[Illustration: DUCT LINE ACROSS 58TH STREET 32 DUCTS]

Construction of the revolving parts of the alternators is such as to
secure very great strength and consequent ability to resist the
tendency to burst and fly apart in case of temporary abnormal speed
through accident of any kind. The hub of the revolving field is of
cast steel, and the rim is carried not by the usual spokes but by two
wedges of rolled steel. The construction of the revolving field is
illustrated on pages 91 and 92. The angular velocity of the
revolving field is remarkably uniform. This result is due primarily to
the fact that the turning movement of the four-cylinder engine is far
more uniform than is the case, for example, with an ordinary
two-cylinder engine. The large fly-wheel capacity of the rotating
element of the machine also contributes materially to secure
uniformity of rotation.

[Illustration: MAIN CONTROLLING BOARD IN POWER STATION]

[Illustration: CONTROL AND INSTRUMENT BOARD--MAIN POWER STATION]

The alternators have forty field poles and operates at seventy-five
revolutions per minute. The field magnets constitute the periphery of
the revolving field, the poles and rim of the field being built up by
steel plates which are dovetailed to the driving spider. The heavy
steel end plates are bolted together, the laminations breaking joints
in the middle of the pole. The field coils are secured by copper
wedges, which are subjected to shearing strains only. In the body of
the poles, at intervals of approximately three inches, ventilating
spaces are provided, these spaces registering with corresponding air
ducts in the external armature. The field winding consists of copper
strap on edge, one layer deep, with fibrous material cemented in place
between turns, the edges of the strap being exposed.

[Illustration: DUCTS UNDER PASSENGER STATION PLATFORM
64 DUCTS]

The armature is stationary and exterior to the field. It consists of a
laminated ring with slots on its inner surface and supported by a
massive external cast-iron frame. The armature, as has been noted,
comprises seven segments, the topmost segment being in the form of a
small keystone. This may be removed readily, affording access to any
field coil, which in this way may be easily removed and replaced. The
armature winding consists of U-shaped copper bars in partially closed
slots. There are four bars per slot and three slots per phase per
pole. The bars in any slot may be