Photophone transmitter 4074931746 9f996df841 b.jpg
   HOME

TheInfoList



OR:

The photophone is a telecommunications device that allows
transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission *** ...
of speech on a beam of light. It was invented jointly by
Alexander Graham Bell Alexander Graham Bell (, born Alexander Bell; March 3, 1847 – August 2, 1922) was a Scottish-born inventor, scientist and engineer who is credited with patenting the first practical telephone. He also co-founded the American Telephone and Te ...
and his assistant
Charles Sumner Tainter Charles Sumner Tainter (April 25, 1854 – April 20, 1940) was an American scientific instrument maker, engineer and inventor, best known for his collaborations with Alexander Graham Bell, Chichester Bell, Alexander's father-in-law Gardiner Hubb ...
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.Carson 2007, pg. 76–78Mims 1982, p. 11. Bell believed the photophone was his most important
invention An invention is a unique or novel device, method, composition, idea or process. An invention may be an improvement upon a machine, product, or process for increasing efficiency or lowering cost. It may also be an entirely new concept. If an i ...
. 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 haveever made, greater than the telephone".Mims 1982, p. 14. The photophone was a precursor to the
fiber-optic communication Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pref ...
systems that achieved worldwide popular usage starting in the 1980s. The master patent for the photophone ( ''Apparatus for Signalling and Communicating, called Photophone'') was issued in December 1880, many decades before its principles came to have practical applications.


Design

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: 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 A copper indium gallium selenide solar cell (or CIGS cell, sometimes CI(G)S or CIS cell) is a thin-film solar cell used to convert sunlight into electric power. It is manufactured by depositing a thin layer of copper indium gallium selenide soluti ...
photodetector at the focus of a parabolic mirror. The cell's electrical resistance (between about 100 and 300
ohm Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm. Ohm or OHM may also refer to: People * Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm'' * Germán Ohm (born 1936), Mexican boxer * Jörg Ohm (b ...
s) 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 Ernest is a given name derived from Germanic word ''ernst'', meaning "serious". Notable people and fictional characters with the name include: People *Archduke Ernest of Austria (1553–1595), son of Maximilian II, Holy Roman Emperor *Ernest, M ...
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.Grosvenor and Wesson 1997, p. 104. Bell used the name for a while but it should not be confused with the later invention "
radiophone A radiotelephone (or radiophone), abbreviated RT, is a radio communication system for conducting a conversation; radiotelephony means telephony by radio. It is in contrast to '' radiotelegraphy'', which is radio transmission of telegrams (mess ...
" which used
radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (short ...
s.


First successful wireless voice communications

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 The Bell Telephone Company, a common law joint stock company, was organized in Boston, Massachusetts, on July 9, 1877, by Alexander Graham Bell's father-in-law Gardiner Greene Hubbard, who also helped organize a sister company – the New Englan ...
back in Boston, Massachusetts, Bell hired
Charles Sumner Tainter Charles Sumner Tainter (April 25, 1854 – April 20, 1940) was an American scientific instrument maker, engineer and inventor, best known for his collaborations with Alexander Graham Bell, Chichester Bell, Alexander's father-in-law Gardiner Hubb ...
, 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 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 Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
as their light source, practical electrical lighting having only just been introduced to the U.S. by
Edison Thomas Alva Edison (February 11, 1847October 18, 1931) was an American inventor and businessman. He developed many devices in fields such as electric power generation, mass communication, sound recording, and motion pictures. These invention ...
. 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 Wireless telephone may refer to: * Cordless telephone, a telephone in which the handset is portable and communicates with the body of the phone by radio, instead of being attached by a cord * Mobile phone, a portable telephone that can make and rec ...
system, 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 The Graphophone was the name and trademark of an improved version of the phonograph. It was invented at the Volta Laboratory established by Alexander Graham Bell in Washington, D.C., United States. Its trademark usage was acquired successively ...
, they had devised some 50 different methods of modulating and demodulating light beams for optical telephony.Mims 1982, p. 12.


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: 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:Bruce 1990, pg. 337 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 The Bell System was a system of telecommunication companies, led by the Bell Telephone Company and later by the American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America for over one hundr ...
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,
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s and
sunspot Sunspots are phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. Sun ...
s. He later also speculated on its possible future applications, though he did not anticipate either the laser or fiber-optic telecommunications:


Further development

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 The Havel () is a river in northeastern Germany, flowing through the States of Germany, states of Mecklenburg-Vorpommern, Brandenburg, Berlin and Saxony-Anhalt. It is a right tributary of the Elbe and long. However, the direct distance from it ...
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 . They produced units commercially for the German Navy, which were further adapted to increase their range to using voice-modulated ship searchlights.
British Admiralty The Admiralty was a department of the Government of the United Kingdom responsible for the command of the Royal Navy until 1964, historically under its titular head, the Lord High Admiral – one of the Great Officers of State. For much of it ...
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 The German Army (, "army") is the land component of the armed forces of Germany. The present-day German Army was founded in 1955 as part of the newly formed West German ''Bundeswehr'' together with the ''Marine'' (German Navy) and the ''Luftwaf ...
'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 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

