pose that instead of reversing the current by reversing the battery
we arrange to send an alternating current through the coil. That will
have the same effect. For one direction of current flow, the diaphragm
is attracted still more by the magnet but for the other direction it is
not attracted as much. The result is that the center of the diaphragm
moves back and forth during one complete cycle of the alternating
current in the coil.

The diaphragm vibrates back and forth in tune with the alternating
current in the receiver winding. As it moves away from the magnet it
pushes ahead of it the neighboring molecules of air. These molecules
then crowd and push the molecules of air which are just a little further
away from the diaphragm. These in turn push against those beyond them
and so a push or shove is sent out by the diaphragm from molecule to
molecule until perhaps it reaches your ear. When the molecules of air
next your ear receive the push they in turn push against your eardrum.

In the meantime what has happened? The current in the telephone receiver
has reversed its direction. The diaphragm is now pulled toward the
magnet and the adjacent molecules of air have even more room than they
had before. So they stop crowding each other and follow the diaphragm in
the other direction. The molecules of air just beyond these, on the way
toward your ear, need crowd no longer and they also move back. Of
course, they go even farther than their old positions for there is now
more room on the other side. That same thing happens all along the line
until the air molecules next your ear start back and give your eardrum a
chance to expand outward. As they move away they make a little vacuum
there and the eardrum puffs out.

That goes on over and over again just as often as the alternating
current passes through one cycle of values. And you, unless you are
thinking particularly of the scientific explanations, say that you "hear
a musical note." As a matter of fact if we increase the frequency of the
alternating current you will say that the "pitch" of the note has been
increased or that you hear a note higher in the musical scale.

If we started with a very low-frequency alternating current, say one of
fifteen or twenty cycles per second, you wouldn't say you heard a note
at all. You would hear a sort of a rumble. If we should gradually
increase the frequency of the alternating current you would find that
about sixty or perhaps a h