THE HOMEMADE ORCHESTRA 



337 



of a needle is placed in a spiral groove on the face of this disk. 

 The base of the needle attaches to a diaphragm that closes the 

 mouth of a small funnel. A tube leads from the stem of the 

 funnel to the small end of a horn. When the disk is used to 

 make a record, a disk of impressionable material is used in place 

 of the hard-rubber disk. The voice, or other sound, is caught by 

 the horn, travels down the tube, sets the membrane in vibration, 

 and that in turn the needle. As the point presses on the disk and 

 moves by appropriate mechanism in a spiral path, it engraves 

 on the disk a series of tiny hills and valleys. Now, when from 

 this disk a duplicate hard-rubber record is made and is set 

 rotating on the turntable of the instrument, the needle, as it 

 traverses the groove with its inequalities, is made to move 

 exactly as it did when the voice was making the impression on 

 the soft disk. That naturally makes the membrane vibrate, 

 which vibration is imparted to the air and reinforced by the 

 horn; so the sound is reproduced. 



When one talks into the telephone, his voice strikes a metallic 

 membrane and sets it in vibration. These vibrations constantly 

 alter the intensity of an electric current that is passing through 

 the instrument. The current of varying intensity passes through 

 the wire to the receiver, and produces corresponding changes in 

 the force of an electromagnet by means of which another metal 

 disk is set to vibrating exactly in unison with that of the sending 

 instrument. Thus the voice is reproduced so that the person at 

 the distant end of the line hears the speaker. The method of 

 operation of the electrical device in the instrument has been 

 already explained. 



During the war an exceedingly interesting method of locating 

 the position of an enemy gun was employed, dependent upon the 

 velocity of sound. Suppose that in the accompanying figure 

 (Fig. 175), observers with accurate recording apparatus are 

 stationed at points a, b, and c. Each notes the exact time at 

 which his instrument records the arrival of the boom of the 

 gun, and promptly telephones this time to a central station. The 



