. The Ontario high school physics. cribed. When the speedof rotation is slow, and hence the number of vibrations persecond few, the pitch is low, and when it is increased thepitch becomes higher. 204. Determination of Pitch. The number of vibrationscorresponding to any given pitch may be determined byvarious devices. One is the toothed wheel shown in Fig. we wish to find the number of vibrations of a tuning-fork. The speed of rotation is increased until the soundgiven by the wheel is the same as that by the fork. Thenthe speed is kept constant for a certain time—say half aminute—an
. The Ontario high school physics. cribed. When the speedof rotation is slow, and hence the number of vibrations persecond few, the pitch is low, and when it is increased thepitch becomes higher. 204. Determination of Pitch. The number of vibrationscorresponding to any given pitch may be determined byvarious devices. One is the toothed wheel shown in Fig. we wish to find the number of vibrations of a tuning-fork. The speed of rotation is increased until the soundgiven by the wheel is the same as that by the fork. Thenthe speed is kept constant for a certain time—say half aminute—and the number of turns of the crank in this time iscounted and the rotations of the wheel deduced. Then onmultiplying this number by the number of teeth on the wheelwe can at once deduce the number of vibrations per perforated disc may be used in the same way. A more satisfactory instrument is that shown in Fig. 214and known as a siren. It was invented by Cagniard de laTour in 1819. LIMITS OF AUDIBILITY OF SOUNDS 181. A perforated metal disc B rotates on a vertical axis, justabove a cylindrical air-chamber G. Theupper end of the chamber and also the discare perforated at equal intervals along acircle which has as centre the axis of rota-tion. The upper and lower holes correspondin number, position and size, but they aredrilled obliquely, those in tlie disc slopingin a direction opposite to those in the endof the chamber. The tube D is connectedwith a bellows or other blower. When the air is forced into the chamber fio- siren. Air enters the chamber C by and passes up through the holes, the disc is way of the pipe /> andon r L o escaping causes the disc made to rotate by the air-current striking ^ ^ rotate,against the sides of the holes in the di«c, and the more power-ful the air-current the more rapid is the rotation. Vibrations in the air are set up by the puffs of air escapingabove the disc as the holes come opposite each other; and bycontrolling th
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