Optical telegraphy. Historical artwork of French soldiers using a device for optical telegraphy (sending and receiving messages using light). One such early device was the Mangin projector, which was invented in the early 1880s. The visibililty of the signal varied from 20-100 kilometres, and depended on the weather and if it was day or night. Early devices used oil lamps at night, but this later device is using a battery (below lamp) to power an electric light. The soldier at lower right is recording a message. Optical telegraphy was soon replaced by electrical telegraphy. This is a French artwork in a series on the history of electricity, published in the early 20th century.
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