vidual Period 1.00 seconds |
|Average 5 Shortest Individual Periods 1.28 seconds |
|Average 10 Shortest Individual Periods 1.34 seconds |
|Average Entire 100 Calls 2.07 seconds |
+----------------------------------------------------------+

The length of time which the subscriber has to wait before receiving an
answer from the operator is, of course, one of the factors that enters
into the giving of good telephone service, and the times shown by this
test are considerably shorter than ordinarily maintained in manual
practice. The waiting time of the subscriber is not, of course, a part
of the time that is consumed by the operator, and the real economy so
far as the operator's time is concerned is shown in the tests recorded
in Table XI.

CHAPTER XXXII

POWER PLANTS

The power plant is an organization of devices to furnish to a telephone
system the several kinds of current, at proper pressures, for the
performance of the several general electrical tasks within the exchange.

=Kinds of Currents Employed.= Sources of both direct and alternating
current are required and a single exchange may employ these for one or
more of the following purposes:

_Direct Current._ Current which flows always in one direction whether
steady or varying, is referred to as direct current, and may be required
for transmitters, for relays, for line, supervisory, and auxiliary
signals, for busy tests, for automatic switches, for call registers, for
telegraphy, and in the form of pulsating current for the ringing of
biased bells.

_Alternating Current._ Sources of alternating current are required for
the ringing of bells, for busy-back and other automatic signals to
subscribers, for howler signals to attract the attention of subscribers
who have left their receivers off their hooks, and for signaling over
composite lines.

=Types of Power Plants.= Clearly the requirements for current supply
differ greatly for magneto and common-battery systems. There is,
however, no great difference between the power plants required for the
automatic and the manual common-battery systems.

In the simplest form of telephone system--two magneto telephones on a
private line--the power plant at each station consists of two elements:
one, the magneto generator, which is a translating device for turning
hand power into alternating current for ringing the bell of the distant
station; and the ot