258 POWER DISTRIBUTION FOR ELECTRIC RAITLROADS.
greatly in favor of the copper. Xven if old worn out rails
were utilized for the feeders it is an open question whether
the extra maintenance would not eventually more than
eat up the saving in first cost over copper.
Personally the author believes the centre rail con-
struction used at Nantasket to be better than any side rail,
and to have ample insulation for all ordinary cases. It is
quite as easy to put in place, and the great cross section of
the rail is advantageous, since the bonding must be main-
tained in any case and the extra conductivity costs no
more than if it were secured by copper feeders.
Whatever the construction, a third rail supply system
must be protected against danger of accidental contacts,
and the insulators must be kept free of conducting material
—brake shoe dust and the like.
On a large system the electrical load is fairly constant
and, except for the question of branches, can be considered
as nearly uniformly distributed. If the schedule is pre-
served, there is unlikely to be any very great massing of
cars, so that less provision has to be made for wandering of
the load than in street railway service or even suburban
service. This simplifies the computation of the conducting
system greatly. If the rails are thoroughly bonded to the
structure, and preferably also to each other, the resistance
of the return circuit is extremely low. A track constant
of 12 should be quite enough to allow under these circum-
stances, and the power demanded should not often average
over seventy-five kilowatts per train at the power house.
The work of rapid acceleration is the most severe contin-
gency that must be taken into account, for elevated roads
are practically level. ‘This work will usually be not far
from double the average work, at times perhaps a trifle
more.
An elevated structure gives an admirable opportunity
for the use of polyphase motors, since the three necessary
working conductors can readily be provided, and such a
system has been several times suggested. In long roads
an alternating distribution at high voltage might be
