Photophone

thumb|alt=An image of darkened brass historical plaque with a streak of green corrosion running down it, mounted on the exterior side of a brick building. |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

thumb|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 NW in Washington, D.C.

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.

In its ultimate electronic form, the photophone receiver used a photoresistor based on a selenium cell at the focus of a parabolic mirror. 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

thumb|Illustration of a photophone transmitter, showing the path of reflected sunlight, before and after being modulated

thumb|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 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.

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:

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, 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:]]

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 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.

By 1935 the German Carl Zeiss Company had started producing infrared photophones for the German Army's tank battalions, employing tungsten lamps with infrared 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.

References

Footnotes

Citations

Bibliography

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. .
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. .

External links

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)
Téléphone et photophone: les contributions indirectes de Graham Bell à l'idée de la vision à distance par l'électricité (1880–1895

First successful wireless voice communications

Reception and adoption

See also

References

Further reading

External links