Photophone

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The photophone or photophone (Greek) is a wireless light telephone invented jointly by Alexander Graham Bell and Charles Sumner Tainter in 1880 for the long-distance transmission of sound and a forerunner of optical radio relay . The transmitter uses a light source and a device with converging lenses , with a mirror vibrating as a modulator . At the receiver, at the focal point of a parabolic mirror, the light converter was initially a membrane in an ear tube and later a selenium cell in a telephone circuit .

functionality

Bell's photophone (schematic representation)

The photophone initially managed without electricity . It is functionally the same as a telephone except that it used modulated light as a medium and was wireless, while the telephone transmitted modulated electricity via a conductor.

Bell described the simplest form of acousto-optical converter as a modulator, a flat, flexible, thin mirror. If the speaker's voice is directed towards the speaker's back, the mirror vibrates to the rhythm of the sound waves and modulates the light.

The light falls through a lens onto the mirror at the focal point and the reflected beam goes through another lens.

The receiver focuses the beam in the parabolic mirror onto the converter at the focal point, e.g. B. a hard rubber plate in an ear tube. In the last version of the photophone , the selenium cell , invented in 1872, sat here as an opto- electrical converter, together with the telephone receiver invented in 1876 , a volume control and a galvanic battery .

Selenium is a light-sensitive photosemiconductor and an opto-electrical converter. A selenium cell consists e.g. B. from 2 parallel wound flat spirals made of thin brass wire on a selenium-coated mica sheet .

Significance for Bell

The photophone wirelessly transmitted speech and song 19 years before the first radio transmission, and Bell described it as his most important invention on the deathbed, even before the telephone. He shared his enthusiasm with his father:

I heard articulated language through the sunlight! I heard a ray of sunshine laughing and coughing and singing! ... I could listen to a shadow and with my ear I heard the passing of a cloud across the solar disk. You are the grandfather of the photophone and I want to share my joy of success. - Bell in a letter to his father Alexander Melville Bell dated February 26, 1880

literature

  • Alexander Graham Bell: The photophone. Lecture given at XXIX. Annual Meeting of the American Society for the Advancement of Science, Boston, August 1880 . Quant & Handel, Leipzig 1880.
  • Alexander Graham Bell: AG Bell's Photophon. In: Polytechnisches Journal . 238, 1860, pp. 409-413.
  • Jürgen Jahns: Photonics. Basics - components and systems. Oldenbourg Verlag, Munich 2001, ISBN 3-486-25425-1 , pp. 1-2.
  • Clara Völker: Mobile Media. On the genealogy of mobile communications and the history of ideas of virtuality. transcript Verlag, Bielefeld 2010, ISBN 978-3-8376-1372-8 , pp. 166-169.

Web links

Commons : Photophon  - collection of images, videos and audio files