low-tension mains and feeders, the direct current for this purpose
being obtained through the agency of rotary transformers.

There are various methods of producing the alternating current for
transmission purposes. In some cases the generators are themselves
wound for high potential; in others they are wound for 80 volts, and
step-up transformers are used, carrying the current up to whatever
pressure is desired, from 1,000 to 10,000 volts. In other cases
dynamos are used having collector rings for alternating current on one
side and a commutator for direct current on the other side of the
armature, thus enabling you, when the peak in two districts of a city
comes at two different times, to take care of this peak by means of
the same original generating unit, furnishing direct low-tension
current to the points near the central station and alternating current
to the distant points. In other cases, where a small amount of
alternating current is required on the transmission line, it has even
been found economical to take direct current from a large unit, change
it by means of a rotary transformer into alternating current, step up
from 80 to, say, 2,000 volts, go to the distant point, and step down
again to 80 volts alternating, and then convert again by means of a
rotary transformer into low-potential direct current.

The introduction of alternating current for transmission purposes in
large cities is probably best exemplified in the station recently
erected in Brooklyn, where alternating current is produced and carried
to distant points, and then used to operate series arc-light machines
run by synchronous motors, the low-tension direct-current network
being fed by rotary transformers, and alternating circuits arranged
with block converters, and even in some cases separate converters for
each individual customer in the scattered districts.