HANDBOOK OF ACOUSTICS - online book

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

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38
HAND-BOOK OF ACOUSTICS.
Bound this is fastened a strip of dark paper B, ruled with equidistant white lines. C is a 2" objective, giving an image of the white lines, which is viewed through the microscope, D. The tuning-fork (t) to be tested is fastened vertically in a vice, so that one of the prongs is situated in the common focus in such a way as to obscure about one-fourth of the field of view. Thus, on looking through the microscope when everything is at rest, an image of the white lines is seen, and the part of the prong (a, fig. 27 A), of the tuning-fork
Fig. 27 (A).                                        Fig. 27 (B).
partially obscuring them; (6) is a scale fixed in the eye-piece. When the fork is set vibrating and the drum is rapidly rotated, the lines can no longer be separately distinguished; but, just as, in the Graphic method, we found a wave to result from two movements at right angles, so by the composition of the fork's motion with that of the white lines, a wave makes its appearance, (fig. 27, B). If the white lines pass through a space equal to the distance between two of them, while the fork makes one vibration, then the length of the wave will be the distance between two white lines as seen through the microscope; furthermore, if this is the case exactly, the waves will appear stationary. If, however, the drum rotates faster or slower than this, the wave will have a slow apparent motion, either upwards or downwards. As the velocity of rotation of the drum is under the control of the observer, it is easy to keep it at such a speed, that the wave appears stationary, that is, at such a speed that the white lines pass through a space equal to the distance between two of them while the fork makes one vibration. It follows, therefore, that the number of white fines that pass over the field of view during the time of the experiment, is equal to the number of vibrations executed by the fork in that time. An electric pendulum, beating seconds, gives the time, while an electric counter records the num­ber of revolutions made by the drum. A fine pointed tube, filled with magenta, automatically marks the paper on the drum at the