Photophone


Photophone

The Photophone was invented jointly by Alexander Graham Bell and his assistant Sarah Orr 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 first wireless telephone message on his newly-invented photophone. A plaque on the wall of the Franklin School at 13th & K Streets NW in Washington, D.C. reads as follows:


FROM THE TOP FLOOR OF THIS BUILDING
WAS SENT ON JUNE 3, 1880
OVER A BEAM OF LIGHT TO 1325 L STREET
THE FIRST WIRELESS TELEPHONE MESSAGE
IN THE HISTORY OF THE WORLD.
THE APPARATUS USED IN SENDING THE MESSAGE
WAS THE PHOTOPHONE INVENTED BY
ALEXANDER GRAHAM BELL
INVENTOR OF THE TELEPHONE
THIS PLAQUE WAS PLACED HERE BY
ALEXANDER GRAHAM BELL CHAPTER
TELEPHONE PIONEERS OF AMERICA
MARCH 3, 1947
THE CENTENNIAL OF DR. BELL'S BIRTH

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.

In the 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 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 transports 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 links

* [http://histv2.free.fr/bell/bell1.htm Bell's speech] before the American Association for the Advancement of Science in Boston on August 27, 1880, in which he presented the above paper.
* [http://www.dcmemorials.com/index_indiv0001693.htm Photo of the plaque]


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Look at other dictionaries:

  • Photophone — Pho to*phone, n. [Photo + Gr. ? sound.] (Physics) An apparatus for the production of sound by the action of rays of light. A. G. Bell. [1913 Webster] …   The Collaborative International Dictionary of English

  • Photophone — Cette page d’homonymie répertorie les différents sujets et articles partageant un même nom. Photophone : la technique de téléphonie développée par Alexandre Graham Bell dans les années 1880. Photophone : un procédé d enregistrement du… …   Wikipédia en Français

  • Photophone — Als Photophon oder Photophone (griech.) bezeichnet man ein von Alexander Graham Bell und Charles Summer Tainter 1880 erfundenes Instrument zur hörbaren Übertragung der menschlichen Sprache in größere Entfernung mit Hilfe eines Bündels… …   Deutsch Wikipedia

  • photophone — noun a device that transmitted sound on a beam of light …   Wiktionary

  • photophone — ● n. m. ►COMM Téléphone portable doté d un mini appareil photo numérique …   Dictionnaire d'informatique francophone

  • photophone — pho·to·phone …   English syllables

  • photophone —  n.m. ® phototéléphone …   Le dictionnaire des mots absents des autres dictionnaires

  • photophone — ˈ ̷ ̷ ̷ ̷ˌfōn noun Etymology: phot + phone : a device whereby a sound signal (as a voice) is transmitted by causing it to modulate a beam of visible or infrared light which is received by a photoelectric cell, amplified, and reconverted into… …   Useful english dictionary

  • Photophone (telephone-appareil-photo) — Photophone (téléphone appareil photo) Pour les articles homonymes, voir Photophone. Un photophone est un téléphone portable capable de prendre des photos numériques de lui même. Le mot a été forgé selon le même principe que sémaphone, le… …   Wikipédia en Français

  • Photophone (téléphone-appareil-photo) — Pour les articles homonymes, voir Photophone. Un photophone est un téléphone portable capable de prendre des photos numériques de lui même. Le mot a été forgé selon le même principe que sémaphone, le photophone (contraction d appareil photo et de …   Wikipédia en Français


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