[Illustration: SWITCHBOARD FOR ALTERNATING CURRENT BLOCK SIGNAL CIRCUITS--IN SUB-STATION] [Illustration: EXTERIOR OF SUB-STATION NO. 18] [Sidenote: _Alternating Current Distribution to Sub-Stations Power House Ducts and Cables_] From alternators to alternator switches the 11,000 volt alternating currents are conveyed through single conductor cables, insulated by oil cambric, the thickness of the wall being 12/32 of an inch. These conductors are installed in vitrified clay ducts. From dynamo switches to bus bars and from bus bars to group and feeder switches, vulcanized rubber insulation containing 30 per cent. pure Para rubber is employed. The thickness of insulating wall is 9/32 of an inch and the conductors are supported upon porcelain insulators. [Sidenote: _Conduit System for Distribution_] From the power house to the subway at 58th Street and Broadway two lines of conduit, each comprising thirty-two ducts, have been constructed. These conduits are located on opposite sides of the street. The arrangement of ducts is 8 x 4, as shown in the section on page 96. [Illustration: EXTERIOR OF SUB-STATION NO. 11] The location and arrangement of ducts along the line of the subway are illustrated in photographs on pages 98 and 99, which show respectively a section of ducts on one side of the subway, between passenger stations, and a section of ducts and one side of the subway, beneath the platform of a passenger station. From City Hall to 96th Street (except through the Park Avenue Tunnel) sixty-four ducts are provided on each side of the subway. North of 96th Street sixty-four ducts are provided for the West-side lines and an equal number for the East-side lines. Between passenger stations these ducts help to form the side walls of the subway, and are arranged thirty-two ducts high and two ducts wide. Beneath the platform of passenger stations the arrangement is somewhat varied because of local obstructions, such as pipes, sewers, etc., of which it was necessary to take account in the construction of the stations. The plan shown on page 98, however, is typical.