TRANSMISSION OF POWER FOR SUBSTATIONS. 133
toward which modern alternators tend. Itis a 6o k. w.
Westinghouse generator giving a frequency of 6o ~ at 600
r. p. m. ‘This size is wound on occasion for voltage as high
as 5000. 'The armature coils, one per pole, are machine
wound on forms, heavily insulated and sprung into slots in
the armature core. ‘This simple type of generator has been
in use for several years past in connection with a 10,000 volt
line from raising transformers, for sending current twenty-
eight miles from San Antonio Canyon to San Bernardino,
Cal. Like all other generators, the alternator is re-
versible and can be used as a motor. In this function,
however, it possesses some curious and interesting proper-
ties which will be discussed later.
The most characteristic property of alternating current
is its potency in producing inductive action. Such action
depends on changes of magnetic field, which in turn
depend on variation of the magnetizing current, and an
alternating current is in rapid and continuous variation all
the time.
It is this property which gives alternating current its
immense advantages in power transmission, since it enables
changes of voltage to be made very simply and with very
trifling loss of energy, and changes of voltage are abso-
lutely essential to economical transmission over long dis-
tances.
From our original formulee for figuring wiring it at
once appears that for a given percentage of energy lost in
the line, the area of copper necessary varies inversely as
the square of the working voltage. For doubling the
working voltage halves the current corresponding toa given
amount of energy, while for the same percentage of loss it
doubles E, which appears in the denominator of the form-
ulee. The value of even a small increase of working volt-
age appears in the most striking manner in the table of
Chap. IV. For working circuits the voltage is limited to
such a figure as permits of thoroughly successful insula-
tion of the motors under service conditions. In the trans-
mission the limit is imposed only by the conditions of safe
