nges in the
intensity or strength of the incoming signal there are going to be
corresponding motions of the receiver diaphragm. And something to
listen, too, if these changes are frequent enough but not so frequent
that the receiver diaphragm has difficulty in following them.

There are many ways of affecting the strength of the incoming signal.
Suppose, for example, that we arrange to decrease the current in the
antenna of the transmitting station. That will mean a weaker signal and
a smaller increase in current through the winding of the telephone
receiver at the other station. On the other hand if the signal strength
is increased there is more current in this winding.

[Illustration: Fig 64]

Suppose we connect a fine wire in the antenna circuit as in Fig. 64 and
have a sliding contact as shown. Suppose that when we depress the switch
in the oscillator circuit and so start the oscillations that the sliding
contact is at _o_ as shown. Corresponding to that strength of
signal there is a certain value of current through the receiver winding
at the other station. Now let us move the slider, first to _a_ and
then back to _b_ and so on, back and forth. You see what will
happen. We alternately make the current in the antenna larger and
smaller than it originally was. When the slider is at _b_ there is
more of the fine wire in series with the antenna, hence more resistance
to the oscillations of the electrons, and hence a smaller oscillating
stream of electrons. That means a weaker outgoing signal. When the
slider is at _a_ there is less resistance in the antenna circuit
and a larger alternating current.

[Illustration: Fig 65]

[Illustration: Fig 66]

A picture of what happens would be like that of Fig. 65. The signal
varies in intensity, therefore, becoming larger and smaller alternately.
That means the voltage impressed on the grid of the detector is
alternately larger and smaller. And hence the stream of electrons
through the winding of the telephone receiver is alternately larger and
smaller. And that means that the diaphragm moves back and forth in just
the time it takes to move the slider back and forth.

Instead of the slider we might use a little cup almost full of grains of
carbon. The carbon grains lie between two flat discs of carbon. One of
these discs is held fixed. The other is connected to the center of a
thin diaphragm of steel and moves back and forth as this diaphragm is
moved. The whole thing mak