NEW YORK PUBLIC LIBRARY / SCIENCE PHOTO LIBRARY NEW YORK PUBLIC LIBRARY / SCIENCE PHOTO LIBRARY
Sun valve (upper left) on a routing beacon. A Sun valve consists of a central blackened metal rod, surrounded by three polished rods, all enclosed in a glass tube. When the Sun rises the blackened rod heats up and expands to become slightly longer than the polished rods. The expansion triggers a device to shut of gas flow to the beacon. As the Sun sets the blackened rod contracts, starting the flow of gas again. The Sun valve was invented by the Swedish industrialist Gustaf Dalen in 1907, initially for use in lighthouses. Photographed in the USA.
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