Photophone - wikipedia Photophone This article is about Alexander Graham Bell and Sumner Tainter 's optical phone. For the sound - on - film technology, see RCA Photophone. A historical plaque on the side of the Franklin School in Washington, D.C. which marks one of the points from which the photophone was demonstrated A diagram from one of Bell 's 1880 papers The photophone is a telecommunications device that allows transmission of speech on a beam of light. It was invented jointly by Alexander Graham Bell and his assistant Charles Sumner Tainter on February 19, 1880, at Bell 's laboratory at 1325 L Street in Washington, D.C. Both were later to become full associates in the Volta Laboratory Association, created and financed by Bell. On June 3, 1880, Bell 's assistant transmitted a wireless voice telephone message from the roof of the Franklin School to the window of Bell 's laboratory, some 213 meters ( about 700 ft. ) away. Bell believed the photophone was his most important invention. Of the 18 patents granted in Bell 's name alone, and the 12 he shared with his collaborators, four were for the photophone, which Bell referred to as his `` greatest achievement '', telling a reporter shortly before his death that the photophone was `` the greatest invention ( I have ) ever made, greater than the telephone ''. The photophone was a precursor to the fiber - optic communication systems that achieved worldwide popular usage starting in the 1980s. The master patent for the photophone ( U.S. Patent 235,199 Apparatus for Signalling and Communicating, called Photophone ) was issued in December 1880, many decades before its principles came to have practical applications. Contents 1 Design 2 First successful wireless voice communications 3 Reception and adoption 4 Further development 5 Commemorations 6 See also 7 References 8 Further reading 9 External links Design A photophone receiver and headset, one half of Bell and Tainter 's optical telecommunication system of 1880 The photophone was similar to a contemporary telephone, except that it used modulated light as a means of wireless transmission while the telephone relied on modulated electricity carried over a conductive wire circuit. Bell 's own description of the light modulator : We have found that the simplest form of apparatus for producing the effect consists of a plane mirror of flexible material against the back of which the speaker 's voice is directed. Under the action of the voice the mirror becomes alternately convex and concave and thus alternately scatters and condenses the light. The brightness of a reflected beam of light, as observed from the location of the receiver, therefore varied in accordance with the audio - frequency variations in air pressure -- the sound waves -- which acted upon the mirror. In its initial form, the photophone receiver was also non-electronic, using the photoacoustic effect. Bell found that many substances could be used as direct light - to - sound transducers. Lampblack proved to be outstanding. Using a fully modulated beam of sunlight as a test signal, one experimental receiver design, employing only a deposit of lampblack, produced a tone that Bell described as `` painfully loud '' to an ear pressed close to the device. In its ultimate electronic form, the photophone receiver used a simple selenium cell photodetector at the focus of a parabolic mirror. The cell 's electrical resistance ( between about 100 and 300 ohms ) varied inversely with the light falling upon it, i.e., its resistance was higher when dimly lit, lower when brightly lit. The selenium cell took the place of a carbon microphone -- also a variable - resistance device -- in the circuit of what was otherwise essentially an ordinary telephone, consisting of a battery, an electromagnetic earphone, and the variable resistance, all connected in series. The selenium modulated the current flowing through the circuit, and the current was converted back into variations of air pressure -- sound -- by the earphone. In his speech to the American Association for the Advancement of Science in August 1880, Bell gave credit for the first demonstration of speech transmission by light to Mr. A.C. Brown of London in the Fall of 1878. Because the device used radiant energy, the French scientist Ernest Mercadier suggested that the invention should not be