Like all current-carrying parts of the switches, the bus bars are enclosed in separate compartments. These are constructed of brick, small doors for inspection and maintenance being provided opposite all points where the bus bars are supported upon insulators. The photographs on pages 95 and 96 are views of a part of the bus bar and switch compartments. [Illustration: TWO GROUPS OF TRANSFORMERS] The oil switches and group bus bars are located upon the main floor and extend along the 59th Street wall of the engine room a distance of about 600 feet. The main bus bars are arranged in two lines of brick compartments, which are placed below the engine-room floor. These bus bars are arranged vertically and are placed directly beneath the rows of oil switches located upon the main floor of the power house. Above these rows of oil switches and the group bus bars, galleries are constructed which extend the entire length of the power house, and upon the first of these galleries at a point opposite the middle of the power house are located the control board and instrument board, by means of which the operator in charge regulates and directs the entire output of the plant, maintaining a supply of power at all times adequate to the demands of the transportation service. [Illustration: MOTOR-GENERATORS AND BATTERY BOARD FOR CONTROL CIRCUITS--SUB-STATION] [Illustration: 1,500 K. W. ROTARY CONVERTER] [Sidenote: _The Control Board_] The control board is shown in the photograph on page 97. Every alternator switch, every selector switch, every group switch, and every feeder switch upon the main floor is here represented by a small switch. The small switch is connected into a control circuit which receives its supply of energy at 110 volts from a small motor generator set and storage battery. The motors which actuate the large oil switches upon the main floor are driven by this 110 volt control current, and thus in the hands of the operator the control switches make or break the relatively feeble control currents, which, in turn, close or open the switches in the main power circuits. The control switches are systematically assembled upon the control bench board in conjunction with dummy bus bars and other apparent (but not real) metallic connections, the whole constituting at all times a correct diagram of the existing connections of the main power circuits. Every time the operator changes a connection by opening or closing one of the main switches, he necessarily changes his diagram so that it represents the new conditions established by opening or closing the main switch. In connection with each control switch two small bull's-eye lamps are used, one red, to indicate that the corresponding main switch is closed, the other green, to indicate that it is open. These lamps are lighted when the moving part of the main switch reaches approximately the end of its travel. If for any reason, therefore, the movement of the control switch should fail to actuate