es a telephone transmitter such as you have
often talked to.

[Illustration: Fig 67a]

Wires connect to the carbon discs as shown in Fig. 66. A stream of
electrons can flow through the wires and from grain to grain through the
"carbon button," as we call it. The electrons have less difficulty if
the grains are compressed, that is the button then offers less
resistance to the flow of current. If the diaphragm moves back, allowing
the grains to have more room, the electron stream is smaller and we say
the button is offering more resistance to the current.

[Illustration: Fig 67b]

You can see what happens. Suppose some one talks into the transmitter
and makes its diaphragm go back and forth as shown in Fig. 67a. Then the
current in the antenna varies, being greater or less, depending upon
whether the button offers less or more resistance. The corresponding
variations in the antenna current are shown in Fig. 67b.

In the antenna at the receiving station there are corresponding
variations in the strength of the signal and hence corresponding
variations in the strength of the current through the telephone
receiver. I shall show graphically what happens in Fig. 68. You see that
the telephone receiver diaphragm does just the same motions as does the
transmitter diaphragm. That means that the molecules of air near the
receiver diaphragm are going through just the same kind of motions as
are those near the transmitter diaphragm. When these air molecules
affect your eardrum you hear just what you would have heard if you had
been right there beside the transmitter.

That's one way of making a radio-telephone. It is not a very efficient
method but it has been used in the past. Before we look at any of the
more recent methods we can draw some general ideas from this method and
learn some words that are used almost always in speaking of
radio-telephones.

In any system of radio-telephony you will always find that there is
produced at the transmitting station a high-frequency alternating
current and that this current flows in a tuned circuit one part of which
is the condenser formed by the antenna and the ground (or something
which acts like a ground). This high-frequency current, or
radio-current, as we usually say, is varied in its strength. It is
varied in conformity with the human voice. If the human voice speaking
into the transmitter is low pitched there are slow variations in the
intensity of the radio current. If the