named ' photophone ', but ' radiophone ', as its mirrors reflected the Sun 's radiant energy in multiple bands including the invisible infrared band. Bell used the name for a while but it should not be confused with the later invention `` radiophone '' which used radio waves. First successful wireless voice communications Illustration of a photophone transmitter, showing the path of reflected sunlight, before and after being modulated Illustration of a photophone receiver, depicting the conversion of modulated light to sound, as well as its electrical power source ( P ) While honeymooning in Europe with his bride Mabel Hubbard, Bell likely read of the newly discovered property of selenium having a variable resistance when acted upon by light, in a paper by Robert Sabine as published in Nature on 25 April 1878. In his experiments, Sabine used a meter to see the effects of light acting on selenium connected in a circuit to a battery. However Bell reasoned that by adding a telephone receiver to the same circuit he would be able to hear what Sabine could only see. As Bell 's former associate, Thomas Watson, was fully occupied as the superintendent of manufacturing for the nascent Bell Telephone Company back in Boston, Massachusetts, Bell hired Charles Sumner Tainter, an instrument maker who had previously been assigned to the U.S. 1874 Transit of Venus Commission, for his new ' L ' Street laboratory in Washington, at the rate of $15 per week. On February 19, 1880 the pair had managed to make a functional photophone in their new laboratory by attaching a set of metallic gratings to a diaphragm, with a beam of light being interrupted by the gratings movement in response to spoken sounds. When the modulated light beam fell upon their selenium receiver Bell, on his headphones, was able to clearly hear Tainter singing Auld Lang Syne. In an April 1, 1880 Washington, D.C. experiment, Bell and Tainter communicated some 79 metres ( 259 ft ) along an alleyway to the laboratory 's rear window. Then a few months later on June 21 they succeeded in communicating clearly over a distance of some 213 meters ( about 700 ft. ), using plain sunlight as their light source, practical electrical lighting having only just been introduced to the U.S. by Edison. The transmitter in their latter experiments had sunlight reflected off the surface of a very thin mirror positioned at the end of a speaking tube ; as words were spoken they cause the mirror to oscillate between convex and concave, altering the amount of light reflected from its surface to the receiver. Tainter, who was on the roof of the Franklin School, spoke to Bell, who was in his laboratory listening and who signaled back to Tainter by waving his hat vigorously from the window, as had been requested. The receiver was a parabolic mirror with selenium cells at its focal point. Conducted from the roof of the Franklin School to Bell 's laboratory at 1325 ' L ' Street, this was the world 's first formal wireless telephone communication ( away from their laboratory ), thus making the photophone the world 's earliest known voice wireless telephone systems, at least 19 years ahead of the first spoken radio wave transmissions. Before Bell and Tainter had concluded their research in order to move on to the development of the Graphophone, they had devised some 50 different methods of modulating and demodulating light beams for optical telephony. Reception and adoption The telephone itself was still something of a novelty, and radio was decades away from commercialization. The social resistance to the photophone 's futuristic form of communications could be seen in an August 1880 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. However at the time of their February 1880 breakthrough, Bell was immensely proud of the achievement, to the point that he wanted to name his new second daughter `` Photophone '', which was subtly discouraged by his wife Mabel Bell ( they instead chose `` Marian '', with `` Daisy '' as her nickname ). He wrote somewhat enthusiastically : I have heard articulate speech by sunlight ! I have heard a ray of the sun laugh and cough and sing !... I have been able to hear a shadow and I have even perceived by ear the passage of a cloud across the sun 's disk. You are the grandfather of the Photophone and I want to share my delight at my success. -- Alexander Graham Bell, in a letter to his father Alexander Melville Bell, dated February 26, 1880 Bell transferred the photophone 's intellectual property rights to the American Bell Telephone Company in May 1880. 