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1880 __ Photophone
Alexander Graham Bell (1847-1922)
Comment : The Photophone was invented jointly by Alexander Graham Bell and his assistant Charles Sumner Tainter on February 19, 1880. Bell believed the photophone was his most important invention. The device allowed for the transmission of sound on a beam of light. On June 3, 1880, Bell transmitted the world's first wireless telephone message on his newly invented form of telecommunication. The photophone used crystalline selenium cells at the focal point of its parabolic receiver. This material's electrical resistance varies inversely with the illumination falling upon it, i.e., its resistance is higher when it is in the dark, and lower when it is exposed to light. The idea of the photophone was thus to modulate a light beam: the resulting varying illumination of the receiver would induce a corresponding varying resistance in the selenium cells, which were then used by a telephone to regenerate the sounds captured at the receiver. The modulation of the transmitted light beam was done by a vibrating mirror: a thin mirror would alternate between concave and convex forms, thus focusing 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. In their Washington, D.C. experiment, Bell and Tainter succeeded in communicating clearly over a distance of some 700 ft. (about 213 m), using plain sunlight as their light source. The receiver was a parabolic mirror with selenium cells at its focal point. Conducted on June 3, 1880 from the roof of the Franklin School to a building at 1325 L Street, this was the world's first wireless telephone communication (outside away from their laboratory). The selenium cells in the parabolic receiver 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. While Bell had hoped his new photophone could be used by ships at sea and to also displace the plethora of telephone lines that were blooming along busy city boulevards, his design failed to protect its transmissions from outdoor interferences such as clouds, fog, rain, snow and such, that could easily disrupted the transmission of light. Until the development of modern laser and fiber optic technologies for the secure transport of light, factors such as the weather or lack of sunlight inhibited the use of Bell's invention. Additionally, in an era where people heated their homes with fireplaces and transportation was largely horse-drawn, social reticence towards this futuristic form of telecommunications was palpable, as could be discerned in a New York Times commentary: “The ordinary man ... will find a little difficulty in comprehending how sunbeams are to be used. Does Prof. Bell intend to connect Boston and Cambridge ... with a line of sunbeams hung on telegraph posts, and, if so, what diameter are the sunbeams to be ....[and] will it be necessary to insulate them against the weather ..... until (the public) sees a man going through the streets with a coil of No. 12 sunbeams on his shoulder, and suspending them from pole to pole, there will be a general feeling that there is something about Professor Bell's photophone which places a tremendous strain on human credulity.”. (Editorial, The New York Times, August 30, 1880. However at the time of his accomplishment, Bell was ecstatic at his achievement, to the point where he wanted to name his new second daughter "Photophone", which was subtly discouraged by his wife Mabel they)
French comment : On doit la première tentative de communication optique à Alexandre Graham Bell, connu pour l'invention du téléphone. En effet, il mit au point, au cours des années 1880, le photophone. Cet appareil permettait de transmettre la lumière sur une distance de 200 mètres. La voix, amplifiée par un microphone, faisait vibrer un miroir qui réfléchissait la lumière du soleil. Quelque 200 mètres plus loin, un second miroir captait cette lumière pour activer un cristal de sélénium et reproduire le son voulu. Le récepteur de cet appareil était presque identique à celui du premier téléphone. Bien qu'opérationnelle en terrain découvert, cette méthode s'avéra peu utilisée. La pluie, la neige et les obstacles qui empêchaient la transmission du signal condamnèrent cette invention, bien qu'il considérait lui-même que le photophone était sa plus grande invention, puisqu'elle permettait une communication sans fil. (Compiled from various sources)
Source : Bell, Alexander G. (1880), "On the Production and Reproduction of Sound by Light", American Journal of Science, Third Series, vol. XX, #118, October 1880, pp. 305 - 324; also published as "Selenium and the Photophone" in Nature, September 1880.
Urls : http://histv2.free.fr/bell/photophone2.htm (last visited ) http://histv2.free.fr/bell/bell11.htm (last visited ) http://histv2.free.fr/bell/photophone.htm (last visited )

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