BASIC CONCEPTS OF ELECTRICITY FOR COMMUNICATIONS
A more practical telegraph system is illustrated in Figure 2.3. Note that the figure has
just one metallic wire connecting the west station to the east station. The second wire is
replaced with ground. The earth is a good conductor, and so we use earth, called ground,
as the second conductor (or wire). Such a telegraph system is called single-wire ground
return. Such a system was widely in use when my wife and I did a stint for the ITU
(International Telecommunication Union) in Ecuador in 19671969.
This is a similar circuit as shown in Figure 2.2. In this case, when both keys are closed,
a DC (direct current) circuit is traced from a battery in the west station through the key
and relay at that point to the line wire, and from there it is traced through the relay and
key at the east station and back through the earth (ground) to the battery. The relays at
each end, in turn, control the local circuits, which include a separate battery and a sounder
(e.g., buzzer or other electric sounding device). Opening and closing the key at one end,
while the key at the other end is closed, causes both sounders to operate accordingly.
A relay is a switch that is controlled electrically. It consists of (a) wire wrapped around
an iron core and (b) a hinged metal strip, which is normally open. When current flows
through the windings (i.e., the wire wrapped around the core), a magnetic field is set up,
thereby drawing the hinged metal strip into a closed position and causing current to flow
in the secondary circuit. It is a simple open-and-closed device such that when current
flows there is a contact closure (the metal strip) and when there is no current through the
windings, the circuit is open. Of course there is a spring on the metal strip, holding it
open except when current flows.
Twenty years after Morse first demonstrated his telegraph on a BaltimoreWashington
route, telegraph covered the country from coast to coast. It caused a revolution in com-
munications. Its use is still prevalent in many parts of the world, as I discussed above.
What Is Frequency?
To understand more advanced telecommunication concepts, we need a firm knowledge of
frequency and related parameters such as band and bandwidth, wavelength, period, and
phase. Let us first define frequency and relate it to everyday life.
The IEEE defines frequency as "the number of complete cycles of sinusoidal varia-
tion per unit time." The time unit we will use is the second. For those readers with a
mathematical bent, if we plot
y = sin x, where x is expressed in radians, a "sine wave"
is developed as shown in Figure 2.4.
Figure 2.5 shows two sine waves; the left side illustrates a lower frequency, and the
right side shows a higher frequency. The amplitude, measured in this case as voltage,
is the excursion, up or down, at any singular point. Amplitude expresses the intensity at
that point. If we spoke of amplitude without qualifying it at some point, it would be the
A practical elementary telegraph circuit with ground return.