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 disrupt the transmission of light. Factors such as the weather and the lack of light inhibited the use of Bell 's invention. Not long after its invention laboratories within the Bell System continued to improve the photophone in the hope that it could supplement or replace expensive conventional telephone lines. Its earliest non-experimental use came with military communication systems during World War I and II, its key advantage being that its light - based transmissions could not be intercepted by the enemy. Bell pondered the photophone 's possible scientific use in the spectral analysis of artificial light sources, stars and sunspots. He later also speculated on its possible future applications, though he did not anticipate either the laser or fiber - optic telecommunications : Can Imagination picture what the future of this invention is to be !... We may talk by light to any visible distance without any conduction wire... In general science, discoveries will be make by the Photophone that are undreamed of just now. Further development Ernst Ruhmer at his `` photo - electric '' optical telephone system station. ( 1905 ) Although Bell Telephone researchers made several modest incremental improvements on Bell and Tainter 's design, Marconi 's radio transmissions started to far surpass the maximum range of the photophone as early as 1897 and further development of the photophone was largely arrested until German - Austrian experiments began at the turn of the 20th century. The German physicist Ernst Ruhmer believed that the increased sensitivity of his improved selenium cells, combined with the superior receiving capabilities of professor H.T. Simon 's `` speaking arc '', would make the photophone practical over longer signalling distances. Ruhmer carried out a series of experimental transmissions along the Havel river and on Lake Wannsee from 1901 to 1902. He reported achieving sending distances under good conditions of 15 kilometers ( 9 miles ), with equal success during the day and at night. He continued his experiments around Berlin through 1904, in conjunction with the German Navy, which supplied high - powered searchlights for use in the transmissions. The German Siemens & Halske Company boosted the photophone 's range by utilizing current - modulated carbon arc lamps which provided a useful range of approximately 8 kilometres ( 5.0 mi ). They produced units commercially for the German Navy, which were further adapted to increase their range to 11 kilometres ( 6.8 mi ) using voice - modulated ship searchlights. British Admiralty research during WWI resulted in the development of a vibrating mirror modulator in 1916. More sensitive molybdenite receiver cells, which also had greater sensitivity to infra - red radiation, replaced the older selenium cells in 1917. The United States and German governments also worked on technical improvements to Bell 's system. By 1935 the German Carl Zeiss Company had started producing infra - red photophones for the German Army 's tank battalions, employing tungsten lamps with infra - red filters which were modulated by vibrating mirrors or prisms. These also used receivers which employed lead sulfide detector cells and amplifiers, boosting their range to 14 kilometres ( 8.7 mi ) under optimal conditions. The Japanese and Italian armies also attempted similar development of lightwave telecommunications before 1945. Several military laboratories, including those in the United States, continued R&D efforts on the photophone into the 1950s, experimenting with high - pressure vapour and mercury arc lamps of between 500 and 2,000 watts power. Commemorations 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 Marker on the Franklin School commemorating the first formal trial On March 3, 1947, the centenary of Alexander Graham Bell 's birth, the Telephone Pioneers of America dedicated a historical marker on the side of one of the buildings, the Franklin School, which Bell and Sumner Tainter used for their first formal trial involving a considerable distance. Tainter had originally stood on