller office districts and the lines
run merely from the subscribers to the nearest office. There is a direct
and very large gain in this respect, in the multi-office system over the
single office system in large cities, but this is not a net gain, since
there is an offsetting investment necessary in the trunk lines between
the offices, which of course are separate from the subscribers' lines.

Approaching the matter from the standpoint of switchboard construction
and operation, another strong reason becomes apparent for the employment
of more than one office in large exchange districts. Both the
difficulties of operation and the expense of construction and
maintenance increase very rapidly when switchboards grow beyond a
certain rather well-defined limit. Obviously, the limitation of the
multiple switchboard as to size involves the number of multiple jacks
that it is feasible to place on a section. Multiple switchboards have
been constructed in this country in which the sections had a capacity of
18,000 jacks. Schemes have been proposed and p