Part 13 (1/2)

When the sides of the bellows are squeezed together the air ether and the air is coreater, of course, is the pressure hich the enclosed air seeks to escape That it can do only by lifting up, that is by blowing out, the two elastic strips which close the end of the pipe

The air pressure, therefore, rises until it is sufficient to push aside the elastic membranes or vocal cords and thus to permit some of the air to escape It doesn't force the h to let some air out But the moment some air has escaped there isn't so much inside and the pressure is reduced just as in the case of an automobile tire from which you let the air escape What is the result? Theof the pipe What got out, then, was just a little puff of air

The bellows are working all the while, however, and so the space available for the reain becoain sufficient to open the membranes

Another puff of air escapes

This happens over and over again while one is speaking or singing

Hundreds of times a second the vocal cords vibrate back and forth The frequency hich they do so determines the note or pitch of the speaker's voice

What deternificance of the sounds which he utters? This is aof ive you enough of an answer for your study of radio-telephony I as for they are easier to picture than membranes like the vocal cords

Suppose you have a stretched string, a piece of rubber band or a ill do You pluck it, that is pull it to one side When you let go it flies back Because it has inertia[7] it doesn't stop when it gets to its old position but goes on through until it bows out almost as far on the other side

[Illustration: Pl VII--Photographs of Vibrating Strings]

It took so, notit, goes to the string and becoy, its ability to do work This work it does in pushi+ng the air molecules ahead of it as it vibrates In this way it uses up its energy and so finally coain to rest Its vibrations ”da carries it a sinal position We say that the ”a the size, of its vibration decreases The frequency does not It takes just as long for a ser Of course, for the vibration of large a must move faster but it has to move farther so that the tied

First the string crowds against each other the airfurther away, just as fast as theseThat takes place at the rate of about 1100 feet a second When the string swings back it pushes away the molecules which are behind it and so lets those that were being crowded follow it You know that they will AirFollowing the shove, therefore, there is a chance for the molecules to inally

The news of this travels out fro As fast as molecules are able they hbors who are farther away; and these in turn reat 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 y Whatever thissays is repeated by each hbor next farther away First the string says: ”Go back” and eachsays: ”Come on” and each molecule of air obeys as soon as the command reaches him Over and over this happens, as78]

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 itthe wire and then back in the opposite direction The electron next to it does the sa al for such an effect to pass through a crowd of electrons If wevibrate 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 in79 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 hears a note two octaves above the ”funda is capable

It is entirely possible, however, for a string to vibrate siive not only its fundaraphs[8] of Fig 80 of Pl

VII illustrate this possibility

What happens then to the ? They must perform quite complex vibrations for they are called upon to s all trying to push theain at the pictures, of Fig 80 and see that each s placed close together, each vibrating in a different way Each of the strings has a different frequency of vibration and a differentaway fro 81]

Suppose instead of a single string acting upon the adjacent s Suppose the first would81A, the second as in Fig 81B, and the third as in Fig 81C

It is quite evident that the81D

Now take it the other way around Suppose we had a picture of the motion of a78 but was co 81D We could say that this complex motion was made up of three parts, that is, had three component siraphs of Fig 81 That means we can resolve any complex vibratory motion into component motions which are simple