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THE PHONOGRAPHIC METHOD 33
reveal themselves nevertheless in the reproduction of the test inscriptions might be laid to the charge of the phonograph.
In taking the first inscription (Cylinder A) the screw regulating the supply of electricity was adjusted so that the shaft of the phonograph made about 140 revolutions a minute. At this rate the needle took over three minutes to cover the cylinder. About two and a half minutes being, I am told, the standard time, the instrument was running during the inscription of Cylinder A considerably below what is considered its normal rate. In listening to the reproduction the mingling of the air waves from the phonograph and the fork was effected by attaching another pipe by means of a hard rubber Y to that leading from the phonograph diaphragm to the ear-piece, the free end of this secondary pipe being favorably placed for the reception of sound from the fork. In the phonographic note itself the rhythm of intensity which I had noticed in using the instrument before reappeared in some slight degree, seeming now in general a double wave with a period equal to that of the revolution of the cylinder. This rhythm of intensity, accomplishing itself in the course of a revolution of the cylinder, is plausibly to be explained as the result of a slight difference in the nearness of the diaphragm to the surface of the wax at different points in its circumference. This would be the result of any deviation of the cylinder from perfect circularity, or any want of coincidence between its axis of rotation and its axis of form. If this is its origin, some pulsation of this kind is to be generally expected in a phonograph note. In later observations I seemed to notice that at higher rates of the speed of the instrument this pulsation of the note was less distinct. Accompanying the tone I could detect two distinct currents of noise: one irregular, like a stream of sparsely scattered pebbles ; the other a continuous mild grating noise. This latter exhibited the double rhythm sometimes very distinctly; but it seemed to me I could hear it also in the tone itself.
During two days in which I studied Cylinder A I found difficulty in so arranging the apparatus as to bring out the beats between the two notes distinctly. Although I seemed to be able to detect them as a shimmer of sound through the grinding pulsations of the phonograph note, yet I could not always be sure that I was not at times counting the mechanical instead of the auricular beats. Moreover, they sometimes escaped me through their irregularity; now and then becoming unaccountably fast, fusing into a slow whir. Indeed, on attending to the pitch of the note, I found it to be varying perceptibly. Since the speed of the cylinder was below the normal rate, this fact could be interpreted as pointing to one of the limits of the accurate performance of the instru- |
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