VIBRATION FIGURES, are certain figures, formed by sand or very dry saw-dust, on a vibrating surface, which is connected with the sensation of sound in our organs of hearing. If the surface, on which the figures are to be formed, be strewed over with bodies easily put in motion, these, during the vibration, remain on the parts at rest, and are thrown from the parts in motion. The form of the parts at rest, which will be shewn by the sand that remains unmoved, and which, in general, is symmetrical, is called a vibration figure. To produce such a figure, nothing is necessary but to know the method of bringing that part of the surface which you wish not to vibrate into a state of rest, and of putting in motion that which you wish to vibrate. On this depends the whole expertise of producing vibration figures.
Thus take a square piece of glass, pretty thin, and very smooth, such as that used for windows, about four or
five inches over, or even more. Smooth it at the edges on a grinding-stone; strew a little saw-dust over its surface, and lay hold of it gently with the thumb and forefinger of the left hand. Holding it thus by the middle, with the right hand rub a violin bow softly against one of its edges, drawing the bow either up or down in a direction almost perpendicular to the surface of the glass, and you will see a tremulous movement, and the whole dust leap about. If the bow be exactly in the middle of one of the sides, the dust will arrange itself almost in the direction of the two diagonals, dividing the square into four isosceles triangles. If the bow be applied at a quarter only of the distance of the one corner from the other, the dust will arrange itself in such a manner as to be found in the two diameters of the square, dividing it into four equal squares. At other times, when the bow deviates a little, the dust forms a figure like a double C, when the two letters are joined back to back. If the square be held by the two extremities of the diameter, opposite to that against which the bow is applied, the dust will form a kind of oval, one of the axes of which will be the same diameter. If the glass be of a circular figure, and be held by the middle, the dust will arrange itself in such a manner as to form the six radii of a regular hexagon. These discoveries were made by Dr Chladni, about the time that he invented the musical instrument, to which he gave the name of CHLADNI (see that article, Suppl.); and as he found the vibration figures to vary in form with the various tones produced by the vibrating substances, a profection of his experiments may probably contribute to throw new light on the philosophy of musical sounds. We shall therefore give, from the 3d volume of Neus Journal der Physik, by Professor Gren, a few directions for making such experiments.
Any sort of glass may be employed, provided its surface be smooth; and when the plate has acquired the proper vibration, it should be kept in that state for some seconds, by continuing to rub it with the bow. The figures will thus be accurately formed.
Such plates should be procured as are pretty equal in thickness. It may be said, in general, that a plate the thinner it is will be so much the fitter for these experiments, though in this respect there is a certain minimum. In small plates, such as those that are circular, and not above six inches in diameter, the observation is general; but in larger plates too great thinness is prejudicial. Besides, it will be found that very thin glass is commonly very uneven, and must therefore be unfit for the experiments.
In practising the experiments, it will be proper to have plates of different sizes; and the sand employed should not be too fine. In other words, it must be of such a nature that when you incline the glass-plate it may readily roll off, because, in that case, it will be easily thrown from the vibrating parts. It will be of advantage that it be mixed with fine dust, which shews peculiar phenomena during the experiments, as it collects itself at one place of the vibrating part.
The plate must be equally bespattered with sand, and not too thick, as the lines will then be exceedingly fine, and the figures will acquire a better defined appearance.