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SUBSTATIONS. 129
tion. It goes without saying that the curves given in Fig.
64, while closely figured for the assumed conditions are
subject to too much variation to be safely applicable to the
decision of a close case. For preliminary investigation
however they will be found convenient.
At shorter distances than that just assumed the same
apparatus would generally be required and the cost of line
would gradually decrease, but so slowly that the economics
of the case would remain nearly the same. ‘The minimum
cost for the case considered would probably be given by
a main generating station at some convenient point be-
tween A and C, and a substation about midway between E
and B. So long as substation apparatus requires the same
attention as generating apparatus, the usefulness of power
transmission is limited to a comparatively small field. If,
however, alternating current motors come into regular use
so that the only substations required shall be static trans-
formers distributed along theline, the use of more than
one generating station, save in roads on a large scale, will
be needless and wasteful.
Just at present the most promising method of operat-
ing roads of moderate length seems to be the use of direct
feeding at rather high voltage, by boosting, the three-wire or
the like. When the length reaches fifteen or twenty miles,
the choice is between separate generating plants and true
substations with the advantage of the latter slowly increas-
ing with the distances involved. In cases where the
amount of power involved is very great, as in large urban
systems like the West End or in extensive suburban serv-
ice such as is likely to be met in the transformation of
steam into electric service, auxiliary stations are most
likely to give the minimum cost of power, since the size of
each plant can be so considerable that further increase will
decrease the cost of power only to a minute degree. ‘The
greatest future gain in systems of moderate size is to be
sought in the use of alternating motors,
