348 
[April, 
The Phonograph. 
into vibration by being attached by a silken thread to a light 
steel spring, which carried at its extremity a blunt metallic 
point, which was held by the elasticity of the spring against 
the tin-foil covering the cylinder, but with sufficient light- 
ness to allow it to be thrown into vibration as the indenta- 
tions on the tin-foil passed beneath it. 
Mr. Edison has, however, in his more recent instrument, 
of which Fig. 1 is an illustration, dispensed with this second 
membrane, making the one metallic diaphragm do double 
duty, first by receiving vibrations from the voice and im- 
pressing them upon the tin-foil, and afterwards by being 
thrown into vibration by the passing below its projecting 
pin of the indentations so produced, and thus giving out a 
repetition of the original sound. 
The diagram Fig. 2 will serve to illustrate the principle 
upon which the tin-foil is impressed by the aCtion of the 
vibration of the diaphragm, and how the latter is again 
thrown into precisely similar vibration by the movement of 
Fig. 2. 
the embossed foil below it. It represents a magnified sec- 
tion of the tin-foil taken along the line of the indentations, 
showing the position of the pin as it rides over its surface 
while the foil is travelling below it from right to left. The 
indentations, of which three only are shown on the diagram, 
having been produced on the foil by the sonorous vibrations 
ot the diaphragm, it follows, from the construction of the 
instrument, that if the foil be drawn under the pin at pre- 
cisely the same speed as it was travelling when the 
impressions were made upon it in the first instance, it will 
cause the diaphragm to vibrate in an exactly similar man- 
ner to that in which it vibrated under the influence of the 
voice, and, from what was pointed out at the beginning of 
this article, it would therefore emit a similar sound. This 
diagram is, of course, greatly exaggerated, and must be 
taken only as . an illustration of a possible explanation of 
what goes on in the aCtion of the instrument, but which is 
by no means certain. It presupposes that each indentation 
is made up of a minute structural surface, the details of 
which are so small as to be quite indistinguishable under 
