The two periods of motion make a cycle.
A cycle of electron motion will produce a cycle of wave
motion in the ether. Consequently each cycle of current flow
will make a Radio wave.
In a previous paragraph it has been shown that a wire
carrying current has a magnetic field about it and the strength
of this magnetic field varies with every change of current. Also
the magnetic field is at right angles with the current flow. When
a transmitter is in operation and an alternating current applied
so it will alternate between the extreme end of the aerial and
the connection ground there exists an electrical strain, or electrical
field as it is called, between the aerial and ground. In
other words, we have a vertical electric vibration and a horizontal
magnetic vibration near the aerial circuit. If the alternating
current reverses its direction in the aerial circuit rapidly
and uniformly there will be a uniform disturbance in the aerial
circuit which will set up a Radio wave. As these Radio waves
consist of both electric and magnetic vibrations they are called
electromagnetic waves. The magnetic portion of the wave will
be the one in which we shall be interested.
When these electromagnetic waves come in contact with
the aerial circuit of a receiving station, they induce in it oscil-
lations of alternating current, exactly similar to the oscillations
in the transmitting aerial circuit.
TUNING A RADIO RECEIVER
You have often heard of “tuning in” a Radio receiver. You
have no doubt at some time or other wondered what goes on
when you change the position of the dial or dials so as to cut
out one station and bring in another station. In this first lesson
we will only give you a general idea of what happens.
As the proper way to learn anything is to start out with
something we do know and then work to the unknown, we will
start out this chapter on tuning by working with two violin
strings. When two strings are in “tune” we know that they will
make the same sound when plucked, or, when a bow is drawn
over them.
Let us perform some experiments now with violin strings.
We start out with two violins, tuned together, that is, each of
the four strings of the one is exactly in tune with the corresponding
string of the other. Now make any string of violin
number 1 vibrate, either by plucking it or by drawing a bow
