Photophone 51160 225609558 2008-07-14T15:20:21Z DeadEyeArrow 340124 ELs to EL section {{for|the sound-on-film technology|RCA 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 (telecommunications)|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: <blockquote> <center> FROM THE TOP FLOOR OF THIS BUILDING<br> WAS SENT ON JUNE 3, 1880<br> OVER A BEAM OF LIGHT TO 1325 L STREET<br> THE FIRST WIRELESS TELEPHONE MESSAGE<br> IN THE HISTORY OF THE WORLD.<br> THE APPARATUS USED IN SENDING THE MESSAGE<br> WAS THE PHOTOPHONE INVENTED BY<br> ALEXANDER GRAHAM BELL<br> INVENTOR OF THE TELEPHONE<br> THIS PLAQUE WAS PLACED HERE BY<br> ALEXANDER GRAHAM BELL CHAPTER<br> TELEPHONE PIONEERS OF AMERICA<br> MARCH 3, 1947<br> THE CENTENNIAL OF DR. BELL'S BIRTH <br> </center> </blockquote> Of the eighteen [[patent]]s 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 mirror|concave]] and [[convex mirror|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]]. [[Image:Photophone.jpg]] In the [[Washington, D.C.]] experiment, Bell and Tainter succeeded in communicating clearly over a distance of some 700&nbsp;[[Foot (unit of length)|ft]]. (about 213&nbsp;[[Metre|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&nbsp;[[Ohm (unit)|Ω]] and 100&nbsp;[[Ohm (unit)|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 [[interference]]s, such as clouds, that easily disrupted transport. Until the development of modern [[fiber optic]]s, 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 [[telecommunication]]s. == 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 (journal)|Nature]]'', September [[1880]]. ==External links== {{Wiktionary}} *[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] [[Category:Early telecommunications]] [[de:Photophon]] [[it:Fotofono]] [[ru:Фотофон]] [[uk:Фотофон]]