RECORDING SPEECH VIBRATIONS. 15 



wave vary the depth of this groove. In an ideal recorder the depth of 

 the groove would vary directly with the ordinate of the sound wave 

 and a longitudinal cut along the middle of the groove would give the 

 sound curve itself. The curves, however, are systematically distorted. 

 The weight of the recorder rests on the engraving point and bends 

 the diaphragm. In a condition of silence the diaphragm has therefore a 

 curved surface and is in a state of tension ; downward movement diminishes 

 it, upward movement increases it; the movement in the two directions is 

 not equal for the same force, the curve being systematically diminished 

 on the upper side and magnified on the lower one. Opposed to this 

 is the distortion due to the resistance of the wax. In a condition of rest 

 the sapphire tool turns off a chip of a certain thickness; when vibrations 

 act on the diaphragm the tool takes a thicker or a thinner chip according 

 to the positive or negative phase of the vibration. The resistance, how- 

 ever, depends on the thickness of the chip; movement downward is thus 

 systematically diminished in comparison with movement upward. This 

 appears clearly in some of the phonograph tracings by Professor Hermann, 

 of which a specimen is shown in figure 2. 



The phonograph record may be made on a composition cylinder called 

 the "wax" cylinder; the record is reproduced by exchanging the recorder 

 with its cutting tool for a similarly made reproducer with a round sapphire 

 or glass point. Wax records gradually wear out. The original record 

 may be used to form a matrix by galvanoplating ; from the matrix a 

 large number of copies may be obtained by casting in wax or in a much 

 harder material or in celluloid. These are the so-called "molded" and 

 "indestructible" records. Still another modification is the direct celluloid 

 process of Lioret; the surface of a cylinder of celluloid is rendered soft 

 at the moment of taking the record; it is afterwards hardened again. In 

 another modification the phonograph record is made on a disc instead of 

 a cyUnder; a metal matrix is formed and is used as a die to stamp copies 

 in cardboard. 



To make a gramophone record, a recording lever turning on an axle 

 is attached to the center of the diaphragm; its cutting end rests on a 

 rotating disc of wax. The construction and action of the gramophone 

 recorder are shown in figures 3 and 4. The groove made in the wax by 

 the cutting end has sidewise deviations due to movements of the center 

 of the diaphragm in response to the sound waves; the depth of the 

 groove is constant. The wax plate containing the sound line is used to 

 form a matrix by electroplating. From the matrix the black composi- 

 tion discs (called "rubber discs," although they contain no rubber) are 

 produced by hydraulic pressure under heating. These black discs or 



