This device is based upon a well-known principle. If two objects, fashioned from the same metal, and identical in every respect except that one is made light-absorbing and the other light-reflecting, are exposed to daylight, while the former will expand, the latter will remain unaffected. This result is due to the fact that the one which absorbs light transforms it into energy. The acting part of the “sun-valve” therefore is a light-absorber. It consists of a central rod, the surface of which is coated with lampblack, so that its light-absorbing qualities are enhanced as much as possible. The lower part of this rod is connected to a small lever, which opens and shuts an orifice through which the gas passes to the flasher in the lantern above. Around this central black copper rod are three other copper rods, disposed equidistantly. They resemble the former in every respect except that they have no light-absorbing qualities, but they are given polished gold surfaces, so that their light-reflecting properties are raised to the maximum.
This sun-valve is exposed. At the break of dawn, under the gathering intensity of daylight, the central black rod absorbs the luminosity, the amount of which is increased by the light thrown from the gold-burnished outer rods, and, converting it into energy, expands longitudinally. In so doing it forces the lever at the base downwards, closing the opening through which the gas flows to the flasher. In a short while, when the day has broken fairly and there is no further need for the beacon’s services, the gas-feed is cut off entirely, only the pilot burner remaining alight, the gas-supply to this not being affected by the sun-valve. In order to bring the greatest possible pressure upon the lever, the blackened rod is so arranged that it can expand only in one direction—namely, downwards.
Upon the approach of evening, owing to the daylight becoming weaker, the blackened rod contracts, and, the pressure upon the lever being released, the gas commences to flow once more to the burner. It is a small stream at first, but as the darkness gathers, and the shrinking continues, the valve opens wider and wider, until at last, when night has settled down and the copper central rod has fully contracted, the gas-valve is opened to its fullest extent, permitting the greatest pressure of gas to flow to the burner, so that the beacon throws its most brilliant light. This automatic action continues infallibly every dawn and dusk, and is the simplest and at the same time most reliable means of economizing gas during the day that has yet been devised.
There is another feature of this system which must not be overlooked. Suppose, for some reason or other, that the sea becomes shrouded in suffused light, such as might arise from the obscuring of the sun by an overhanging bank of fog or smoke, the beacon comes automatically into service, as the cutting off of the daylight must bring about a contraction of the blackened copper rod controlling the valve.
The central rod can be adjusted to any degree of sensitiveness, by means of a screw, while protection of the vital parts is insured by enclosure within a heavy glass cylinder. The first apparatus of this character was tested by the Swedish authorities in 1907, and proved so successful that it is now in service at all the exposed unattended lighthouses in Swedish and Finnish waters; while it has been adopted, also, very extensively by the United States, more particularly for the lighting of the lonely stretches of the Alaskan coastline and of the Panama Canal.
Of course, the saving of gas which is rendered possible by the use of the sun-valve varies according to the season of the year. During the winter, when the nights are long, the saving may not be very marked, but in the summer, when darkness does not last more than four or five hours, the economy is very noticeable. According to the experience of the Swedish authorities, the average saving of gas during the year varies from 35 to 40 per cent., as compared with similar lights not fitted with this device.
But there is another factor which is influenced to a very appreciable degree by the utilization of the sun-valve. By cutting off the light when it is not required, the capacity of—i.e., the duration of service upon—one charge is lengthened, and this in the case of an isolated light is a very important consideration. In fact, with the “Aga” system wherein the sun-valve is combined with the flasher, it is possible for the light to work a round twelve months without the least control or necessity for intermediate inspection, and at as low an annual charge as £2 15s., or about $14.
THE LAGERHOLMEN LIGHTHOUSE.