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 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:

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

Of the eighteen patents 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 and 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.

In the Washington, D.C. experiment, Bell and Tainter succeeded in communicating clearly over a distance of some 700 ft. (about 213 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 Ω and 100 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 interferences, such as clouds, that easily disrupted transport. Until the development of modern fiber optics, technology for the secure transport of light inhibited use of Bell's invention.

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, September 1880.

External links

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.
Photo of the plaque