ACOUSTICS.
1. On the .acoustic Figures produced by the Vibration of Plates.
Galileo long ago pointed out a method of showing the vibration of plates held horizontally, by covering the plate with fine sand, which accumulated in those parts of the vibrating surface which were at rest. In the year 1787, M. Chladni of Wirtemberg made a great number of experiments on the nature of the vi brations produced by plates of glass of different shapes. In our article Acouslics, we have done little more than mention these experiments, and have given a few specimens of the acoustic figures produced in these experiments in Plate II. Fig. 5, 6, 7, 8, 9; but as the subject is highly interesting, and as the experiments may be easily repeated, we shall make no apology for resuming the subject.
The plates used in such experiments should be thin plates of good window glass, from three to ten inches in diameter, having their edges ground to the shape required. The powder to be used may be very fine sand, but Professor Oersted, whom we have had the pleasure of seeing repeat many of Chladni's experi ments, prefers the Lycopodium powder.
In order to damp the vibrations of the plates in par ticular places, we have only to pinch them between the finger and thumb of the left hand in the place di rected, and then set them a vibrating by drawing a fiddle bow over their edge. Sometimes another place may be damped by the application of another finger of the left hand, or any points of the plates may be press ed against one or more fixed objects.
These methods of damping by the fingers, and by pressing against fixed obstacles, are shown in Plate CCCCLXXXIII. Fig. 1. 2, 3.
In Fig. 1. one part only is damped by being pinch ed between the fingers and thumb; but it is easy to damp another part by any of the three unoccupied fin gers of the same hand.
In Fig. 2. the centre e of an elliptical plate is damp ed by pressing it with the thumb against a fixed point, such as a piece of cork, and two points at its circum ference are damped by the application of two fingers.
In Fig. 3. the point e is damped by bringing the elliptical plate against an upright fixed obstacle a b, and the two points c, d are damped by the two fingers.
In place of using the fingers alone, particular places may be damped by means of a wooden vice, Fig. 4. furnished with small wooden screws, or it would be easy to construct a more complicated one in which the arms moved round joints, so that a number of points could be easily pinched at the same time.
1. If we take a square plate, Fig. 5. and pinch it in the centre, while it is put into vibration by drawing the bow near one of the angles, the sand will accumu late as in the figure, and the sound will be the gravest of all. Sometimes the figure changes into four curves, which join the middle of the sides of the plate.
2. If the square plate pinched at the centre is made to vibrate by drawing the bow along the middle of one of its sides, the sand will accumulate as in Fig. 6. and the sound will be less grave than in the preceding case.
3. if the plate Fig. 7. is pinched at N, and the bow applied at F in a line perpendicular to the side AB, the sand will arrange itself in three parallel lines.
4. But if it is pinched a little farther from the edge, as at N, Fig. 8. the lines will change into curves as in the figure.
5. If the square plate is held at N, and the bar ap plied at F, Fig. 9. two parallel lines, and one perpen dicular line will be produced as in the figure.
6. If a circular plate, Fig. 10. is pinched at its centre, and also at another point of its circumference, and if the bow is applied 45° from this last point, a cross will be produced as in the square plate, Fig. 5. The tone is here the gravest that the plate can produce. Fig. 10. may be produced merely by pinching the plate at its centre.
7. Fig. 11. may be produced by the very same pro cess as the last, merely by drawing the bow more gently and rapidly over the point of 45°. Sound less grave.