108 POWER DISTRIBUTION FOR ELECTRIC RAILROADS.
At present both stations are worked in the ordinary way.
These roads are the longest yet deliberately built and
worked for interurban service, although longer ones are
under construction and there are longer continuous routes,
the outgrowth of adjacent lines.
The third class, true substations, is just coming into
existance, as it is a result of the extension of interurban
work. It includes all cases of power transmission to dis-
tributed stations and at present involves the use of motor
generators or their equivalent. Whenever alternating
motors for railway service shall be thoroughly worked out,
this class of substation work, freed from the present neces-
sity of rotating apparatus, will be likely to lead the others
in importance. At present it is being used rather tenta-
tively as a substitute for the second class of substation.
The best example of this substation work in -~onnec-
tion with power transmission is the Lowell (Mass.) one
shown in Fig. 63. The total length of the line is nearly
fifteen miles and all the power is supplied from the main
power station A, which is equipped with four roo k. w.
composite generators giving either three phase or continu-
ous current. A bank of raising transformers gives a 5500
volt, three phase current on the line. This is transmitted
to the two substations B, nine miles from A, at Ayer’s
Mills, and C, fifteen miles from A at Nashua, N.H. These
substations are duplicates. Each contains two 75 k. w. ro-
tary transformers, together with the necessary switchboards
and bank of reducing transformers. Station B was started
first and supplies the middle section of the line, while the
Lowell end forms part of the regular street railway system
and is fed from A. C, the Nashua substation, feeds the
terminal portion of the line and provides the local service in
Nashua, replacing a steam power station. At A, the gen-
erators are operated regularly in parallel and in each sub-
station the rotary transformers are run in parallel.
The details of some of these typical substations will
be taken up later; for the present purpose it is enough
to outline them sufficiently to emphasize the economic
