An unattended light had to save its own fuel
A beacon on a remote rock must shine when a ship needs it and keep working between service visits. Burning acetylene continuously wastes much of the fuel during daylight. Gustaf Dalén’s sun valve, developed in 1907, used the warmth of sunlight to close the gas supply by day and reopen it as evening arrived. It performed the switch mechanically.
This was one part of Dalén’s work on lighthouse lighting at AGA. A flashing apparatus supplied short bursts of gas instead of a continuous large flame. A small pilot flame remained available to light the bursts. Flashing reduced fuel use, and the timing could give different beacons recognizable patterns. The sun valve added another saving: the main light could rest through the bright part of the day.
The black rod warmed more than its shiny neighbours
Inside a protective glass enclosure, one blackened metal rod stood among three shiny rods. All were exposed to much the same surroundings. Their surfaces handled sunlight differently. The black surface absorbed more of the incoming radiation and became warmer; the polished surfaces reflected more and stayed relatively cooler.
Heating makes a metal rod expand. The warmer central rod therefore lengthened more than its shiny neighbours. A mechanical connection turned this small relative movement into closure of the gas valve. At dusk the central rod cooled, the difference in length decreased, and the valve reopened. The returning gas could be lit by the pilot flame.
The useful quantity was the difference between the rods. A change in the general air temperature tends to affect the neighbouring rods together. Sunlight adds a larger warming effect to the blackened one. The design used a comparison built into the instrument to make its response to radiation more distinct from its response to the ambient temperature.
Imagine a cool, sunny morning. Air temperature alone might suggest that a thermal switch should remain in its night state. Here the direct radiation warms the absorbing rod relative to its references, so that the mechanism can still respond to daylight. Conversely, the rods approach one another’s temperature as illumination falls. Comparing neighbouring surfaces was a practical way to make the sunlight itself matter.
A small movement could command a much larger flow
Tekniska museet describes a rod expansion of only two thousandths of a millimetre as enough to extinguish the beacon. The exact adjustment belongs to the particular mechanism, but the scale conveys the engineering problem: a very small thermal movement had to operate a reliable gas control. The valve did not need sunlight to provide the energy of the flame. Sunlight only had to move the mechanism that admitted fuel.
The valve also needed time to warm and cool. A cloud passing across the sky changes the incoming radiation before the rods have fully changed temperature. Their thermal response gives the switch a physical timescale. Adjustment of the mechanism determines when the relative motion becomes large enough to change the gas supply.
This is environmental control through a sensor and an actuator embodied in metal parts. The sun valve responds to incoming daylight. It does not continually measure the beacon’s brightness, compare it with a target and correct an error using a modern electronic controller. The gas flasher and the daylight switch carry out different jobs within the lighting system.
The award recognized a working lighting system
The 1912 Physics Prize recognized Dalén’s automatic regulators used together with gas accumulators for illuminating lighthouses and buoys. Its scope was broader than one rod responding to sunlight. Fuel storage, controlled flashing and daylight shutoff had to work together to make unattended lighting practical.
The presentation speech stressed the growing need for navigational lights and the difficulty of continuously supervising every installation. Dalén’s work answered that concrete problem. A useful beacon had to last through a long interval away from a workshop, using its stored gas at the right times. The cleverness of the sun valve lies in how little apparatus it needed to make that decision.
The physical idea is still easy to hold in mind: shine the same light on two different surfaces, let the absorbed energy produce a measurable difference, and use that difference to move a valve. The distant lamp becomes automatic through an ordinary material property, arranged with unusual care.