Page:Elementary Text-book of Physics (Anthony, 1897).djvu/190

176 made by König are mounted are of such dimensions that the enclosed body of air will yibrate in unison with the fork, but they are purposely made not quite of the dimensions for the best resonance, in order that the forks may not too quickly be brought to rest.

A membrane or a disk, fastened by its edges, may respond to and reproduce more or less faithfully a great variety of sounds. Hence such disks, or diaphragms, are used in instruments like the telephone and phonograph, designed to reproduce the sounds of the voice. The phonograph consists of a mouthpiece and disk similar to that used in the telephone, but the disk has fastened to its centre, on the side opposite the mouthpiece, a short stiff stylus, which serves to record the vibrations of the disk upon a sheet of tinfoil or wax moved along beneath it. The wax is in the form of a cylinder mounted on an axle moved by a screw working in a fixed nut, so that when the cylinder revolves it has also an endwise motion, such that a fixed point would follow a spiral track on its surface. To use the instrument, the disk is placed in position with the stylus attached and slightly indenting the wax. The cylinder is revolved while sounds are produced in front of the disk. The disk vibrates, causing the stylus to indent the wax more or less deeply, so leaving a permanent record. If now the cylinder be turned back to the starting-point and then turned forward, causing the stylus to go over again the same path, the indentations previously made in the wax now cause the stylus, and consequently the disk, to vibrate and reproduce the sound that produced the record.

The sounding-boards of the various stringed instruments are in effect thin disks, and afford examples of the reinforcement of vibrations of widely different pitch and quality by the same body. The strings of an instrument are of themselves insufficient to communicate to the air their vibrations, and it is only through the sounding-board that the vibrations of the string can give rise to audible sounds. The quality of stringed instruments, therefore, depends largely upon the character, of the sounding-board.

The tympanum of the ear furnishes another example of the facility with which membranes respond to a great variety of sounds.