Page 245 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 11 Acoustic Devices 207

Key Key

Line Relay Station Line
Line Line
Battery Battery
Sounder Sounder

Local Battery Local Battery

Earth Ground Earth Ground
Station A Station A
Figure 11-52 Two-Way Telegraph System with Relay Station

Telephones Battery Battery

Although we take them for granted, the telephone is probably
the most important communication technology ever invented. Transmitter Transmitter
Coupling Coupling
In the early days of the telephone, coverage was limited to Transformer Transformer
local areas and was in many ways, a novelty. The telegraph
was still relied on to communicate over long distances. The
Receivers Line Receivers
advent of the carbon microphone and a reliable receiver made
Figure 11-53 Two-Way Telephone Circuit
voice communications possible. Figure 11-53 shows a basic
telephone circuit. When you speak into the transmitter
(microphone), the sound is reproduced on both receivers
(loudspeakers). This basic system made it possible to have a
normal conversation over great distances. and business in the early 1900s. To make a call, the subscriber
Figure 11-54 shows a schematic for a two-way telephone cranked the magneto and the operator’s ringer would sound.
system with magneto ringers. Cranking the magneto (generator) The operator would pick up and ask what number the caller
on one set, will activate the ringer on the other set. This addition would like to be connected to. The operator would ring that
to the basic voice made it possible to produce a loud call at the number and connect the two lines together.
other end of the line alerting the party that they are being called. Military phones of the World Wars were simply a basic tele-
Figure 11-55 shows an illustration of an early wall hang- phone with magneto installed into a leather or canvas case, as
ing phone. These units were placed in thousands of homes shown in Figure 11-56. These were a complete, self-contained

Battery Battery

Transmitter Transmitter
Coupling Coupling
Transformer Transformer

Receiver Receiver
Ringer Ringer
Magneto Magneto

Hook Switch Line Hook Switch

Station A Station B
Figure 11-54 Two-Way Telephone Circuit with Magneto Ringers