TABLE XII
Subscribers' Waiting Time
| Number of Calls Tested | 100 | |
| Longest Individual Period | 5.20 | seconds |
| Average 5 Longest Individual Periods | 4.64 | seconds |
| Average 10 Longest Individual Periods | 3.80 | seconds |
| Shortest Individual 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.