One of the most powerful electric beacons in the world. Its maximum candle-power is 43,000,000.
The installation is not widely dissimilar from that used at the Isle of May. It comprises two De Meriten dynamos in duplicate, while the lamps are of the modified Serrin-Berjot type, using carbons, not of circular section, but with fluted sides. This shape was introduced by Sir James Douglass, who contended that the former type did not produce the requisite candle-like steadiness of the flame so essential to lighthouse illumination. The dioptric apparatus was of the sixteen panel type, so that the rays were thrown out in sixteen brilliantly white horizontal spokes. To one approaching the lighthouse at night-time, the effect in the sky was somewhat curious. It recalled a huge and illuminated cart wheel or Catherine wheel, lying flat on its side, throwing its rays to all points of the compass in a steadily moving circle. This practice had been borrowed from the French, who went so far as to introduce a twenty-four panel system, and, as in France, the St. Catherine’s light, when first brought into service, was not a complete success. The French considered that, by distributing the light through as many panels as possible, the question of bringing the flashes into action at short intervals would be facilitated, ignoring the fact that by so doing the intensity of each ray was impoverished. In other words, with the twenty-four panel light each panel only received and threw out one-twenty-fourth part of the volume of light emitted by the arc. Similarly, in the St. Catherine’s light only one-sixteenth part of the light produced was thrown through each panel. A few years ago the optical system was replaced by an apparatus having fewer panels. The light thrown from the Isle of Wight pharos, with its beam exceeding 5,000,000 candle-power, represents a marked advance upon the oil light which it displaced, and certainly it ranks as the most brilliant light in the English Channel.
A few years ago another magnificent light was brought into service in the North Sea by the installation of electricity in the lighthouse of Heligoland. With characteristic Teuton thoroughness, the Germans discussed the question of the illuminant for this beacon in all its bearings, and resolved to introduce the most powerful light possible. This decision was influenced by the dangerous character of the waters washing the island, as it is flanked on all sides by highly perilous ridges and sandbanks, which must become accentuated owing to the heavy sea-erosion that prevails.
The German authorities investigated the various electrical installations that had been laid down for lighthouse work, with a view to discovering the most suitable system, the advantages and defects of existing electric lights, and how the drawbacks might be overcome most successfully. Meantime the famous Siemens firm discovered a means of grinding glass mirrors into parabolic form, and this discovery was accepted as the solution to the problem.
In this type of mirror the back is silvered. The metallic polished surface is protected completely from mechanical injury and from all possibility of tarnishing. The inventors claim that mirrors so prepared are able to compete successfully with lenses and totally reflecting prisms—in fact, it was maintained that the silvered glass parabolic mirror possessed the advantages of greater reflecting power and enhanced accuracy, with less divergence of the beam of light.
Owing to the perfection of the lenses and prisms system of lighthouse optics, the introduction of arc lights in conjunction with parabolic mirrors was received with considerable hesitation. In order to dispel these doubts, the above-mentioned firm forthwith embarked upon an elaborate series of comparative tests at Nuremberg to ascertain the relative value of the two systems, and as a result of these experiments they concluded that quite as good an effect is obtainable with the arc and parabolic mirror as with the best examples of any other method.
Accordingly, the authorities decided to install the system in the Heligoland lighthouse. They stipulated that the intensity of the beam of light should be at least 30,000,000 candle-power, with a maximum current of 100 ampères. The duration of the flash was to be one-tenth of a second, followed by eclipses of five seconds’ duration.
The electrical engineering firm entrusted with the contract fulfilled these conditions by mounting three searchlights spaced 120 degrees apart upon a rotating platform. That is to say, each light is projected outwards from a point equal to a third of the circumference of a circle. The mirror diameter was settled at 75 centimetres (29½ inches) and the focal length at 250 millimetres (10 inches), the current being taken at 34 ampères when the table made four revolutions per minute.
Subsequently a fourth searchlight was introduced into the apparatus, for the purpose of practical experiments and observations concerning the duration of the light-flash. This fourth unit was mounted above the three searchlights, but in the axis itself. It is so disposed that its flash comes midway between any of the two below, and it is arranged to rotate three times as quickly as the main group of lights. Accordingly, the duration of the flash thrown from the fourth searchlight is only one-third of the flash thrown by the others—that is, one-thirtieth of a second. This lamp is provided with all the necessary mechanism for keeping it in steady rotation at the increased speed, and for drawing current from its feed-cable.
Before the installation was placed in the lighthouse at Heligoland, it was submitted to searching tests at the Nuremberg works of the builders. These trials proved that with a current of only 26 ampères the average intensity was as high as 34,000,000 candle-power, with a maximum of nearly 40,000,000 candle-power; while with 34 ampères the average intensity rose to approximately 40,000,000, with a maximum of nearly 43,000,000 candle-power. Accordingly, the terms of the contract were fulfilled completely.