the roof of the school building and transmitted to Bell at the window of his laboratory. The marker did not acknowledge Tainter 's scientific and engineering contributions. On February 19, 1980, exactly 100 years to the day after Bell and Tainter 's first photophone transmission in their laboratory, staff from the Smithsonian Institution, the National Geographic Society and AT&T 's Bell Labs gathered at the location of Bell 's former 1325 ' L ' Street Volta Laboratory in Washington, D.C. for a commemoration of the event. The Photophone Centenary commemoration had first been proposed by electronics researcher and writer Forrest M. Mims, who suggested it to Dr. Melville Bell Grosvenor, the inventor 's grandson, during a visit to his office at the National Geographic Society. The historic grouping later observed the centennial of the photophone 's first successful laboratory transmission by using Mims hand - made demonstration photophone, which functioned similar to Bell and Tainter 's model. Mims also built and provided a pair of modern hand - held battery - powered LED transceivers connected by 100 yards ( 91 m ) of optical fiber. The Bell Labs ' Richard Gundlach and the Smithsonian 's Elliot Sivowitch used the device at the commemoration to demonstrate one of the photophone 's modern - day descendants. The National Geographic Society also mounted a special educational exhibit in its Explorer 's Hall, highlighting the photophone 's invention with original items borrowed from the Smithsonian Institution. See also Atomic line filter Free - space optical communication History of telecommunication Laser microphone Mie scattering Modulating retro - reflector Optical sound Optical window Photoacoustic effect Radio window Rayleigh scattering Semaphore line Visible light communication Volta Laboratory and Bureau References Footnotes Jump up ^ The demonstration model was a replica in principle but not identical to Bell and Tainter 's model. The commemorative model transmitter was a thin mirror cemented to a short aluminum speaking tube, and its receiver was a silicon solar cell and audio amplifier, both installed in a lantern light housing. Citations Jump up ^ Bruce 1990, pg. 336 Jump up ^ Jones, Newell. First ' Radio ' Built by San Diego Resident Partner of Inventor of Telephone : Keeps Notebook of Experiences With Bell Archived 2002 - 02 - 19 at the Wayback Machine., San Diego Evening Tribune, July 31, 1937. Retrieved from the University of San Diego History Department website, November 26, 2009. Jump up ^ Bruce 1990, pg. 338 ^ Jump up to : Carson 2007, pg. 76 - 78 ^ Jump up to : Groth, Mike. Photophones Revisted, ' Amateur Radio ' magazine, Wireless Institute of Australia, Melbourne, April 1987 pp. 12 -- 17 and May 1987 pp. 13 -- 17. ^ Jump up to : Mims 1982, p. 11. Jump up ^ Phillipson, Donald J.C., and Neilson, Laura Bell, Alexander Graham, The Canadian Encyclopedia online. Retrieved 2009 - 08 - 06 ^ Jump up to : Mims 1982, p. 14. Jump up ^ Morgan, Tim J. `` The Fiber Optic Backbone '', University of North Texas, 2011. Jump up ^ Miller, Stewart E. `` Lightwaves and Telecommunication '', American Scientist, Sigma Xi, The Scientific Research Society, January -- February 1984, Vol. 72, No. 1, pp. 66 - 71, Issue Stable URL. ^ Jump up to : Gallardo, Arturo ; Mims III, Forrest M... Fiber - optic Communication Began 130 Years Ago, San Antonio Express - News, June 21, 2010. Accessed January 1, 2013. Jump up ^ Clark, J. An Introduction to Communications with Optical Carriers, IEEE Students ' Quarterly Journal, June 1966, Vol. 36, Iss. 144, pp. 218 - 222, ISSN 0039 - 2871, doi : 10.1049 / sqj. 1966.0040. Retrieved from IEEExplore website August 19, 2011. Jump up ^ Bell, Alexander Graham. `` On the Production and Reproduction of Speech by Light '', American Journal of Science, October 1880, Vol. 20, No. 118, pp. 305 -- 324. Jump up ^ Grosvenor and Wesson 1997, p. 104. Jump up ^ Ernest Victor Heyn, Fire of genius : inventors of the past century : based on the files of Popular Science Monthly since its founding in 1872, Anchor Press / Doubleday - 1976, page 74 Jump up ^ Mims 1982, pp. 6 -- 7. Jump up ^ Mims 1982, p. 7. Jump up ^ Mims 1982, p. 10. ^ Jump up to : Mims 1982, p. 12. Jump up ^ Editorial, The New York Times, August 30, 1880 Jump up ^ International Fiber Optics & Communication, June 1986, p. 29 Jump