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160 EXPERIMENTAL STUDY OF BEATS. [VII. § 78.
In addition to the alternations of intensity which characterise beats, they also contain variations of pitch. The existence of such variations is both theoretically demonstrable and experimentally recognisable, but they are too minute to require examination here1.
78. The most direct way of studying the beats of simple tones experimentally is to take two unison tuning-forks and attach a small pellet of wax to the extremity of a prong of one of them. The fork so operated on becomes slightly heavier than before; its vibrations are therefore slightly retarded, and its pitch lowered. When both forks are struck and held to the ear, beats are heard. These will be most distinct when the forks' tones are exactly equally loud, for in this case the minima of intensity will be equal to zero, and the beats will therefore be separated by intervals of absolute, though but momentary, silence. The increase in rapidity of the beats, as the interval between the beating tones widens, may be shown by gradually loading one of the forks more and more heavily with wax pellets, or by a small coin pressed upon them. If it is desired
1 The reader may, if he wishes to pursue this subject, refer to a Paper, by the author, on ' Variations of Pilch in Beats,' in the Philosophical Magazine for July, 1872, from which Fig. 56 has been copied. |
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