HANDBOOK OF ACOUSTICS - online book

A complete view of Acoustical Science & its bearings on music, for musicians & music students.

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148
HAND-BOOK OF ACOUSTICS.
Thus, when the vibrating air columns are in the same phase, the india-rubber membranes will vibrate simultaneously in the same direction, so as to expel the gas with greater force, and thus pro­duce a very elongated flame. On the other hand, when they are in opposite phase the membranes will move simultaneously in opposite directions, and thus, neutralizing one another's effects, their move­ments will be without influence on the flame. The latter will therefore rise and fall with the beats, of which indeed, they are the optical expression. By the aid of a rotating mirror, the separate vibrations of the flame may also be observed as explained on page 5.
Instead of the organ pipes referred to above, two of the singing flames described in Chap. I, fig. 3, may be used; but in this case, beats from overtones, as well as from the fundamentals, will, in all proba­bility, be heard. In order to vary the pitch, one pipe should be supplied with a sliding tube, as shown in the left hand pipe of fig. 78.
Two unison tuning-forks may be used to produce beats of varying rapidity. The pitch of one may be lowered by attaching pieces of bees-wax to its prongs, or, if the forks be large, by fastening a threepenny piece, by means of wax, to each prong. With large forks, these beats also may be optically expressed. A pencil of light from the lamp L (fig. 79), passes through the lens I, and then strikes against a little concave mirror fastened to one prong of the fork T. From this mirror it is then reflected to a similar mirror attached to the fork T1, and is finally received on the screen A. When the forks are at rest, only a spot of light appears on the screen; but if one fork is set vibrating, this spot lengthens out, to form a vertical line of light. Now let both forks vibrate together with equal amplitudes, and in the first place suppose them to be in unison: if they are in exactly the same phase, the line of light will be twice as long as at first; if in opposite phase, the line will be reduced to a spot: in any intermediate phase, the line will have a length intermediate between these two extremes. In the next