these formulas are used. You notice there are now only two
formulas, for we already know the value of v; it is 300,000.
    
A broadcasting station is radiating a wave whose frequency
is 750 kilocycles per second. What is the wavelength of this
wave? The first formula gives us the answer; in place of f put
the frequency 750 and we get
l = 300,000 / 750 = 400 meters
    
Once again, suppose we are receiving signals on our receiv-
ing set, which has a wavelength of 300 meters. What is the
frequency of the transmitting station? This time use the second
formula. Putting 300 for l, we get
f = 300,000 / 300 = 1,000 kilocycles
PRODUCTION OF RADIO WAVES
    
In the case of water waves we know very well what caused
them. Something dropped into the water caused the disturbance
in a similar way Radio waves may be produced. The disturbance,
in the case of Radio, is produced in the ether. The only known
thing that is capable of affecting the ether (name given to the
medium presumed to permeate all matter and space) is the electron
and the only way that electrons can produce waves in the
ether is by making them move rapidly to-and-fro, so that they
vibrate or oscillate. Wires are mostly used to guide the electrons
along a given path. These free or moving electrons produce
electromagnetic waves in the ether and the motion of the waves
produced are similar in every respect to the motion of the electrons
within the guiding wires. In order to radiate the Radio
waves to great distances, the wire is raised to a high position
and it is then called an aerial or antenna.
    
The current from a battery flows steadily and only in one
direction through the circuit. That is direct current. There
are no back and forth motions in such a circuit. There is, however,
another form of electric current known as alternating
current which does not flow steadily in one direction. Alternating
current continually and regularly reverses its direction
of flow. There is a period of forward motion followed by a
period of reversed motion.