up ^ Carson 2007, pg. 77 ^ Jump up to : Bruce 1990, pg. 337 Jump up ^ Bruce 1990, pg. 339 Jump up ^ Hecht, Jeff. Fiber Optics Calls Up The Past, New Scientist, January 12, 1984, pp. 12 -- 13. Jump up ^ Carson 2007, pp. 77 - 78 Jump up ^ Carson 2007, pg. 78 Jump up ^ Cover page Technical World, March 1905. Jump up ^ `` Correspondence : Wireless Telephony '' ( October 30, 1902 letter from Ernst Ruhmer ), The Electrician, November 7, 1902, page 111. Jump up ^ Wireless Telephony In Theory and Practice by Ernst Ruhmer, 1908, pages 55 - 59. Jump up ^ Mims 1982, pp. 14 -- 17. Jump up ^ Hecht, Jeff. `` Yarns From The Technological Jungle : Siliconnections : Coming Of Age In The Electronic Era '', New Scientist, February 27, 1986, pp. 50 -- 51. Jump up ^ Mims 1982, pp. 6 & 12. Bibliography Carson, Mary Kay ( 2007 ). `` chapter 8 ''. Alexander Graham Bell : Giving Voice To The World. Sterling Biographies. 387 Park Avenue South, New York, NY 10016 : Sterling Publishing Co., Inc. pp. 76 -- 78. ISBN 978 - 1 - 4027 - 3230 - 0. OCLC 182527281. Bell, A. G : `` 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. Bruce, Robert V Bell : Alexander Bell and the Conquest of Solitude, Ithaca, New York : Cornell University Press, 1990. ISBN 0 - 8014 - 9691 - 8. Mims III, Forest M. The First Century of Lightwave Communications, Fiber Optics Weekly Update, Information Gatekeepers, February 10 -- 26, 1982, pp. 6 -- 23. Grosvenor, Edwin S. and Morgan Wesson. Alexander Graham Bell : The Life and Times of the Man Who Invented the Telephone. New York : Harry N. Abrahms, Inc., 1997. ISBN 0 - 8109 - 4005 - 1. Further reading Chris Long and Mike Groth 's optical audio telecommunications webpage Ackroyd, William. `` The Photophone '' in `` Science for All '', Vol. 2 ( R. Brown, ed. ), Cassell & Co., London, circa 1884, pps. 307 - 312. A popular account, profusely illustrated with steel engravings. Armengaud, J. `` Le photophone de M. Graham Bell ''. Soc. Ing. civ. Mem., year 1880, Vol 2. pp. 513 -- 522. AT&T Company. `` The Radiophone '', pamphlet distributed at Louisiana Purchase Exhibition, St Louis, Missouri, 1904. Describes the photophone work of Hammond V Hayes at the Bell Labs ( patented 1897 ) and the German engineer HT Simon in the same year. Bell, Alexander Graham. `` On the Production and Reproduction of Sound by Light : the Photophone ''. Am. Ass. for the Advancement of Sci., Proc., Vol 29., October 1880, pp. 115 -- 136. Also in American Journal of Science, Series 3. No. 20, 1880, pp. 305 -- 324 ; Eng. L., 30. 1880, pp. 240 -- 242 ; Electrician, Vol 5. 1880, pp. 214 -- 215, 220 - 221, 237 ; Journal of the Society of Telegraph Engineers, No. 9, 1880, pp. 404 -- 426 ; Nat. L., Vol 22. 1880, pp. 500 -- 503 ; Ann. Chim. Phys., Serie 5. Vol 21. 1880, pp. 399 -- 430 ; E.T.Z., Vol. 1. 1880, pp. 391 -- 396. Discussed at length in Eng. L., Vol 30. 1880, pp. 253 -- 254, 407 - 409. In these papers, Bell accords the credit for the first demonstrations of the transmission of speech by light to a Mr AC Brown of London `` in September or October 1878 ''. Bell, Alexander Graham. `` Sur l'application du photophone a l'etude des bruits qui ont lieu a la surface solaire ''. C.R., Vol. 91. 1880, pp. 726 -- 727. Bell, Alexander Graham. `` Professor AG Bell on Selenium and the Photophone ''. Pharm. J. and Trans., Series 3. Vol. 11., 1880 -- 1881, pp. 272 -- 276 ; The Electrician No 5, 18 September 1880, pps 220 - 221 and 2 October 1880 pps 237 ; Nature ( London ) Vol 22, 23 September 1880, pps. 500 - 503 ; Engineering Vol 30, pps 240 - 242, 253, 254, 407 - 409 ; and Journal of the Society of Telegraph Engineers Vol 9, pps 375 - 387. Bell, Alexander Graham. `` Other papers on the photophone '' E.T.Z. No. 1, 1880, pps 391 - 396 ; Journal of the Society for the Arts 1880, No. 28, pps 847 - 848 & No. 29 pps 60 - 62 ; C.R. No. 91, 1880 -- 1881, pps 595 - 598, 726, 727, 929 - 931, 982, 1882 pps 409 - 412, 450, 451, 1224 - 1227. Bell, Alexander Graham. `` Le Photophone De La Production Et De La Lumiere ''. Gauthier - Villars, Imprimeur - Libraire, Paris. 1880. ( Note : this is item # 26, Folder # 4, as noted in `` Finding Aid for the Alexander Graham Bell Collection, 1880 -- 1925 '', Collection number : 308, UCLA Library, Department of Special Collections Manuscripts Division, as viewable at the Online Archive of California ) `` Bell 's Photophone ''. Nature Vol 24, 4 November 1880 ; The Electrician, Vol. 6, 1881, pps. 136 - 138. Appleton 's Journal. `` The Photophone ''. Appleton 's Journal, Vol. 10 No. 56, New York, February 1881, pps. 181 - 182. Bidwell, Shelford. `` The Photophone ''. Nature., 23. 