Design
Bell and Tainter's invention was based on the discovery of photovoltaic properties of certain materials by A. E. Becquerel in 1839. The Photophone used crystalline selenium cells at the focal point of its parabolic receiver. This material's electrical resistance varies inversely with the illumination falling upon it, i.e., its resistance is higher when it is in the dark, and lower when it is exposed to light. The idea of the Photophone was thus to modulate a light beam: the resulting varying illumination of the receiver would induce a corresponding varying resistance in the selenium cells, which was then used to regenerate the sounds captured by the telephone receiver.
The modulation of the transmitted light beam was done by a mirror made to vibrate by a person's voice: the thin mirror would alternate between concave and convex forms, thus focusing or dispersing the light from the light source. The Photophone functioned similarly to the telephone, except that the Photophone used modulated light as a means of transmitting information, while the telephone relied on a modulated electrical signal carried over a conductive wire circuit.
Bell described it in his writings:
| “ | We have found that the simplest form of apparatus for producing the effect (transmitter/modulator) consists of a plane of mirror of flexible material against the back of which the speaker's voice is directed. Under the action of the voice the mirror becomes alternately convex and concave and thus alternately scatters and condenses the light. | ” |
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