Photophone
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On June 3, 1880, Alexander Graham Bell transmitted the first wireless telephone message on his newly-invented photophone. Bell believed the photophone was his most important invention. The device allowed for the transmission of sound on a beam of light. Of the eighteen patents granted in Bell's name alone, and the twelve he shared with his collaborators, four were for the photophone.
The photophone used crystalline selenium cells as the receiver. This material's electrical resistance varies inversely with the illumination, i.e., its resistance is higher when it is in the dark, and lower when it is lighted. The idea of the photophone was thus to modulate a light beam: the resulting varying illumination of the receiver would induce corresponding varying resistance in the selenium cells, which could be used by a telephone to regenerate the sounds captured at the receiver. The modulation of the light beam was done by a vibrating mirror: a thin mirror would alternate between concave and convex forms, thus focussing or dispersing the light from the light source. The photophone functioned similarly to the telephone, except the photophone used light as a means of projecting the information, while the telephone relied on electricity.
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In one experiment in Washington, D.C., Bell and his co-inventor Charles Sumner Tainter succeeded to communicate clearly over a distance of some 700 ft. (about 213 m), using plain sunlight as the light source. The receiver was a parabolic mirror with the selenium cells in its focal point. The selenium cells had an electrical resistance varying between 300 Ω and 100 Ω.
Although the photophone was an extremely important invention, it was many years before the significance of Bell's work was fully recognized. Bell's original photophone failed to protect transmissions from outside interferences, such as clouds, that easily disrupted transport. Until the development of modern fiber optics, technology for the secure transport of light inhibited use of Bell's invention. Bell's photophone is recognized as the progenitor of the modern fiber optics that today transport over eight percent of the world's telecommunications.
Literature
- Bell, A. G.: "On the Production and Reproduction of Sound by Light", American Journal of Sciences, Third Series, vol. XX, #118, October 1880, pp. 305 - 324; also published as "Selenium and the Photophone" in Nature, September 1880.
External link
- Bell's speech (http://histv2.free.fr/bell/bell1.htm) before the American Association for the Advancement of Science in Boston on August 27, 1880, in which he presented the above paper.