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Photophone[edit]

Main article: Photophone

Bell and Tainter's Photophone receiver, one part of the device to conduct optical telephony.

The Photophone, also known as a radiophone, was invented jointly by Bell and his then-assistant Sumner Tainter on February 19, 1880, at Bell's 1325 'L' Street laboratory in Washington, D.C.^[8][30]

Bell believed the photophone was his most important invention. The device allowed for the transmission of sound on a beam of light. On April 1, 1880, and also described by plaque as occurring on June 3, Bell's assistant transmitted the world's first wireless telephone message to him on their newly invented form oftelecommunication, the far advanced precursor to fiber-optic communications. The wireless call was sent from the Franklin School to the window of Bell's laboratory, some 213 meters away.^[17][31][32]

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".^[33]

Bell transferred the Photophone's rights to the National Bell Telephone Company in May 1880.^[34] The master patent for the Photophone (U.S. Patent 235,199Apparatus for Signalling and Communicating, called Photophone), was issued in December 1880,^[32] many decades before its principles could be applied to practical applications.