switchboard and other auxiliary apparatus.  In designing the sub-stations, a type of building with a central air-well was selected.  The typical organization of apparatus is illustrated in the ground plan and vertical section on pages 101, 102 and 103 and provides, as shown, for two lines of converters, the three transformers which supply each converter being located between it and the adjacent side wall.  The switchboard is located at the rear of the station.  The central shaft affords excellent light and ventilation for the operating room.  The steel work of the sub-stations is designed with a view to the addition of two storage battery floors, should it be decided at some future time that the addition of such an auxiliary is advisable.

[Illustration:  CONTACT RAIL BANDS]

The necessary equipment of the sub-stations implies sites approximately 50 x 100 feet in dimensions; and sub-stations Nos. 14, 15, 17, and 18 are practically all this size.  Sub-stations Nos. 11 and 16 are 100 feet in length, but the lots acquired in these instances being of unusual width, these sub-stations are approximately 60 feet wide.  Sub-station No. 12, on account of limited ground space, is but 48 feet wide and 92 feet long.  In each of the sub-stations, except No. 13, foundations are provided for eight converters; sub-station No. 13 contains foundations for the ultimate installation of ten converters.

[Illustration:  DIRECT CURRENT FEEDERS FROM MANHOLE TO CONTACT RAIL]

The function of the electrical apparatus in sub-stations, as has been stated, is the conversion of the high potential alternating current energy delivered from the power house through the tri-phase cables into direct current adapted to operate the motors with which the rolling stock is equipped.  This apparatus comprises transformers, converters, and certain minor auxiliaries.  The transformers, which are arranged in groups of three, receive the tri-phase alternating current at a potential approximating 10,500 volts, and deliver equivalent energy (less the loss of about 2 per cent. in the transformation) to the converters at a potential of about 390 volts.  The converters receiving this energy from their respective groups of transformers in turn deliver it (less a loss approximating 4 per cent. at full load) in the form of direct current at a potential of 625 volts to the bus bars of the direct current switchboards, from which it is conveyed by insulated cables to the contact rails.  The photograph on page 102 is a general view of the interior of one of the sub-stations.  The exterior of sub-stations Nos. 11 and 18 are shown on page 107.

[Illustration:  CONTACT RAILS, SHOWING END INCLINES]

The illustration on page 108 is from a photograph taken on one of the switchboard galleries.  In the sub-stations, as in the power house, the high potential alternating current circuits are opened and closed by oil switches, which are electrically operated by motors, these in turn being controlled by 110 volt direct current circuits.  Diagramatic bench boards are used, as at the power house, but in the sub-stations they are of course relatively small and free from complication.