A photographophone is a device that was first developed by

Ernst Ruhmer Ernst Walter Ruhmer (15 April 1878 – 8 April 1913) was a German physicist. He was best known for investigating practical applications making use of the light-sensitivity properties of selenium, which he employed in developing wireless telephony u ...

Berlin Berlin ( , ) is the capital and List of cities in Germany by population, largest city of Germany by both area and population. Its 3.7 million inhabitants make it the European Union's List of cities in the European Union by population within ci ...

, Germany in 1900. The Photographophone could record and reproduce speech and music through a

celluloid Celluloids are a class of materials produced by mixing nitrocellulose and camphor, often with added dyes and other agents. Once much more common for its use as photographic film before the advent of safer methods, celluloid's common contemporary ...

film. The process started by speaking into a

microphone A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and publ ...

. The electrical signal from the microphone through a

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

supplied electric current from a battery pack that caused a corresponding variation in the light of an arc (later used an incandescent lamp). The light from the arc lamp passes through the cylindrical lens slot which created sharp white lines on the moving sensitive film. This film, after being taken out of the box and developed, shows a series of perpendicular striations parallel to one another, which are really a photographic record of the sound waves originally entering the telephone transmitter. To reproduce the sound an projector directs light through the film traveling with the velocity equal to that with which the record is made. Behind the film a sensitive selenium cell is mounted receiving the variations in light producing a variation in its resistance and a corresponding effect in the telephone receivers connected. "It is truly a wonderful process: sound becomes electricity, becomes light, causes chemical action, becomes light and electricity again, and finally sound."''Wireless Telephony In Theory and Practice''
by Ernst Ruhmer (translated from the German by James Erskine-Murray), 1908, page 39.

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BY ERNST RUHMER, Scientific American, July 20, 1901 Display technology History of film Film and video technology Film sound production Audiovisual introductions in 1900 {{film-tech-stub