Glossary
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Glossary Definition: Photophone
The photophone was originally invented jointly by Alexander Graham Bell and his assistant Sumner Tainter on February 19, 1880. Then on June 3, 1880, Alexander Graham Bell transmitted the first wireless telephone message on his newly-invented photophone. Bell believed the photophone was his most important invention. The device allowed for the transmission of sound on a beam of light. 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.