INTRODUCTION TO TELEPHONE SYSTEMS
Components of Telephone Systems.
To provide satisfactory service, the telephone system must include, besides transmitters and receivers,
components such as ringers, switchboards, and transmission lines. This chapter explains the overall functions of these
additional components and shows how they are used in a number of common telephone systems. Later chapters will
present in detail the principles of their operation.
Simple Telephone Circuit.
a. The simplest telephone circuit is obtained by connecting a transmitter to a receiver, as in figure 23. In such a
circuit, the transmitter may be located a considerable distance from the receiver, perhaps several miles away, and yet a
person speaking into the transmitter at station A can be heard by another person at the receiver at station B. One-way
telephone communication is effected.
b. One-way communication serves for the transmission of intelligence in one direction, but it is inadequate for
most of the purposes for which the telephone is used. Two-way conversation is indispensable. Figure 24 shows how
simply this can be arranged. A receiver is added at the transmitting end and a transmitter at the receiving end. With two
transmitters and two receivers, so connected, the voice of a person speaking into either transmitter can be heard in both
receivers, and two-way communication is effected between stations A and B.
c. Although the circuit in figure 24 can be used as the basis for a simple telephone system, its usefulness is
limited. How can a person at station A signal someone at station B to come to the phone so that conversation may begin?
Although the circuit shown in figure 24 does not provide a means for alerting a receiver of an incoming call, is arranged
simply as shown in figure 25. At each station, A and B, signaling (ringing) circuits are added, and these make it possible
for a person at either station to signal the other station when conversation is desired. A signaling circuit includes a ringer
(bell or buzzer) and a hand generator. A person at station A, wishing to talk with someone at station B, turns the crank of
the hand generator. This generates an ac voltage which send a signal current over the transmission line to operate the