HÄLLÖ

The lighthouse on Hållö was built in 1842. For a very long time the Swedish coast was sparsley and feebly illuminated, until in the second half of the nineteenth century a number of Swedish inventions in the field of lighthouse technology pointed the way forward: a lamp running unattended for ten days on vapourized oil, a propeller which was turned by the heat of the lamp and supported screens to produce the desired signal from the lighthouse, an incandescent mantle which gave an intense light from vapourized paraffin, Venetian blinds which flashed out the lighthouse characteristic.

Gustaf Dalén completed his opening and closing device for automatically regulating the supply of acetylene gas in 1906. The lighthouse flashed out its signal for months without human intervention, as long as the gas lasted. But gas was expensive and Dalén's next invention extinguished the gas during the hours of daylight.

The solar valve earned him the Nobel Prize for Physics in 1912. He completed the unmanned lighthouse with an automatic mantle changer and a development of the regulating device, which was now able to drive the heavy lens apparatus.

In modern Swedish lighthouses, closing devices and lights run on electricity. The gas pipe has been replaced by solar cells and batteries and the solar valve by the photoelectric cell. But if the Frenchman Fresnel were to look down from his place in heaven, he would recognize his invention of the 1820s in today's acrylic resin lens.