Because of their remote outpost, lighthouse technology often did not rely on electricity and the internal combustion engine, both widely available at the beginning of the 20th century. Instead the lightstations relied on traditional mechanics capable of great precision.
More than a century before the Split Rock Lighthouse was built, United States lighthouses had begun to revolve their lights - then composed of a variable number of wick-based oil lamps - to produce the regular flashing signals. This rotation was accomplished by a clockwork mechanism with a gear box attached to the base of the lens pedestal and connected to a cable that had to be wound by hand every two hours through the night. The cable supported a column of disk weights that dropped by gravity through the hollow mast at the center of the tower. Their vertical descent was translated into the horizontal rotation of the lens assembly.
In 1822 French physicist Augustin-Jean Fresnel developed the annular or ring-shaped lens. The light could now be flashed by rotating only the lens assembly while the light source itself remained stationary. By the 1890s a tray of mercury was used as a bearing surface. With the Split Rock lens literally floating in 250 pounds of liquid mercury, it became possible to rotate the mammoth, 1,500 pound apparatus fast enough so that only two lens panels were necessary to produce a frequent signal. Since fewer individual panels means less diffusion of light, a substantial increase in the brightness could be obtained from the same lamp.
Reflecting and refracting prisms recovered 60 percent of the light emitted from the fixed source 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.
Still a relatively new innovation in 1910, oil vapor lamps were installed at Split Rock, making the light one of the most powerful of the more than 400 lighthouses and beacons shining on the Great Lakes. A brass fuel assembly of initially a double and later a single tank fit snugly against the lens pedestal. Filled with kerosene, the tank was pumped up by hand at intervals each night with enough air pressure to operate the light until dawn. The kerosene, vaporized by being passed over a Bunsen flame, created a light dramatically more efficient and powerful than the old concentric wick lamp that burned the fuel directly. The glare, officially visible for 22 miles, could be seen by fishermen as far away as Grand Marais, more than 60 miles distant.
When the lightstation and fog signal were electrified in 1940, the kerosene vapor lamp was retired and replaced by a 1,000-watt light bulb.
Explore fast facts about the Split Rock light station and fog signal.