ious curvatures, and
made of malleable cast iron, are used. The photograph on page 100
illustrates the arrangement of cables in one of these manholes.

[Illustration: OPERATING BOARD--SUB-STATION NO. 11]

In order to further diminish the risk of interruption of the service
due to failure of power supply, each sub-station south of 96th Street
receives its alternating current from the power house through cables
carried on opposite sides of the subway. To protect the lead sheaths
of the cables against damage by electrolysis, rubber insulating pieces
1/6 of an inch in thickness are placed between the sheaths and the
iron bracket supports in the manholes.

[Sidenote: _Cable
Conveying
Energy from
Power House to
Sub-Stations_]

The cables used for conveying energy from the power house to the
several sub-stations aggregate approximately 150 miles in length. The
cable used for this purpose comprises three stranded copper conductors
each of which contains nineteen wires, and the diameter of the
stranded conductor thus formed is 2/5 of an inch. Paper insulation is
employed and the triple cable is enclosed in a lead sheath 9/64 of an
inch thick. Each conductor is separated from its neighbors and from
the lead sheath by insulation of treated paper 7/16 of an inch in
thickness. The outside diameter of the cables is 2-5/8 inches, and the
weight 8-1/2 pounds per lineal foot. In the factories the cable as
manufactured was cut into lengths corresponding to the distance
between manholes, and each length subjected to severe tests including
application to the insulation of an alternating current potential of
30,000 volts for a period of thirty minutes. These cables were
installed under the supervision of the Interborough Company's
engineers, and after jointing, each complete cable from power house to
sub-station was tested by applying an alternating potential of 30,000
volts for thirty minutes between each conductor and its neighbors, and
between each conductor and the lead sheath. The photographs on
page 98 illustrates the construction of this cable.

[Sidenote: _Sub-Station_]

The tri-phase alternating current generated at the power house is
conveyed through the high potential cable system to eight sub-stations
containing the necessary transforming and converting machinery. These
sub-stations are designed and located as follows:

[Illustration: DIAGRAMS OF DIRECT CURRENT FEEDER AND RETURN CIRCUITS]

Sub-station No. 11--29