Note 19 — THE PHONOGRAPH.

I give the text of M. Cros’ sealed paper, opened by his request, at the Académie des Sciences, December 3, 1877:—‘Speaking generally, my process consists in obtaining traces of the movement to and fro of a vibrating membrane, and in using this tracing to reproduce the same movements, with their intrinsic relations of duration and intensity, either on the same membrane or on one adapted to give out the sounds which result from this series of movements.

‘It is therefore necessary that an extremely delicate tracing, such as may be obtained by passing a needle over a surface blackened by fire, should be transformed into a tracing, capable of sufficient resistance to guide an index which will transmit its movements to the membrane of sound.

‘A light index is fastened to the centre of a vibrating membrane; it terminates in a point (a metallic wire or tip of a feather) which rests on a surface which has been blackened by fire. This surface forms part of a disk, to which the double action of rotation and rectilinear progression has been given. If the membrane is at rest, the point will trace a simple spiral; if the membrane vibrates, there will be undulations in the spiral, and these undulations will represent the precise movements of the membrane in their duration and intensity.

‘By a well-known photographic process a transparent tracing of the undulations of the spiral can be represented by a line of similar dimensions on some resisting substance, tempered steel for example.

‘When this is done, this resisting surface is placed in a turning machine which causes it to revolve and advance with a velocity and motion similar to those by which the registering surface was actuated. A metallic point if the tracing is concave, or a grooved index if it is in relief, is kept upon the tracing by a spring, and the index which supports this point is connected with the centre of the membrane which produces the sounds. Under these conditions, the membrane will be actuated not by the vibrating air, but by the tracing which guides the index, and the impulses will be precisely similar in duration and intensity to those to which the registering membrane was subjected.

‘The spiral tracing represents equal successions of time by increasing or decreasing lengths. There is no inconvenience in this, since the turns of the spiral are very close together, if only the circumference of the turning circle is used; but then the central surface is lost.

‘In all cases the tracing of the helix on a cylinder is much more satisfactory, and I am now trying to make this idea practicable.’