1881, pp. 58 -- 59. Bidwell, Shelford. `` Selenium and Its Applications to the Photophone and Telephotography ''. Proceedings of the Royal Institution ( G.B. ), Vol 9. 1881, pp. 524 -- 535 ; The English Mechanic and World Of Science, Vol. 33, 22 April 1881, pps 158 - 159 and 29 April 1881 pps. 180 - 181. Also in Chem. News, Vol. 44, 1881, pp. 1 -- 3, 18 - 21. ( From a lecture at the Royal Institution on 11 March 1881 ). Breguet, A. `` Les recepteurs photophoniques de selenium ''. Ann. Chim. Phys., Series 5. Vol 21. 1880, pp... 560 - 563. Breguet, A. `` Sur les experiences photophonique du Professeur Alexander Graham Bell et de M. Sumner Tainter '' : C.R. ; Vol 91., 1880, pp 595 -- 598. Electrician. `` Bell 's Photophone '', Electrician, Vol. 6, February 5, 1881, pps. 136 - 138,183. Jamieson, Andrew. Nat. L., Vol. 10, 1881, p. 11. This Glasgow scientist seems to have been the first to suggest the usage of a manometric gas flame for optical transmission, demonstrated at a meeting of the Glasgow Philosophical Society ; `` The History of selenium and its action in the Bell Photophone, with description of recently designed form '', Proceedings of the Philosophical Society of Glasgow No. 13, 1881, * * * Moser, J. `` The Microphonic Action of Selenium Cells ''. Phys. Soc., Proc., Vol. 4, 1881, pp. 348 -- 360. Also in Phil. Mag., Series 5, Vol. 12, 1881, pp. 212 -- 223. Kalischer, S. `` Photophon Ohne Batterie ''. Rep. f. Phys., Vol. 17., 1881, pp. 563 -- 570. MacKenzie, Catherine `` Alexander Graham Bell '', Houghton Mifflin Company, Boston, p. 226, 1928. Mercadier, E. `` La radiophonie indirecte ''. Lumiere Electrique, Vol. 4, 1881, pp. 295 -- 299. Mercadier, E. `` Sur la radiophonie produite a l'aide du selenium ''. C.R., Vol. 92, 1881, pp. 705 -- 707. Mercadier, E. `` Sur la construction de recepteurs photophoniques a selenium ''. C.R., Vol. 92, 1881, pp. 789 -- 790. Mercadier, E. `` Sur l'influence de la temperature sur les recepteurs radiophoniques a selenium ''. C.R., Vol. 92, 1881, pp. 1407 -- 1408. Molera & Cebrian. `` The Photophone ''. Eng. L., Vol. 31, 1881, p. 358. Preece, Sir William H. `` Radiophony '', Engineering Vol. 32, 8 July 1881, pp. 29 -- 33 ; Journal of the Society of Telegraph Engineers, Vol 10, 1881, pps. 212 - 228. On the photophone. Rankine, A.O. `` Talking over a Sunbeam ''. El. Exp. ( N.Y. ), Vol. 7, 1920, pp. 1265 -- 1316. Sternberg, J.M. The Volta Prize of the French Academy Awarded to Prof. Alexander Graham Bell : A Talk With Dr. J.M. Sternberg, The Evening Traveler, September 1, 1880, The Alexander Graham Bell Papers at the Library of Congress Thompson, Silvanus P. `` Notes on the Construction of the Photophone ''. Phys. Soc. Proc., Vol. 4, 1881, pps. 184 - 190. Also in Phil. Mag., Vol. 11, 1881, pp. 286 -- 291. Abstracted in Chem. News, Vol. 43, 1881, p. 43 ; Eng. L., Vol. 31, 1881, p. 96. Tomlinson, H. `` The Photophone ''. Nat. L., Vol. 23, 1881, pps. 457 - 458. U.S. Radio and Television Corp. `` Ultra-violet rays used in Television '', New York Times, 29 May 1929, p. 5 : Demonstration of transmission of a low definition ( mechanically scanned ) video signal over a modulated light beam. Terminal stations 50 feet apart. Public demonstration at Bamberger and Company 's Store, Newark, New Jersey. Earliest known usage of modulated light comms for conveying video signals. See also report `` Invisible Ray Transmits Pictures '' in Science and Invention, November 1929, Vol. 17, p. 629. White, R.H. `` Photophone ''. Harmsworth 's Wireless Encyclopaedia, Vol. 3, pp. 1541 -- 1544. Weinhold, A. `` Herstellung von Selenwiderstanden fur Photophonzwecke ''. E.T.Z., Vol. 1, 1880, p. 423. External links Look up photophone in Wiktionary, the free dictionary. Bell 's speech before the American Association for the Advancement of Science in Boston on August 27, 1880, in which he presented his paper `` On the Production and Reproduction of Sound by Light : the Photophone ''. 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Question: when did alexander graham bell invented the photophone
Answer: February 19, 1880