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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--29-33 City Hall Place. Sub-station No. 12--108-110 East 19th Street. Sub-station No. 13--225-227 West 53d Street. Sub-station No. 14--264-266 West 96th Street. Sub-station No. 15--606-608 West 143d Street. Sub-station No. 16--73-77 West 132d Street. Sub-station No. 17--Hillside Avenue, 301 feet West of Eleventh Avenue. Sub-station No. 18--South side of Fox Street (Simpson Street), 60 feet north of Westchester Avenue. [Illustration: SWITCH CONNECTING FEEDER TO CONTACT RAIL] [Illustration: CONTACT RAIL JOINT WITH FISH PLATE] The converter unit selected to receive the alternating current and deliver direct current to the track, etc., has an output of 1,500 kilowatts with ability to carry 50 per cent. overload for three hour