HANDBOOK OF ACOUSTICS - online book

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ON INTERFERENCE.                             149
hia. 79.
place, suppose the forks are not in exact unison; as we have already seen, they will be at one moment in the same phase, then gradually diverge till in opposite phase, and again gradually converge to the same phase. The line of light will vary coincidentally; at one moment being of considerable length, then gradually shortening till but a mere spot, and then lengthening again. The beats, of which this alternate lengthening and shortening is the optical expression, will at the same time be heard.
If the beam of light in the above, instead of falling on a screen, be received on the revolving mirror of fig. 3, the separate vibrations will, as it were, be visible, and will appear as represented in fig. 79 of, in which the varying amplitude of the sinuosities corresponds to the varying intensity of the resultant sound.
If no better apparatus be at hand, beats may be studied on the pianoforte, by loading one of the two wires of a note with wax, and then striking the corresponding key; or they may be observed by stretching two similar strings on a violin, and after bringing them into unison, throwing one more or less out of tune ; but in these cases, as the tones are compound, the matter is complicated by the beats of the overtones.
If two tuning-forks are nearly, but not quite in unison, and the vibration number of one of them is known, it is easy to ascertain the vibration number of the other, by counting the beats between them, provided we know which is the sharper of the two. For example, suppose we have a standard C fork producing exactly 512 vibrations per second, and on sounding it with another fork, we find that in half a minute, 90 beats are counted. Now 90 beats per half minute, is at the rate of three beats per second; but we know