PHOTOPHONE, the name given an ap paratus for transmitting articulate speech to a distance along a beam of light. It was first de scribed in 1880 by Alexander Graham Bell, known in connection with the telephone, at the Boston meeting of the American Association; but already in 1878 its inventor had announced the possibility of "hearing a by means of a similar agency. It has never been com mercially introduced. The success of the photo phone depends on the peculiarities of the metal selenium. Crystalline selenium offers a high degree of resistance to the passage of an elec tric current; it is eminently sensitive to light; and the resistance is less when exposed to light than in the dark, being in some cases only a 15th in the light of what it is in the dark. Pro fessor Bell, his friends and assistants devised some 50 forms of apparatus, for so varying the transmission of light to prepared selenium as to produce audible sound. In the photophone found most serviceable the transmitter is a plane mirror of silvered microscope glass or thin mica; the receiver, fixed at a distance with out any connection, is a parabolic reflecting mirror, in the focus of which is placed a sensi tive selenium connected in local circuit with a battery and telephone. When the appa ratus is used, a strong beam of light is con centrated by a lens in the plane mirror; the speaker directs his voice against the back of this mirror, which is thrown into vibrations corresponding with those of the voice. The
reflected beam of light, to which corresponding vibrations are also communicated, is directed through a lens to the receiving mirror, and creates in the selenium cell a rapidly variable current, which at the end of the telephone at tached becomes audible again as vocal sound. When first described, the photophone had been used effectively with a distance of 230 yards between transmitter and receiver. The rays of the oxyhydrogen light, or of an ordinary kero sene lamp, suffice for transmitting articulate speech. The loudest sounds obtained from the photophone were produced by means of a per forated disc, noiselessly revolving so as rapidly to interrupt the light in transmission. It was also found that a very audible sound could he Procured from the selenium without the aid of telephone and battery. A beam of intermit tent light will produce a strong musical note from the selenium. Further experiment showed that selenium is not the only substance thus sen sitive to light. Still louder sounds than these obtained from the selenium directly, though not articulate, were got from diaphragms of hard india-rubber and of antimony; and sounds of varying intensity were given out by many other substances, including gold, silver, platinum, cop per, zinc, lead, paper, parchment and wood. Consult Bell, 'The Photophone) (1878). See