Seeing by electricity, the earliest notions (1878-1880)
The 1873 discovery of the effects of light upon the resistance of a selenium bar is important historically, not so much for any practical value which selenium might have had for the purpose, but for the glut of schemes and proposals which were made for television systems in the years which followed. Selenium, which belongs to the sulphur and tellurium family, is a nonmetallic element and was first discovered by Berzelius in 1817 in a red deposit found at the bottom of sulphuric acid chambers when pyrites containing selenium was used. Like sulphur it exists in several modifications, being obtained as a dark red amorphous powder, as a brownish black glass mass, as red monclinic crystals or as a bluish grey, metal-like crystalline mass. In its natural state, selenium is almost a nonconductor of electricity, its specific conductivity being forty thousand million times smaller than that of copper, but Knox, in 1837, found that on being annealed it became a conductor having a large resistivity compared to that of copper. It was this property which led to its use in certain experiments by Willoughby Smith, and to the discovery which he reported a letter to Mr. Latimer Clark, then Vice-President of the Society of Telegraph Engineers.
Seeing by electricity, the earliest notions (1878-1880), Page 1 of 2
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