to another and for tuning the circuits to a certain frequency in
conjunction with the variable condensers, the capacities of which
are varied by operating a knob on the panel. At the rear of the
set (top of the picture) you will notice a power supply unit and
a rectifying tube. The power supply unit consists of a special
transformer which changes the ll0-volt alternating current supplied
from the line to the required lower and higher Values for
the operation of the set. The lower voltages are required by
the filaments of the tubes. The rectifying tube converts the
current to pulsating direct current and a filter circuit smoothes
out the pulsating direct current delivered by the rectifying tube
and makes it pure D.C. and noiseless in action before it is applied
to the plates of the vacuum tubes. A resistance unit is also used
in the power supply to reduce the high rectified voltage to the
values required by the plate circuits.
The Radio receiver which has just been described and
illustrated is an A.C. set; that is, it is a Radio receiver operated
by power taken from the house lighting system.
A battery operated receiver is very similar to this except
that it uses different vacuum tubes and does not use a power unit
or rectifying tube but receives its power direct from batteries.
Many of the parts, such as coils, tubes, condensers, etc., in
modern Radio receivers are very well concealed by shielding.
Fig. 15 will give you a fair idea of what a modern A.C. screen
grid set looks like until in later lessons we dissect the set more
carefully and learn all of its details.
All together this short description may seem rather unsatisfying
more than likely a thousand questions are cropping up in
your mind. “What is a screen grid tube?” “What is a pulsating
current and a filter circuit?” Or, “What does this unit do
when the set is operated?” Or, again, “Why is it necessary
to have some condensers variable and some fixed?”
It is the object of this course to teach you all of these things
thoroughly. The subject of Radio is a long one, but you will
understand fully how all of these things work as they are
described in detail in various advanced lessons of the course.
For the present, familiarize yourself with the names and appearances
of the various parts which enter into the make-up of a
Radio receiver.
We are now nearly ready to conclude this first lesson. Before
we do so, however, we wish to advise you that if there is anything
in this lesson that has escaped you, or if there are things
