= Sun valve
{wiki}
A <sun valve> automatically restricts a gas light's supply in daylight and allows it again as illumination falls. In <Gustaf Dalén>'s design, a blackened rod and three polished rods respond differently to sunlight. Their relative <thermal expansion> operates the gas valve through a membrane; a pilot flame permits relighting. The museum dates this design to 1907.<Gustaf Dalén – Beacon Lighting by Tekniska museet>[Sun valve: date, black rod, three polished rods, membrane and daylight/night operation]{ref}
The useful comparison is between two thermal responses. Both kinds of rod expand when warmed; a dark surface can absorb more incident <radiant power>, so its <temperature> rises more. Comparing their extensions can reject a common ambient-<temperature> change while responding to illumination. The mechanism converts <light> into <heat>, <heat> into a difference of <length>, and that difference into a change of gas flow. <Differential thermal model of a sun valve> makes those assumptions quantitative.
\Image[/sun-valve-differential-rod-and-lighting-chain.png]
{title=Differential rod expansion controls daylight shutoff in a gas-lighting system}
{description=Original functional schematic, not a scale reconstruction or copied patent drawing. The three polished rods supply a mechanical reference for the dark rod. Daylight shutoff, pilot ignition and repeated flashing have separate functions. The linkage is symbolic; no historical rod dimensions or measured displacements are specified.}
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