1878.] 
249 
The Phonograph . 
comparatively high powers of the microscope, and yet must 
be sufficiently pronounced to impart to the diaphragm 
through its projecting pin those minute variations of vibra- 
tion by which the proper form is given to the sound-bearing 
waves of the air, and the exaCt quality of the sound is con- 
veyed to the ear. It is almost impossible to conceive that 
microscopical striae (for they can be nothing more) upon 
such a substance as tin-foil can impart, by mechanical 
means, to a diaphragm as rigid as that employed in the 
phonograph, such niceties of motion ; but the phenomena 
connected with the telephone have shown that metallic 
diaphragms are capable of imparting to the air, and the 
human ear is capable of detecting, sonorous vibrations whose 
amplitude is so minute as to have been altogether unsus- 
Fig. 3. 
>■ 
peCted before. Some idea of the minuteness of the inden- 
tations may be formed from Fig. 3, which is printed from 
an electrotype cast of a piece of the foil, and is therefore a 
facsimile on a plane surface of the marks recorded by the 
instrument. 
It will readily be understood that, in order to obtain a 
perfect reproduction of the original sounds, the tin-foil must 
travel below the diaphragm at precisely the same speed as 
it was turning when it was receiving the impressions, and 
therefore in Mr. Edison’s second instrument, which we have 
been describing, the heavy flywheel was added to render the 
speed of rotation as uniform as possible ; but in turning the 
instrument by hand it is impossible, notwithstanding this 
addition, to insure the surface speed of the cylinder being 
always the same. In order to meet this requirement Mr. 
Edison has since applied clockwork mechanism for driving 
