When the transmitted Radio wave cuts the receiving antenna
(aerial) illustration in Fig. 13, it starts the electrons in the
aerial wire moving immediately which we now know to be a
very small electric current, This current is naturally at a very
high frequency. It is one of the jobs of the receiver to reconvert
into sound waves this high frequency current which is called
Radio Frequency (R.F.) (upon which is impressed the sound
waves) so that when it reaches the earphone or loudspeaker, it
can become audible to the human ear. This lower frequency of
sound is called “audio frequency” (A.F.). The conversion can
be accomplished in one method by means of a crystal. We will
learn later on just what there is about various crystals which
Fig. 13----Broadcasting and receiving process.
enables them to make these signals audible. In modern sets this
conversion is accomplished by means of a single vacuum tube,
called the detector tube, about which we shall also learn a great
deal more later on.
As we said before, the signal in the form of an electric
current as it comes from the aerial in most cases is very weak.
If the receiver is fairly near to a powerful Radio station, the
incoming signal may be strong enough to be heard by means
of a simple receiver using earphones. But now-a-days we can
hear signals from stations clear across the country while using
a loudspeaker. How is this accomplished? Evidently the signal
must be strengthened or "amplified." Both R.F. and A.F. can
be amplified. This is accomplished by means of amplifying
tubes. A single tube amplifies the signal only a little. Several
