Page:Elementary Text-book of Physics (Anthony, 1897).djvu/179

§ 138] a card against the teeth of a revolving wheel. With a very slow motion the card snaps from tooth to tooth, making a succession of distinct taps, which, when the revolutions are sufficiently rapid, blend together and produce a continuous tone, the pitch of which rises and falls with the changes of speed. Savart made use of such a wheel to determine the number of vibrations corresponding to a tone of given pitch. After regulating the speed of rotation until the given pitch was reached, the number of revolutions per second was determined by a simple attachment; this number multiplied by the number of teeth in the wheel gave the number of vibrations per second. The siren is an instrument for producing musical tones by puffs of air succeeding each other at short equal intervals. A circular disk having in it a series of equidistant holes arranged in a circle around its axis is supported so as to revolve parallel to and almost touching a metal plate in which is a similar series of holes. The plate forms one side of a small chamber, to which air is supplied from an organ bellows. If there be twenty holes in the disk, and if it be placed so that these holes correspond to those in the plate, air will escape through all of them. If the disk be turned through a small angle, the holes in the plate will be covered and the escape of air will cease. If the disk be turned still further, at one twentieth of a revolution from its first position, air will again escape, and if it rotate continuously, air will escape twenty times in a revolution. When the rotation is sufficiently rapid, a continuous tone is produced, the pitch of which rises as the speed increases. The siren may be used exactly as the toothed wheel to determine the number of vibrations corresponding to any tone.

By drilling the holes in the plate obliquely forward in the direction of rotation, and those in the disk obliquely backward, the escaping air will cause the disk to rotate, and the speed of rotation may be controlled by controlling the pressure of air in the chamber.

Sirens are sometimes made with several series of holes in the disk. These serve not only the purposes described above, but also to compare tones of which the vibration numbers have certain ratios.