ABSTRACT
The end of chapter 8 contained the story of the photophone, an invention patented by Alexander Graham Bell but only produced in small numbers. It suffered from two major limitations. First, the carrier for the acoustic waveform was sunlight, a commodity unavailable at night and during bad weather. Secondly, the light travelled in straight lines through the air, so that the beam had to be aimed accurately at the receiver and could not penetrate smoke, fog or heavy rain. In contrast, the telephone utilized an electric current, which could be sent along flexible copper cables regardless of the weather or the time of day. Bright reliable lamps and cables capable of guiding light would have made the photophone available twenty-four hours per day.
