lecules to move back and even to
occupy more room than they had originally.

The news of this travels out from the string just as fast as did the
news of the crowding. As fast as molecules are able they move back and
so make more room for their neighbors who are farther away; and these in
turn move back.

Do you want a picture of it? Imagine a great crowd of people and at the
center some one with authority. The crowd is the molecules of air and
the one with authority is one of the molecules of the string which has
energy. Whatever this molecule of the string says is repeated by each
member of the crowd to his neighbor next farther away. First the string
says: "Go back" and each molecule acts as soon as he gets the word. And
then the string says: "Come on" and each molecule of air obeys as soon
as the command reaches him. Over and over this happens, as many times a
second as the string makes complete vibrations.

[Illustration: Fig 78]

If we should make a picture of the various positions of one of these air
molecules much as we pictured "Brownie" in Letter 9 it would appear as
in Fig. 78a where the central line represents the ordinary position of
the molecule.

That's exactly the picture also of the successive positions of an
electron in a circuit which is "carrying an alternating current." First
it moves in one direction along the wire and then back in the opposite
direction. The electron next to it does the same thing almost
immediately for it does not take anywhere near as long for such an
effect to pass through a crowd of electrons. If we make the string
vibrate twice as fast, that is, have twice the frequency, the story of
an adjacent particle of air will be as in Fig. 78b. Unless we tighten
the string, however, we can't make it vibrate as a whole and do it twice
as fast. We can make it vibrate in two parts or even in more parts, as
shown in Fig. 79 of Pl. VII. When it vibrates as a whole, its frequency
is the lowest possible, the fundamental frequency as we say. When it
vibrates in two parts each part of the string makes twice as many
vibrations each second. So do the adjacent molecules of air and so does
the eardrum of a listener.

The result is that the listener hears a note of twice the frequency that
he did when the string was vibrating as a whole. He says he hears the
"octave" of the note he heard first. If the string vibrates in three
parts and gives a note of three times the frequency the listener he