Waves and ripples in water, air, and æther

by Sir J. A. Fleming

Such being the case, you will see that if the glass plate is placed behind the flame at a certain distance, the flame at once stops flaring and becomes quiescent. If, however, the plate is moved to or from the flame by a very small distance equal to about the one-twelfth part of an inch, the tall flame at once drops in height and begins to flare. If we move the plate steadily backwards by equal small distances, we find the flame alternately quiescent and waving.

The explanation of this effect is that it is due to the interference between the direct and reflected sound-rays. The waves of air are turned back when they meet the glass in such a manner that the crests of the arriving waves are made to coincide with the hollows of the reflected waves, or, to speak more correctly, the zones of condensation of one are coincident with the places of rarefaction of the other. When the glass is adjusted so that this happens, all air-wave motion just in front of it is destroyed, and hence the sensitive detecting flame remains quiescent. If, however, the glass is moved nearer to or further from the flame, then the condensations of the reflected wave may be made to fall in the same places as the condensations of the arriving wave, and in that case the disturbance is doubled, and not destroyed.

Fig. 52.

A little model may be made which will help the reader to grasp this point. Cut out a piece of paper in the form shown in [Fig. 52] to represent a wave. Bend back the paper on itself at the dotted line ab, and let one half represent the arriving wave, and the other the reflecting wave. It will be seen that in this case the crests of the incoming wave are obliterated by the hollows of the returning wave. If, however, the paper is bent back at cd, then the crests of the reflected and incident waves conspire, and there is no interference.

Whenever we can produce interference in this manner between two sets of sound-rays, or light-rays, or rays of any other kind, we have the strongest possible proof that we are concerned with a wave-motion; because in no other way that we can understand is it possible that a destruction of sound by sound can take place by, so to speak, superimposing two sound-rays, or a destruction of light by bringing together two rays of light.

We may, then, conclude our discussion of this part of our subject by examining the manner in which vibrating bodies communicate a different form of wave to the air. As already explained, we are by our ears enabled to appreciate the fact that the air is thrown into a wave-motion, and that this wave-motion may consist of waves of great or small wave-length, and great or small amplitude. But we are able to do something more—we are able to detect a difference between the form of two waves, so that if represented by a wavy line of light, as you have seen, the nature of the outline of that line impresses itself upon our consciousness. Nothing is more remarkable than the extraordinary delicacy of the ear in this respect. Amongst all our scores of friends and acquaintances we recognize each by a quality of voice which we speak of as harsh, melodious, sympathetic, rasping, penetrating, or clear. This is not altogether a matter of enunciation or vocalization, for if different persons pronounce correctly the same vowel-sound, we can detect a great difference between their voices. We have, then, to ask wherein this difference consists when considered with respect simply to what goes on outside of us in the air.

Great light was thrown on this by the invention and perfection of the phonograph and telephone, and also a more recent and wonderful invention, variously called the micro-phonograph or telegraphone. You have all heard a phonograph speak, or sing, or reproduce music. In its original form the Edison phonograph consisted of a cylinder covered with tinfoil, against which pressed lightly a steel point attached to the centre of a metal disc. In its modern form, as improved by Edison, Bell, Tainter, and others, it is a far more perfect instrument for recording and reproducing sound. It now consists of a cylinder covered with a composition similar to very hard soap. This cylinder is carried on a metal drum, and caused to revolve by clockwork slowly and very uniformly. A metal arm carries an elastic metal disc called a receiving diaphragm, and to the back of this is attached a very delicate cutting-tool like a small chisel. By means of a screw the chisel and diaphragm are made to travel along the cylinder, and if no vibration is given to the disc the tool cuts a spiral on the recording cylinder, which is a clean groove with smooth bottom ploughed out of the soft composition. If, however, we speak or sing to the diaphragm, the air waves cause it to vibrate, and this makes the tool cut a furrow, the bottom of which is irregular, the undulations corresponding exactly to the movements of the diaphragm. Thus, if we could look at the section of the furrow, we should see it undulating like a miniature switchback railway, each up-and-down corresponding with one vibration of the diaphragm. In this manner we store up a record of air waves on the hard-soap cylinder. In the next place, to reproduce the sound, another diaphragm with a trumpet mouthpiece has at its back a little pointed lever or set of levers, one extremity resting upon the bottom of the irregular furrow.

Then, if the cylinder is so set that this reproducing diaphragm travels over the record cut by the receiving diaphragm, we have a motion communicated to it which is the exact facsimile of that which produced the furrow. Accordingly, the reproducing diaphragm gives back to the air impulses which reproduce the same wave-trains, and therefore the same speech or song, as that which created the record.

We may in this manner record any human utterance and receive it again, word-perfect, months or years after it was made.[25]