listen to
your own voice.

Notice that in the dictaphone the air waves of your voice are all
concentrated into a small space as they go down the tube. At the end
of the tube is a diaphragm, a flat disk which is elastic and vibrates
back and forth very easily. The air waves from your voice would not
vibrate the needle itself enough to make any record; but they vibrate
the diaphragm, and the needle, being fastened rigidly to it, vibrates
with it.

[Illustration: FIG. 98. Making a phonograph record on an old-fashioned
phonograph.]

In the same way, when the reproducing needle vibrates as it goes over
the track made by the cutting needle, it would make air vibrations too
slight for you to hear if it were not fastened to the diaphragm. When
the diaphragm vibrates with the needle, it makes a much larger surface
of air vibrate than the needle alone could. Then the tube, like an ear
trumpet, throws all the air vibrations in one direction, so that you
hear the sound easily.

EXPERIMENT 58. Put a clean white sheet of paper around the
recording drum, pasting the two ends together to hold it in
place. Put a small piece of gum camphor on a dish just under
the paper, light it, and turn the drum so that all parts will
be evenly smoked. Be sure to turn it rapidly enough to keep
the paper from being burned.

Melt a piece of glass over a burner and draw it out into a
thread. Break off about 8 inches of this glass thread and tie
it firmly with cotton thread to the edge of one prong of a
tuning fork. Clamp the top of the tuning fork firmly above
the smoked drum, adjusting it so that the point of the glass
thread rests on the smoked paper. Turn the handle slightly to
see if the glass is making a mark. If it is not, adjust it so
that it will. Now let some one turn the cylinder quickly and
steadily. While it is turning, tap the tuning fork on the
prong which has _not_ the glass thread fastened to it. The
glass point should trace a white, wavy line through the smoke
on the paper. If it does not, keep on trying, adjusting the
apparatus until the point makes a wavy line.

[Illustration: FIG. 99. A modern dictaphone.]

Making a record in this way is, on a large scale, almost exactly like
the making of a phonograph record. The smoked paper on which a tracing
can easily be made as it turns is like the soft wax cylinder. The
glass needle is like the recording