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"Four years after the invention of the telephone, Alexander Graham Bell caused flurries of excitement with another invention, which he described in a series of essays and lectures in the US and Britain during the autumn of 1880. The device was what he called the 'photophone'. It depended upon the discovery made by Willoughby Smith in 1873, during the course of work on the Atlantic undersea telephone cable, that the resistance of the material selenium, which was ordinarily extremely high, in fact varied with the action of light, exposure to light lowering the resistance of the material."