To understand how Radio signals can be sent and reproduced
from a receiving set, a study of wave motion and its characteristics
is essential. In transmitting voice or music by Radio,
sound waves are converted into electrical vibrations, which are
then again converted into Radio waves (magnetic waves), and
finally change back into sound waves.
Sound waves cannot be seen. Likewise, electromagnetic
waves used for Radio communications cannot be seen; neither
can their effect be heard until transformed by receiving ap-
paratus. However, we are fortunate in having many other ex-
amples of wave motion, which can be seen and therefore studied.
Then the knowledge gained can be applied to those wave motions
Which cannot be seen.
For example, if a long rope, lying on the floor, is shaken
sideways briskly, by moving it to the left and then to the
original position, a single wave-like motion is given to it, which
will travel down its length as shown in Fig. 11(A). If the rope
is shaken twice, two waves will be started which travel away
to the other end, keeping always the same distance apart, this
being shown in Fig. 11(D). If the shaking is repeated harmoniously
(rhythmically), a continuous wave motion is started
which transmits the energy imparted by the hand to the other
end of the rope. Try this experiment yourself by taking a
flexible rope or hose, twenty feet or more long, and shaking it
sideways as it lays along a straight line on the floor. This will
give you an opportunity to study the wave motions which now
will be described.
In order to understand the properties of waves, and these
properties apply also to electromagnetic waves, we must know
something about the terms used to describe them. A clear
understanding should be had of the terms; medium, velocity,
amplitude, cycles, wavelength, period and frequency. A1I these
are necessary to describe a wave motion. No doubt you have
already used some of these words yourself, to describe other
actions besides wave motion, so see how they fit in with your
present knowledge of the terms.
All types of waves, including heat, light, water, sound, and
Radio, are produced in mediums which are needed to allow vibra-
tion or oscillation to exist. That "something" through which a
wave passes is called the medium. In Fig. 11, the rope is the
