A Photophone receiver and headset, one half of Bell and Tainter's optical telecommunication system of 1880

The photophone (also known as a radiophone) is a telecommunications device which allowed for the transmission of both articulated sounds and normal human conversations 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 1325 'L' Street laboratory in Washington, D.C.^[1][2] Both were later to become full associates in the Volta Laboratory Association, created and financed by Bell.

On April 1, 1880, Bell's assistant transmitted the world's first wireless telephone message to him from the roof of the Franklin School to the window of Bell's laboratory, some 213 metres (700 ft) away.^[3][4][5]

Bell believed the Photophone was his most important invention. Of the eighteen patents granted in Bell's name alone, and the twelve he shared with his collaborators, four were for the Photophone, which Bell referred to as his 'greatest achievement', writing that the Photophone was "the greatest invention [I have] ever made, greater than the telephone".^[6]

The device was a precursor to fiber-optic communications that came into widespread use during the 1980s.^{[citation needed]}

Design[link]

Illustration of a Photophone transmitter, showing the path of reflected sunlight, before and after being modulated

Bell and Tainter's invention was based on the discovery of photovoltaic properties of certain materials by A. E. Becquerel in 1839.^[7] The Photophone used crystalline selenium cells at the focal point of its parabolic receiver.^[5] 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 was then used to regenerate the sounds captured by the telephone receiver.

The modulation of the transmitted light beam was done by a mirror made to vibrate by a person's voice: the thin mirror would alternate between concave and convex forms, thus focusing or dispersing the light from the light source.^[5] The Photophone functioned similarly to the telephone, except that the Photophone used modulated light as a means of transmitting information, while the telephone relied on a modulated electrical signal carried over a conductive wire circuit.

Bell described it in his writings:^[7]

History[link]

A diagram from one of Bell's 1880 papers

Bell transferred the Photophone's rights to the American Bell Telephone Company in May 1880.^[8] The master patent for the Photophone (U.S. Patent 235,199 Apparatus for Signalling and Communicating, called Photophone), was issued in December 1880,^[5] many decades before its principles could be applied to practical applications.

Demonstration[link]

Illustration of a Photophone receiver, depicting the conversion of modulated light to sound, as well as its electrical power source (P)

In their Washington, D.C experiment, Bell and Tainter 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 invented by Edison. 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 from the window as had been requested.

The receiver was a parabolic mirror with selenium cells at its focal point.^[5] Conducted from the roof of the Franklin School to Bell's laboratory at 1325 'L' Street, this was the world's first wireless telephone communication (away from their laboratory), thus making the Photophone the world's earliest known radiophone and wireless telephone systems. The selenium cells in the parabolic receiver had an electrical resistance varying between 100 and 300 Ω (ohms).

Reception and adoption[link]

The telephone itself was still something of a novelty, and radio was decades away from commercialization. The social reticence to the Photophone's futuristic form of communications was palpable, as could be discerned in an 1880 New York Times commentary:^[9][10]

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 moniker).^[4] He wrote somewhat ecstatically:^[4][11]

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.^[4] Factors such as the weather and the lack of light inhibited the use of Bell's invention.^[4] 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 II, its key advantage being that its light-based transmissions could not be intercepted by the enemy.

Bell later also speculated on the Photophone's possible future applications, writing prior to any perception of the laser, and which also vaguely foretold fiber-optic telecommunications:^[11]

He also pondered its possible scientific use in the spectral analysis of artificial light sources, stars and sunspots.

Legacy[link]

A plaque marker on the side of the Franklin School in Washington, D.C. which marks one of the sites where the Photophone was demonstrated

In 1947 on the centenary of Bell's birth, the Telephone Pioneers of America dedicated a historical marker on the side of one of the buildings, the Franklin School, that Bell and Tainter used for their successful trial, which was also the world's first wireless telecommunication.

See also[link]

• Atomic line filter
• Extremely high frequency
• Free-space optical communication
• History of telecommunication
• Laser microphone
• Mie scattering
• Modulating retro-reflector
• Optical window
• Radio window
• Rayleigh scattering
• Semaphore line
• Visible light communication

Notes[link]

References[link]

Further reading[link]

• 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 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, pps. 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, 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, 5 February 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, pps. 212-223.
• Kalischer, S. "Photophon Ohne Batterie". Rep. f. Phys., Vol. 17., 1881, pp. 563–570.
• Mackenzie, Catherine C. "Alexander Graham Bell", Houghton Mifflin Company, Boston, p. 226, 1928.
• Mercadier, E. "La radiophonie indirecte". Lumiere Electrique, Vol. 4, 1881, pps 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, pps. 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, pps. 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, pps. 1541-1544.
• Weinhold, A. "Herstellung von Selenwiderstanden fur Photophonzwecke". E.T.Z., Vol 1, 1880, p. 423.

External links[link]

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".
• Long-distance Atmospheric Optical Communications, by Chris Long and Mike Groth (VK7MJ)

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