removed from the armature without
removing the frame. The alternators, of course, are separately
excited, the potential of the exciting current used being 250 volts.

As regards regulation, the manufacturer's guarantee is that at 100 per
cent. power factor if full rated load be thrown off the e. m. f. will
rise 6 per cent. with constant speed and constant excitation. The
guarantee as to efficiency is as follows: On non-inductive load, the
alternators will have an efficiency of not less than 90.5 per cent. at
one-quarter load; 94.75 per cent. at one-half load; 96.25 per cent. at
three-quarters load; 97 per cent. at full load, and 97.25 per cent. at
one and one-quarter load. These figures refer, of course, to
electrical efficiency, and do not include windage and bearing
friction. The machines are designed to operate under their rated full
load with rise of temperature not exceeding 35 degrees C. after
twenty-four hours.

[Illustration: THREE-CONDUCTOR NO. 000 CABLE FOR 11,000 VOLT
DISTRIBUTION]

[Sidenote: _Exciters_]

To supply exciting current for the fields of the alternators and to
operate motors driving auxiliary apparatus, five 250-kilowatt direct
current dynamos are provided. These deliver their current at a
potential of 250 volts. Two of them are driven by 400 horse-power
engines of the marine type, to which they are direct-connected, while
the remaining three units are direct-connected to 365 horse-power
tri-phase induction motors operating at 400 volts. A storage battery
capable of furnishing 3,000 amperes for one hour is used in
co-operation with the dynamos provided to excite the alternators. The
five direct-current dynamos are connected to the organization of
switching apparatus in such a way that each unit may be connected at
will either to the exciting circuits or to the circuits through which
auxiliary motors are supplied.

The alternators for which the new Interborough Power House are
designed will deliver to the bus bars 100,000 electrical horse power.
The current delivered by these alternators reverses its direction
fifty times per second and in connecting dynamos just coming into
service with those already in operation the allowable difference in
phase relation at the instant the circuit is completed is, of course,
but a fraction of the fiftieth of a second. Where the power to be
controlled is so great, the potential so high, and the speed
requirements in respect to synchronous operation so exa