Popular Science Monthly/Volume 25/June 1884/Professor Dvorak's Sound-Mills


PROFESSOR SILVANUS P. THOMPSON has made known, through the columns of "Nature," an interesting series of experiments by Professor V. Dvorák, of the University of Agram, in the production of an apparatus which should rotate under the influence of sound-waves in the same way as the radiometer introduced by Professor Crookes rotates under the influence of rays of light and heat. The same idea was suggested independently to several men, among whom were our countrymen, Professor A. M. Mayer, of Hoboken, and Mr. Edison, all of whom have made in the matter researches of great scientific interest. Professor Dvorák has devised four kinds of "sound-mills," as they may be called, two of which depend on the repulsion of resonant boxes, and two on different principles.

One of the instruments is represented in Fig. 1. It consists of a light wooden cross, balanced on a needle-point, and carrying four light resonators—hollow balls of glass, forty-four millimetres in diameter,

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Fig. 1. Fig. 2.

with an opening of four millimetres at one side, and responding to the note g', or the middle G, of the piano-forte (= 392 vibrations). When this note is forcibly sounded by the tuning-fork, the air in the resonators vibrates in response, and the apparatus begins to rotate. Rotation will take place even if there is only one resonator, properly balanced; but the phenomenon is more marked and certain if there are four.

A second style of apparatus—the "rotating resonator"—is represented in plan and elevation in Fig. 2. It consists of a short cylindrical box of stiff paper, having four projections, each of which bears at its side a short open tube of paper. It is hung on a silk fiber, and is supplied with a small needle, projecting below to steady the motion during its rotation.

The operation of these instruments depends on the principle which has been pointed out by Lord Rayleigh and Professor Mayer as well as by Professor Dvorák, that "when sounds of great intensity are produced, the calculations, which are usually carried only to the first order of approximation, cease to be adequate, because now the amplitude of motion of the particles in the sound-wave is not infinitely small as compared with the lengths of the sound-waves themselves. Mathematical analysis shows that under these circumstances the wave of the pressures in the condensed part, and in the rarefied part of the sound-wave, is no longer equal to the undisturbed atmospheric pressure, but is always greater. Consequently, at all nodal points in the vibrations of the air in tubes or resonant boxes, the pressure of the air is greater than elsewhere; and therefore any resonator closed at one side and open at the other is urged along bodily by the slight internal excess of pressure on the closed end." The apparatuses therefore rotate by reaction.

To produce vibrations of sufficient intensity. Professor Dvorák uses heavy tuning-forks mounted on resonant cases, and excited electrically. For this purpose he places between the prongs of the fork an electro-magnet, in which the core is composed of two plates of iron, separated by a sheet of paper, and cut of such a breadth as to lie between the prongs without touching them. The core is overwound with insulated copper wire, as shown at E, Fig. 3, and the electro-magnet is mounted by a bent piece of wood, a b c, upon the sounding-box, K, of the fork. The wires are connected in a circuit with the battery, and with the electro-magnet of a self-exciting tuning-fork of the same note. The sounding-boxes of the forks must not touch the table, but the arm a b c is clipped at about the point b in a firm support; and particular care must be taken to have the wood of the resonant boxes tuned into exact accord with the tone of the fork and of the air within the cavity of the box.

The third apparatus is the "sound-radiometer," and was described by Professor Dvorák before the Imperial Viennese Academy in 1881. It is more simple than the two instruments previously described, but its cause of action is less easily explained. It is shown in Fig. 4. It consists of a light cross of wood pivoted by a glass cap upon a vertical needle, and having attached to its four arms four pieces of white card, perforated with holes which are depressed conically on one side and raised at the other, so as to present a surface something like that of a nutmeg-grater. Each card has twenty-five holes thus pierced, and the

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Fig. 3. Fig. 4.

whole apparatus a hundred holes. The rotations are more rapid if the cards are set on obliquely in the fashion shown at E in the figures, with the burred sides outward. The rotations are produced when the "mill" is set in front of the resonant box.

Dvorák's fourth apparatus is called by him an "acoustic anemometer," and is represented in Fig. 5. It consists of a little "mill" of simple construction, h i k, the vanes of which are made of small pieces of paper or card slightly curved, and a sounding-box, c d f g, placed a little way from it, while between them is held an ordinary Helmholtz's

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Fig. 5.

resonator having its wide opening, b, turned toward the box, and its narrow opening, a, toward the mill. The stem of the tuning-fork is inserted in the socket F of the sounding-box. The internal increase of pressure induced by the vibrations of the tuning-fork through the sounding-box in the resonator at a has the effect of driving a jet of air gently against the sails of the mill, which consequently rotates. The two-aperture resonator of this apparatus may be replaced by a resonator having but one aperture, which may be formed of a glass ball cut away at one side and cemented to a glass plate having a small hole in the center. When the air ejected from the mouth of the resonator is examined by the method of mixing smoke with it, and then viewing it through slits cut in an open disk, the currents are seen to consist of a series of vortex rings. A variation of this anemometer may be made by taking a card pierced with a hundred holes and placing it between the resonant box and the "mill," when the latter will rotate in the wind which passes through the conical holes.

The machines of Mayer, Mach, and others, are closely akin to those of Professor Dvorák in design and action. Mr. Edison also has contrived a phonometer, or instrument for measuring the mechanical force of sound-waves produced by the human voice, in which the vibrations produced in the phonograph-diaphragm by a sound made in the mouth-piece propels a finely-cut ratchet-wheel with considerable velocity. With this device Mr. Edison has "literally accomplished the feat of talking a hole through a deal board."