First Transmission on Alexander Graham Bell's Photophone

On June 3, 1880, Alexander Graham Bell transmitted the first wireless telephone message on his newly invented photophone from the top of the Franklin School in Washington, D.C. Bell believed that the photophone was his most important invention.

The device allowed the transmission of sound on a beam of light. Of the eighteen patents granted in Bell's name alone, and the twelve that he shared with his collaborators, four were for the 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.
Diagram from Bell's 1880 paper.

Of the eighteen patents granted in Bell's name alone, and the twelve he shared with his collaborators, four were for the photophone.

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.