SUBSTATIONS. 123
have a better load factor and more continuous service, ren-
dering it more easy economically to run separate generat-
ing stations. When the service in any part of a road is
decidedly discontinuous and gives a poor load factor, that
substation will give the best economy which has the lowest
fixed charges for interest, depreciation and labor. Hence
in this class of work power transmission with true substa-
tions, is likely to give better results than independent gen-
erating stations. Coming mnow again to this class of true
substation, the only well developed example is found in the
pioneer power transmission plant of the Lowell & Sub-
urban Electric Railway running from Iowell, Mass., to
Nashua a distance of nearly fifteen miles. Fig. 63 shows
a sketch map of the route. The first step was the. trans-
mission of power from the Lowell generating station, A, a
little over nine miles to B, the substation at Ayer’s Mills.
A little later the generating station which had for some
time done service at Nashua was shut down and replaced
by a substation, C, fed from the transmission line from
Lowell. ’The need of this terminal substation was largely
due to the local traffic in Nashua which is a place of some
20,000 inhabitants; and a heavy summer suburban traffic
extending from Lowell to a pretty lake and picnic ground
five miles north called for ample power in the initial sec-
tion of the road. Hence power was transmitted direct to
Nashua for the load there and also toan intermediate point
which could supply the line in the intermediate section
and help out the suburban loads at each terminus.
The generating station in Lowell is common to the
local service of the railway line and to the transmission
apparatus proper, which was added when the long distance
line was undertaken.
The generating apparatus for the transmission plant
consists of four 100 k.w. composite generators delivering
either direct current at 500 to 550 volts or three phase cur-
rent at about 320 volts. In this case the three phase side
only is in regular use, and the current is transformed in a
bank of substation raising transformers to a pressure of
