Abstract
THE transmission of speech by light was first “realised nearly a quarter of a century ago by the invention of Prof. Graham Bell's photophone, a full description of which will be found in NATURE, vol. xxiii. p. 15. The transmitting instrument contained a small silvered disc or diaphragm of thin microscope cover-glass, which was clamped around the circumference like the diaphragm of a telephone. The receiver was a large parabolic mirror, at the focus of which was fixed a selenium cell in circuit with a telephone and battery. A beam of light from the sun or an electric lamp was reflected by the silvered diaphragm to the parabolic mirror, which concentrated the rays upon the selenium cell. The speaker's voice was directed upon the back of the diaphragm, causing it to vibrate in correspondence with the sound waves; the rapid changes in the curvature of its surface which accompanied the to and fro movements of the central parts of the diaphragm varied the concentration of the light upon the selenium, and since the conductivity of the selenium varied with the illumination, sounds were produced in the telephone similar to those by which the transmitting diaphragm was agitated.
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References
A detailed description of his apparatus is given in Journ. Inst. Electrical Engineers, vol. xxx. p. 240.
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BIDWELL, S. Photo-Telephony . Nature 69, 373–374 (1904). https://doi.org/10.1038/069373a0
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DOI: https://doi.org/10.1038/069373a0