Lighthouse Technology

The mechanics

More than a century before the Split Rock Lighthouse was built, U.S. lighthouses had begun to revolve their lights. At the time, the lights were composed of a variable number of wick-based oil lamps, and revolving them produced 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.

The Fresnel lens

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.

Split Rock Lighthouse used such a lens. While 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, the lighthouse could achieve a substantial increase in brightness 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.

The light

When oil vapor lamps — still a relatively new innovation in 1910 — were installed at Split Rock, the light became one of the most powerful beacons shining on the Great Lakes. A brass fuel assembly of 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 resulting beacon, officially visible for 22 miles, was claimed to have been seen by fishermen as far away as Grand Marais, more than 60 miles away.

When the lightstation and fog signal were electrified in 1940, the kerosene vapor lamp was retired and replaced by a Fresnel lens.

The fog signal

Audible fog signals have been used for hundreds of years, in forms as varied as bells and gongs, steam-powered whistles, and cannons, guns, and other explosives.

In the 19th century, engineers began to explore ideas to automate and mechanize the signals. By the time the Split Rock Lighthouse was built, a new device called a diaphone became the standard fog signal apparatus. Originally invented as an organ stop, it used a vibrating air stream to create a powerful low-frequency tone that could be heard miles away.

At Split Rock, the lighthouse keepers would sound the fog signal at 20-second intervals whenever visibility dropped below about five miles.