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The photophone (later given the alternate name radiophone) is a telecommunications device which allowed for the 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 in Washington, D.C.〔Bruce 1990, pg. 336〕〔 Both were later to become full associates in the Volta Laboratory Association, created and financed by Bell. On June 3, 1880, Bell's assistant transmitted a wireless voice telephone message from the roof of the Franklin School to the window of Bell's laboratory, some 213 meters (about 700 ft.) away.〔Bruce 1990, pg. 338〕〔Carson 2007, pg. 76-78〕〔〔Mims 1982, p. 11.〕 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 (have ) ever made, greater than the telephone".〔〔Mims 1982, p. 14.〕 The photophone was a precursor to the fiber-optic communication systems which achieved worldwide popular usage starting in the 1980s.〔〔〔 The master patent for the photophone ( ''Apparatus for Signalling and Communicating, called Photophone'') was issued in December 1880,〔 many decades before its principles came to have practical applications. == Design == The photophone was similar to a contemporary telephone, except that it used modulated light as a means of wireless transmission while the telephone relied on modulated electricity carried over a conductive wire circuit. Bell's own description of the light modulator:〔 The brightness of a reflected beam of light, as observed from the location of the receiver, therefore varied in accordance with the audio-frequency variations in air pressure—the sound waves—which acted upon the mirror. In its initial form, the photophone receiver was also non-electronic. Bell found that many substances could be used as direct light-to-sound transducers. Lampblack proved to be outstanding. Using a fully modulated beam of sunlight as a test signal, one experimental receiver design, employing only a deposit of lampblack, produced a tone that Bell described as "painfully loud" to an ear pressed close to the device. In its ultimate electronic form, the photophone receiver used a simple selenium cell at the focus of a parabolic mirror.〔 The cell's electrical resistance (between about 100 and 300 ohms) varied inversely with the light falling upon it, i.e., its resistance was higher when dimly lit, lower when brightly lit. The selenium cell took the place of a carbon microphone—also a variable-resistance device—in the circuit of what was otherwise essentially an ordinary telephone, consisting of a battery, an electromagnetic earphone, and the variable resistance, all connected in series. The selenium modulated the current flowing through the circuit, and the current was converted back into variations of air pressure—sound—by the earphone. In his speech to the American Association for the Advancement of Science in August 1880, Bell gave credit to the first demonstration of speech transmission by light to Mr. A.C. Brown of London in the Fall of 1878.〔〔 The French scientist Ernest Mercadier suggested that the invention should not be named 'photophone', but 'radiophone', as its mirrors reflected the Sun's radiant energy in multiple bands including the invisible infrared band.〔Grosvenor and Wesson 1997, p. 104.〕 For a period of time the invention also used the latter name. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Photophone」の詳細全文を読む スポンサード リンク
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