On March 3, 1947, the centenary of
Alexander Graham Bell Alexander Graham Bell (, born Alexander Bell; March 3, 1847 – August 2, 1922) was a Scottish-born inventor, scientist and engineer who is credited with patenting the first practical telephone. He also co-founded the American Telephone and Te ...
's birth, the
Telephone Pioneers of America Pioneers, a Volunteer Network, founded and more commonly known as the Telephone Pioneers of America, is a non-profit charitable organization based in Denver, Colorado in the United States. The association was organized in Boston in November 1911 ...
dedicated a historical marker on the side of one of the buildings, the Franklin School, which Bell and
Sumner Tainter Charles Sumner Tainter (April 25, 1854 – April 20, 1940) was an American scientific instrument maker, engineer and inventor, best known for his collaborations with Alexander Graham Bell, Chichester Bell, Alexander's father-in-law Gardiner Hubba ...
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 Melville Bell Grosvenor (November 26, 1901 – April 22, 1982) was the president of the National Geographic Society and editor of '' The National Geographic Magazine'' from 1957 to 1967. He was the grandson of telephone inventor Alexander Graham Be ...
, 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 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.Mims 1982, pp. 6 & 12.


See also

*
Atomic line filter An atomic line filter (ALF) is a more effective optical band-pass filter used in the physical sciences for filtering electromagnetic radiation with precision, accuracy, and minimal signal strength loss. Atomic line filters work via the absorption ...
* Free-space optical communication * History of telecommunication * Laser microphone * Mie scattering *
Modulating retro-reflector A modulating retro-reflector (MRR) system combines an optical retro-reflector and an optical modulator to allow optical communications and sometimes other functions such as programmable signage. Free space optical communication technology has eme ...
* Optical sound * Optical window * Photoacoustic effect *
Radio window The radio window is a range of frequencies of electromagnetic radiation that penetrate the earth's atmosphere. It plays an important role in astronomy; up until World War II, astronomers could only use the visible and near infrared portions of the o ...
* Rayleigh scattering *
Semaphore line An optical telegraph is a line of stations, typically towers, for the purpose of conveying textual information by means of visual signals. There are two main types of such systems; the semaphore telegraph which uses pivoted indicator arms and ...
* Visible light communication * Volta Laboratory and Bureau


References

Footnotes Citations Bibliography * * Bell, A. G: "On the Production and Reproduction of Sound by Light", ''
American Journal of Science The ''American Journal of Science'' (''AJS'') is the United States of America's longest-running scientific journal, having been published continuously since its conception in 1818 by Professor Benjamin Silliman, who edited and financed it himself ...
'', 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. . * 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. .


Further reading



*Ackroyd, William. "The Photophone" in "Science for All", Vol. 2 (R. Brown, ed.), Cassell & Co., London, circa 1884, pp. 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 H T 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 A C 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 A G 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, pp 220–221 and 2 October 1880 pp 237; Nature (London) Vol 22, 23 September 1880, pp. 500–503; Engineering Vol 30, pp 240–242, 253, 254, 407–409; and Journal of the Society of Telegraph Engineers Vol 9, pp 375–387. * Bell, Alexander Graham. "Other papers on the photophone" E.T.Z. No. 1, 1880, pp 391–396; ''Journal of the Society for the Arts'' 1880, No. 28, pp 847–848 & No. 29 pp 60–62; C.R. No. 91, 1880–1881, pp 595–598, 726, 727, 929–931, 982, 1882 pp 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 th
Online Archive of California
*"Bell's Photophone". Nature Vol 24, 4 November 1880; The Electrician, Vol. 6, 1881, pp. 136–138. *Appleton's Journal. "The Photophone". Appleton's Journal, Vol. 10 No. 56, New York, February 1881, pp. 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, pp. 158–159 and 29 April 1881 pp. 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, pp. 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, pp. 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, pp. 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, pp. 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


Bell's speech
before the
American Association for the Advancement of Science The American Association for the Advancement of Science (AAAS) is an American international non-profit organization with the stated goals of promoting cooperation among scientists, defending scientific freedom, encouraging scientific respons ...
in Boston on August 27, 1880, in which he presented his paper'' "On the Production and Reproduction of Sound by Light: the Photophone"''.
Long-distance Atmospheric Optical Communications, by Chris Long and Mike Groth (VK7MJ)Téléphone et photophone: les contributions indirectes de Graham Bell à l'idée de la vision à distance par l'électricité
{{Telecommunications Alexander Graham Bell History of telecommunications History of the telephone Optical communications Photonics