Split Rock Lighthouse Beacon
Split Rock Lighthouse is the most popular and photographed lighthouse on the Great Lakes.
Built in 1910 Split Rock Lighthouse is topped with a large, steel lantern which features a third order, bi-valve type Fresnel lens, so called due to its resemblance to a mollusk shell. manufactured by Barbier, Bernard and Turenne Company in Paris, France. The lens floats on a bearing surface of liquid mercury which allows near frictionless operation. The lens is rotated by an elaborate clockwork mechanism that is powered by weights running down the center of the tower which are then reset by cranking them back to the top. When completed, the lighthouse was lit with an incandescent oil vapor lamp that burned kerosene.
Reflecting and refracting prisms recovered 60 percent of the light emitted from the fixed lighte and concentrated it into two beams that emerged from the central magnifying portion of each lens panel. The characteristic flash that swept the horizon once every 10 seconds at Split Rock was created by adjusting a mechanical governor on the clockwork mechanism.
By virtue of the tower’s location atop the cliff, the lens boasted an impressive 168-foot focal plane, and was visible for a distance of 22 miles in clear weather condition.
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