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The Photophone

Alexander Graham Bell was known for investigating new ideas throughout his life. In 1879, he began work on an alternative to the electrical telephone: the photophone. This device would use beams of sunlight to transmit speech to an electrical receiver similar to the receiver in the telephone.

The invention depended on crystals of photo-sensitive selenium, which responded to the amount of light falling on them by conducting more or less electricity. A person’s voice was projected through a glass tube towards a thin, flexible mirror. The ensuing vibrations would distort the mirror and create variations in the beam of light striking the selenium cell, which in turn led to the varying of the electrical current flowing through the circuit. The fluctuating currents were then changed into speech by the photophone’s receiver. This is similar to the principle of variable electric current used by Bell for transmission of sound signals in his telephone.

By 1880, Bell could observe audible effects over a distance of 270 feet (82 metres), and was optimistic that the photophone would replace electrical transmitters that depended on the use of wires. He patented his invention in that year. However, the technology depended on transmission through the air and was not reliable. Clouds, for example, could easily disrupt the process. As well, attempts at increasing the distance between transmitter and receiver were unsuccessful. The two parts of the photophone had to be in sight of each other to be able to function.

In 1881, Bell turned his attention to a related project, the spectrophone, which would produce audible signals from infrared light. He then became interested in improving the phonograph and abandoned further work on the photophone.

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