Did you ever take the end of a cane in your hand, raise it up over your
head, and then bring it down suddenly and sharply, so that it nearly
touched the ground, as though you were about to strike something? If
not, try it now with a thin walking-cane or with a pine stick about
three feet long and one-half inch thick, and you will find that there
is a peculiar sound given out. It is not the stick that makes this
sound, but it is owing to the fact that you have caused the air to
vibrate, or tremble, and thus give out a sound.
[Illustration: Fig. 10]
If you strike a tuning-fork sharply you will see the ends vibrate and a
sound will be given. If you put your fingers on top of a silk hat and
speak near it you will feel vibrations of your voice.
Every time you speak you cause vibrations of the air; and the louder
and higher you speak the greater the number of vibrations.
Suppose you take a thin piece of wood in your hands (say, for instance,
the lid of a cigar-box cut in the shape shown in the picture, Fig. 10)
and hold it about two inches from your mouth and then speak. You will
feel the wood tremble in your hand. This is because the vibrations of
the air cause the wood to vibrate in the same manner. These vibrations
are very minute and cannot be seen with the naked eye, but they
actually take place, and could be measured with a delicately balanced
instrument.
[Illustration: Fig. 11]
Now let us try another experiment in further illustration of this
principle. We will take a tube about three inches long and one and
one-half or two inches in diameter. This tube may be made of cardboard.
Now cut out a piece of thin cardboard which will just fit over one
end of the tube. This piece we will call the "diaphragm." Fasten the
diaphragm by pasting it with two strips of thin paper to the tube.
These strips of paper should be fastened only on the ends, and the
middle of the paper allowed to be slack, as shown in the picture, so
that the diaphragm may work backward and forward easily. Take a small
shot about the size seen in the sketch and tie it to a single thread of
fine silk, then let it hang as shown in the sketch (Fig. 11), so that
it will only just touch the diaphragm. Now, if you speak into the open
end of the tube the diaphragm will vibrate and the shot will be seen
to move to and from it according to the strength of the vibrations. If
we could by any means make a diaphragm in another tube reproduce these
same vibrations, we should hear the same words respoken, if the tube
were held to the ear.
[Illustration: Fig. 12]
While the vibrations caused by the human voice are too minute to be
seen, it may seem surprising that they can be made to produce power.
This is done by an ingenious mechanism called a Phonomotor, perfected
by the great inventor Thomas A. Edison, of whom every one has probably
heard. This mechanism, when spoken or sung at (or into) immediately
responds by causing a wheel to revolve. No amount of blowing will start
the wheel, but it can instantly be set in motion by the vibrations
caused by sound.
The Phonomotor (which is shown in the engraving Fig. 12) has a
diaphragm and mouthpiece. A spring, which is secured to the bedpiece,
rests on a piece of rubber tubing placed against the diaphragm. This
spring carries a pawl that acts on a ratchet or roughened wheel on the
fly-wheel shaft. A sound made in the mouthpiece creates vibrations in
the diaphragm; the vibrations of the diaphragm move the spring and pawl
with the same impulses, and as the pawl thus moves back and forth on
the ratchet-wheel it is made to revolve.
The instrument, therefore, is of great value for measuring the
mechanical force of sound waves, or vibrations, produced by the human
voice.